)
B.P Tissot D.H. Welte
r '
PetroleumFormation andOccurrence
I
SecondRevisedand EnlareedEdition
With 327Figures
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PRO.IECT COFY
FEB2 5 1985 La Habra,Californ ia
Springer-Verlag Berlin HeidelbergNewYorkTokyo1984
Bsnl.{anoP Trssor Professor InstitutFnncaisdu P6trole du Petrole andEcoleNationaleSup6rieure 92506Rueil,France 4, avenuede Bois-Pr6au, Professor Drerrtcs H. WELTE JiilichGmbH Kernforschungsanlage Geochemie Institutliir Erdtilund Organische Postfach1913, 5170Jiilich,FRG and mbH IES Gesellschaft ftir IntegrierteExplorationssysteme Kartauserstr 2, 5170Jiilich,FRG
NewYorkTokyo BerlinHeidelberg 2. Auflage Springer-Verlag ISBN3-5,10-13281-3 NewYotkTokyo BerlinHeidelberg 2ndedjtionSpringer-Verlag ISBN0-38713281-3 ISBN 3-54t}{8698-6L Auflag€ SpfidgeFverlagBerlin tleidelberg NewYork Tokyo Berlin HeidelbergNewYork Tokvo ISBN 0-38748698-6lstedition Springer-Verlag
Library ofCongress Catalogingin Publication DalaTissot, B. P (Bernard P), 1931. Pelroleum formation and occurrence.Includes biblio' graphiesand indeJL1. Petroleum - Geology.2. Gas, Natural - Geology.l. Geochemical prospecting-L welte, D.lt. (Dielrich H.) 1935-.II. Title. TN870.5.T51.1984 5532'8. 84-10484. This work is subject10 copy.ight. All righls are reserved,whether the whole or pan of the material is concerned.specificallythose oflranslation, reprinting, re_useof illuslrations. broadcasting, reproduction by photocopying machine or similar means, and sloragein data banks-Under $ 54 ofthe German Copyright Law wh€re copiesare made "Verwenungsgesellschaft Won", Munich for orher than private use, a fee is payableto Berlin Heidelberg1978and 1984. O by Sprioger-Verlag Printed in Germanx The use of registerednames,trademarks,etc. in this publication does not imply' even in the absenceof a specific statement,that such names are exempt from the relevanl protectivelaws and regulationsand therefor€free for generaluse Typ€sertingand prinling: Beltz Offsetdruck,H€msbach/Bergstr. Bookbinding: Konrad Triltsch,GraphischerBetrieb, wiirzburg 2132l3110-5432r0
r
Foreword
The publicationof this book PerroleamFormationand Occurrence by BernardTissotandDietrichWeltewill indeedbe welcomedby petroleumgeologists, petroleumgeochemists, teachersand studentsin thesefields.andall otherswho areinterested in the origin and accumulationof hydrocarbonsin nature. It is indeed a privilegefor us to havethe oppofiunityof sharingwith thesetwo eminentscientists the wealthof informationthey haveacquired and developedduring long careersdcvoted to concentrated scholarlystudyand practicalinvestigation of the nature,origin, and occurrence of petroleum. ProfessorBernard Tissotgraduatedfrom the Ecole Nationale Supdrieuredes Mines in 1954and from the Ecole Nationale Supdrieuredu Pdtrolein 1955.In 1955he receiveda D.E.S. in geologyfrom the Universityof Grenobleandthenbeganresearch work on petroleumgeologyat the InstitutFranEais du P6trole.He wasmadeheadof the Departmentof Geochemistry in 1965,and since 1970has also been teachingorganicgeochemistry at the EcoleNationaleSup6rieure du P6trolewherehe becameProfessor in 1973.ProfessorTissothas had a broad and variedbackgroundof practicalexperience.He hasbeena memberof exploration teamsin France,New Caledonia,and Sahara.ln 1960-1963 he headeda missionof the IFP to the Departmentof National Developmentof Australia.Outstanding amonghis achievements has been the use of the Paris basin as a laboratoryin the developmentof an understanding of the relationbetweentemperatureand petroleumgenesis.In recentyearshe hasdevoted considerableattention to North America and has oublished srudieo s n t h e o r i g i no f t h e A t h a b a s claa r s a n d sa n d i h e U i n t a Basin oil. Togetherwith J. Espitali6he is responsible for the development of newpyrolysistechniques for andinstrumentation the identificationof petroleumsourcerocks and their stageof maturationand a mathematical modelfor the thermalevolution of organicmatter in sediments.He is author or co-authorof numerousoutstanding scientificpaperson originandmigrationof petroleum publishedin both French and English in various neriodicalsand books. ProfessorDietrich Welte received his Ph. D. Degree in 1959 from the Universitvof Wirzburg in geolog.v and chemistry.He
Foreword
geochemist in the Hague thenworkedfor threeyearsasa research the origin of oil and Company on for the ShellInternationalOil where he established a Wiirzburg gas. In 1963he returncdto geochemistry. In 1966 he received researchlaboratoryin organic the President's awardfrom the AmericanAssociationof Petroleum Geologistsfor his outstandingpaper on Relationbetween Petroleumantl SourceRock. During 1966 he visited various in the USA andin 1967took a andindustriallaboratorics academic Geochemist andtowardthe endof his oositionasSeniorResearch Coordinatorfor Exploraitay wasgiventhe functionof Research tion with ChevronOil FieldResearchCompanv.Whilebeingwith research Chevron he was working on geological-geochemical projectsin California,the Gulf Coast.andotherareasin America. ln 1970-1972hc taughtat the Universityof Gottingenandsince and Oil and 1972hasbeenProfessorof Gcology.Geochemistry. Coal Depositsat thc Instituteof Technologyal Aachen.During his scientificcareer ProfessorWelte has made numerousoutstandingcontributionsto the knowledgeof the geochemistry of petrolcum,publishedin German and in Englishin various periodicalsand books,in additionto numerousprivatereports MostrecentlyProfesresultingfrom hisoil companyconnections. post of the newly founded of Director sor Welte acceptedthe at the Nuclear Institutefbr PetroleumandOrganicGeochemistry ResearchCenterJtilich,althoughhe will continuesometeaching in Aachen. Although residentin Europe and presentlyworkingin European institutions.Professors Tissot and Welte have both been frequenl sojournersin America and have long participated and geologists. activelyin meetingsof American geochemists on problemsof petroleumgenesis Both haveworkedextensively in Americaandhavepublisheda numberol papersin the Bulletin of the American Associationof PetroleumGeologists.Many on the Geologvof confercnces readerswill recallthe outstanding in Banff,Albertain May FluidsandOrganicMatterin Sediments specifictopics 1973at whichfor severaldaysthe authorspresented in this subjectareaand fieldedquestionsand commentsfrom a Both Tissotand Welte pickedaudienceof petroleumgeologists. haveparticipatedand aidedgreatlyin work on the hydrocarbons of coresfrom the JoidesDeep Sea Drilling Project, and are membersof thc JoidesPanelon OrganicGeochemistry. asfar backas scientists The originof petroleumhaschallenged associated was by many to be supposed the 17thcenturywhen it "phlogiston". a matter From being simply with the mysterious of the with development of scientificcuriosityit became, the petroleum industrv. a subject of vital practicalimportance. knowledgeof which has often meant the differencebetween success and failurein explorationfor petroleum.Much progress
Foreword
VII
hasbeenmadebv thecombinedeffortsofgeologists, geochemists, and geophysicists, but the subjectis still a dynamiconewith many baffling unknownsand uncertainties. Perhapseven more fillei w i t h u n c e r t r i n t i er .n d u n k n o w n a s t t h e p r e s i n td a y .a n dc l e a r l y just as critically important to practicaloil exploration,is thi mannerof migrationand accumulation of petroleum. T i \ s o tr n d W e l t ea p p r r r a q, 5 S. 1 , , r a c c t; o l o n l ) a sg c o c h e m i s t s but alsowell-armedwith geologicalknowledge.The book starts (Part I) with the productionand accumulation of organicmatter - l h c h i r : r cf c e d s t o ( k f ( ' r p c t r ( ) l c u mT. h e n [ o l l , i w st h e r e r v c r i t i c l l P l r t I l o n t h e t r a n s [ ' ) r m a t i (o] nf r h i r o r g a n i cm a t t e rt ; kerogenandthento oil, or gas.lmportantaspects of theformation of coal and oil shalesare also treated.part III dealswith the formationofoil andgaspoolsandthe knottyproblemof migration of petroleumfrom sourceto reservoir.Part IV is concerned *ith the compositionand classification of petroleum.,.geochemical fossils",and the relation bctweencharacterof petroleumand gcologicalenvironmentand subsequent alterationof oil in the reservoir.Finallv,Part V coversthe practicalidentificationand evaluationof sourcerocks. the correlationbetweenoils and betweenoils and sourcerocks,and an evaluationof the Dractical u s e s( ) l g c o c h e m i s t Irn) p e t r o l e u m exploration. The Tissot-Welte volumeis a clear,practical.readableaccount. h r l w o o f t h e w o r l d . l e a d i n gr e s e a i c h e ri n s r h e f i e l d ,o f t h e formation,migrationand accumulation of petroleum,primarily from the geochemical viewpoint.but adequatelyseasoned with geology.The text is further clarifiedby a wealthof tablesand f i g u r c sa n d i s a m i n eo f r e f e r e n c ien f o r m a t i o nT. h e t o D i c sa r e d e re l o p e di n a l o g i c Ii ra n dr e a s o n a bul ea yw i rh f u l lc r e d i ft u ro th e r and sometimesdivergentviews.One of the many meritsof the T i s s o t - W e l theo o ki s i t : o p e n - m i n d eadp p r r ; r c h to problema s nd its avoidance of a dogmaticattitude- the truemeasureof a reallv d e c pk n o w l e d g o eI a s u b j c c tA . p a r t i c u l a r lryt r o n gp o i n l i s t h ; attentionwhichthe book givesto practicalprocedures and to the complexinstrumentation rcquiredin modernpetroleumgeochemicalinvestigations, aswell as to the moretheorelicalaspects of the.ubjecr The bookwill fill a verl'muchneededplacebothasan advanced classroom t e x r .a n da r a g u i d ea n dr e f e r i n c ew o r ko n t h es u b i e c t . u h i c hs h o ul d h e i n rh eh a n d so f a l lp r a cilc i n gp e r o l e u mg e o l o g i5s r and geochemists. Thesewill undoubtedlyemerseas bettei oil f i n d e r sf o r l h e r e a d i n go f t h i 5v o l u m e . Holrrs D. HEDBERG Professor of Geology(Emeritus) PrinceronUniversity,1978
Prefaceto the SecondEdition
The first edition of Petroleum Formation and Occurrence was publishedabout 6 yearsago. In the preface,we expressedthe hoDe t h a t t h i s h o o k m r v h c l p t r )p r o m o l c I h e i n l e g r at i o n o f o r g a n i cg e o _ chemistry into the geosciencesand improve communication between geologists and more chemicallv oriented researchers inrolred in practicap l e t r o l e u me x p l o r a t i o n \. 4 e h e l i e v ew e h a v c indeed contributcd toward attainingthosegoals.Furthermore. we are very pleased to see both how rapidly this field of petroleum s c i e n c e si s g r r r u i n g l n d l u n o t c i t s i n t l u e n c co n g e o s c i e n c eisn g e n e r : r l I. t h r r sh e e o m cn e c e \ \ u r vt o e x p a n dt h e s c o p eo f t h c b t r o k in order to includc important new topics. suctr as biological m a r k e r r .t l . l sg e n e r l r i o n .h e a v l o i l s r n J i a r s r n d s . m i g r a t i , r n " a n , . l m o d e l i n g .n a r t i c u l i r r l \g e o l o g i c arl n d g e o c h e m i c am l o d e l i n gw i t h high speedcomputers.lt is evident that computer modelingii here to stay. and mav verv well revolutionize the field. The comouter can be used as an cxpcrimental tool to test geologicalideai and h l p o t h e s e ru h e n e v e ri r i s p o s s i h l er o p r o r i d i a d c l u a r e r o [ r w u r e for normallv verv complicated geologicalprocer.ei. The enorm_ ous advantages oftered by computer simulation of geoloqical p r o c c s \ c \i r r e t h u l n r , p h r r i c a l o r p h r s i c o - c h c m i c aplr i n c i p l e , , - a r e r i , r l a t e J a n r l l h l l , ) r r h ( [ i r \ r t i m c t h e g c o l o g i c a il i m e l a c l r ) r . alwaysmeasuredin millions ofvears rather than in decades.canbe handled with high speed computers with large memories. Thus. the age of true quantification in thc geoscienceshas arrived. We b e l i e r et h a t r h i sc o m p u r c r - a i J e d..1 u : i n r i t a r i vaep p r o a c hu i l l h a r e an economic and intellectual impact on the petroleum industrv. mainll on cxpl,'ratiol L)ur c,'ncqp1o 1 t p e i r o l e u mg e n e r a l i o ; . mlgratlon. and accumulationare becoming increasinglyquantifi_ able and hencecan be used as predictive tools in the seirc-hfor oil and gas. This approach helps to cut costs not only in mature exploration areaswhen looking for ..overlooked.,petroleum. but a l \ . ) i n h r g h - r i \ kl r ( r n t i e rr r e l \ . T h e i n t e l l e c t u arl e . r u l ct o u l r lb e I better chancefbr a much-ncededsynthesisof the principal fieldsin g e o s c i e n c es.u c h: r sg c o h r g v g . c r r p h r s i c ra. n d g e o c h e m i . , t l .W i t h this second edition ol our book. wc hope to contribute both to s c i e n r i f i c a l lm r o r c a d r a n c e da n t l h e n c ei i r k - d i m i n i s h i n ee x o l o r a l i o n s l r a t c g i e sa. n d l r l . o t r r t h e g r o u i n g r l c m l n , - lI o r a s r n e r g e t i c view of the qeosciences.
X
Prefaceto the SecondEdition
We havenot only updatedthe previoustext, but haveadded completelynewchapterson gas,heavyoilsandtar sands.distribution of world petroleumreserves,casehistorieson habitatof petroleum.and geologicaland geochemicalmodeling.Other chaptersor sectionshavebeenenlargedand rearranged. suchas thoseon geochemical fossils(biologicalmarkers).primarymigration. asphaltenes and resins.coal as a possiblcsourcerock, new techniquesof sourcerock characterization. and oil-sourcerock correlations.We havealsoincludcdmore than 270new references,added84 new illustrations.and modifiedothers.The index hasbeenconsiderably improvedandenlarged. Both of us are indebtedto co-workersand colleagues in our organizations. the InstitutFrangaisdu P€trole(lFP). Rueil Malmaison, and the Kernforschungsanlage-Jiilich (KFA). Special thanks for scientificadvice and critical commentsgo to P. Albrecht.B. Durand. D. Leythaeuser. R. Pelet.M. Radkeand J. Rullktttter.One of us (D.H.W) wantsto acknowledge the verv fruitful cooperationwith scientistsfrom the ShengliOil Field Research Institutein the People'sRepublicof China.We areboth thankful that permissionwas grantcdby the ShengliOil Field ResearchInstituteto publishintbrmationon the commonwork aboutthe Linyi Basinin NE China. The greathelpwe receivedfrom Mrs. R. Didelez(lES). Mrs. B. Hartung (KFA) and Mrs. G. Tramblay(lFP) ior organization, coordination andeditingofthemanuscript ofthe secondeditionof our bookis alsovcry thankfullyacknowledged. Finally.wewantto thankour wivesagainfor theirpatiencewith usduringour workon the secondeditionof our book.
J u l y .1 9 8 4
B. P. Tlssor D. H. Wgr-re
Preface to theFirstEditron
The subiect treated in this book Petroleum Formation and Occurrence rstrulyinterdisciplinary. It hasitsrootsin suchdiverse fieldsof scienceas biology,oceanography. and, mostimportant, in variousbranchesof chemistryand geology.Thoseconcerned eitherin academicor industrialwork. with research. or Dractical p r o h l e m sa s s o c i a t ewdi l h p e t r o l e u mh. a v el o n g r e c o g n i z erdh i s fact.A steadilyincreasing numberof thesepeopleis lookingfor a comprehensive sourceof informationon the variousaspectsof petroleumexploration. For a numberof yearswe hadbeengivingseminars in different countrieson the origin,migrationandaccumulation ofpetroleum. It wasfrom theseseminarsthatthe ideato write a bookwasborn. After discussing this matter,it wasclearthat sucha book should not be written by many authors,who might eachbe expertin a particularficld. but that it shouldbe writtenjust bv the two of us as generalists, evenif it meantan enormousamountof work. In thiswaywe had hopedthe book wouldbe easierto read,andthat the many facetsof this difficult subjectcould be better understood. It is our wish that people in the academicworld, advanced studentsin geosciences and chemistryor other branchesof science,wouldbenefitfrom thisbook.The book mavalsohelnto i n t e g r a toer g a n i g c e o c h e m i s lm r yo r ei n t ot h eg e o s c i e n cl eh \a nh a s beenthc caseup to now. Most of all. however,we hopethat this book is of use for thoseworking in petroleumexplorationand fields relatedto it. For a long time there has been a lack of communication betweengeologists activein practicalexploration, and researchers more involvedin chemicallyorientedlaboratory work: we hopeto havebridgedthat gap, Throughour book we want to demonstrate that the searchfor petroleumcanbenefitgreatlyfrom theintegrationandapplication of theprinciplesof petroleumgeneration andmigration.For many yearsthe decisionto drill a well waslargelytakenon the basisof the recognitionof suitablestructures.Then the selectionof a structurewasbasedmainlyon intuitionand generalexperience, because vervlittle informationwasavailablewhetheror not a trap would containhvdrocarbons. A svstematic utilizationof the new
XII
Prefaceto the First Edition
understanding of petroleumformationandoccurcomprehensive ratein rence,as presentedin this book, can improvethe success predictingpetroleum-filledstructuresand hencedecreasethe financialriskof drilling.To followthisconceptrequiresthatin the future petroleumgeologistsmust acquiresome knowledgeof petroleumgeochemistry. To this end the teachingof organic geochemistry has to be developed.We hope that this book can coveredin serveas a basictext for the petroleumgeochemistry sucha course. The completionof the book would have been impossible without the help of our co-workers.colleaguesand friends, in our organizations at the InstitutFranqaisdu Pdtrole especially (IFP). Rueil-Malmaison. the Kernforschungsanlage-Jtilich (KFA) and the Rheinisch-Westfalische TechnischeHochschule Aachen(RWTH). We are highly indebtedto P. Albrecht. Ch. C o r n f o r dW , . Dow.B. DurandG , . E g l i n t o nA, . H o o d ,R . P e l e t , J. Williamsand M. A. Yiikler. who criticallyreadand reviewed parts of the manuscript.We thank Mrs. R. Didelez for her during preparationand organizationof indefatigableassistance Lastbut not Ieastwe wantto thankour wives the final manuscript. for their patiencewith us whileworkingon the book. Finallyspecialthanksgo to H. D. Hedberg,who read all the usthroughoutthe text.gaveveryvaluableadvice,andencouraged work.
April. 1978
B. P.Trssor D. H. Welre
Contents
ProductionandAccumulationof OrganicMatter: A GeologicalPerspective
Part I
ChapterI
ProductionandAccumulationof Organic Matter:The OrganicCarbonCycle
- TheBasisfor MassProductionof 1.1 Photosynthesis O r g a n i cM al t e r 1.2 The OrganicCarbonBudgetDuringthe Historyof the Earth 1.3 The OrganicCarbonBudgetin theBlackSea S u m m a ray n dC o n c l u s i o n Chaprer2
Evolutionof theBiosphcre
andBacteria 2.I Phytoplankton 2.2 HigheP r lants Historyof theBiosphere 2.3 Geological S u m m a ral n dC o n c l u s i o n Chqpter3 3.1 3.2 3.3
ofModernAquatic BiologicalProductivity Environmenl.
PrimaryProducers of OrganicMatter FactorsInfluencingPrimaryProductivity PresentPrimaryProductionof theOceans S u m m a rayn dC o n c l u s i o n
Chapter4
of the Biomass: ChemicalComposition Zooplankton, Bacteria,Phytoplankton, H i g h e rP l a n t s
4 .I P r o t e i nasn dC a r b o h y d r a t e s 4.2 Lipids 4 . 3 L i g ni n a n dT a n n i n of 4.4 QualitativeandQuantitativeOccurrence in Bacteria, ImportantChemicalConstituents Phytoplankton, ZooplanktonandHigherPlants
7 1l I J
14 14 17 19 20
21. 21. 28 30
31 31 34 44 45
Contents
4.5
andTheirEffectson Biomass NaturalAssociations Composition S u m m a ray n dC o n c l u s i o n
Chapter5
Processes andtheAccumulation Sedimentary o t O r g a n i cM a tt e r
50 53 55
Richin OrganicMatter, FossilandModernSediments Implicatron andTheirGeological Organic 5.2 The Roleof DissolvedandParticulate Ma er for Sedimentary Organic 5.3 AccumulationMechanisms Matter S u m m a rar n dC o n c l u s i o n
) l
R e l e r e n c teosP a r t|
63
5.1
Part II
ChapterI
..
.
7he Fateof OrgunicMatterin Sedimentary Basins: Generationof Oil and Gus Diagenesis. Catagencsis andMetagenesis of OrganiM c atter
l.l Diagenesi. 1.2 Catagcnesis l.l M e ta g e n e . iasn dM e l a m o r p h i \ m S u m m a rar n dC o n c l u s r o n Chupter2
Early Transformation of OrganicMatter: The Diagenetic Pathwavfrom Organisms to FossilsandKerogen Geochcmical
2 . 1 S i g n i f i c a na n dM a i nS t e p s o f E a r l y ce r r m l t n : [ , i o nr Trr 2 . 2 B i r ) c h e m i cDr le g r a d a t i o n 2.-l PrrlvcrrnJensltion 2.4 lnsolubilization of OrganicMatterin Young 2.5 IsotopicComposition Seelimenlr 2.6 ResultandBalanceof Diagenesrs S u m m r r yl n d C o n c l u s i o n Chapter3 3.1
59 6l
Fossils andTheirSignificance in Geochemical PetroleumFormation
VersusCatagenesis: Two Different Diagenesis of Hvdrocarbons in theSubsurface Sources
69 69 7l 12 73
71 71 75 IJl 85 89 90 92 93 93
Contents Inheritedfrom Living Organisms. Hydrocarbons Directlyor Throughan Early Diagenesis: Fossils(BiologicalMarkers) Geochemical 3 . 3 n - A l k a n eas n dn - F a t t yA c i d s 1 . 4 I s o -a n dA nt c i s oA- l kr r n e . 3.5 C'1-branchedAlkanes .1.t' Acycliclroprcnoids 3 . 7 T r i c v c l i cD i t er p e nr,i d s in Triterpcnoids:Occurrence 3.8 SteroidsandPentacyclic RecentandAncientSediments DuringDiagenesis 3.9 Fateof SteroidsandTriterpenoids a n dC a tr g e n e . i s - 1l.0 O r h e rP o l )t e r p e n e 5 3.11 Aromatics 3.12 OxygenandNitrogenCompounds and 3.13 Kerogen.the PolarFractionof Sediments, of of CrudeOilsasPossible Sources Asphaltenes F o s s iM l olecules S u m m a rat n dC o n c l u s i o n
3.2
ChapterI
andClassification. . Kerogen:Composition
4.I DefinitionandImportanccof Kerogen 4 . 2 I s o l ailo no f K e r , r g e n of Kerogen 4.3 Microscopic Constituents 4.4 Chemicaland PhysicalDeterminationof Kcrogen structure 4.5 ChcmicalAnalvsis 1.6 Physical Analysis 4.7 GeneralStructureofKerogen 4.8 DepositionalEnvironmentand Compositionof Kcrogen:the EvolutionPaths 4.9 Conclusion S u m m a ral n dC o n c l u s i o n Chapter5
From Kerogento Petroleum
5.1
Diagenesis, Catagenesis andMetagenesis of Kerogen
5.2 5.3 5.4 5.5 -5.6
Experimental Simulationof KerogenEvolution StructuralEvolutionof Kerogen Formationof Hvdrocarbons DuringCatagenesis . IsotopeFractionation andKerogenEvolution ExperimentalGenerationof Hydrocarbons from OrganiM c atcrial S u m m a rar n t lC o n c l u s i o n
98 100 110 110 111 I 16 117 121 1.26 126 127
129 130
131
l3l 132 133 139 140 142 I17 151 159 1-59 160 160 169 I74 176 189 192 198
XVI
Contents
Chapter6
Formationof Gas
199
6 . 1 Constituenls andCharacterization of PetroleumGas . 199 6 . 2 GasGeneratedDuringDiagenesis of OrganicMatter 20L 6 . 3 GasGeneratedDuring Catagenesis andMetagenesis o f O r g a ni c M a l l e r 204 6 . 4 GasOriginatingfrom InorganicSources 201 6 . 5 Occurrenceand Compositionof Gas in Sedimentary Basins:Exampleof WesternEurope 208 6 . 6 Distributionof Gasesin Sedimentary 2r3 Basins 1t,1 S u m m a ray n dC o n c l u s i o n Chapter7
Formationof Petroleumin Relationto GeologicalProcesses. Timingof Oil and Gas G e n e r ai o l n
215 215 218
7.1. GeneralSchemeof PetroleumFormation Ratio . 1 .2 GeneticPotentialandTransformation Versus 7.3 Natur€of thc OrganicMatter. Gas Provinces Oil Provinces 7.4 Temperature, Timc andPressure 7.5 TimingofOiland GasGeneration 7.6 ComparisonBetweenthe Time of SourceRock DepositionandtheTime of PetroleumGeneration. S u m m a ray n dC o n c l u s i o n
225 228
C h a p t e r S C o a l a n d l t s R e l a t i o n t o O i l a n d G .a s. . . . .
229
8.1 8.2 8.3 8.4 8.5
GeneralAspectsof CoalFormation T h eF o r m a t i o o nfPeal Coali[icationProcess CoalPetrography PetroleumGeneration S u m m a rav n dC o n c l u s i o n
Chapter9
Oil Shales:A Kerogen-Rich Sedimentwith Value . . . PotentialEconomic
9 . 1 H i 5 l o r i cIa 9 . 2 Definitionof Oil Shales.Oil ShaleVersusPetroleum 9.3 9.4 9.5 9.6 9.7 9.8
SourceRock C o m p o s i t i oonf O r g a n i cM a l l e r C o n d i t i o nosf D e p o s i t i o n O i l S h a l eD e n s i t y P y r o l y so i sf O i l S h a l e s Oil Yield; Compositionof ShaleOil Oil ShaleDistributionsandReserves S u m m a ray n dC o n c l u s i o n
Referencesto Part II
219 222 223
229 230 234 241 253 254 1<,1
251 256 258 259 259 260 26r 266 267
Contents Part III
ChapterI
The Migration and Accumulation of Oil and Gas
An Introductionto Migrationand Accumulationof Oil andGas
S u m m a rar n dC o n e l u s i o n Chapter2
. . . 2 9 3 . . . 2 9 5
Ph1'sicochemical Aspectsof Primary
. . . 2 9 6 2.1 2.2 2.1 2..4
T c m p e r aulr ea n dP r e s ' u r e C , , m n u .o, n F l ui d . Possible Modesof PrimaryMigration S u m m a ruvn dC o n c l u s i o n
Chapter3
. . . 2 9 6 . . . 3 0 1 . . . 3 0 7 . 309 . 323
Geologicaland Geochemical Aspectsof P r i m a r rM i g r a t i o n
3 . 1 TimeandDepthof PrimaryMigration Changesin Compositionof SourceRock Bitumen VersuC s r u d eO i l 3 . 3 Evaluationof GeologicalandGeochemical Aspectsof P r i m a r rM i g r r ri to n Conclusions andSuggestions on PrimaryMigration S u m m r r rr;n d C o n c l u s i o n C h u p t eIr
S e c o n d a rMyi g r a t i o na n dA c c u m u l a t i o n . . .
325 325 33t) 333 338 340 341
,l.l
The Buoyant Riseof Oil and Gas VersusCapillary Pressures . . 1.2 Hydrodynamics andSecondary Migration . . . .1.3 Geologicaland Geochemical Implicationsof Secondary Migration . . . . 1.1 Terminationof Secondary Migrationand A c c u m u l a t i o n oOfi l a n d G a s . . . . . . ,1.5 Distances of Secondary Migration . . . . S u m m aar ny dC o n c l u s i o n .......356 Chapter5
ReservoirRocksandTraps,the Sitesof Oil and GasPools
312 344 317 351 35,1
J)/
5.1 ReservoirRocks 5.2 Traps S u m m r r ya n dC o n c l u s i o n
358 360 365
References to Partlll
366
Contents
XVIII Part IV
The Compositionand Classificationof CrudeOils and the Influenceof GeologicalFactors
( lnprer I
Comporitionof Cru.JeOils
...
375
.. 375 1 . 1 PetroleumVersusSourceRockBitumen . for CrudeOil 1 . 2 AnalyticalProcedures . . . 3 7 5 Characterization . . . 3 7 9 in CrudeOils 1 . 3 MainGroupsof Compounds in CrudeOils . . . 3 8 2 I . r t PrincipalTypesof Hvdrocarbons . . . . l 9 l t 1 . 5 SulfuC r ompounds . 401 I . 6 Nit rogen Compounds 403 . 1 . 1 O x v g e nC o m p o u n d s l . t i High MolecularWeightN. S .O Compounds:Resins . ,103 a n dA r p h a l t cn c s . ,108 i co m p o u n d s 1 . 9 O rg a n o m c t a l lC Analysisof Main CrudeOil Constituents ' 1 1I 1.10 Covariance . 'l 14 S u mm a r va n dC o n c l u \ i o n
-
C h a p t e2r 2.1 2.2 2.3 2.1 2.5 2.6
C l a s s i l i c a t i rornfC r u d eO i l s
115 .... General 116 Historical 116 Basio s f P r o p o s eCdl a s s i f i c a t io fnC r u d eO i l s . . . 117 . . . . . . Classification of CrudeOils C h a r a c t e r i s t i c s o f t h e P r i n c i p a l C l a s s e s o f C r u. d e4O1 i9l s . . 122 . . ConcludingRemarks . . . . '123 SummarvandConclusion
Chapter3
Fossilsin CrudeOils and Geochemical asIndicatorsof Depositional Sediments EnvironmentandGeologicalHistory
of FossilMolecules Significance Fossilsas lndicatorsof Geological Geochemical Environments Fossilsas Indicatorsof Early 3.3 Geochemical Diugenesis Fossilsas IndicatorsofThermal 3.4 Geochemical Maturation 3.5 Presentand FutureDevelopmentin the Useof GeochemicalFos.ils S u m m a ra\ n dC o n c l u s i o n
3.I 3.2
.115
121 124 126 132. :133 ,136
Contents ChapterI 4.1 4.2 4.3 4.1
Geological ControlofPetroleumTvpe
Generaland Geochemical Regularities of Composition Geochemical Regularities Relatedto the E n v i r ( )m n e n to I D e p o s i t i o n Geochemical Regularities in Relationto Thermal Erolutirrn ConcludingRemarkson CrudeOil Regularities S u m m a ravn dC o n c l u s i o n
Chupter5
PetroleumA lteration
139 440 450 457 457 :159
5 . I T h e r m aAl l t e r a t i o n 5.: Deasphalting 5.3 Biodegradation andWaterWashing S u m m a ray n dC o n c l u r i o n
460 161 463 469
Chapter6
170
6. I 6.2 6.3 6.1 6.5 6.6
HeavyOilsandTar Sands
Definitions C o m p o s i t i oonf H e a v vO i l s SpecificGravityandViscositv OriginandOccurrence of HeavyOils . World Reserves andGeological Setting . . . . . Vllorization o f H e a v vO i l s . S u m m ; r ravn dC o n c l u s i o n
170 472 477 ,r80 182 483
R el er e n c c s
Purt V
Chapter I I.I l.l 1.3 l.,l
Oil and Gas Exploration:Application of the Principlesof Petoleum Generationund Migrution ldentification ofSource Rocks
495
A m o u n l r r IQ r g s n l qY 1 1 g r 495 T v p eo f O r g r n i c M a l l e r 197 Maturation of the OrgeinicMatter 515 Conclusionson Characterizationof PotentialSource
Rockr S u m m u rul n dC o n c l u s i o n Chapter2
5.10 546
Oil andSourceRockCorrelation
5,18
2 . 1 C r r r r e l ai ol n P l n r m e l e r s 2 . 2 O i l - O i lC , r r r e l ai ot n E x a m p l e s 2 . 3 Oil-SourceRockCorrelationExamples
5,19 551 561 570
S u m m r r yr n d ( - o n c l u . ' i o n
Contents
Chapter3
LocatingPetroleumProspects: Application of Principleof PetroleumGenerationand Migration GeologicalModeling
571
573 3.1 Acquisitionof theGeochemical Inlbrmation. 3.2 FirstConceptualModelof PetroleumGcnerationin a 575 Basin 3.3 NumericalSimulationof the Evolutionof a 516 Basin- Geological Modeling . Sedimentary 581 S u m m a rav n dC o n c l u s i o n Chapter4
4.I 4.2 4.3 1.1 4.5
4.0 4.7 4.8 4.9
Modeling:A Ouantitative Geochemical Approachto the Evaluationof Oil and Gas Prospects
583
Necessityof a QuantitativeApproachto Petroleum 5U3 Baslns Potentialof Sedimentary MathematicalModel of Kerogen Degradationand 585 H r d r o c a r b uG n eneration GeneticPotentialof SourceRocks.Transfbrmation 5{t9 Ratio . . . . . . . . . . . . . .590 Validityot the Model of the ActivationEnergiesin Relationto Significance I h cT ) p eo f O r g a nj c M a tt e r Modelto Petroleum Applicationof the Mathematical 593 Exploration Reconstruction of theAncientGeothermalGradient 596 60,1 M i g r a t i o nM o d e l i n g . . . 607 Conclusion 608 S u m m a ray n , JC o n c l u s i o n
C h u p t e5r
H a h i t aot f P e t r o l e u m
.
610
Habitatof Petroleumin the ArabianCarbonate . 6lI Plat[orm . 615 5.2 Habitatof Petroleumin YoungDeltaAreas 5.3 The Linyi Basinin the People'sRepublicof China . 624 5.4 Habitatof Gasin theDeepBasinof WesternCanada. 628 . 639 S u m m a ray n dC o n c l u s i o n 5.I
Chapter6
6 .I 6.2 6.3
The Distributionof World Oil andGas Reserves and Geological- Geochemical Implications
T nr o d u cilo n Geological Settingof Oil andGasReserves Age Distributionof PetroleumReserves
6,11 641 612 648
Contents 6.4
Significance of the Age andGeotectonic Distribution of Petroleum andCoal 6.5 Richness of Sedimentary Basins.Role of GiantFields a n dC i a n tP r o v i n c e s 6.6 UltimateWorld Oil andGasResources 6.7 Paleogeography asa Clue to FutureOil and Gas Provinces SummaryandConclusion
656 660
R e f e r e n cteosP a r V t . .
667
SubjectIndex
...
662 666
Part I Productionand Accumulationof OrganicMatter: A GeologicalPerspective
ChapterI Productionand Accumulationof OreanicMatter The OrganicCarbonCycle
Production, accumulationand preservationof undegradedorganic matter are "organic prerequisites for the existenceof petroleumsourcetocks.The term matter" or "organicmaterial",as usedin this book, referssolelyto material comprisedof organicmoleculesin monomericor polymericform deriveddirectly or indirectly from the organicpart of organisms.Mineral skeletalparts, suchas shells,bones,and teeth are not included.First, organic matter has to be andpreserved andthereafterit mustbe deposited synthesized by livingorganisms part geological of the sedimentary further events, Depending on in sediments. organicmatter may be transformedinto petroleum-likecompounds.It is important to realizethat during the historyof the earth the conditionsfor synof organicmatterhaschangedconsiderably. thesis,deposition.andpreservation
- The Basisfor MassProductionof Organic I .1 Photosynthesis Matter is a noteworthy The emergence of photosynthesis as a worldwidephenomenon historical event with respectto the formation of potential sourcerocks. Tbe photosyntheticprocessconvertslight energyinto chemicalenergy.Photosynthesis is basicallya transferof hydrogenfrom water to carbondioxideto produce organicmatter in the form of glucoseand oxygen.The oxygenis freed from the water molecule and not from carbon dioxide. From glucose,autotrophic organismscan synthetizepolysaccharides, suchas celluloseand starch,and all other necessary constituents. A simpleform of the equationof photosynthesis reactionis givenin FigureI.1.1.
u,----cuH'rou.60;-6H2o 6 co2 -t2H2d j, 674 kcal
( Glucose)
r Polysaccharides Fig. I.1.1. Equati()l) of ph()toslrnthesis. Glucose. relatively rich in errergy,is formed by green plants with the help of sunlight (h . v). Oxygen is by-product of this proccss
Productionand Accumulation of Organic Matter. The Organic Carbon Cyclc
N
,\
nfi-"s 9HzFF
'\(,"r''
H-c. i'I I
-.4c\ -.)\^,,,'k - zc7.t'k-.7t 1-.7tq.,t*.
0
cH3
t cH:
lsoprenoidside chain
.
t cH3
.
,rt:f
N
"
,- t:9,l
H N H N
\c:cn
l,i-cl
I
-ru. "l .' '":1"
l chg
HC:CH
b
P o r p h i nn u c l e u s
Fig. I.1.2. (a) Molecule of chlorophyll. rhe light-absorbinggreen pigment ofplantsand a prerequisite for photosynthesis.The phytyl side chain of the chlorophyll moleculc is an important source for chainJike isoprenoid compounds in sediments and oils. Likewise. the nitrogen-containing porphin nucleus is the precursor for porphyrins. (b) Structure of Dorphin nucleus
Photosynthesisis the basic processthat accomplishes the massproduction of organicmatter on earth. Primitive autotrophicorganisms,suchasphotosynthetic bacteriaandblue-greenalgae,werethe first organisms responsible for thismass production.A basicprerequisitefor photosynthesisis the light-absorbinggreen pigmentchlorophyll(Fig.I.1.2).In primitiveautotrophsit occursin a relatively free state in the cell of the organism.In more highly evolvedplants it is concentratedin chloroplastsin green leaves.These chloroplastsare complete photosyntheticfactories. The oldestrecordedformsof organiclife areabout3.1 to 3.3billionyearsold and are bacteriaand algae-likebodiesfrom the SwazilandGroup in SouthAfrica (Schopfet al., 1965).However,it is possiblethat life on earthis at leastasold as the oldestknownrocks- 3.7 billion years. It is assumedthat approximately2 billion yearsago,photosyntheticproduction of organicmatter was fairly well establishedworldwide,and this time servesasa zero referencepoint. Before it was reached,anotherbillion yearsprobably was required for the isolated occurencesof most primitive organismsto spread sufficientlyfor photosynthesisand,hence,for massproductionof organicmatter prevalent. to be universally A summaryof geologicaleventsconsideredto be importantin the organic carbon cycle is presentedin Figure I.1.3. Without water there is no life. Therefore,abundantlife, evenon a mostprimitive level,wasnot possibleon earth prior to about4 billion yearsago,whenwater becamea commonsubstanceon the earth'ssurface.During that primordialtime,the atmosphere wasreducing,i. e., there waspracticallyno free oxygen.
-The 1.I Photosvnthesis Tim. in million
G.ologic
Basisfor MassProductionof OrsanicMatter
l m p o r la n l . v c n t s d u r i n g hislory ot th..arih
500
l,
1000
l3
1500
Fig. I.1.3. Events supposedto be of importance for the evolution of life during the earth's history. Mass production of organic matteronearthdid not occur prior to 2 billion yearsago when photosynthesiswasestablishedas a worldwide phenomenon
:l
;t: 2000 2500
: :
.! E
;
3000
Bact.ria and p miliv. alga. A b i o l o g i c a l -c h . m i c a l
3500
End ot firsl q.osyn-
t 00 0
Larg.r quantiti.s of wat.r on surtac. ol
a500 Origin of .arth
It is generally agreedthat the early earth's atmospherewas devoid of free oxygen,and that it contained H2,CHa,NH3,N2 and HrO. However,this view is hypothesisbeingespeciallyquestioned.In not unopposed,the methane-ammonia connection with this hypothesis,Calvin (1969) relers to an abiological or chemicalevolution that started more than,1 billion yearsago. When primitive organismsfirst appearedabout 3 billion years ago, they probably utilized the abiologically produced organic molecules as a source of energy to maintain metabolism.Therefore, the first organismsare assumedto have been heterotrophic. However,the growingpopulationof heterotrophsprobablycouldnot be supportedforever in this way. It is arguedthat, by the time theseorganismshad almost depletedthe reservoirof abiologicallyproducedorganic matter, photosynthesisdevelopedas a secondsourceof energy. In this way, heterotrophicorganismsthat were ableto usesunlightasan extra sourceof energy could becomeindependent,and with further evolution could escapethe food shortage.Certainpurpie-coloredbacterialiving todayshowthese properties.They can act like heterotrophsand utilize organic compounds,and The they alsocontain the greenpigmentchlorophyll to carry on photosynthesis. asperformedby bacteria,did not produceorygen. oldestform of photosynthesis, Photosyntheticbacteria are anaerobic.Instead of using HrO as a hydrogen donor, they can use H2Sand excretesulfur rather than oxygen.
6
Productionand Accumulation of Organic Matter. The Organic Carbon Cycle
Certain -blue-greenalgaethat evolvedfrom photosyntheticbacteriaprobably w€re the first oxygen-producingorganisms.Although there are a number of photosynthetic pigments,nonecancompletelyreplacechlorophyll(Fig.I.1.2)in photosyntheticorganisms.Chlorophyll moleculesabsorb light energy, which elevateselectronsto a higher energylevel. This gain in energyis transferredto other molecules. Oxygen is believedto have been toxic to organismsof that time. However, a reducing environment assured that divalent iron was abundant in aoueous solutions.This iron could act asa sink for the oxygenproducedasa by-productof photosynthesis.It is very likely that the well-known banded ironstonesof the Precambrian are formed by this interplay between photosynthesisand a subsequentoxidation of iron to a trivalent form, with precipitation of the insolubleoxides(Cloud,1968). Autotrophic, photosyntheticorganismswere superior to heterotrophsand consequentlysoon dominated the biological realm. As stated before, about 2 billion yearsago,photosynthesis emergedasa worldwidephenomenon.Herewith the foundationfor the food pyramid and the evolution oahiqherforms of life was laid. It is arguedthat afterthiseventthe atmosphere of the earthslowlybecame oxidizing,i. e., free molecularoxygenbecameavailable. Photosynthesis makesuseof lhe energycomingfrom sunlight,employingonly a narrowbandof the sun'stotal radiation.The portionof thespectrum utilizid by most photosyntheticorganismsis between40fi0 16g966 A. which nearlveouals the portion of light visible to the humaneye.Rayswith shorterwavelengthsand higher energyare evenharmful to life. Different partsof the visiblelignt spectrum may be utilized_by different photosyntheticorganisms.The poriion-used is rleterminedby the type of pigment an organismemploys(Fig. I.1.4). Ir enables photosyntheticalgaeandbacteriato live at clifferentdepthlevelsin the samebodv of water.Life in deeperwateris correlativeto useof iongerwavelengths. ....... - ---
.9 o
.u nl E.
length in Angstrijm unit s
Green algae Greenbacteria Red algae Purple bacteria
Fig. I.1.4. Differentpans of the light spectrummay be utilized by different photosyntheticorganisms. This enables photosynthetic algaeandbacteriato live at differentdepth levels in the same body of water. (Adapted from Tappan and Loeblich, 1970; reprintedwith permissionand from publication of the GeologicalSociety of America)
1.2The OrganicCarbonBudgetDuringthe History of the Earth
'7
1.2 The OrganicCarbonBudgetDuring the History of the Earth For a massbalanceof carbon used in photosynthesisduring the history of the earth, it is necessaryto add up all organic carbon presenton earth in various repositories,suchas oceanwatersandsediments.The total estimatedamountof organic carbon and graphite, which formerly representedsedimentaryorganic (Welte,1970).A morerecentestimateby 6.4 x 10151 carbon,is approximately Hunt (1972) is about twice as high. However, Hunt includes in his balance calculation"organic" carbon in basalticand other volcanic rocks, as well as in granitic and all metamorphic rocks. The biological origin of much of this "organic" carbon is questionable. Most of the carbonon earth is concenlratedin sedimentaryrocksof the earth's crusl. Part of it is fixed asorganiccarbon,and a greaterpart ascarbonatecarbon. It is estimatedthat 187oof total carbonin sedimentaryrocksis organiccarbonand that 827oof sedimentarycarbonis bound in the form of carbonates(Schidlowski e t a l . ,1 9 7 4 ) . A relationshipof courseexistsbetweenorganiccarbonand carbonatecarbon. The atmosphericCO2-reservoiris in a constantexchangewith the hydrospheric CO2-reservoir.From aquaticenvironments,carbonatesmay be precipitatedor depositedby organisms(shells,skeletonsetc.) to form carbonatesediments. Conversely,carbonaterocks may be dissolvedto contribute to the equilibrium reactionbetween CO3 , HCO; and CO2 in waters.Primary organic matter is formed directly from the atmosphericreservoir by terrestrial plants, or by photosynthesisof marine plants from dissolved CO, in the hydrosphere. Terrestrial and marine organicmatter, in tum, is largelydestroyedby oxidation. Thus CO, is returned for re-circulation in the system. A simplified sketch showingthe main processesand pathwaysconcemingthe elementcarbonin the earth's crust is given in Figure I..1.5.Only an almost negligibleportion of the organic carbon in the earth's crust, including the hydrosphere,is found in living organismsand in a dissolvedstate.The major part of the organiccarbon (5.0 x 10'st) is fixed in sediments. Another sizeablepart ofthe organiccarbon is (1.4 x 10'50,mainly,in theformof graphite-like materialor metaanthracite, (Table origin I.1.1). fixed in metamorphicrocks of sedimentary If it is correct that all this organic carbon has been formed either directly or indirectly by photosynthesisduring the earth'shistory,there shouldhavebeen a correspondingamount of oxygen liberated simultaneouslyaccording to the equationof photosynthesis(Fig. I.1.1). This amount must be accountedfor by free oxygen, together with formerly free orygen presentlyused by oxidation processesof substanceother than biologicalorganicmatter. At present,we find free oxygen in the atmosphere (in air 20.95 vol 7o) and varying amounts dissolvedin oceanwaters(generalrange2-8 ml Or per l). Formerly free oxygen is found in both dead and living organic matter. However, most formerly free oxygenhasbeenutilized by the oxidation of variousformsof sulfur andiron. This oxygenis bound today in sulfatesand in oxidesof trivalentiron, andis distributed throughoutthe earth'scrust, includingthe hydrosphere(Table I 1.2). As statedbefore,it is believedthat the primordial atmosphereof the earthwas reducingandthe elementssulfur andiron occurredonly in divalentform. Oxygen
Producrion and Accumulation of Organic Matter. The OrsanicCarbon Cvcle
""9i, r E:i;!
: ;
I rt.o.6!j
;.!
.:i..'f .1*.
€ 'E
c !
l 9 ;
"
I
E 6
1
9 ; g t :.9 d
-
c :
g -i+
= :-. ; 9
S
.E;
\
c ; i
./t i'
> 5 : 2 / i ,
- :id ':i ! c9re:l 5
\ )\
1"\
s\
>,x ? E .
iE: A.a o 9 / 9
d i
i F c
f,! produced by photosynthesis,therefore,was usedto oxidize sulfidesto sulfates and divalent to trivalent iron. The total free and formerly free oxygenfound on earthamounlsto approximately16.9 x 10t5t.
1.2The OrganicCarbonBudgetDuringthe Historyof theEarth in I0 lst. TablcI.l. t. Organiccarbonin theearth'scrustexpressed (After wcltc. 1970)
0.003 5.0
Organisms and dissolved organic C Sediments Metamorphic rocks of sedimentary origin (80% of all melamorphic rocks) Total organic cartx)n
1.4
Table I.1.2. Frce and formerly free oxygen in the earth's crust. cxpressedin l0 l st exclusivctlf oxygen in carbonatesand silicates (After weltc. 1970)
1.18 0.02 0.l6 2.6 10.2 2.1 16.9
Atmosphere Occans Biological CO2 Dissolvedmarine SOI Evaporitic SOa2 FeO '--- Fe2O3 Total oxygen
The ratio of the total quantitiescalculatedfor oxygen(16.9 x 10'st) and organiccarbon(6.4 x 10r5t)is similarto the massratio of theseelementsin the CO, molecule:
rOr \c
32 12
^..\ |
I formerlyfree oxygenX 10r5t [0rst in ...k= rocks x 10'' carbont. organic"*b"t t -c";t.
16.9
\
6.4
l
This balance calculation for oxygen and organic carbon on the basis of photosynthesisindicatesto us that most of the oxygennot bound in carbonates and silicateshas indeed been produced by photosynthesis.Therefore, there should be a relationshipbetweenorganiccarbon in fossilsedimentsand oxygen levelsol paleoatmorpheres. Making use of so-calledhalf-massagesof sedimentaryrocks,as givenby Garrels and Mackenzie(1969), an accumulationrate for organiccarbonof approximately3.2 x 106t y I was calculatedon the basisof the previously mentionednumbers.The presentannualmarineproductionof organiccarbonis estimatedto be 6 x 1010t(Vallentyne,1965). With this annual marine -4, production,the total globalpreservation of organiccarbonof 10 or 0.01% during the earth'shistorycan be calculated.Although it is difficult to estimatethe true preservation, it seemsto be safeto assumethat it is lessthan0.17o.Menzel and Ryther (1970)alsoestimatedthat about0.17oof the annualproductionof organic matter is buried in surfacesediments.Only this tiny fraction of organic
[0
Productionand Accumulationof Organic Matter. The OrganicCarbon Cycle
matter is preservedin sediments,whereasthe remainderis recycled,mainlyin the euphoticzone of the top water layer in the oceans.This is why oceanographers speakof a closedsystemwith respectto living phytoplanktonand CO2in ocean waters.In a studyon the origin andfateof organicmatter in the Black Sea,Deuser (1971) found a preservationrate of 4o/c.However, this value of 47o has to be considered as an upper limit, which is reached only under such favorable condilionsasare found in the Black Sea.Theseconditionsare anoxygen-freeand fairly calm water body without scavengingbenthic life at the bottom exceptfor anaerobicbacteria.The sedimentationof certainpetroleumsourcerockshasvery likely takenplaceundersimilarconditions. In this connectionthere is frequentlyan alternationofenvironmentsfavorable for production and preservationof organicmatter and those in which much less organicmatter is preservedin sediments.A Soodexampleof this is the seriesof finely laminatedsediments,with altemating layersrespectivelyrich and poor in organic carbon, describedby Ross and Degens (1974) in young Black Sea sediments. The cycle of organic carbon in nature is shown in Figure I.1.6 There is a p r i m a r ys, m a l lc y c l e( 1 ) w i t h a t u r n o \ . cor f a b o u t2 . 7 t o 3 . 0 x 1 0 1 2otf o r g a n i c carbon,anda half-lifeof daysup to tensof years.Thereis alsoa secondary,large andwith a half-lifeof cycle(2) comprisingan estimatedquantityof 6.4 x 101st, the tiny leakageof years. interconnected by The two cyclesare severalmillion
Cycle o f o r g a n icca r b o n 0rganic mall.t tix.d in tossll sedrm.nis, c0al and kero9.n
soils andx.dimcntsr profoundly Rll.r.d.
0.ad
planls,anrmals 0 r g a n i cm a t l . r I n m . t a m o r p h i cs . d r m . n t s , mainly as malaanthra$t. and graphil..
Cycle I
C y c l e2
Fig. L1.6. The tow major cycles of organic carbon on earth. Organic carbon is mainly recyclcdin cycle 1. The crossoverfrom cycle I to cycle 2 is a tiny leak that am')unt\ only tt) 0.Ol -0.1 7. to the primary organic productivity ( Afier weltc' 1970)
1.3The Organic Carbon Budget in the Black Sea
about 0.017oto 0.17c of the total organiccarbon,representing oxidationof sedimentaryorganicmatter to COr. For our considerations, the largersecondary cycleis of greaterimportance.Once the organic matter hasentereda sediment, its long-termfate is mainlvgovemedby tectonicevents.In otherwords,phases of subsidence and increasein burial,or phasesof uplilt and erosion,determine whether the organic content of a sedimenl is preservedand transformedinto petroleum, or is eroded and oxidized. If organic matter completesthe second cycleduringthe birth. evolution.andendof a geosyncline,it undergoesincreasing burial, andis subjectedto diagenesis. catagenesis, andfinally metamorphism.The processes of diagenesisand catagenesis are of prime importancein the formation of petroleum.
1.3 The OrganicCarbonBudgetin the Black Sea The BlackSeamayservehereasa modelcase forconditionsprevailingduringthe formation of sourcerock-typesediments.The subsequentconsiderationsfollow mainlythe findingsand thoughtsof Deuser( I 97 I ). In the Black Sea,the main sourceof organicmatter is in situ pholosynthesis (Fig.I.1.7).A dominantroleis playedby thesmallest unicellularalgae.The main groupsof marine life dependdirectly or indirectlyupon their rate of production. About 100 g organiccarbonm 'yr-' have been producedby photosynthetic organismsover the entire water-coveredarea during the last 2000 years.In addition, a certainamountof organiccarbon(lessthan 107o)is introducedmainly as detrital material by rivers and the Seasof Azov and Marmara. Shimkusand Trimonis(1974)estimated the contributionof organiccarbonto theBlackSeaby rivers to be about one third of the total input. Furthermore,chemosynthesis servesas a sourceof organiccarbon.This is the organiccarbonresynthesized by autotrophicbacteria,the amountof which is not known exactly.Il is estimatedto be lessthan 15gCo,, m-2yr r. Most of the organic carbon produced in and carried into the Black Sea is oxidizedto CO2 in the top 200 m due to respiration,and is returnedto the hydrosphere-atmospheresystem. The amount immediately returned to the primary organiccarboncycle(Fig. I.1.6) and thus availableagainfor photosynthesis,is probablycloseto 8070.A smallportion is alsocarriedawayinto the Seas of Marmara and Azov. The remainderis lransferredinto the anoxicwatersbelow the top 200 m zone.There it is subjectmainlyto chemicaland microbialattack.It seemssafeto assumethat generally80o/oto95Vo otthe organiccarbonis recycled in the top water column, where there is photosyntheticactivity. In the anoxiczone, Deuser (1971) assumeda steady-state situation.The organic matter being oxidized due to sulfate reduction,and due to lossesfrom solubilizationand fossilizationis balancedby an equal influx of organicmatter from above,where oxygen-bearingwatersprevail. About one fourth of the organiccarbonintroducedinto the anoxiczoneof the Black Seais buried in the sediments.and thusfossilized.This amountsto asmuch as 470 of the total organiccarbon input of the Black Sea.This is certainly more
12
ProductionandAccumulationofOrganicMatter'The OrganicCarbonCycle
c a t t e ri n B l a c kS e a S o u r c eosf 0 r g a n im Photosy.thesis
N{'
/../S wiih 02
.
Ch€mosynthesis
with H2S.
k ea i ca t t eirn B l a c S t a t eo f o r g a n m Seas of
Respiration
Azov
and
N.\st P a r t i c u l a t e F l ux 0 x r d a l i o n{ d u r i n gS 0 1
S o l u b i lz a l
Sclm2lyeal
Fig.I.1.7. The organiccarbonbudgetof theBlackSeaduringth"elast2000yea$' Top: as g m-'yr-' Bo!!om:Iale ol organlcmatter' of organicmattercalculated so-urces ALt
than the averageamount of organiccarbon normally fixed inmarine sediments' the The main reasJnfor the higherpreservationrate in the Black Seais probably a certain to orygen of absence in the matter of orginic slower degradation .Up The Doint.a hLh sedimentationrate can alsohelp to preserveorganicmaterial' to is hard i-por,un.! of thosetwo factorsin the preservationof organicmatter is It localities' qr'"riity, *d certainly varies in different environmentsand is more obuioui thut in areasiuch as deltas, the high rate of sedimentation important than an anoxic water zone, whictr should be of greater influencein closed.staenantbodiesof water. As a basisfor comparisonwith the Black Sea'
andConclusion Summary
13
the Amazon River dischargesabout 10101organic carbon per year (Williams, 1968).This is about 10Otimes more than the total annualproductionof organic carbonin the Black Sea.The Amazon River representsapproximately207oof the total $orldwide river runoff. Organic maner in q-atersis arbitrarily subdividedinto particulate(living and deadt arrl "disolr ed" organicmatter (in generalparticles<1p). Approximately llr-t ..-rithe total organiccarbondischargeof the Amazon River is in a dissolved sr-:re.rbe rest is panic'ulateorganic matter. Whereas the particulate organic mener crrrFd inro the seais more significantlocally,the dissolvedorganicmatter j:Dl : *rler dttribution due to mixing of water masses.No matter what the :rLrEs!\€-{are that lead to preservationand fossilizationof organicmalter, they r. J prrme importance for the formation of potential source rocks.
SummaryandConclusion Photosynthesis is the basisfor massproductionof organicmatter. Abour 2 billion years ago in the Precambrian,photosynthesisemerged as a worldwide phenornenon. Herewith the folndation for the food pyramidand the evolutionofhigher forms of life waslaid. The enrichmentof molecularoxygenin the atmosphereof the earthis a direct consequence of photosynthesisand the massproductionof organicmatter. During the earth'shistoryand on a globalscale,the averagepreservationrate ofthe primary organicproduction,expressedasorganiccarbon,is estimatedto be lessthan 0.l7o. The upper limit of the preservationrate oforganic carbonto be found in certailt oxygen-deficient environmentsfavorablefor depositionof sourcerock-typesediments is about47o.
Chapter2 Evolutionof the BiosPhere
petroleumprecursors(kerogenand we know of the occurrenceof petroleumand Michigan' usA) rhroush the Share' (Nonesuch i;;;;.-";;;;;1i-e ;l;;;; and bacteria' and up to the Devonian, mainly marine phytoplankton C;;;t assource served have could zooplankton and benthonicalgae ;il;;;;;;;6t
fromland matterderived rn"t"uti"t, t"irestrialorganic ffi;;;iil ;;;;i*.. level and kind of uit"rnutin"'o*""' rn" evJlutionary ;ffi;-;ii".i-;; and may be of decisiveinfluence on- the type contributing sourceorganrsms rock Therefore' one must amount of Detroleumgenerateoin a certain source or ttt. biospherein connectionwith the formation H.io"r;i;;;1;iioriot petroleum.
2.1 PhytoPlanktonand Bacteria the main producersof organic Some 2 billion years ago in the Precambrian' bacteria Throughout the "".il.t *"i" Ufir"-gree"nalgae and photosynthetic" uni Silurian, a variety of marine phytoplanktonic 6".i.f*'bJ";i;lun of organlc the dominant-sources "'rga.iaa-, U"","da, and blue-greenaigaewere andspreadsufficientlybythe carbonuntil land ptunt, upp"ut"J on th"econtinents that wen today manne Middle Devonian (Zimmermann, 1959)' It is estimated for 50% to 607oof the world organic ohwoplanktonanObu.t".iu u." '"tpontible production(Vallentyne'I 965)' *olii;Iu-si; iaibon andI-oeblich
ranla1 ;i"itntr".. unutt"i, rappan(1e68).and throlghgeological phltoplankton of iossil trs"?0ii"* ".ilui"Jtn" uu"nJ'un"e l'-Phytoplankton I 2-' Figure in is given
representation time. A diagramatrc theEarlyPaleozoic, preca.uan,inJreaseatnroughout trre in started Droduction and Pirmo-Carboniferous ;".piv i" r-""-p""onian' During i;#;",c',"il Late in occurred maximum Another low wasgenerally Triassictimes,productlon Inthe Early theendof theCretaceous' at abruptly dropping Jurassic-Cretaceous, Paleocene I'ate in rapidlv increased lt *u. tc-i *tv low ;il;;";:;Ju"tiln in the a,maximum Finally' Oligocene uguin'inttl" and Eocene,anddecreased of productlon iai*"n" "^ilJ-iir.i*". tofr"wedby a deilineto the presentlevel pttdY"l"ilY jt::ambrian-Earlv p"riod of iigh "byptrvtoplantion -thuracteristic planktonsuch as organic-walled puLJtoi"j *'u. gouemed "acritarchs",andgreenalgaeTheyhadno skeletons algaelvarious "acritarch" ;1";;";; Thename ot ottt"t tineril substa-nces "il;i"'.iticu, ;;pffi;;
2.1PhvtoDlankton andBacteria
l5
Estimaled lolal amornl ( a !en!.altrlnd)
0 i s t r i h u t i so int h r c s 0 e ct 0 t l y p e0 t 0 r g a n i s m s
-i E'.s "\it
! - ; - E - !i 1 H
.!-6-.,: 6
; l t r I _ ; : r = i ; o o * \
roo -
-;.1 _ 3
l
350
roo E rlo
E
550 600 6t0 700
Fig.I.2.1. Variationin abundance of fossilphytoplankton groupsandtotalphytoplankton duringgeologicalpast.(Adaptedfrom Tappanand Loeblich,1970);reprintedwith permission andfrom publication of the Geological Societyof America) refersto a purelymorphologicalgroupof rounded,polygonalor elongate,smooth or omamented,acid-resistantorganic-walledvesicleswhich are probably algal cysts. Their origin is still being debated. The second maximum in Late Jurassic-Cretaceous was dominated by calcareousnannoplankton,including coccolithophorids(small unicellular plants with a calcareousskeleton) and dinoflagellates.Siliceousphytoplankton,especiallysilicoflagellates and diatoms, appearedin the Late Cretaceousand became increasinglyimportant in the Cenozoic. Fossil records are inadequate to quantify the productivity of bacteria throughoutgeologicaltime. Becauseof microscopic(or submicroscopic) size,and the lack of hard parts,they are rarely fossilized.However,examplesof fossilized bacteriahave been recordedfrom all geologicalsystemsincludingthe Precambrian. Fossilizedbacteriaare often associatedwith orsanic matter suchas Dlant tissues. and animaland insectremains(Moore, l9b9j. Most fossilized baiteria were genericallysimilar to present-dayforms occurringin comparableenvironments(Schopfet al., 1965;Moore, 1967). Bacteriaand blue-greenalgae,both unicellular,are the only organismsthat do not possessmembrane-boundorganellessuchasnucleiwithin the cell. They are, therefore,calledprokaryotes,and are distinct from all other organismswith cell nuclei,which are called eukamotes.
EvolutionoftheBiosphere
16
Bacteriaandalgaeundoubtedlyareandalwayshavebeenecologicalpioneers. in theirphYsiologYThis enabled Bacteriaespeciallyshowan enormousversatility ubiquity' Bacteria may be irl.- i" rit! "rrn"'.t everywhereand guaranteedtheir orboth' p^roduction)' oxygen it"i"i"".pt i", ""atrophic (photosyrihesiswithout upon entirely relying success' fr"v "i"'"n .t,"anding eximple of evolutionary Actime' geological thioughout their versatility, and apparently unrestricted and bacteria
i.ilr'g ,. Zi;n
morethani00 :P"1:'.:1 ttsi6, :rl,qt.',1e64), organic matter in
soils and related organismsare presently ttnown to attack situati'onhaschangeddrastically sediments.There is no reasonto assumethat this debriswerefirst available' organic of *nen Iutgeiamounts .i"'."irt"i;t".".uiiun, to phvtoplankton in l; ;it;t.."d iater, d"ud bacteria are secondonly ;;;ll matter ultimatelv buried and preservedin sediments' ;;il;;il;i;-tg*ic Thefundamentalrelationstrrpinthefoodchainwithinthepyrarnidoflife between autotrophic causesa direct correlation in occu"ence and distribution of zooplankton and heterotrophic zooplankton' The biomass ;;;;;;tki""
(Bosorov Productivitv orhighphvtoplankton ueligh ln areas ii&Hil;;t;o of emergency the since trasixisted relations"hip 1960).-'this il il;;;, and era foraminif unicellular rttrt" iitl"mbriin, suchas ffit;#,il;;;t*it#. likeworms' of theanimalkingdom' otherorganisms to "r*;pplies ii t"JrlrLfi. to the respect with landmarks "tJ" ".t[iopoa.. Thereire a few ili#il; and emersence th€ including andoiieiinvertebrates' ;;;;;"-;1;.;ptankton (ordovician-Silurian)' an Paleozoic ;;;;;;.f ;;"i.lites during the Earlv during the Cambrian' Ordovician trilobites of occasionallymasslveoccurrence r<
<
e i : 9
a o
z
z
. 8 X F " x E 8
g -q :r gF 3z ;E: !; = i: !3: !i B SaEFg <
<
ro
o
o
o(
e
u
(After Moore' 1969) Fie. I.2.2. Main groupsof microfossilsin the maine environment'
2.2HigherPlants
l7
and Silurian, aswell as the explosiveemergenceof foraminifersduring the Late Jurassic.Planktonic foraminifera must be consideredas major contributors of organic matter to certain marine sediments.Their quantity and occurrence, however,seemto be controlledprimarily by phytoplanktonproductivity,i. e., by the availability of a food-source (Tolderlund and 86, 1971). More highly organizedanimals,suchasfishes,contributeso little organicmatter to sediments that they can practically be ignored. However, the larval stages of most invertebrateshaveprobablycontributedvaryingamountsof organicmattersince the Cambriantime. A quantitativeestimateof the contributionof variousgroupsof organismswith high productivity and biomassduring geologicalhistory is difficult. It existsfor only a few categoriesof organisms,such as phytoplankton (Fig. I.2.1). Main groupsof microfossilsandtheir possibleorganiccontributionto sedimentsin the marine and nonmarineenvironmentsare shownin Figure I.2.2 and FigureI.2.3. These figures are not intended to show the continuity of an environment,but rather the kind of organismavailablewhen the environmentdid occur (Moore, 1969). Microfossils rich in organic matter are black, while forms in which inorganic skeletal material predominatesare stippled. The appearanceof a givengroup is representedby a narrow area and a more commonoccurrenceby a wider area.No attempt is made to show relative abundance.
::i;;:F
Fig. I.2.3. Main groups of microfossils in the nonmarine environment. (After Moore, 1969)
2.2 Higher Plants Following phytoplankton and bacteria, higher plants are the third important contributor to organic matter in sediments.As can be seen in Figure I.2.4, remains of higher plants appear in sedimentsof Silurian age and have been
Evolutionof the Biosphere
Fig.I.2.4. The evolutionof vascularlandplants.DuringLate Silurianthe first higher thecontinents' andconquered fromthe marineenvironment plants,psilophyta, emerged 1969) (ModifiedafterZimmermann, common relics since Devonian time. The precursorsof higher plants evolved throughout the Precambrian,Cambrian and Ordovician' In order of developmentjhese precursorsincludeblue-greenalgae,green algae,and finally higher algaesuchasseaweedand kelp. They lived in the marineenvironment'Evolution a-smallnumberof of land plantsstartedin the Siiurian.The sporerecordsuggests diversity through increasing continuously with Silurian, in the types land-plant Devonian. the in materialof Silurianage In this connection,relativelyrich sporeassemblages (Wenlock-Ludlow) from North Africa are of specialinterest,becauseof their early age,with respectto the evolutionaryhistory of land plants(Richardsonand Ioannides,1973). According to the macrofossilrecord, it was not until latesl Silurian that primitive Pi"ilopsida(Cooksonia),belonging to the. division of Pteridophyta' ionquered the continents.SomePsilopsidaalsolived in the marineenvironment' Theie primitive plants were possiblyleaflessand rootless,but certainly had a vascularsystem. During the Early Devonian, other groupsof Pteridophytaevolved(Zimmermann, 1i59). In the Middle Devonian,due probably to an.explosiveevolution' mostclassesof vascularplantshad alreadyappearedasprimitive representatlves' In the Late Devonian,Psilopsidabecamescarce,while other Pteridophyta,such as Lycopsida (Cyclostigma),Sphenopsida(Sphenophyllum),and early fems
2.3Geological Historyof the Biosphere
19
(Pteropsida)dominatedthe land flora. Contrary to the Silurianland plants,Late Devonian plants had small leaves,roots and secondarywood. The land flora of the Late Devonian is in many respects similar to the flora of the Early Carboniferous.During Early Carboniferous,the first Equisetalesand seedferns (pteridosperms)appeared.Lepidodendronbecamea commonplant. During the Late Carboniferous,this type of land flora reacheda culmination in variety of forms and quantity. Shrubsand larger treesoccurredmore and more frequently during Late Devonian and Carboniferous,eventuallyforming denseforestsand hencelarge massesof wood, now partly fixed in extensivecoal seams. Toward the end of the Paleozoic,duringthe PermianAge, the gymnosperms, a new divisionof plants,emergedfrom earlierbeginnings.They includedprimarily such classesas Coniferales, Ginkgoales, Cycadales and Bennettitales, and dominated the flora until the Cretaceous.Because of this dominance of gymnosperrns,the time-span between Late Permian and Early Cretaceousis called the "era of gymnosperms".Pteridophytadid not play an important role during this time. A final important turning point in the evolution of plants wasreachedduring the Early Cretaceous.The characterof land vegetationwasseverelychangedby the sudden appearanceof angiosperms,which quickly dominated the flora. Althougb present-dayvegetationshowsa wider variation in angiospermsthan that during the Upper Cretaceous,the sametlpe ofplantsstillcoversvastareasof the continents.The sketchedevolution of land Dlantsshown in Fisure I.2.4 is takenfrom Zmmermann( 1969).
2.3 GeologicalHistory of the Biosphere The evolution of the biosphere and the geologicalhistory of the earth are intimately interrelated, and cannot be considered separately. For a short comprehensiveunderstandingof both, partly on a hypotheticalbasis,we follow largely the ideas and interpretationsof Biilow (1959). Throughout geological history, the evolution of flora was alwaysone stepaheadof faunalevolution.At all evolutionarylevels,plantsinvadednew ecologicalsystemsfirst, andwerelater followed by animals. During the Early Paleozoic,when algae dominated the biosphere,only minor quantitiesof marine invertebratesexistedcomparedto the plant biomass.The tremendoussurplusin productivityof lower plantsat that time is representedby dark and black marineshales,rich in organicmatter,which are the "normal" open marine sedimentsof the Cambrian,Ordovician and Silurian tlmes. In later periods,similar shaleswere lesscommonin marine environments,and situations.Examplesof their occurrenceswere limited to specialpaleogeographic thesespecialsituationsare land-lockedmarinebasinssuchaslagoons,andlarger bodiesof water, suchasthe early Atlantic Oceanin the Middle Cretaceoustime. In the open marine environmentafler the Siluriantime, a kind of equilibriumhad developedbetweenplant production and a plant-consumingfauna.The normal surplus in plant (mainly algae)productivity was thus terminated. The overall
2li
Evolutionof the BiosPhere
not seemto be evolutionarylevel of the Late Silurianmarineflora andfaunadoes of the conquest with the of evolution level from today's marine il;;iif;ti of dominance the and Devonian' Silurian tV f *a plants during late .."ii*"i. ot sort some until to-disappear' began -u.in. otgJni" productivityslowly was reached between-marine and lerrestrial productivity in the .o"ifit.irri from open C]"tu""ou. ti-". Xter the Silurian,the surplusof productivityshifted of the most marine enuironmentto coastalareas,and to the paralicbasinswhere gymnosperms of iate paleozoiccoal depositshavebeen found.with the advent of the very "."i itr" ""0 of the Pileozoic, and especiallywith the appearance plant Cretaceous' adaptable,and hencesuperior,angiospermsduring the Early of the deposits coal Large areas sumlus productiviryshiited to co-ntinental shift. to this attest basins, d.J,"..,i"i "",r i"riiary Age, whichoriginatedin inland
SummaryandConclusion producerof organic starting in the P'ecambriantill the Devonian, the sole primary amount of increasing an Devonian the Since phytoplankton. marine a"tt".'*ut plants At present terrestrial by higher production contributed been has orimarv produce about it".i"J p'i'V.pf"*ton and rerrestrialhigher plants are estimatedto contributors most important four the carbon Quantititively, "quui",ioirnti or otganic plants and to'organic matter in sedimentsare phytoplankton,zooplankton' higher solittle to average on the contribute bacteiia.Higher organizedanimals,suchaifishes, neglected be practically they can that mutlr in sidiments -'iilo.atttg orsanic a.tendencyto be high in to tft" fawsof the food ihain, zooplanklon.shows where "r"ui of higfi phy,oplanktonproductivity.Heterotrophicbacteriaare abundant of food as a source oreanicmatter is available
Chapter3 BiologicalProductivity of ModemAquatic Environments
The biologicalproductivity of aquaticenvironments,especiallymanne envrronments, is of great importance to the formation of potential oil sourcerocks. Although the primary productivity of organicmatter in aquaticenvironmentsis presently in the same range as in subaerial environments - due to the wide-spreadoccurrenceof land plants- the chancefor presewationof organic matter in sub-aquaticenvironmentsis far greater.In sub-aerialenvironments, free accessof air, together with the presenceof moisture, allows growth and action of bacteria, and hence a breakdown and destructionof organic matter. However,in sub-aquaticenvironments,the depositionof fine-grainedsediments limits accessof dissolvedmolecularorygen.Thus, the activity of aerobicbacteria comesto a halt when the limited amountof oxygentrappedin the sedimentsis exhausted.In this connection,it is also important to realize that air contains 2l7o (vol) of oxygen,whereaswater normally containsonly a few ml of oxygen per l. Therefore,in practicalsense,organicmatteris only preservedandfossilized in sub-aquaticsediments.
3.1 PrimaryProducersof OrganicMatter Primary production of organic matter from inorganic components in the present-daymarinerealm mayserveasa key to the geologicalpast.In addition,in certain areas, large amounts of organic matter have been contributed by terrestrialhigher plants.This, however,was not possibleprior to the emergence of land plants during Silurian-Devoniantime. As stated before, the main producers of organic matter in the marine environmentare the variousgroupsof unicellular,microscopicphytoplanktonic organisms.The main groupsare diatoms,dinoflagellates,Cyanophyceae(bluegreen algae) and very tiny, naked phytoflagellatesor nonmotile cells, the nannoplankton.They generallyrange in sizefrom 0.002 mm to 1 mm and may form coloniesup to 5 mm in diameter.The nannoplanktonand ultraplankton comprisespeciesmeasuringabout 10p or less(key, 1970).All theseorganisms are equipped with mechanismssuch as flagellae or spiny extensionsof their skeletonsto preventor slow down their sinkinginto the darknessofgreaterwater depths.Someorganisms,such as certain diatoms,even produceoil droplets to lower their averagespecificweight, giving them additionalbuoyancy.Examples of the most important phytoplanktonicand zooplanktonicorganismspresently foundin the oceansare shownin FisureI.3.1.
BiologicalProductivityof
22
ModernAquaticEnvironments
{ 1
t '1
t-J
0.1mm
Fie.l'3'l.Microplanktonicolganismsoftheoceans.(AfterKrey,1970).I.Dinoflage|. micansl2 Dinophysisacuta,3 Peridinium diveryens'4 Ceratium iulu.,-t p-tor"it^m "iipo, Distephanus ll silicoflagel.lata,ei7 S Ceratiumfurca,6'Cirotiumfru'as n^r. nnrtiro, 1O ,f"rutr^. ltl. Diatomeae: 8 toscinotliscusconcinnus,9 Rhizosoleniat'aer6sensis' -rtiiroroUrfo 13 Ditylum mobiliensis' 12 Bifutulph.ia decipiens, 11 Chaetoceras setigera, "Nits;chia closterium. lV Protozoae: 15 Tintinnopsiscompqnul4"l6 iri[nr*"Uii, 14 vl Copepodae: Titrtinnopri,,ubuloto.V. Cirripediae: 11 Nauplius'tar'verrucaStrijmia 18 Temorulongicomis are by definition Benthonic organisms, such as large kelp and rock.weed' depths usually water of shallow toireas attached to the 6ttom and are restricied aworldwide on but importance local of maybe Phytobenthos ..i"-""JtglO;. annual present is a there that It is estimated ignored. scale it can iractically be
in the and0 2 x 10etphytobenthos "t"Jr"tl- 6i lso x 10'1;f phytoplankton bcean(Krey,1970).
Chapter3 BiologicalProductivityof Modem Aquatic Environments
The biologicalproductivity of aquaticenvironments,especiallymanne envlronments, is ol great importance to the formation of potential oil sourcerocks. Although the primary productivity of organicmatter in aquaticenvironmentsis presently in the same range as in subaerial environments - due to the wide-spreadoccurrenceof land plants- the chancefor preservationof organic matter in sub-aquaticenvironmentsis far greater.In sub-aerialenvironments, free accessof air, together with the presenceof moisture, allows growth and action of bacteria, and hence a breakdown and destructionof organic matter. However,in sub-aquaticenvironments,the depositionof fine-grainedsediments limits accessof dissolvedmolecularorygen.Thus,the activity of aerobicbacteria comesto a halt when the limited amountof oxygentrapped in the sedimentsis exhausted.In this connection,it is also important to realize that air contains 21Vo(vol) of oxygen,whereaswater normally containsonly a few ml of oxygen per l. Therefore,in practicalsense,organicmatteris only preservedandfossilized in sub-aquaticsediments.
3.1 PrimaryProducersof OrganicMatter Primary production of organic matter from inorganic components in the present-daymarinerealm mayserveasa key to the geologicalpast.In addition,in certain areas, large amounts of organic matter have been contributed by terrestrialhigher plants.This, however,was not possibleprior to the emergence of land plants during Silurian-Devoniantime. As stated before, the main producers of organic matter in the marine environmentare the variousgroupsof unicellular,microscopicphytoplanktonic organisms.The main groupsare diatoms,dinoflagellates,Cyanophyceae(bluegreen algae) and very tiny, naked phytoflagellatesor nonmotile cells, the nannoplankton.They generallyrange in sizefrom 0.002 mm to 1 mm and may form coloniesup to 5 mm in diameter.The nannoplanktonand ultraplankton comprisespeciesmeasuringabout 10p or less(Krey, 1970).All theseorganisms are equipped with mechanismssuch as flagellae or spiny extensionsof their skeletonsto preventor slow down their sinkinginto the darknessoI greaterwater depths.Someorganisms,such as certain diatoms,even produceoil dropletsto lower their averagespecificweight, giving them additionalbuoyancy.Examples of the most important phytoplanktonicand zooplanktonicorganismspresently foundin the oceansare shownin FisureI.3.1.
BiologicalProductivityof Modern Aquatic Environments
22
tt 1
,7
(After Krey' 1970) I DinoflagelFir. I.3.1. Microplanktonicorganismsof the oceans 3 micans,2 Dinophvsis ocula' ::t:1i.nil:f !',::!:2' t;i^;, l-i;.;;"";r"m :,::::'::: Distepharuts II Silicofl^Eel'lat^e:,7 triposvar.atlantico,5 Ceratiumlurca,6Vlratiumla'rls g 'on'inn"s' 9 Rhizosoleniafaerijsensis'lO "iiil^o,i"^i" sieculum. 111.Diatomeae: co"ilro'!i"^ 13 Ditvtum 12 Bidlyle!.ta .no!l\:ns:' 'Ptoto'ou"' setiseru, rr Chaektcerasdecipierc' 16 camponula' Tintinnopsis 15 closte'ium Iv ii'**,chia iiiii*"iiii, Copepodae: Yl' var' vetruct Striimia i'rl***""ri, ,"U"t*t V Cirripediae: 11 Nauplius l8 Temoralongicornis
Benthonicorganisms,suchaslargekelpandrockw eed'arebydefinition of st'attowwaterdepthsusually attachedto the botto. u,td u'" t"ttii"leaio'uttut but on aworldwide rhvtou"ntttot mufbeot locatimportance ;;;;;;i6;. a presentannual is theJe that scaleit can practicallyU" lgn*"j' iiit"stimated in the phvtobenthos
;"rlil
#;i;so.- t'rot & pr'vtfr""kion and0 2 x 10et
ocean(KreY, 1970).
3.2 FactorsInfluencingPrimaryProductivity
Although still controversial,bacteriaareconsideredto be ofimportance,along with phytoplankton,withrespectto biomassandorganicproductionin the marine environment.However, the overall production of heterotrophicbacteriashould be lessthan the productionof phytoplankton,sincethe productionof consumers of organicmatter cannotexceedthe level of primary production(Datsko, 1959). Exceptionsto this are caseswhereothersourcesof food are availablefor bacteria, suchasdissolvedandparticulateorganicmatterderivedfromcontinents.Sorokin (1971), for example,observedthat the production of bacterioplanktonunder 1 m2 in tropical ocean waters may exceed the primary production by phytoplankton.The role of bacteriain the biologicalproductivityis to decompose dead organic matter and convert its decay products into forms suitable for assimilationby aquaticplants.Furthermore,bacteriathemselvesserveasfood for aquatic animals.Bacteria in the ocean are partly suspendedin the water and partly attachedto plankton organismsand particlesof detritus.Great quantities of bacteriaoccur alsoon the seafloor, probablywith the exceptionof very great water depths,and in the top layer of sediment(Bordovskiy, 1965) in shallowto moderatewater depths. Phltoplankton, the primary producer of organic matter, forms the basic member of the food chain and hence of the pyramid of life. Diatoms, dinoflagellates,and coccolithophoresare the main producersin this sequence. Next membersin the food chain are vegetarianssuchas smallcrustaceans (e.g., copepodes,seeFig. I.3.1), that live on ph).toplanktonicorganisms.Then follow larger animals,such as herring and mackerel.In a food chain of this tlpe the quantitativerelationshipbetweenphytoplanktonand mackerelis about 1000:1. It is interestingto comparethis with the relationshipbetweenprimary organic production and organic matter buried and preservedin marine sediments.This relationshipalso showsa ratio of about 1000:1. Tasch (1967) presentsinformation on the geologicalage range of existing primary producers.Dinoflagellatesprobablyoccupiedfirst place,andcoccolithophoressecondplaceamongprimaryproducersin Mesozoicoceans, excepttoward the end of the Cretaceous,whenmarinediatomsroseto first placeamongprimary producers.Acritarchs are supposedto be remains of other unicellular algae, which were also primary producers throughout the Palezoic and in Late Precambriantime.
3.2 FactorsInfluencing Primary Productivity Biological productivity in the marine environmentis mainly controlled by light, temperature,and chemicalcompositionof seawater, especiallywith regard to mineral nutrients such as phosphate and nitrate. These parameters are interrelated in a very complex mannerto physiographicfeaturesof the ocean, such as morphologyof the oceanbasins,oceancurrentsand mixing of different water bodies. A verticalprofile or hypsographiccurveof the earth'ssurface,showingrelative distribution of surface area, is usetul for an understandingof the following
BiologicalProductivityof Modern Aquatic Environments 0Mill.km2
LAND
'!
SEA
t 9 o - 0
5 o
a j
l0O 16l Mill krn2
curveof the earth'ssurface(lefr)and relativedistributionof Fig. I.3.2. Hypsographic anddepthlevels(right).The two maximaofthe curveto therightreflectthe elevations dualismbetweencontinentsand oceans,i.e., continentaland oceaniccrust.(After 1950) Kuenen,
discussion.FigureI.3.2 showsthe relativeproportionsof surfaceareasat various elevationson land and depths beneaththe oceansover the entire earth. Two distinct maxima appearon this curve, one at atnut 100 m abovesealevel, and anotherbetween4000 and 5000 m below sealevel.The first one includesthe old continentalmasses,and the secondthe bottom of the deep sea.This reflectsthe well-known dualism between continentsand oceans,and betweencontinental and oceaniccrust (Seibold,1974).The classand distributionof seasby water depth are as follows: areaswith a water depth of 0-200 m are conventionally called shelf (7.57o of the sea floor): water depths of 200-4000 m are characteristicof the continental slopes(8%) and include the continental rise (57o); from 4000 m-5500 m is the deep sea abyssalplain (77.57o);and below 5500 m are the so-calledtrenches(27o).Shelf,continentalslope,andcontinental rise are togethertermedcontinentalmargin.If primaryproductivityof the oceans is seenin relation to the abovedistribution of oceanicareas(Table L3.1), the importanceof coastalwaters,which includeprimarily the shelfregionsandpart of the continentalslopes,is evident.A mapof the shelfareasis shownin FigureI.3.3.
to theopenocean.(Adaptedafter of coastal watersascompared TableI.3.1. Productivity Krev.1970) Area
Area in km2 x 106
326 Open ocean Coastalwaters 36 Areaswith 0.4 upwelling
Averaged productivity Ing Lm _yr
Total productionin 10't
50 100
16.3 3.6
300
o.l
25
3.2 FactorsInfluencingPrimaryProductivity 0
30
60
90
1 2 0 1 5 0 t 8 0 150 t20
s0
120 150 180 150 120 90
!
:
. I I
:
t.
I Continental shelf "
I
0 ii
30
E0
60
Fig. I.3.3. Shelf areasof the world. Water depth in shelf areasis O 2O0m.About7.57rof the sea floor belongs to shelf areas
ii All organic matter in the oceansis ultimately producedby unicellular algae through photosynthesis,except for very small contributionsby pelagicmacrophytes.The spatialvariation in photosyntheticproductionis mainlycontrolledby the supplyof light and nutrients.In general,growth is limited by supplyof light at the oceansurfaceonly in polar regions,at depth in the openocean,and in turbid coastalwaters (Menzel, 1974). Outside these regions,rates of production are controlled by the supply of nutrients,and this in turn is influencedby recycling and circulationthat replenishor removenutrient supplyfrom the euphoticzone of the water column.The interrelationshipbetweenoccurrenceof phosphate,an important nutrient, and growth of phytoplanktonin the SouthAtlantic oceanis shownin FigureI.3.4. In the oceans,9070or moreof the producedplant matteris eatenby grazingorganisms(Menzel,1974).As plantswillgrow only in the lighted upper layersof the ocean,the conceptof the euphoticzonehasbeenelaborated. The euphoticzone is defined asthe zone of highestphotosyntheticproductivity near the surfaceof the oceans.and takesinto accountsuchfactorsasillumination and nutrients. An exact depth cannot be given, as it varies from one area to anotheraccordingto local conditions.Normally, the euphoticzoneis situatedin the top 200 m of surfacewaters.The majority of organic matter production, however,is concentratedin the upper 60 to 80 m of the water column. If thermal or salinitystratificationstabilizesthe water in the euphoticzone,the rate of nutrient replenishmentis very slow (e.g., in subtropicalregions).Rapid mixing, on the other hand, may continuallyfertilize the euphoticzone,but plant cells may be transportedby the mixing currentsout of the light zonebefore cell division and reproduction can take place. Balanced optimal conditions are frequentlyencounteredin regionswith water upwelling (Table I.3.1), suchasin
26
PHOSPHATEI(ms/ m') @ El @ I
BiologicalProductivityof Modern AquaticEnvironments
. ) 2-g 9-22 22-35 >35
P L A N K T o N<5 (1000c€tts/t)Z, 5-10 @ 10-50 @ 50-100 >100 I
Fig. I.3.4. Distribution of phosphateandplankton in surfacewatersofthe SouthAtlantic ocean.(After Dietrich, 1963; taken from Debyserand Deroo, 1969)
Fig. L3.5. Depth distribution of phosphate and oxygenin an area of upwellingoff southwestAfrica. There is a characteristicdeDletion of nutrientr.hereexemplitied by rhedepletion of phosphate,due to intense phytoplankton growth in near-surfacewaters.It is replenished from belowby upwelling.(AfterEmery, 1963)
certain areasof westemcontinentalshelves.The resultingdepth distribution ot seawater properties,as found in an areaof upwellingoff SouthwestAfrica (Fig. I.3.5), has been summarizedby Emery (1963). It is characterizedby a depletion of nutrients due to intensephytoplankton growth in near-surfacewaters and a replenishment of these by upwelling, locally supplementedby addition of nutrientsfrom rivers.Good situationsfor productionmay alsooccurwherethere are altematingperiodsof water stabilization,when phytoplanktonbloomsin the euphoticzone, and periodsof remixingby storm or tidal activity,when nutrients
:
3.2Factors Influencing PdmaryProductivity
i
are replenishedto allow the next bloom.Theselatter conditionsoccurfrequently in the protected coastal areas of British Columbia (Strickland, 1965). Water mixing, either in short or longerintervals,is thus of greatinfluenceon biological productivity. The relative constancyof chemicalcompositionof the ocean walets, 3.5Vo salinity by weight (Table I.3.2) indicatesgood mixing on longerterms,at leastof the top 200 m of surfacewater. In these superficialwater layers,wind-driven currentson the averagecomplete the mixing within a few decades.In deeper waters,wind is not the dominatingfactor, but rather it is sinkingmassesof cold water from the polar regions.This cold, healy water creepsunderneathwater of lesser density and moves toward lower latitudes. This processalso causesa long-termmixing, but it takes 1000or 2000 yearsto be completed.Temperature in oceanwatercis particularly interrelatedwith illumination and water mixing. Hence,it is difficult to isolatethe effect of temperatureon primary productivity. However, there are "optimal" temperaturesfor variousphytoplanktonspecies. Dinoflagellates,for instance,prefer warm tropical waterswith temperaturesof 25'C or more, whereassiliciousdiatomsand radiolariadominatethe cold waters (5- 15'C), especiallyin the polar regions.The differentiationinto variousspecies of organismsis by far greater in warm tropical watersthan in cold waters.The sameis observedwith salinitychanges.Areaswith lowersalinities,suchasregions of brackish water with salinities of 5-70lnn, are characterizedby minima in number of species.However, a reduction in the number of speciesdoes not necessarily meana reductionin productivity. When consideringchemicalcompositionof seawater, it mustbe kept in mind that it is normally not salinityasa whole,but availabilityoI nutrientsthat is most important for productivity. When the concentrationof a nutrient becomestoo low, it will influence photosynthesis,rate of cell division and even chemical compositionof plant cells. Aside from COr, nitrogen, phosphorusand trace organics (vitamins) are essentialfor plant growth. Nitrogen is consumedas ammonia (NHa+)or nitrate (NO;), and phosphorusas phosphate (PO43-).
i I
i
1
Table I.3.2. Main inorganic components of sea water, expressedin molecular form, showing quantityof salt dissolvedinll
Formula NaCl McCl, MgSOa CaSOa K2SO4 CaC03 MgB12
27.2 3.u 1.6 1.3 0.9 0.1 0.08 34.98
27
Biological Productivity of Modern Aquatic Environments
28
Among the trace organics,vitamin Br2 is important. Generally in the marine environment, nitrogen as nitrate is the most important element,becauseit is usually the first to be stripped from the water by growing organisms.In a few locations,phosphatemay be depletedbeforenitrate, but this is more commonin lakes and rivers than in the marine environment (Strickland, 1965). Marine basins,isolatedmore or lessfrom the open oceans,show grosswater properties that deviatemore or lessfrom thoseof open oceanwaters (Fig. I.3.6a, b).
0 x y g ei n o c e a rnr a t e(rm l l l l 1
(
2
(
\,* \
3
--7 / Atl€ntid
<\.
t
5
6
ocoan
I ) l I
l n d i an
Pacifi,
tttt,.
EI
I
Bry o{
Kaoe
Pacilic
G
EI
-0.30 H2s( o.oo )
oco.n
Fig.I.3.6. (a)Verticaldistribution of oxygenin variousocean basins.(After Dietrich, 1963). (b) Vertical distributionofoxygen (ml I l) in waters of a closed bay (Kaoe, Halmahera Isfands).(After Kuenen,1942)
3.3 PresentPrimaryProductionof the Oceans As describedin the preceding pages,primary productivity in the oceansis influencedby many interrelatedparameters,especiallylight, nutrients,andwater mixing. Knowledge of these parameters,together with direct productivity measurements,provided the information needed for preparation of a map showing intensitiesof primary biological production in the world oceans(Fig.
r.3.7).
J.3 PresentPrimary Productionof the Oceans
29
produciviry Fig.I.3.7.Primarybiological (org.carbon. , yr-,) in rhcworldoceans. Productivity is relatively highin coastal areas. cspecially in zones of upwelling alongthe westcoaslof the AmericanandAfricancontinents. (Adaptedfrom DebyserandDeroo, 1969)
Many researchershaveobserveda so-calledlandmassor islandeffect,where, as a broad generalization,primary productivity is greater near the coast of continentsand islands,or evenover submergedridgesandbanksthan in the open ocean(Strickland, 1965). The reasonfor this phenomenon,which occursat all latitudes,is not yet fully understood.It could be that grazing,andhenceapparent reduction of productivity, are for some reason more continuous and more effective in the open ocean than in coastalareas.Strickland(1965) has summarizedthe generalpictureof productivityon a regionalbasis.The two most important extremes,barren oceansand zonesof upwelling,are cited below (see a l s oF i g .I . 3 . 7 ) : Barrenoceans:Most of the tropicaloceansbetweenlatitudes10" and 40', and stagnanttro?ical gyres like the SargassoSea, are very unproductive (about 0.1 g Cm'" dav'). However,as this rate persistsmost of the year, annual productionsof 50 g Cm "'can be expected. Continentalshelf upwelling:T}rc effect of prevailingwinds and of the Coriolis force on oceancurrentsoff the westcoastof continentsmaycauseupwelling,and thereforegreatfertility (2 g Cm r0day I andmore). The primary productivityin freshwaterlakesprincipallyobeysthe samelawsas in the marine environment. Due to the fact that smaller water bodies and shallowerdepth are involved, climatic influencesand influencesfrom the surroundingland are generallymuch more pronounced.
30
BiologicalProductivityof Modern Aquatic Environments
SummaryandConclusion The biologicalproductivity in the marine environmentis mainly controlledby light, temperature,aird mineral nutrienls, suchasphosphatearld nitrate. The larger part of biologicalproductionis concentratedin the upper 60 to 80 m ol the water column.The biological productivity of coastalwaters (100 g C"om-2a-r) is on the averageabout twice ashigh asthat of the openocean,Most productiveare areaswith waterupwelling (300g C..rm-'a-t), suchas in certainparts of Westerncontinentalshelfs.
Chapter4 ChemicalCompositionof the Biomass:Bacteria, Phytoplankton, Zooplankton, HigherPlants
In the preceding chapters, we have learned that bacteria, phytoplankton, zooplankton(especiallyforaminifersand certaincrustaceans), andhigher plants are the main contributorsof organicmatter in sediments.Natural associations of these various groups of organismsin various faciesprovincesthus determine composition and type oi organic matter to be depositedand incorporatedin sediments. From our point of view, we are mainlyinterestedin biomasscompositionfrom such natural associationsoccurring in open waters of the shelf areas, the continentalslopesand continentalrises.In general,theseincludethe continental marginsand large land-locked,isolatedbasins(marginalseas)suchasthe Black Sea, the Baltic Sea, the MediterraneanSea, and smaller lagoonal-typewater bodies.As will be shownlater, the lipid and lipid-like fractionsof organismsare among the chemicalconstituentsplaying a dominant role in the formation of petroleum. Hence, in the following chapter, we will concentrate on the composition of the biomass,with special emphasison lipids and lipidJike constituentsof bacteria, phytoplankton,zooplanktonand higher plants. However, at present,only limited quantitative information is availableon natural associations of organismsandtheir grosschemicalcomposition.When discussing compositionof the biomass,it must be kept in mind that a sedimentdoesnot contiin a true imageof the original populationof organismsin the water column above. Moreover, the chemical composition of organismschangesprior to incorporationin a sediment.As far astheir soft partsare concerned,basicallyall organisms are composedof the same groups of chemical constituents,i.e., proteins,lipids, carbohydratesand ligninsin higher plants.
4.1 Proteinsand Carbohydrates Proteinsare highly ordered polymers,made from individual amino acids.They accountfor most of the nitrogen compoundsin organisms.Proteinsconstitute suchdifferent kinds of materialsasmusclefibers,silk and sponge.In the form of enzymes,they catalyzebiochemicalreactions within andoutsideofcells.They are of specialinterestin biologicalmineralizationprocesses, suchasthe formationof shells,wherethey act asorganicmatrices,and are thus intimatelyassociatedwith the mineral phase.In the presenceof water, insolubleproteins may be broken down, due to the action of enzymes,to their water-solublemonomers,amino
32
Chemical Composition of theBiomass: Bacteria, Plankton,HigherPlants
acids.In Figure I.4.1, this processof hydrolysisof the peptidebond (a) is shown togetherwith somestructuralexamplesof amino acids(b). Carbohydratesis a collectivename for individual sugarsand their polymers. They include the monosaccharides, di-, tri- (oligosaccharides) and polysaccharides.The name carbohydrateis derived from the empiricalformula C" (HrO)", which suggestshydrated carbon. This empirical formula, however, has no particularchemicalsignificance,sincecarbohydratesarein factpolyhydroxylated compoundsand not just hydratedcarbonaceous substances. Carbohydratesare amongthe most abundantconstituentsof plantsand animals.They aresourcesof energy and form the supportingtissuesof plants and certainanimals.Cellulose and chitin are amongthe most prominent polysaccharides occurringin nature. The polysaccharidecelluloseconsistsof 2000 to 8000 monosaccharideunits. Wood is composedof cellulose(4O-6OEa)and lignin. Higher plants synthesize the largestamount of cellulose,whereascertain algae,seaweedsand diatoms
Peptrde bond
Neut.ol qmino ocid!
Ttt
H
H-C-COOH
I
N-
A fH" CH;C-COOH
d-Alonine
H n .N
9Hr-ci'2
Prolins CHz CH-COOH
H
H
a
Acidicominoo",0"
,l no Hooc-cHtcHtC-;ooH
Glutomic ocid
I
Asporlic ocid
NILt . HOoC-CH2-C-cooH
|l Bosic omino ocid3 Ornithine
T", H2N-CH2-CHtCHtC-COOH H
b
Lysne
H2N-cHacH-
T"t
cH.cHz-c-cooH FI
Fig. I.4.1. (a) Hydrolysisof peptidebond. (b) Structuralexamplesof neutral,acidicand basicamino acidsfound in geologicalenvironments
.1.1Proteins andCarbohydrates
33
appear to be devoid of cellulose (Percival, 1966). Cellulose and chitin have analogousstructures consisting basically of glucose and glucosamineunits, respectively(Fig. I.a.2). Some algae, especiallybrown algae (Phaeophyta), contain as much as 407o (d.y -t.) of alginic acid, a carbohydratederivative. Pectin, a similar carbohydrateand like alginic acid, a polyuronid, is a frequent componentof higher plantsand bacteria(Fig. I.4.2). Polysaccharides are largely water-insolublesubstances.They are, however, converted by hydrolysis into water-solubleC6 and C5-+ugars
cHzoH
f\,"vd \'L-( .-.o/ '
,
lt.o"
,
l.o"
f\r"tn-\,(:"y"{-\
Y_o/.o'Y
Yo' .o'.4 cH2oH
or2oH
I
it'o--cHzoH
Cellulose
c00fl
coocfts
Peclin
cooH
..t",.CX"'t-gx "r(1)-"\qx"z cooH Alginic ocid
cH2ofl
-".Y
/-o,,.,o
HsCOC-Ntl
cH2oH
C h i fi n Fig. I.4.2. Macromolecularcarbohydratederivatives(e.g., cellulose,pectin,alginic acid and chitin) occurringin supportingtissuesof plantsand animals(free verticalbarsin forrnularepresentOH-groups)
34
ChemicalCompositionof the Biomass:Bacte a, Plankron,Higher Plants
4.2Lipids The term lipids, as defined by Bergmann(1963), is appliedto all organism-producedsubstances that arepracticallyinsolubleinwater, but areextractableby one or another of the so-called fat solvents.These solventsinclude chloroform, carbontetrachloride,ethers,aliphaticand aromatichydrocarbons,and acetone. Lipids encompassfat substances, such as animal fat, vegetableoils, and waxes suchasthoseoccurringassurfacecoatingsof leaves.Fatsareutilizedin the energy budget of organisms.Waxes, however, are primarily designedfor protective functions. Naturally occuning fats are mixtures of various triglycerides, which are classifiedchemicallyas esters.Glyceridescanbe hydrolyzed,usuallyby aqueous sodium hydroxide, to give glycerol and the componentlatty acids,which are obtained as sodium salts(Fig. I.4.3a). Fatty acidsof natural fats have an even number of carbon atoms becausethey are synthesizedbiochemicallyfrom C, units (acetateunits). Most frequently occurringare fatty acidswith 16 and 18 carbon atoms,palmitic and stearicacid respectively(Fig. I.a.3b). Unsaturated fatty acids (Fig. I.4.3c) are common, especiallyin vegetable oils. From a physiologicalpoint of view,fats andoils havea highenergycontent.Therefore,in animalsand plants they are usedas energystorage.Seeds,sporesand fruits are especiallyrich in lipids. Among algae, diatoms are known to contain great quantities of lipids, sometimesup to 707o on a dry weight basis.A high lipid content results,especiallyif the algaegrow under nitrogen-deficientconditions and in cold water. Natural waxes,sucb as bees' wax and plant waxes,are mixtures of various constituents.Waxes differ from fats in that glycerol is replaced by complex alcoholsof the sterolseriesor by highereven-carbon-number aliphaticalcoholsin the Cr6to C36range.Thesealcoholsare often found in excessof the acids,which are also even-numberedand range from C24to Ca6.Plant waxesalso contain hydrocarbons, especially long-chain n-alkanes having a predominance of moleculeswith odd carbonnumbers. In additionto thesetypical lipids,there are a numberof lipid-like components suchasoil-solublepigments,terpenoids,steroids,andmanycomplexfats,suchas certain phospholipids(phosphatides).A prominent biochemicalbuilding block, the basicunit of manyof thesecomponents,is the isopreneunit, consistingof five C-atoms(Fig.I.4.4). Isopreneunits, due to the conjugateddouble bonds,may polymerizeto form chains and rings. A great variety of these chain-like and cyclic isoprenoid compounds is known to occur in plants and animals. For a long time, the individual monomer isopienewas not found in nature.However,it wasrecently discoveredin small quantities in certain plants. It is a highly volatile, reactive hydrocarbon. Molecules containing 2 isoprene units are called terpenes (or monoterpenes,Ctu); those with 3 units sesquiterpenes(C,); with 4 units diterpenes (Czo); with 6 units triterpenes (C:o); and those with 8 units tetraterpenes (Cno). Natural gums, like caoutchouc and gutta, which are producedby certainhigherplants(EuphorbiaceaeandSapotaceae, respectively), are polyterpenes.There are indications(Brooks and Shaw,1972)that sporopol-
4.2Lipids
r,rc-tcart-{/-.lo-at,
no-c1z
.o
i:"i--,ii;|7i2i-+.#l:-:i,.'"'"-r'".'na
Triglyceride
Glycerol
Flttyocid (sodiurnsoltl
2H2! lc z l zc l z cH 2 cH 2 H c c c c ta' ta' ta' ta' ta' ta' ,^..r^.,r..,^n.a' Y Hz Hz A2H2
o H oI c-o" ..,^,, .,,,
H2 H2 H2
( Simptified formuto)
Polmitic ocid(Cr. H32o, Hz t1Z HZ HZ t1Z Hz Hz HZ On
o
c c c c c c c c c - o H / \/ \/ \/ \/ \./\./\./\./ -
Sieoric ocid (ctsH36oe)
H2 H2 H2 n '
ntt\
/t\
c
/"\
/t\
c
t12 H2
/to-r"\
c
H2
c
H2
/t\
H2
ot
,rt\ ,rt\ /t'-
c
H"
c
c
H2
H2
.-^s -", ,r'r,r-r,r"rz"t
Id2
I^2
(c'" H"oo.) Oleicocid
o H 2 H 2 H 2l
H
H2 t",tz H H H a a a - a -,., a /\ /\ /,, /o H C c c c a Hz Hz H2
( Simplifiedformulo)
n' "' ". fl c
0H
,^,/V\r'\,r\,r\.4/Vc-
Hz Hz H2 H2 H? H2 HZ Hz
9 I t12 t12
t.
u
^l
Linoleicocrd
o H
H
H
H
H
H
H 2 H 2 H 2
I I (c,"Hof,z) Hrc, ,%-i.c,,9,-ocrzc":fr A A r.i r9-o" e v v C C
Hz
Hz
Hz
,t rii iirri-i,
-/ \-/ \,/""\/ Hlc c c H
Z
9H
,
Linolenic ocid
Hz Az Hz Az
' - a ' H".o.) tz-z' " o - \ / - \ 1-on l#r \ , /rio!,, riic7 i ' {c.
9. H
,
\9 H
,9
9 H
Arocfiidonicocid
Fig. I.4.3a c. Main fat and oil componentsin organisms.(a) Triglyceride,(b) palmiticacid and stearicacid, (c) variousunsaturatedacids
4.2Lipids
35
,rr_.rr^r, -{'-z -"y,
r
* i:-:[..'n"'-''"''n-'o:lr:ii::z3-ii: Triqlyceride
a
Glycerol
Futlyqcid( sodiumsdtl
HzHzHzlzlzxzlzl. /tt H' a
o
/ut /ct /"r. /"t ,tt rtt ,t-o* C C C C C C C C HZ HZ Hz Hz H2 H2 H2
( Simplifiedformulo)
Polmitic ocid(c|6H!2 oJ tlz tlz
H2 H2 tl2 H2 H2 H2 !
c c c c c c c c c - o H / \/ \/ \/ \./ \./ \./ \./ \./ -
,
Hz Hz taz H2 H? Hz H2 H2
r"v\,^..r,\,r\r\rr\,rc-
Steoric ocid (cE H3602)
otr
( Simplifiedformuto)
o
!, l, |, ll
f;z lz lz l_f
n"'\
/'\
c
/'\
/t\
c
c
/"":r\
c
c
/t\
/t\
c
/t\
o"
/t'-
d
c
Oleicocid
H2 |-l2H2H"
H2 HZ H2 H2
(c,a Hrooa)
o
! . 1 .1 |
|
H
r{ a
! -a ! a
oH (cEHr&) Linoleic ocid
9, ,rtt zt\ zt\ l E" ?r" fr" 3"
/ \ / \ / " ' . , / o H C H^ H^ H^
! -a | a
H 2 H 2 H 2l
H
H
o Ha H? Ha ll
oH (cE \u1-7%-*17c"-,zcr 2"":"\ A frc fr brgc c c c c Hz
Hz
H 2H z H H
tr\ I
H
Hz H
H
H
"
tr\ t"F[\ /r,-,Fr rc,:"c, l-.r C C C C n2
H,
Linoleoic ocid
Hz Hz Hz Hz H
H,
H,
"
r r j
z.i /c-oH H,
H!Oo2)
c
H 2
(czoH3eo2 )
Arodridonicocid
Fig. I.4.3a-<.Main fat and oil componentsin organisms.(a) Triglyce de,(b) palmiticacid and stearicacid, (c) variousunsaturatedacids
36
ChemicalCompositionof theBiomass: Bacteria, Plankton,HigherPlants
||"C H ,C
FU
//
. .cH"
T N
I .A H
V
CHt
Fig. I.4.4. (a) Isopreneunit. Building block of many naturalbiological components,suchassteroids,terpenesand carotenoids.(b) Terpinolen, a naturallyoccurringcyclicmonoterpene, consisting of two isoprene units
is a polyterpene. lenin,an importantconstituent of sporeandpollenmembranes, The varioustypesof isoprenoidsor terpenoidsmay occurasa linear arrangement of isopreneunits or cyclizedinto from 1- to 5-ringsystems.Terpenoidsmayoccur ashydrocarbons,alcoholsor their esters,4ldehydes,ketones,acidsand lactones. They exhibitvariousdegreesof unsaturation. Monoterpenes(C1e)occur abundantlyin higher plants and in algae.They are consideredas metabolicallyunstable compounds(Francis, 1971). It is worth mentioning that monoterpenesare especiallyprominent in plants of arid-type climates,enrichedin essentialoils.An exampleis roseoil, a costlyfloweressence,
\hrious
Monoterpenes
f f"'i.., "4 fr' f"' G€rrnico.id
af""{' {"
Fig. I.4.5. Examples of monolerpenes,mamly occurring in essentialoils of higher plants
37
4.2 Lipids
cHr
C H-: CHg | I C=CHCH,CHzC=CHCH.CHrC:CHCH.oH
Lrl\
I c;lc"o/c9x.
cH.'
cHaancli<Szc-aro 9Hs
". ,z\\
9Hr
| | CHzCH2C=CHCH2CH2 C=CHCHz OH /CCHz ^un3 ,, /
H.c-c-ctt.
a
b
of a sesquiterpene, famesol,a commonisoprenoid Fig.I.4.6a andb. Structural example isomets alcoholof higherplantsandof bacteria(in a boundform).(a)Naturallyoccurring asprecursor of dicyclicsesquiterpenes of farnesol.(b) Farnesol
which is mainly composedof geranioland citronellol.The chemicalstructuresof these and other monoterpenesin various oxidation stagesare given in Figure I.4.5. In addition to being derived from plants,they are also formed by certain arthropods(Francis,1971). Sesquiterpenes(C15), consisting of 3 isoprene units, are representedby farnesol,an acyclicisoprenoidalcohol (Fig. I.4.6), and the correspondingolefin farnesin, both of which are found in many plants. Farnesolis a wide-spread compound,but it occursonly in very small concentrations.In certain bacteria, chlorophyll is esterifiedwith farnesolinsteadof phytol (Figure I.1 2). Monocyare commonlyoccurringcomponentsof the plant clic and dicyclicsesquiterpenes kingdom. Compoundswith a larnesol-t1peskeletonare consideredasbiochem! compounds Sesquiterpenoid cal precursorsof numerouscyclicsesquiterpenoids. probably fulfil hormonalregulations.They occur mainly in plant material. Diterpenes (C2n)are common constituentsof higher plants. They form the main part of resins,especiallyin conifers. In resins,they occur together with phenylpropanederivatives,such as coniferyl alcohol (Fig. I.4.15a). As will be seen,coniferylalcoholis a basicconstituentsof Iignins.Most diterpenesexhibita cyclicstructurewith two or three rings(Fig. I.4.7). Among the acyclicditerpenes, phytol (Fig. I.4.7a) is the most important component. It is part of the very abundantchlorophyll molecule.Many phasesof plant growth and development "gibberellins" (Fig. La.S), which can be grouped are affected by the so-called
aYt*.*
aI Y >\\..2
r^Yo"
.+
aJ,,T.q'" aA/ ><\2
Hooc><\_/
a
b
c
Phytol
Mo n o o l
Abi.tic ocid
Fig. I.4.7 a-{. Structuml examples of acyclic (a), dicyclic (b) and tricyclic (c) diterpenoids. Diterpenesare commoncoostltuents of higher plants
38
ChemicalCompositionof the Biomass:Bacteria,Plankton,Higher plants "
C2o-Gibborellint
Fig. I.4.8. Structural examples of gibberillins, which occur in small quantitiesin all higherplants.(After Macmillan,1971)
Ct9-Gibborelling
among the cyclic diterpenoids.They occur, usually in small quantities,in all higherplants(Macmillan,1971). Triterpmes and related.steroids are avery important group of cyclic isoprenoid compoundscontaining6 isopreneunits.Most triterpenoidsare pentacyclic,while steroidsare all tetracycliccompounds.Triterpenoids,as comparedto steroids, generallyoccur more commonlyin higher plants.Steroidsare very abundantin the animal and plant kingdom. A common biochemical precusor for the various triterpenes is squalene (C36tI5s).Squalene(Fig. I.4.9) is a poly-unsaturatedhydrocarbon,frequently occurring in plant and animal tissue.It was first isolatedby Tsujimoto (1906) from the unsaponifiablefraction of liver oils of various sharks. Thereafter.
Squolen€
-
al-K
affXV
Oleonone-Type
Urson€-Type
Lupone-Type
Fig. I.4.9. Squaleneand origin of most important types of pentacyclictriterpenes. Squaleneoccursin relativelylargequantitiesin most organisms
39
4.2Lipids
squalenehas been found in varied quantities (sometimesin concentrationsof severalpercent)in the unsaponifiablematter of most organisms,that havebeen analyzedfor its presence.A schemefor the origin of the most important typesof pentacyclictriterpenes (ursane, oleanane, lupane) from squaleneis given in Figure I.4.9. A number of prominent triterpenes,found in organisms,are consideredas direct precursorsof hydrocarbonsin fossil sedimentsand petroleum (Fig. I.a.10). Although the various triterpenoidsare very abundant in higher
CHz H-C) . .
Clo , O.onolic ocid(plonft)
Cao,Ursolic ocid (plont3)
Oleonone series
Cao,Lupeol(plonl6)
Ursone s€ries
Lupone senes
cao, oiPloptens (forn3,Uuo-gro6n ol9oo, bocterio)
( protozoon) Cao,Toirohymonol
CJo,lsoorborinol(plonts)
(17P,2lP)H-Hopone series
Gommocerone geries
Arborone s6riss H_C
cH-
HO F:C
Cao'64-Onoce.in( plonl3)
-"cHr
Clo, Loaoslgrol(onimol li33uq!)
Tetrocyclicirilerpenes Fig.I.4.10. Prominenttriterpenesof living organismsasprecursorsof fossilt terpenoids
40
plankton,Higherplants Chemical Composition of theBiomass: Bacteria,
plants,it shouldbe emphasizedthat one kind, the hopaneseries(Fig. I.4.10), is found in relatively large concentrationsin bacteria and blue-g.een algae (Dorsselaer,1975). The other triterpenoids,especiallythose with five 6-membered rings,seemto be restrictedlargelyto higher plants. The tetracyclic triterpenoid lanosterol and its derivatives, like other terpenes, are modified biochemically, usually by addition or loss of
" 3
Cz7, Cholgslorol (onimols, plonlB)
C2B,Ergo3lorol (yeo3l)
Hrc
cis
cHr cHl
C2a,C,omp€3t€rol (ptonts)
cl-tl
cH_
Cz9, P-Sito3lerol (plonts)
CH:
cHr
HO
Cz?,FunOill€rol ( funoi) fungi)
Ca4,Cholicocid (bil€of hichgronimol!)
Fig. L4.11. Structuralexamplesof prominentsteroids.Positionsmarked(. ) denotesites where a methyl group hasbeen lost
4.2Lipids
4l
methyl groups.The loss of methyl groups leadsto the important group called steroids.Theseare found throughoutthe plant and animal kingdom.Steroidsare rare, but not absent,in bacteria and blue-greenalgae(Ensmingeret al., L973; Metzner, 1973). However, it is not yet completely clear whether these prokaryotic organismssynthetizesteroidsthemselves,or whether they contain them becausethey were consumedwith their food while they lived heterotrophically. Structural examplesof commonly occurring steroidsare given in Figure I.4.11. Positions where methyl groups have been lost are marked by dots. Cholesterol (Cr) is quantitativelythe mostimportantsterolof admals;however, it has also been found in many other organismseven including bacteria. Ergosterol (Crr) is most abundantin yeastand other fungi, but has also been discoveredin certain algae.Fucosterol (C2r) is very common in diatoms and various kinds of algae, such as Phaeophyceae, Chrysophyceaeand especially Rhodophyceae(Metzner, 1973). Campesterol (Cr) is found in varioushigher plants (Karrer, 1958).Sitosterol (Crr) and stigmasterol(C2r)are typical of, and very abundant in, higher plants. Sterolshaving the same carbon number and structural arrangement,such as fucosterol,sitosteroland stigmasterol(all Cr,.-., steroids)cannotbe differentiatedonce they havebeentransformedto saturated steranes. The mostimportant representatives of the group of tetraterpenes(Cap),made from 8 isopreneunits, are the carotenoidpigments.There is a large variety of carotenoidpigmentsin nature. Becauseof their similarity in structure,they are named after the most common member of this group, carotene.Caroteneis a polyunsaturatedhydrocarbon,composedof isopreneunits. Oxygen-containing compoundsof this type are called xanthophylls.The carotenoidsusuallyhave either a red or yellow color becauseof the great number of conjugateddouble bondsin their molecules.FigureI.4.12 showsstructuralexamplesof carotenoids suchaslycopeneandFcarotene, whichoccurin plants.Carotenoidsare abundant in algae,often reachingconcentrationsof about0.27oto 0.8% (dry wt.). In some species, concentrations may even rise to 5Vo or more, Carotenoids are concentratedin certain parts of higher plants,suchas in fruits and leaves. H3c\...cH3 9H. cH3 H3C,r",zcH. 9Hr 9Hr = cHc:cHcH =cHcH:ccH =cHcH=ccH: n.c/'\cct=cnc =cHcH cHcl-\c H2 HzC:..,C\,. cHz crlJ
.c--,,,CHz Hsc c{z P-Coroiene
H3C\..-CH3 gHr gHr H3C\.'/CH3 9Hr 9Hr - --?H =CHC CxCt:CnC: CHCH CHCH-CHgH:i6H:6HgH:CgH-CHCH Cf-
cv .c . cHz
..c.., :cHz cHZ
CH3
H:C Lycopene
Fig.1.4.12. Structuresof importantcarotenoids,lycopeneand&carotene.Lycopene,Cao H56,is the red pigmentof the tomato and manyother fruits.B-carotene,C4oHs6,is oneof severalcaroteneisomersoccuringin many lower and higher plants
42
theBiomass: Bacteria, Plankton,HigherPlants Compositionof Chemical
Another important group of pigments,which are also oil-solubleand bear an isoprenoidchain, are the chlorophyll derivatives.Chlorophyllsareconcentrated occurs(seeChap.I.1.1). In algaeand in the partsof plantswherephotosynthesis plants, in leavesof higher averagecontentsof 0.270to 1% (dry wt.) chlorophyll are reported. The chemicalstructure of chlorophyll is shown in Figure I.1.2a. Chlorophylls may be consideredas derivatives of the porphin nucleus.The structure consistsof four pyrrole rings, linked together by methine (-CH:) bridges (Fig. I.1.2b). Petroleumporphyrins are also derivativesof the porphin nucleus,i. e., of chlorophylls. T\e phospholipidsor phosphatidesconstitute an integral and quantitatively important part of many membranesat all evolutionary levels. They are also concentratedin the more active tissues(liver, kidney etc.) of animals.Many membranesconsistof protein-phospholipiddoublelayers.A diagramaticsketch of the architectureof a membraneis givenin FigureI.4.13a.Phospholipidsdiffer chemicallyfrom simplefats in that they containa phosphoricacid grouping,and frequently a basicnitrogen in addition to fatty acid components(Fig. I.4.13b). Suberinand cutinplay an important role in protectivetissuesof plants,suchas in cuticles,cork, and spore walls. Cutin and waxesare depositedmainly on the outer side of plant cells, i.e., on the surface,whereassuberin is formed and depositedinside plant tissuesand cell walls. Suberinand cutin act as protection against evaporation and promote wound healing. Like waxes, they are not homogeneoussubstances. They are accompaniedby lignins,tannins,long-chain aliphaticcompounds(e.g., almhols) andtriterpenes.Suberinandcutin arehighly resistantto microbial or enzymaticattack, oxidation, and chemicaltreatment. They consistmainlyof polyrnerizedand crosslinked structuresof fatty acidsand alcohols.Dicarboxylic and hydroxy acidswith 12 to 26 carbon atoms play an important role amongsuberinconstituents.Cutin seemsto be a mixtureofhighly cross-linkedhydroxyacidswitha dominanceof 16, 18 and26 carbonatoms.Plant cuticulaecontainf rcm 5OEoto 9OEosrtin (Metzner,1973).The detailedchemical
I zoi
t.
.
f
R'-CO-O-CH
n2-co-o-ix o l n
H-C -O-P -O -CH2-CH2-NH2
t5A l I
H
O
H
20x
r
a
b
Fig.I.4.13. (a) Exampleof architectureof membranes,consistingofprotein-phospholipid double layers.Membranesare required on all evolutionarylevels.Especiallyprimitive organisms,suchasbacteria,consistto a largeextentof membranematerial.(b) Structural exampleof phospholipid(c-cephalin; Rr and R2 are hydrocarbons)
.1.2Lipids
struclures of suberinandcutinarenot yet known.Matic (1956)proposeda cutin model with fairly random intermolecularesterificationsbetweenhydroxy acids andthe carboxylgroups,asshownin FigureI.4.14.Hydroxy acidstypicalof cutin and suberin seem to be {ound only in higher terrestrial plants ranging from Psilopsida to angiosperms (Eglinton,1976).
I
o 8 0 H H H H I T I " H H H H H H H H / z O H2C-C-C-C-C-
C-C-C-C-C-C-C-C-C-C-C-C-C-
t -
I
Ht.H 'j
HCH
I
HCH
otl OH H H H H H H H H ] I H l iH H H H O-c-C. c-C-C-C-C-C C-C-C-b-C-C-C-C-C-CH. ' H H H H H ' l n H B H H " " " l c H o l
r,in l H
I HCll I
r
\
"J"
HCH
I
HCH
I
I
o-cti
HCH
l
/
8CH
/
t / HC-O I
xcx
I
HCH
I , I HCH o-cft
HCH
t
Ho-cH
t
/
/
xcn
I
I
HCtl
HCfl
I
I I HCH
HCH
rin I HCH
I
HCH
HCH
xcn HCH
I HO-CH2
e
H HH H Hr r H n " , , " " " " I | c-c-c- c-c-c-c-c-c- c-c-c-c- c-c:o
O:c
I
OH
Fig. 1..1.14.Structure of cutin (takcn trom Hitchcock and Nichols. l97l).Cutinoccursin protective tissuesof plants. Plant cuticulaeconsist largely of cutin
ChemicalCompositionof the Biomass:Bacteria,Plankton,Higher Plants
44
4.3 Lignin and Tannin Two related substances,lignin and tannin, need emphasishere becauseof their wide-spreadoccurrenceand, hence, their geochemicalsignificance.Both are characterized by aromatic (phenolic) structures. Aromatic compounds are usuallynot synthesizedby animals,but are very commonin plantstissues.Most photosyntheticorganismsderive aromatic ring systemsfrom monosaccharides, whereasin heterotrophicbacteriaand fungi the synthesisfrom C2-unitsseemsto predominate. However, both ways of synthesisare possible in either case (Metzner, 1973). The nameslignin and tannin are collectiveterms,and do not refer to clearly defined substances. Lignin oco.rs as a three-dimensionalnetwork located betweenthe cellulose micellesof supportingtissuesofplants.Basically,ligninis a high molecularweight pollphenol, consistingof units constructedfrom phenyl-propanederivatives. From an evolutionarypoint of view, lignin appearedfor the first time ln mosses. Apparently, typical lignins exist that show a phylogeneticdifferentiation. In plants, lignins are synthesizedby dehydration and condensationof aromatic cH20H
cH, ot-t
cH20H
t -
t -
CH
CH
CH
CH
CH
t -
ll CH
a-\ I
*".-.
,..o$o.n.
I OH
Coniferyl
Sinopyl
p-Coumoryl
olcohol
olcohol
olcohol
a o
Fig. I.4.15. (a) Aromatic plant alcohols,important building blocksof lignin. (b) Partofa lignin molecule.Lignin occursasa three-dimensionalnetwork betweencellulosemicelles of supportingtissuesof plants.It is very abundantin higherplants.(After Metzner, 1973)
4.4 Occurrenceof ImportantChemicalConstituentsin Orqanisms
45
alcohols such as coniferyl, sinapyl and coumaryl alcohol (Fig. I.4.15a). A schematicstructureof part of a lignin moleculeis shownin Figure I.4.15b. Tannins are quantitatively much less important than lignins, but are widespread.Their name is derived from their ability to tan hides.In compositionand behavior, they are intermediate between lignin and cellulose.They possess carboxyl and phenolic groups on their aromatic structures. The chemical compositionof tannins varies considerably,but all are complexderivativesof gallicor ellagicacid (Francis,1954).Structuralformulasof both acidsaregivenin Figure I.4.16. According to Francis (1954), tannins are found principally in higher plants, but also in fungi and algae.In higher plants, they are especially concentratedin the bark (up to 177o) andin leaves(up to 6.57o). Aside from these quantitativelymore important aromaticcompounds,there are entire seriesof phenolsand aromaticacidsand their derivativeswhich occur throughoutthe plant kingdom. COOH
I
a-r Ho,\2\o* OH
Gollic ocid
E l l o g i co c i d
Fig- I.4.16. Structureof gallic and ellagic acids,principal building blocks of tannins.(After Francis,1954)
4.4 Qualitative and Quantitative Occurrenceof Important ChemicalConstituentsin Bacteria,Phytoplankton,Zooplankton and Higher Plants Although all organismsare madeup of the samegroup of chemicalconstituents, there are considerabledifferencesin chemicalcompositionbetweencertainkinds of organism.For instance,there is quite a differencebetweenthe mostprominent marine plantssuchas tiny unicellularplanktonic algae,and the vast majority of terrestrial higher plants. The organic matter of marine plankton is composed TableI.4.1. Themainchemical constituents of marineplanktonin percentof dryweight. (After Krey,1970)
Diatoms Dinoflagellates Copepods
24-48% 41-48Ea 71,77Ea
Lipids
Carbohydrates
Ash
2-lO% 2- 6Vo 5-1980
0-3lVa 6-36Vo o- 4Eo
30 59Eo t2-77 Eo 4- 6%
,16
Plankton,HigherPlants Bacteria, of theBiomassr Composition Chemical
mainly of proteins (up to 507o or more), a variableamount of lipids (generally between5 and 25 %) andvariableamountsof carbohydrates(generallynot more rhan407o).Table I.4.1 listsaveragequantitiesof the main chemicalconstituents of common marine plankton. Compared to these quantitatively important marine organisms, higher terrestrial plants, including the majority of trees, are composed largely of areincorporated Both constituents cellulose(30to 50olo)andlignin(15 to 25olo). plants, and hence for functions fulfil supporting mainly in skeletalstructuresthat protein of higher content The planktonic organisms. are not neededin aquatic, In the following or less. averages 3 7o as 107o, but plants may be as high terrestrial discussion.basicinformation about the chemicalcompositionof relevantclasses of organismswill be describedin so far as it is available. Baaeria, lhe most primilive organisms,are extremelyadaptableand, hence, relatively variable in their chemicalcomposition.They contain about 80oloor rnor" *it".; the rest is organicmatter. On a dry wtJightbasis.bacteriaconsistof and 17csulfur'With about507ocarbon,10-15o/onitrogen,2-6% phosphorus respectto types of organic compounds,bacteria are composedof about 50olo proteins,207a cell wall material (membranes)and 10ololipids; the rest is ribonucleic and desoxyribonucleicacids (Schlegel, 1969). Membranes are composedof a considerableproportion of lipidsandlipidJike constituents(Table polyphospolysacch4rides, I.4.2). Many bacteriaare able to storefat substances, with 27 to 29 of avariety sterols phatesandsulfur.In the lipid fractionof bacteria, These sterols 1973). (Laskin and Lechevalier, found iarbon atomshave been (Crr) (Cur) and stigmasterol (Cn), ergosterol belongmainlyto the cholesterol in found (Fig have been I.4.10) series of the hopane types.Recently,triterpenes range generally in the are from bacteria (Dorsselaer, Fatty acids 1975). bacteria of C16to C2n.The most abundantones seemto be branchedfatty acidsof isoor anteiso-configuration(methyl branching I or 2 atoms, respectively,from tcrminal C-atom) with carbonnumbersfrom 14to 18. Hydrocarbonsoccurin the form of openchains,mainlywith 10 to 30 carbonatoms.
Table 1.4.2. Membrane constituentsof bacteria. (Aftcr Schlegel,1969)
Type of organic compound
Percent of membranes ot Micloco((uslysodeikticus
28-3'7 Lipid (9) (neutral) (phospholipids) (28)
Polysaccharide
l5 20
purplebacteria
,10 50 (l0-20) (3 0 )
5 3 0
-l.JOccurrence oflmportantCh€micalConstituents in Organisms
47
Phytoplankton,as pointed out before, is the most important producer of organicmatter in the aquaticenvironment.The main organicconstituentsof two important algal g'oups, diatoms and dinoflagellates,were presentedin Table I.4.1. Environmentalconditions,such as water temperatureand nutrients,may influencegrosscompositionof algaemorethan eitherclassorspeciesdifferences. Composition of the lipidfractionof diatomsisgivenin TableI.4.3.The amountof free uncombinedfatty acids,which comprisesmore than half of the total lipid fraction, is relativelyvery high, especiallywhen comparedto animal tissue. Table.I.4.3.Composition of lipid fractionof (AfterClarkeandMazur,1941) diatoms. Uncombined fatty acids Combined fatty acids Nonsaponifiable fraction Alcohols Hydrocarbons
s9 82o/o 1-t1qa t2-29% 3 7% 3-140/a
The distribution of major sterolsand carotenoidsvariesconsiderablyamong algalclasses (Goodwin,1971).Chlorophytae (greenalgae)preferentiaJly contain ergosterol,but also contain other sterols,such as cholesteroland fucosterol.In Rhodophytae(red algae)cholesterolis the main sterol,while in Phaeophytae (brown algae),it is fucosterol.The concentrationof sterolsin algaemaybe ashigh as 0.37o on a dry weight basis.The relative amountof carotenoidsis highestin Phaeophytaewhereit may be asmuctras6 mg g-r (0.6%) dry weight (Goodwin, 1965). The major fatty acids present in algae are all saturated or unsaturated monocarboxylicacidswith straight even-numberedcarbon chainsin the C12to C26range.Long-chainfatty acidsare prominentcomponentsof both storedfats and of polar lipids (phospholipids)of biomembranesin algae.Branchedfatty acidsof the iso- and anteiso-type,if present,are usually minor componentsof algae,in contrastto their concentrationin bacteria.Palmitic (C,u)andstearicacid (Crr), together with their unsaturatedcounterparts with the same carbon numbers,are the most abundant(Erwin, 1973). Polyunsaturatedfatty acidsare much more commonin algaethan in higherplants. The concentrationof hydrocarbonsin the lipid fraction of algaeis generally 3-5%. T\ey consist of saturatedand unsaturatedhydrocarbonshaving both straightandbranchedchains.Marine algaesynthetizen-alkanesin the rangefrom n-Cp to n-Cr2(Clark, Jr., 1966). Although the ratio of even-to odd-numbered homologsis frequentlycloseto one,thereare numerousexampleswhere n-C15or n-C,7,or botb, are the dominatingalkanes(Youngbloodet al., 1971).Thesetwo often representmore than 907o of the entire homologousseries.Stranskyet al. (1968) have investigated the hydrocarbon content of the freshwater alga Scenedesmwquadricanda.Over 6O7oof the hydrocarbonfraction consistedof unidentifiedsaturatedhydrocarbons.The rest wascomposedof membersof the n-alkanehomologous series,of 2-methyl(iso) and 3-m;thyl (anreiso)branched
48
Bacteria,Plankton'HigherPlants of the Biomass: ChemicalComposition
alkanes, and of olefins, mostly 1-alkenes. The n-alkane distribution was Jo-inui"a by n-C11.Gelpi ei al. (1968) observed olefinic straight-chain hydrocarbonswith 17 to 33 C-utorn. in two algae, Botryococats braunii (golden-brown alga) and Anacystismontana(blue-greenaLlga)Thesehydrocar[ns exhibited a-piedominanceof the odd-numbered Crr, Crr, C2nand C3' molecules.The toial content of thesehydrocarbonswas in the range0'1-0'37o dry weight. Maxwell et al. (1968) found that the-hydrocarbonsextractedfrom "ontistedsolei of two olefinicstraighrchaincompounds(C,,H,t)' Bitryoiorr
(Afterl-eeet al ' copepods. of lipidfraction(wt. %) of calanoid TableI.4.4. Composition 1970) Fraction
Calanus helgolandicus wild type
Trace
Hydrocarbons Wax esters (incl. any sterol esters)
Gaussiap nceps wild type
3'7
30
14
17
Triglycerides Polar lipids (free acids,cholesterol, monoglyceridesetc.) Phospholipids
45
1'7
Total lipid (percentdry wt.)
15
28 . 9
There is relatively little information on the detaiiedchemicalcompositionof prominentzooplankton,suchasforaminifersandcopepods(seealsoTable I 4 1)' i-Io*"u"., sincezooplankton,especiallycopepods,feed directlyon phytoplankton, there is a certain resemblancebetweenthe lipid fraction of phytoplankton and zooplankton.Copepodsconstitutethe largestsinglefraction of zooplankton in most waters.Comioiitlon of the lipids of calanoidcopepodswasinvestigated by l-ee et al. (19?d). The lipid fraction of certain wildtype copepodswas unusuallyhigh, with almost30% on a dry weightbasis.The effectof nutrition on the total am;unt of fat and the rapid depletionof this fat during starvationled to the conclusionthat lipids, especiallywax esters.sewe in copepodsas a reserve energy store. The compositi,onof the copepodlipids, accordingto Lee et al' lipid fraction are of special itezbj, i. giu"n ln Tabl; I.4.4 The wax esieisin the acid constituentsare madeof fatty int".e.i, bJcu,t " both their alcoholsand their atoms' Total carbon carbon or more Ions-chain carbon skeletonswith 18
4.4 Occurrence of Important Chemical Constituents in Organisms
49
numbers,alcoholplus acid,in thesewax esterswerefound to rangefrom 30 to 44. According to Lee et al. (1970), the wax estersof marine copepodsare very probably the main sourceof long-chainalcoholsobservedin marine sediments and in someoceansurfacelioids. Blumer et al. (1963, 1964) reviewed occurrenceand distribution of the isoprenoidhydrocarbonpristane(2,6,10,14-tetramethyl-pentadecane) in marine organisms.They found that pristane is a major componentof the body fat of copepods,andmaybe usedfor buoyancyto conserveenergythat would otherwise be usedto maintaina suitablepositionin the water columnby swimming.Pristane from calanoidcopepodsmay be the major primary sourceof this hydrocarbonin the marineenvironment. Terrestrial higher plants are the fourth major source of organic matter in sediments.As indicated before, the bulk material of higher plants, especially shrubsand trees,is composedmostlyof celluloseand lignin (together50-707o). Quantitatively,lipids and proteins are only of seconddryimportance.However, certain parts of higher plants, such as barks, leaves,spores,pollen, seedsand fruits, may be highly enrichedin lipids and lipid-like substances. This latter fact, together with the resistanceof these plant parts to mechanical,chemicaland biochemicaldegradation,is largelyresponsiblefor the contributionof terrestrial plant material to aquaticsediments,and especiallyto marine sediments.The fat content of seedsand fruits variesgreatly amongvariousplants (about l-50Va); hence,averagenumbersare meaningless.Pollenusuallycontain between2 and SVo fats. Leaves contain considerableamounts of lipids and lipidJike sub(waxes,cutin,suberin,etc.). stances Lipids derived from higher plants may be characterizedby a number of features.The n-alkanesin the rangefrom Cr0to Ca6showa strongpredominance of odd-carbon-numberedover even-numberedn-alkanes(bv a factor of 10 or more). This predominanceis especiallyapparentfrom n-Cr, to n-Crr, with a strong predominanceof the n-alkanes n-C21,n-C2e,and n-C31.The lowest molecularweight n-alkanereportedin plants (Koons et al., 1965), is n-heptane (C7H16),and the highestis dohexacontane(C62H126). Even-numberedaliphatic alcoholswith 24 to 36 carbon atoms are relativelycommon,especiallyin plant waxes.Other typical componentsof higherplantsare phenoliccompounds,such asconiferyl alcohol(Fig. I.4.15a) and others.Straight-chainsaturatedfatty acids with an even number of carbon atoms in the range from C, to C26are quite common. However, by far the most prominent fatty acids (Fig. I.4.3b) are palmitic (Cr6)and stearicacid (C1s).Straighrchainunsaturatedfatty acidsmost commonlyfound(Fig.I.4.3c)have14, 16, 18,and20 carbonatoms.Again acids with 18, and to a lesserdegreewith 16, carbon atomsare the most prominent. Various kinds of even-numberedhydroxy acidswith 12 to 26 C-atoms,resulting from degradation of cutin and suberin, are also typical contributions from terrestrialhigherplants(Eglinton,1976). In leaves and fruits, two kinds of pigments (Fig. I.4.12) may occur in concentrationsup to 17o.Among the terpenoidsbuilt from isopreneunits (Cs), the monoterpenes(Fig. I.a.5), e.g., in essentialoils, and the diterpenes(Fig. L4.7), e.g., in resins, are frequently observed. Most t'?es of pentacyclic triterpenoids(Fig. I.4.10) are essentiallyrestrictedto higherplants,especiallyto
50
Bacteria'Plankton,HigherPlants of theBiomass: Composition Chcmical
The main sterolsin higherplantsare sitosterol the more specializedangiosperms. and stismaiterol.both with 29 C-atoms,but the latter havingone doublebond in the alipathicsidechain(Fig.I.4.11).
and Their Effectson Biomass 4.5 Natural Associations Composition The chemicalcompositionof the biomassin a givenarea,and at a giventime, is mainly dependenton the physicaland chemicalenvironmentof the biological habitit and the evolutionarylevel of the organisms.Determiningenvironmental facto$ include light, temperature,nutrients, and water currenls' as far as the acuatic realm is concerned. Conditionsin limnic water bodieswill be quite different from either restricted marine basinsor the open ocean.The more important environmentalfactorson land are climaticcondiiions,morphologyof the landscape'andthe availabilityof nutrients.The time factor,in a geologicalsense,is introducedby the stratigraphic age, or in other words, by the evolutionarylevel of the organismsrn questron' of plantsandanimalsresultfrom various Irioie or lesstlpical naturil associations combinationsof environmentalfacton through time and space' Before the Devonian time, the bulk of the biomasswascomposedof bacteria, blue-green algae, and higher algae living in the marine environment' A Precainbrianilgal coal, the so-calledshungite (Lake Onega, USSR), may be consideredas J respresentativefossil example of one of the earliest natural associationsof organisms. Natural associalionsthroughout the Cambrian, Ordovician and even in the Silurian(seealsoChap.I.2), bear the imprint of the marineenvironment,andare practicallydevoid oi higher plants This meansthat, in the fossilcounterpartrn sedimenti,we do not expectgreatquantitiesof lignin and its derivatives'AIso we expectto find no cuticuiesand plant waxes,rarely spores,no pollen, no diatoms, and no foraminifers. From a structural chemicalpoint of view, we expect no pronouncedodd predominanceamongthe higher n-alkanes,and only a limited quantityof long-chainaliphaticmoleculeswithmorethan 25 C-atoms'We expect to finO-u variely of steroid compounds,but relatively few and.only very small quantitiesof triierpenoids,with the exceptionof the nearly ubiquitoushopane series(Fig.I.4.10). Appioiimately' since the beginningof the Devonian, a greater variation in natuiil associaticnsof organismscan be observed.This is due to a gradually increasingdifferentiationin theplant andanimalkingdom(Fig I 2 2 to I'2'4) and to the coiquest of the continenisby plants.A limited correlationexistsbetween the geological facies of a given sedimentary unit and natural associations' Ho*-ever,Tora number of reisons,it is merelya similarityandnot a match'Aside from the evolutionaryfactor that influencesnatural associations,there are the of factorsof plant and animalgeography.and the complicated-interrelationships of the The law the facies on an imprint leave not necessarily that do chaini food food chain, and the quantitative importance of primary producers such as phltoplankton are the two principal reasons why natural associations'as
.1.5NaturalAssociations andTheirEffectsonBiomass Composition
51
discussedhere, haveto be consideredas alterationsof, or additionsto the basic stock of organisms,which is alwaysformed by phytoplankton and bacteria.In general, three basic natural associationsamong remnants of bioloeicallv producedorganicmatter can be observedin aquatii sediments: 1. Remnantsof algaeand zooplanktontogetherwith microorganisms (mainly bacteria). 2. Remnantsof higher plants,stronglydegradedand partly oxidized,and deeply reworkedby rnicroorganisms. 3. Remnantsof higherplants,liltle to moderatelyaltered. Of course, combinationsbetween these nat;ral associationsare Dossible. especiallyby the addition of terrestrial higher plants to nearshore marine sediments.A schematicsummaryof naturalassociations of organicmatterdurins geological hisloryis givenin FigureI.4.17. So far, thereis little preciseknowledgeof differencein chemicalcompositionof the biomassdue to variousnatural associations. Only a few extremesrecognized in the fossilrecordcanbe skelched.Varyingcontributionsfromhigherterr'esr rial plants to an adjacent marine water body with autochthonbusbacterial, ptrytoplankton,and zooplankton communiriesmay serve as an example.The chemicalconsequences of sucha changehavebeendescribedearlier.Aside from differences in lipid- and hydrocarbon-typecomposition (e.g., wax alcobols, alkanes),a changein the grosschemicalcompositionof the biomass,i. e., a change in the atomic H/C ratio, may be observed.Predominantlyland-derivedorganic matter with a high content of lignins and carbohydrateshas a H/C ratio i; the range of 1.3 to 1.5, whereasautochthonousmarine plankton with large protein and lipid fractionsreachesa H/C ratio of about1.7 to 1.9 (seealsoFigs.'II.4.12 and II.7.6). The principal reason for this difference is that the land-derived organicmatter is more aromaticand richer in oxygenthan the marineplanktonic biomass. A consequenceof such differenceswith respectto fossil sedimentshas been describedby Breger and Brown (1968).who interpretthe variablehydrogen contentof kerogenin shaleswith wide regionaldistributionasrelatedto a varying terrestrial (plant) influence.Contributions from terrestrialDlantsto the marina e-nvir^onment can also be recognizedby a shift in rhe dr,C-iangetoward lighter r3C/r2Cratios(Sackett,1964; Nissenbaum and Kaplan, 1g7t\ tor the organic carbon in marine sediments.Marine organiccarbongenerallyis isotopically heavier than contemporaneousterrestrial organic carbon. The ranee of this dilferencevaries.On I heaverage. marineorganiccarbonis heavierby aLut 5 per mille. Quantitative data on compositionol the biomassin certain bodiesof water, with respectto the type of organismsfound there, is scarce.Two examplesare givenin Table I.4.5 andI.4.6, for the Black Seaandthe CaspianSea.resoectivelv. Phyloplanktonand or bacteriahaveto be considered asihe mainoroducersof organicmatter,ascanbe deducedfrom TablesI.4.5 andI.4.6. The ohvtoolankton in the Black Sea are composedmainly of diaroms. dinofiagellatesand coccolithophorids.The relativedistributionof theseorganismschangesseasonally and regionally(Shimkusand Trimonis,1974).In shallowwater areasof the BlackSea(lessthan100m),largequantities oforganicmaterialarealsoproduced
Plankton,HigherPlants Bacteria, theBiomass: ChemicalCompositionof
)z
u* - ' , . * _ I - t
\
higher plants (lower of €volution)
I
(I*>,-<S?"-ril\.
( + higher plants higher of evolution)
level
/\l\*--{
*\"*-r*\' 1 . "i
--r{----ii
t r . l
.l
'{
* phytoplankton (algae) and
M a i n n a t u r a l a s s o c i a t i o r so f o r g a n r c Sact eria, algae
and
SILURIAN
CRETACEOUS - RECEN T
zooPlankto.l
d e E a d e d , p a r l l y o x l d i z ed reworked by mrcroorganrsrns
l i t l l e 1 o m o d e r a l e l Ya l t € r e d
Fig. I.4.17. Main natural associationsof organic matter in aquatic sediment during geologicalhistory.CambrianthroughSiluriansedimentaryrocksmainlycontarnremnants 6t baiteria, algie and zooplankton Devonian through Jurassicsedimentary rocks generallycontair remnantsofalgae,zooplanktonandbacteria,but in additionsomehigher to Recentsedimentary flant maner, especiallyin near-shoreenvironments.Cretaceous and frequentlya and bacteria organisms planktonic less relatively generally iontain iocks greaterpart of higherplant remains
53
Summaryand Conclusion
Table I.4.5. Compositionof biomassandproductivityof varioustypesof organismin the Black Sea.(After Wassojewitch,1955) Organisms
Biomass,106t
Productionper year ok 1061
Plankton Phytoplankton (without bacteria) Zooplankton Benthos Makrophytae Zoobenthos Bacteriain water Bacteriain sediment Fish
15.0 13.5 1.5 40.0 20.0 20.0 30.0 lo.0 1.0
45 80 40 40 12,00018,000 6,000- 8,000 o.t7
Total
96.0
> 23,000
2,' 745 2,700
1,3.22 13.0 0.22 0.38 0.19 0.19
57.60 2u.tr0 100
Table I.4.6. Composition of biomass and annual production in the CaspianSea.(Taken from Bordovskiy, 1965, after Datsko, 1959)
Group of organisms
Biomass dry wt. ( 1000r)
Annual poduction dry wt. (1000t
Phytoplankton Bacteria Zroplankton Zoobenthos Phytobenthos Fish
650 32O 500 4.500 375 1,800
200,000 80,000 15,000 18,000 3'75 900
by phytobenthos.In the northwestarea,up to 97oof the entirebiomassis formed by a singlekind of algae(Phylloporanevrosa). Finally, it is stressedthat we havemerelyscratchedthe surfaceof a vastamount of undiscoveredinformation pertaining of biomasscompositionand how it is relatedto sedimentof variousfacies.
SummaryandConclusion The typeoforganic matter depositedandincorporatedin sedimentsdependslargelyon the naturalassociationofthe variousgroupsoforganismsin differentfaciesprovinces, Fro6 the point ofview of petroleumformation, suchnatural associations which occur on the continentalmarginsand in large landJocked,isolatedbasins(marginalseas) deservethe sreatestattention.
54
ChemicalCompositionofthe Biomass:Bacteria,Plankton,Higher Plants
All organismsare basicallycomposedof the same chemicalconstituents:lipids, proteins,carbohydratesand ligninsin higher plants.However,there are very characteristic differenceswith respect to relative abundanceof compound and detailed chemicalstructure. With respectto the formation of petroleum, the lipids are most imponant. Lipids encompass fat substances. waxesand lipid-like components,suchas oil-soluble pigments, terpenoids, steroids and many complex fats. A prominent buildingblockof manyof thesecornponentsis the isopreneunit (5 C-atoms),consisting of four C-atomsin sequence,and one methyl sidechain. A fundamental difference exists between the chemical composition of marine planktonicalgaeandterrestrialhigherplants.The organicmatter ofmarine planktonis mainly composedof proteins(up to 50% and more), a variableamountof lipids (5 to 25Vo),and,generallynot more than 407ocarbohydrates.Higher terrestrialplantsare largefycomposedof cellulose(30 to 5070)and lignin (15 to 25%). Both constituents fulfil mainly supportingfunctions. and are not neededin aquatic.planktonicorganisms.Lignin is the major primary contributor of aromaticstructuresin organicmatter of Recentsediments. Predominantlyland-derivedorganicmatterwith highconqentsoflignin and carbohydrates has H/C ratios around 1.0 to 1.5 and is more of an aromatic nature. Organic mate al mainly derived from autochthonousmarine plankton reachesH/C ratios around 1.7 to 1.9 and is more of an aliDhaticor alicvclicnature.
Chapter5 Processes Sedimentary andthe Accumulationof OreanicMatter
The accumulationof organic matter in sedimenl is controlled by a number of geologicalboundaryconditions.It is practicallyrestrictedto sedimentdeposited in aquatic environments,which must receive a certain minimum amount of organicmatter. This organicmatter can be suppliedeither in the form of deador living particulate organic matter or as dissolvedorganic matter. The organic material may be autochthonousto the environmenlwhere it is deposited,i. e., it originatedin the water column aboveor within the sedimentin which it is buried, or it may be allochthonous,i. e., foreignto its environmentof depositon.Both the energy situation in the water body in question and the supply of mineral sedimentaryparticlesmustbe suchasto allow a particularkind of sedimentation. If the energylevelin a bodyof wateris too high,eitherthereis erosionofsediment rather than deposition, or the deposited sediment is too coarse to retain low-densityorganicmaterial.An exampleis a beachareawith strongwaveaction. Furthermore,in coarse-grainedsediment,amplediftusion of oxygenis possible throughthe wide open pores.On the other hand,if the levelof energyis very low, too little sedimentis supplied,and there is, Iike-wise,no appreciableorganic sedimentation.Examplesof this type occur in certainparts of the deepsea. Once these boundary conditions are satisfied,the accumulationof organic matter in sedimentis dependenton the dualismbetweenprocesses that conserve and concentrateand thosethat destroyand dilute organicmatter.
5.1 Fossiland Modern SedimentsRich in OrganicMatter, and Their GeologicalImplication Here,we areprimarily interestedin analyzingandunderstandingthe formationof those sedimentaryrocks that are potential source sedimentsfor petroleum. Although we do not intend at this momentto definepotentialsourcerocksof petroleum, only fossil sedimentwith a minimum amount of organic carbon is considered. For various reasons,a lower limit is set at about 0.57o organic carbon for detrital rocks and at about 0.37ofor carbonateand evaporite-t)?esediment.A large vadety of sediment derived from widely differing geographic and stratigraphicprovenanceshas been analyzedby numerousresearchers.From their data, a number of facls can be deducedwith respectto the formation of sedimentrich in orsanicmatter.
56
of OrganicMatter andtheAccumulation Processes Sedimentary
In a comprehensivestudy of bituminous rock sequences,Bitterli (1963) concludedthat paleogeographicturning points, which resultedin either transgressionsor regressions,were especiallyfavorable for the depositionof such sediment. According to him, most bituminous rock sequencesbelong to a paleogeographicsetting where transitional (brackish) or alternating marine/ freshwaterfaciesprevailed.However,fully marineconditionswerenot excluded. Furthermore,Bitterli statedthat organic matter richnessis not inherent in any with fine-grainedsediments.This particularlithofacies,but tendsto be associated latter fact was emphasizedby Hunt (1963), who demonstratedin a studyof one t'?e of sediment that the smaller particles, apparently due to their greater with a largeramountof organicmatter(Table adsorptioncapacity,are associated
r.5.1). Table I.5.1. Variationin organicmatterwith particlesizein vikingshaleof Canada.(After Hunt. 1963) Particlesize
Average wt. 7. organic matter
Siltstone Clay (2-4 1t) Clay (lessthan 2 p)
1.79 2.08 6.50
In the caseof modern sediment.a number of additionalfactscan be deduced concerningthe depositionof sedimentrich in organicmatter. Shimkusand Trimonis (1974) observedthat areasof high concentrationof organic carbon (more than 37o) in modern Black Seasedimentdo not coincide t" ') ,"iih ,.eus of high primary organicproduction (more than 0.2 g Cm duy However, there was a good correlationbetweenlarge amountof organicmatter andhigh concentrationof CaCO, (more than 304/o).From this, it wasconcluded that the accumulationof organicmatter and CaCO3were both controlledby the materialsfrom terrigenoussources. changingcontributionsof clastic-argillaceous Thus, the concentrationof organicmatter and CaCO3would be low in areaswith material,due to a dilution effect. high ratesof depositionof clastic-argillaceous This eiiect is strongestat the periphery of the depositionalbasin, where the production of organicmatter is relativelyhigh. In addition, the data of Shimkus and Trimonis (1974) show that high organic carbon content in sediment is correlatablewith high concentrationsof subcolloidalparticles(< 1 prm),andwith the distribution of illite plus montmorillonite. This relationship probably should be investigatedin connectionwith the abundanceof suspendedmaterial (> 0.7-1.0 pm), which is composedof an appreciableamountof organiccarbon (1,5-757o, average 307o). The importance of suspendedmatter with respect to transportation and sedimentationof organicmatter wasalsorecognizedby Meadeet al. (1975) and
5.2TheRoleof Dissolved andParticulate OrganicMatter
5'7
Milliman et al. ( 1975).In coastalwatersof the Atlanticocean,off the coastof the northeastemUSA andoff the Amazon River delta,suspendedmatter wasfound to contain large amounts of organic matter, generally from 40 to 8070. In nearshoreareasof the Amazon River delta, the organic part of the suspended matter ranged from 10 to 25Vo and in offshore areasfrom 50 to 807o. This suspendedmatter, being highly enrichedin organic material, shows a density gadation. The more organic material, the lower is its density. l-ow-density material can more easilystay in suspension. Therefore,the settlingout of these particles will be inversely related to the energy level of a body of water. Low-density organic matter will be carried away into quiet waters,unlessit is either oxidizedor consumedby bacteriaor other scavengers prior to deposition. The chancesfor destruction of organic matter are greater in longer water columns,especiallyif there is enoughorygen (Figs.I.3.5, I.3.6a). However, if there is a stratifiedwater columnwith oxygenrestrictedto the surfacewater layer and HrS in most of the remainingwater column, asoccursin the Black Seaor other landlockedbasins(Fig. I.3.6b), there is a greaterchancefor preservationof organicmatter.There arenumerousexamplesof sediment,rich in organicmatter, which were depositedal great depthsin water columnsexceeding1000 m, asin the Black Sea, MediterraneanSea. Gulf of Mexico and the Cariaco Trench (Mclver, 1975). In someof thesecases,the absenceof oxygenapparentlyis not necessaryfor the preservationof a considerablepart of the organicmatter. The importance of allochthonousorganic material in mqdem sediment is exemplifiedby the occurrenceof greatquantitiesof pollen in bottom sedimentof the Black Seanear the westernshore(Roman, 1974).Although clearlymarine, this sediment contains more land-derived than marine particles among the microscopicallyidentifiable fraction of organic matter. Distribution of pollen t)?es is related to the distanceof the samplefrom the shoreline.
5.2 The Role of Dissolvedand ParticulateOrganicMatter Relatively large amountsof dissolvedorganiccarbon (DOC) are found in both sea water and freshwater(seeChap. I.1.2). Dissolvedorganiccarbon,which actuallymeansdissolvedorganicmatter that is determinedascarbon,is defined by oceanographers asorganicmatterthat passesthrough sieveswith 0.4 or0.8 pm pore size.The organicmaterialretainedon the sievesis calledparticulateorganic carbon (POC). Menzel and Ry.ther(1970) statethat the vertical distribution of POC and DOC in the oceansis practicallyhomogeneousbelow a depth of 200 to 300 m; POC is present in concentrationsfrom 3 to 10 pg C l-' and DOC in concentrationsfrom 0.35 to 0.7 mg Cl I (Fig. I.5.1). They alsostatethat above this depthconcentrationsare variable,with POC directly proportional and DOC inverselyproportionalto primary productionof organicmatter.From thesedata, together with the observationthat an oxygen minimum generally occurs in shallowdepthsusuallyin the rangeof 200 to 300 m, it is concludedthat the cycling of organicmatter is almostentirely restrictedto relativelyshallowsurfacewaters. Below this depth, apparentlyno cyclingof organicmatter occurs,which may be
and the AccumuIation of OryanicMatter SedimentaryProcesses
.58
Particulate organiccathon l W/ll 0
5
J
5
25
(mg,/l) Dissolved organic carbon 0 .2 .1
6
8 1 . 0m g l l
5 0- 1 0 0(0y s l l ) l 0 1 0 0( 1 9 , / l)
CI
l U UU
Fig. I.5.la and b. Average depth distribution of particulate organic carbon (a) and of dissolvedorganiccarbon (b) in variousareasof the Atlantic and PacificOccan. (Modified after Menzel and Ryther, l97O)
due either to the biochemicallyinert nature of the compounds,or to a concentrationbelow a thresholdlevelfor utilizationby bacteria.It mayalsoresult from bacteriabecominginactivebelow a certaindepth level,due ro increasing pressure(Pelet,1976). Mineral particles,especially clay minerals,settlingthrougha water column containing dissolved organic matter may get coated with (polar) organic compounds.The adsorptionof aminoacids,sugars,phenolicsubstances andother organic compoundson surfacesof clay minerals and carbonate particles in seawaterhasbeendemonstrated by Baderer al. (19b0),Chave(1970),Degens (1970),Mitterer (1971)and manyotherc.This adsorptionprocesscan alsobe consideredas a conversionof dissolvedorganic matter to a particulate form. Breger ( 1970) pointsout that complexhigh molecularweightorganicmaterialsin seawatermay accountfor a very high percentage of total organicmatter.He further indicated that interactions among these substancesmay lead to phenomenasuchascoprecipitation,adsorption,flocculation,or protection.The frequently observed inverse relationship between grain size of sedimentary particlesand amount of organic matter for both modern and ancientsediment
5.3 Accumulation Mechanismsfor SedimentaryOrganicMatt".
59
certainly has to be consideredin connectionwith these interactions.Carbon isotopestudiesby Gormly and Sackett( 1977)on DOC, POC, andorganicmatter depositedin modernsedimentof the Gulf of Mexico point in the samedirection. According to these authors, the adsorption of dissolvedorganic matter by suspendedmineral particles may play an extremely important role in the extraction of organic matter from water and its subsequentincorporation in bottom sediments. All theseobservationsare in line with a proposedpath of kerogenformation; (1) degradedcelluarmaterial------> (2) water-soluble complex,containingamino acidsand carbohydrates------' (3) fulvic acids-----(4) humic acids-----+ (5) kerogen (Nissenbaum andKaplan,1972;Welte,1974).Partlydegraded cellularmaterial, or even whole fragmentsof organismsmay be incorporatedin this processof kerogen formation. Altered particulate organic matter resulting directly from degradationof organismsmay also serveas an adsorbentfor organics.
5.3 AccumulationMechanisms for SedimentaryOrganicMatter From the foregoingdiscussion,in addition to information presentedin previous chapters,a conceptfor the accumulationmechanismsof sedimentaryorganic materialha\ beendeveloped. GeneralJy, marinephytoplankton represents themainsourceof organicmarter in marine sediments,whereasin someshallowwater regionswith sufficientlight for photosynthesisthe main sourceis marine phytobenthos.In both instances, bacteriathat reworkdeadorganisms haveto beconsidered asanadditionalmajor sourceof organiccarbon.In certaintypesof sediment,especiallythosedeposited in nearshoreareas and deltas, allochthonousland-derived organic material, representedby pollen, sporesand other plant debris,may be anotherdominant contributor. An apparenlbut by no meanscompellinginterrelationshipexistsbetweenthe quantity of organicmatter suppliedto a depositionalenvironmentand the concentrationof organicmatter depositedin the sediment.The supplyof organic matter is greatestin areasof high primary productivity, i. e., along continental margin,around"land masses", andespecially areasof upwelling(seealsoChap. I.3.3, Figs.1.3.,1and I.3.7). Howevcr,this supplyfactorbecomesmodifiedby thoseprocesses responsiblefor conservationand concentrationof organicmatter on one hand,andby thoseresponsiblefor destructionand dilution on the other. Under geologicalconditions prevailing on the earth's surface, all organic matter is unstable.The conservationof organic matter is aidedby a number of factors. Great productivity of organic matter has a tendency to create environmentswith low oxygen concentration(Eh below * 100 mV), or with oxygendeficiencyaccompaniedby the production of HrS (Eh negative,below 0 mV) by sulfate-reducing bacteria.Decayof organicmaterialanditsconversion to CO2 are responsiblefor this drop in oxygen concentration.This process frequenllyis the causefor an oxygenminimum at the botrom ofthe euphoticzone oI seawater.
60
of OrganicMatter andtheAccumulation Processes Sedimentary
Low oxygenconcentrations,or absenceof free oxygenand presenceof H2S, deoressthe rate of destructionof organicmatter. Under suchconditions,it is not further mineralizedby direct oxidatlonto CO2.Nevertheless.the destructionof organicmatter is continuedat a lower rate by anaerobic,heterotrophicbacteria. The adsorptionof either dissolvedor particulateorganicmatter on surfacesof mineral particlesenhancesits stabilizationin two ways.First,it is betterprotected againstbiological consumption,and second,it settlesmore rapidly through the water column.The residencetime an organicparticle spendsin an oxygen-containing water column is also important. Therefore, in shallow waters up to a few hundred meters depth, conditions are better for accumulationof organic matter than in very deepwaters.A similareffect may resultfrom stratificationin deeperwaters, e.g., due to a pycnoclineor thermoclinethat limits the oxygen exchangeand thus can causeoxygendeficiencyin lower levels.Lack of orygen also prevents the consumption of organic matter by aerobic, heterotrophic organisms,which may be active in areasof grazingor bioturbatjon in bottom sediment.The consumptionof organic matter by heterotrophicorganismsis a major factor in destructionof organicmatter. The rate of deposition of sedimentaryparticles plays a key role in all sedimentaryprocessesand, hence,also in the preservationandconcentrationof organicmatter.Assuminga constantsupplyof organicmatter,its concentrationin sedimentwill be inverselyrelatedto rate of depositionof mineral particles.The resulting dilution effect is well known. The relationship,however, is not that simple, becauseit is modified by other factors,suchasgrain sizedistribution of mineral particles, length of water column and residence time in it, and consumptionof organicmatter by heterotrophicorganisms.Empirical observations indicatethat, with increasingrateof deposition,the concentrationof organic matter in sediment passesthrough a maximum, and only at higher rates of deoositiondilution is effective. Ouantitative information on this complicatedproblem is not available.Good examplesof the dilution effectaredepocentersof deltassuchasthe Nigerwherea relativelylargesupplyof both allochthonousandautochthonousorganicmaterial provides comparatively low average organic carbon values (0.3-0.87o) in fine-grained sediments.A depocentertypically is an area where most of the sedimentaryload of a river is depositedover a prolonged period of time. In adjacentareasthat do not receiveso much clasticterrigenousmaterial,average organic carbon valuesof equivalentsedimentare higher (O'7-l.zVo\. A similar general situation with respectto organic matter sedimentationand dilution by mineral matter was noted in the Gulf of Mexico, where similarly low average organiccarbonvalueswere reported (Dow and Pearson,1975). The lipid fraction of organic matter, which is of special importance for petroleumsourcerocks,behavessomewhatdifferently in sedimentaryprocesses than nonlipid material. The bulk of the lipid fraction is highly water-insoluble, whereasthe bulk of the nonlipid fraction, i. e., carbohydratesand proteinaceous compoundsand their derivatives,are water-solubleor hydrolyzable.In addition, many highly resistant parts of organisms,such as membranes,cuticles,wax coatings,spores,pollen and cork, are enrichedin lipidJike substances(seealso Chap. I.4.2). As a result,there is a generaltendencyfor the lipid fractionto
Summary andConclusion
61
survivein particulateorganicmatter (POC) rather than in the dissolvedorganic material (DOD). Organismsdepositeddirectly in sediment,of course,include their lipid content as well. Therefore,it may be deducedthat the contributionof particulate organic matter to sedimentmay be more important for a potential source rock than is the contribution from dissolvedorsanic matter. Data bv Gormly and Sacketr(1977) point in the samedirectionlThey found rhat the d13C-values of the lipids of POC aremoresimilarto the averagelipid composition of sedimentthan are the 613C-values of the dissolvedlipids.Likewise,thev found the drrC-valueof rotal DOC to be very similarro the valuefor averagerotal organiccarbonof underlyingsediment.Averagedt3C-values of lipidsin sediment were lighter by about 6 per mille than the averagetotal organiccarbon. Bitterli's (1963) observationthat transgressions and regressions are favorable for the depositionof sedimentrich inorganicmatter,is explainedin the foregoing discussion about supply and preservation of organic matter. Depositional transgressionsand regressionscombine such favorableconditionsas relalively shallowwaterswith an amplesupplyof autochthonousandallochthonousorganrc matter, and zonesof depositionof medium- to fine-grainedsediment. In a seriesof papers,Emery (1960, 1963, 1965)hascomprehensivelystudied recentenvironments,which are of interestin determininghow a potential source sedimentof petroleumis formed. On the continentalshelves,the following areas of quiet water are important: lagoons,estuaries,and deep basinsof restricted circulation.Examplesof suchenvironmentsare found in southemCalifomia, the Gulf of California, parts of the East Indies,the Caribbean,and perhapspartsof northem Canada and tbe Eurasiatic shelf between Norwav and the Taimir Peninsula. Another ty?e of environment favorable for the accumulationof organic matter includesthe continentalslopes.There, the organic matter may reachvery high concentrations,becausethe rate of depositionof detrital silt and claysis suchthat an optimalbalancebetweendilution a;d conservationoforganic matter is maintained.Seawardof the continentalsloDe,concentrationsoforsanic malterin sedimentcommonlydecrease. A specialsiiuationmayexistjustbeyond the baseof the continentalslope,whereorganic-richsedimentcan accumulateas a result of sliding down the slope. Finally, it is mncluded that organic-richsedimentmay occurwhereverthere is sufficientsupplyof organicmalter, reasonablyquiet waters,and an inlermediate rate of sedimentationof fine-grainedmineral particles.
Summaryand Conclusion The depositionof sedimentsfich in organicmatter, i. e., such as contain more than abott 0,5Voby weight of organiccarbon,is restrictedto certainboundaryconditions. Such sedimentsare depositedin aquatic€nvironmentsreceivinga certain minimum amountoforganic matter. In subaerialsediments,organicmatter is easilydestroyedby chemical or microbial oxidation. The supply of organic matter is high along the conlinentalmargins,becauseof high primary productivity of coastalwatersand/or a high input of allochthonousland-derivedterrestrialplant mate al.
62
Sedimentary Processesand the Accumulationof Organic Matter
Balancedoptimum conditionsbetweenthe energylevelin a body ofwater andrate of sedimentationare neededto preserveand concentrateorganicmatter in sediments. The clay-sizemineral fraction becomeseasilycoatedwith organicmatter. This finegrainedfraction and suspendedparticulateorganicmatter ate of a low density,and are carriedawayfrom water bodieswith a high energylevel to more quiet waters.There, depositionof fine-grainedsedimentslimits the accessof dissolvedmolecularoxygen, andthereforeincreasesthe chancesfor preseruationof organicmatter. If, however,the rate of s€dimentationis too high, organic matter is diluted, and a sedimentlow in Qrganic matteris deposited. Favorableconditionsfor the depositionolsedimentsrich in organicmatter are found on the continentalshelfsin areasof quiet waters,suchasin lagoons,estuaries,and deep basinsof restrictedcirculation.Another environmentfavorablefor the accumulationof organicmatteris the continental slopes. The lipid fraction of organic matter behavesdifferently in sedimentaryprocesses than nonlipid material.Contraryto the nonlipid material,the bulk ofthe lipid fraction is highly water-insoluble.Furthermore, many resistantparts of organisms,such as membranes,cuticles,spores,pollen etc. are enrichedin lipid-like substances. Therefore, the lipid fractionof organisms tendsto survivein the particulateorganicmatter rather than in the dissolved organic matter. It may thus be deduced that the contributionofparticulateorganicmatteris more importantfor a potentialsourcerock thanthat of drssol\ed orsanicmalter
References to PartI
Bader,R. G., Hood,D. W., Smith,J. B.: Recovery of dissolved organicmatterin seawater and organicsorptionby particulatematerial.Geochim.Cosmochim.Acta 19. 236_243 ( 1 9 6 0) Bergmann,W.: Geochemistry of lipids.In: International Seriesof Monographs on Earth Science. Berger,I. A. (ed.).Oxford-London-New York,paris:pergamonpress,1963, Chap.12,pp. 503-542 Bitterli, P.: Aspectsof thc genesisof bituminousrock sequences. Geologieen mijnbouw 42, \83 2O1(1963\ Blumer,M., Mullin, M. M., Thomas,D. W.: Pristanein zooplankton. Science14O,974 ( 1 9 6 3) Blumer,M. Mullin, M. M. Thomas,D. W.: HelgolandcrWiss.Meeresuntersuchung 10, 1 8 7r 1 9 6 4) Bogorov,V. G., Vinogradov,M. Y. E.: Distrubtionof the zooDlankton biomassin the centralPacific.Tr. Vses.cidrobiol. Obshchcstva AkaLt.NaukS.S.S.R. 10, (1960) Bordovskiy,O. K.: Sourcesof organicmatter in marinebasins.MarineGeol. 3,5-31 (1965) Breger,L A.: What you don'tknowcanhurt yotr:Orgtniccolloidsandnaturalwatcrs.ln: OrganicMatter in NaturalWarers.Hood,D. W. (ed.).Symp.Univ. AlaskaSept.2-4 1961t, Inst.Mar. Sci.:Occasional Publ.No. 1.563-574(1970) Breger, L A., Brown, A.: Distribution and tlpes of organic matter in a barred marine basin.Trans.N.Y. Acad.Sci.25,741 755 (t963) Brooks, J., Shaw,G.: Geochemistryof sporopollenin.Chem. Geol. 1rO, 69 8'j (lgj2) Biilow,K. v.: Die Verflechtung von Erd- und Lebensgeschichte. In: Die Entwicklungsgeschichte der Erde.Autorenkollekriv (ed.),Leipzig:Kosmograph, 1959,pp. 438 451 Calvin,M.: ChemicalEvolution.Oxlord: ClarendonPress.1969 Chave,K. E.: Carbonate-organicintcractionsin seawater.In: OrganicMatter in Natural Waters.Hood, D. W. (ed.). Symp.Univ. Alaska Sept.2 4 1968,Inst. Mar. Sci.: Occasional Publ.No. 1,373-385(1970) Clark, R. C., Jr.: Occurrence of normalparaffinhydrocarbons in nature.WoodsHole Oceanogr. Inst.WoodsHole,Mass.,Ref.no.66 34, unpubl.manuscript (1966) Clarke,H. T., Mazur,A.: The lipidsof diatoms.J. Biol. Chem.126,283-289( 1941) Cloud, P. E., Jr.: Atmosphericand HydrosphericEvolutionon rhe primitive Earrh. Science, 160,729 736 (1968) Datsko, V. G.: OrganicMatter in Soviet SouthernWaters. Moscow: Izd. Akad. Nauk. (in Russian)1959 S.S.S.R.. Debyser,J., Deroo,G.: Faitsd'observation sur la gendsedu p6trole.Rev.Inst.Fr. pdtr., 24, 1,21-18 (1e69) Degens,E. T.: Molecularnatureof nitrogenouscompoundsin seawaterandrecentmarine sediments. In: OrganicMatter in NaturalWaters.Hood, D. W. (ed.).Symp.Univ. AlaskaSepl.2 4 1968,Inst.Mar. Sci.:Occasional Publ.No. l,77-105 (1970)
ReferencestoPartI Deuser,W. G.: Organic-carbon budgetof the BlackSea.Deep-Sea Res.,18,995-10O4
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Dietrich, G.: GeneralOceanography.New York-London: Wiley Interscience,1963 Dorsselaer,A. v.: Triterpdnesde s6diments.Ph. D.thesis,Univ. Strasbourg1975 Dow, W. G., Pearson,D. B.: Organicmatter in Gulf Coastsediments.OffshoreTechnol. Conf.Dallas.PaperOTC 2343(1975) Eglinton,G.: Personal communication, Universityof Bristol(1976) Emery, K. O.: The Seaoff SouthernCalifornia.New York-London: Wilev Interscience. l9tr0 Emery, K. O.: Oceanographicfactorsin accumulationof petroleum.proc.6th World petr. Congr.Frankfurt,Sec.I, Paper42, PD2, 483-491(1963) Emery, K. O.: Characteristicsof continentalshelvesand slopes.Am. Assoc.petr. Geol. Bull. 49, 1379- | 3E4( I q65) Ensminger, A., Dorsselaer, A. v.,Spyckerelle, Ch.,Albrecht,p., Ourisson, G.: pentacyclic triterpenes of the hopane type as ubiquitous geochemicalmarkers: origin and significance.In: Advancesin Organic Geochemistry,Tissot, B., Bienner, F. (eds.). Paris:Technip, 1973, pp.245-260 Erwin, J. A.: Crmparative biochemistryof fatty acidsin eukaryoticmicroorganisms.In: Lipids and Biomembranesof EukaryoticMicroorganisms.Erwin, J. A. (ed.),pt.2. New York-London: AcademicPress,1973 Francis,M. J. O.: Monoterfrenebiosynthesis.In: Aspectsof Terpenoid Chemistryand Biochemistry. Goodwin,T. W. (ed.),pt. 2. Irndon-New york: Academicpress,1971, pp. 2948 Francis,W.: Coal, its Formation and Composition.Inndon: Edward Arnold, 1954 Garrels, R. M., Mackenzie,F. T.: Sedimentaryrock types: Relative proportions as a function of geologicaltime. Science163, 57O 571 (1969) Gelpi,E., Oro, J.,Schneider, H. J., Bennet.E. O.: Olefinsof highmolecular weightin two microscopicalgae.Science161, 700-701 (1968) Goodwin,T. W.: Distibution of carotenoids. In: Chemisrryand Biochemistry of plant Pigments.Goodwin, T. W. (ed.), pt. 4. Irndon-New York: Academicpress,1965,pp. t27 -132 Goodwin, T. W.: Algal carotenoids.In: Aspectsof TerpenoidChemistryand Biochemistry. Goodwin,T. W. (ed.).London-NewYork: AcademicPress,1971,Chap.11, pp. 315-356 Gormly, J. R., Sackett,W. M.: Carbon isotopeevidencefor the maturationof marine lipids.Proc.Tth Int. Meet.Organ.ceochem.Madrid(1975) Hitchcock,C., Nichols,B. W.: Plant Lipid Biochemistry.London-Newyork: Academic Press.1971 Hunt, J. M.: Geochemical dataon organicmatterin sediments. In: Vortrageder 3. int. wiss. Konferenzfiir Geochemie,Mikrobiologie und Erdiilchemie,Budapest.Bese,V. (ed.),394,1963 Hunt, J. M.: Distribution of carbon in crust of earth. Am. Assoc.Petr. Geol. Bull. 56. 2273-22'71(r9'72) Karlson,P.: KurzesLehrbuch der Biochemie.Stuttgart:Thieme, 1970 Karrer, W.: Konstitution und Vorkommen der organischenPflanzenstoffe.Basel-Stuttgart:Birkhauser,1958 Koons, B., Jamieson,G. W., Cierezko, L. S.: Normal alkane distribution in marine organisms:possiblesignificanceof petroleumorigin. Am. Assoc.Petr. Geol. Bull.49, 3 0 1 - 3 1 6( 1 9 6 5 ) Krey, J.: Die Urproduktion des Meeres.In: ErforschungdesMeeres.Dietrich, G. (ed.), Frankfurt:Umschau,1970,pp. 183-195
References to Part I
65
Kuenen,Ph.H. (1942):Citedin: Sall6and Debyser:Formationdesgis€ments de p€trole. Paris:Technip,1976 Kuenen,Ph. H.: MarineGeology.New York: JohnWiley and Sons,1950 Laskin,A. J., Lechevalier, H. A.: Handbookof Microbiology,Cleveland,Ohio: CRC Press(a divisionof the ChemicalRubberCo.), 1973 Lee, R. F., Nevenzei, J. C., Pfaffenh
Referencesto PartI Sackett,W. M.: The depositionalhistory and isotopic Organic Carbon compositionof Mar. Geol.2, 173-185(1964) marinesediments. Schidlowski,M., Eichmann,R., Junge,C. E.: EvolutiondesiridischenSauerstoff-Budgets Umschau22,703-7O7(1974) und Entwicklungder Erdatmosphdre. Schlegel,H. G.: AllgemeineMikrobiologie. Stuttgart:Thieme, 1969 of fossil Schopf,J. W., Barghom,E. S.,Maser,M. D., Gordon,R. O.: Electronmicroscopy (1965) l3u5 years old. f59, Science bacteriatwo billion Geologie.Brinkmann,R., Louis, Seibold,E.: Das Meer.In: Lehrbuchdcr Allgemeinen M., Seibold,E. (eds.).BandI, FestlandMeer,Kap.9 16.Stuttgart: H., Schwarzbach, , p .2 9 1 - 5 1 1 E n k e ,1 9 7 4 p in BlackSea.In: The BlackSea Shimkus,K. M.. Trimonis,E. S.:Modernscdimentation Geology,ChemistryandBiology.Degens,E. T., Ross,D. A. (eds.).Memoir20.Tulsa, Okfahoma:Am. Assoc.Petr. Geol., 1974,pp. 249-218 Sorokin, J. I.: On the role of bacteriain the productivityof tropical oceanicwaters.Int Rev.Ges.Hydrobiol.56,1-48 (1971) Stransky,K., Streibl,M., Sorm,F.: Lipid hydrocarbonsof the alga.CollectionCzechoslov. Chem.Commun.33, 416-424(l96ll) Strickland,J. D. H.: Productionof organicmatterin the primarystagcsof marinefood Riley,J. P.,Skirrow,G. (eds.).London-NewYork: chain.In: ChemicalOceanography. AcademicPress,1965,Vol I, pt. 12,pp.478 595 Tappan, H.: Primary production isotopes,extinctionsand the atmosphere.Paleogeogr' Paleoclimatol. Paleocol.4,187 210 (1968) Tappan, H., Loeblich,A. R., Jr.: Geobiologicimplicationsof fossilphytoplankton evolutionand time-spacedistribution.In: Symp.Palynologyof the Late Cretaceousand Early Tertiary.Kosanke.R. M., Cross,S. T. (eds).East Lansing:Geol Soc Am' MichiganStateUniv., 197O,pp.247 31O Tasch,P.: The problem of primary production in the seasthrough geologictime. Rev. Paleobot.Palynol.1,283-290 ( 1967) in the distributionof planktonicf()raminifera Tolderlund.D. S.. 86. A. W.: Seasonal westemNorth Atlantic. Micropaleontology 17,291 329 (1911) Tsujimoto (1906): Cited in: OrganicChemistry.Fieser,L. F., Fieser,M. (eds).New York: Reinhold.1956 Vallentyne,J. R.: Net Primary Productivityin Aquatic Environmcnts Goldman,C R. (ed.).Berkeley:CaliforniaUniv. Press,1965,p. 309 N. B. (1955): Cited in: Seibold,E.: Das Meer. In: I.ehrbuchder Wassojewitsch, Allgemeinen Geologie.Brinkmann,R. et al.(eds.).Stuttgart:Enkc,Bandl, pt. 9, 1974, p p .2 9 1 - 5 1 1 auf der Kohlenstoffund die Entwicklungder Photosynthese Welte,D. H.: Organischer 57, 17 23 (197O) Erde. Naturwisscnschaften andkerogen, Welte, D. H.: Recentadvancesin organicgeochemistryof humic substances 1973.Tissot,8.,Bienner,F (eds). a review.In: Advancesin OrganicGeochemistry 1 9 7 4 ,p t . 1 , p p . 3 - 1 3 Williams, P. M.: Organic and inorganic constituentsol the Amazon River' Nature (London)218, 937-938(1968) and unsaturated Youngblood,W. W., Blumer,M.. Guillard,R. L, Fiore,F.: Saturated (1971) 201 Biol.8, 190 algae. Mar. in marine benthic hydrocarbons Stuttgart:Fischer,1959 der Pflanzen. Zimmermann,W.: Die Pbylogenie eineUbersicht.Stuttgart:Thieme'1969 der Pflanzen, Zimmermann.W.: Geschichtc Zobell, C. E.: Action of microorganismson hydrocarbons.Bacteriol. Rev. l0' 1 (1946) Sect. 4th Int Congr'Microbiol.,Copenhagen' Zobell,C. E.: Soilandwatcrmicrobiology, v I I . 4 5 3( 1 9 4 7 ) Zobell. C. E.: Geochemicalaspectsoi the microbialmodificltion of carboncompounds, Contrib.1703.34,1653(1964) ScrippsInst.Oceanogr.
Part II \\e\l\estQrgrs\tMsttersrSe(\rnerr\ss1 Basins:Generationof Oil and Gas
Chaprer1 Diagenesis, Catagenesis andMetagenesis of OrsanicMatter
The physicochemicaltransformation of organic matter during the geological history of sedimentarybasinscannot be regardedas an isolated process.It is controlled by the same major factors that also determine the variations of composition of the inorganic solid phase and of the interstitial water of the sediments:biological activity in an early stage,then temperatureand pressure. Furthermore, organic-inorganicinteraction can occur at different stagesof the sedimentsevolution.Nature and abundanceof organicmatter may result in different behaviorof the mineral phase,shortly after deposition;compositionof mineralsand structureof the rock may influencecompositionand distributionof organicfluid phasesat depth.An excellentpictureofthe conditionsofdeposition and early historyof sedimentshasbeengivenby Strakhov(1962),and will be frequentlyusedin the followingparagraphs. A general scheme of evolution of the organic matter from the time of depositionto the beginningof metamorphism,is shownin Figure II.1.1. To understandthe discussion,the followingstagesof evolutionare considered: diagenesis, catagensis, metagenesis andmetamorphism. The acceptance olthese termsin this book is somewhatdifferentfrom that of Vassoevich et al. (1969, 1974).The two scalesof evolution are shownin Figure IL1.2, with referenceto the equivalentcoal ranksand to the stagesof petroleumgeneration.
1.1 Diagenesis Sediments depositedin subaquatic environments containlargeamountsof water (porosityamountsto about80% in claymud at 5 cm depth,i. e., wateris60%by weightof total sediment),minerals,dead organicmaterial(contemporaneous autochthonous or allochthonous, and reworked),and numerouslivingmicroorganisms.Sucha rnixture resultsfrom varioussedimentaryprocesses and primary componentsof very differentorigins;it is out of equilibriumand therefore is aprocessthrough unstable,evenif microorganismsare not present.Diagenesis which the system tends to approach equilibrium under conditions of shallow burial, and through which the sedimentnormally becomesconsolidated.The depthintervalconcernedis in the order of a few hundredmeters,occasionallyto a few thousandmeters.In the early diageneticinterval,the increaseoftemperature and oressureis small. and transformationsoccur under mild conditrons.
70
of OrganicMatter Catagenesis andMetagenesis Diagenesis,
0 2040@80rooo 2040 @ 80r@ .-.-.Wolerconleol (reighl9") Compositon ol disseminoled orqonrcmolial 0 1 2 f , 4 5 -Vilrinilc cllectonc€(%inoil)
fromthefreshly deposited of theorganic matter, Fig.ll. 1 I . General scheme of evolution aminoacids,IT. fulvicacids, zone.Cl1:carbohydrates,,4A: sediment to themetamorphic (nonN, S, O: N,S,Ocompounds H,4: humicacids,L: lipids.HCr hydrocarbons, hydrocarbons) is microbial oneof the mainagentsof transformation Duringearlydiagenesis, that live in the uppermostlayerof sediments activity.Aerobic microorganisms consumefree oxygen.Anaerobesreducesulfatesto obtainthe requiredoxygen. of organicmatter,whichin theprocess The energyis providedby decomposition is usually ammonia andwater.The conversion is convertedinto carbondioxide. E7, decreases partly same time in muds. At the carriedout completelyin sandsand CaCOr slightly.Certainsolidslike the organodetrital abruptlyandpH increases with authigenic together and re-precipitate, dissolve, reach saturation and SiOz mineralssuchassulfidesof iron, copper.leadandzinc,siderite,etc. Within the sediment,organicmaterialproceedsalso towardsequilibrium. "biopolymers"(proteins,carbohydrates) are Previousbiogenicpolymersor andearlydiagenesisThen destroyedby microbialactivityduringsedimentation their constituentsbecomeprogressivelyengagedin new polycondensedstructures ("geopolymers")precursingkerogen.When depositionof organicmatter derivedfrom plantsis massivecomparedto mineralcontribution,peatandthen coal)are formed.The mostimportant brown coals(ligniteand sub-bituminous is methane.In additionorganicmatter hydrocarbonformed during diagenesis
1.2Catagenesis
71
M o i nsloges of evollion
Cool
r,of
{r9i!,!97|l
ITU
trl7t)
xvx
?
I 6
Sub C bitumrnous;-
----{
0.5 8
Hiqh
q
9 to 12
.la5)
15
(40) I - (35)
Ol
10
10 r5
12
biluminous;
6 '.l a?A
2.Q lo 2.5 30 35 40
!6 18
5
2A
Fig.II.1.2.Mainstages ofevolution oftheorganic matter. Thestages usedbyVassoevich (1969, 1974) andtheLOM scale of Hoodet al.(1975) areshown for comparison, andalso theequivalent coalranks.(Adapted fromVassoevich 1969, 1974, Intenational Handbook of CoalPetrology, 1971, Hoodet al. 1975,Stachet al. 1975) produces C02, H2Oandsomeheavyheteroatomic compounds duringlaterstages of diagenesis. The end of diagenesis of sedimentaryorganicmatter is most conveniently placedat the level where extractablehumic acidshave decreased to a minor amount.andwheremostcarboxylgroupshavebeenremoved.Thisis equivalent to the boundarybetweenbrown coal and hard coal,accordingto the coalrank classificationof the InternationalHandhook of Coal Petiology (1971). It corresponds to a vitrinitereflectance of about0.5%.
1.2 Catagenesis Consecutive depositionof sediments resultsin burialof previousbedsto a depth reachingseveralkilometersof overburdenin subsidingbasins.This meansa considerable increase in temperature andpressure. Tectonics mayalsocontribute to thisincrease. For thisstageofsedimentary evolution,we suggest the useofthe word catagenesis, proposed by Vassoevich(1957), and also used by Strakhov (1962).Temperaturemay rangefrom about50 to 150'Cand geostaticpressure due to overburdenmayvary from 300to 1000or 1500bars.Suchincreaseagain placesthe systemout of equilibriumand resultsin new changes.
and Metagenesisof OrganicMatter Diagenesis,Catagenesis
Composition and texture of the mineral phasesare conserved,with some changesmostly in the clay fraction. The main inorganic modification still concernsthe compactionof the rock: water continuesto be expelled,porosityand permeabilitydecreasemarkedly; normally salinity of interstitial water increases and may comecloseto saturation. Organicmatter experiencesmajor changes:throughprogressiveevolutionthe "wet gas" and kerogen produces first liquid petroleum; then in a later stage condensate;both liquid oil and condensateare accompaniedby significant amo]unts of methane.Massiveorganicdepositsprogressthroughthe variousranks oI coal, and also producehydrocarbons,mostlymethane. The end of catagenesisis reachedin the range where the disappearanceof aliphaticcarbon chainsin kerogenis completed,and where the developmentof an ordering of basickerogen units begins.This correspondsto vitrinite reflectance of about 2.0 which, accordingto various coal classifications.is approximatelythe beginningof the anthraciteranks. Sincetheseare severechangesin the organicmaterial, and sincewith further of petroleumandonly Iimitedamountsof evolutionthereis no moregeneration methane,thispointseemsto be at a naturalbreak.Therefore,we proposeto draw and to call the here an additionalborderline,one that terminatescatagenesis, subsequentstagemetagenesis.
and Metamorphism 1.3 Metagenesis The laststageof the evolution of sediments,which is known asmetamorphism,is reached in deep troughs and in geosynclinalzones. Here temperature and pressurereach high values; in addition, rocks are exposedto the influence of magma and hydrothermal effects. Petroleum geology, however, is only concerned with the stage precursing metamorphism,which has been variously characterized and designated as early metamorphism, epimetamorphism, we shall are concerned, anchimetamorphism, etc. As far asorganicconstituents ot organicmatter' refer to this stageprecursingmetamorphismasmetagenesis Minerals are severelytransformedunder those conditions:clay mineralslose their interlayer water and gain a higher stage of crystallinityl iron oxides containingstructuralwater (goethite)changeto oxideswithout water (hematite) occur,like the formation etc.; seveiepressuredissolutionand recrystallization ofthe originalrock structure'The of quartzite,and may result in a disappearance of organic roci< reachestemperature conditions that lead to the metagenesis and matter. At this stagethe organicmatter is composedonly of methane acatbon residue, where some crystalline ordering begins to develop. Coals are transformed into anthracite. True conditionsof metamorphismrcsultin greenschist,and amphibolitefacies develoDment.Coal is transformed into metaanthracite,which has a vitrinite reflectinceof more than 4Vo.'the constituentsof the residualkerogenare convertedto sraohiticcarbon.
Summaryand Conclusion
73
SummaryandConclusion The three main stagesof the evolution of organicmatter in sedimentsare diagenesis, catagenesis and metagenesis. 1. Diagenesisbeginsin recentlydepositedsedimentswheremicrobialactivityis one of the main agentsof transformation,Chemicalreaffangementsthen occur at shallow depths:polycondemationandinsolubilization.At the end ofdiagenesis,the organic matter consistsmainly of kerogen. 2. Catagenesis resultsfrom an increasein temperatureduring burial in sedimentary basins.Thermal degradationof kerogenis responsiblefor the generationof most hydrocarbons,i. e., oil and gas. 3. Metagen€sisis reachedonly at greatdepth. However,this last stageof evolutionof organic matter begins earlier (vitrinite reflectance approximately 2Vo) than metamorphismof the rnineralphase(vitririte reflectanceof about47, corresponding to the beginningof the greenschistfacies).
Chapter2 of OrganicMatter: EarlyTransformation The DiageneticPathwayfrom Organismsto Fossilsand Kerogen Geochemical
and Main Stepsof Early Transformations 2.1 Significance
in the youngsedlment' andresidence processes The timecoveringsedimentation cycle The firstfew the carbon in l stage specia a ver) represents deposited. freshly *"t.r., oi sedimeni,just below tlie water-sedimentcontact' representthe interfacethrough which organic carbon passesfrom the biosPhereto the in this.zone of the geosphere.The- residencetime of organic compoun-ds iedimentarycolumnis long comparedto the lifetimeof the organisms'but very short comiared to the clurationof geologicalcycles:a l-m sectionoften andlaterin such i00 to 10000years.Duringsidimentationprocesses, represents degrees by.varying to alterations organicmaterialis subjected youngsediments, changed, Iargely is composition Lf miirobial andchemicalacrions.As a resultits within historypredetermined andits futurefateduringthe restof the geological h i s l o r y k l i t s s u b s e q u e lnet m p e r a t u r e t h ef r a m e w o r o with When comparingthe nat!re of the organicmaterialin young.sediments point is that the striking was derived' it which from that of the livingoiganisms biogenic the particularly and organisms' of these mostof the usu;l c;srituents lipids'andligninin Proteins,carbohydrates, havedisappeared. macromolecules. basis'of the aLn ash-free on weight' dry total rhe nearty nill"t ptunt.amountto ofthe amount total The environments or subaerial bi6masilivingin subaquatic not usuallv is young sediments very from be extracted ian au.. .oInporind,that results situation This less often and material, organic ofthetotal morethan20c/o by bacteriainto individualaminoacids' of the macromolecules from degradation and sugars,e"lc.As monomers'theyareusedfor nutritionof the microorganisms' material' forminglargeamountsof brown tnE.e.iauebecomespolycondensed, humicacids' resembling and NaOH, in diluted partly soluble ^Astimeandsedimentationploceed'thesetlimentisburiedtoseveralhundr insolubleasa result progressively of meters.Mostof theorganicmaterialbecomes hydrophilic superficial with lossof associated of in.reasingpolycondJnsation young sedimatter.from iun"tionulgioupi. fitit completelyinsolubleorganic "humin" by thefew called mentshasriceivedlimitedattentionuntil recentlylt is soils ln ancientsedimentsthe who haveworked on sub-aquatic soil scientists (HCl' iniotuUt"organicmatter, calledkerogen,is obtainedby demineralization as matter' organic for ancient is not degradative HF) of the rick. This procedure 'al rather is The situation shales and oil (1977)on coals iiro*n Oy Durand et be differeni in young sediments'where an importantpart of-th-ehumin can progresfraction hydrolyzable The hydrolyzedby thii procedure(Huc' 1973).
2.2Biochemical Degradation
'15
sivelydecreases with burial.Thushumin,collectively with otherinsolubleorganic mattersuchaspollen,spores,etc. maybe considered asa precursorof kerogen, but the termsrLrfiin andkerogenare not strictly equivalent.Petroleumgeochemistsconsiderkerogenasthe main sourceof petroleumcompounds. The whole processis referredto hereasdiagenesis andleadsfrombiopolymers synthesizedby living organismsto geopolymers(kerogen)throughfractionation, partial destructionand rearrangementof the building stonesof the macromolecules. For convenience in the discussion. threesteDswill be considered in t h e f o l l o w i n gp a r a g r a p h( sF i g .I L l . l . 1 : Biochemicaldegradation - Polycondensation - Insolubilization It has to be realizedthat the first and the secondsteDfollow in immediate succession, resultingin a zonewhereboth processes are acliveat lhe sametime. Thiszonecomprises to someextentthe waterandthe top Iayerof the sediment. Nevertheless. smallor minuteamountsof aminoacids, Iipidsand sugarsare still found even in very old rocks. Insolubilizationmay also start early for some organicmaterial,but humic acids,which are solublein dilute NaOH, are still foundat severalhundredmetersdepthin sediments containingabundantdetrilal materialof continentalorigin.
2.2 BiochemicalDegradation The four constituents of the youngsediments are minerals,organicmatterfrom deadplantsor animals,interstitialwater,and livingbenthicorganisms. The last onesusuallyincludea wide rangeof forms, but microorganisms are the most activeand widely distributedin shallowwater sediments. They are usuallyless importantand activeunderdeepoceanicconditions. a) MicrobialActivity - mostly bacteria,actinomycetes, Microorganisms fungi and algae - are abundantin sub-aerialsoils,waters,and sedimentsdepositedunder moderate waterdepths.Their normalactivityis the decomposition of organicmatter,thus providingenergyand/ormaterialto build up the constituents of their cells. In aquaticenvironments, bacteriaseemto be especially important:they are presentin both seaand lake waters.In certainsites,they are abundantin the uppermosthalfmeterof the youngsediment.As burialcontinues, the quantityof bacteriaand other microorganisms decreases rapidlywith depth(TableII.2.1). Sincephotosynthesis is impossible insidethe sediment,the energynecessary for thesynthesis of microbialconstituents is obtainedthereby degradation ofexisting carbon compounds(Debyser, 1969), essentiallythrough respirationunder aerobic conditionsand fermentation under anaerobicconditions.The material utilized for synthesisis also normally taken from existingorganiccompounds.
The DiageneticPathwayfrom Organismsto GeochemicalFossilsand Kerogen TableII.2.l 1. Quantity of bacreriain NorthwesternPacificbottom sediments(Atter 1952) Limberg-Ruban, Quantity
Total number of bacteria (mill. per g sediment)
Level (cm)
Surface of sediment
30 '70
10.1 2.6 0.6
(.c,)
Spores
8tt.6 '71.7 '7 5.6
'7.'7 ,1.3 1.4
3.'7 18.0 23.0
2. Ouantity of bacteria in a recent sediment from San Diego Bay(Calif )' (After Zobell and Anderson, 1936)
Depth (cm)
Anaerobicbacteria Aerobic bacteria (per g sediment) (per g sediment)
0- 3 4- 6 14-t6 24-26 44-46 66-68
l,160,000 14,000 8,900 3,100 5,700 2,300
74,000,000 314,000 56,000 10,400 28,1O0 1,200
Anaerobic/aerobtc ratio
1:64 l:21 l: 6 l: 3 1:5 l: 2
As nutritionof bacteriaoperatesin an osmoticway,theorganicmaterialhasto be dissolvedbefore it can be used.The necessarydecompositionof the organic providprocesses' throughenzymatic by microorganisms matteris accomplished heterotrophic for a base and energy of as a source ing the materiai to be uied providingaminoacidsand arehydrolyzed, pr"ocesses. Proteinsandcarbohydrates degradation' a less active of obiect are the iugars; lipids and lignin materialcango very the organic of the destruction in oxid-izingenviionments, compoundsto organic oxidize to oxygen use molecular far: aerobic bacteria action may That (respiration) energy liberate and to water, and carbon dioxide oceantc deep (Fig. 1) Under II.2 is destroyed matter organic last until all to be seems activity bacterial as different, is somewhat the situation conditions, environment. reducedby hostile In fine-srainedsediment,like shales,siltsor fine carbonatemuds,the pore quickly a closedenvironment,whereinterstitialwater isseparated spacebecoimes fiom the overlyingsei or lake water. Most of the oxygenconfinedin that spaceis exhaustedty ier6tic activity and anaerobicconditionsare rapidly established' first on a resirictedscale,around decomposingorganisms,then on a wider scale by (Fig. II.2.1). Under theseconditions,oiganicmatteris partly decomposed oxidahydroxides; ie.i'"ntation, sulfatesare reducedand also ferric and other
'77
2.2 BiochemicalDegradation
Preservation in lineclay or carbonate mud
0estruction in a poroussediment dcp0sited inder aerobicconditions Water
Dissolved 02
particles 0rganic
Fig. II.2.1. Preservation or destructionof the organicmatterin a freshlydeposited sediment.In a fine clayor carbonatem]ud(top), porewaterbecomes a nearlyclosed microenvironment. Thereis no replenishment of oxygen,andanaerobic conditions are rapidlyestablished, first on microscopic, thenon a macroscopic scale.In a poroussand depositedunder aerobicconditions(bottom),ftee circulationof water containins dissolved oxygenresultsin the destruction of the organicmatter tion-reductionpotential is loweredbelow zero and often droosaslow as _ 150to - 4 0 0 m V ( S t r a k h o v1. 9 6 2H : uc. l97l). With increasingdepth,the organicmatterdepositedin marine,fine-grained sedimentsnormally goesthrough three successive zones(Claypooland Kaplan, 1974): - an oxidizing zone, where free oxygen is still available for oxidation of the organicmatter;thiszonemay be absent,whenthe depositionenvironmentis a l r e a d ya n o x i c ; an anaerobic sulfatereductionzone,afterall molecularoxygenhasbeenused; - an anaerobicmethanegenerationzoneafter mostof sulfate hasbeenreduced. Anaerobicconditions,however,are first establishedon a microscopic.then on a macroscopic scale(Fig.II.2.1).Thensulfatereductionandmethaneseneration are not mutuallyexclusive.Thus there may be someoverlap,with;espect to depth, of the variouszones.For instance,Doose(1980)observedin sedimentsof SantaBarbarabasin,off California,that the sulfatereductionzonechansesto the methanegenerationzoneat about1.5m depth,with some50 cm overlap. Sulfate-redu-cing b actetia(Desulfovibio) ltilize oxygenfrom SOI- andreduce the sulfur to S'- underanaerobicconditions.This normallyhappensbelowthe water-sedimentinterfacewhereinterstitialwater is separatedfrom the overlying
The DiageneticPathwayfrom Organismsto GeochemicalFossilsand Kerogen
Actionol sulturotadiring h.cteria Actionol st|ll.te reducing
Ilo sullur oxidiring bacteria
with dissolved no 02 Sedimenl b
A c t i o no f s u l l . t e t e d u c i n gb a c t e t i a
andbottom in-sediments sulfide of hydrogen anddestruction Fig.II.2.2a,b. Formation and destrucformation between is an equilibrium there witers.(a) Openseaconditions: oxygenin bottom watercirculation:no dissolved tion of hydrogensulfide.(b) Restricted bacteria.Thus,thereis no of aerobicsulfur-oxidizing waters.ihis iesultsin the absence andcontainH2S anoxic which are waten, in bottom of hydrogensulfide destruction free water: bacteria extract the limited amount of sulfate initially present in interstitialwater or providedby the slow processof diffusion The zoneof sulfate reductionis normallyconfinedwithin the upperlevelof sediment,andthe water columnabovethe sedimentdoesnot containhydrogensulfide.Seaor lakewater usuallycontainsdissolvedoxygen,and hydrogensulfideemanatingfrom the sedimentinterface is oxidized igain into sulfate, under aerobic conditions,by other types of bacteria (Thiobaciltus)' Thus, an equilibrium is established betweensupplyand destructionof H2S:Orr (197a)(Fig. 11.2.2)ln somecases' however.ttii bottomwater layerof the seaor lake becomesdevoidof oxygen, and sulfur-oxidizingbacteriacannotwork in suchanaerobicconditions except water free the waters Furthermore photosyntheticcolored bacteriain shallow may sulfur and free H2S quantities of large Lecomessubtectto sulfatereduction; anaerobic except disappears' life the benthic be oroducedfrom seawater;and miciobes,partly due to lack of oxygen,and partly due to toxicityof HrS (Fig' il.2.2). The phenomenonmay occur for variousreasons,but it is due mainly lo restrictionsin water circulationandto eutrophicconditions.Restlictionsin water circulation may prevent oxygensupply of water layers. For examplethe Black Sea, nearly septrated from the MediterraneanSea, receivesmore freshwater Thisresultsin a low salinityand from rainsandriversthanis lostby evaporation. Thereforethe water a low densityof surfacewaters,thuspreventingconvection. oxygen Anaerobic of deprived remain waters bottom and is stratified column by aerobic sulfurdestroyed is not but it H2S, generate abundant bacteria oxidizing-bacteria.Eutrophic conditionsresult from superabundantsuppliesof
2.2 Biochemical Degradation
79
organlcmatter or nutrients(phosphate,nitrate) to lake or seawaters,as is presentlyhappeningin Lake Erie due to human pollution. populationsol bacteriaandotherorganisms thatliveat theexpense of theorqanicmatteranduse dissolvedoxygengrow considerably. They su-hsequently eliiinate oxygenfrom the bottomwaters.The sameeffectcanbe postulated for fossilsedimenis which, earlierin geological history,had receivedan abundantsupplyof organicmatter, asin somelacustrinebasinsor in tropicalswampsfoundtodayin certaincoastal areas. Sulfur is not incorporatedin the bacterialcell. In clay muds,whereiron is usuallyabundant.it is likely that sulfur readilyrecomhineswirh iron to form hydrotroiliteand troilite, which are slowlyconvertedto pyrite. In carbonate muds.however.whereiron is muchlessabundant,sulfurmayrecombinemainly with residualorganicmatter.Massiveincorporalionof sulfurinto sediments in certainconfinedenvironments with H.S-saturated bottom water may lead to a subsequent recombination with organicmatterduringlatediagenesis andfinally explainthe origin of a sulfur-richpetroleumformedduringcatagenesis. Fermentationis an anaerobicprocessby which bacteria,insteadof using molecularoxygen(respiration).useoxidizedformsof organicmatter,particularly carbohydrates. Celluloseis degradedenzymatically by bacteria,iuch as Clostidia (Doose, 1980),producingthe low molecularweightprecursorsfor methaneformation(acetate.bicarbonate) and the final stageis a reduclionof carbondioxideor acetateby methane-generaring hacteria,suchasMethanobacteria. In marinesedimentscontainingorganicmatter.Doose(19g0)observedthat methanegenerationbecomesimportantonce the contentof sulfatehas been largclydiminishedby anaerobicsulfate-reducing bacteria.This observationis interpretedas resultingfrom a competitionfor hydrogen(issuedrrom organlc matter) betweenthe sulfate-reducing bacteriaand the methaneproducers.In fact,Oremlandand Taylor (1978)havedemonsrrated by laboratoiyincubations of sedimentsthat methanogenic bacleriacannotreallv competcwith sulfate_ reducingbacteria:althoughthe t\aoprocesses are not muruallyexclusive, sulfate reductionis largelypredominantas longassulfateis ahundanrlyavailable. Anaerobicfermentationhas long been consideredby variousauthorsas a possibleway to formation of petroleum.as methanegenerationwas well established throughthisprocessin swamps(marshgas). Laboratorvexpeflments u erecarriedout on fattyacids.carbohydrates andnaturalmuds.The resultsshow that methaneis the singlehydrocarbonproducedin major quantities.Other hldrocarbons(ethan€,.propane,butane)show very low concentrations only, bctween10-r and 10 h, in the gas generatedin the experiments(Davis and S q u i r e s1. 9 5 3B . o k o v a .1 9 5 9 ) . When conditionsare particularlyfavorable,the formationof methanemav reachan importantlevel.Lake Kivu, in CentralAfrica.is an examoleofmethane :ormationby bacterialreductionof carbondioxideon the bottom of the lake Louis. 1967).The oxygencirculationis restrictedbecausethe lake is deen 'JEOm) and the valleywassealedoff by recentlavaflows.The total volumeof l ateris about-580kmr andat depthsgreaterthan265m it containsdissolvecl gas :(rmprisingCOr (75%), CHa (24V0),H.. N2, Ar (l%). The toral amountof
80
FossilsandKerogen to Geochemical Pathwayfrom Organisms The Diagenetic
methaneis evaluatedto be 57 billionm3undernormaltemperatureandpressure conditions.In fossilsedimentsbacterialdegradationof organicmatter maybe the "dry gas" fields at shallowdepths,like the largegasfield discoveredin sourceof of WesternCanada(MedicineHat, depth:310m; Upper Cretaieoussandstones suggest of WesternSiberia.Theseexamples andin the Cretaceous CH^:96.ZVo\ that under certain conditions, such as in the case of abundant depositionof cellulose,an important part of the organicmatter may be convertedto methane' Carbon isotopemeasurementshaveconfirmedthis assumptionin severalcases' The isotopic aspectsof methanegenerationby bacteriaare discussedin Section 2.5 of this chapter. Biogenic formation of gas in sedimentarybasinsand its significancein t-ermsof gasreserveswill be further discussedin Sect.6 2 It is difficult to ascertainwhich limiting factor will stop the degradationof organicmatter by bacteriaunder anaerobicconditions:lack of nutrient from the remainingorganicmaterial,toxicity of productsfrom their own metabolism,etc. In deep oceinic conditionsthe hostileenvironment(pressure,temperature) might preventaerobicand anaerobicbacteriafrom beingvery active. b) Freeor HydrolyzableOrganicCompoundsin the YoungSediment As macromolecules,like proteins and polysaccharides'are degradedby microbial activity, individual amino acids and sugarsmay be found in the young though,their Surprisingly, sediment,in additionto fattyacidsandhydrocarbons. total amount is rather low: even in the top layer of the sediments,most of the nor extractableby organicmaterial(about75 to 95%) is neitherhydrolyzable organicsolventsr.This materialwill be consideredbelow (Sect.2.3) underthe "humin". genericnamesof "humic compounds"and Amino acids, mostly hydrolyzable and not free' are generally the most organiccompoundsln the abundant(Fig.II. 2.3)amongthefreeor hydrolyzable top 0-15 cm layer of a reducingfjord, the SaanishInlet in British Colombia 670of the totalorganiccarbon.In the Bering (Iirown et al., 1972),theyrepresent "readily organicmatler" reaches25Va hydrolyzable Sea(NW PacificOcean)the of the organicmatter(Huc 404/o (Bordovskiy,1965)andin the centralBlackSea to include also the likely figures are et al., 1977). However, these latter below. In all cases' as defined or humin, acids fractionof humic hydrolyzable the fjord sediment, in rapidly decrease at depth these compounds however, in the Black Sea slowly m, and more 17o at 35 than wherethey amountto less m. at 500 to ca.207c amount wherethey still sediments, with deplh from 3.87oof organic Sugarsshow a still more abrupt decrease down to 0.027oat 44 m, in a core from interface carbon at the water-sediment (Vallentyne, 1963). PacificOcean The total amount of free or combinedfatty acids, hydrocarbons'terpenes' sterolsandpigmentsis generallybelow17oof the organiccarbon,althoughsome and 1 In additionwe mustbe carefulwhenconsideringanalysisof proteins,carbohydrates microorganisms of the normal constituents are the as these sediments, lipids in recent living in the sediment(Debyser,1969)
2.3 Polycondensation
o . r . +
81
% of o/qonr( motter s u g o r ( P o c i fc O c 6 o n ) A m i n o o c i d 3( R o d u c i n q t r o . d , B r i r i s h C o t u m bCi oo)r u s 4/3a B y d r o t y s d b tO e r q o n i cm o l r e r{ B e n n q S . o ) Foity ocids Reducins rjord I Hyd.ocorbons ( C o r € s4 / 3 A ) I Bnl.h Cotumbio
Fig IL2.3. Variation of aminoacids, sugars andlipidsin Recent sediments, asa tunction of depth(wt. % of thetotaloreanic matter).(DatafromVallentyne, 196j,Bordovskiy, 1965. Brownet al.. 1972) exceptions, like algaldeposits, mayoccur.Thereis a decrease of thetotalamount ot these compoundswith increasingdepth. This d""r"u.. .uy be due to consumption by bacteriaor to thefactihat thesecompoundibecorne ooundin the rnsoluble organicfractions.In additionto.that,thefreemoi"J"., .u.f, ur rt".otr, terpenes,etc., are subjectto chemicalchanges in the upperlayerof sediments. Thesechangesdo not affect the carbon skJleton, Uutirnp.ou'eir,e stability of thesemolecules (chap. II.3). FinaIythereis evidence fo."u'.iyr*nrrorrnutionuy microbraldecomposition and resynthesis ot t y,t.o.uJon .t'uinJ ini,"ua .t uf., 1971).This results,for instance,rn changes ofih" f."gtl, oiii. ;ri;hatic chains.
1.3 Polycondensation theorganic. materiat in youngsediments isneither hydrolyz_
*: iii": l-,."1 by .rbte, nor extractable organicsolvents.Most tiee
or tryO.otyzaUie compounds
82
FossilsandKerogen to Ceochemical The DiageneticPathwayfronrOrganisms Fig. tI.2.,1. Flow diagramof the separation of humicmaterial
H'L2N
I
,rr*,
|
,ro,
disappearat shallowdepth; and their residue,which is not directly usedby becomesquickly incorporatedinto new polymeric.insoluble micioorganisms, structures. Techniquesusedby soil scientists(Fig. 11.24) appliedto Recentsediments (sub-aquaiic soils)resultin extractionof humicmaterial.The extractionwith a ( 17c), and mixtureof dilutesodiumhydroxide(0 1 N) andsodiumpyrophosphate to /a/vic similar yields compounds precipitation by acicl separation a subsequent leaves extraction The (insoluble in acids). humic acids and (sofuble inaci)s) cclrls humic the terms (insoluble In the following, in NaOH). to hamin similar a resiiue andfulvicwill bc usetl.insteadof humic-likeandfulvic-likematerial The sumof t h e h u m i ca n dt u l ri c a c i d su i l l b e c r l l e dh u m i cc o m p o u n d s . of the organic In subaerialsoils.humic acidsresultfrom polycondensation residueof microbial (fungi. bacteria.etc.) metabolism,under more ot less of phenolsis an importantprocess oxidizingconditions.Oxidativecondensation (Flaig. 1966). Addition of nitrogenouscompoundsoccurs mainly through iandim polymerizationof free radicalslike semiquinone(Flaig' 1972) The structureoi soil humic acidshas been describedby Swain (1963):they are madeof nucleibearingreactivegroups,connectedby bridges polycondensates i.luil"i or. simpleor condcnsedcycles.aromatic.naphthenicor heterocyclic: bridgesarc mostly oxygen, sulfur' peptidic or methylenebonds: the most impirtant reactivegroupis OH Molecularweightsof humicacidscovera wide .onn.. .u. 10.000t;100.000,and the sizeof particlesmay reach10uA'
1.3 Polvcondensation
83
In subaquaticsediments.and also in the overlvingwater column,similar proccsses of polycondensation occur,underaerobicandanaerobic conditions.In suchsedimcnts humicacidsmaydevelopby polycondensation of autochthonous. mostlvplankton-derived material(protein,carbohvdrate. andlipid derivatives). and/orallochthonous land-derived material(mostlyligninandcellulose). Amino acids and carbohvdratesmay react rapidly. through the Maillard reaction (Maillard.1912)to producea polycondensatc. linkedby peptidicbond.similarto humic acids (Nissenbaumand Kaplan. 1972,Hoering and Hare, 1973).In additionto that.theremavbe someadmixtureof detritalir dissolved humicacids from continentalorigin. The structurcof humicacidsformedin subaquatic sediments is ratherdifferent f r o m s o i lh u m i ca c i d s P . h e n o l i co n s r i t u c nar si e l e s sa h u n d a ni tn m a r i n eh u m i c acids.On the contrarvtheirH/C atomicratiois highe;*ith valuesof 1.00to l.-50. comparedwith 0.-50 to I .00in soilhumicacids.Thislesultsfrom the abundancc of aliphaticchainsand alicvclicringsin marinehumicacicls.whichis clearlvshown o n I R s p e c t r o s c o (pFvi g .I L 2 . 5 )b v t h e i m p o r t a n cocf C H . . C H 1b a n t l . . lrround l9()1).1,150and 1375cm 11.The role of peptidicbondsis confirmeclbv the i m p o r t ; r n cocf r h e c o r r e s p o n d i nl m g i d e l i r n t l l l i n f r r r c d l r . r n r l s( l t , j t i a n r l l 5 . l t tc r ' , . u h i c h i s g r l ' a t e ri n m a r i n ch u m i ca c i d so f s h a l l o rov r i g i nt h a n i n
F i g . I I . 2 . 5 . C o m p a r i s oonf IR spectraof humic acids fronr tcrrestrialsoils. and humic acitls from Recent marine sedimcnts (Huc. 1973). Idcntification of humic acids: Terrestri roils. A rendzin.B brown soil.acid.C podzol.Marllc seclitnents: D and I'Francc. Atlantic coast. E Eastern Meditcrranean. G WestAfttctr. ldentilicdtio ol lR bands: I uliphatic C H, 2: C = O. -l and .i amidcs..l aromaticC=C.6COof polysaccharides
84
to Geochemical Pathwav fromOrganisms Fossils andKerogen TheDiagenetic
terrestrialhumicacids(Huc andDurand.1974).The sulfurcontentalsoappears to be higherin marinehumicacids,Carboxylgroupsare abundant,asshownby the strongabsorptionat the 1700cm-t IR band. Rashidand King (1969)havestudiedthe molecularweightdistributionof the humicmaterialfrom the Scotianshelf.Fulvicacidscovera widerangefrombelow 700to 10.000mol. wt. Molecularweightsof humicacidsextendmuchfurtherand someare greaterthan 2.10o. Fatty acidsmight be fixed on humic material.as aliphaticestersubstituents
o
by Burlingameet al. (1969)for the C O bonds)in a similarway. assuggested kerogenof the GreenRiverShales. Thisprocess resultsin lixationof alkylchains of unsaturated fatty acidsmight in the polvmerichumicmaterial.Polymeriz;rtiori conditions alsooccur.althoughthishasnot beenprovin undermild temperature of the youngsediment. proccss.astheylack Hydrocarbons are not affectedbv the polycondensation the necessary functionalgroupsto be linked to the humic material.However. varioustypesof organiccompounds, includinghydrocarbons, maybe attachedto humicmaterialbv weakerbonds:physicaladsorptionor hydrogenbonds. ma1' The abundance andnatureol humicandfulvicacidsin Reccntsediments vary. accordingto the conditionsof environment.In placeswherecontinental materialfrom highcr plantsis often the runoff is considerable. allochthonous Gulf Coast.Bayof mainsourceof organicmaterial:Nigerdelta,Texas-Louisianai Bengaland Amazon fan in modernenvironmentslLower Jurassicof Luxembourg (Hagemann,1974)in ancientsediments.In suchcasesfulvic and humic acidsaregcnerallyabundant.They mayincludehumicacidsformedin continentransportedto the basinof deposition.Allochthonous tal soilsandsubsequentlv materialmay also includereworkedorganicmattererodedfrom ancientscdimentsand transportedby riversto the basinof deposition.For instance,Gadel and Ragot(1974)measuredthe amountof anthracitematerial.erodedfrom the Carboniferousof the Alps and incorporatedin the Recentsedimentsoff the Rhonedelta. marineorganicmatteris likelyto bc thc major In otherplaces.autochthonous constituent.For instance.in the SaanichInlet sedimentsof British Colombia (Brownet al.. 1972)the averagccarbonisotopicratioof humicacidin sediments is -22.5"/,",. comparedwith -29.1% in humic acidfrom neighboringsubaerial arescen soil.and l9%"for thetotalorganiccarbonof plankton.Otherexamples in the occurrenceof humic acidsin sedimentsfar awayfrom any coastand in sedimentsof the Antarctic area, where terrigenousorganicsupplyfrom the thereis more continentis unlikely(Bordovskiy.1965).However,on the average, humic materialproducedfrom terrestrialorganicmattcr than from autochthonousmarinesources. In additionto the attachmentof organiccompounds,humic acidsmay also collectandfix heavymetals,suchas U, V, Ni, Cu andothers.
2.4 Insolubilization
85
2.4 Insolubilization Decompositionand polycondensation of organicmaterial,mostlyoccurringin the first few metersof sediment,resultin macromolecules that reDresenr more than90% of the totalorganicmatterin youngsediments. However,a certainpart of the macromolecules still consists of NaOH-solublecompounds: in the upper layerfrom 0 to 10m the amountof fulvicplushumicacidsrangesfrom 10to i0% of the total organicmatter, the highestvaluesbeing recordedin deltaicor estuarineterrigenousmuds(Huc and Durand, 1974).With increasing depthof burial,thesewide variationsare reduced,because the fulvicandhumii aciis are beingconvertedto insolublehumin (Huc and D_urand, 1972). hsolubilizationoccursduring diagenesis ofsedimentson a depth and time scalemuchwiderthanthe previoussteps:severalhundreds,or evenoccasionally thousands, of meters,oneto severalmillionsof years.With increasing depth,the insolublehuminpredominares overfulvicandhumicacids(Fig.II.2.6).Also, the fulvicacid/humic acidratiodecreases (Fig.II.2.7).The organicmaterialbecomes m_orecondensed,asshownby a generaldarkeningand an increaseof absorption of visiblelight. Thesefactscan be interpretedas a resultof an increased polycondensation, accompanied by eliminationof a sufficientpart of the superficialhydrophilic
ReducingFjord BrifishColumbio (ofterBrownet 01,1972)
urr 1()()0
BeringSeo ( oflerBordovskiy,l965 )
20
40
% ot lotol orgonichott.r -
60
80
too
---------|
Fig. 11.2.6.Evolutionof the organicconstituents in Recentsediments as a functionof depth. F,4..fulvic acids. ll,4. humic acids, F/ or Kr humin (or kerogen). (Data from Bordovskiy.1965:Brown et al., 1972)
86
to Geochemical Fossilsand Kerogen The DiageneticPathwayfrom Organisms
o--___l 8oYof Eenin ( WeslofAfrico)
E
I o or 0201 0405 ---'-----'---------------'-_FA /NA toto
06 07 ---->
Fig. I1.2.7. Variationof the fulvic acidrhumic acidratio in a corc fronr the Bay of Benin (west Africa). (Datafrom IIuc. 1973)
functions.Thusthc organicmatcrialbecomesprogressively insoluble.It evolves from fulvic acid to the humic acid type, and then to the completelyinsoluble humintype.A similarprocesshasbeenpreviouslyproposedby Swain(1963)for humicmaterialof soils. Sevcralkinds of functionalgroupsare clearlyaffectedduring the phaseof insolubilization. Two stagescanbe tentativelyseparated. An initialstagecovers the first tensof meters.Duringthattime,the evolutionof the humicacidfraction (Huc andDurand.1974)showsa progressive lossofpeptidicbonds.accompanied b y a p r o g r e s s i vdei s a p p e a r a nocfet h e 1 6 5 0c m ' a n d1 5 4 0c m ' I R b a n d s( a m i d eI andI I ). Furthermore.there is a decrease of hydrolvzable nitrogen andalsoof the N/C ratio.The eliminationof nitrogenfrom tht:solidorganicphasehasalsobeen recordedby Bordovskiy( 1965)and Kempet al. (1972)on the total dry scdiment ( F i g .I 1 . 2 . 8 a ) n dB r o w ne t a l . ( 1 9 7 2o) n t h eh u m i ca c i d sA . s a r e s u lo t f nitrogen e x p u l s i o nt.h e a m n r o n i an i t r o g e nc o n t c n ti n p o r eu a l e r i n c r e a s e D s .u r i n gt h e sameintervalcarboxylicandaliphaticgroupsarelittleaffectedandmayrelatively increasefor this reason(Huc and Durand. 1974).A secondstep is much less known. as it covers the interval from tens to hundredsof meters. where adequatelycored intervalsare the exception.Over this intervalthe oxygen contentis reduced:O/C ratio rangesfrom 0.3 to (1.6in humicacidsfrom young sedimentsand decreases to 0.1 to 0.2 in kerogenof relativelyshallowancient sediments(500-1000m depth). The oxygen decreaseis mostly related to eliminationof carboxylicacidgroups(Fig. II.2.9). The succession of fulvic acid to humic acid to humin seemsto be the most plants probableevolutionwith respectto organicmatterderivedfrom continental incornoratedin deltaicor estuarinemuds: there the initial contentof humic
2..1lnsolubilizatron
87
BeringSeo
LokeOntorro
{ ofterBordovskiy,1965 )
(ofler Kempet oi 1972)
Organic Nitrogen
8
1 6 2 4 ! 2 4 NH4-Nlroqen{89/l)
a1 0 4 o i c N ( % o l d r y! . d i n ! n t )
0
0 200 4@ 600 F i x e d l . r c h o n gc€No H b 4N i t r o q e n { p p mt o d r t r a i q h or l s e d . l
0z o 0 2 0 4 0 ' 6 o q o i , cN ( % o t d r ys e d r h c)n l
Fig.11.2.8. Organic andammonia nitrogen in Recent sediments fromtheBeringSeaand LakeOntario.(DatafromBordovskiy. 1965, Kempet al.. 1972)
compoundsis high. However,a cefiain part of the organicmatter may be incorporatedin the sedimentthrough a different pathway.which does not comprisethe fulvicor humicacidstage.On the one hand.part of the insoluble fractionmav be of purelydetritalorgin.i. e.. reworkedfrorn a previousorganic deposit. On the other hand. there are geologicalsituationswhere humic compoundsamount to only a minor part of the organicmatter in the young sediment.For instancein the centralBlack Sea.the upper level,which correspondsto the well-known"euxinic" environment(0-70 cm, (!7000 years), containsonly 10-15% of humicplusfulvicacids.whereas85 907aofthe organic matteris completelvinsolublein alkali.A deepersedimentarv level(70-330cm. 700tI-13.800 years),whichwaslaid downwithoutan anoxicwatercolumnon top of it. containslessorganicmatter.but ca. 357aof it is composedof humicand fulvicacids.versusca.65% insoluble1DebyserandPelet.1q77).Theseexamples suggestthat some autochthonous marine organicmatter depositedin a very reducingenvironmentneverpassedthroughthe fulvicor humicacidstage.The samecould applyto materialmostlymadeof lipids.like somealgalkerogens.
88
The DiageneticPathwayfrom Organismsto GeochemicalFossilsand Kerogen
I
o3
Fig.II.2.9. Elementary composition andmainfieldsof occurrences of humicacidsand kerogens fromRecentor immatureancientsediments. Themajortypesof kerogenf1,11, ,f11) andwillbedescribed in andtheirpathsofevolution(crrops)areshownfor comparison Chapter IL4 suchas the Coorongiteof Australia.The respectivefieldsof humic acidsand immaturekerogens, shownin FigureIL2.9, arein agreement with thisinterpretatlon. Whateverthe pathof organicdiagenesis maybe,the resultis theformationof a polycondensate. This is insolublein alkali and it can be termed"humin", bv analogywith humin from soils(Fig. II.2.4). It usuallycomprisesmost of the marineautochthonous organicmatterin thedepthrangegreaterthan 50or 100m. Huc and Durand (1977) analyzerlancient sedimentsfrom the diagenesiszone (burial depth200-1000m). They reportthat the insolublefractionamountsto morethan 907cof the marine-derived organicmatter,whereashumicplusfulvic acidsamountto Iessthan 107o.The distributionis morevariablein land-derived organicmatter,wherehumicplusfulvicacidsstill amountfrom 5 to 70%. They alsolastdown to a greaterdepth.For example,in the Upper Cretaceous ofthe Doualabasin.Cameroon,extractable humicacidsmaystill amountup to 57oof the total organicmatterdown to a depthof 1000m. The generalstructureof humin is alreadycomparable to what is knownfrom the structureof kerogen,whichcomprises mostof the organicmatterat depth. The main differenceis that an importantpart (15 to 40%) of humin in young sedimentsis hydrolyzable. With increasingburial the hydrolyzable fraction decreases, as shown by Huc et al. (1977)on the Black Seasediments.This observation indicatesthatsomestructuralchanges occurat depthtowardsa more resistantstatusof the organicmatter.In kerogenof ancientrocks,the hydrolyz-
2.5Isotopic Composition of Organic Matterin YoungSediments
89
able fraction is insignificant.Therefore,humin shouldbe consideredas the precursorof the kerogen,as definedby petroleumgeochemists.
2.5 lsotopicCompositionof OrganicMatter in YoungSediments The distributionof carbonisotopesin humin dependson the originalisotopic composition of the sourceof organicmatter,hndon isotopefractionationduring the processwhichleadsto humiccompoundsqnd humin.Galimov(1973,1978) hasshownthatthe isotopedistributionin biogenicmolecules is directlyrelatedto the chemicalstructures.For example,carbonof aliphaticchainand methoxyl group is depletedin rrC, whereascarbonof carbonyl,carboxyl,phenolicand aminegroupsis enrichedin "C. comparatively(Abelsonand Hoering, 1961, Galimov,1973.1978.Vogle-rand Hayes,1980).As a result,Iipidsare relatively lighter (i.e.. enrichedin ''C) and proteinsand carbohydrates heavier(i.e., enrichedin "C). The variationsare usuallyreportedas : C 1s a m p l e - 1 r ' C , r 2 C ) . t a n d a r d 1r'C d(.i,=#r1000. ('C/''C)standard The mostcommonstandardis the "PeedeeBelemnite"or PDB. Isotopicvariationsare alsoobservedwith respectto hydrogenandits heavier isotopedeuterium(D). The standardfor hydrogenisotopesis ,,standardMean OceanWaters" or SMOW. Similarlyas for carbon,the isotopevariationsare as expressed
6D7* =
(D/H) sample-(D/H)standard (D/H)standard
x 1000.
The mainchemicalconstituents (lipids,carbohydrates, etc.)ofterrestrialplants are usuallyisotopicallylighter(i. e. depletedin rrC.1 than thoseof marineplants 1l978). (Silverman,1967,Galimov A similareffecthasnot beenobservedwith respectto hydrogenand deuterium(StillerandNissenbaum, 1980).For carbon, the globaldifferencerangesfrom 5 to 10%c. This distinctionreflectsthe isotopic composition of the carbon source for photosynthesis:marine plants utilize carbonatecomplexesin seawater,whereasterrestrialplants use atmospheric carbondioxide,with a lowerdi3C.The difference is reducedin kerogenofancient sediment,but it still amountsto from 3 to 5%o.Duringtheprocess ofpolycondensationandinsolubilization, whichleadsfrom biogenicconstituents to fulvicacid, humicacidandhumin,thereis a slight,but systematic, enrichmentin the lighter isotope r2C. This effect results from the elimination of carboxylic and other functional groups and from increasingpolymerization (Abelson and Hoering, 1961.Galimov,1973,1978).Theredoesnot appearto be a significant fractionation of hydrogenisotopesduringthis process(Reddinget al., 1980). methaneresultsin an importantisotopefractionaGenerationof biochemical tion. The dlrC of biogenicmethaneis about30 to 50%o lowerthanthe dr3Cofthe o r g a n i cm a t t e r .i . e . . t h e m e t h a n ei s e n r i c h e di n t h e l i g h t r 2 Ci s o t o p eO . n the
90
andKerogen Fossils to Geochemical ftomOrganisms TheDiagenetic Pathway
is enrichedin contrary.the carbondioxidealsogeneratedby microorganisms, '3C.Thus.the isotopecompositionof kerogenis little affectedby thisprocess' of observedthat generallycarbonisotopiccomposition Furthermore.it has'been yoxng sediments' is in as it range same is in the kerogenfrom ancientsediments of The !enerationof biochemicalmethaneresultsin an importantfractionation in is depleted methane biogenic in the hydrogen The also. isotopes hydr6gen of concentration deuterium to the compared :l60%l, approximately deuteium by the associatidwaterswhich are the most likely sourcefor hydrogen(Schoell'
le80).
\
the poisibilityof methaneoxidationin near-surface A specialremarkconcerns concliiions.with changesof the isotopiccompositionwhich may result rn bacteriaare misinterpretationof the origin of this gas. Methane-oxidizing shownthat (1981 havc al andsoils Colemanet ) in voungsedimenis widespreacl hydrogen and carbon -isotopes' culturesof these bacteriafractionatedboth ''C resultingin the rcsidualmethanebeingenrichedin the heavy.isotopes andD' dD valueof methaneis ca' l0 timesgreaterthan In oartiiularthe changein the -Ihus. if biogenicmethancmigratesinto an aerobic the changein the drrClalue. environrientand becomespartialtyoxidizedby bacteriathe residualgasmight show a d!rC comparablet; that of thermogenicgas:however'comparisonof of thissituation(Coleman dlrC and dD shouldin manycaseallowidentification e t a l . , l 9 t lI ) . A comparableobservationwas reportedby Doose (1980)in younBmarine zone(whichbeginsat sedimentifrom California.Abovethe methane-generation in the sulfatelou,ralues to.very decreases of methane quantity the 1.5m depth) v a l u em a y r e a c h d ' ' C t h e i n r t C : e n r i c h e d i s m e t h a n e t h i s z o n c a n d reducing -20%. ilnthe uppersliceof sediment'ascomparedto -80 or - 90%"between1'5 enriched becomes and2 m. At the iametime.the isotoperatioofthe bicarbonate methaneoxidationby for anaerobic Thesedataareinterpretedasevidence in 12C. bacteria. sulfate-reducing of low quantitiesof Theseremarfsshouldbe keptin mindwhenthe occurrence soilsor groundwatersis to be interpretedin terms methanein marinesecliments, or inlandwatersprotecfor hydrocarbonprospection of origin and significance tion.
2.6 Resultand Balanceof Diagenesis As a result of microbial activity in water and in subaquaticsoils' biogenic polymershavebeendegraded,thenusedasmuchaspossiblefor the metabolism Thus,evenin finemuds,a partof theorganicmatterhasbeen bfrni..ootgunitrns. consumedlnd has disappearedthrough conversioninto carbon dioxide and of themicrobial the constituents water.Anotherpart hasbLenusedto synthesize unassimilable The residue cycle. the biological into cell,andthusis ieintroduced which is polycondensate' a new in incorporated now is by microorganisms mild under process occurs chemical (F\g. This 11 2.10) insoluble:Eerogen of increase of the the influence pressure thus conditions: and temDerature
2.6 Resulrand Balanceof Diagene\is
91
llighly orginired bioporymers
lndividu.l mon0m0rs
\/ C02.H20
Hslslo0rnGous randon qaop0rYmaas
Fig.II.2.10.Fateof organic material duringsedimentation anddiagenesis. resulting in twomainorganic fractions: kerogen andgeochemical fossils temperatureandpressureis likely to be subordinate, comparedto the natureof the original organic constituents.This view is confirmedby the resultsof cxperimentalevolutiontests:heatingorganicmatterunderinert atmosphereis ableto simulatethe transformations occurringat greaterdepths- catagenesis and metagenesis but not diagenesrs. Besideskerogen,organicmatterstillcomprises at theendof diagenesrs a mrnor amountof free hydrocarbons and relatedcompounds. They havebeensynthesizedby living organismsand incorporatedin the sedimentwith no or minor changes.Thus, they can be consideredas geochemical fossils,witnessingthe depositional environment. Severalauthorshavestudiedthe relationship betweenthe originalamountof organicmatter depositedin a freshsedimentand the amountthat was finallv preservedwhen the sedimentwas fossilized(Strakhovand Zolmanzen.1955. Bordovskiy.1965.Hartmannet al., 1973).Their calculations werebasedon the
92
FossilsandKerogen to Geochemical Pathwayfrom Organisms The Diagenetic
of iron determinationof the expenditureof organicmatter for the reduction matter organlc original the of to 50% 15 from that ihow The results compounds. decaY may be lost by microbial in ti has been observedthat many fine-grainedsedimentswhich are rich of Toarcian orsanic matter. such as EoceneGte"n Riu"t Shaleand Lower scale This w;;i;;" E;t.p.. are extensivelylaminateddown to a millimeter werenot disturbedby benthic onlyiltf sediments lr*inutlon canbe preserued intensively tr;;;;i"g urganisms.Burrowingorganisms'when present'would matterthey organic layeri ln additionto consuming reworkthi top sedimentary to come in contactwith sedimentaryor organic oxygen molecular allow also On^the contrary'the .urton. fnu.. they favor further aerobicdegradation anoxiccondiTherefore' ut a"nla of Uantttoni.fuunuwould favorpreservation tionsinthewatercolumn.asintheBlackSeatodav'enhancethepreservationof alsobecause orguni. rnu,,.., not only becauseof the reductivecharacter'but of abundance an respect, this In life. thSseconditionsare hoitile for benthonic envtronunfavorable an suggest might rock benthonicfossilsin a sedimentarv ment for petroleumgeneration.aspointedout by Hedberg(1964) in e.g"ni. methan; is the singlehydrocarbonwhich ma'v be generated evolutlon' matter of organic stage over the diagenetic abundance
Summaryand Conclusion carbohydrates Diagenesisof organicmatter leadsfrom biopolymers(proteins' lrpids, called collectively geopolymers to iynth"esizedby plants and animals) ;;"li;;l; sediments' ancient in material kerogJn,which is the main organic microorganni'opofyt".t tu.ft as proteiis and carbohydrat€sare.firstdegradedby nutrition' for their used are whtch someof ismsinto individualaminoacidsandsugars, polymerisation and polycondensation by recombines The residuenot usedby microbes transformato form brown compoundscomparableto fulvic and humic acids These tions occur mostly ai or near the water sedimentinterface or hundredsof lnsolubilizationof the previousconstituentsoccursover the first tens and loss polycondensation lncreasing of sediments burial ptogressive rn.i"rr,-auting from leading insolubilization of functional groups are responsrblefor a progressive to humic acidsand finally to kerogen' '-"niog"ni" fulvic t",ft^n" is the singlehydroJarbonwhich may be generatedin abundance diagenesis. during
Chapter3
Geochemical FossilsandTheirSisnificance in PetroleumForqation
3.1 Diagenesis VersusCatagenesis: Two DifferentSourcesof Hydrocarbons in the Subsurface Diagenesisin voung sedimentsresultsin two main organicfractionsof very different quantitative importance: kerogen amovnts to the bulk of organic matter,whereassomefree molecules of lipidsincludehydrocarbons and related compounds. Thesemolecules havebeensynthesized by livingorganisms andget trapped in the sedimentwith no or only minor change(Fig. II.3.l). They comprisespecificcompoundsof relativelyhigh molecularweight(Fig. II.3.2), and can be consideredasfossilmolecu.les, or geochemical /ossi/s.Suchmolecules will be consideredlater in greaterdetail (Sect.3.3ff.) They representa first sourceof hydrocarbons in the subsurface. Kerogenalsoincludeslipidmaterialboundto thepolycyclicnetworkwhichwill be described in the nextchapter.From the kerogen,hydrocarbons aregenerated at depth,duringcatagenesis, as a resultof the thermaldegradationof kerogen (Fig. II.3.1). To a certainextentsomeof thesehydrocarbons alsoresemblethe originalbiogenicstructure,but mostof the newlyformedhydrocarbons are of a more simplestructure.They are metastableunder subsurface conditionsand comprisealkanesand 1- or 2-ringcycloalkanes or aromaticsof low to medium molecularweight(Fig. II.3.2). They representa secondsourceof hydrocarbons in the subsurface. The questionthat immediatelyarisesis whetherboth sources of hydrocarbons contributeto petroleumaccumulations, and which is the main source.Some contributionof the molecules inheritedfrom livingmaterialis well established by the occurrenceof characteristic molecules,like steroids,triterpenoids,etc. in Recentandancientsediments andin crudeoils.Someauthorshaveexpressed the view that the first sourceof hydrocarbons is the main one, and that mostor all hydrocarbons of a crudeoil wereformedat an earlystageof sedimentary history. That opinionprevailedfor sometime amongresearchers engagedin petroleum cxploration,stimulatingmany disputesabout the conditionsof petroleum generation.its depthand time. The alternatives of thisdiscussion were: petroleum generationat shallow depth: diagenesis(first tens or hundredsof meters)versuscatagenesis, at a depthgreaterthan 1000m, - early Eenerationimmediately following deposition of sediment versus/ate generationat a time whena lot of the overlyingreservoirandcoverrocksare deDosited.
94
GeochemicalFossilsand Their Significancein PetroleumFormation Living orgonaEms
C.rbo'
la!nan
li?id.
T
it1.:i.;r + Eeccnt sedimont
A/ t
rorG Principal ol oilforn.tion
l HC
0 il Crude
C,
Zone ol gaslormation
in geological withregardto theevolution Fig.II.3.1. Sources ofhydrocarbons situations, of organicmatter.Geochemical fossilsrepresent a first sourceof hydrocarbons in the (blacksolidarrows).Degradation subsurface of kerogenrepresents a secondsourceof (grcydottedortotes) hydrocarbons
- bactericlonginof petroleum,asmicroorganisms areveryactivein the shallow freshly depositedsediment,vercusthermqLor thermocatalyticdegradationof kerogenat depths,wheremicroorganisms are no moreactive. that havenowled to theconclusion Quantitativeandqualitativeconsiderations are hydrocarbons whichare moreor lessdirectlyinheritedfrom livingorganisms definitelypart of crudeoils, but they representonly a minor part. The bulk is generatedlater at a depth where thermal degradationof kerogenbecomes i m p o r t a n(tF i g .I I . 3 . 3 ) : a) Quuntitativeevaluationson hydrocarbonsin Recent sedimentshave been madeby Smith(1954),Hunt (1961,1972) andBlumer(in: Philippi,1965),andare shelfandopen shownin TableI1.3.1.All thevaluesfor coastalbasins,continental
-1.1Diagenesis VersusCatagenesis: Two DifferentSources of Hydrocarbons
95
d ,/^\,/^\,,\
,,-\,/\/^\./^\.,,\r1?/.\/\
\--..tl.]
|
.'fY .f.0- ' .vw .,oJF'
,\ t
e \../
l
,d
A "d..
Fig. II.3.2a h. Examplesof petroleummoleculesfrom two differentsourcesin the subsurface: Left. (a c) examplesof hydrocarbons and relatedmoleculesof biological origin: (a) n-heptadecane: n-alkaneoccurringin algae,(b) ergostane: steranederiving trom a precursorsterolpresentin veast,(c) isoarborinol:triterpenoidalcoholfrom a tropicalplant. Rtgrri (d-h) examplesof low molecularweight hydrocarbons, without biogenicstructure..-genelat€d at depthfrom the degradationof kerogen| (d) l?-heptane,(e) methvlcyclohexane. (f) methylcyclopentane, (g) toluene,(h) xylene
marine basinsare in the range of 20 to 100ppm (on dry weight basisof rock). The only exceptionsare encounteredin peatsand in some reducingbasinsor trenches close to the coast linc: 100 ppm off the California coast. 100 to 140 ppm in the Cariaco Trench (Venezuela). In deep oceaniccores from the Deep Sea Drilling Project, hl,drocarbon yields range from 10 to 40 ppm (Hunt, 1972).
Composition of HC C4 io C8 n-rlhnrs ctcloplni.ns, ctcloh.r.n. lon il.lY. tllylhlnr. t
i,-/-:-,-,/2/-
=-==_i=={ 7;;=t-= -:'= --.--.--.-: :---n,-;;7::".':V o-';-_::=--._1=:.konltrctn //
vA==t
,--:i 7.:,d:s,,,r,\1
+
I
Anci.ni 3.dimcnl 0ilEGrs li.lds
Fig.11.3.3.Differences ofthe hvdrocarbon contentin Recentandancientsediments. The low hydrocarboncontent of the Recentsedimentsis insufficientto accountfor the disseminated hydrocarbons plusthe pooledoil andgas in ancientsediments
96
GeochemicalFossilsand Their Sienificancein PetroleumFormation
Table. II.3.1. Average valuesof hydrocarbons,non-hydrocarbonbitumen, and organic carbonin Recentsediments
Origin
Type of sediment
Number Extractable Organic AverugeHC AverageOrg.C Author of bitumen carbon (mg/g) (ppm) samples HC
Soils(Texas) Peats(Florida) Freshwaterlake Conlinental (Louisiana) Mud flat (Sabine Pass,Texas) LagunaMadre, Texas Coastal l-ouisiana Califomia (estuarypond) I-ake Maracaibo Mississippi Mississippi Mississippi l-ouisiana PelicanIs (0 100 ft) Orinoco Orino{ic)
-
6
non HC
0 1f(,
1.2 37.O
4l
0.3
2
20 30 800 68
6{i00 1060
20
3 3 10
10
10
iz
3
1.2 0.8
7 l0
0.8
2
65 32 30
255 275 0.s 0.9
40
Philippi(196s)
Philippi(196s) Smith(1954) Smith(1954) Hunt (1961) Philippi (1965) Philippi(196s) Smnh ( 1954) Philippi(1e65) Smith( 1954) Smith( 1954) H u n t( 1 9 6 1 )
0.5 4.9 0.9
6 3
Smith(195.1) Hunt ( 1961) Philippi(1965)
461
0.7
Hunt (1961)
:)/)
1.4
Hunt ( 1961)
t.2
Philippi(196s)
20 10-40 -
4 l6
5
210 709 555
Philippi(1e65) Philippi (196s)
Philippi (1965)
0.4
55 60
1
Deep DSDP (Various marine locations) ChubascoTrench Off W. Africa CariacoTrench CariacoTrench Off California
l4
135
tuo
; Deltaic
Carbonate muds
5 0.9
0.9
80 80
Grandelle (18 s3ft) Open Gulf of Mexim Marine Gulf of Mexico Mediterranean
(%)
Hunt (1972)
0.1-0.3
1.7 23 31 3't6 1 . 5 105 1250 2 . 0 3.4 140 100 2.1
1 2 5 5
Philippi(196s) Smith( 19s4) H u n t( 1 9 6 1 ) Philippi(1965) Philippi(1965)
3.1 Diagenesis VersusCatagenesis: Two DifferentSources of Hydrocarbons
97
Table II.3.2. Average valuesof hydrocarbons,non-hydrocarbonbitumen, and organic carbonin ancientnonreservoir sediments Number Extractable Organic Average HC of bitumen carbon AverageOrg-C Author (ppm) samples (mg/g)
Type of Origin
HC 200 formations lrom 60 sedi menraryoasrns carbonates Rocksof
281
100
from 18 sedimentary basins
shales and silts
-:
151
Hunt (1961)
0.9
Vassoevich(1967)
0.45
(1u67) Vassoevich
0.2
Vassoevich( 1967)
668
860
780 1.82
Institut Franqais
418
930
810 2.lo
du Pdtrole (unpublished)
1260 1220 1.90
shales 118 not specified
Hunt (1961)
lrrdp
orlglns all type!
1.65
340 400 0.18
sllts
Sourcerocks
(%)
very
-
9 source rocks formations
300 600
shales
_ various
791
non HC
315
not 261 specifiedto 2360
440 0.67 0.53 ro 3.67
34 to 101
Philippi(1965)
Disseminated hydrocarboncontentsln anclentnonreservolrsedimentsare shown in Table II.3.2. They average300 ppm in shalesand 340 ppm in carbonates, accordingto H unt ( 1961). ln petroliferous series,valuesobserved by Philippi(1965)in nonreservoir rocksare in the rangeof 300to 3000ppm. From our evaluations,basedon 668 samplesfrom sourcerocksof l8 petroliferous basins,the averagehydrocarboncontentis above800ppm. The hydrocarbonyield of Recent and ancientpetroliferoussedimentsis comparedin FigureIL3.3. It is evidentthat the hydrocarbon conrentof Recent sediments is by no meansableto explainthepresence of oil andgasfieldsplusthe hydrocarbonsof disseminated bitumen in nonreservoirrock of petroliferous sequences. Thermaldegradalionof kerogenduringcatagenesis, resultingfrom burial of sedimentsto a depth level of one to severalthousandmeters,has generatedthe greater part of crude oil hydrocarbons(perhaps907c), and probablyalsoa Iargepart of thosehydrocarbons constitutingnaturalgas. An alternativehypothesis would be the generationof hydrocarbons at depth from the nonhydrocarbon fractionextractable from youngsediments. e. g., acids. However,thesecompoundsare alsobiogenic,and their distributionis roughly comparable to that of the hydrocarbons of Recentsediments. Thus.the qualitative aspects. discussed below,applyaswell to thishypothesis,
98
in Petroleum Formation Geochemical Fossils andTheirSignificance
b) Qualitativecompositionof hydrocarbonin Recentsedimentscomparedto crudeoils,providesa confirmationof the fact that petroleumhydrocarbons are mainlygenerated laterandnot directlyinheritedfrom organisms. Low molecular weight alkanes(Ca-Cs)and cycloalkanes (cyclopentane and cyclohexane) are almostabsentfrom recenlclay and carbonatesediments(Dunton and Hunt, 1962:Erdman. 1965),althoughvery smallamountshavelocallybeendetected in marinecoresamplesof shallowdepth(Hunt, 1974).The sameappliesto low molecularweightalkyl-benzenes (Erdman,1958,1961, and alkyl-naphthalenes 1965).All thesemolecules areimportantconstituents ofcrudeoilsandofancient rock bitumen.even in Upper Tertiarv bedswhere sufficientlyburied. A few polynuclear aromatics suchasperylene.chrysene andfluoranthene arepresentin Recentsediments.with little or no substitution,whilstcrudeoils showa wide varietyof substituted polyaromatics (Aizenshtat,1973;TissierandOudin.1973). Hydrocarbonsalreadypresentin Recentsedimentsare the sourceof a verv minorfractionolcrude oils.Nevertheless, theyareofgreatinterestto us,asthev represent theheritagefrom.livingorganisms andmaybe regardedasgeochemical generfossils.beinghelpfulie definingbiologicalenvironments. Hydrocarbons atedlater, at depth.throughthermaldegradation of kerogen,are the sourceof the bulk of crude oils; however. they are likely to provide only limited informationon the originalorganicmaterial.
Directlyor 3.2 Hydrocarbons Inheritedfrom Living Organisms, (Biological Throughan Early Diagenesis: Fossils Geochemical Markers) The occurrence of hydrocarbons andcloselyrelatedsubstances is knownfrom a wide rangeof living organisms:manyof them havebeenfound in Recentand ancientsedimentsand in crude oils. Treibs (1934),in his pioneeringwork, identifiedporphyrinsin crudeoils and suggested that they may originatefrom chlorophyllof plants.Smith(1952,1954,1955)showedthat somehydrocarbons areincorporated in the Recentsediments of the Gulf of Mexico.Sincethat time, numeroustvpesof hydrocarbons or substances with a similarcarbonskeleton havebeentracedfrom the livingorganisms into Recentsediments: for example Erdman(1958and 1965)studiedlipids,sterolsandcarotenoids in theMississippi delta,in marshesand in offshoresediments of SouthernCalifornia;Kidwelland in Recentsediments of Hunt (1968)foundsaturatedand aromatichydrocarbons (1959,1961)reportedsaturated the Pedernales areain Orinocodelta;Meinschein hydrocarbons, includingsteranes andtriterpanes, andpolycyclicaromaticsfrom gaseous and Iiquid sedimentsof the Gulf of Mexico;Emery(1960)investigated hydrocarbons in sedimentsof SouthernCalifornia;Blumer et al. (1963,1964) isolatedpristanefrom Recentmarinesediments. andtracedit backto the phytol chainof chlorophyllthroughzooplanktonfed on phytoplankton.Many others. or closelyrelated sincethat time, have found varioustypesof hydrocarbons molecules in youngsedifients. Suchmolecules maybe derivedfrom terrestrial(mostlyplants),marinepelagic (mostlyplankton).marinebenthonic(algae,bacteriaand other microbes),or
3.2 Hydrocarbons lnheritedftom LivingOrganisms: Geochemical Fossils
fl.
99
P,"aurro,= fossilmolecule n C29H60
'"'""'..J,QjPY ,A.YJ b.
Pr""ur"or'ffii
fossilmotecuta
- - - - - - -cool-,,'\.,,^\,..\,^,^.,.\. n C15N32
n Cl6lallr acrd
,.\f"'-'oot
, r-r)
.YY" 'vT'... H
)r-' a , at1
arr" ...,.^'-.'r ar^+-\
*W
"n-* '-
C h oe . t l r o
'
,."
.'/
Yw
/^\2\
|
.vW \-+._.J I
Lholpllpne .o q q,d,
lY-Y
IY\ \-\_J
Fig.IL3.4aandb. Examples of geochemical fossils(a) molecules of biologicorigin. withoutchemical (b) molecules alteration: (Occurrences inherited withminorchanges. reportedby Albrechtand Ourisson,1969;Seifert,1973;Rubinstein et al., 1975; Spyckerelle, 1975) limniclife. The carbonskeletonof the molecules is preserved andtheycaneasily be linked to somemajor structuraltypes:steroids,cyclicor acyclicterpenoids, etc. They stemeitherdirectlyfrom livingorganisms, i. e., without anychemical alteration,or indirectlywith only minor changeswhich occur mostly during diagenesis in the youngsediment:lossof functionalgroups,stabilization of the moleculeby hydrogenation, aromatization, etc. (Fig.II.3.4). We proposeto use the expression"geochemical fossil" to designatea moleculesynthesized by a plantor an animal:the moleculebeingunchanged or havingsufferedonly minor changes. with preservation subsequent of the carbonskeleton.Thesemolecules "biolosical are alsocalled markers".
100
Geochemical FossilsandTheirSignificance in Petroleum Formation
At depth,the biogenicmolecules suffernot only thermaldegradation, but also dilution with newly formed hydrocarbonsresultingfrom kerogendegradation. However,identificationof the moststableinheritedmolecules remainspossible for a longtimeanddepthspan,i. e., intotheancientsediments andcrudeoiison rhe followinggrounds.Silverman(1967)andWelte(1969b, 1972)havedemonstrated by meansof isotopemeasurements andopticalrotationthatthe fractionofcrude oils boiling between400 and 500'C, which corresponds to molecularweights between350and 450,hasthe highestconcentration of thesefossilmolecules.It includes,in particular,tetracyclic andpentacyclic structures with 26 to 35carbon atomsand n-alkanesin the samerangeof molecularweight.The geochemical fossilsare usuallyrestrictedto a smallnumberof predominantmolecules of each type. On the contrarv,moleculesissuedfrom kerogendegradationare mostly abundantin the low to mediummolecularweightrange(below25 carbonatoms) andcovera wide spectrumof eachstructuraltype, In somecasescertainclasses of compoundsof biogenicorigin,e. g., straightchaincompounds, isoprenoids, terpenes,sterolsor porphyrins.maybe linkedto the youngkerogenby relativelyweakbonds,andlaterreleased duringthe initial stageof kerogendegradation(seeChaps.II.3.13and II.5.4). The occurrenceof biologicalmarkersin sedimentsand crude oils and the comparisonwith moleculessynthesized by Iivingorganisms hasbeendiscussed manytimesandpartrcularly by EglintonandCalvin(1967),Maxwellet al. (1971). Blumer(1973)andWhitehead(1973).The laboratoryandfield simulations may help to determinethe fate of biogeniccompoundsin geologicalsituations (Eglinton,1973).The mainclasses of compoundwill be considered successively, with respectto origin and occurrence of specificmolecules.
3.3 n-Alkanesand ,x-FattyAcids J.J.0 Varioussourcesof linearaliphaticchainsin organisms havealreadybeen discussed (Chap.I.4.4).The distributionof thosemolecules bearsthe imprintof their biochemicalsynthesis,i.e., predominanceof ceftain medium to high molecularweightcompounds with specificcarbonnumbers,Iike fatty acidswith an evennumberof carbonatoms.or n-alkaneswith an odd numberof carbon atoms.The preservationof such molecularfeaturesin ancientsedimentsis commonlyobserved, althoughit is progessively obliteratedwith increasing depth of burial and age. A statistical studyon the occurrence of odd andevenpredominance of higher n-alkanes(> C2n)basedon 1302ancientsedimentand crude oil samplesis presentedin TableII.3.3, accordingto Tissotet al. (1977).From the tableit can be seenthat the existenceof an odd-carbon-number predominance is rather common.An evenpredominance is lessfrequent,but by no meansexceptional. 3.3.1 Odd-carbon-numbered n-alkanesof high molecularweight (n-C25-n-C4) are frequentlyreported from Recent detrital sedimentswith an important contributionof continentalrunoff andcomprisingboth claymineralsor siltsand
3.3 r-Alkanes and n-Fatty Acids
101
Table II.3.3. Odd and even-carbon-number predominances of higher n-alkanesin sediments andcrudeoils.(Data from Tissotet al., 1977)
Age
Paleozoic and Triassic
Jurassic
Cretaceous
Tcrtiary
All ages"
Number of samples
231
lu5
589
289
t302
odd
Modcrate 38
predoml nance
Strong
124
predomlnancc Strong
l1
l5
' ll samples:age unknown.
organicmaterialfrom plants(Fig. I1.3.5).They havebeenfound by Bray and Evans(1961)in variousRecentsedimentsfrom offshoreSouthernCalifornia. bays and continentalshelf of the Gulf of Mexico and freshwaterlakes.The predominanceof moleculeswith an odd number of carbon atoms can be measuredby the "CarbonPreference Index" (CPI), i. e., the ratio,by weight,of odd to evenmolecules2. The ratio variesfrom 2 to 5.5 in the varioussamoles studiedby these authors.Other studiesof Recent sedimentsfrom offshore Louisiana(Meinschein,1961),San FranciscoBay (Kvenvolden,1962)and in numerousother locationshaveconfirmedthesefisures. 2 Bray and Evans (1961) computed the CPI over the C2a-C31range, i.e.,
+ cet- lf lusrI cr' -r c'" c'' * <:- ' 2 C2" C2s r * lc-a
Cr,j
Crz
c2s+c27+c2e+cI+cal c26+c2s+cro+c32+cr4 Philippi( la65) prclcrredrn u\( lhe ralio 2 Czs Scalan and Smith (1970) introduced another coefficient, which is a sort of moving averagc,centeredon C;12, and showingthe variation of the odd even preferencewith the increasingmolecular weight
I c , r r ' c , - ? c , , ,l,r r t ' [
4 C , .| + 4 C , . , j
102
GeochemicalFossilsand Their Significancein PetroleumFormation
BlockS€o1462K (0.55m) E
- Block Seo (055rn) 1462K --- l{orregion Seo
level) XL8(Surtoc€
"-9 .9
* T
3
8
232726232423 22 21 20 €
18 17 1615
32 31 3029282726 25 2423 22 2t 20 19 18 17 16
AllonlicOceon(oft i,louriiooio) (O5Om) 12392-1
l,Junber pernolecul! ofcorbon oloms
-Alloniicoceon
(oftMouritonio)12392 1(050n)
-BolticSeo 0 8 5(92 m )
ze zt 26 25 24 23 22
BollicSeo0B 59 (2m)
33
3 2 3 1 3 0 ? 9 z 8 n 2 6 2 5 2 42 3 2 2 ? t n E $
14 16 18 20 22 24 26 28 3a l,lumberof corbonoloms0ermolecula
Fig. IL3.5. Dist butionsof n-alkanes in Recentsediments of variousorigin.Gas (/e/t)showing chromatograms of saturated hydrocarbons thepeaksofn-alkanes, withtheir (right). carbonnumbers.n-Alkanedistributioncuryesde vedfrom gaschromatograms (AfterDastillung. 1976,Debyser et al., 1977) Thesehydrocarbons arederivedfrom cuticularwaxesof thecontinentalhigher plants.Odd-carbon-numbered n -alkanesmay be eitherdirectlysynthesized by plants the or derivedthroughan early(iagenesis(defunctionalization) from the even-numbered acids,alcoholsor esters.When both marine and continental
3.3r-Alkanes andr-FattyAcids
103
organicmatter are incorporatedin a sediment,the continentalcontribution usuallvdefinesthe n-alkanefingerprint,especially in the C2qC3jrange.Thisis dueto a higherproportionof the n-alkanesin continentalorganicmatterthanin marineplanktonicmatter.This explainswhy so manyRecentmarinesediments displaya "continental"fingerprint(Fig.II.3.5). However,the observation is not necessarily valid for areasof pure carbonaledepositionwithout terrestrial contribution,asno odd-carbon-number predominance (withveryrareexception) is found in the alkaneshigher than n-C1sfrom contemporaneous marine organisms (Koonset al., 1965;ClarkandBlumer,1967).As a result,Powelland McKirdy (1973)haveshown$e absence of odd-carbon preference in that range of molecularweight.evengjxhallowdepth,in Tertiarycarbonate serieswithlittle or no contributionfrom continentalplantsdepositedaroundAustralia. In ancientdetritalsediment(Fig. IL3.6), the importantpredominance of the originaloddn-alkanesof highmolecularweightis preserved in relativelyshallow shalesandsilts,wheredegradation of kerogenhasnot yet started,andwherethe hydrocarboncontentremainslow: in a Lower Cretaceous (Aptian) shalefrom N .A q u i t a i n eB a s i n
Los Angeles Basin P h ipl p i , 1 9 6 5
u
30 25 20 15
l 5 l 8m
10 5
!
: ?
:
t
Niger delta 9
0 t5
6 I pl 2219m
i
E
3
:
2460m
*19
]
15
15
zttr
I
Cr33 3332m
S e a lS e a c h 3290m
15 20 25
15 20 25 30 35
15
Corbonoiomsper molecu
Fig. II.3.6a-c. Changeof z-alkanedistributionswith depth (a) Recent and fossil sediments from Nigerdelta.Samples noted1200and2300m arefrom theancientTertiary delta.Valuesarerelatedto organiccarbon(pelet.1974).(b) UpperMioceneto pliocene, Los AngelesBasin,California.Valuesare expressed as percentof n-alkanes(philippi, 1965).(c) Lower Cretaceous. North AquitaineBasin,France.Valuesare expressed as percentof r-alkanes(Tissotet al.. 1972)
104
Geochemical FossilsandTheirSignificance in Petroleum Formation
1518m depthin North Aquitainebasin,r?-C27 is still five timesmore abundant than n-C26or n-C26(Tissot et al., 1972)1in an Upper Cretaceousshalefrom Gabon,WestAfrica, at 1155m depth,n-C2eis four timesmore abundantthan z-C4 or n-C3e;in a fosstlEquisetumof Triassicage,Knocheand Ourisson(1967) found that n-C2j, n-C25,n-C27(predominant)and n-C2ewere the four main components of the hydrocarbons in almostthe sameproportionsas they are in contemporaneousEquisetum. However, such an important predominanceof severalsuccessivg odd n-alkanesseemsinfrequentin rocksof that age.It is more usuallyencoun/eredin Lower Cretaceousand youngerbeds,on the basisof our observations on 1302samplesfrom 18sedimentary basinsin TableII.3.3 (Tissot et al.,1977).This fact may be due to an increasing maturityof older rocks,but may also derivefrom an increasein diversityand abundanceof higherplants associated with the development in the Lower Cretaceous of angiosperms time. Also, it is perhaps favored by the occurrenceof important detrital seriesin continentalmarginsduringCretaceous and Tertiary. Furthermore,odd n-alkanesderivedfrom higherplantsoccuralsoin oilshales like the GreenRiver shales(Robinsonet al., 1965),the veryshallowMesseland Bouxwillershalesin the Rhinevalley,or the M6natshalesin CentralFrancewith CPI from 2.5to 30 (Albrecht.1969;Arpino, 1973).An odd-carbon preference of the n-alkanesis alsoknownfrom lignites(Arpino, 1973)andcoals(Welte,1967: Leythaeuser and Welte. 1969;Brooks,1970). In crudeoils, the high molecularweightn-alkanesinheritedfrom terrestrial plantsare normallydilutedby hydrocarbons from kerogendegradation, andthe CPI is around1.0.However,someoils,probablyderivedmainlyor solelyfrom L o w e r C r e l o c e o u cs r u d eo r s M o q e l l o nB o s i n ( S . A m e r i c o )
-
non moflne crudeo ,
L o w e rC r e l o c e o u s , W e sAl f r i c o ----
no. mofl.e crude oil,
Eoce.e-Uinlo Bosin
t
+
+
-
s 6
=
I
30
15 20 25 ---------------per Corbon oloms molecule
30
Fig. II.3.7. n-Alkanesdistributioncurvesof threecrudeoils de ved from continental plant material,probably extensivelyreworkedby bacteia. Onthe left, a commontype of crude oil, derived from marine organic matter, is comparedto the nonmarinecrude oil occurrinsin the equivalentformationof the samebasin
-1.3/,-Alkanes andr-FattyAcids
105
terrestrialorganicmaterialwhich has been reworkedby bacteriaand other microbes,still showa largeamountof high molecularweightn -alkanes(> C2e) FigureII.3.7 showssomeexamplesof such with a moderateodd predominance. from oils found in Lower Tertiaryof the Uinta basin,USA, Lower Cretaceous from West partsof the Magellanbasin.SouthAmerica,and Lower Cretaceous Africa. 3.3.2 Odd-carbon-numberedn-alkanesof medium molecular weight, mostly n-C15and n-C,1, rlay also representin somecasesa direct heritagefrom the prdsentin algaeand from the relatedacids.In ancientrocksthe hydrocarbonq distinctionis possibleonly when n-C15and/or n-Ctt are largelypredominant n-C'oandn-C1s(Fig.IL3.8). Thissituationhasbeenfoundin comparedto n-C1a, rocks from the coastalbasinof West Africa (Spyckerelle, Lower Cretaceous 1973),in someDevonianbedsand in the ColoradoGroup of the northeastern fromthe flankofthe westernCanadianbasin(Alberta),andin the UpperJurassic northeastern shelfof the Aquitainebasin.Most of theselocationsareconsistent with the hypothesis of a shelfthat is coveredwith benthonicalgae.In upperbeds of the Green Rirer shales(Mahoganybeds), n-C'7 is the main n-alkane constituentin the mediummolecularweightrange.
, I
U i n t oB o s i n Eocen€
' - 1 I
9 1 5m
?425 m
p esr m o l e c u l e Corbonolom
Fig. II.3.8. r-Alkane distribution curvesobservedin Eocenesedimentsof the Uinta Basinand Lower Cretaceous of West Africa. The predominanceof n-C15and n-C11is derivedfrom algal contribution.The organicmatterfrom Uinta Basinshowsa mixtureof r-alkanesfrom algae(Crs, Cri) and heavier r-alkanes from higher plants (C2?, CC:q)
GeochemicalFossilsand Their Sienificancein PetroleumFormation
106
Fig. 11.3.9.Predominance of odd numbered,-alkanesof mediummolecularweightin two crudeoils. (After Martin et al., 1963)
JohnCreek
:
g
P i n eU n i t
6
5
r
0
1
5
2
0
2
5
3
0
3
5
- --) C o r b o no l o m sp o r m o l e c u l e
agefrom the UnitedStates.Martin In severaloilsof loweror middlePaleozoic of the odd et al. (1963)found an unusualdistributionwith somepredominance n - a l k a n efsr o m C 1 1t o C ' , ( F i g .I I . 3 . 9 ) . predominanceare reported from 3.3.3 Severalcasesof even-carbon-number carbonateor evaporitesediments andoccasionally from crudeoils(Fig.II.3.10). and Paleoceneand Eocenecarbonatesfrom SouthTunisiashow a systematic importantpredominance of evenn-alkanes from n-C,6to n-C30.Similarobservations have been made in some other sediments:Zechsteinfrom NW Germany (Welte and Waples,1973),a few samplesof the Upper Jurassicfrom north Aquitaine basin(n-C26to n-C1s),Corbiculashalesof Oligoceneage from the Rhine valley (Welte and Waples,1973).A few Tertiary crude oils from the Mediterraneanarea show the samecharacter,but this is rather infrequentin crudeoils: Amposta,Spain(Albaigesand Torradas,1974)and Greece(Tissot et al., 1977).Douglasand Grantham(1974)reporteda similardistributionin Wurtzilite.a nativebitumenfrom the Uinta basin. with a high phytane: This particulartype of distributionis usuallyassociated pristaneratio,whereasan equalabundance of phytaneandpristaneor a pristane predominance is much more frequentin other sediments.Welte and Waples (1973)suggestthat in very reducingenvironments, reductionof n-fatty acids. alcoholsfrom waxes,and phytanicacid or phytol is prevalentover decarboxylaof even-carbon-numbered n-alkanemolecules tion, resultingin predominance (CPI predominance phytane < of over pristane.In over odd molecules 1) and a of odd results in a majority less reducingenvironments,decarboxylation of pristaneover phytane.A typical n-alkanes(CPl > 1) and a predominance exampleof reductionis givenby Knocheet al. (1968),who foundin a fossilplant Voltzia (Coniferules)a hydrocarbonfraction consistingalmost wholly of one
1.3 r-Alkanesand r-Fanv Acids
t ;;::;
I
107 Fig. tl.3.l{1. Predominanceof even /l-alkanes in rockextracts from theLower Tertiarv of Tunisiaand thc Rhine Vallev.andin a Miocenecrudeoilfrorl Greece. y.,/aicd1ba,'s are isoprenoid hvdrocarbon distributions
( , R h i n €q r o b e n ) o qoce"e 9?1n
I
-
r :3;;"j"" Corbon o i o m sp e rm o l e c u l e
component:n-C1rH.r.In waxesof closelyrelatedlivingplants.a-C,8H5s is never an importantcomponent,but the n-C:salcoholis prisent and could be the p r e c u r \ otrl l t h e r r - C . n llk;rne. Shimoyamaand Johns(1972)proposedan alternativeinterpretationof the e v e nn - a l k a n ep r e d o m i n a n c e T .h e y c a r r i e do u t e x p e r i m e n t adle g r a d a t i oonf n-fattyacidsin the presence of montmorilloniteor calciumcarbonate.The two mrneralsseemto havedifferent catalyticeffect: decarboxylationwith lossof onc ciubonatom is favoredbv montmorillonite, whilebetacleavage with lossof two carbonatoms is lavored by calcrumcarbonate.Therefore,an originalevencarbon-numbcred fattv acidwouldgeneratemostlyan oddn-alkanein shalesand a n e v e nr r - a l k a nien c a r b o n a t e s . Actualll. the two interpretations proposedare not conflictinA,asthe mechanismproposedbv Weltewouldoccurduringdiagenesis in youngiediment,whilst the interpretationof Shimoyamaand Johnsrefersmostly to the caragenenc transformations occuringat depth.However,two remarkssuggest that reduction is a morefrequentprocessthan betacleavage: - the comparisonbetweenthe fossilplantand the presentequivalent, madeby K n o c h ee t a l . ( 1 9 6 8 ) . - the association of even-numbered n-alkaneswith the C1 isoprenoid(phytane) derivedfrom the C.ualcohol(ph)'tol),and not with the C,,1isoprenoid.
108
Formation in Petroleum FossilsandTheirSignificance Geochemical
In Recent sedimentsa certaineven predominancelimited to C16and C1s n-alkaneshas been observed(Simoneit,1975;seealsoFig. II.3.5, Norwegian doesnot extendto highermolecularweights,asheavier Sea).The predominance inheritedassuchfrom plantsareprevalentoverthoseissuedfrom oddn-alkanes. an eliminationof the functionalgroupof acidsor alcoholsin youngsediments arc an important 3.3.4 Long-chainn.alkaneswithoutodd or evenpredominance Paleozoicto ranging from lower parl of orgafti€rnatterin someancientsediments, fraction of many importanl constitute an Tertiary(Tissotet a!..1977).They also extends often distribution of n-alkanes (Hedberg, 1968). The high-waxcrudeoils other microbial from bacterial and probably derived They are up to C$ or C.o. with a waxes.possiblyfrom reworkinghigherplantwaxes.They are associated whicharetypicalof a bacterialorigin(seeSect. seriesof iso-andanteiso-alkanes bacteriaandyeast, non-photosynthetic 3.4):FigureII.3.11. In contemporaneous with morethan 20 carbon Han et al. (1968)foundthat long-chainhydrocarbons Important atomsamountto more than 50oloof the nonaromatichydrocarbons. of bacterialbiomasspossiblyresultedfrom degradationof large accumulation quantitiesof higher plants in paralic or intracontinentalbasins.such as the Tertiaryof lndonesiaor the Eoceneof the Uinta Basin. 3.3.5 n-fatty acrds have been identified in numerousRecent and ancient sediments.When large proportionsof terrestrialmaterialaccumulatedin a restrictedlimnicenvironment,n-fattyacidswith a rangefrom C24to Ci: andwith predominance arefound.Thisis identicalwiththe range an even-carbon-number for this oi piantwaxes.MesseiandBouxwillershales(Arpino,1973)areexamples acids main fatty the subordinate, kind of facies.Wherecontinentalrunoffis only ln organisms of marine from fats probably derived rangefrom C1ato C22andare g., Ct8 e C16. predominance, an even-carbon-number thairange,theyshowagain mudsfrom the PersianGulf (Welteand C1 in Recentcalcareous andsometimes Tanner basin,off southernCalifornia from the in muds Ebhardt. 1968),Cr6 (Kvenvolden,1970).A singlefatty acidmay evenbe the majorconstituent'such 1973)Usuallybothmarine from Gabon(Spyckerelle, asCr6in LowerCretaceous contributeto then-fattyacidsdistibution.Thisresultsin sources andcontinental two ranges,C16-Crg(from marine or continentalsource)and C'1-Cp (from continenlalsourceonly). This is the casein Recentmudsfrom the SanNicolas delta,in carbonatemudsfrom basinoff southernCalifornia.from the Mississippi (Simoneit' the Floridabay (Kvenvolden,1970),and in manyoceanicsediments
197s).
wherethe evenpredomiThis situationis alsofrequentin ancientsediments, evolution.In rocksolderthan Lower with catagenetic nancenormallydecreases the secondmode of r-fatty acids(C2a-C,r)is poorly represented Cretaceous, This is the caseevenin sampleswhereC,a,C16,C' are abundant,with an even: odd ratio of 3 or 4, indicatingthat thermalmaturationhasbeenonly moderate' This siruationoccurs.for instance,in the PermianBone Springlimestoneand Lamarlimestonefrom Texas(Kvenvolden,1970)As a similarsituationoccursin the accumulation respectto n-alkanes,we canextendour previousobservation: predominance, but or even with an odd of not only long-chainC1-C,j n-alkanes only range, occurs same carbon also even-numberedfi-fatty acids in the
3.4Iso-andAnteiso-Alkanes
109
occasionally in immaturesediments olderthan Cretaceous. Thisis againrelated to lhe evolutionof higherplants.On the contraryC1ato C26n-alkaneJand ,?-fatty acids, which are mostly derived from plankton and benthonicalgae, are presumedto have been abundantlyavailablesinceprecambriantime. Indeed their predominance is foundin sediments rangingin agefrom lowerpaleozoicto Recent. 3.3.6 .Comparisonbelweenn-alkaneand n-fattyacid distributions.On comparison, the distributiohof n-alkanes,free fatty acids,and fatty acidsbound to kerogenin youngsedimentsare generallyratherdifferent.The distributionof n-alkanes is usuallydominatedby odd-numbered C2aC33 alkanes.The fattyacids boundto kerogencontainmostlyC,5-C1s, whereasfree fatty acidsrnavinclude b o t h r a n g e so f m o l e c u l aw r e i g h r .b u l r h e y a r e c o m p a r a t i v e Il ye s sa - b u n d a n t . Obviously.a certainamountof n-alkanesis directlyinheritedfrorrrorganrsms, becausethey are known to occuras suchin cuticulir waxes,for example.The balanceis likely to be derivedfrom acidsand/oralcohols,boundsomihowto kerogen.The alkanesreleased from kerogenmaybe of variouschainlengthsand includeabundantalkanesof mediummolecularweight(< C1).
3.4 Iso- and Anteiso-Alkanes Iso- (2-methyl-)and anteiso-(3-methyl)alkanes are associated with n-alkanesin plantwaxes,wheretheycomprisea comparable numberof carbonatoms(about 2-531) with an odd predominance. A distributionof isoalkanes comparableto their occurrencein plantshas been reportedby Arpino (1973)in an Eocene sedimentfrom the Rhine valley.The iso- and anteisoacids are also known in bacterialwaxes,wheretheycovera widerrangeof molecularweight.In thelower part of the GreenRiver shalesfrom the Uinta basin,2-methyl-and 3-methylalkanesform a homologous series,withoutoddor evenpredominance, in theC2jC3xrange,sometimes extendingup to Ca5.Theyareprobablyof microbialorigin. The samecompoundscommonlyoccurin the high-waxcrudeoils (Fig. II.3.11). Iso-andanteiso-fatty acidsof lowermolecularweightrepresent a largefraction of bacterialacids,whereastheyaremuchlessabundantin higherorganisms. They have been reported in the lipids of marine organismsand also in Recent sediments. The relatedisoalkanes werefoundby Johnset al. ( 1966)in ancientoils andsediments. and their occurrence mav be ratherfreouent.
3.5 C16-branched Alkanes SeveralC,n-isoalkanes whichare relativelyabundantin someancientsediments andcrudeoils,suchas2,6-dimethyloctane and2-methyl-3-ethylheptane, maybe derivedfrom the terpeneconstituents of essentialoils frorn higler plants,as suggested by Mair (196a)and Mair and Ronen (1966, 1967);(Fig. IL3.12). Althoughthey are particularlyabundantin the poncaCity crudeoil (0.55and
110
Geochemical FossilsandTheir Significance in petroleumFormation 2
+t :€
E=ul i,. :SEEEE{ I P
t 6
o o
b f o z
<1-
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l----__]:l?H II t : i ;EEt-l
E
- 1 _ 1= 3
|
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<-
3 g p e l o r n l o1s0 7 .t q 6 ! a r $
.9
:
:
t
:
o
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;Es
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.9
:
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il
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I r
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o
o
o
se
i - , . -=3. i a-t
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l
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s+!
E=+{ ll .l I
t
l
"
l
E
-
o
lH pe,on.oslr % tq6.[
_
Fig. 1I.3.11.Examplesof high-waxcrudeoilsprobablyderivedfrom continental organic matterextensively reworkedbv microorganisms. High molecularweightn -alkanesare abundant,but they do not show a marked odd or even predominance.Note the homologous seriesof 2-methyland3-methylalkanesidentifiedby solidcircleson the gas chromatogram of the iso-cvcloalkane fraction(boftom).A normalcrudeoil from Paris Basinis shownfor comparison
u1
3.6 AcyclicIsoprenoids ilaturalterpenes+
a\
\,ctrron
A'
Geraniol , 9q eI r l 0 r l o ir o s € n
Fossilmolecutes
Fig. IL3.12. Possible origins of Cp branchedalkanesfrom terpenesof essential oils
' - A 2,6-dimelhyloctane
I
-
O -\ Lrmonene l L e m 0 n , 0 r a ln g e
) U I
2methyl-3ethylheptane
0.64%volumerespectively) theyhavenot beensoughtin a sufficientnumberof differentcrudesto appraisetheir valueasgeochemical fossils.
3.6 AcyclicIsoprenoids A wide range of linear or cycliccompounds.formally built up from several isopreneunits,is knownin livingplantsand bacteriaand to a smallerextentin animals.They occur as hvdrocarbons.alcoholsor other derivatlvesor rn a
,i,
.rIl ri2
Ln
l \ n f , r c n su n r t id! r
combinedstate,suchasesterin the phytylchainof chlorophyll(seeChap.I.4.2) or glycerolethersin bacterialmembrane.In sedimentsand crudeoils, similar structuralarrangements havebeenfrequentlyrepofted,but theyoccurmostlyas fully saturatedaliphaticor alicvclicmolecules, or theymay includeone or more aromaticcycles.Polycyclicstructures will be dealtwith in Sections3.7 and3.8. Generally,in geochemistry the term "regularisoprenoid"is restrictedto an acyclic.branched,saturated moleculewith a methylgroupon everyfourthcarbon atom,irrespective of the totalnumberof carbonatoms:sucharrangement implies a *headto tail" linkageofthe isoprenegroups.The bestknownandmostcommon isoprenoids arepristane(2, 6, 10,14tetramethylpentadecane) andphytane(2,6, 10, 14 tetramethylhexadecane). In additiona wide spectrumis alreadyknown from Ceto C25,and othersmight be detectedin rhe future (Fig. II.3.13).Aside from the regularisoprenoids, the term "irregularisoprenoid"is usedfor some
1t2
FossilsandTheir Significance in PetroleumFormation Geochemical
,,\,^-A-^../...,-
C 1 5( F a r n e s)a n e Fig.II.3.13. Principalmolecules
,,(.,^_\r^_)..",,.'.
cl6
of regularisoprenoidhydrocarbons present in ancient sedimentsandcrudeoils
C l 8 { N o f p r i s t)a n e j.=^-J...
c19 (Pristane )
-1.,..^.-f..-,.---J.-,.....-t-
c20 {Phvtane )
,(,^'r\r--
moleculeswith "head-to-head"or "tail-to-tail"linkage,suchas squalaneand where lycopane,whichareknownto occurin certaingeochemical environments, they are probablyderivedfrom squaleneand lycopene(Chap.I.4.2. Figs.I.4.9 a n dI . 4 . 1 2 ) . In livingorganisms, the mostwidespread sources of isoprenoidchainsseemto be the Cl phytol,a diterpeneboundthroughan esterIinkagein the chlorophl'll They molecule.andC15,C26,Cj6andCoqisoprenoids from bacterialmembranes. will be successively discussed below(a) and (b). Non-polarlipids of methanogenic bacteriaalso includea Cr. irregularisoproductsup to squalane (Holzer prenoid,the Clnsqualene andits hydrogenation (hydrocarbons, alcohols, el al.,1979).Otheracyclicterpenesandsesquiterpenes aldehydes. ketones)are alsowidelyknownin plantsandflowers.Sesterterpenes are rather rare (Spyckerelle,1973),but squalaneis an acyclictriterpenefrom sharkliver oil, and lycopeneis an acyclictetraterpene from tomatoesand other fruits. In additionto theseolefins.Clark andBlumer (1967)foundpristane(C'e isoprenoidalkane)at a low concentration in marinebenthonicalgae,belongingto the Laminariales, and in planktonicalgae.Zooplanktoncontainsan assemblage of pristane,phytane(C'eandC1 isoprenoid alkanes)andvariousrelatedalkenes. probablyderivedfrom phvtolof the chlorophyll like zamenesand phytadienes. throughfeedingon phytoplankton(Blumerand Thomas.1965,1971). In Recentmarinesediments,pristaneand phytaneare present,their abundancedependingon the localenvironment.Isoprenoidacidsand an isoprenoid alcohol.dihl'drophytol,havealsobeenidentified.In ancientsediments,crude oils and coals. isoprenoidsfrom Ce to C:. have been identifiedin various concentrations. They generallyseemto be more abundantin marinesedimenls where comprisingautochthonous organicmatter than in nonmarinesediments of Canada,eastern organicmatteris of terrestrialoriginonly (LowerCretaceous SouthAmericaandWestAfrica). The lowercarbonnumberCe-Csisoprenoids thatrangeof havebeenscarcely reported( Gohringet al., 1967),possiblybecause molecularweightis not frequentlyanalyzedon rockextracts.The mediumC15to C25isoprenoidsare presentin most sediments,includingPrecambrian,and oils: Bendoraitisand Hepner (1962),Welte (1967),Han and Calvin (1969), Spyckerelle et al. (1972),Wapleset al. (1974).The p staneCre,phytaneCr6,and sometimes the C'6 isoprenoid,are the mostabundant,whereasC,1,C22andC2a are scarce.Pristaneand phytaneare also suspectedto occur as mixturesof
3.6Acycliclsoprenoids
ll3
stereoisomers in severalrocksand crudeoils (Maxwellet al.. 1972).Isoprenoid acidshavealsobeenidentifiedin few sediments suchas GreenRiver shalesor Serpianoshale. a) Isoprenoids from a PhytolSource The phytol side chain of chlorophyllis a widespreadsourceof isoprenoid structures in the biosphere. The stereochemistry ofisoprenoids < C,6andrelated acids in sedimentsis compatiblewith an origin from the phytyl chain of chlorop{(Maxwell et al., \912, 1973:'Cox et al., 1970,I9i2). Animals, particularly2ooplankton,feed on plants,and seemable to produceseveral
Phytol
m
l
l
lWi\,,'crr,zf-oH R
_,..
i H
Phylol l*0r
i -
P h y l e no icci d l-COr
t -
Prislene
l.tt,
a
t Prislone
CHr I
rl'
R-CH2-C=CH-C _;H CHr l-C=CH2 R-CH2
Prisr.n. ---l-
=t-:r=l:o!9!L\/!9!2(t3!!: =l+H.::r.."""--: =l-:
CH" R-CH2-CH-CH3
P'i"i;-.:
W a t c rw i t h d i 3 s o l v e 0 d2
Phylol
'\t, \ R'CH2-C.CH- CH20H
liYaicrwith H?S
I
CHr t-CH-CH2-CH20H 0ihydfophylol R-CH2
*-"
:
l-rro i.Ht
Phylone
CHe tR - C H 2C- H - C2H- C H 3
X r r o g o n: E
Frs. 11.3.14aand b. Reductiveand oxidativepathwaysfrom phytol to Cre and Cro :rrprenoids. Phytoldiagenesis initiatedin the presence of oxygen(a), in the absence of : \ \ s e n( b )
114
Formation in Petroleum andTheirSignificance Fossils Geochemical
saturatedorunsatulatedisoprenoidhydrocarbonsfrom.thephytylchain'as (Blumer and "i"".i U" the occurrenceoi these moleculesin zooplankton with isoprenoids of itti, putn Irruyu. u *ay of direct introduction ii".."t,'iqo-il. 20 or lesscarbonatomsin sediments from chlorophyll Anotherpossibility,however,is the incorporatiorof phytol into hydrogenated quickly phytol is in a vervreducingenvironment, in seclimenti: ( p h y t a ne) h y d r o c a r b o n i n t o t duclion O i tr , t r o o t ' v t o lf ,o l l o u e db y a i u b s e q u e nr e i s f o l l o u ed a c i d p h l t a n i c t o o x i d a l i o n ;iygenate,l'en,ironment. i;';;;fti. processes < C'e) (isoprenoids fragmentation (pristane)or t,n O..utt,o*ylotio"n the more or lessreduiing characterof lhe local environment' 'otltir"e ,{c$rdins to by W-elteand Waples ' as 5uggested may be preclominait i, "'rr"r-,il-.
in fresh phvt'rl iiJtiilrrg'ii:.i+r. i't'. rir.,slepof thetraniiormation.of ( andMaxwell 1974)' who
trr*ttated by in situexperimentsof Brooks i.ot#ittit it--i"U.fed phytol into scaledsedimentcorestrom Recentlacustrine i"p.l.J identifiedvarious environments.After-two to eight weeksot incubationthey phvtvl and l".f"Oing dihydrJphvtol'a. C'g isoprenoigf,:l:""' i".p.""Jt.' microbiological that iinriropttytyt estersind pLytadieneThis resultsuggests w e e k s ' a f e w i n a c t i v e h e e n a l r e a d y h a v e pruaa.t", ofchlorophyllat on hydrogenolysis On the otherhand,laboratoryexperiments confirmed that iave, (Bayliss' 1968)' t".p",,iu" f,igh pie..ure and high -pristani the phytyl from and phytani,-cun.!9^Pt:tluttd ,u?u.ui"oisoprenoicls. in C2n'C1e' resulted has "truin.ol.".i heatingof pure phytoi lAlbrecht' 1969) Using olefins' plus some isoprenoids i:,". c ^. c, and pJssibiyC,olsaiurated ;d-riir.J pnyi.r'in coniactwith differentc1ays, De Leeuwet al (1914'1977) ketones'acidsandother alcohols'aldehydes' oUtuln"Jl.opt"nnidhydrocarbons, products. ' (< CrJ may alsobe derivedfrom natural Low molecularweightisoprenoids suchasfarnesoland sesquiterpenes acyclic with 15carbonatoms,i. e., molecules from C'1 to with isoprenoids is different problem "rr".i"r"A hydrocarbons The of structure phytol Furthermore'1he from C... whichaie not likely to originate Spvckerell;(i973) to eliminatethe possibilitvof ;:,'r;;'c..lt;p;.";ili'allowei processmay that a comparable un'orininiro*.tqualeneor lycopeneHe suggests
i;;;'i?;; ilii;ii"
with25 Jnof'o* naturalmolecules c -c," isoprenoids
io C21-C2.isoprenoids.some sesterterpenoid carbon atoms (sestenerpenes, ind inseti*u", andthe samestructurehas leavet fromhngi, alcoholsareknown beenfoundin furan and ubiquinonederivatives' from BactedalMembranes(Archaebacteria) b) Isoprenoids to be a third majorlineof descent' havebeenrecentlydiscovered Archaebacteria Fox et al ( 1980) They are besideseubacteria1"trueoacteria")andeukaryotes: alsoby a by a iundamentalgeneticdifference(nucleicacids)and characterized compositionof their membranelipids' different -' (deRosaet al ' of glyceroIethersin archaebacteria ftt"t. "* ti"infy composed al Kushw:ha.et ' 1981)'andnot tsiz, lsso; Torna6eneand l-ang*;rthy, 1979; hydrocarbon Furthermore eukaryotes ".t"r. u, tttayu." in otherprokarlotesa;d
i . 6 A c v c l iIcs o p r c n o i d s
ll5
chainsboundto glycerolare generallybranched.Archaebacteria may represent ancestrallife forms and they are presentlyconfinedin specificecologicniches correspondingto extreme environments: thermoacidophilic,halophilic, (anaerobic,methane-producing) methanogenic bacteria.The Iatter, and especiall_v thoseliving in the youngsediment,are a mostprobablesourceof fossil lrpros.
I HO\
o-Rt
FO-BI
ifo-'' I 0 - 8 , - 0 1.
r- 0H
ff
T0-nr-0'1 H6Jo-Rz-o-J a
cro I
z-.-----^,^---1. 2 ---\---1--^__\_
\
C2n
lRr
I
z%tccol
z f f i t ' l *r ,f rr-r^.}.r^-:! z \-^-^?^J
2--
^
f i z c r a l
2
{-*"^-^,--^.
cro 82 i
c
d Fig. t1.3.l5a-d. Branchedandisoprenoidalkanesprobablyderivedfrom membranes of mrcroorganisms. (Adapt€dfrom Chappeet al.. 1982.and Aquino Neto et al.. 1982.)(a) Glyceroldi- and tetraethersin membranelipidsof archaebacteria. (b) Ethers,derived irom (a). boundto the kerogenor asphaltene matrix.(c) Branchedandisoprenoid alkanes obtainedfrom (b) by selective cleavage of elherbondsin kerogenor asphaltenes: R,: C'5isoalkaneand C,uisoprenoidalkanefrom monoethers R:: Cl,risoprenoidalkanesfrom diethers:they are typicalof archaebacteria R!: C1,branchedalkanefrom diethers,not vet foundin livingorganisms. (d) Tricvclohexaprenane. precursor a possible of the Cteto C.utricyclicterpaneseries.nor r et foundin livingorsanisms
116
Formation in Petroleum Fossils andTheirSignificance Geochemical
havebeenrecentlyidentifiedand In geological environments. fossilmolecules they occurin kerogenand in the relatedto membranelipidsof archaebacteria: polar fractionof Recentand ancientsediments,and crudeoils: Michaelisand Albrecht (1979).Chappeet al. (1979,1980,1982).Among them.mostremarkchains,the structureof ableare glyceroltetraetherscomprisingCan-isoprenoid which is characterized by a "head to head" junctionof the C:0sub-units(Fig. I1.3.15).This macrocycleprobably plays the same role in archaebacterial membranesas sterolsin eukaryoticones, i.e., to improve the mechanical properties.Among the Cruisoprenoidunits.the acyclicCaebi-phytaneusually pr€qominates. rings althoughthe structuresincludingone or two cyclopentane are a\o found. chain,the Cr5(2-methyl)alkane.anda Otheretherscomprisethe Cr6-phytane C1,chainresultingfrom the "headto head"junctionol the previousone. The related isoprenoidhydrocarbonsup to Cao,with the "head to head" junctionhavebeenfound in crudeoils:Moldowanand Seifert(1979).Albaiges (1980). They are probablv issuedfrom degradationof the elhers during have been known in archaebacteria catagenesis. Severalother hydrocarbons (Gardner oils: squalane and identifiedin marine sedimentsand/orcrude C1 and various Whitehead.1972:Brassellet al., 1981).C2.irregularisoprenoids branchedalkanes(Brassellet al., 1981).C4 lycopane.identifiedby Kimbleet al. ( 1 q 7 4m ) a r h a r ea c o m p a r a b locr i g i n . or throughan Apart from isoprenoids directlyinheritedfrom livingorganisms there is a definitepossibilitythat the isoprenoid early diagenesis in sediments. chainsmay be boundto the kerogenmatrix.for instanceby an esteror an ether degradationof releasedat depth during catagenetic bond. and subsequently kerogen.This hypothesis will be discussed in Chaptertl.5.
3.7 TricyclicDiterpenoids Tricyclicditerpenoidshave beenfound in sedimentsof the Deep SeaDrilling Projectbl Simoneit(1975.1977).They havebeeninterpretedasbeingderived from diterpenessuchasabieticacid.andthey are thoughtto represent €vidence for the contributionof higherplantmaterial.Abietic acidis alsothe mostlikely precursortbr saturatedtichteliteandtriaromaticretenefoundin peatancllignite ( F i g .u . 3 . 1 6 ) . A seriesof tricyclicterpanes extendingfrom C,eto C1nwasreportedby Aquino Neto et al. (1982)and Ekweozorand Strausz(1982)in someancientsediments and crudeoils.Three of them. with 19 and 20 carbonatoms.wereidentifiedas probabledegradationproductsof a C1 tricyclohexaprenol (madeof a tricyclic nucleus,plus a long isoprenoidchain) (Aquino Neto et al., 1982).This last compoundis as yet unknown.but couldbe formedanaerobically from regular hexaprenol,which is a universalcell constituent.ln this case.the biogenic precursormight be againa constituentof bacterialmembranes (Fig. IL3.15d). Tricl'clic molecules.saturatedor aromatic,are widely presenttn ancrent sedimentsand crude oils. but thev are generallvsubordinatein the saturated
3.8 Steroidsand PetacyclicTriterpenoids:Occurrencein Sediments
Hydrocarbons Salurated
I I
llon arom.lic unsaturaled
Aromatics
117
Non hydrocarbons
.1'\
dY )-
c00rl
F chteIle
-ffi
/^.1
Retene
o e h y d r o a b i e l ai cc d
|
r,
Cholestane lSienne)
( T rr e r p a n e l
All-17 Chot"onene (Strrene)
l , 4 o n o a r o mc aslt e r o i d
H 0 pl 7 ( 2 1 ) - e n e ( T n r e r p e )n e
Tfa i r o m a t trrcl e r p e n o r d
5 0 C h o l a nai cc r d l S l e ' o , dc a ' b o x y la| (c , d)
i dl c o h o l ) 1 T rl e r p e n o a
Fig.II.3.16.Examples of polycyclic molecules present in ancient sediments andcrudeoils (Maxwell et al.. 1971;Rubinstein et al., 1975; Tissotet al., 1972; Seifeft,1975: Albrecht andOurisson. 1969;Spyckerelle, 1975;Ludwig.1982) fraction.In someshalesfrom the UpperDevonianofNorthernAfrica.theLower Cretaceousof the Northern Aquitaine Basin, France, the Cretaceousand Tertiaryof West Africa, etc., the distributionof cycloalkanes by ring number showsa definitemaximumcorresponding to three-ringmolecules,which may representtp to 40Vo of the total cycloalkanesand 20Vo of the saturated hydrocarbons. In some cases,like shalesof the Lower Carboniferous-Upper Devonianof EasternSahara,the aromaticfraction showsa high conteni of tricyclicdiaromaticsof the tetrahydrophenanrhrene group with i distinctpre_ dominanceof C1eH2o. Othertricyclicandalsodicyclicaromaticmolecules foundin ancientsediments may_bederivedfrom triterpenesby fragmentation and dehl,drogen ation.They are C,2or C13(di- or tri-methyl)naphthalene and C,uor C17(di- or tri-methyi) phenanthrene.
1.8 Steroidsand Pentacyclic Triterpenoids: Occurrence in Recentand AncientSediments In this section.we shall reviewthe first and also the most recentreportsof polycvclic molecules belongingto thesteroidor triterpenoidclasses, or obviouslr Jerivedfrom them.Theiroccurrences will be successivelv reportedin Recentani
118
Formation in Petroleum FossilsandTheirSignificance Geochemical
ancientsedimentsand in crude oils Then their mutual relationship,and the will he discussed of thevariousmoleculartypesin subsurface progressive changes I I . 3 . 9 . in Section Steroidsform a group of compoundswidely distributed in living organisms (Fig. I.4.11),wheretheyplay an importantbiologicalrole: in particularsomeof th.tn a." a constituent of the membrane lipids of eukaryotes All naturally steroidscomprisea nucleusofperhydro1,2cyclopentanophenanthrene occurring -methyl groups,one alkyl chain, and functionalgroups Pentacyclic bearing tiilerpinotds ire madi of 6-memberedringsonly or of four 6-memberedand one 5-mf,mberedring. Tbey normally comprise 30 carbon atoms, and have been reported to occur frequently in plants, and also in prokaryotic organisms ln paiticular, triterpenesof the hopaneseries(Fig. II 3.16) are.a widespread ionstituent of thc membranelipids of prokaryotes(eubacteria,blue-green algae).Varioustetracyclictriterpenesalsooccurin animalsandplants'
a) RecentSediments theoccurrence andErdman(1964)described Schwendinger In Recentsediments, (Erdman, 1965), carbon organic of the total 0.01 7. to that may amount of sterols may amount present and are also bonds, double form without reduced Stanols.a of to asmuchas10-5070of the sterols(PeakeandHodgson,1973).The presence the correspondinghydrocarbons,steranes,together with triterpanes,was tn a reportedby Meinschein(1961).who analyzed4- and 5-ring cycloalkanes Recentsedimentfrom the Gulf of Mexico.More recently,Gagosianet al (1980) in of the varioussteroidcompounds andtransformation analyzedthe occurrence Black Sea (WalvisBay, of threedifferentmarineenvironments Recentsediments of a seriesof triterpaneswith the and WesternNorth Atlantic).The occurrence pentacyclichopaneskeletonin variousRecentsedimentswasdefinitelyestabiistt"O'UyKim6le (1972)and van Dorsselaer(1975).Spyckerelle(1975)found tetra- and pentacyclicpolyaromatic.probably derived from triterpenes,in lacustrineand marine Recentsediments.Later tetracyclicdi- and triaromatic moleculesderived from the loss of one ring in pentacyclictriterpeneswere situations et al. (1977)in comparable by Spyckerelle characterized
b) AncientSediments Steranesor pentacyclictriterpaneshavebeen reportedto occurfrequentlyin buriedto shallowdepths(Fig ll 3 16) Both have ancientimmaturesediments River shales,by Burlingameet al. (1965)'Hills et al' in Green beenidentified (1966),Hendersonet al. (1968)and Andersonet al. (1969),The Messeland Bouxwillershalesfrom the Rhine valleycontainseveraltriterpanes'but onlv Albrecht (1969)and Arpino (1973).The triterpanes smallamountsof steranes: from of molecules series,whichincludea succession to the hopane belong mostly forms(Ensminger to C35,with two mainstereoisomeric C27toC3zandsometimes et al. 1974).Furthermore,thesehopanederivativesshowan ubiquitousoccur-
in Sediments anclPetacyclic Triterpenoids: Occurrence J.ll Steroids
119
rencein varioustvpesof sedimentof any age(van Dorsselaer et al.. 1974).The shallowsamples of the LowerToarcianshalesfrom the ParisbasincontainC., to (Califet-Debyser C1,steranes andtriterpanes andOudin,1969).Fromthosesame shales.Rubinsteinet al. (1975)and Huc (1976)identifiedsterenesand triterpenes:Tissotet al. ( 1971. 1972)havealsoidentifiedC1 to Cjnmonoaromatic and diaromaticsteroidsand reported some possibleC27to C30monoaromatic triterpenoids. Similarly,Albrecht(1969)reportedthe occurrence of mono-.di-. tri- and tet{q-aromaticmoleculesrelated to pentacyclictriterpenes.These compounds ha\e beenidentifiedby Spyckerelle (1975)and Greiner(1976),and resultfrom the jprogressive aromatizationof triterpenesof the hopaneseries (Greiner et al. 1976).Sincethat time it has been observedthat mono- and triaromaticsteroidsare rather commonin ancientsedimentsand crude oils. Monoaromaticsteroidsare sometimes aromatizedin the A-ring (Hussleret al.. 19llI ). and frequentlyin the C-ring(Ludwiget al., 1981a). Triaromaticsteroids arealsowidespread andtheyusuallybecomeprevalentovermonoaromatics with
a-Triterpanes Lupane series Opening of A-rinq
b -Steroids Monoarornatics lG-dngl R ..>.^.-\-
616
@R2 R r =H orRz=CHr 8 r = G H r R 2= H
R , = H R ,= C H " -' R 1 = C H 3 R ,= ; Fis. II.3.l7a andb. Examplesof alteredtriterpenes andsteroidsfoundin sediments and ;rude oils.(a) Degradedtriterpanes by openingthe E-ring(hopaneseries)or the A-ring ,lupaneseries).(b) Monoaromatic (in A-ring or C-ring)and triaromaticsteroids
Formation in Petroleum Geochemical FossilsandTheirSignificance
120
increasingthermalmaturation(Ludwiget al., l98l b; Mackenzieet al.. 1981): F i g u r eI L 3 . 1 7 . Tetracyclicmoleculesderived by degradationfrom pentacycllctrrterpenes a C.oto C., naphtheneseries(17, havealsobeenidentifiedin ancientsediments: loss of E-ringin hopanes(Trendelet from the probablyderived 21-secohopanes) derivedfrom the tetracyclics and aromatized al.. 1982):saturated,unsaturated in deltaicor widespread are series and amyrins lossof A-ring in h\pane,friedelin (Spyckerelle et al . 1977i oils crude the related iacustrineaniient {edimentsandin 3.ll Corbetet aI..1980;Schmitteret al.,1981);FigurelI Pentacyclictriterpaneshave also been reported from lignite-(Jarolim e1al', aromatized 1963,1965)and coal (van Dorsselaer,1975) The corresponding (Jarolim et al ' Czechoslovakia from present in lignite are also hydrocarbons 1969) and Herout, 1965;Streibl
c) CrudeOils triterpaneshavebeenidentified These In crudeoils, steranesand pentacyclic for natural for theopticalrotationobseryed areprincipallyresponsible molecules hydrocarbonmixtures:Hills andWhitehead(1966),Louis(1964,1968),Hills et werefirst reportedby O'Neal and steranes ui. 1tlOS,1970).The monoaromatic Hood (1956).Sincethat. the mono-anddi-aromaticsteroidshavebeenfoundin oils from SouthAmerica(Tissot oils,includingLower Cretaceous severaicrude et al., 1972)and Lower Pileozoic oils from the Eastern Sahara(Tissot et al ' to be widespread 1974).Later, mono-and triaromaticsteroidswererecognized and Cyclopentenophenanthrene "on.iitu.nt, of crudeoils, asmentionedabove (1957) and Douglas and by Carruthers its methylderivativehavebeenidentified (1962). Mair and Martinez-Pico
H0
ft
C2z-hdns-ac d,
C24-lEns-actn,
C24'1,?rrbrl€ acrds,
Fig. 11.3.18.Possibleorigin of steroid acids from animalbile acids(Seifen. 1973)
3.9Fateof Steroids andTriterpenoids DuringDiagenesis andCatagenesis
121
Rearrangedsteranes(diasteranes) havebeenobservedin severalcrudeoils and ancientsedimentswhich have already gone through partial diagenesis (Ensminger et al., 1978).In the LowerToarcianshalesof the Parisbasin,thevare presentin immaturesamples.This supposedto derive from the diasterenes hypothesis isgonfirmedbv simulationon clayminerals(Sieskindet al.. 1979).
d) Other RelatedSubstances Closelyrelatedsubstances, includingalcohols,acidsand ketones,have been repoftedfrom sedimentsand fossilfuels(Fig. IL3.l6). In particular.Albrecht ( 1969)foundin theMesselshalesoftheRhinevalleytheassociation ofa pentacyclic triterpenoidalcohol.isoarborinol, andthe corresponding ketone.arborinone.In thenearbyBouxwillershale,Arpino (1973)foundtriterpenoidketonesandacids, but no alcohol.A seriesof Cjl to C3j acidscontainingthe pentacyclic hopane skeletonhas been identified by van Dorsselaer(1975)in various types of sediments. Oxygenatedtriterpenes(alcohols.ketones)havealsobeenreported from peat and lignite (Ikan and Mcl-ean. 1960;Jarolim et al. 1961,1963). Tetracyclic acidsderivedfrom triterpenes (lupane,friedelinandamyrinseries)by openingthe A-ring wereidentifiedin deltaicsediments by Corbetet al. (1980). Stanolsand stanonesare also reportedby Mattern et al. (1970)in Messel shales,but few or no sterolsseemto be present. Seifert et al. (1971, 1972) and Seifert(1975)have determinednumerous differentcarboxvlicacidsin a youngCaliforniancrudeoil. A prominentfeatureis the relativeabundance of the steroidacids.whichamountin thisDarticularoil to morethan0.037cof the totalcrudeoil. The relativeabundance of the isomersof C12andC11steroidacidsareinterpretedby Seifert(1973)asderivingfrom animal b i l ea c i d s( F i g .I I . 3 . 1 8 ) .
3.9 Fateof SteroidsandTriterpenoidsDuringDiagenesis and Catagenesis The origin of thesenumeroussteroidsor pentacyclictriterpenoidsfound in sedimentsand crude oils is obviouslythe naturalll' occurringsterols and triterpenes.The variousmoleculartypesidentifiedwill be discussed later with respectto their significance in termsof environmentof depositionor stageof thermalevolution(Chap.IV.3.) However,severalaspects of the occurrence and distributionof thesecompounds in sediments andcrudeoilsrequirean explanation at thispoint, namely: - the relativeabundances of steroidsandtriterpenoids, - the fate of the biologicalmarkersin relationto the originalconfiguration of functionalgroupsand C:C bonds, - the changesin stereochemistry due to isomerizationduring diagenesis or catagenesls.
122
Formation in Petroleum Fossils andTheirSignificance Geochemical
molecules'ascomparedto the usual - the occurrence of extendedor dealkylated C:rCr1 sterolsand Ciutriterpenes. a) RelativeA\dances of SteroidsandTriterpenoids mayvary Thel sometimes of steroidsancltriterpenoids The relativeabundhnces may be predominantSteroids group one cases in other but occursimultaneously, shalesof the Paris Toarcian Lower in the triterpenoids ihan are more abundant ln the contrary' Gulf Persian of the rocks source the Cretaceous basinor in triterpanes' abundant contain vallev. Rhine in the shalcs. Bouxwiller and Messel but onlv a smallproportionof steranes.Both typesare fairlv abundantin the GreenRiver shales(Kimble et al.. 197'1) mayoriginatein part of sediments triterpanes thatpentacvclic It is considered in particular areprokaryoticorganisms: from plants.However.the mainsources for the responsible is mainly microiial life in the upperlaversof youngsediments van by (1972) and ubiquitoushopanes,as proposedby Ensmingeret al' partly and from animals partly et al. (1974).Sierinesmaybe derived Dor.sselaer from plants.as aireadyproposedby Seifert(1973)regardingsteroidacids As in the contributingbiomass'C21and terresirialanimalsare uiually insignificant aquatic from the autorhthonous derived C,ssteroidsfound in sedimcntsmaybe from either be derived may steroids whilst C2e lifi. i. e., zoo-andphytoplankton' cases sterols ln some phytoplanktonalgac and continentalplantsor irom brown algae For instance' especially canbe tracedbackto the contributingorganisms' sapropellayeras Black Sea of the sterol major the Boon et al. ( 1979)identified sterolin some major is the which dinosterol' alcohol' the C1,,4-methylsteroidal where sediments some in of steranes quasi-absence The only. dinofligellates being sterols from result instances in some present might are triterpines synthesize to unable which are microorganisms, certain out by taken seleciively thesecompounds.However,this absencegenerallyresultsfrom the original compositionof the organicmatter. Stiroid-richsedimentsare usuallymarine or lacustrine'and their kerogen belongsto typesI or II (for definition,seeSection4 8) whichare mainlyderived from iutochthonousorganicmatter.ln this respect.planktonmay be the major to ubiquitous Thusthe ratioof steranes conditions. sourceof sterolsin geological material: the main hopanesin crude oils may indicatethe origin of -source it is low when ratio is high' terrestrial plinktonic whenthe sterane/hopane b) Fateof BiologicalMarkersin Relationto Their OriginalStructure The fate of the functionalgroups,includingC=C bonds' which occur in the biogenicmolecules.is of particularinterestand may influencethe subsequent evoiutionof the biologicaimarkerstowardthe varioushydrocarbon(saturated' types.In this respectsteroids and non-hydrocarbon mono-and poly-aromatics) and triterpenoidsbehavedifferently L Ster;ls are rather easilyconvertedto saturatedstanolsand to the correall of which steranes' andsaturated sterenes unsaturated hvdrocarbons. sDondins
3.9Fatcof Steroids andTriterpenoids DuringDiagenesis andCatagenesis
123
are known in Recentor ancientsediments. A high contentof steranesis often observedin marinesediments depositedin a reduiingenvironment.with a high concentration o\sulfur compounds.This observationmay suggestthat a veiy redu_cing andhyd\ogenating environmentlcadsto sterane.wheiJaslessreducing conditionsof depdsitionleadto sterene.whichmaybe subsequently convertedtJ eitheraromatics.or saturates. The process of aromatization of steroidsalsotakesplacein sediments. whereit mav start from ring A or. more frequently.from iing C and terminatewith trraromaticmolecules of the cyclopenteno-phenanthrene t]'pc (nomenclature of ringsis shownin FigureIL3.l7). Aromatizationin ring C mav start in ancient sedimentsfrom stereneprecursors.Such mc,noaromatic stei.ridshave been identifiedbv l-udrviget al. ( l98l a). The relatedtriaromaticsteroiqscorrespono to the rearrangemcnt or lossof a methvlgroupandoffer threealternati\esll uct u r es w i t h I - m c t h _ "o"r| .; l - m e t h v lo. r n o n t e t h vol n r i n gA ( L u d w i gc t a l .. l 9 gI b : M a c k e n z i e t a l . . l 9 8 l ) . M o n o - a n d t r i a r o m a t i cs t e r o i d so f t h i s u r o u n a r e widespread in ancientsedimenfs and crudeoils. Lessfrequentarc monoaromatic (cvcleA) steroids,whichhavebeenreported b v H u s s l eer t a l . ( 1 9 8 1 ) i n C r e t a c e o u s b l a c k s h a l e s c o r e d i n t h e A t l a n t i c d u r i n s t h e D e e pS e aD r i l l i n gP r o j e c(t D S D P ) .T h e s es h a l ecso n t a i na r i c ho r g a n i cm l t r i r o f type_ll(up to 30% organiccarbon): theyweredepositedin anoxic. vervreducing conditionsand thev still belongto the diagenesis stage. A suggestion was made that aromatizationof steroidsstartingin ring A is restrictedto confinedreducingenvironments, whereasaromatization startingin ring C would occur in the more commonopen environments(Hussleret al., 1981).In all casesaromatization mightsubsequentlv proceed(C to A or A to C) and generatetriaromaticsteroids.For instance,Mackenzicet al. (19g1)have shownthat the ratio triaromatic/monoaromatic steroidsincreases with thermal maturationin the Lower Toarcianshalesof the parisBasin.This asoectwill be f u r t h e re m p h a s i z ei nd C h a p t e rIsV . 3 . 4a n dV . L 3 . 2. The fate of triterpenoidsduring diagenesishas been summarizedby S p )c k e r c l l e( 1 9 7). .5 - Pentacvclic triterpenes with an oxygenatcd functionalgroupat C-3,areknown from higherplants:lupane.friedelinand amyrinsseries.The corresponding moleculesare known in sedimentsas alcohols.ketonesand aromaticor naphthenoaromatic hydrocarbons. but only rarelyassaturatedhydrocarbons. Theseobservations suggestthat conversionof triterpenealcoholsto triterpanesis moredifficultthanthe sterolto steraneconversion. In fact,tetracyclic productsresultingfrom the openingor the lossof ring A seemto be rather common in the alkane, alkeneor acidic fractionsof deltaicor lacustrine sediments. andalsoin crudeoil derivedfrom terrestrialorganicmatter(Corbet et al.. 1980:Schmitteret al., 1981).Thisphotomimetic rransformation maybc controlledby microorganisms during diagenesis. A further stepwould be a progressive aromatization of ringsB to D (nomenclature of ringsshownin Fig. L4.10) generatingin particularthe tetracyclicdiaromaticand tnaromatic derivatives from the amyrinsgroupcharacterized in sediments andcrudeoils bv Spvckerelleet al. (1977).Another possiblewav of conversionof C-3
124
Formation in Petroleum FossilsandTheirSignificance Geochemical
by lossofthe compounds wouldbe to yieldunsaturated triterpenes oxvsenated \ t h e A - r i n g :i n i t h s t a n i n g a r o m a t i z a t i o n i u i c " r i o n qglr o u pa n dt h e nu n d e r g o as lhe such formed' would be ini, .u." lp-.ntu.y.licaromatizedtriterpenes byJarolim in lignite reported derivatives ofmono-to pentaaromatic successiqir et al. (1965). - :-aesoxytriierpenes of the hopaneseries,without a functionalgroup in C-3 position.are kno*n from ferns,mosses.bacteriaandtlue algae They have as acids'saturatedhydrocarLeenreportedin Recentand ancientsediments lnfact' thepentacyclichydrocarbons aromatic and hopene bons.unsaturated cf oils mainlyconsists crude and of sediments (triterpanes) saturatedfraction from the but the hopaneseriesAromitizationwouldnot startfrom the A-ring' bond is locatedin hopeneII' D-ring.wherethe unsaturated terpaneswith 24 to 27carbonatoms'possiblyderived Furthermore.tetrac,vclic of sedimentsand crudeoils' constituents widespread from hopanes.are ratler showedthat thev couldbe and structure their it""a.i'", al. (1982)identified b1'microbialactionat an either E cycle' the by opening derivedfrom hopanes catagenests' during degradation by thermal or earlystageof diagenesis. Due to Isomerization c) Changesin Stereochemistry and Isomerizationat chiral centres.either part of the ring systemof steroids action This itit"tp.nniO. or locatedin the sidechains.is now well documented the structureThefirstreportsconcerned i..uii in u .ttung.of thestereoisomeric junction is This blE iing iu*tiJn in hopaneswhichchangesduringdiagenesis (BB-hopanes)' sediments andin youngimmature u.*tty ii, in Uiog.nicmolecules and in ancientsediments ;hilsimaturatio; resultsin the transsiereochemistrys ( 1 9 75)' ( 1 9 7 4 ) ' v a n D o r s s e l a e r r t al ) :n s m i n g e e . r u a " o i t t ( c B - h o p a n e sE junction the biogenic of the ring affect ensminser(i9;+). A similarprocesicould and ursanes(Whitehead'1973)' oleananes havebeenmadeat a laterstageof evolutionin the Comparableobservaiions also isomerization (Mackenzie et al ' 1980)Furtherrnore' rin!s.vsi.mof,teranes part of and diagenesis during hopanes and steranes o.iu^ in the sidechainsof measuring for indices proposed.as been have changes ."ir!""..i, (ibid.). These in greaterdetail ,nutirutionof .o*ce rocksandirude oilsandtheyarediscussed V . 1 . 3 a n d I V . 3 . 4 i n C h a p t e rIsV . 3 . 3 . d) Occurrenceof Extendedor DealkylatedMolecules 29 carbon triterpenoidsnormallycontain30 and sometimes Naturalpentacvclic in estrone atoms atoms.Thereis morevariationamongsteroids(from 18carbon and stigmasterol to 29 in ind adrenosterone andestradioland 19in androsterone od-hopaneis 3 The notationfB-hopaneis usedfor l7 B-H. 21p-H hopane.The notation hoPane usedfor 17a-H. 2l PJ{
3.9Fateof Steroids andTriterpenoids DuringDiagenesis andCatagenesis
I25
sltosteroland 30 in d\osterol). The distributionis much wider in ancient sediments andcrudeoilsItriterpanes or oxygenated triterpenes occurfrom 27 to 32 carbonatoms,and sometimes up to 35 carbonatoms;steranesanoaromatlc steroidsoccurwith 27to 30carbonatoms.Othertetracyclic molecules, whereone or severalcvclesare aromatized, occurwith 17carbonatoms,andarefrequently a b u n d a nwr i r h l 9 l o 2 3 . Severalpointsneedexplanation: - the occurrence of triterpanesbeyondC4 - structureand origin of the C,r-C1 aromatizedtetracyclics, ie.. compnsrng mostly a partly aromatizedcyclic carbon skeletonwith some methvl sub_ stituents. discoverrof tetrahvdroxvbacteriohopane. a C15pentacl,clic polvalcohol (._The F t j r s t eer t a l . . 1 9 7 1 ) a. n d t h e p r o b a b l ee x i s t e n coef J t h e rs i m i l a rc o m p o u n d s amongnaturalsubstances providean explanationfor the occurrenceof triter_ paneswith morethan30carbonatoms.RohmerandOurisson(1976)haveshown that this classof compoundsis rather ubiquitousin prokaryotes(bacteriaand blue-green algae).wherebacteriohopane polyolsare an importantconstituent of the membranelipids.Accordingto the condirionsof depoiirionand diagenesis, the Cjs precursorsmay yield a seriesof alkanesand aiiclsextendineto C.. in confinedanoxicenvironments, andC12in the morecommonopan"nu]ronaant. Thereis. however,little doubtthar in all casesthe hopaneserieswith extended sidechainfoundin sediments andcrudeoilsis of prokiryoticorigin(Ourissonet al..1979). Among the aromatizedtetracyclics, Tissotet al. (197,1) reportedthe impor_ tanceof C11C2,aromaticsteroidsin verymatureLowerpaleozoiccrudeoilsfrom the Sahara.Algeria.The sameconstituents wereidentifiedbv Mackenzieet al. (1981)in the deep samplesfrom the ParisBasin as mono- and triaromatic stcroids.Theiroccurrence maybe explainedby varioushypotheses. A concentration of particulartypesof steroidsin that rangeof molecularweightmight be considered asseveralsteroidsare knownwith 18to 21 carbonatoms.However. thesesteroidstoday belongmainly to the group of sex hormonesor corticosteroids,and thesecanmakeonly minor contributions to sedimenrs. Bile acitls. comprisinga steroidstructurewith 24 carbonatoms,may alsobe considered. Microbiologicaldealkylationprocesses are nor excluded.Ho*"ue.. an abiotic. purely thermal disproportionation is the most likely way of derivationfrom ClrC..psteroids:the eliminationof methylgroupsand of the main alkyl chain, alongwith subsequent formationof alkanes.wouldbe balancedbv aromauzarron o f o n e o r m o r ec y c l e sI.n f a c t ,M a c k e n z i e t a l . ( 1 9 E 1s) h o w e da c o r r e l a t l o n betweenthe increasing catagenesis in the Parisbasin.markedby burialclepth, and the proportionof C11molecules in the triaromaticsteroids. Other aromatized tetracyclicshave a different structure and origin. Spyckerelle ( 1977)identifiedin the Messelshalessometetracyclic mono-,di- ind triaromaticC"i to C2,compounds derivedthroughdegradation ofthe A-ringfrom pentacyclic triteryenescontainingan oxygenated functionalgroupin C-3,during diagenesis, This schemeof degradation,possiblyinitiatedbv microorganrsms. may constituteanotherpath to the C1aC.1aromatizedtetracyclics.
126
Formation in Petroleum GeoclemicalFossilsandTheirSignificance
of arewidelydistributedcompounds steroidaand triterpenoids In conclusion, of ahundance relative biogenicoriginwhichcarry a wealthof information The may compounds of certain steranesand triterpanes,or the specificoccurrence such rearrangements, givea clueto the originof the organicmatter.Subsequent provide informaprogressive aromatization' and is changesin stereochemistry processesThe use of steroidsand triterand catagenesis tion on diagenesis in Part IV, Chapter3' purposes will be discussed penoidsfor thesevarious presented V, Chapter2. in Part will be whereastheir usefor correlation
3.10 Other Polyterpenes Carotenoidsare pigmentscomprisingeight isopreneunits.They are Cancompounds.occurringin continentalplantsandin algae(Fig l.4 12).Theyhavebeen both lacustrine(Lyubimenko'1923)andmarine identifiedin Recentsediments. and Erdman(1963)and Erdman(1965) (Traskand Wu, 1930).Schwendinger foundthecarotenoidcontenttobearound5.l0'(asafractionoforganiccarbon) 5 The corresponding in swampsediments,and 3 to 80.10 in marinesediments. are probablywidely presentin ancientsedimentsFor saturatedhydrocarbons in severalcasesandis reportedin Messel example,lycopanehasbeensuspected shaleiby Iiim6le et al. (1974).Murphyet al (1967)foundperhydro-B-carotene' and Gailegos(1971)identifiedtwo isomersof this compoundin the branched wheretheyareabundant extractfrom the GreenRivershales, cyclichydrocarbon in particularhorizons(Mahoganybeds).
3.11 Aromatics is low Furtherin Recentsediments of aromatichydrocarbons The abundance of low molecularweightaromatic more. Erdman(1961)hasshownthe absence from variousorigins Thisis in contrastto their in Recentsediments hvdrocarbons and crudeoils. imoortancein ancientsediments A few polycyclicaromatic hydrocarbons(PAH) were first reported br Meinschein(1959)in marine sedimentsof the Gulf of Mexico. Thomasand nodules.Later Blumer (1964)idcntifiedpyreneandfluoranthenein manganese Tissier and Outlin (1973) identified by mass spectrometryand spectroperylene'benzo-8.9-fluoranchrysene,triphenylene, fluorimetrvfluoranthene. in nonpollutedsedimentof the Bay of Ve.vs theneandtracesof 3.4-benzopyrene (France):Figure I1.3.19.With the improvementof analyticaltechniquesit becameobviousthat PAH from Recentsedimentsrepresenta complexassemblage of the parent compoundsand their alkyl homologs(Youngbloodand B l u m e r .1 9 7 5 ) . of PAH the concentrations In the deeperandolderlayersof Recentsediments approach with in PAH concentrations are generallyvery low. A drasticincrease contributionof anthropogenic to the Earth'ssurfaceis indicativeof an enhanced
3.12Oxygenand NitrogenCompounds
rn
/fr
121
O...,..t
qCr/"5r Fluoranthene
4Y\
YY
.r)
Perylene
Chrysene
'q
a*4
Triphenylene
Fig. IL3.19. Polyaromatic hydrocarbonsfound in an nonpollutedRecentsediment of the bav of Veys, France (Tissier.1974)
%o
Benzo8,9 tluoranlhene 3,4-Benzopyrene
PAH within the pasr75 100years(Miiller et a\.,1977:Wakehamer al., 1980). Depth profilesof individualPAH may be usedto discovertheir source.e. g.. it hasbeenshownthat peryleneconcentrations increase with depth,suggesting an in-situ formation mechanism(Orr and Grady, 1967;Brown et al., 1972: Aizenshtat.1973;Wakeham,1977;Wakehamet al.. 1980).Perylenemay be derivedfrom polyaromaticprecursorssynthesized by living organisms,e.g.. fungi.For instance.Aizenshtat(1973)suggests thatperyleneis derivedfrom land organismsand is incorporatedin sedimentwith the detrital mineralfraction. Recentlysubstantialamountsof perylenehave beenobtainedfrom Namibian Shelfsediments(Wakehamet al., 1980),whereinput of terrestrialmaterialis thoughtto be minimal.Thus.the precursoris not necessarily of terrestrialorigin. Other PAH may havebeenformedin naluralfires.Their distributionvaries littlc over a wide range of depositionalenvironments,suggesting a common mechanismof generationby combustionof land plantsand dispersionby air transport(Blumerand Youngblood.1975;Youngbloodand Blumer.1975). Finally,aromatizedditerpenoids. steroidsand triterpenoids with one to four aromaticringsarewidespread (Spyckerelle, 19751Simoneit. 1977;Laflammeand Hites. 1978)and havebeendiscussed already(Sects.3.7. 3.8 and3.9).
3.12 Oxygenand NitrogenCompounds They are likell'to includemanygeochemical fossils.but their studyis still very incomplete.Lincar and branchedacids.ketonesand steroidacidshave been itlentificdin a limitednumberof sedimentsor crudeoils( Seifertet al.. 19'/1,I972: Seifert,1973.1975).Nitrogencompounds (pyridines,quinolines.carbazoles) are probablyskeletalfragmentsof plant alkaloids(Whitehead.1973). Pigmentscontainingthe porphinskeleton(four pyrroleringslinkedtogether bv four methine -CH:bridges) are very importantin the plant and animal kingdom.Thev includemainlvchlorophyll,the greenphotosynthetic pigmentof plants(Fig.L 1.2),andhemin.the red pigmentofanimalblood.Chlorophylland heminare magnesium and iron chelates, respectively.
128
Ueochemrcal tossilsandTheir Significance in PetroleumFormation VanadYl.:
\anadyl etioporphyrin Itr ( m e s o e l r o p o r p hI y f r n
desox0phylloerylhro?tioporphyfln
C2H5 H
CH3
C2H5 H
c2H5
.
H
:N'.. V
\ru'
zN = O
/
Hgc H
\{
...,/a.A i
cH3
(Y Y') ,N1
,>N., . v / \ ,= o, t ,
\\
H H
c2 H5 H
c2Hs
H 5 c2
cH3 H
CH3
cH3
H
Fig. II.3.20. Examples of porphvrins found in ancient sedimentsand crude oils
Petroleumporphyrinsalso containthe porphin skeleton.They have been extensively studiedsinceTreibs(1934)provedtheir biologicaloriginandrelated them to the chlorophyllor hemin molecules.They have been identifiedin a sufficientnumberof sediments andcrudeoils to establisha wide distributionof thesegeochemical fossils:Hodgsonet al. (1967),Baker (1969),Blumer (1965, 1973),Bakerand Palmer(1978).Two recentreviewsof porphyringeochemistry havebeenpresentedby Maxweller al. (1980)and Baker(1980). The major type of fossilporphyrinsare desoxophylloerythroetioporphyrin (DPEP),whichcontainsan isocyclic ringin additionto thefour pyrrolerings(Fig. II.3.20),and mesoetioporphyrin or erioporphyrinIII withoutthe isocyclicring. In addition,threeminor typesareobserved:di-DPEP.rhodo-etioandrhodoDPEP(BarwiseandWhitehead,1980).Fossilporphyrinsareeithermetal-free or mostlychelatedwith nickelor vanadyl(VO) in ancientsediments andcrudeoils. Cu-porphyrins havealsobeenreportedin deepseasediments. wherethevmaybe an indicatorofoxidizedterrestrialorganicmatter:PalmerandBaker(1978).Gacomplexes andAl-porphyrinshavebeenfoundin bituminouscoal:Bonnettand (1980).Theseauthorsalsoisolatedmanganese Czechowski (lll) and iron (III) porphyrinsfrom a lignite. The transformation of chlorophyll,or hemin.is likelyto startduringsedimentationor earlvdiagenesis. Magnesium or ironwouldbe lostat an earlvstageanda re-chelationwith vanadylor nickel may stabilizethe moleculeand insureits preseruation.Bergayaand Van Damme (1982)have studiedthe stabilitvof metalloporphyrinsadsorbedon clay minerals:Mg-porphyrinis particuiarly unstable,whereasNi- and VO-porphyrinare amongthe most stablein this environment.The hydrolysisof the phytol mav alsooccurat that stage.Later, reductionof thevinyl andcarbonylgroups,anddecarboxylation of the carboxylic acid groups lead to the formation of petroporphyrins.Evidenceof thermal conversion from DPEP to etio type hasbeenpresentedby Didyk et al. (1975). and alsoby Maxwellet al. (1980)in the Toarcianshalesof the ParisBasin. An alternativebiologicalsourceof fossilporphyrinscan also be proposed accordingto somerecentdiscoveries of microbiologist R. K. Thaueret al. They
3.13Kerogen andAsphaltenes a$Possible Sources of FossilMolecules
129
identifiedin a methanogenlc Archaebacteria(Methanobacterium thermoautotrophicum)the first naturalnickelcomplexwith a porphinoidskeleton(Pfalzet al.. 1982).This possibleorigin might explainthe widespread occurrence of Niporphyrins.asthesebacteriacommonll'livein youngsediments. More work is obviouslyneededto find out whethervanadylcomplexmay alsooccur. The massspectraof fossilporphyrinsshowa distributionof molecularweights suggesting a seriesof alkyl-substituted porphyrins.For instance,Barwiseand Whitehead(1980)observedporphyrinsrangingfrom C:5to morethanC50,with a maximumat Cr2,in a concentrate ofvanadylporphyrinsfrom Boscancrudeoil. A transalkylation betweenpairsof porphyrins.or betweenporphyrinsand other compoundshas been proposedto explain this distdbution (Baker. 1966). Alternatively.Blumer and Snyder(1967)suggested that porphyrinstructure could be integratedinto kerogenand later regeneratedwith variouschain lengths.Later Mackenzieet al. (1980)showedthat C.,sC1oporphyrinsare generated below2000m in the Toarcianshalesof the ParisBasinandconsidered them to ariseby thermalcrackingof the kerogen.Quirke er al. (19U0)studied poryhyrinswith extendedsidechainsand concludedthat theseporphyrinswere formed by bindingchlorin precursorsvia the vinyl substituentonto kerogen: subsequently the porphvrin productswere generatedas a result of cracking kerogenduringcatagenesis. This is likelyto be the majorpathwayto porphvrins foundin petroleum.However,Casagrande and Hodgson(1976)foundhomologousseriesalsoin near-surface sedimentfromtheBlackSea;thisfactsuggests that someof thesehomologsaregenerated duringthe veryearlystagesof diagenesis. The recent developmentsin analyticalmethods(high performanceliquid chromatographv, gas chromatographv. massspectrometry,nuclearmagnetic resonance)make possiblea precisestructuralidentificationof porphyrins (Eglintonet al.. 1980:Maxwelletal., 1980).Thusnewprospects areopenforthe geochemistry of porphyrinsand alsofor their use as correlationor maturatron indicators. Otherpigments.namedfringelites,havebeenfoundby Blumer(1960.1962)in a fossilcrinoid of Jurassicage. They are poll'hydroxyquinones. which can be linkedto similarpigments(aphin,hypercin)in presentlvlivingorgunisms.
3.13 Kerogen,the PolarFractionof Sediments, and Asphaltenes of CrudeOils asPossibleSourcesof FossilMolecules Most of the geochemical fossilsreviewedin this chapterhavebeentracedfrom organismsthrough Recent sedimentsinto ancientsedimentsand crude oils. However.thereis evidencethat certainmoleculeswith a biologicalfingerprint are producedduring the thermalevolutionof kerogen.On the one hand.the (relatedto organiccarbon)of moleculeslike odd-carbon-numbered abundance r-alkanes,isoprenoids. iso-and anteiso-alkanes, increases with depthin several geologicalsituations.On the other hand, experimentalevolution of some previouslv generates extractedkerogens compounds with a biologicalfingerprint. e.g.. odd-numbered n-alkanesof highmolecularweight(Welte.1966.on Messel
130
-\ Formation in Petroleum FossilsandTheirSignificance Geo\hemical
shalesof the Douala (Albrecht,1969,on Upper Cretaceous shales),isoprenoids et al , 1977' on Yallourn basin),and steroidsand triterpenoids(van Dorsselaer lignite; Gallegos,1975.on GreenRiver shales). 'A from observationhas been madeby pyrolysisof asphaltenes cornparable also and squalene yields normalcrudeoils. heavyoils andtar sands:asphaltene naturallv with, those not identical steranesand hopanes,iimilar to, although Dresentin crudi oil (Rubinsteinet a\ .1979',Sammanet al ,1981) Selective from crudeoils' cleavageof etherbonds,when appliedto kerogen'asphaltene isoprenoids alkanes,and yields branched and th-epolar fractionof sediments, 1982)' al.' (Chappe et whichcan be tracedbackto archaebacteria with a functional to considerthatbiogenicmolecules Thus.it seemsreasonable bonds,e g ' by chemical matrix or asphaltene kerogen fixed on the group may be The variable evolution by thermal released and subsequently !rt..i o, "the.s, position of the cleavagemight explain some of the -alkylationor dealkylation bbserved.comparedto actuil biogenicmolecules(Blumer, 1973) This process steroids' could generatesomeof the homologousseriesof acyclicisoprenoids, andcrudeoils,but triterpinoids,porphyrins.etc that occurin ancientsediments are not presently'knownin living organismsFurthermore,there may be less conversibnof bi,ologicalto nonbiologicalstereochemicaltypes of steroidsand (Huc, 1978;Rubinsteinet al ' 1979i triterpenoidsin kerogenor asphaltenes matrixmayresult Sammanet al.. 19Ul).Thustrappingin kerogenor asphaltene of the stereochemistry. in a betterpreservation A final remark concerns the prominence among fossil molecules of the constituentsdesignedto ensuremechanical,thermal or chemicalpreservationof otherprokaryotes (Caeisoprenoids). the cells:membrinelipidsof archaebacteria higher plants from waxes. cuticular (sterols); (hopanes)and eukaryotes seem constituents These etc exine, weight); spores (n-alkanes of highmolecular during occurring alteration and chemical io be more resisiantto microbiological Furthermore,someof them with a functionalgroup may be early diagenesis. rathir ea-ily trappedin the kerogenor asphaltenenetwork' thusinsuringa better includingthat of the originalstereochemistry' preservation.
Summaryand Conclusion by plantsor animalsandincorporatedin Geochemicalfossilsare moleculessynthesized sedimentswith only minor changes.In particular.the carbonskeletonofhydrocarbons or other lipids is pieserved.ThJse moGculesrepresentonly a minor fraction ofcrude oils. However.they are of greatinterestto geologistsand geochemists'asthey provide information on the original organicmaterial. Alkanes, fatty acids, terpenes,steroids,and porphyrins are th€ major groupsof geochemicalfosiils. They cin be tracedfrom Recentto ancientsedimentswhcre they by other hydrocarbons irogressively suffer thermal degradation and/or dilution 'generatedat greaterdePths K"rog"n ir'u porrible additionalsourceof geochemicalfossils Somelipids may be trapped"inthe kirogen network, or alternativelyboundto kerogenby chemicalbonds' Thesemoleculesare releasedfrom kerogenwith increasein depth and temperature'
Chapter4 Kerogen:Composition andClassification
4.1 Definitionand Importanceof Kerogen The term kerogenwrll be usedhere to designatethe organicconstituentof the sedimentarv rocksthat is neithersolublein aqueousalkalinesolventsnor in the commonorganicsolvents.This is the most frequentacceptance of the term kerogen.and resultsfrom a direcr generalizarion to othei rock tvpesof the definitionby Bregcr(1961)in carbonaceous shalesand oil shales.However,it shouldbe kept in mind that someauthorsstill restrictthe namekerogento the insolubleorganic matter of oil shalesonly, becausekerogenoriginallywas appliedto the organicmaterialfoundin Scottishshales,whichyieldedoil upona destructive distillation.Sucha distinctionseemsvery artificialfrom a geochemical point of view, as the definitionof "oil shale"is itself mostlyan economic concept(a rock ableto providecommercialoil producrsby heating)andsubject to variations.accordingto the progressof technologyand the fluctuationof petroleumeconomy. A few authorsseemto usethe term kerogenfor the total organicmatterof sedimentary rocks.It is hereunderstood thatthefractionextractable withorsanic solventsis calledbitumenand that the term "kerogen"doesnot includesoluble b i t u m e n( F i g .I L 4 . 1 ) . As pointed out before, the early form (precursor)of kerogen in young sediments is the insolublematerialthat is alsocalled"humin" by soilscientists, althoughitscomposition is differentfrom compounds presentin continental soils. The maindifferencebetweenhuminof youngsediments and kerogenof ancient sedimentsis the existenceof an importanthydrolyzable fractionin humin;this fractionprogressively disappears at depth.In geologicsituations, thereis usually an informationgap, with respectto kerogenevolution,at relativelyshallow depthsof burial. Observations on core samplestaken by oceanologists often coverthe depthsfrom 0 to 10m (0 to 30ft). By contrast,coresamples takenbythe oil industryusuallystartat 500or 1000m depth.Somewellsdrilledon behalfof the JOIDES programmay help to fill the gap,althoughmanysectionscoredin deepoceanicbasinsshowlittle or no organiccontent.The observations reported in ChapterIL4 referto the compositions andpropertiesof kerogento the extent that they can be analyzedbelowthe previouslymentioneddepthgap. Furthermore, most of the considerations are related to the amorphousfraction of kerogen,that usuallyrepresents the bulk of the kerogen. It shouldbe notedthat the definitionusedbv Durand 0980) "the fractionof sedimentarv organicmatterwhichis insolublein rheusualorganicsolvents"isnot
and Classification Kerogen:Composition
132
Totolrock )
totol orgonicmotter> Heorf kolecu/cs conlotntogC,t/,0,5, // Uatecut0l relgnl
AsPhof tenes,f/
tl olet504 u'uot
+Resins
rocks sedimentary matterin ancient organic of disseminated Fig.IL4.1. Composition is strictly equivalent,exceptin ancientsediments,once the early diagenesis completed.In Recentsedimentsextractablehumiccompoundsare includedin the kerogendefinedby Durand and excludedfrom our definition.In ancient usuallystudiedin the petroleumindustry(i. e., belowthe samplinggap sediments mentionedabove),both definitionsbecomeequivalent:Fig. 11.4.1 Kerogenis the mostimportantform of organiccarbonon earth.It is 1000times more abundantthan coal plus petroleumin reservoirsand is 50 times more abundantthan bitumen and other dispersedpetroleumin nonreservoirrocks (Hunt, 1972).In ancientnonreservoirrocks,e.g., shalesor fine-grainedlimeusuallyfrom 80to 99% of the organicmatter,therest stones,kerogenrepresents beingbitumen.
4.2 Isolationof Kerogen The first and probably main hindrance when studying kerogen is to isolate withoutnoteablealterationof the generalstructure.This kerogenquantitatively preliminary isolationfrom the inorganicmaterialis requiredfor most physicalor when minerals- generallyfar morc which are constrained chemicalanalyses, abundant are present. Physical methods of separation based on difference of specific grariq (sink-float method) or differential wetting of the kerogenand mineralsby tuo immiscibleliquids,suchasoil andwater(Quassmethod),havebeenreviewed\ Robinson(1969).The advantageof thesemethodsis the absenceof chemical alterationof kerogen,but recoveryis normally incompleteand thus a fractiona-
4.3Microscopic Constituents of Kerogen
133
tion of kerogenmayoccur,generallyfavoringthe largerorganicfragments.These methods,however,aresuccessfully usedby somepetrographers, to reducepyrite contentwithoutappreciable alterationof ancientkerogens(DurandandNicaise. 1980). Destructionof the inorganicmaterial by hydrochloricand hydrofluoric acids has been widely usedand can hardly be avoided,if the aim is a quanutanve recoveryof kerogen.However,the acid treatmenthasto be performedunder mild conditions in a nitrogen atmosphere(Durand et al.. 1972). Chemical procedures shouldbe restricted to thatstepin orderto avoid,asmuchaspossible, an alterationof the chemicalstructureof kerogen.In particular,decantation in variousliquidssuchas CClaor CHCI3hasto be preferredto the eliminationof pyrite by oxydation(HNO3) or reducrion(LiAiH4, NaBH4.erc.) treatments, whichoxidizeor reducealsokerogen.Ferricsulfatehasbeenusedsuccessfully in verv acid conditionsto eliminatepyrite from ancientkerogens(Durand and Nicaise.1980).The procedurecannotbe applied,however,to kerogensfrom Recentsediments or from the earlvdiaeenesis interval. Pyriteis by far the mostfrequeniminlral remainingafterrrearment.followed bv verv fine-grainedrutile and zircon; however,the latter do not hinder the kerogenanalysis.A part of the pyrite is in intimateassociation with kerogen, e.g., the framboidaltypeof pyritedescribed below.Thus,it cannolbe removed withoutalterationof kerogen. During HF treatment,somecomplexfluoridesmay be formed,whichcanbe preventedby adequateoperatingprocedure,or subsequentlydissolvedby concentrated HCI (Durandand Nicaise,1980). A completeoperatingprocedurefor chemicalisolationof kerogenhasbeen providedby DurandandNicaise(1980),with a description ofthe atparatusused for routinekerogenisolation. Alpern (1980)comparedthe originalcontentof the principalconstituents of the organicmatterin five Toarcienshalesof the parisbisin wirh their contentin two organicconcentrates preparedby the chemicalprocedureof Durand and Nicaise,and the physicalmethodusedby coal petrographers. respectively. He found that a large proportion of the organicparticles,particul;rly the finer fraction(e.9., the finelydisseminated fluorescent materialwhichamountsto ca. 807c of this particularorganic matter), is lost by the physicalmethod of preparation.On the contrary,there is a preferentialconcentrationof larger fragments,particularlyidentifiablemicroorganisms.
-1.3MicroscopicConstituents of Kerogen Techniques of microscopic examination(naturallight reflectance and transmittance,or UV fluorescence) can be appliedto both kerogen-containing rock and previously isolatedkerogen.Microscopic observations havebeenin usefor along timein coalpetrologytoidentifythevariouscoalmacerals andevaluatetheirrank alongthe carbonization path that leadsfrom peat to anthracite.More recently, thesemicroscopic techniques havealsobeenappliedto studythe finelydissemi-
134
andClassification Kerogen:Composition
rocksin order to determinetheir gradeof natedkerogensin other sedimentary Much work has been devotedto that evolution(reflectancemeasurement). aspect,which will be dealt with later. Identificationand diagnosisof kerogen particleshas, however,receivedattentiononly more recently.First, isolated and coal petrographic work hasbeendone,mostlyas a resultof palynological studies:Combaz (1964), Staplin (1969),Alpern (1970),Hagemann(1974). ( 1975b), RaynaudandRobert(1976).Later,severalattemptswere Teichmueller approachto the identificaview of the microscopic madeto providea systematic tion of organic constituents:Rogers(1979), Combaz(1980).The principal conceptsare briefly reviewedhere, whereasthe practicalapplicationwill be in ChapterV.1. discussed
1.3.1 LocatingKerogenin Rocks rocksis difticult,evenb1 Locatingthe organicmatterin commonsedimentary statewithout becausemostof it is in a finelydispersed microscopic techniques. clear contours.Defined organicremnants(suchas algae.spores.pollen and vegetaltissues) usuallyamountto a minorpart of the organicmatterwith a range of a few percentof the total kerogen.However,in certainraretypesof rockslike torbanite.tasmanite,etc. (Fig. IL4.2) theyoccurin greatquantitiesln average and electron sedimentaryrocks.the combinedapplicationof UV fluorescence organicmaterialis fluorescent microprobehasshownthatmostof the amorphous at shallowdepthandcanbe locatedthroughthistechnique.Someminerals'such forms,or the Ca diagram but theircrystalline ascalcite,alsoexhibitfluorescence, obtainedwith electronmicroprobemakesthe distinctioneasy. allowthe Besidessuchstudiesof definedorganicremnants,thesetechniques namel!' matter, of organic observationof a secondfar more abundanttype bedded between laminated or often organicmatterwhichis finelydisseminated, mineralparticles,and especiallyclay minerals.In FigureII.4.3, fluorescence showsthe sizeandthe textureof thismaterialin the LowerToarcianshalesof the suchasthoseusedto purifycoal Parisbasin.It is obviousthatphysicaltechniques in separation) are of little assistance (fine grinding and density macerals separatingthis type of kerogenfrom the minerals.In fact, thesetechniquesandrecoveryof onlr appliedto shalesof the Parisbasin,resultin a concentration of the total organicmatter(Sect.4.2). a smallpercentage makes andfluorescence of reflectionmicroscopy Furthermore,the association evident the intimate relationshipbetweenpyrite and organic matter. For
Fig. II.4.2 a and b. Examplesof microscopicaspectsof kerogen(Photo A. Combaz).(a) Thin sectionsob6ervedin transmitted light. Left: boghead, Lower Permian, Autwl France;eachelongatedyellow body is a colony of Botryococcus(Chlorophyceaealgae)Rrglrr. faminatedcarbonatesourcerock, Devonian,Alberta; the dark brown bandsarc "saPropelic" q laminae rich in organic matter. (b) Kerogen concentrates.Left: "humic" organicmatter.Maestric+ amorphousorganicmatter,Turonian,Senegat.R€ht. tian.Senesal
I I
1.3 Microscopic Constituents of Kerogen
r.
'(
136
andClassification Kerogenr Composition
coatedwith example,pyrite commonlyoccursin the form of microcrystals organic matter and associatedto form sphericalaggregates(framboids). The microscope by the electronscanning is clearlyvisualized shapeof the aggregates behave as traps for trace (Combaz, 1970; Fig. II.4.4). Such assemblages elements,asprovedby electronmicroprobe.The association of organicmaterial andpyriteis sointimatethatit is not surprising thatno physicochemicaltechnique is ablefully to separateone from the other.
4.3.2 ldentification isolatedfrom the mineralsshowsthat, Examinationof kerogenconcentrates exceptin certaintypesof sedimentsformed by massiveaccumulationof a single rocks;the of kerogenis commonin sedimentary typeof organism.heterogeneity size,shape.colorandstructureofthe organicremnantsvary.Kerogenappearsto of organic be an association of variouskindsof organicdebris.The tiny fragments to an are not susceptible matteroccurringin finelylaminatedshalesor carbonates by directly or UV may show some structure. Largerfragments exactdiagnosis. and may be (acidic. oxidative treatments), artificial etching fluorescence, or after identifiedas spores,pollensand plant tissues,algae,etc. Gelifiedtissuesare frequent;other fragmentsinclude resinousexcretions,sclerotes.or various mlcroorganlsms, basedon opticalexaminationhasbeenproposedby A generalclassification and the individualorganicconstituents Combaz(1980).who first characterizes remnantsof terrestrialplants(sporesand pollen groupsthem in major classes: grains.vegetalfragments);algae and related microflora (Acritarchs,Leioscolonialand benthicalgae)l Dinoflagellates, phaeridaceae and Tasmanaceae, and miGraptolites,Scolecodontes microfauna(Chitinozoa,Gigantostraceae, granular.subcolloicroforaminifera ) ; and the amorphousfraction(aggregates' it becomes and association. dal, pelliculargelified).Basedon their occurrence whichcanbe comparedwith the typesof organic possibleto definep alynofacies, matterdefinedupon chemicalanalvsisof kerogen(Combaz.19tiO) More generalterms are often used for kerogenby petroleumgeologists. confusionalongthis line betweendescription Unfortunatelythereis sometimes "humic" and "sapropelic"organicmatterare and interpretation.For example with the respective typesof organicmatterin poor andgoodoil termsassociated
of theParisBasinby > of kerogenin theLowerToarcianshales Fig.IL4.3a-c. Observations UV fluorescencemicroscopy(Photo B. Alpern). (a) Fluorescenceof kerogenin rocks (polishedsection). tetr abundantfluorescenceallows one to locat€ the finely disseminated organicmatter in a shallow immature shale (F6cocourt).RiSrrr fluorescencehas in a deepshaleburiedat 2500m (Bouchy).(b) Kerogenconcentrate almostdisappeared (c) Kerogenconcentrate isolatedfrom shaleby techniques. isolatedfrom shaleby chemical and pollen grainsare physicaltechniques.Larger fragmentsof algae(Tasmanaceae) proportionally more abundantthanin the originalrock
'1.3Microscopic Constituents of Kerogen
Fig. IL,1.3a-c
137
llu
andClassification Composition Kerogen:
'l'hey are also basedon palynologicaltype preparation.examinedin sources. transmittedlight (Fig. IL4.2). Humic organicmatter refersto kerogenwhere organicmatter to be a majorcontributor.Sapropelic planttissuesareconsidered kerogen.exhibitingan is commonlyusedto designatea finely disseminated amorphous,cloudy, or vaporousshapeafter acidic destructionof minerals optical of kerogenshowsthat the sapropelic analysis However,physicochemical to the is comparable typemayincludesomekerogenwhosechemicalcomposition n.8). h u m i ct y p e( S e c t i o 4
1:!
(framboids): of pyriticmicrocrystals aggregates spherical Fig.IL4.,1. Kerogencontaining and the microscope' Silurianof Lib],a.The photographwastakenby electronscanning is framboid of a single pm Below: thedetail picture . 70 is ca horizontaldimensionof the with a thinfilm of organicmatter'The diameter shown.Eachgrainis a pyritecrystalcoated
oftheframboldsisca.i0,am,andthesizeoftheindividualgrainsis03to06,rrm(Comb 1970)
.1.4ChemicalandPhysical Determination of KerogenStructure
139
Observationsmadein reflectedlight are usuallydescribed in termsof liptinite, vitinite andinertinite.This order correspondsto increasingvaluesof reflectance in the samepieceof rock, Liptinite maceralsare consideredto be derivedmostly from algae (alginite)or spores(sporinites),with occasionalcontributionsof cutin, resins,waxes,etc. Vitrinite and inertiniteare usuallythoughtto have issuedfrom higherplant tissues,the structureof which is still recognizablein the oxidizedinertinite,whereasvitrinite has passedthrough a gelificationstage. Other constituentsare more difficult to relate to a specificorigin: bitumites (Alpern, 1970),bitumens(Robert, 1971),bituminiteand exudatinite(Teichmucller,1974).Theyareprobablynot strictlyequivalent, despitethesimilarityof names. The combinationof reflectance andfluorescence microscoDv sometimes makes possiblethe distinctionbetweenreworkedandcontemporaneous organicmatter in sedimentarysequences. The organicconcentrates of rockscontaining"sapropelic"kerogen,taken at shallowor moderatedepth,normallyshowa strong fluorcscence. However,somefragments differby theabsence of fluorescence and by strong reflcctances.Thesefragmentsfrequently representreworked organic materialthat oncehadbeenburiedto greatdepth,andthereforehasgainedhigh reflectanceand has lost fluorescence. Obviously,this organicmaterialcomes from the erosionof older rocks. As highly carbonizedorganicmaterial(like anthraciteor other high-rankcoal) is fairly resistantto weathering,it can be transported to the new depositional basinand mixedwith youngorganicdebris. Hagemannhas shown the occurrenceof "trimacerals",which are a typical reworked detrital componentof coal. There are also typesof organicparticles, which show a high reflectancee.g., fusinite,althoughthey have never been buried deeply.The high reflectanceof theseparticlesis probablycausedby oxidationprocesses at the sudace,eithermicrobiallyor by abiologicalmeans. Evenpeatscontainsuchhigh reflectingparticles.
4.4 Chemicaland PhysicalDeterminationof KerogenStructure Differentphysicaland chemicaltechniques of analysisare ableto givevaluable information on the structure of kerogen. However, as shown in the previous discussion,kerogen is composedof different basic constituentsin varying proportions.Some of them can be identified as macerals,the rest being frequentlyan amorphousmass.Therefore,an analysisof a groundrock sample resultsusuallyin a globalfigure,possiblyintegratinga wide rangeof individual compositionsfrom different constituents.The only exception is the electron diffraction method, which givesa true imageof the particlestested,evenin the sizerangeof severalangstroms.The problemof integratingthe variousproperties of different constituentsis particularly acute when reworked material, already deeply transformed, is reincorporatedin a new sediment.For this reason, a preliminarymicroscopic examinationis alwaysadvisable. From a quantitativepoint of view, amorphouskerogenis usuallyfar predominant over recognizableorganic debris. Therefore, all chemical and physical
140
Kerogen:
Compc.ition
and Cl.r.si(icatron
results reviewed below can be considered as mostly related to amorphous kerogenunlessotherwisestated. The principal techniquesof chemicaland physicalconstitutionalanalysishave beenreviewedby Robinson(1969)for kerogenof the GreenRiver formation, and by Durand et al. (1972)and Espitali6et al. (1973)for the Lower Toarcian workingon several shalesof the Parisbasin.More recentlya groupof scientists, of organicmatterfrom differentgeologicalorigin, provideda evolutivesequences and their contributionto completereviewof the principalanalyticaltechniques of kerogenstructure:Durand and Monin (1980),Durandthe understanding Souron(1980),Rouxhetet al. (1980),Oberlin et al. (1980),Marchandet al. (1980),Vandenbroucke (1980).
4.5 ChemicalAnalysis a) Elementalanalysisshowsthat carbon and hydrogenare the main atomic constituentsin all types of kerogen(Table 11.4.1).For 1000carbon atoms, hydrogenvariesbetween500 and 1800atoms,dependingon the type and the evolutionof the organicmatter.The next abundantatom, oxygen,amountsto only 25 to 300atoms.Usually,nitrogenandsulfuratomsare lessabundant,5-30 sulfurand 10-35nitrogenatomsper 1000carbonatoms. b) Bitumenextraction.Bitumen canbe separatedfrom kerogenby extractionwith organicsolvents.It is reasonableto expectdifferent kerogensto yield different haverelativelylow bitumens.Among the constituents of bitumen,hydrocarbons to mediummolecularweights(generallybelow 500).Therefore,they are not likely to providevaluableinformationon the structureof the insolublekerogen. (thepartof bitumeninsoluble like asphaltenes On thecontrary,largerstructures, pentane hexane, heteroatoms, with a molecularweightof several in or containing thousands), may providemore information.Analysesthat are feasibleon both etc.) show a and kerogen(elementalanalysis,IR spectroscopy, asphaltenes broadsimilaritybetweenthesematerials,andalsoa parallelvariationwith burial depth.Asphaltenes, beingsolublein severalsolvents,are easierto analyze.A generaldescription of asphahenes from petroleumandof their structureis given are composedof aromaticsheets, by Yen (1972).He notesthat asphaltenes heterocycles, and aliphaticchains.They are similarto smallentitiesof kerogen. Because of their lowermolecularweight,theyarepartiallysolubleandthusmore are easilyanalyzedthan kerogen.Compositionand structureof the asphaltenes discussed in ChapterIV.1.8. c) Oxi(lstion, hydrogenolysisand pyrolysis are three methods of degrading whichareeasierto analyze. kerogento form lowermolecularweightcompounds, potassium perman(1969) have shown that alkaline Robinson andco-workers ganateoxidizesthe kerogenof the GreenRiver formation into oxalic andvolatile acids and CO2. Using chromic acid, Burlingameet al. (1969)were able to on kerogen.Philp and Calvin (1977) determinepart of the chainsubstituents
\N\\$\\t\\\t\tt\\Nt$\t$\t\-\\K.NK\$\\\Ktt$\NgN\stu\\\\\ B aj a C a l i f o r n i a .
4.5 ChemicalAnalysis
t4 i,
Table II.4.1. Elementarycompositionof sometypical kerogens 3D uasln g I or countrv gi.
Piceanc€basin
Uinta basin
Uinta basin
Uinta basin
Australia Australia France
Formation and/or age
Green River shales, D Eocene Green River shales, D Eocene Green River shales, C Frcene Green River shales, C Eocene Coorongite, Recent D Tasmanite, Permian D Aulunboghead, Permian C
Parisbasin L. Toarcian D Parisbasin L. Toarcian C N. Saharabasin Silurian C N. Saharabasin Silurian M Esthonia(USSR)Kukersite, Ordovic. D Rhine Gratren Mess€lshales, D F,ocene
III
Douala basin Douala basin Doualabasin
lJ. CretaceousD U. CtetaceousC U. CretaceousM
Yugoslavia
Lignite, Plioceneb D Rn : 0.43Ea Coal, Carboniferousb C Rm : 1.22Vo Anthracite, Carboniferousb M Rm:2.'75Ea
Germany
Germany
Weidrt qa
c
H
O
N
Atonic Ea H
O
N
76.510.010.30.6 2.6 37.258.33.7 0.3 0.5 15.9 9.1 8.4 3.9 2.6 38.755.93.2 1.1 0.5 80.9 8.6 4.4 3.8 2.3 42.4 54.0 1.8 1.0 0.8 a2.7 1.1 8.3 t.7 3.2 58.8 34.9 4.4 t.O O.g
77.5r0.8 9.3 0.4 2.0 35.960.43.2 0.2 0.3 75.9 9.4 8.8 2.1 3.8 38.4 56.73.3 0.9 o..l a2.2 9.9 4.7 1.3 2.5 39.85?.71.5 0.5 0.5 12.6 45.4 80.6 lrs.4
7.9 72.42.1 1.9 40.3 52.55.2 1.r 5.0 2.3 0.2 48.4 48.42.1 5.9 6.4 3.4 3 . 8 50.344.13.0 3.5 5.6 2.1 6 3 . 63 1 . 03 . 1
1.0 1.0 r.1 0.0 0.9 1.8 0.9 1.4
73.5 8.3 15.60.4 2 . 2 39.553.66.3 0.2 0.4 69.3 8.3 18.02.6 1 . 8 37.453.71.3 1 . 2 0 . 4 't2.7
6.0 19.02.3 0.0 45.244.8a.8 1.2 0.0 83.3 4.6 9.5 2.1 0.2 55.138.94.7 1.2 0.0 91.6 3.2 2.9 2.0 0.3 68.129.01.6 1.3 0.0
68.6 5.1 21.2 2.6 2.5 46.0 41.2 1o.i 1.5 0.6 88.3 5.0 3.9 2.0 0.8 57.5 39.3 1.9 l.l
0.2
91.3 3.2 3.2 1.8 0.5 68.0 28.9 1.8 1.1 0.2
Evolution stages:D diagenesis;C catagenesis; M metagenesis. Rtn reflectanceof vitrinite.
I42
Kerogen:Composition andClassification
Vitorovii et al. (7974,1977)useda methodof stepwiseoxidationof kerogen from Aleksinac shale(Miocene, Yugoslavia)by alkaline permanganate.Oxidation productsof eachstepwere analyzedseparatelyby gaschromatography-mass spectrometry.They provedthis kerogento consistof two typesoforganic matter: one is more aromatic;the other, mainlyof aliphatictype, is more abundant.A generalreviewof the main oxidativestudiesof kerogenis presentedin Vitorovic (1980). Pyrolysis with a subsequent analysis of the pyrolysis products by gas wasalsousedwith the sameobjective.In particular,Espitalid chromatography et al. ( 1977)developeda pyrolysismethodfor classificationof the varioustypesof kerogen, based on the relative contents of hydrocarbonsand carbon dioxide generatedduring pyrolysis.Horsfield and Douglas(1980)pyrolyzedvarious kerogenconstituentsand coal macerals.B€har et al. (1984) have studied chromatography showinga great by pyrolysis-gas kerogensand asphaltenes similarityin composition.Furthermore,the pyrogramsare diagnosticof the different kerogentypes. Hydrogenolysishas been extensivelyused by coal chemists. d) Functionalanalyslshasbeendevelopedto evaluateoxygenfunctionalgroups. For the Green River formation,Festerand Robinson(1966)determinedthe distributionof oxygenbetweenthe variousgroups:carboxyl157o,ester25Ea. hydroxyl,carbonyl,amide6.5Vo,unreactiveoxygen(probablyether) 52.5%. Later, Robin and Rouxhet (1976)combinedreactionsselectiveof functional groupswith IR spectrophotometry to evaluatethe respective amountsof these groups,and especiallythoseincludingoxygen.Their respectiveamountsvary greatlywith increasingdepth. A selectivedegradationof etherbondsenabled Michaelisand Albrecht (1979)to identify acyclicisoprenoidstructurespresentin kerogen,and to relatethem to membranelipidsof archaebacteria.
4.6 PhysicalAnalysis a) Electron Diffraction and X-ra, Electrondiffractionhasbeenusedby Oberlinet al. (1974)to showthe degreeof organizationofcarbon structuresin amorphouskerogenat differentscalesfrom 5 of to 500A and their evolutionduringburial.The methodshowsthe existence stacksof two to four more or lessparallelaromaticsheetsin kerogen.The to lessthan diameterof an aromaticsheetis between5 and 10A, corresponding ten condensed aromaticcycles.In additionthe aromaticsheetcanbearaliphatic or functionalgroups,but thesegroupsare not seenby thistechnique.The most frequentnumberof sheetsper stackis two, but sometimes threeor four occurThe interlayerdistancerangesfrom 3.4A to 6 or 7 A in shallowkerogensbut it is relativelynarrowin deepones,with a maximumfrequencyfrom 3.4 to 4.0 A. In shallowkerogensthe orientationof the buildingblocksis completelyrandom. depth,largeraggregates or clusterstendto emerge;the aromatic With increasing sheetskeep their size (5-10 A) and individuality,but becomeprogressiveh
,1.6Physical Analysis
143
parallel.The extensionof such aggregates may reachg0-500A in very deeo samples.Howeverthe orientationof the aggregates are in mutualdisorder, The electronmicro-diffractiontechniquehas been usedto study the artificial carbonizationof industrial substances. It allowsone to locate carbondiffracting structuresdown to 10 A or less.The dark field imagetechniqueis particularl! interestingfor kerogenstudies:a diaphragmplacedin the focalimage-planegivei way only to the beamsat a preselected 2 O angle,corresponalng to (bOZ;o1ttre carbon lattice. Thus an imageon a black backgroundis fbrmed, where tire only illuminatedareasare thosewherea carbonaceous structureis Dresent.The useoi this techniquein combinationwith normal electronmicroscoDy on the same samplemakespossiblethe locationof thesestructures(Fjg.ILri.5). The latticefringe imagetechnique,usinginterferencesof normalanddiffracted beamsin combinationwith a very high magnification (x 5,000,000_g,000,000), allowsa visualization of the edgeof the individualaromaticcarbonsheets.Thui. it becomespossibleto measurethe distanceand the diameterof the associated aromaticsheets(Fig. ILa.6). A review of electron diffraction techniquesand their application to under_ standingkerogenstructureis providedby Oberlinet al. (1ggb). More classicX-ray diffractionhas been used successfully in coals and in petroleum asphaltenes.A carbon crystallineorganizationis known to emerge progressivelyin coalswith increasingrank from lignite to anthracite.On the othir hand,Yen et al. (1961)haveusedX-raydiffractionandothertechniques in order to proposea structureof the asphaltenes. b) InfraredSpectroscopy (Absorption) Thistechniqueallowsan evaluationofthe relativeimportance ofcarbonyland/or carboxylgroupsversusaliphaticchainsplus saturatedrings.However,rr ls not possibleto discriminate betweenthe lattertwo. The progressive disappearance of carbonylandaliphaticgroupsis alsoindicativefor the diagenetic andcatagenetic evolutionof kerogens. Infrared spectraof kerogencloselyresemblethe spectraobtainedfrom coalor asphaltenes. The mostlikely assignment of the variousbandsis shownin Figure II.4 7,. based on publicationsof King et al. (1963),Erdman (19?5), Tltlll Espitali6et al. (1973)andRobin( 1975,1976).ihe mainzoneiof inleresrseemro be: a wide asymetricbandcenteredat 3430cm r, relatedto OH groups(phenolic, alcoholic,carboxylicOH); - a strongabsorptionresultingfrom severalfine bandsbut showingmostly two maxima(2920and 2855cm -'), relatedto CH, and CH3aliphaticgroups; a wide band with a maximumaroundt 710cm-r, attributedto vaiiouj C=O groups(ketones,acids,esters): a wide band with a maximumaround1630cm r, mostlyrelatedto aromatic C=C, althoughthere is definitelysome contributionof other srrucrures: olefinic"C=C,particulartypesof C=O groups(bridgedquinoniccarbonyl), and of free water:Robin and Rouxhet(1976i:
Kerogen:Compositionand Classification
l,l4
n'-\\ = \\s ,,
.ttt
u,1
)
7=
=
1,_rr)s)=--7 \\:\ =
\\ ,
\\):'
\\ ll ll \\
t-\\
lllumindtedareos Fig. IL,l.5a c. Electronmicrographof kerogen:dark field technig\e, the stacksof where kerogen' (af Immature structures are relatedto carbonaceous diagenesis to the corresponds sample The random. at aromaticsheetsarestillorientated (b) Verr started not have rearrangements 'vet ;;;;g;;..i; boundarl-.and catagenetic aromatlc clusters these Within appear' or clusters matrirekerogen.where aggrcgatcs metagenesrs the catagenesis to corresponds sample The tl.",titr"",a p"*ff"l orieritatiin. of the representation arecompleted1c1Schematic rearrangements catagenetlc boundaI,vi (riqht) (lelt) (b) (a\ and to respectively ori.ntntinnof ihc aromaticshietscorresponding ( A . O b e r l i na n dJ . B o u l m i e r )
-1.6Phvsical Anrl\iis
l.l5
t!!
:=-*,'
t00I Fig.ILJ.6a r:. Electronmicroqraph ofkerogen:latticefriDgetechnique. Thistechnique. usrnginte.ferenccs .f normalanddiffractcdbeams.allowsi visualizaiion of the aromatic sheets.Theseare markedin the micrographbv a succession of severalparallelblackand whitestripcs.(a) Iop: vervmatulekerogencorresponding to thecatage;esis-metagenesis boundar),(vitriniteretlectance 2.4.,i). Bottorn:dcvelopmentof the-aromattcsheetsin anthracite is shownforcomparison: (b)naturalanthracitc; (c)anthracite heatedto 1000.C (A. Oberlinand J. Boulmicr)
ion undCla'silicrtion Kerogen:Composit
I46
CH2tCH3
2OOO
i6OO
Fig. II.,1.7. Typical lR spectrumof kerogen(tlpe ll) The-/No.inre/i show specific abiorptionbandsof two other kerogcns:(a) 720 cmrI relatedto long aliphaticchains mostlyoccurringin type-l kerogen;(b.)930to 700cm relatedto aromaticCH (bending out oi plane)mostlyoccurringin very maturekerogen.(Adaptedfrom Robin 1975)
t, an absorptionband at 1455cm due to CH3 and to linear and cyclicCH1 r. g r o u p sa. n da n o t h e rb a n da t 1 3 7 5c m r e l a l e dt o C H , o n l y : r, a very wide band from 1400to 1040cm that includesC-O stretchingand OH bending; of weakbandsfrom 930to 700cm-t, relatedto variousaromatic a succession on the numberof adajcentprotons: CH (bendingout of plane)anddependent a 720cm t band.due to aliphaticchainsof 4 or morecarbonatoms.
c) NuclearMagneticResonance in asphaltenes (NMR) hasbeenappliedto dissolved Nuclearmagneticresonance some of their degreeof aromaticity.Recently, liquid phasefor characterization new possibilitiesof applying NMR to keroge-nwere offered by the Cross NMR. This techniqueoperPolarization,Magic Angle Spinning(CP/MAS)13C carbon ateson the solidphaseandallowsan evaluationof the aliphatic/aromatic alterationof the thermal characterize in kerogen.Denniset al. (1982)appliedit to black shalesin the EasternAtlantic by basalticintrusions.Miknis Cretaceous et al. (1982)usedthesametechniqueto evaluatethegeneticpotentialofoil shales'
,1.7GeneralStructureof Kerogen
t47 Fig. II.4.8. Structural model of macromolecular arrangementsoccurringin kerogen(type II). Topl immature kerogen, corresponding to a low evolutionstage.Bottom: the samekerogenafter reachinga high evolutionstagethroughcatagenesis. (After Tissorand Espitali6,1975)
-
Aromolc cycLes
-
,\r.
solumred cycres
^
Feterocyces r
A,phot c cho ns
Thereare, however,limitationsto the interpretationof NMR specrraoue to the complexityof kerogen(Miknis er al., 1982). d) ThermalAnalysis Differential thermal analysis(DTA) has been applied to kerogen by von Gaertnerand Schmitz(1963).More recentlyEspitaii6et al. (1973Jusedboth DTA and thermo-gravimetric analysis(TGA) for stuclyingkerogen.These techniques maygivesomeindicationof the structureof keroqen,but their main c o n t r i b u l i oins t o l o l l o wr h ee v o l u t i o n o f k e r o g e nt C h a p .I I . 5 , ) . When combinedwith a techniquefor characteriiingthe productsfrom pyrolysis,however,TGA providesvaluableinformationon the kerosenstructural type. For example,Souron et al. (1974, 1980)usine TGA ind mass spectrometry,have shown the relative importanceof aliphatic.naphthenic andaromaticstructures in severaltypes ofkerogen,andalsotheirchanges during catagenetic evolutionof the lowerToarcianshalesfrom the parisbasin.Comoa_ rabletechniqueshavebeenusedby Juentgenon coal,and by Marchandet al. (1974)on sporesand pollens.
4.7 GeneralStructureof Kerogen(Fig. II.4.8) A generalstructureof kerogenmay be proposedasa resultof the dalacollected from variousanalyticaltechniques.The proposedstructureappliesmainly to amorphouskerogenbecauseof the large predominanceof amorphousover
andClassification Composition Kerogen:
148
kerogenis a structuredmaterialin the kerogensstudied.Thc averageamorphous lt consistsvery probablyof nucleithat are macromoleculc. three-dimensional crosslinkcdby chain-likebridges Both thc nuclei and the bridgesma1'bear functionalgroups.ln addition.lipid molcculescanbe entrappedin the kerogen matrix.asin a molecularsieve. a) Nuc/eiare formedby stacks.comprisingtwo to four more or lessparallel aromaticsheets.Eachof the sheetsor layerscontainsa relativelysmallnumber heterocycles (lessthan 10) of condcnsedaromaticrings. includingoccasional such c1'clic of The diameter containingnitrogen.sulfur andpossibl.voxygcn two per is frequently of lalers is smallerthan l0 A. The number assemblages in shallow wide range with a 3-.'1 A. greater than stack.anJ thc interlaycrdistancc The kerogcns buried in deepll J.7 A arouncl kerogen.and a narrowmaximum organrzatlon whose increastng of kerogen. blocks stacksare the basicbuilding with depthdetermincsthe e\olutionarytrendof the organicmattcrin sedinrents' on Thesenuclcibearalkl'l chains(linearor Iith few shortbranchcs)substituted g r o u p s . v a r i o u s f u n c t i o n a l a n c l r i n g s . r i n g s . n a p h t h e n i c the suchas: b) Bridgesma)'consistof structuralarrangements - l i n e a ro r b r a n c h e da l i p h a t i cc h a i n s :- ( C f l r ) " - . a t t a c h c dt o t h e n u c l e ia s substitucnts oxygenor sulfurfunctionalbonds:kctones:-C-
o ester: C-O
. c t h e r : O - . s u l f i d e-: S
. or disulfide: S-S
o - combinationof an aliphaticchainR with a functionalgroup.e g . aliphatic e s t e r-: C - O - R
o on nucleior chains,may includevarious c) Superficialfunctions.substituted g r o u p ss u c ha sh y c l r o x v -l :O H , c a r b o x y l-:f - n " m e t h o x y-: O C H r c t c .
o that kerogenshavemolecularsievepropertiessimilar d) Therc are indications to thoseobservedin coal.A hint in this directionis the fact that sampleswhich have first been cxtractedexhaustively.and thcn treatedby acid for kerogen isolationreleasedadditionalhydrocarbonmolcculesupon a ncw extractron. Oxygenseemsto play a particularrole in the structureof kerogen.It may of kerogenbv weight in shallow,immaturesediments amounttofrom l0to25c:/c The highestvaluesoccurin detritalformations.wherethe input of terrestrial of the Doualabasin. plantsis important.e. g.. kerogenof the UpPerCretaceous An evaluationof the proportionof carbonvland type IIl, see Section11.,1.8. carboxl'lC=O groups.versusother oxygencombinationscan be made by comparingthe l7l0 cm ' C = O bandon I R spectra.with thetotaloxygcncontent. I n t h e L o w c rT o a r c i a ns h a l eos f t h e P a r i sb a s i n( F i g .I I . a . 9 ) .o x y g e na m o u n ttso depth With incrcasing an averagcof l0'.,ib1"weightin shallowimmaturesamples. thc C - O bandsis rcduceclto zero. itnd an averageamountof 511oxygenstill
4.7 GeneralStructureof Kerogen
149
t5 O x y g e n( w e r g h r% )
F i g , I 1 . 1 . (C, o m n a r i r o n n t o \ \ p e n c o n t e n t ( e l e m e n t a rav n ad nI R a lavbs si so)r p t i b oa nn d C : O a r l T l t Jc m | ( K l T l U ) k: c r o g eonf t h eL o w e T r o a r c i asnh a l e p s .a r i st s a s i n( D . ata fromRobin.1975)
remainsin kerogen(Espitalidet al.. 1973).Thus,abouthalfof theoriginaloxvsen was presentin carbonl'lor carboxvlgroups.Computedas C=O,-theseli[ilc groupsaccountfor up to I 0 7. by wcightof thetotalimmaturekerogenof theparis basin.The proportionwouldbe ca.20 to 30Vcin kerogenof detrilalseries.with abundantcontributionof hither plants(typeIII, Douila basin).The restof the oxygenis likely to bc presentin ether bonds.in phenols.or in heterocvclic \ l r u c lu r e \ . Otherobservations. alsomadeon the Lower'I'oarcian shales of theparisbasin. providesomeinformirtionon thc locationof thc C=O groups.At a depth of 2 5 0 0m . w h e r eb i t u m e ni s a b u n d a n rt h . e l 7 l 0 c m ' C = ( ) b n n i w a sf o u n dt o b e more pronouncedin the total organic matter - especiallythe extractable bitumenfraction- than in the kerogen.This fact suggests ihat carbonvland carboxylgroupsare morefrcquentlvrelatedto aliphaticchainsand smallcyclic structures thanto polvcondensed aromaticandheterocyclic nuclei.Furthermore. the inverserelationship (Fig. ll.4. l0) betweenthe amountof bitumenqenerated andthe intensityof the C= O infraredbandin kcrogenis an indirectprool t hatin thisparticularshalentanl'petroleumconstituents are boundto kerogenby ester hondings. Nitrogenof peptidicbondsandfunctionalgroupsseemto havebeenremoved d u r i n gs h a l l o wd i a g e n e soi sf t h c o r g a n i cm a t t e r( C h a p .I L 2 ) . T h e r e m a i n i n g nitrogen.in kerogengenerallvamountsto 2 to 3Vc b1, weight and mav bi essentiallv presentin heterocvclic condensed structures.becauseit is releasecl onlv to a limited extentduring catagenesis but more abundantlyduring hightemperaturepyrolvsisexperiments. e.g.. Fisherassal,s and oil-shaleretorting. However.a certain proportionof nitrogenis still presentin the condensed polvaromaticnucleiof the mostcvolvedkerogensobtaineduponpyrolvsis.
150
Kerogen:Compositionand Classification
tI
between Fig. II.4.10. Relationship the amount of hydrocarbons extracted from the Lower Toarcian shales of the Paris Basin and the intensity of specific IR absorption bandsofthe insolublekerogen:Topr C = O b a n da t 1 7 1 0c m - ' ( K 1 7 1 0 ) . Bottom: aromatic CH bands from 930ro 700cm-r (K 930-700).These of aliphatic aredueto desubstitution chainsfrom aromaticnuclei.(Data from Tissot et al., l97l: Robin, 1975)
I
o
; " l
; l 6
: a
;t i l 6 l 3.I = o o
^\oN>"-
t'"9
d
-
Hydrocorbonsgeneroted------->
( m g N C/ g o f o r q o nc c o r b o n ) m e o s u r €odnt h e s o l u b l eb r l u m e n
Sulfur is probablypresentin both forms, asan atomicconstituentof heterocyas it is released cles,andpossiblyassulfideor disulfidebonds.Duringcatagenesis and later ashydrogensulfide. derivatives, benzothiophene There is definitelysome relationshipbetweenthe respectivestructuresof from crudeoils.Kerogens from bitumen,andasphaltenes kerogens, asphaltenes from bitumen are especiallysimilar,judgingfrom elemental and asphaltenes etc. Therefore,this type of asphaltenehas to be analysis,IR spectroscopy, thought of as small fragmentsof the kerogenstructure.Although they have an overallcompositionsimilarto that of kerogen,the molecularweightof asphaltenesfrom bitumenis smaller:therefore,they are solublein the usualorganic solvents. Asphaltenesprecipitatedfrom crude oils have been studiedby numerous by Yen andtheir structurehasbeendescribed physicalandchemicaltechniques, (1972).The overallarrangement is somewhatsimilarto thoseof kerogensandof from bitumen.The oxygencontentis ratherdifferent,however,as asphaltenes shown by elementaryanalysis(Table 1V.1.8, Chap. IV 1) Oxygen atoms kerogenor bitumen represent1.5-10% of the total numberof atomsconstituting This differencecan be but i.57o or lessin crudeoil asphaltenes. asphaltenes, evolution during catagenetic bonds explainedby the eliminationof heteroatomic the most of migration efficiency and formationof petroleum,and alsoby a low asphalticfraction. oxygenated
4.8 Depositional Environmentand Composition of Kerogen:Evolutionpaths
l5l
4.8 DepositionalEnvironmentand Compositionof Kerogen: the Evolution Paths The chemicalstructureofa shallowimmaturekerogen(relativeimportanceofthe different nuclei, bonds and functions) varies with the oriein;l mixture of organismsand with the physical,chemicaland biochemic-al conditionsof deposition. These differencesare attestedby microscopicexaminationof the constituents, elementaryanalysis.lR spectroscopy, degradativeoxidationand varrous experiments of pyrolysis. The variations inherited from the vouns sedimentremainsubstantial for shallowancientsediments, but becomeprosresl sivelyweakerwith increasing burialdepth.However,the differences canstiil be re_cognized over a longtime anddepthspan,i. e. , until the beginningof the zone wheregasis generatedby thermalcracking. The globalatomiccomposition of the threemajorelements(C, H, O) is shown . in FigureIL4.11, whichis a graphof atomicH/C versusO/C ratios.Thisfieure providesa usefulapproach to classification ofkerogens(Forsman,1963;Mcl-ver, 1967;Welte, 1969;Durandet al., 1972).Kerogenstakenat variousdepthsfrom th_esameformation normally group along a curve, calledhere an evotuiion path. Closelyrelatedenvironments of depositionresultin the samepath (Tissot;t al., 1974).These different curvesstart with different hydrogen:oxygen rarlos, accordingto the original organicmaterialand conditionsof deposition.The curvescome together for very deep samplesas the kerogenapproaches1002o carbon.Thisplot wasfirst usedby van Krevelen(1961)to characterize coalsand their coalificationparh (Fig. II.4.12).We proposeto call rhistype of diagrama "van Krevelen diagram". The extremetypesof disseminated organicmattercorrespondon one hand (type I) to kerogen,which is rich in aliphaticstructuresand consequently in hl9r9q.n'' as in somealgaldeposits;on the other hand (type III) to kerogen, whichis rich in polyaromatic nucleiandoxygengroups,like oiganicmattermade of plantsfrom terresrrialorigin (Fig. IL4.11). a) TypeI relersto kerogenwith a highinitialH/C atomic(ca.1.5or more)anda , low.initialO/C ratio (generallysmallerthan0.1).Suchkerogencomprises much lipid material,particularlyaliphaticchains.The contentof polvaromaticnuclei andheteroatomic bondsis low, comparedwith the othertypesof orgon,"*u,,... The small amount of oxygenpresentis mainly found in ester bonds.When subjectedto pyrolysisup to 550or 600.C, the kerogenproducesa larger yield of volatileandTorextractablecompoundsthan any other type of kerogen(up ro g07o by weightfrom shallowimmaturesamples)and likewisea higheryieid of oil. Paraffinichydrocarbons are predominantover cyclicstruclures. The highproportionof lipidsmay resulteithei (l) from a selectiveaccumulation of algalmaterialor (2) from a severebiodegradation of the organicmatter (other than lipids and microbialwaxes)during deposition.The first source includesorganicrich sedimentsmade up mostly of algae,particularlythose derived from lacustrineBotryococcusand associatedfoims (Marchand et al., 1969),e.g., Autun andCampinebogheads, torbanitefrom Scotland.cooronqite from South Australia,and their marine equivalents,such as tasmanitef;m
Kerogen:Compositionand Classification
tJz
/'= ,,?/l-
TyPr
I
oorl" o' hJn.c (oo s (ot.r Du'ond .r o ,'976) B o u d o n . s o l r h e l r e l d0 l i e r o q e n Evollt on poth. ol the p.inc pol lyP.s ol k.rog.n \ro-
Aga tnd /or lormation Alqo k.rog.ns (Bokrococcu3,.tc Lvorious o 5hoL.s Lor.r Too.cion shol€s
l
It
Brain,country
G . . n R i v . r l h o e ! ( P o l e o c e n-. E o c . n . )
Poris,Froncc,WG.rmon) Sohorc,Al9.rioondLibro
Lo*.r Moonvill! 5hol.. L o w . r M o n n v i r lseh o i c s ( M c | v . r , 1 9 6 7 )
Fig. IL4.11. Pdncipaltypesand evolutionpathsof kerogen:typesI, Ilandlllaremost frequent. Kerogens of intermediate composition also occur. Evolution of kerogen burialis markedby an arrowalongeachevolutionpathI, II with increasing composition and IIL We proposeto namethis type of diagramafter van Krevelen
,1.8Depositional Environmentand Composition of Kerogen:EvolutionPaths
E
153
Fig. IL4.12. Evolution paths of maceralgroupsin coals. (After van Krevelen,1961)
Tasmania.The secondsourceincludessomerich oil shales.wheredisseminated organicmatter has been extensivelyreworkedby microorganisms, so that the kerogenis derivedmostlyfrom a biomass of reworkedandothermicrobiallipids. Thissituationseemsto occurparticularlyin lacustrine environments. The prolific oil shalesof Colorado.Utah and Wyoming(GreenRiver shales)seemto result from a combinationof both algaland microbiallipids.The occurrence of type-I kerogenis relativelyrare comparedto the othertypes. 6) Ty"peII is particularlyfrequentin many petroleumsourcerocksand oil shales,with relativelyhighH/C andlow O/C ratios.The polyaromatic nucleiand theheteroatomic ketoneandcarboxylicacidgroupsaremoreimportantthanthey are in type I, but lessthan in type III. Ester bondsare rather abundant(Fig. II.,1.13).Saturatedmaterialcomprisesabundantnaphthenicringsand aliphatic chainsof moderatelength.Sulfuris alsopresentin substantial amounts,located in heterocycles andprobablyalsoassulfidebond.In the associated bitumen,the cyclic structures(naphthenic,aromaticor thiophenic)are abundantand the sulfurcontentis higherthan in the bitumenassociated with the othertypes. Type-lI kerogenis usuallyrelatedto marinesediments wherean autochthonousorganicmatter,derivedfrom a mixtureof phytoplankton, zooplanktonand microorganisms (bacteria),hasbeendepositedin a reducingenvironment. The yieldfrom pyrolysisof thistypeof kerogenis lowerthantypeI, but it may still provide commercialoil shales.For example,on pyrolysis,the Lower Toarcianshalesof the Parisbasinprod,]uce ca.607oof theinitialweightof organic matter. In dispersedstate,thesekerogensare the sourcematerialof a great numberof oil or gasfields:DevonianandColoradogroupof Cretaceous agefrom WesternCanada.Paleozoicfrom North Africa, someCretaceous and Tertiary sourcebedsof WestAfrica, Jurassicof WesternEuropeand SaudiArabia, etc. c) lvpe 11-1 refersto kerogenwith a relativelylow initialH/C ratio (usuallyless than 1.0)and a high initial O/C atomicratio (ashigh as0.2 or 0.3).This type of
Kerogen:Composition and Classification
15,1
;
Fig. II.4.13. Amount ofoxygenengagedin variousfunctionalgroupsin 9 d 9 shallowimmature samplesof different kerogens:Eocene.GreenRiver - 9 r o : - i shales.Uinta Basin(type I): Lower o o o 6 o t Toarcian, ParisBasin(type II); U. t o Cretaceous, Douala Basin (type + + + + Ill). Type I containslittle oxygen, which is mostly engagedin ester TypeI Toioloxygen=4.6%groups.Type III containsmuch oxygen in C=O bonds:acids.ketones. Esterbondshavenot beendetected. It alsocomprises a largeamountof noncarbonyloxygen.probablyengagedin heterocycles. in quinones, Type1I or in etherbonds,methoxylgroups, % Totqlorygen=10.3 etc. (After Robin. 1975)
I E
I :
g
g I
-9
s .9
t5
Typelli Tolol oryqen.275olo
kerogencomprisesan importantproportionof polyaromaticnuclciand hcteroatomicketoneandcarboxylicacidgroups,but no estergroups(Robin, 1975;Fig. II.4.13). Noncarbonyloxygenis possiblyincludedin ether bonds.Aliphatic groupsaresubordinate constituents in varyingproportionsof the organic matter. They consistof a few longchains(morethan25 carbonatoms),originatingfrom higherplantwaxes,somechainsof mediumIength(15to 20 carbonatoms)from vegetablefats, methyl groupsand other short chains.This type of kerogenis comparatively lessfavorablefor oil generationthanare typesI or II, althoughit may provideconvenientgassourcerocks,if buriedat sufficientdepth.It is also lessproductiveon pyrolysis.It doesnol includeany oil shales. Type-III kerogenis derivedessentially from continentalplantsand contains muchidentifiablevegetaldebris.Plantorganicmatteris incorporated in sediment eTtherdirectlyor throughits alterationproductsin soil humic acids.Microbial degradationin the basin of depositionis usuallylimited due to important sedimentationand rapid burial, becausethis type is rather frequentin thick detritalsedimentation alongcontinentalmargins.Examplesof type-lII kerogen are found in the Upper Cretaceous of the Doualabasin(Cameroon)and in the
4.8 Depositional Environmentand Compositionof Kerogen:Evolutionpaths
155
I
Atomicratio 0/C ------* Fig.II.4.14.Elemental analvsis ofsomekerogens fromtheCretaceous blackshales ofthe NorthAtlanticcoredduringtheDeepSeaDrillingprogram. Themainevolution pathsof typeI. lI andI I I kerogens areshown.GrcupA rcfefito $arineorganicmatter; groupB to a mixtureof marineandterrestrial organic matter,someof thesamples beingpossibly degaded(rightpafl);groupC to te:f:'estrial moderately degraded organicmatter;group l) to residual organicmatter.All samples belongto the diagenesis stageof evolution. (Adapted fromTissotet al., 1979)
lower Mannvilleshalesof Alberta, and alsoin someTertiarydeltas,suchasthe Mahakamdeltain Indonesia. d) In additionto thesethreemajor typesof kerogen,and their admixtures, there is _alsoa residual type of orgaiic *utter, i'hi.h is characterizedby abnormallylow H/C ratiosassociated with highO/C ratios.Thissituationwas,for instance,met in severalcorestakenin the North Atlantic Oceanas Dartof the Deep SeaDrilling Project(DSDP): kerogensisolatedfrom many Cretaceous sediments of the Bay of Biscay(leg48) andsomeof the Blake-Bahama basin(les 44)showa H/C ralio aslow as0.5or 0.6,associated with a highO/C ratio(0.25t6 0.30)at shallowdepthsof burial(200to 1000m): FigureII.4.14,groupD; Tissot et al. (1979,1980).Suchlow valuesof IVC ratio are normallyfound only in sedimentswhich havebeen buried at greatdepthsand, in suchcase,they are associated with low O/C rarios(lessthan 0.10).Infraredspectroscopy (Caitex, 1979)hasconfirmedthe abundanceof aromaticnucleiand oxygen-containing goups andthe absence of aliphaticchainsin thistypeof kerogen:FigureII.4.15. Furthermore,optical examinationof the same samplesmainly ihows coaly fragmentsof oxidizedorganicmatter or charcoal,in transmittedlieht, and i n e r t i ni t e i n r e f l e c r eldi g h t . This residualtype of kerogenexhibitinga very low H/C ratio and a relatively _ high O/C ratio cannotgenerateany hydrocarbons andis considered thereforeto be one form of "dead carbon" in the senseof petroleum generation.The term "inertinite"is originallyreferringto a comparable propertyof thiskind of organic
Kerogen:Composition and Classification Fig. IL,1.15. Infraredspectrometry of four typicalkerogens.The symbolsreferto samplesobtainedfrom the Cretaceous black shalesof the North Atlanticand shownin Figure tI.4.1,1(Tissotet al., 1979)
o
<
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whereit wasobservedthatin thecoking material.It is derivedfrom coalscience, processcertainparticlesin the cokingcoaldid not reactandwerehencetermed organic inertinitic.Obviously,thereareseveraloriginsof thiskind of nonreactive rocks:it maybe reworked.oxidized materialwith a low H/C ratio in sedimentary weatherorganicmaterial;it maybe inertiniticmaterialstemmingfrom subaerial ing or biologicaloxidationin swampsandsoils.Examplesof thiskind arefound examinationof highamongthe above-cited DSDP samplesand on microscopic rcflectanceparticlesin Recentpeats.In all cases,this type of kerogenhasno potentialfor hydrocarbongeneration. From a practicalpoint of view,anotherform of "deadcarbon"is that organic materialthat has lost its hydrogenthrougha naturalhydrocarbongeneration processand can be observedas micrinite(seealsop. 243)in petroleumsource rock. In suchcasesboth H/C and O/C ratiosare verylow. This concepthasalso et al. (1979)when givinga massbalanceof the been adoptedby Leythaeuser organiccarbonin sourcerocksmainlybasedon Rock Eval pyrolysis. e) General remarks. Although type-Il and -III kerogensrepresent rather frequentsituations,the composition of kerogenmaycoverotherareasin Figure II.4.11betweenthe evolutionpathsI andIIl. Theremaybe differentreasonsfor includingmixtureof marine that situation:variabilityof contributingorganisms, and terrestrialinputs,and alsodifferentconditionsof preservation
,1.8Depositionai Environment andComposition of Kerogen:Evolutionpaths
15'l
A first sourceofvarrability may resultfrom the predominantcharacterof some contributingorganisms. For example,a marinesediment,deposited in a reducing environmentmay contain abundantdiatomsunder temperateor cold water conditions,and dinoflagellates or coccolithophorids in warm seas.The relative abundance of zooplankton,suchascopepods,may introduceanothercausefor variability.The mainconstituents of thesevariousgroupsof organisms, shownin Table I.4.1, explainthat the originalcompositionof the relatedkerogensmay vary, althoughthey all belongto type IL Furthermore,some kerogens,althoughmarine,show a relativelyhigh O/C ratio, probably due to an important contributionof plant debris from the continent,like the Viking shales,Cretaceous from Alberta (Mclver, 1967),or some Cretaceousblack shalesof the North Atlantic. They occupy an area betweentypesII and III on the van Krevelendiagram:part of groupB, Figure I I . 4 .1 4 . Finallv, another cause of variabilitv is suspected.althoughinsufficiently documented:biochemical and chemicaldegradation of the organicmatterprior to or duringdepositionmavresultin a decrease of the H/C ratioandan increase of the O/C ratio. An extremecaseis obviouslythe residualtype of organicmatter presentedabove,whichhassuffereda strongsubaerialoxidation,and no longer presentspotentialfor hydrocarbongeneration.Moderatelyoxidatedmarineor marine/terrestrial organicmattermay eventuallyresultin an increase of the O/C ratio (> 0.2) with a loweredH/C ratio (ca. 1.0)still sufticientto generatesome hydrocarbons andparticularlygas.Among the blackshalesof the North Atlantic (Fig. II.4.14)the sampleslocatedon the right handsideof groupB (O/C 0.20to 0.30)mightresultfrom suchsituation. Faciesvariationsin sedimentmaycausea changeof the kerogenfrom onetype to anotherwithin a givenhorizonor formation.For instance,BregerandBrown (1962)showedin the Chattanooga shales(Kentucky,Tennessee andAlabama)a progressive changein kerogencomposition. In the areacloseto the shoreline at the time of deposition,kerogenshowslow hydrogenvalues.Thisis attributedto materialof terrestrialorigin.depositedin a relativelyoxidizingenvironment.To the north, i.e., basinward,the hydrogencontent increasesdue to a higher proportionof marinephytoplankton.Upon pyrolysis.the oil yieldvariesdirectly with the H/C ratio of the kerogen. When comparingdisseminated organicmatterto coal maceralsby usingthe van Krevelendiagram(Figs.IL4.11 and II.4.12).it can be seenthat the main typesI. II and lll broadlyfall inlo the sameareasof the diagramasdo alginite. exiniteandvitdnite,respectively. Thiscomparison is quitereasonable for alginite and some of the type I kerogens,which are made up mostly of algae,like Botryococcus shales. It is alsovalidfor vitriniteandtype-III kerogen.In termsof coal chemistry,thesekerogenshaveH/C, O/C distributionscomparableto the grosscomposition of peat,ligniteandcoal,andespecially to thevitrinitemaceral. This is understandable, as peat is primarilymadeof the samevegetaldebris. depositedin a confinedenvironment,wherebiodegradation is alsolimited for other reasons(settingup of antisepticconditions). However.the comparison, basedon elementary composition, hasno meaning with respectto type-IIkerogenandexinites.Thiskerogenresultsfrom a complex
andClassification Kerogen:Composition
158
mixture of organicdebris,which have undergonebiochemicaland chemical whereasexineshave preservedtheir shapeand individuality. reorganization, Furthermore, this constituentis likely to accountfor a minor part only of the organic matter depositedin a marine environment. The relationshipbetween palynofaciesobservedin transmittedlight (amorphousor sapropelic,humic facies)andchemicalcompositionof kerogenis rather higherplantdebris. of land-derived complex.Humicmaterial,mainlyconsisting normally falls into the areaof type-III kerogen.Amorphousfaciesresultsin fact whichhavelosttheir identity. from severealterationof the originalconstituents, Thus,if manyamorphouskerogensbelongto type lI, someothersundoubtedly belong to type III. Floculatedhumic acids,for instance,stemmingfrom a under the colloidalsolutionwill undoubtedlyhavean amorphousappearance will groupthem At the sametime,however,theirelementalanalysis microscope. along a low evolution path. This amorphousmaterial is often mistakenas potential. indicativeof a "sapropelic"facies,with goodhydrocarbon An exampleof the variabilityof amorphousorganicmatterwith respectto kerogentypesis shownin FigureIL 4. 16.The spreadof the H/C andO/C ratiosis coveringthe entirefield andhenceclearlyshowsthat thereis a varietyof origins for amorphousorganicmaterial.Powellet al. (1982).using58 samplesof the Basinand the CanadianArctic Islands.showeda low level Beaufort-Mackenzie of correlationbetweenthe contentof amorphousmaterialand the chemical analysisof kerogen. They attributed this situationmainly to a failure to amorphous and hydrogen-rich betweenhydrogen-poor distinguishconsistently organlcmatter.
.J
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0.1
o.2
0.4
0.5
Atomicratio 0/C + of 21 kerogensfrom "amorphous"organicmatter.The Fig. 1I.4.16.Elementalanalysis sarnplesare from various origins; they all belong to the diagenesisor early catagenesis areshownfor comparison. stages. The mainevolutionpathsof typeI, II andIII kerogens (After Durandand Monin, 1980with additions)
Summaryand Conclusion
159
4.9 Conclusion All physicaland chemicalmethodsfor kerogenanalysisconvergeto provide a coherentpictureof the organicmatterandits evolution.In particular,elemental analysis provides the framework within which other methods can be used (DurandandMonin, 1980).Thereare,however,limitationsto its use,especially for the advancedstagesof thermal evolution. The composition ofa shallow,immaturekerogendependson the natureofthe organicmatter incorporatedin the sediments,and on the extentof microbial degradation.In turn, the compositionof kerogen,particularlyin respectto hydrogen(aliphaticchains)and oxygen(functionalgroups),determinesthe geneticpotentialof a sediment,i. e., the amountof hydrocarbons that can be generatedduringburial. The mostclassicsourcerock, containing"marine organicmatter, depositedin a reducingenvironment",broadlycorresponds to type II and hasa high genetic potential.On the contrary,continental organicmatterdeposited in environments where biodegradationis limited by quick burial, correspondsto type III andhas only a moderatepotentialasfar asoil is concerned. Mostkerogens, whenplotted on the van Krevelendiagram,fall on or betweenthesetypes.Type I is much less frequent and referseither to selectivelybiodegradedorganicmatter, enrichedin lipids, or to rocks consistingalmostentirely of organicmatter suchasalginites.
Surnmaryand Conclusion Kerogenis the ftaction ofthe organicmatter in sedimentaryrockswhich is insolublein organic solvents,whereasbitumen is the soluble part. Kerogen is a macromolecule rnadeof condensedcyclic nuclei linked by heteroatomicbondsor aliphaticchains. Different types of kerogen can be recognizedby optical examination and physicochemicalanalyses.Three main typesseemto accountfor most existingkerogens. They are characterizedby their respectiveevolution path in the van Krevelen (H/C, O/C) diagram. Type-I kerogencontainsmany aliphaticchains,and few aromaticnuctei. The H/C ratio is o ginallyhigh, andthe poJentialfor oil andgasgenerationis alsohigh. This type of kerogenis eithermainly derivedfrorn algallipids or from organicmatte! enrichedin lipids by microbial activity. Type-IIkerogencontainsmore aromaticandnaphthenicrings.The H/C ratio andthe oil andgaspotentialare lower thanobservedfor t)?e-I kerogenbut still very important. Type-II kerogenis usuallyrelated to marine organic matter depositedin a reducing environment,with medium to high sulfur content. Type-III kerogencontainsmoslly condensedpolyaromaticsand oxygenatedfunctional grcups,with minor aliphaticchains.The H/C ratio is low, andoil potentialis only moderate,althoughthis kerogenmay still generateabundantgasat greaterdepths.The O/C ratio is comparativelyhigherthan in the other two typesof kerogen.The organic matter is mostly derivedftom terrestrialhigherplants. Resid.ualkerogenmay consistof reworked, oxidizedorganicmaterial, or inertinitic materialstemminghom subaerialweatheringor biologicaloxidation.It is one form of "dead carbon" and hasno potential for oil or gas.
Chapter5
FromKerosento Petroleum
As sedimentation and subsidence continue,temperatureandpressurerncrease. In thischangingphysicalenvironment,the structureof the immaturekerogenis no longerin equilibriumwith its surroundings. Rearrangements will progressivelytakeplaceto reacha higher,andthusmorestable,degreeofordering.The steric hindrancesfor higher ordering have to be eliminated.They arc, for instance.nonplanarcycles(e.g., saturatedcycles)and linkageswith orwithout heteroatoms, preventingthe cyclicnucleifrom a parallelarrangement. This constantadjustmentof kerogento increasing temperatureand pressure resultsin a progressiveeliminationof functionalgroupsand of the linkages betweennuclei(includingcarbonchains).A widerangeofcompoundsis formed. includingmediumto low molecularweighthydrocarbons, carbondioxide.water. hydrogensulfide, etc. Therefore.the petroleumgenerationseemsto be a necessary consequence of the driveof kerogento adjustto its newsurroundings by gaininga higherdegreeof order with increasing overburden. In unmetamorphosed sedimentary basinsthisprocessof rearrangement is not carriedto the end. The levelof regionalmetamorphism is neededto reachthe graphitestage,which is the stableconfigurationunder high temperatureand pressure.
5.1 Diagenesis, Catagenesis and Metagenesis of Kerogen Kerogenis a polycondensed structureformedunder the mild temperatureand pressureconditionsof youngsediments and metastable undertheseconditions. Therefore.its characteristics seemto remainrather constant.even in uncient sediments. aslongastheyarenot burieddeeply.A typicalexampleis providedbv the lowerCarboniferous lignitesof Moscow.Althoughtheyareabout300million rank is stillverylow, because theyhaveneverbeen vearsold. their carbonization burieddeeperthan200m (Karweil.1956).In mostcases. however.assedimentation and subsidence proceed,kerogenis subjectedto a progressive increaseof temperatureand pressure.lt is no longer stableunder the new conditions. Rearrangements occur during the successive catagenesis. stagesof diagenesis. and metagenesis towardthermodynamic equilibrium(Fig. II.5.1). Such evolution has been studiedin detail in variousrock seouenccs. and particularlyin the DevonianandCretaceous of WesternCanada(Milver. 1967). theTertiaryof Louisiana(Laplante.1974),theLowerToarcianshalesof theParis
5.1 Diagenesis. Catagenesis and Metagenesis of Kerogen
161
cI ,9
Alomic rolio O/C
Vitrinitereflectance
,-,1
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Fig. 11.5.1.Generalschemeofkerogenevolutionfrom diagenesis to metagencsis in the ran Krevelcn cliagram.Approximateraiues of vitrinite reflectanceare shown for conparison Basin. the Silurian of thc Sahara (North Africa.). the Upper Cretaceousof the Douala Basin (Wcst Africa) and the Eocene Green River shalesof the Uinta Basin. ln thesc studies.observationson the natural transformationin a geological \rtuation were comparedwith experimentalevolution through laboratory simulatlon . As the ma in stcpsof kerogenevolution are the samefor all tvpesof kerogcn.
162
FromKerogento Petroleum
w e s h a l dl c s c r i b ien d e t a idl e g r a d a t i oonf t v p el l . w h o s ce l e m e n t a rcl o m p o s i t i o n . y u s i n gF i g u r eI I . 5 .l . t h e s ee v o l u t i o snt c p s i si n t e r m e d i a b t ce t w e e o n t h e rt v p e sB canbe extendcdto the other typesof kcrogen. Kerogenis transformedwhensubjcctedto highertempcratureand pressure. $ here sequencc is in a sedimentar.v An idealsituationto studythistransformation parametersexceptburial remainconstant.The all geologicaland geochcmical Lower Totrrcianshalesof the ParisBasin(Fig. I1.5.2)are rich in orgrnic matter (organiccarbonvariesfrom 3 to 15l/c)and seemto meet theserequirements (LouisandTissot.1967).The shalcsrepresenta well-defincdAmmonitezonein theupperpartofthelowerJurassicsediments,datedaroundltlO.l0nlears.Their constantover mineralogical composition,includingclayminerals.is reasonably most ol the basin.as is their fossilorganiccontentsas well. As thc structural the burial historyand historyof thc basinis simple.it is possibleto reconstruct computethe maximumburial depth of the formationat evcry locationin the basin.The availabilityof a sctof coresfrom variousdepthsin the basincnablesa transtbrstudyby microscopic, chemicalandphysicalmethodsof the progressive mationof kerogcnas a functionof maximumburialdepth.The detailedresults . u r a n de t a l . ( 1 9 7 2 )E. s p i t a l i e i t al.( 1971). a r er e p o r t e di n T i s s o e t t a l . ( 1 9 7 1 )D Tissoe r ral.(1974). ln order to cxtendthe resultsto deepcrgeologicalconditions(the maximum depthof the Lower Toarcianshalesis slightlybeyond2500m). someadilitional data from thc sameshalescoredin WcsternGermanyare included.'IheLower Silurian shalesof the Sahara(North Africa) will also be considered.They unit whichcovcrs stratigraphic andratherhomogeneous rcprescnta well-defined environmentis rathcrsimilarto that of the Lower a wide area.The deoositional and,accordingto elemenToarcianshalcs(blackshales.containingTasmanites) characteristics of the sameevolution tary and IR analvsis.the kerogencxhibits ( t y p e a l . . 1 9 7 4 ) . path tt I I . s e eF i g .I L . 1 . 1 l :T i s s o e The degradationof kcrogenas a functionof burial hasbeenobscrvedb-"-the mentioncdin ChapterIL,1.Thc maintrendis a phvsicalandchcmicaltechniques contentof kerogen.Horvevct.threesuccesslve incrcasc of the carbon continuous stepsare ilistinguishcd. of oxygcnancla of kerogenis markedb)' an importantdccrease a) Diaganesi.s dcpth. When refcrringto correlativeincrcaseof carboncontentwith increasing results t h ev a nK r c v e l e nd i a g r a m( F i g s .I I . 5 . l a n dI L 5 . - l ) .t h i ss t a g eo f e v o l u t i o n in a slight dccreaseof HIC and a nrarkeddecreaseof O/C ratios. Intrared to thatthc diminutionoloxygcnis dueesscntially spectroscopli hasdemonstrated . . vc r l m b i n a l i oonf t h ep r o g r e s s i veel i m i n a t i oonf C : O g r o u p( l 7 l 0 c m ' b a n d ) B ) ave s .o b i ne t a l . ( 1 9 7 7 h t h e a n a l y s efso r f u n c t i o n agl r o u p sa n dI R t e c h n i q u e R shownthe respectivebchaviorof the'"ariousgroupscontainingC=O. namcl!' that acidsand ketonesare more affectedthan esters. Kerogenreflectanceis rather scattered.dependingon the type of organic debris:ca.0.5olo or belowon vitrinite.lesson exinite.Freeradicalsremainscarce lp. is usuallylow; Xp 5 5.lU-uu. e. m. CGS/gof as paramagnetic susceptibility, lp valuemay be somewhathigherin type-III kerogen. organiccarbon.fhg ln the Paris Basin the final stage of diagenesisis reachedin location corresponding to maximumdepthof burialfrom 800to 1200m (sampleA in Fig.
5.1Diagenesis. Catagenesis andMetagenesis of Kerogen
163
1L5.3).Intermsofpetroleumexploration.thisstagecorrespondstoanimmurure kerogen.and little hvdrocarbongenerationhas occurredin the sourcerocks. However.large quantitiesof carbondioxideand water and also some heavv heteroatomic(N. S. O) compoundsmay be producedin relation to oxygcn climination. Displacementin thc van Krevelen diagram and experimental . . 2 )c o n f i r mt h e s er i e w s . e v o l u l i o n( S e c 5 b) Catagenesis, the secondstageof kerogendegradation. is visiblein deeper samples andmarkedby an importantdecrease of the hydrogencontentandof the II/C ratio (from 1.25to about0.5 in type-Il kerogen).due to generarionand 'l'able relcaseof hvdrocarbons. [I.5.1showsthe variationof atomiccomposirion. from the stage of diagenesis(burial depth 780 m) ro the variousstepsof (burialdepth21170 catagencsis and 2510m in the ParisBasin.ca. 3000m in thc Sahara)anduntil the beginningof metagenesis (ca.4000m in the Sahara).Figure IL5.3 showsthecorresponding evolutionpathfrom sampleA (diagenesis) to B, C and D (catagcnesis) and E (metagenesis). During catagenesis the mainfeatures on IR spectraare a progressivc reductionof aliphaticbands.correlativeto the H/C lowering(Fig.I L 5.,1).andthe appearance of aromaticCH bandsfrom 930to 700cm I. The appearance of thesearomaticbandsis dueto a desubstitution on aromaticnucleiand possiblyto an increasing aromatization of naphthenicrings. A carefulstudv of the aliphaticbandshas also proved that long chainsare prcferentiallvrcmoved. resultingin a relative increaseof methvl groups. compared t o t h e t o t a l s a t u r a t e sdt r u c t u r e(sR o b i n .1 9 7 5 R : o b i ne t a l . . 1 9 7 7 ) . Infrared spectraalso show the eliminationof the residualC:O band to be completedrapidly:subsequently the O/C ratio remainslow and constant(ca. 0.05),suggesting that the remainingpart ol oxygen(restrictedto heteroc!clicor cther groups)is not affected.The OiC ratio may even increaseby relative concentration.due to a preferentialeliminationof hvdrogenand carbon.rrC N M R o f s o l i dk c r o g e np r o v i d e cs o m p a r a b lien f o r m a t i o nosn t h e e l i m i n a t i oonf saturatedhvdrocarbonstructuresand the increaseof aromaticit)'during catagenesis. Conclusions are similarto thoseobtainedfrom lR spectroscopl-. P a r a m a g n e tsi cu s c c p t i b i l iitnvc r c a s epsr o g r e s s i v etlov 5 0 o r 6 0 . 1 0' r u . e . m . CGS/gof orqaniccarbon.This erowingabundance ol free radicalshasprobablv t h e s a n eo r i g i na s t h e i n c r e a sien a r o m a t i cC H b a n d s i.. e . . d e s u b s t i t u t i o an d aromatization. Vitrinitereflectance increasing alsoincreases. gentlyat first.then more rapidll from about 0.5 to 2%. The diffcrencein reflectancebetween v i t r i n i t ea n do t h c rm a c e r a l se. g . . c x i n i t ev. a n i s h cbse v o n dI . 5 . l . H o r v c v e rt .h e mutualarrangemcnt of the stacksof aromaticsheetsrcmainsrandomlydistributed.asnotedlrom electronmicrodiffraction. Somcof the numericalvaluesmentionedfor type-lI kerogensare differentin othcr t),pesol kerogen.For instancc.the rangeof variationof the H/C ratio d u r i n gc a t a g e n e si si sw i d c ri n t v p e - lk e r o g c n( 1 . 5t o 0 . 5 )a n dn a r r o w eirn t y p eI I I ( 0 . 8t o 0 . 5 )c o m p a r e w d i t h t y p eI I ( F i g .l l . 5 .I ) . T h e p a r a m a g n e tsi u csceptibility of tvpe-lll kerogen.whichis morearomatic.is oftenhigherthanobseNedon t),pe I o r I I u t t h e s a m ed e l t h o f h u r i a . . In termsof pctroleumexploration,the stageof catagenesis corresponds to the mainzoneofoil generationandalsoto the beginningof the crackingzone.which produces"wet gas"with a rapidlyincreasing proportionof melhane.
From Kerogento Petroleum
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5.1 Diagenesis,Catagenesis and Metagenesisof Kerogen
165 Fig. I1.5.3. Evolutionof the elemental composition of type-IIkerogenduringburial: Lower Toarcian shalesfrom ParisBasinandW. Germany, Silurian shales from the Sahara(Algeria and Libya). Evolution stages:z{ end of diagenesis; B D catagenesis; (Modifiedaft metagenesis. ter Tissotet al.. 1974)
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50
oOXh
Fig. IL5.2. Map showingthe outcropsof the Lower Jurassic of the ParisBasin.andthe locationof surfaceand subsurfacesampling.The Lower Toarcianshalesare situatednear the top of the LowerJurassic outcrops.Isobaths showthe presentdepthto the baseof the shales.(After Tissotet al., 1971)
166
From Kerogento Petroleum
Table II.5.1. Catageneticevolution of the elementarycompositionof kerogens(ype II) from the lowerToarcianshales,Parisbasin,andfrom the Silurianshales,Northern Sahara A. Atomic comDosition
Basin
a
D C
Vacherauville C6sarville Bouchy
M
AB1 oR1
Paris
N. Sahara
I-ocation
Maximum burial depth (m)
780 20'70 2510 ca. 3000 ca.4000
o
N
40 53 5 t 4 3 5 3 3 1 48 48 2 | 56 64
39 3t
3 3
s
Total
100 1 0.08 100 0.04 100
0.2 1.9 100 0.9 1.4 100
B. Atomic compositio, (relatedto 100 carbonatoms) Vacherauville Cdsarville Bouchy
Paris
N. Sahara
C M
AB1 OR1
780 2070 25lO ca.3000 ca.4000
1 0 0 1 3 0 1 3 2.5 2.5 248 100 r23 7 2.3 0.2 233 100 100 4.5 2.3 0.1 20'l 100 70 100 48
o.4 3 178.4 t.4 2,2 1,57
" Stages (finalstage),C Catagenesis, of evolution:D Diagenesis MMetagenesis. c) Metagenesis ol kerogencanbe observedin verydeepsamples or in locations with a high geothermalgradient.Eliminationof hydrogenis now slow and the residualkerogenusuallyconsists of two carbonatomsor moreout of thrceatoms (H/C < 0.5). as in samplesfrom the Saharathat are buriedto,t000 m (Table I I . 5 . 1 ,a n ds a m p l eE i n F i g . I I . 5 . 3 ) .I n e x t r e m ec a s e st h e c a r b o nc o n t e n m t ay reach9lVa by weightand the H/C atomicratio is only 0.4. Aliphaticand C=O bandshave vanished,and aromaticC:C is the main band remainineon IR spectra t F i g . I l . 5 . 4 .h o l t o m J . The beginningof metagenesis corresponds approximately to a vitrinitereflectanceof2c/o:at thisstage,a major rearrangement of the aromaticnucleroccurs and is clearlyobservedby electronmicrodiffraction.The stacksof aromatic layers.previouslydistributedat random.now gatherin clusterswith a preferential orientiation(Fig. IL 1.5).The sizeof the clustersis from 100to 200A in typeI l k e r o g e nI.t i s ^ s m a l l e i nr t y p eI I l , l e s st h a nl J OA , a n dl a r g e ri n t y p eI , w h e r ei t may reach500A or more (Oberlinet al., 1974;Boulmieret al.. 1977).At the sametime dispersionof the interlayerspacingdecreases (Oberlinet al.. 197-5). This major changeis accompanied by other importantevents,in rcspectto vitrinitestructureand alsoobservedby IR and electronspin resonance (ESR) characleristics. Beyondthe levelof2Vcreflectance. anisotropy ofvitrinitebegins to appear.Aromatic CH bands (from 930 to 700 cm-r) and paramagnetic
5.1 Diagenesis,Catagenesis and Metagenesisof Kerogen
167
Fig. I1.5.4. lnfrared spectra showing evolution of typ€-II kerogen during burial (Lower Toarcian.ParisBasin; Silurian, Sahara)
o @
o @ \!-/
t000 2600
18001600140012001000800
susceptibility both reach a maximumat abo}t 2o/avitrinite reflectance,then d e c r e a sm e a r k e d l y( D u r a n de t a l . , 1 9 7 7R ; o b i ne t a l . , 1 9 7 7 ) . All thesedatapointto the progressive development of a higherdegreeof order polycondensation, bv increasing moreregularspacingof the aromaticIayersand appearance of a preferentialorientationover areasrangingfrom 100to 500 A, comprisinghundredsor thousands of individualstacks,smallerthan 10A. In termsof petroleumexploration,the stageof metagenesis is entirelysituated in the drv gaszone. Resultsobtainedby opticaltechniquesof examinationsupportthis general scheme of kerogendegradation throughdiagenesis, catagenesis andmetagenesis. Microscopicexaminationof kerogen from various depths showsa progressive evolutionof the sporesand pollen with respectto their stateof preservation (shape,ornamentation etc.) and color (Correia,1969).In the ParisBasinthey changefrom vellow in shallowsamplesto orange,red, and finally brown. In deepersamplesfrom the Saharathey losepart of their morphological character andbecomeblack. UV fluorescence of organicmaterialshowsa markeddecrease with depth(Alpern et al., 1972).Both phenomenaare interpretedasa resultof increasing condensation of the kerogen.Reflectance of the variousmacerals also changesas a functionof burial.The valuescorresponding to the threestepsof
From Kerogento Petroleum
168
as a Fig. 1I.5.5. Reflectance function of depth during evolutionof type-lll kerogen of the Upper Cretaceousof the DoualaBasin.Valuesare measuredon true vit nite particlesand on a cement of vitrinite type. (Reflectance measurements made by Alpern, in: Durand and Espitali€.1976)
0iagenesis
E I OOO
o
Cetagencsis
Melagenesis
o h/./otl/c
ceher/ /n te.ogeo
3 R e l l e c l o n cien o i l ( % )
kerogenevolutionhavebeenshownin FigureII 5 1. andan exampleof evolution as a functionof depth is presentedin Figure II.5.5: the reflectancehas been of measured on vitrinite(andvitriniticcement)of the kerogenin the Cretaceous the DoualaBasin(typelll). Additionalinformationon the evolutionof kerogenis providedby dfferential (TGA). Combustionin anal1srs (DTA) and thermogravimetric thermalanalysis an oxygen atmosphereshows a strong exothermicDTA peak. which shifts depthof the sample(Espitali€et al., with increasing towardJhighertemperature rank ln with increasing are madeon a coalsequence 1973).Similarobservations
A 1 6 0
B
I I
.p I
..+---t
I
d
.9
- *
.--o-:: OF 150
250
550
.. 550
D
Fig. IL5.6. Thermogravtof kerogen metricanalysis type ll (temperatureinc r e a s eo f 4 " C m i n - ' ) . Kerogensfrom the Paris Basinandfrom the Sahara are arrangedin order of increasingevolution.The lettersA to E refer to samples shown on Figure IL5.3. (After Tissotet al., 1914)
5.2Experimental Simulation of Kerogen Evolution
ftg
atmofphere.the weightlossmeasuredbv thermogravimetric analvsis fjtltroCgl (TGA) (Fig.ll.-5.6)amounrsro 707c(sampleA) in shallowk-erogens ot theparis Basincorresponding to the stageof diagenesis. During catagen;sis. the weight lossdecrcases regularlyto 55% (sampleB). then 40%(iampieC), andfinallvto 20% (sampleD). Jn very deep samplesfrom the Sahara(sampleE). where netagenesis is reached.the weightlossis depressed to lessthan l0%. The TGA temperature corresponding to the maximumdegradation ratealsoincreases with depth. This generalevolution schemeis in agreementwirh the hypothesisof a progressiveelimination.through bond breaking,of the noncondensed con_ stituento s l k e r o g e n( e . g . , a l i p h a r i c h a i n sa n d s a t u r a t e o dr a r o m a t i c y c l e s . singleor associated in a smallnumber).the residuebecomingprogressively more condensed and lesssensitiveto degradation.
5.2 ExperimentalSimulationof KerogenEvolution Much experimentalwork has been carried out by heating kerogenin the laboratory.^Some of theseexperiments wereconductedbv ind-ustries asa part of the searchfor adequateraw materinlsto produceoil from oil shalcs. Transformations occurringover geologicalperiodsare in esserrcc nor acces_ sibleto humanexperiments. aswe shallneverbe ableto accountfor the millions of vearsinvolvedin the naturalprocesses. Therefore.in the laboratorvit is nccessaryto increasetemperaturesin order to acceleratethe reacfion.s and compensate fbr time.However.because a valiclbasisfor comparison hasnot yet been established, we are not yet able to determinewhich time_remoerarure correspondcnce should be used. For this reason.the first objectiveof such laboratorvexperiments is to determinethe extentto whichtheyare represenra_ l r v co l n a t u r t lt r i l n \ f o r mtai o n . In thisregard.experimental assays of thermalevolutionhavebeencarriedout on.shallowsamplesfrom the threemaintypesof kcrogen.The shallowsamples. artificiallvtransformcdbv heating experimcnts.ari then comparedto the naturallvtransformedsamplesfrom greatcrdepthsof the samebasin.These expcnmentsare basedon thcrmogravimctric analysis with temperatureprogram o l J ' ( - 6 i n u n L l e ri n c r t a l m o s n h c r icn a n u t t e m p lt o r c p r e s e nat p r o g r e s s i v e A, regularrimescorresponding to sampletempciatureot Jtt0. ::tt to t11.,1 600"C.the elementarycompositionand physicalpropeitiesof the kerogen(lR spectroscopv. electron diffraction.electronspin resonance.reflectan"ce) are measured.The compositionof the reactionproductsis evaluatedbv mass spectrometry(Souronet al.. 1971)or by catharometerand flame ionlzation detectors. Again we shallreviewin detailthe resultsrelatedto type_llkerogen.which provide us with the generalschemeof artificialevolution.The shallowand immaturekerogenfrom Fdcocourt,in the Lower Toarcianshalesof the paris Basin,hasbeensubjectedto suchexperiments in orderto comparetheresulting kerogenwith samplesburiednaturallyto greaterdepths.Somi of the resultini
I7O
FromKerogento Petroleum
are shownon FigureI1.5.7.and they may be comparedto the natural analyses to FiguresII.5 3 to II 5 6. In addition.Figures evolutionuponburialby reference dataobtainedby heating of somesignificant I I.5.8and Ii.5.9 showa comparison all threeof the maintvpesof kerogen(1. II and III) underidenticalconditions weight loss are Although the degradationof kerogenand the associated from distinguished be stagesmay threesuccessive continuouiand progressive. These three (type II) the laboratoryheatingof immatureFdcocourtkerogen stagesare describedin the followingparagraphs. i) Heatingto ca. 350'C resultsin a rather smallweightloss.due mostlyto oroductionoi waterandcarbondioxide.asshownby massspectrometryOptical darkeningof definedorganic ixaminationof the samplesshowsa progressive Elementalanalysisand fluorescence. of UV decrease debriswith a correlative with a diminutionof the associated of oxygen a loss show infraredspectroscopy c o m p o s i t i oH n/C' O/C is T h e e l e m e n t a l 1 1 . 5 . 8 ) . ( F i g s . i L 5 . 7 a n d C=O Uand (Fig ll 5 3) m to 1500 of 1000 to depths buried samples to natural comparablc The degreeof orderingof the carbonnuclei,as measuredby electronmicrodiffraction. remainsas low as it is in the shallownatural samples Likewise' of kerogenis still weak reflectance Artificial evolutionat about350"Ccan be comparedwith the final phaseof in sedimentarybasinsThus the first artificialheatingstage(350"C) diagenesis latediagenesis can be correlatedwith subsurface its major degradation b) From ca. 350'C to 470-500'C.kerogenencounters unit (Fig ll 5 7) The stagc.The weightlossis maximalper time or temperature andparticularlyaliphaticsThe sampleseems productsare mostlyhydrocarbons by Robinson plasticandis possiblymelted,asalreadyobserved to swell.becomes changes composition (1969) whcn heating Green River shales Elemental decrcase bands CHj aliphatic CH1. to ca.0.5and iapidly:the H/C ratio decrcases (Fig ll 5 9)' mirkeclly,whereasaromaticCH bandsappeardueto desubstitution then esters' first' removed are acids disappears: The C-O band progressively (Fig eliminated progressively are ketones whereas which are more stable. paramagnetic and the to 2Vr:. np progressivcly increases II.5.u). The reflectance e to 50 b0 10 u e.m. CGS/gof organiccarbon The decpest susceDtibilitv naturalsampleof the LowerToarcianshalesof the ParisBasin(Bouchy2500m' to ca. 100'C) has a compositionand a structuralorganization corresponding equiviilentto 400"C in laboratorvexperiments Other kerogensof similar comnositionfrom Silurianof thc Saharathat have been deeply buricd and over 130'Cfor a long periodof time' asD in subjectedto naturaltemperatures to correspondrng Fisure11.5.3.havea compositionand structuralorganization the end of that phase(470 500"Cin experiments) occurringin This step of evolutioncan be correlatedwith the catagenesis as reasonably sedimentaivbasins.and the laboratoryassayscan be considered duringthat step However'thc situaof kerogentransformation representative tion is rathercomplexwith regardto the bitumenproductsformedduringthe laboratory assaysas comparedwith oil and gas generatedduring natural As temperatureconditionsare much higherin laboratoryexpericatagenesis. and crackingreactionsof menisthan in sedimintarybasins,disproportionation the generatedbitumenmay proceeddifferently'accordingto the experimental
5.2 Experimental Simulationof KerogenEvolution
t7l
t
I
a o
3
2 -
s
9
a a
I
;
iE
(*,"0,,n0,,,0",o,,,,
I
, ; *
_-----
r€mperoture (.c) _-
_________.t-
rr 0t0 _.-Atomic o/c ---
0.20 }-
Intraredspectrometry
a
i*t\
1l\ \4 h\ \ ,\.. A,\\
A ,\\
0nqin.l sample unhealed
3500c
4000c
4200c
. ^ . \ 4 5 0 c0 5000 c 1000 ?600 1300 1400 1000 <wove number (cm,r )_
Fig. I1.5.7. Artificial cvolutionol type-ll kcrogen. A shallowsampleof the Lower Toarcian shalesfrom Fdcocourt. ParisBasinwasheatedat a constantrate of,1'cl min Lup to the tenperature indicated.Elementarvcompositionmav be comp , r r e du r t h I - i e u r el l . 5 . t . a n d l R \ p ( ; l r o m ( l r \ wlth Fi{ureIL5.;1.Reflectance is measureo on rne bulk of orcanicmatter.which is not of vitrinite tl-pe.-fhus.reflectance valueslessthan 1.5-27e cannot be comparedwith a scale of vitrinite reflectance. (After Tissotet al.. 1974)
procedure(bitumenremainingin the reactioncell or recoveredin a cool traD. etc-).Therefore,the samestageof kerogendegradation may resultin different r ieldsof heavyconstituents and mcdiumto Iighi hydrocarbons, accordingto the particularexperimental condition".
172
From Kerogento Petroleum Fie. II.5.8. Eliminationof the variousfunctional groupscontainingoxygenduring artificial degradationof kerogen:ketones'acids, of immaturekerogenof typeI, esters.Samples lI and Ill wer€heatedto varioustemperatures up to 600"C. K 1710is the intensityof the absorptionband related to carbonyl and carboxylC=O groups.(After Robin, 1975)
and small' c) Beyond 470-500"C,the weight loss to 600"C is ve,ryslow removed been groups have Hoiu*.i, as most aliphaticchains-andfunctional sheets of aromatic slacks occurs pr*iou.ty. a major structuralrearrangement be can A and to 200 I00 of range within the satherto form cluste.sor aggregates (on IR bands CH aromatic time, same .icrodiffr"aitio-n At the :;;;1y ";;". drop (Fig IL5 9) whereasreflectance susceptibility spectra)and paramagnetlc can be paired with the tiansformations these it"; 2 n 3o/o.All ;;;;;;" FigureII 5 3' andthis E in as samples, otr.t*,ion, madeon very deepnatural melagenesls' to ""iig"i.. intervalof artificialevolutioncorresponds by ri S.aundII.5.9 allowa co1nl!so1 of someoftheresultsobtained type-Il heating by obtained with those lll of types I and n.uiing t"-g"* quantitative ;;;"g;r. Qu?ti,u,lu. evolution is quite comparable.However' in the original differences to duL other the to type urp.it. uury from one heating' upon generated products of amount total i'he ."'"tp"titt.i "f f."rogen. for type III The ui rn'.u.u."a by the iveight loss, is greatestfoi type-I and least is highir for typesI and ll' andlowerfor typelll' in orooortionof hYdrocarbtns dlropof the aliphaticIR bandsand H/c ratioon ;i,i the respective ;;;;;;";; (8as:oil hldr.ocalbons,ratio Figure I1.5.9.Furthermore,the light to m-edium. to Contrary iuiirl itrr* for type I, moderatefor type Il, and high for tyPelll
5.2 ExperimentalSimulationof KerogenEvolution .9
9
r
6
: I
]
s
1'73
€
g
o l.-
d o)
II
m 400 600 ("C) Temperoture
Fig. II.5.9. Changing composition of kerogenduringartificialevolutron rn heating experiments. Lelr:weightlossandatomicH/Ciatio.Rr8f,r: IR absorption Oands related to aliphatic-groups (K 2920) andto aromatic CH (K 930-7'00). Surpt.r'oiir.utur" t..og.n of typesI. II. III wereheared upto temperarures shown, asin Fiiu.eff..i.S.(AfterRobin, 1975) hydrocarbons, the proportionof carbondioxidegenerated is higherfor type |II k_erogen in agreementwith the total drop of carbonylfi LunJ liro*n in Figure IL5.U.Furthermore.the transformation of type-IlI l..og"n i, i.ogr"ssrve,and extendsover a wide temperature range, wtrile the inteival is narrow and the t c m p e r a t u roel m a x i m u mw e i g h tl o s si s h i g h e rf o r l y p e - l keroeen. A very important result of the h^eating .*p.iir.nt, ir'rh. ,t"p_oy_rt.p coincidence of the kerogensresultingfrom ihe ripiO, trightlmpl.aru.e exper,_ mentalevolutionandthe slow,low_temperature naturalevolutibn.It is posiible to associate one stageof laboratory_simulated degradation to eachstageof the parh..Therespective resuttsof etimenratanalysis. IR ipeoros_ ::::ril.-::liti." copy. rhermogravrm€t ric analysis andelectronmicrodiffraction are idenlicaland provethe identityof compositionandstructureof the two paireJkerogens. This conclusionallowsone to make an evaluationof ttre OegriOation potentlalof a qlven.kerogenin the deeppartsof the basin, by using-shallow or outcropplng samplesfrom the sameformation. However,thereare limitationsto the useof artificialheating expenments. ln general.sarisfacrory simulaliunsare oblainedfrom samplesin"'f, irr". r"i"* i.r . eithertype I kerogenor typesII and III kerogenswhich :llq.l_::1.t.",. navealmo^st comptetedtheir diagenesis. Other trials,madeon more rmmature sampres or.rypesll andIrI - e. g., a maximumburialdepthca.200-300 m in the rvresset snates(typeU) or in the Doualabasinshales(typeIII) andalsothe brown coalsoftheMahakamdelta- resultedin a systematic siriftof th; OiC ratiodueto
1'74
FromKerogento Petroleum
a higheroxygencontext.Thisresultis interpretedasbeingdueto thepreferential process) duringpyrolysis. eliminationof oxygenaswater(a hydrogen-consuming ascarbon eliminated is mostly where oxygen situations geological to ascompared Thus the 1983) Vandenbroucke, and Tissot 1980; (Monin et al., dioxide eliminationof oxygenis crucialfor the completionof diagenesisAs diagenesis is occursat rathershallowdepthandmild temperature'the temperatureincrease cannotbe Diagenesis probablynot the singledeterminingfactorin diagenesis. id.quut"ly simulatedby heating.For instance,the variousreactionsof oxygen eliminationmay havedifferentkinetics,someof them havinga highertemperathan others.Furthermore.there may be an influenceof the ture dependencc conditions(openvs confinedenvironment).Finallyexperimental experimental oil vs gas)of evolutioncannotbe readilyappliedto predictthe nature(especially generated. hydrocarbons
5.3 StructuralEvolutionof Kerogen h a tb u r i a l i c ' t n c r e a s c T h e s eo b s e r v a t i o nass.a w h o l e. l ea dt o t h ec o n c l u s i ot n temperature- constitutcsthe dcterminingfactorin in pressureand especially risepromotesthe formationof the evolutionof organicmatter.The temperature of petroleum.at the mainconstituent bitumen.andparticularlyof hvdrocarbons. the expenseof kerogen(LouisandTissot.1967;Tissotet al.. 197' / Consideringthe generalstructureof kerogenproposedabove(cyclicnuclei' bearing alkyl chains and functionsand linked b1' heteroatomicbonds and (Fig. I1 4.8) As burial. is suggested aliphaticchiins). the followingmechanism and thus temperatureand pressureincrease'the structureof the immatule conditions' kerogenis not anymorein equilibriumwith the newphysicochemical and aromatization increasing toward take placeprogressively Rearrangements high under configuration The stable of an orderedcarbonstructure. development tempeiatureand pressurewould be graphite,but this stageis neverreachedin However,the buildingblocksof kerogen.each sediments. nonmetamorphosid sheets.tend to b€comeprogressively aromatic nucleusmadi of two or more hundredsA. Electronmicrodifone or several up to covering parallelover areas sheetsgatherto form aromatic of the stacks shown that clearly iraction has parallelarrangement a reaching of nuclei number that the or clusters, aggregates *iitrln ttre aggregateincreases,and that the regularityof interlayerspacing increases. of orderhaveto of suchhigherdegrees for development The sterichindrances be eliminated.Thcy include,for instance'nonplanarcycles(e g ' saturated cycles).heteroatomicbonds,or aliphaticlinks which prevent-Ihecyclicnuclei to dldge'1e-tis, In a firststage,corresponding fiom forminga parallelarrangement. and roughlyin order of ascending heteroatomiibondsare broken successively rupture energy.startingwith some labile carbonyland carboxylgroups(like keionesand acids).Heteroatoms'especiallyoxygen' are partly removcdas volatile products:H2O. COr. The rupture of these bonds liberatessmaller structuralunitsmadeof oneor severalboundnucleiandaliphaticchainsThese of bitumens.The largeronesarestructurall! fraementsarethe basicconstituents
5.3StructuralEvolutionof Kerogen
n5
similar.tokerogenbut of lower molecularweightand thereforesoluble(MAB extractl,asphaltenes). The smalleronesarelinear,branchedandcychchydrocarbons and closelyrelated heterocompounds, such as thiophenederivatives. During.thefirst (diagenetic) stage,the largerfragmentscontainingheteroatoms. especially oxygen(resins.asphaltenes, MaB eitract), are predJminant.Their etlmrnatron, and that of CO2and H2O, causethe oxygenand sulrurcontentrn kerogento decrease drastically. As temperaturecontinuesto increase,the kerogenreachesthe stageof catqgenesis. More bondsof varioustypesare broken,like estersand alsosome carboncarbonbonds.within the kerogenand within the previouslygenerated fragments(MAB extract. asphalteneietc.). The na* irugrn"n,, generated hecomesmallerand devoidof oxygen:therefore,hydrocarbons are relatively enriched.Thiscorresponds first to the principalphaseof oil formation.asnamei bv Vassoevich(1969).and then to ihe stageof '.wet gas,,and condensate generatlon.At the sametime, the carboncontentincreases in the remaining kerogen,due to the eliminationof hydrogen.Alipharicand alicvclicg.uup, u,! partly removedfrom kerogen.Carbonyland carhoxylgroupsare completely eliminated,and most of the remainingoxygenis inciuAlain ether bonds and possiblvin heterocvcles. When the sedimentreachesthe deepestpart of the sedimentary basins, temperatures becomequitehigh.A generalcrackingof C_C bondsoccurs,both In Kerogenand rn ttrrumenalreadygenerated from it. Aliphaticgroupsthat were stilpresentin kerogenalmostdisappear. Correspondingly, low irolecularweight compounds,especiallymethane,are released.The remainingsulfur. when presentln kerogen,is mostlylost,andH2Sgeneration may be important.Thisis the principalphaseof dry gasformatron. Oncemostlabilefunctionalgroupsand chainsare eliminated,aromatization andpotycondensa tionof the residualkerogenincreases, asshownby alterationof opticalcharacteristics (e.g., sporesandpollenbecomeblack)and by IR spectra. Parallelarrangement of the aromaticnucleiextendsover *id" ur"a, from g0 to 5U{,A, tormingclusters.Physicalproperties evolveaccordingly (highreflectance, erecrrondtllractton).Suchresidualkerogenis unableto continueto generate hydrocarbons. asshown.by the negativereiultsof thermogravimetric assiys.This stageis reachedonly in deep- or veryold- sedimentarylasins andcorresponds Io metagenesis. Thesevariousstagesare shownon Figure1I.1.2with their correspondence to . vitrinitereflectance Ievels,coalranksand hydrocarbons generated. TableIL 5.2 presentsthe atomicbalanceof the organiJmattertransformation by catagenesis from a shallowimmaturesrage(700m) to a deeperone (2500m) situatedin the principalzoneof oil formationin the parisBasin.The figures foi kerogen, hydrocarbons,resins,asphaltenesand MAB extract. result from isolationof the constituents and elementalanalysis.Water and carbondioxide result from laboratoryexperiments(see above),simulatingthe evolution of '1 Heavy bitumen, extractedby methanor-acetone-benzene mixture. Itcan tJeconsidered a beingmade_ofheavier,or more polar, asphaltenes than thoseextractedby chloroform or comparable solvents.
From Kerogento Petroleum
176
Table II.5.2. Atomic balanceof the total olganic matter in the evolution from 700 m to 2500 m, correspondingto 1000 carbonatoms(lower Toarcianshales,ParisBasin) 700 m (Vacherauville)
Burial depth (location) C Kerogen Hydrocarbons Resins Asphaltenes MAB extract Coz Hzo HzS N2 Total
H
940 1220 t20 23 1 1 1,'7 2 1 t.2 0.2 t4 1.4 0.3 1 3 t7 22 3 1 3.7 0.7
2500m (Essises,Bouchy)
H 24 806 8'76 106 1 8 0 0.1 33 4l 22 0.2 0.2 28 3 5 5
O
N
40 1 8
1 . 6 0.4 1 . 6 0.4 3 . 4 0 .6 10 'l02 51
5 0.1 0.1 0.2
18
36
; 1000 1306 126.324.2 24.5 1000 1293 11L.624.4 23.4
kerogen.Hydrogensulfideand nitrogenhavenot beenmeasuredand are only a se s t i m a t i o nl o s r b a l a n c i npgu r p o s e s included As a conclusion,it can be emphasizedthat the main feature in kerogen extendsover of a carbonorder' whichprogressively evolutionis the emergence elimination The temperature widerareas,andbecomesstrongerwith increasing rangeof a wide of the formation results in of the sterichindrancesto ordering carbon weight hydrocarbons, to low molecular compounds,includingmedium generation petroleum the Therefore, etc. dioxide, water. hydrogensulfide, of the constantadjustmentof kerogento consequence appearsto be a necessary overburden. with increasing new temperatureconditions
During Catagenesis 5.4 Formationof Hydrocarbons 5.1.1 General of the thermal degradationof kerogenrequiresthe A better understanding knowledgeof the evolutionof the total organiccontentof the sediment.For instance,the carboncontentof immaturekerogenof the ParisBasinat a burial depthfrom 400to 800m amountsto 9470of the totalorganiccarbonof the rock. while kerogenburiedat 2500m in the centralpart of the basinaccountonly for 81% of the total organiccarbonrecoveredin the cores.The balance,6 and197r by extractablebitumenof the total organiccarbon,is represented respectively from kerogen(TableII.5.2) whichhasbeengenerated In order to observethe formationof petroleum,we usuallymeasurethe total + resins+ asphaltenes bitumenextractablefrom the rock. i.e., hydrocarbons
5.4 Formationof HydrocarbonsDuring Catagenesis
t7'7
(Htl + R + A) or hydrocarbons only (HC). The totalorganiccarbonbeingC1,we express: a) The transformation ratio of organicmatterinto petroleumas: Bitumen HC+R+A Co
Co
- bitumenratio
b) The transformation ratio into hydrocarbons as: HC ^ : hydrocarbonratio Bitumenratio usuallyrangesfrom 0.02to 0.20,i.e., 20 to 200mg/gof total organrccarbon.Hydrocarbonratio rangesfrom 0.01 to 0.15. i.e.. from 10 to 150mg/gof total organiccarbon. Actually,the useof the ratios"carbonof bitumen/Ce.' and ..carbonof HC/C6" wouldbe better.but mostdatain literaturelackthe carbonanalvsis on extracts. Such ratios can be consideredas significantif numeroussimples taken at randomfrom a given formationat about the sametlepth (for iniranceover a 10-20-mcoredinterval)showa reasonably constantvalue.regardless of thetotal organiccontent.Examplesfrom varioussections in the Mowry shaleof Wyoming (Schrayer andZarrella,1966),from theLowerToarcianshalesof parisBasinand from Lower Mioceneof WestAfrica (Fig. II.5.l0) showthisrequirementto be satisfied.
Low€rToorcion PorisBosin
T€.liory WestAfrico
W e , D er r0 1 ; 0 e dnht e r v o , 1 1 5 8 , 1 1 5 9 m
WeI X (nol releosed)j0epth mleftol,l524-1562m
6
s -
;Q
+
/(
u _
5 _-,_
*
l 0 % Orgoniccorbon
u __,
1 '>
2 % Orgonic corbon
Fig. 11.5.10.Significance of the ratio of bitumento organiccarbon.This rauo remarns reasonably constantover a shortinterval.despiteconsidlrablevariations of the absolute amountof organicmatter. Examplesare shownover coredintervalsof 1l m (Lower Toarcian.ParisBasin)and 38 m (Tertiary.WestAfrica) respectively
FromKerogento Petroleum
118
5.1.2 Presentarionof SomeTvpical Examplesof PetroleumGenerution Burial of sedimentsis responsiblefor a temperatureincrease.which in turn of the tbrmation of hydrocarbonsEarlyobservations the generation determines basinscamefrom Larskaya of petroleumas a functionof burial in sedimentary bedsfrom Azov-Kuban und Zhubt"u (1964),working on Mesozoic-Cenozoic of theParisBasin.and basin:Louis(1964).workingon the LowerToarcianshales Philippi(1965).workingon Mio-Plioceneof Los Angelesand VenturaBasins -H C + R + A and' They noted an incteasewith depth of the bitumenratio ;HC by an increasein hydrocarboncontentof accompanied hydrocarbonratio = v0 the total extractedbitumen. As alreaclystated.with regardto kerogenevolution'a studyof.theinfluenceof burial on petroleumgeneiationin sedimentsrequiresa situationin which all CretoceousI
Tefirory
of Fig. I1.5.1l. Reconstruction burial histories (dePth versus for deePsamPles geologicaltime) from variousformations:Lower Toarcianof the ParisBasin.UPper Cretaceousof the Douala of the Basin.Paleocene-Eocene Uinta Basin.Mioceneof the Los AngelesBasin
5..1Formationof Hydrocarbons DuringCatagenesis
Ij9
geologicaland geochemical parametersremain constantexceptdepth. Four exampleshavebeenthe objectof detailedgeochemical studiescomprlsingboth quantitativeandqualitativeaspects. Theiroverallgeological conditionsandtype ol organicmatterare ratherdifferent. a) The caseof Lower Toarcianshalesof the paris Basin has alreadvbeen discussed in Section-5.l. The kerogenbelongsto typeII, whichis rnostfrequentin sourcerocks.The detailedresultsare reportedby Louis and Tissot(1967)and T i s s oe t t a l . ( l 9 7 t) . b) The Upper Cretaceousfrom the Douala Basin (Cameroon)is a thick sedimentarysequencein which the natureof the sedimentsand conditionsof depositioncanbe considered asreasonably constantfrom Turonian Coniacianto Campanian (90-70millionyears):Dunoyerde Segonzac (1969).Thecorrespono_ ing depthintervalis 80(H200m in the Logbabawells,siudiedby Albrechtand Ourisson(1969)and Albrechtet al. (1976).The kerogenbelongito typeIII. c) The GreenRiver formation(both the Eoceneand the assJciated FlacstafT -up member)of the Uinta basin.Utah. has reachedmaximumburial depths ro 6000m. Kerogencompositionis reasonablyconstantand belongsto typi I. Formationof petroleumhasbeenstudiedby Tissoret al. (197g). d ) T h e C e n o z o i cb e d so f t h e L o s A n g e l e sa n d V e n t u r ab a s i n s( C a l i f o r n i a ) . studiedb1'Philippi (1965),rangefrom Recenrto upper Miocene(dbout0 12 million vears).The sedimentation is supposedto haui beenof rarnerconsranr composition.The depth variesfrom 0 to 3500m in the Los Angelesbasinto -1600 m in the Venturabasin. The rockscontainingorganicmatterconsistof shalesand silts in the paris. Douaiaand Los Angelesbasins.They rangefrom shalesto marisin the Uinta 'l'he basin. burial historvin the deepestpart of eachbasinis shownon Fisure I I . 5 I. 1 . 5.1.-l QuantitutiveAspecx(Fig.II.5.l2) At shalbw depths.the bitumensand hydrocarbons are fairlv low. l.hebitumen riltro
HC+R+A
c,,
is onlv ca. 30 mg/gin the Doualabasinand 40 mc/s in thc
ParisBasin.Hvdrocarbonratio is 5 to 15mg/gin the Los AngelesandVentura b-asrns. around l0 mgig in the paris and I)ouala basins.and somewhathigher (30 mg/g)in the Uinta basin. Bitumenandh1'drocarbon ratioschangelittle downto depthsof ca. 1500m in Parisand Doualabasins.more than 3500m in Uinta basin,2400m in the Los Angeles basin and still deeper in the Ventura basin. According to local seothermalconditions.thesedepthscorrespond,respectively, to about 60"C (ParisBasin).70'C (Doualabasin),100.C(Uintabasin)and 113"C(LosAngetes and Venturabasins).Beyondthat threshold,the transiormation ratrosincrease ratherabruptlyin the Doualabasin,moregentlyin theotherbasins.The bitumen ratio reachesmaximumvaluesas high as 1g0mg/gat 2500m and 100mg/gat 2100m, respectively for the paris and Douala baiins.At the sametime. the proportionof hydrocarbons increases with respectto heavyheterocompounds
180
From Kerogento Petroleum
Paris Basin L.Toarcia n K€rogentypeII
0ouala Basin U.Cretaceous
uintaBasin Eocene
Kerogentype III
Kerogentype I
LosAngeles Basin lVlio-Pliocene ( P h i l i p p i , 1)9 6 5
1 :
9
6
2000
e '=
100
150
0
50
m g / g o f l o l o l o r g q n i cC L;J
Ct5* hyd/aco/,bons
=
Don hvdloco/bons
lreslnst
asDho/tenes)
Fig.IL5.12. Formation (resins of hydrocarbons andnonhydrocarbons andasphaltenes containing N, S.O) asa function (cf.alsoFig.11.5.1 of burialdepth,in different basins I ). 'PZOF" Themajorstepsof evolution of the organic matteraremarked:"immature". (principalzoneof oil formation).and "ZGF" (zoneof gasformationby cracking). Corresponding temperatures areshownaccording geothermal gradients. to present In the DoualaBasin.the first temperature is the presentonethe secondtemperature is a paleotemperature calculated (1975) according to TissotandEspitalie (resins,asphaltenes).and the hydrocarbonratio reachesmaximum values rangingfrom 70 mg/gat 2100m in the Doualabasin(kerogentypeIII) to 100mg/g at 2500m in the ParisBasin(kerogentype II) and to 150mg/gar 5200m in the Uinta basin(kerogentype I). At greaterdepthin the Doualaand Uinta basins,both bitumenandhydrocarbon ratios decreaseprogressively to very low values(below 10 mg/g). This disappearance of liquid hydrocarbonsand other petroleum constituentsis attributedto cracking.whichproduceslow molecularweighthydrocarbons (gas) and a carbonresidue.This is in agreementwith the occurrence of gas,mostly melhane,in Logbaba wells. The decreasein liquid hydrocarbonsand the generationof methaneat great depth have been recognizedin severalother tbrmationsof varioussedimentary basins,like the Upper Devonianof Western Canada(Deroo et al., 1973)and NorrhernAfrica (iissot et al.. 1974)and the Lower Cretaceous and UpperJurassic of the Aquitainebasin(Le Tran, 1972).It has not been observedin the Parisand Los Angelesbasins,as the depth of investigationis not sufficienl to reachthis stage.
i l
II
5.4Formationof Hydrocarbons DuringCatagenesis
1g1
However,the molecularweightdistributionof hydrocarbons in the parisbasin showsthat the light hydrocarbonsincreasewith depth more abruptly than mediumand heavyones.Hydrocarbons < C15,whichare very scarceat si!a[o* depth.may amountto asmuchasone third of the total hydrocarbons at 2500m. This observationis in agreementwith the hypothesisof an overwhelming occurrence oflight hydrocarbons at greatdepth.In the SouthAquitainebasin.Li Tran (1972\observed,after a maximumal 3500m (about 100"C),a marked decreaseof the bitumenratio, accompanied by an abrupt increaseof gaseous hvdrocarbons and H,S at 4200m (about 120.C).In the Devonianof Northern AlbertaandNortheastern BritishColombia,Evanset al. (1971)notedan abrupt increase of methanecontentin cuttingsgasat depthcorresponding to 100.C.All thesefeaturespointtowarda crackingoforganicconstituents undeiconditionsof lncreaslng Iemperature. The existence of two criticaltemperature thresholds in sourcerocks- the first onereprcsenting a lowerthrcsholdto massive generation of tiquidhydrocarbons. and the seconda lower thresholdto crackingand gas generation_ led ( 1969)to the conclusion Vassoevich that in betweenthosethrisholdsthereis the principalzoneoJoil formatiort.Followingthe earlydiagenesis of organicmatter s^9dir-n:nts. subsequent evolurion will thus include the successrve sreps ll.yo!lq (Fig. II.1.2) of hydrocarbons formation. a\ Diagenesis an immaturestagewith little change:hydrocarbons arescarce.Theyare more or less directlv inherited from living organismsand comparableto the m o l e c u l epsr e s e nitn I h e y o u n gs e d i m e n l . b\ Catasenesis t h e p r i n c i p azl o n eo f o i l f o r m a t i o n , - the zone of gas formation,where ,,wetgas" with increasing proportionof methaneis generatedin largeamountsthroughcracking. c) Metagenesis - someadditionalgenerationof hydrocarbons (mainlymethane)from kerogen; liquid hydrocarbons previouslygeneratedare alsocrackedand convertedto gas, - stageof slructuralrearrangement of the residualkerogen. 5.4.4 QualitativeAspects:Compositionof Hydrocarbonsand N, S, O-Compounds I t i s o b v i o u sf r o m F i g u r e sI L 5 . l 3 , I I . 5 . 1 4 ,a n dI L 5 . 1 5 ,t h a tt h e p r o p o r t i o nosf hnear and cyclic hydrocarbonsgeneratedduring the principalphaseof oil formationdependon the type of kerogen. In the ParisBasin,type-IIkerogengenerates abundantcyclichydrocarbons of saturatedandaromatictypes,whereasnormalandiso-alkanes arecomDarativelv fessimportant(.20%of the hydrocarbons). In the DoualaBasin,the hydrocaibonsgeneratedfrom type-lll kerogencontainabundantn-alkanescomparedto typeII; the proportionof n- plusiso-alkanes amountsto 40% of hydrocaibons. In
From Kerogento Petroleum
182
E ! 6
E
.E
30
100
mqlq of lolol orqonrc corbon
asa typesof kerogen in different alkanes of normalandbranched Fig.t1.5.13.Formation in the Paris.DoualaandUintaBasins(same iui.tion ot burialdepth.asexemplified carbon in mgperg organrc asin Fig.II.5.l2).Valuesareexpressed formations morelinearplusbranchedalkanesthan the Uinta basin,type-Ikerogengenerates of the are major constituents iroth. normal and iso-alkanes' cyclicmolecules:" (607c). hydrocarbons ' of We shallnow presenta brief reviewof thevariationsamongthe mainclasses generatlon petroleum and their relationto the variousstagesof hydrocarbons a) Alkanes in shallowsamplesresemblethe n-alkanesalreadyoccurringin Recent n-alkanes is generallyimportant' The role of high molecularweightmolecules sediments. and the distributionstill reflectsthe depositionalenvironment ln the Upper of the Tertiaryof the Los Angelesand Venturabasinsor the lower Cretaceous organic North hquitaine basin.wherethere are contributionsfrom terrestrial of predominance matterde;ivedfrom higherplants.a strongodd-carbon-number main of the-threshold burial to the highern-alkanesis obseivedduringincreasing The CPI is still around4 downto 2400m in California' generation. hydro"carbon andaround5-at1500m in North Aquitainebasin(Fig lI 3 6) Wherer-alkanes
5.4 Formationof HydrocarbonsDuring Catagenesis
183 Kerogen lypem
E -9
E E t
UpperCreloceous DouoloBosin
rhg/g of toiol orgonic corbon -,-=
--'>
Fig-II.5.14. Formationof cyclichydrocarbons (aromatics andcvcloaikanes ) asa tunction of burial depth. in rhe samebasinsand formaljonsar in Figure IL5.l3. Values arc expressed ln mg per g organiccarbon
U i n f oE o s i n ( T y p e I) +
PorisBosin ( Typetr )
D o u o l oB o s i n ( TypeItr ) +
f
\
lt,r,',t
L-x:r'"
il |jlrh,lJ
,,,.f,H,,,., Fig. 11.5.15.Compositions of hydrocarbons generatedfrom the three main tvpesof kerogenat the depthof maximumoilformation.Areasareproportionalto the respectivc amountof eachhvdrocarbon classper g organiccarbon
FromKerogento Petroleum
184
orthe derivedfrom higherplantsaremixedwith thosefrom eitherphytoplankton is smallerbut still microbialbiomasslivingin sediments, the odd predominance distinct. amountsof new alkanesare In the principalzoneof oil formation,substantial generated disappearwith lessor no oddpreference5, thuscausingtheprogressive anceof the odd/evenpredominanceby dilution. Thusthe CPI is loweredto about a shiftof 1. In addition,alkanesof Iowmolecularweightarefavored.Thiscauses the maximum in the n-alkane distribution curve towards a smaller number of carbon atomsper molecule.Thesechangescan be seenclearlyon n-alkane distributioncurvesof samplesfrom the Los Angeles(Fig. II.3.6) and Douala basins. Where conditionsare not suitablefor the generationof new n-alkanes (unfavorablecompositionof kerogen,low geothermalgradient,or very short time of burial), however,the odd-carbonpreferencemay last down to a great depth.In the Neocomianshalesof the North Aquitainebasin,France,wherethe majorpart of the organicmatteris composed of carbonaceous debrisfrom higher plants,the preferencefor odd-numbered molecules remainsat any depthhigher thanit is in goodsourcerocksof upperJurassic age(TableII.5.3).In the Ventura basin, California (Philippi, 1965), where the geothermalgradient is low (26.6'C/km)and the ageof sedimentsis Mioceneto Recent,the CPI is over 2 down to 4000m. On the contrary. the absenceof a moderateto strongodd-carbonpreferenceis a consequence ofcatagenesis. Someyoungandshallowsediments not necessarily show a distributionof n-alkanescontainingmany low to medium molecular weightmoleculesand few alkanesbeyondC25.The odd preferenceis slightor absent.This situationoccursin Pliocenebedsof the Pannonianbasinat depths, 400-800m wheretemperaturehasalwaysbeenlow. The immaturecharacterof thesesedimentsis confirmedby the occurrenceof interbeddedsedimentary
TableII.5.3. Compared valuesof theCPI asa functionof depthin the Jurassic(sourcerock) and lowerCretaceous (nonsource rock)of the NorthAquirainebasin Depth
Jurassic
Irwer Cretaceous
< L500m
1.4
2.2
1500-2500 m > 2500m
1.7
1.0
1.3
on for thisfact.For example,aliphaticchains,substituted 5 Theremaybe variousreasons kerogen nuclei, may be broken at various positions. Degradation of wax esters experimentallyproducesodd ,-alkanes and even fl-alkenes,which are subsequentlr conve ed to even r-alkanes(Esnault, 1973).
5.4 Formation of HydrocarbonsDuring Catagenesis t5 I - - - t ] l
t to 5
N]
l 474 n
15 20 25 30
t5
5
il N
185
Fig. IL5.l6a-e. Distribution of r-alkanes in Pliocenesediments of the PannonianBasin.The absenceof an odd/evenpredominance in sedimentsat relativelyshallowdepths470.725 and 810m is not due to catagenetic evolution.This is proven by a strong odd predominanceoccurring at intermediatelevels(650 m) or even deeper (1030m) (Tissotet al.,19'11)
650rn
6 2 0 2 5 3 0
i
S =
s
725m
1 5 20 25 30
c
8 1 0m
t5
l O 3 Om
e
15
20
25
_________)'
30
Numberolcarbonatoms
layers,containingorganicmatterfrom plantswith a strongodd-predominance of t h e h i g h e r r - a l k a n e( sC P I2 t o 3 ; F i g .I I . 5 . l 6 ) . At greaterdepth.wherecrackingbecomesimportant.distributioncurvesare moreand moredominatedby lightermolecules in all typesof sedimcnts. Some aclc/lc isoprenoidsare also inheritedfrom living organisms.either directly or through an early diagenesis. As the abiogeniC formationof such peculiarstructures is unlikelv.one wouldexpectthe isoprenoids to be dilutedby newly formed n- and iso-alkanesin relation to the abundanceof alkanes generatedfrom kerogendegradation.A plot of isoprenoidsh -alkanesas a functionof increasingcatagenesis showsthat the isoprenoidcontentdecreases with depth'and with increasing hydrocarbongeneration,but in a differentwav from what couldbe expected.During burialto the temperaturethresholdof thi 6 The pristane:n-C17ratio may showmore complexvariations,due to the changeof 11-alkane distributionwith depth.
186
FromKerogen to Petroleum
generation. mainhvdrocarbon no changeis noted.ln thefirstpartof theprincipal the ratio of isoprenoidto total saturatedhydrocarbons zonc of oil formation. This result indicatesthat somcisoprenoidalkanesare remainsnearlyconstant. may be generated The being duringcatagenesis. generationof ncw isoprenoids explainedbv the fixationol isoprcnoidacidsor alcoholsasesterson the kerogen by Burlingameet al. (1969).During the of the Recentsediment.as suggcsted principalstageof oil formationsuchbonds.weakerthan C-C bonds,would be interprewouldbe liberated.An alternative morerapidlybrokenandisoprenoids tatiou of the phenomenonwould be the existenceof isoprenoidmolecules released. trappedwithin the kerogenand subsequentlv As oil generationcontinues.fewer and fewer isoprenoidchainsof this type would be available.and they are no longerproduccdwhen the oil generation curve reachesits maximum. Therefore. the isoprenoid: tl-alkanes ratio becomesimporlant. decreases. At greaterdepth.crackingof the hydrocarbons is seriouslyaffected. and thc distributionof isoprenoids b) SteroidsandTriteroenoids Thesetetra-and pentacyclic molcculesof biogenicorigin are mostlypresentin or partlyaromatizedhydrocaras full_nsaturated.unsaturated voungsediments andalcohols(seeSect.3.8).At shallow bons.or asrelatedstructurcs suchasacicls depth and down to the temperaturethresholdof principaloil fbrmation.their distributionremainsrathersimilar.The ring analysisof nonaromatichydrocarshowsa high proportionof 4- and 5-ringmolcculcs. bonsby massspectrometrv c o m p r i s i nm g o s t l ys t e r a n es. t c r c n c .t r i t e r p a n ea. n d t r i t e r p e n et v p c si n m r n \ shallowsamples.as in shallowsamplesof the ParisBasin.They are distributed c s s c n t i a lilny t h c c a r b o nn u m b e r a n g eC . - C r , ,( F i g .I L 5 .1 7 ) .A s i m i l a sr i t u a t i o n occurswith respectto aromaticsteroidsand triterpenoids.At thc cnd of the rliageneticstage.unsaturatcdpolvcvclicshave been convertcdto saturatesor aromatics. During the principal stagc of hydrocarbonformation. thc abundanceof h_'-drocarbons or br polvcvclics decreases. eitherby'dilutionwith ncwll gcnerated progressive decrease with depth is casc of found in the A typical degradation. ( T i s s o t I 9 7 l : E n s m i n g c c r t al.. s h a l e o s f t h e P a r i s B a s i n c t a l . . l - o w c rT o a r c i a n 1 9 7 7 )w i t h a c o r r e l a t i v er e d u c t i o no f t h c o p t i c a la c t i v i t ) . I { o w c v c r .s o m e andtriterhvdrocarbons. includingprobablvsteranes tctrac!clicandpentacvclic p a n c sh. a v eb e e nr e p o r t e di n a P r e c a m b r i asne d i m e n(tB u r l i n g a mcct a l . . 1 9 6 5 ) . Generalll. aromatizcdforms seemto be more stable.as thev are still rather from EasternSahara(Algeria).In these abundantin lowcr Paleozoic secliments old sedimentsfullv saturatcdstcranesor triterpanesare not detected(l'issot ct al.. 197,1). c) AromaticHydrocarbons depth.butat The ratioof aromatichydrocarbons to organiccarbonincreaseswith carbon. In the rate does ratio of total hvdrocarbons to organic a smaller than the
5.4 Formationof Hydrocarbons DuringCatagenesis Distribution of Composition telracyclrc of Saturaled HC cycloalkanes saturatedhydrocaftons
ffi :-------
6
l
o
l
g;F^l
l ;o 6
€
f: rooo
E'
:
.Ir
20
E
s
t0
:
g
l',{--_l
I':.SJ C o r b on ! m b e r
0istribulion of aromatic and naphthenoaromalic molecules
il,, g
-J
Aromalrc HC
187
9 - - 6
Cycloolkones
0
E 2000
I 2500
0 20 40 60 m9 soi HC
g,orqontT
0 2040 60
-3 Corbon number
Fig.II.5.17. Occurrence of the :l- and5-ringcycloalkanes fie/r]andofthe corresponorng (right)in thc Lower Toircian th"r.., naphrhenoaromatics i;ri.-B;;i;. The amountof \ll urirted J n Lnl r p h t h ( i , Jr r\ m a tl cp , , l v c \ . l i cd\ e cel a \ e \w r t hd ( p , n .e i i t , " r . . r , n r . ., t . r e r a \ e i l m n u nnt l c h . n _ l t k u r ? -a n dr . r r a l k a n ei n . r.rearcs
Paris Basin. total hvdrocarbons increasefrom 50() 700 m to 2500 m bv a 10:l
hl'dnrcarbon b sy t 5 : 1 .a n da r o m a r i ctsy o n t ui , l . t n p u i t i . u t , , r . ltnc l f o :aromatrc:saturtted ,lrlu1l*d ratio decreases in the 2000-2500m ranseof the paris Basin. probabll'duc to the crackingof side chainson urornoi. nucler(l,ig. II.-5.17). Je:r:a:ing. importanceof steroid and rriterpenoidrvpe ....1I.11,-1,,-'t Ih.: uirh dcprh.thcre is a gcneraltendencvto favor. ar great ::liJt:lll.ir::In,irrcs purcl\ irrr)mrrie r 1 p e . :i r l k y l - h c n z t , n_en. i r p h r h a l e n c . r_r (prn ( n i r n _ rl jnl lr.e n c\l\l'^r lt hI t e \ \c,h r i n so l r h o r r l e n g t h ( l _ 3 c a r b o n a t o m s i n a d d i t i o n t o t h e c v c l i c nucleus).The lattcr moleculesmav result in part from tfr.,lnuL.r"ll-,"g',,i stcroidsand triterpenoicis with their methvlsubstituents. d) GlobalCompositionof H1'drocarbons Thcse,various aspects ol the catagenetic evolutionresultin a strongtendencyfbr , r - J n o t s o - a t k t n etso I n c r e i l \ et h e i r r e l a t i r ei m p o r t a n c e . . . r m p n r e d u i fh o t h e r d u r i n gr h e p r i n c i p asl l a g eo f o i l f o r m a l i r ) n :I rI 8 ucl,urjrlllpcs. lTissoe r t at.. t9/ll | l 5 . r t i ) l ' o r y cc) r i cm o l e c luc s .e i t h er s a t u r a t eodr u n s a t u r a t e d . a r ec o n s l o e r _
From Kerogento Petroleum !
3
o
!
e
2 Essisss
l Vochorouvillo
E a
6
; 6 E z
o O
z
a
I
o
o
o ---------)
o
a
a
a
a
a
O
a
a
a
1 2 4 4 5 Numb€r of oromolic clrles
0 --------|
ta ,4od/us p.opa///ort/
I !,e
a
a
a
r 2 3 4 5 Number of oromolic cycles
/a Ddss % aflolo/ hrd.ocorbors
"r\4 3 Lcarnd
"P /tP z
&
@"
ot L-a
@
@8 #
d
@
@
dr
N
4
& a
in theLower typesof hydrocarbons of thevariousstructural Fig.I1.5.18.Abundance iSht: adeep immature sample; shallow and Lef: a shales of the ParisBasin. Toarcian proportionalto Theradius ofthecirclesis of oilformation. sample fromtheprincipalzone onthebottom typesareshown type.Themainstructural of eachmolecular theabundance (Tissot et al.. 1971.modified) ablylessfrequentin thisstageof evolutionthancomparedto the amountthatwas inherited f r o m b i o g e n im c o l e c u l eosf l i v i n go r g a n i s m s . Beyondthe principalphaseof oil formation,thus when crackingbecomesa more stablelow molecularweight dominatingprocess,the thermodynamically of sediprevail.Therefore,in an advancedstageof catagenesis hydrocarbons ments,only the light gasesand benzenesurvive(Leythaeuseret al.. 1980). (saturatedand aromatic amountsof light hydrocarbons Accordingly,increasing
5.5IsotopeFractionation andKerogenEvolution
1g9
compounds) aregenerated from the kerogenand/orfrom the bitumenpreviouslv generated at thesematurationlevels.Due to the strongtemperature dependence of their formationreactions. rheiranalysis represents i sensitive indicatorfor the \drocarbon q:ler{lon processin the main phaseof petroleumgenerarron (Tissotet at..1971:philippi, 1975). e) N. S.O Compounds Thesecompoundsinclude(1) resinsand asphaltenes, whichare extractiblewith chloroform,and (2) other heavy asphaltenic compouncls, which can only be extractedby a methanol-benzene or methanol-acetone-benzene (MAB) mixture. From Table IV.l.8 in ChapterIV.1, it is clear that resinsand asohattenes of bitumenhavean averageelementary composition whichis differentfrom that of resinsand asphaltenes of crude oils. Oxygen.nitrogenand sulfur are more abundantin the resinsand asphaltenes from shallowbitumen.but their abun_ danceprogressivelv decreases with depth and becomesmore similarto those observedin crudeoil constituents. Whenplottedon a H/C versusO/C diagram. asphaltenes tiom the bitumenof a given formationfollow an evolutionpath broadlyparallelto the kerogenevolutionpath previouslydescribed; the sameis true for resins. The.followinginterpretationis proposed.During the first stepsof kerogen . degradationrather large fragmentsof kerogenari released,such as resins, asphaltenes andMAB-extract.Theyhavea composition broadlvsimilarto thatof the kerogen and still include structureswith various tvpis of bonds. As temperaturecontinuesto increase,other bondsare broken in the remainins kerogenand in the break-downproducls.Therefore,the compositionof th! break-downproducts,whichis originallydifferent,becomes progressively more similarto the compositionof crude oil suchas resinsand isphaltenesduring catagenesis. Due to that evolution,hydrocarbons of low to midium molecular weight are likely to be produced in the sameway as they are produced from kerogen.Thus,N, S, O compoundsappearasposiibleintermediates in someof the reactionpathwaysfrom kerogento petroleum.
5.5 IsotopeFractionation and KerogenEvolution The break-downof kerogenduringcatagenesis involvesthe splittingof various typesof bondsandthe release particularlvhydrocarbons. of smallermolecules. It has beenobservedthat the carbonof the bitumenreleasedbv this processis isotopically lighterthancarbonof the relatedkerogen{in the rangeof 1 to 4%c). The same appliesto pooled oils derivedfrom the disseminatidbilumen as c.omp?red to its sourcekerogen.Thisisotopefractionationmay be explainedby the relationship established by Galimov(1973)betweenisotopedistributionand chemicalstructure(Chap.IL2.5). Bitumen,especially thehydrocarbon fraction. is derivedfrom chemicalgroups,originallydepletedinrt. suchas aliphatic chainsor saturatedcycles.This isotopiccharacteris preserved whenhydrocirbon molecules are releasedfrom kerocen.
From Kerogento Petroleum
190
Fig. IL5.19. Schematicrepresentationof carbon isotope ratio changes duringpetroleumformationandevolution (Silverman.1967)
II E l
{ l
.:l
I
Lighler
Table II.5.4. C-Isotope fractionation during formationof CHa in pyrolysiserperiments (500"C).(After Sackett,1968) Sourcemolecule
613C/oo Cfla versusA
Propane N-butane N-heptane N-octadecane Crudeoil, Penn.-Okla.
- 3.8 - 9.1 - 9.6 -23.6 -25.8
Correctedvalue accordingto least square methodfor a straightlinefunction. A similareffect.i.e.. a tendencytoward enrichmentof the light hydrogen isotopeversusdeuteriumis observedin the hydrocarbons released. The rangeis largerthanin the caseof the carbonisotopcfractionationandthereis alsomore variation.Valuesas high as a 40%"depletionin D may be obtained(Schoell. 1980). M e t h a n eg e n e r a l l ys h o w sa s t i l l l o $ e r d r t C . i . e . , t h e d i f f e r c n c eb e t w e e n isotopiccompositionof methaneand the sourcematerialis largerthan between crude oil and its sourcematerial.Methaneand other low molecularweight hydrocarbons are mostlygeneratedby splittingof C-C bondsin kerogenor in petroleum.Silverman(1961, 1967)and Sackett(1968)pointed out that the prod[ctionofmethaneresultsin anisotopefractionation. Thisis basedon thefact that thcre is lesscncrcvreouiredto breaka rrC-rrCbond than a 'rC-':C boncl. Hence in thermal ciacking 'rc-rrc bond rupturc occurs about 87, more f r e q u e n t l y( S t e v e n s oent a l . , 1 9 4 8 ;B r o d s k i e i t a l . . 1 9 5 9 )T . h u s ,t h e m e t h a n e generatedis enrichedin 'tC. comparedto heavierhydrocarbons and kcrogen: Table IL5.,l and FigureIL5.l9. Becauseol the productionof isotopicallylight methane.the sourcemoleculeis relativelyenrichedin the heavy ''C isotope. Furthermore,becauseof the relativelylargehydrogencontentof the methane molecule.the source material becomesdehydrogenated. which necessarily resultsin unsaturationand the creationof more reactivedoublebonds.This in
5.5IsotopeFractionation andKerogenEvolution
191
turn wouldleadto polymerization reactions of the residualmolecules, givingrise to.componentsof even higher molecularweight. Silverman(1967,1971t has followed this line of thought in explainingthe observedcarbon isotopic compositionof narrowdistillationfractionsfrom crudeoils.He pointsout that a minimumin the carbonisotopedistributioncurveis observedin the molecular weightregionwhichcontainsthe highestconcentration of biologicallyproduced carbonskeletons.This view is illustratedin FigureIL5.20, wherethe highest opticalactivity- witnessof biogeniccompounds - is obscrvedin the fraction havrngthe lowesrd''C. The generalschemeof crude oil evolutionand the accompanying changes in isotoperatiosof thedegradation productsareshownin F i g u r eI I . 5 . 1 9 . Sackett(1978)showedthat the productionof methaneresultsin a hvdrogen isotopefractionationas well. Laboratorvsimulationexperimentsindicated a significant_ enrichmentof the light hydrcrgenisotopein generatedmethane. althoughthe predictedfractionationundernaturalconditioiscouldnot be made with any reliabilit)'.Carbonisotopefractionationis associated with the clcavage ol a carbon-carbon bond to form a methyl frec radicalwhich subsequentlv combineswith a hydrogenatomto form methane.The hydrogenatomis formed by cleavageof a carbon-hydrogen bond so that one_fourthoi the hvdrogensin methanehasparticipated bothin a carbon-hydrogen bondcleavage anda carbon_ hydrogenbond formation.The rate-determining stepis presuirablythe bond cleavage and thereforeresponsible for the fractionation. With increasing evolution of the source rock (measured bv depthof burialor . vitdnite reflectance), a progressive increaseof the drrC and dti of methaneis generallyobserved. In otherwords,the differencebetweenthe isotopiccomposi_ tion of methaneandthatof the sourcematerial(coalor kerogen)is progressively
l3c/t2c Rotht
t
Resid@n 6Js.c '08
+3.
|
,oeE
a
105 "
'04.E +?
*oz -P
*or.E -01 -0 2 500 A v e r o gder n i l o l i o i l e n p e r o t u r(e. C )-
| I
550 600 -, -)
Fig- II.5.20. Changesin optical rotation and carbon isotoperatio with increasing molecularweight(boilingpoint) of distillationfractionof a ciude oil. Highestopticai activitvis observedin the molecularweight rangefrom 350 to ,150.corresponorng ro distillationtemperatures 420to 550.C.Tetra-anJ pentacyclic ring structures fall in that range.Carbonisotoperatiosexhibita minimumin the samefraction.Isotoperauosare relatedto PetroleumStandard-NBs-22. which is 29.4%"lower than tOl_1. latter Silverman.1967;Welte. 1972)
192
FromKerogento Petroleum
lowered(Colomboet al., 1966;Galimov, 1973;Stahl, 1975;Schoell,1980). Sackett(1968) has shown that the isotopefractionationeffect in methane lengthof the carbonchainin the source generationdecreases with decreasing crackingtemperatureup to a temperatureof molecule,and alsowith increasing for thefactthatmethane 500"C(Sackett,1978).Both factorscouldbe responsible and sourcematerialare isotopicallymore similarat greaterdepthof burial or higher rank levels.Furthermore,higher rank levels of organicmatter also representa higher degreeof aromaticity.This changein chemicalstructure toward higher aromaticity would also result in a lower isotope fractionation betweenmethaneand its sourcematerial (Galimov, 1973)with progressing The relativelylow isotopefractionationobserved and metagenesis. catagenesis for gasesderived from more aromaticcoalsas comparedto gasesderived from lessaromaticmarinekerogenmightbe explainedsimilarly.
5.6 ExperimentalGenerationof Hydrocarbons from OrganicMaterial Experimentaiheatingof organicmatterhasbeencarriedout by manyauthorsln order to support the theorieson thermal generationof petroleum.Results concernedwith the evolutionof kerogenhavebeenpresentedin Section5.2. Althoughexperimental heatinghasprovedto be a convenientmeansof simulating kerogenevolution,the simulationmay not be asgoodwith respectto liquid procedures. and gasproducts,whichdependsomewhaton the experimental pure are basedon differentartificialor naturalsubstances: The experiments Recentmuds, extractedfrom livingorganisms, chemicalcompounds, substances carriedout on purechemical Experiments andkerogenfrom ancientsediments. of the possiblecatalyticeffectof minerals. compoundsallowan investigation 5.6.1 PureChemicalCompounds Fatty acidshave been the object of many experimentssinceit wassuggested offatty Decarboxylation pointedout thattheycouldbe a sourceof hydrocarbons. authors! by numerous is well-known mechanism studied acidsby heating a includingBogomolovet al. (1960,1961,1963),Eismaand Jurg (1964),Shimoyama and Johns (1971, 1972) and Almon (1974).Degradationof alcohols (Sakikana,1951).esters,ketones(Frost, 1940;Demorestet al., 1951)and 1l958) (Levi andNicholls, wasalsostudied,sinceall of thesegroupsof aldehydes oxygencompoundsare knownto be presentin the originalorganicmatter.The form in which they are incorporatedin sedimenls,however,is not clear.Other and polyterpenes substances. suchas chlorophyll(Bayliss,1968),aminoacids, (Erdman.1965)havealsobeenstudied. between200and300"Chasshown acidat temperatures HeatingC2IHa3COOH of mineral that n-alkanesof variousmolecularweightare producedin presence catalysts(Eisma and Jurg, 1964;Shimoyamaand Johns, 1972).With clay
5.6Experimental Generation of Hydrocarbons fromOrganic Material
193
minerals (kaolinite, Ca-montmorillonite),the main n-alkane constituentis C1,H.,0. resultingfrom directdecarboxylation. With calciumcarbonate,the main n-alkaneconstituentis C.nH1..suggesting that beta cleavageof the acid is the mostlikelyprocessof n-alkaneformation.BesidestheseDredominant n-alkanes with20or 2l carbonatoms,thegeneration of otheralkaneswith shorteror lonqer carbonchainsis observedin heatingexperiments of longduration.Theyprobably resultfrom crackingand polymerization. Unsaturatedhydrocarbons and othei fatty acidsthan the originalC,THIrCOOHhavealsoheenobserved. Almon (1974)investigatedthe mechanisms of fatty acid decarboxylation. Anhydroussmectitefavorsthe formationof branchedalkanes.with a distribution of isomersapproachingthermodynamicequilibrium.The presenceof water favorssomesort of free radicalreactionand resultsin a strongpredominance of then -alkanes: sucha distributionof alkanesiscomparableto thesituationknown i n c r u d eo i l s . Thermal and catalyticcrackingof n-octacosane (nC,sH56hydrocarbon) was studiedby Hendersonet al. (1968)in orderto simulatethe theimalalterationof n -alkanedistributionin sediments. Erdman (1965) has shown that thermal degradationof a polvterpene (f-carotene)and an aminoacid(phenylalanine) resultsin a productionof low molecularweight alkvl-benzenes and alkvlnaphthalenes. which are absentln Recentsediments.
5.6.2 NaturalRecentSubstance.\ Experimentson naturalsubstances go asfar backasAlbrechtand Engler.who heatedfishoilsat theendof thelastccntury. In morerecentworks,main11, marine mudswereused.For instance,ErdmanandMulik (1963).bv heatingmuds,have producedlow molecularweighthydrocarbons. in particularalk!,lbenzenes. rhat areabsentin Recentsediments. Harrison(1976)studiedtheevolutionoffree and boundfattyacidsb1'usingpyrolysisofa modernsediment.Ishiwatariet al. ( 1976) heatedkerogenfrom a voungmarinesedimentcollectedin the Tannerblsin. offshore California. Kerogen producedliquid productsand gas comprrsrng initially carbon dioxide and water, then methane.other hvdrocarbonsand hvdrogen.n-Alkaneswcre mostl) generatedhv successire reactionsthrough intermediateproducts (heavier, probably heteroatomicbitumen). Separate heatingof thc mainorganicconstituents of thesedimentshowedthat the bitumen (extractable with bcnzene-methanol)is responsible for58% of n -alkanesgener_ ated.kerogenfor 4lVc and,humicacidsfor 0.47conlv. 5.6.-1Kerogenof Ancient Sediments Numerousexperiments ofheatingancientsediments, whichhavepreviously been subjected to an exhaustive extraction,haveshownthatkerogenis abletoproduce new hydrocarbonswhen heatedat a highertemperaturethan the maximum reachedduringits burialhistory.The industrialtreatmentof oil shalesconsists of
FromKerogento Petroleum
194
a pyrolysisto generatethe shaleoil. This oil, obtainedfrom 350to 500'C,hasa ratherdifferentcompositionfrom a normalcrudeoil. For example,unsaturated (N, S. O) are abundant. and heterocompounds hydrocarbons A seriesof experimentshas beencarriedout on the shallowand immature M e s s esl h a l e so f t h e R h i n eV a l l e l ' ( W e l t e 1 . 9 6 6A 1 l b r e c h t ,1 9 6 9 B 1 a j o re t a l . , 1969).on the Lower Toarcianshalesof the ParisBasin(Debyserand Oudin. Durand,in: Tissotand Pelet,1971)and on the Yallournlignitefrom Australia (Connan.19741 van Dorsselaer.1975). The Messelshales.rich in organicmatter,weredepositedin the RhineValley during Eocenetime and havea maximumthicknessof 150m. The maximum depth of burial is 200 to 300 m. so their kerogenis still very immature(e.g., hydrocarbonratio is only 7 mg/gof organiccarbon).After exhaustiveSoxhlet extraction.Welte (1966)heatedthe rock to varioustemperatures from 190to 490"C. The bitumen content increasesregularlywith heatingtemperature: hydrocarbons, and especially saturatedhydrocarbons. are favoredwith increaspoint is the distributionof n-alkanes ing temperature.A particularlvinteresting ( F i g .I L 5 . 2 l ) . I n t h e n a t u r abl i t u m e na s t r i k i n go d d p r e d o m i n a n cwea sp r c s e n t from rClj to rClr. A weaker,but still definitepredominance of odd n-alkanesis notedin the new bitumenwhichwasgeneratedbv heatingto 21(I 300"C.but no odd predominanceof r-alkaneswas observedin bitumensproducedabove 30t)"C.ln additionto the particulardistributionof n-alkancs,Albrecht (1969) showedthatheatingthekerogenof MesselshalesalsoproducedC1.,C1o,C1*,C,e. and C.,1isoprenoids. a 350 I I I | I I I
3OO
; E E
150
.: n
w
w
22
?4
C-olomsper
Fig. II.5.21. Distributionof normalalkanesin the naturalbitumenextractedfrom the ranging by heatingthe shalesat temperatures andin the bitumengenerated Messelshales. from 210to.190'C(Welte.1966)
5.6Experimental Generation of Hydrocarbons from OrganicMaterial
195
Connan(1974)heatedYallourn lignite and showedthe formationof odd_ numberedn-alkanesand even-numbered n-alkenesby decomposition of wax esters.Decarboxylation of fatty acidsgeneratesthe odd-numbered n-alkanes, whilst the alcoholproducesthe evenn-alkenes.In the sameexpenments,van Dorsselaeret.al. (1977) reported also the tbrmation ol some pentacvclic triterpaneswhichwere probablypresentin the kerogenmatrix.By t eatingttre Green River shales.Gallegos(1975)showedsome lenerationof rerpanesancr steranes. althoughthey are comparatively lessabundantin pyrolysisoil than in the naturalbitumen. All theseresultssuggestthat ,.ghost"structures,inheritedfrom the original . biogenicmolecules,can be either attachedto the kerogenby chemrcalbonds (e.g., linearor isoprenoidchainslinkedby an esteror eth;r bond)or entrapped in cavitiesof the kerogenmatrix (asin a molecularsieve)and then released-upon temperaturerncrease.Thus moleculesretainingcharacteristic structures(iso_ prenoidchains.steroidor triterpenoidpolvcycliC nuclei)or distriburionparrerns (odd n-alkanes)may be producedduring the first stageof the catigenetic evolution.Foilowingthat lineofthought,Seifert(197g)co-rnpared the hydrocar_ bonspresentin crudeoilswith thosegenerated by pyrolvsisof kerogen.in ortler to identify the sourcerock responsible lor generationoi the oil. With the Messelshale.Bajor et al. (1969)carriedout heatingexperiments wirh . increasing temperatures in order to simulatea progressive buiial. Theyshowed, as..afunction of temperature.a progressiveincieaseof the liquid'bitumen followedby a decreaseof liquid bitumenexplainedby crackingand massrye generationof light hydrocarbons.This succession is in agreeient wrth the progressive evolutionobservedin sedimentary basinsasa fu-nction of burial. Experiments on LowerToarcianshalesof t-heparisBasinareof two different types. A first set was carriedout on shallowimmaturesamplesin order to determinethe influenceof time and temperatureon bitumenand hydrocarbon generation(TableIl.5.5): after90, 180,and270da1,s. the amountgenerated was TableII.5.5.Bitumen(hydrocarbons + resrns+ asphaltenes) generated byhealing lower Toarcian shales (Fecocor.rrt: paris basin) at 180, 2OO and 22O"C
Time (days)
Bitumen generated(mg/g of original organiccarbon) at 180"Cat 200"C ar 220"C
1 3 10 30 90 180 270
5.9 6.6 6.6 8.8 10.4 10.7 12.8
6.8 8.7 9.2 14.8 12.2 22.2 2t.2
9.2 13.1 14.0 21.4 30.0 30.4 36.2
196
FromKerogento Petroleum
aboutthreetimesmore at 220'C than at 180"C.In addition,generationof high seemsto precedetheformationof hydrocarmolecularweightheterocompounds reactionsin which heavy bons.The latter would mostlyresultfrom successive (Louis and Tissot,1967).This act as intermediateproducts heterocompounds lshiwatari et al. (1976)reported the results of interpretationis confirmedby above. from various samples on LowerToarcianshales, In a secondsetof experiments after exhaustive conditions were heated under standard depthsin the basin from shallow generation more bitumen They resulted in Soxhletextraction. undergone some which had already deep rocks than from immaturesamples duringtheir naturalburial The role maturationandhadproducedhydrocarbons on the sameshalesof the ParisBasin of pressurehasbeentestedin experiments pressure varyingfrom a few barsup to 800bars.The hydrocarbons with geostatic studied generated areapparentlyof the sameorderof magnitudeat all pressures might be slightlylessabunsuchas resinsand asphaltenes, Heterocompounds, role Thisis an indicationof the relativelysubordinate dant underhighpressures. of pressurein the formationof oil. wereinvestigated by heatingkerogenof More recently.two importantaspects situations.Lewan et al. ancientsedimentsin conditionscloserto subsurface of water("hydrouspyrolysis"),the observed ( 1979)showedthat in the presence to that of crudeoils generated is morecomparable distributionof hydrocarbons than it is without water. In pafticularno olefinsare generated.when water is Dresent. The role of the mineralmatrix wasobservedin pyrolysisconditionsby two Espitalidet al. (1980)comparedthe productsyieldedby seriesof experiments. in an ancientshale,the isolatedkerogenfrom the shale,and heatingthe kerogen proportionsof minerals,suchasillite, smectite, with varying the samekerogen for clay minerals,suchas illite, are responsible etc. They observedthat some generated during (> non-hydrocarbons Crr) and ofheavy hydrocarbons relention In turn, the pyrolysis,thus providingan effluent rich in light hydrocarbons. produce light hydrocarprogressively cracked and are heavy constituents trapped bonsanda carbonresidue.if heatingis continued.HorsfieldandDouglas(1980) showedcomparableresultsby pyrolyzingvariouskerogensand coal macerals of minerals. (vitrinite,sporinite,alginite)in the presence
5.0.4 Ignificance ol HeatingExperimenrs cannotbe compareddirectl) derivedfrom heatingexperiments The conclusions basins conditionsin sedimentary and withoutgreatcareto subsurface is to provethat kerogenis ableto The mainresultof the heatingexperiments form petroleumconstituentsunder temperatureincrease Laboratoryexperiasthereareprobablyimportant mentsarenot ableto simulatenaturaldiagenesis, factorsotherthantemperatureat thatstage;however,theyprovidea convenient with respectto kerogenevolution The resultsare in simulationof catagenesis of petroleumgenerationdeducedfrom the agreementwith the mechanisms basins.Furthermore.experiments of casehistoriesin sedimentary observations
5.6Experimental Generation of Hydrocarbons from OrganicMaterial
lgj
havealso provedthat biologicalmarkerscan be fixed to kerogenby chemical bonds,suchas esteror ether bonds,or entrappedin the kerogenmatrix and subsequently releaseduponcatagenesis. Laboratoryresultshavealsoshownthat time and temperaturecan, to some extent.compensate for eachother.The importantroleofthe heavyheteroatomic compounds,includingasphaltenes, is confirmed.Someof them may be already presentat the stageof earlydiagenesis, whereasthe major part resultsfrom the thermalbreakdownof kerogen.Heavyheteroatomiccompoundscontributeto the generationof hydrocarbonsand other constituenrsof low to medium molecularweight.ln thatrespecttheycanbe considered asintermediate products in someof the reactionpathwaysfrom kerogento hydrocarbons. The possiblerole of minerals,especially clayminerals,is not clear.Catalytic inlluenceof cla1,mineralsis only possibleif thereis an intensecontactguaranteed betweenthe reactingorganicmoleculeand the clay surface.Such a contact bctweenthebulk of the kerogenandclaymineralsis not possiblebecause theyare both solid materials.Thereforea catalyticeffecton the breakdownof kerogen can be largelyexcludcd.Free petroleumcompoundsliberatedby non-catalytic thermalbreakdownof kerogen,however.cancomein contactwith claysurfaces andcanbe adsorbed. Thereaftera catalyticreactionis easilypossible. In pyrolysis experimentsthis effect is observed.At presentit is difficult to assessthe importanceof catalyticreactionsunder geologicalconditions.Finally, more experiments are probablynecessary to measurethe role of waterin the reactions generating light or mediumhydrocarbons. It can be observedthat an intermediate situation,betweenlaboratoryexperimentsandcatagenesis dueto burialin sedimentary basins,is providedby igneous intrusionsin sediments.There, volcanicflows, dikes or sills provide high temperatures overa shorttimerange.Catagenetic transformation of kerogencan be observedto a certaindistanceof the igneousrock andpetroleumis genirated. Thissituationhasbeenreportedby Hedberg( 1964)andMcIver(1967)andoccurs in SouthAfrica, SouthernColorado, Argentina,etc.andmorerecentlyin cores takenduringthe Deep SeaDrilling Program:Simoneitet al. (1978,1931).
198
From Kerosen to Petroleum
SummaryandConclusion During burial of sediments,the increasein temperature results in a progressive reafiangementof kerogen: 1. During the last part of diagenesis,heteroatomicbonds and functional Sroupsale eliminated. Carbon dioxide, water and some heavy N, S, O compounds are released. In terms of petroleum exploratio!, soutce tocks are consideredas immature at this stage. 2. During catagenesis,hydrocarbon chains and cycles are eliminated. Thus, flIst formed.This stagecorrespondsto the mainlycrudeoils andthengasare successively principalstageof oil formation. and alsoto the principalstageof wet gasformation. 3. During metagenesis, a rearrangementof the aromaticsheetsoccurs.The stacksof aromaticlaye6, previouslydistributedat random in kerogen,now gather to form larger clusters.At this stage,only dry gasis generated. The amount and the compositionof hydrocarbonspresentin sourcerccks change progressively,as a function of increasingevolution. During diagenesis,moleculesof steroids, biologicalorigin predominate,includingcertainn-alkanesand iso-alkanes, and terpenes. Duing catagenesis,medium to low molecular weight hydrocarbon become predominant, and particularly n- and iso-alkanes.Where metagenesisis reached,methaneis practicallythe only remaininghydrocarbon.
Chapter6
Formationof Gas
6.1 Constituents and Characterization of PetroleumGas Thereis a widevarietyof naturalgasoccurrences whosccomposition anomooes of formation mav be rather diffcrent. Howevcr. in this chaptcronlv natural petroleunlgas will bc considered.While methane(CH.,) is aluays ;r major constituentof thc gas.othercomponents mav be present: - hvdrocarbonsheavierthan methane(mostl)'ethaneC.Ho. propaneC,H3. b u t a n eC , H 1 , 1 ) . - carbondioxideCOl: hydrogensulfidcH,S, - n i t r o g c nh. v d r o g e na. r g o n .h e l i u m . (liquidhydrocarbons condensate dissolved in the gas:thevareseparated when the gasreachessurfaceor shallowsubsurface positions). The originof gaseous constituents is multipleandmaybe not onlv organic,but alsoinorganic(atmospheric air. volcanicand geothermalgases,radioactivity). Characterization of naturalgaseshasto rely on a smallnumberof parameters. as the number of their constituents is relativelysmall,comparedto the largc varietyofmoleculesfoundin crudeoil. Furthermore,dry gasis mainlycomposed of methane.with minor fractionsof nonhydrocarbon gases. The main parameters usedto characterize gasare the following: the methanecontent.and alsothat of higherhydrocarbons, when they are presentlthis is convenientlyexpressedby the Cr/tC" ratio (methane/total hvdrocarbons), - the content,of CO2,H:S, N, and other nonhydrocarbon gases,if present. - the carbon isotopecompositionof hydrocarbongas, usuallyexpressedas d''C(%.) (for definitionsee Chap. II.2.5) in particulardr3C,refers to rhe isotopiccompositionof methane. - the hydrogenisotopecomposition ofhydrocarbongas,usuallyexpressed asdD (fbr definitionseeChap.II.2.5). The principalcharacters of hydrocarbon gasesgenerated duringthesuccessive stagesof kerogenevolutionare shownin Table II.6.1, accordingto the data recentlypresentedby Srahl(1975),Galimov (1980),Schoell(1980).Rice and Claypool (1981).and Tissot et al. (1982).Both the first and the lasr stage (diagenesis and metagenesis) generatedry gas.i. e.. methanewith a very low contentol higherhydrocarbons: C,/)C">0.97. However.thesetwo stagesare casilydifferentiatedby their respective isotopicfractionalion:the drrC,ratio of
Formationof Gas
200 of hydrocarbongasesgenerated Table 1I.6.1. Pdncipalcharacteristics evolution. (After Stahl, 1975; of kerogen stages during the successive Galimov,1980;Schoell,1980;Rice and Claypool,1981) Main stagesof evolution
Diagenesis
Dry gas
cr/tc,
-90 > 0.97 to -55
< -180
-55 to -30
< -140
> 0.9'/
-40 to -20
- 150 to - 130
Wet gas
Metagenesis Dry gas
dD
< 0.98
Oil-associatedgas Catagenesis
drrcr
the earlv.biosenicmethanerangesfrom -90 to -55%o'whereasthe metagenetic -40 and-20%".The dD of methanecould methaneshois drrC valuesbet-ween provideadditionalvaluableinformation,asshownin TableII.6.1. Theintermedigas' with a wet gas.includingoil-associated generates ate stageof catagenesis ratios isotopic 98. The Cr/-tC"<0 variabli proportionof higherhydrocarbons: values. 6r3C,and dD showintermediate studyof the drrcr measuredon methaneand the C'/IC" Thus, a systematic of pooled gases,as and relativelysimplecharacterization a first ratio allows ( 1975).Galimov data of Stahl from the is compiled which I1.6.1 shownby Figure (1980),Schoell(1980),Rice and Claypool(1981) This illustrationshowsthe stagesof gasgenerationduringthe evolutionof sediments: threesuccessive - biogenicgas.generatedduringdiagenesis, - t h e r m agl a sg e n e r a t eddu r i n gc a t a g e n e s i s .
tdrlt ollDnctir! J@ lrinlt l.rot.r.trrtir! [@
and Fig.IL6.1. Relativeabundance isotopic compositionof methanein gasesfrom biogenicorigin (diagenesis) and thermogenicorigin (catageThe original nesisand metagenesis). data used for compilation of this figure are shown,with references,rn FigureII.6.7
6,2 Gas GeneratedDuring Diagenesisof the OrganicMatter Fig. IL6.2. Successivestagesof gasgenerationin sediments,after Claypool and Kaplan (1974), Demaison and Moore (1980), Galimov (1980) and Rice and Claypool(1981).Isotopiccompositionof kerogenand metbaneare shownafter Galimov(1980),with minor changes
- thermal crackinggas generatedduring the late part of catagenesis and metagenesls. The principaleventsresponsible for gasgenerationin sediments are shownin FigureII.6.2, accordingto Claypooland Kaplan(1974),Demaisonand Moore ( 1980),Galimov( l9tt0),RiceandClaypool( 1981),togetherwith a comparison of the isotopiccompositionof kerogenand the relatedmethane(Galimov,1980). Theseaspects will be reviewedin the followingSections6.2 and6.3. It shouldbe notedthat the isotopiccharacterization of methanepresented in TableII.6. I andFigureI1.6.1is basedon pooledgases.The greatestcareshould be taken when applyingtheseresultsto smallamountsof gasfound near the surface.The isotopiccomposition of low quantitiesof methaneoccurringin soils, groundwaters andyoungsediments maybe alteredby actionof microorganisms. In particular,oxidation of methane by methane-oxidizing bacteriautilizes prefercntiall)rrC. as comparedto rrc. Thus the residualmethanemay be 'C (see enriched in a h o v eC . h a p .1 1 . 2 ] .
6.2 GasGeneratedDuring Diagenesis of the OrganicMatter 6.2.1 Hvdrocarbons The uppersliceof a youngsedimentcontainingorganicmatteranddepositedin a normal.open and oxygenated sea.may well be aerobic(Fig. IL6.2). However. the sedimentbecomesanaerobicat very shallowdepth.first at a microscopic. then at a macroscopicscale.and bacterialreductionof sulfateis initiated, generating Sr . The availableamountof sulfateis quicklyloweredundernormal conditions.andthe sedimententersthe zoneof methanegenerationby bacterial degradationof the organicmatter whoseultimatestep is carbon dioxide or acetatereductionby methanogenic bacteria.Thisbiogenicmethaneshowsa very low isotopicratio dlrcr : 90 to -55%". The succession of theseeventsis discussed in ChaoterII.2.2.
202
Formation of Gas
A comparisonof the isotopicratio of the kerogenandof the relatedmethane showsthat suchbiogenicmethaneis highly depletedin the heavierisotopeof carbon.asits drrCris ca. 30 to 50%clowerthan the Atrc of the originalkerogen. of the kerogenis only slightlychanged,asthe However.the isotopiccomposition and COr (rrCglobal svstemsimultaneouslygeneratesCH4 (1rC-depleted) (Galimov.1980). enriched),providingsomeinternalcompensation Favorableconditionsfor generationof abundantbiogenicmethaneare the following.accordingto Rice and Claypool(1981): - eliminationof oxygcn. low sulfatecontent. - moderatetemperatures: tempera75'C seemsto be the maximumacceptablc with is ratherin agreement ture.evento thermophiljcbacteria;thisassumption oil degradation by othertypesof bacteria madeon subsurface the observations ( s e eb e l o w .C h a p .I V . 5 . 3 ) . - sufficientspaceavailableto the bacterialbodies(1-10 pm). i.e., a moderate stageof compaction. and alsofrom observations madein sedimcntary From theseconsiderations, at depthshallowerthan basins.it is assumedthat mostbiogenicgasis generated 1000m. However. there are situationswhere active generationcould have persistedat greaterdepth. (ethane.propane,etc.) generated by The proportionof higherhydrocarbons bacterialactivityis very low. as comparedto mcthane.However.a systematic ratio in the corestakenaspart of the Deep examinationof the ethane/methane with depthtrom l0 r to SeaDrilling Projecthasshownthat thc ratio increases : 10 at depthsrangingbetween500and1500m: FigureII.6.3(RiceandClaypool. l98l). This progressivechangeis interpretedas a result of mixing biogenic methanewith someearlvproductsof the thermalevolutionof kerogen. of biogenicgasis shownin Table of the majoraccumulations Thc composition e t s .r l n g i n g I I . 6 . 2a n di n F i g u r eI L 6 . 4 .T h e ya r cf o u n di n r e l a t i v e lyyo u n gs c d i m n which neverexperienced high temperatures. from Cretaceous to Pleistocenc, r than 1300m in the majorlieldsof Western Presentreservoir dcpthsareshallowe Siberiaand in the GreatPlainsof Canadaand United States.Biogenicga: may
Fig.IL6.3. Ethaneto methaneratioof naturalgasftom somewellsof the Deep SeaDrilling Project,and ftom the Frio sandsin Gulf Coast(RiceandClaypool,1981)
6.2 GasGeneratedDuringDiagenesis of the OrganicMatter
203
Fig.IL6.,l. Relativeabundance andisotopiccompositionofmethanein variousgases from biogenic origin. Stratigraphicagesgiven denotethe ageof the reservoirrock. (Data from Schoell,l9g0; Rice and Claypool,1981)
TableIL6.2. Compositionof the major accumulations of biogenicgas.(Adaptcdfrom RiceanclClaypool.1981:wirh addirionaldatafrom Schoeil.19;g0) Countn' Uasinor tield Reservoir age D e p r h( m )
c .tc,
'C (t;.) d L\timaled
U ,S .S .R , W Siberia Crelaceous 70{)-ll00 > 0.99 68 10 58 tl
Canada S.E. Alberla Crctaceous r00- 1000 > 0.99 - 6li to -60
u.s.A. Ilalt N. GreatPlains Po Valley Cretaceous Pliocenc 120-810 1200 1000 > 0.97 > 0.97 76 to -55 0..1
U ,S .A . Gull of Mexico Plcistocenc ,160-21J00 > t\.97 -69 to -55 0.1
il0ir x mi)
.rlsolre tbund lt greaterdcpthsin youngersediments. suchas thc plioceneof \ o r t h I t a l l a n dt h eP l e i s t o c e n o cf t h eG u l fo f M c x i c ow . h e r ea v e r v v o u n a gg e a n r l .i lou {eothcrmalgradicntntakethe sediments still immaturedouinto 2E00m. 6.l. 2 N ctnht drocsrbons \onh1'drocarbongasesmav also be generatedduring the stageof diagenesis. oarticularlvcarbon dioxide and nitrogen. Carbon dioxide itav result from ructerialactivityat shallowdepth degradingthe organicmatter in sediments. \larsh gasmav containI to I\Vc COl (Sokolov.1974).However,at thtsstage iarbondioxideis lrkelyto be largelydissolved in subsuiface waters.Later,whcn thermaldegradation of kerogenis initiated,at depthssuchas500to 1500m. the main featureof diagenesis is the eliminationoi functionalgroups.including
204
Formationof Gas
to carbonvlandcarboxylgroups.whichgenerateabundantCOl: thiscorrcsponds decrease of the O/C the eliminationol oxygenlrom kerogenand the associated is directlvrelatedto the originalcomposition ratio.The amountof COl generated of kerogenand increases from type I to type III. whichis ableto generatelarge quantitiesof carbondioxide.In factthe labileC:O groupsmav amountto 30'Z by weight of some type III kerogens(see above.Sect.,1.7).Karu'eil (1969) reportedthat I kg of coalis ableto generateup to 75 I of COr. Nitrogen is sometimesfound in association with shallowbiogcnicgas (for instancein Alberta: Hitchon.1963).Althoughnitrogenmay be a remnantof air trappedin intcrstitialwatersof sediments. anotherpossibilityis that diagenetic ammoniamay be oxidizedto N: by meteoricwaterscarryingoxygen. Hvdrogensultlde is also producedby sulfate-reducing bacteriain young s e d i m e n t(sC h a p .I 1 . 2 . 2 )H . o w e v e ri.t i s n o t p r e s e r v eadn du s u a l l yr e c o m b i n e s verv quicklv:eitherwith sulfatcto producelrce sulfur.or with iron to Elenerate sulfides(mainlv in detrital sediments).or with organic matter (mainly in carbonate-evaporite sediments).
6.2.-l Inportutrce o.l BiogenicGas The world reservesof biogenicmethanehave been estimatedbv Rice and C l a y p o o(ll 9 U l) t o l 7 x l 0 ' - m ' ( 1 7 , 0 0 0 b i l l i o n c u b i c m e t e lrns )a.d d i t i o nt o t h i s . newreserves couldbe discovered in placessuchastheGreatPlainsof Canadaand theUSA. The presentevaluationamountsto ca. 20% of the total world gasreserves estimatedto be 77.-5x l01rmr. However.thisfigureis heavityinflucncedbv the h u g ea c c u m : i ) . a n dt h e ul a t i o n so f b i o g e n igca si n W e s t e r n S i b er i a ( c a .l l x 1 0 r m ligure couldbe quite dilTerenton a regionalbasis:e. g.. in WesternEuropethe fractionof biogenicgasis approximatelv 7 % versus93?1of thermalgasgenerated d u r i n I c l r t r e c n c sri n' d m c l a u n t c.i..
6.3 GasGeneratedDuringCatagenesis andMetagenesis of OrganicMatter 6.3.I Ht'dro<'arbtnt Gas Catagenesis is primarilythe stageof oil generation. Horveve r. formationo f liquid hydrocarbons is alwal'saccompanied by somegenerationof hvdrocarbongas. especially The occurrence C,-C1.whichwerevirtuallvabsentduringdiagenesis. of C,-Cr hvdrocarbonsis even a characteristicfcature diagnosticof the catagenesis zone:an examplefrom WesternCanada.wherethismethodhasbeen p a r t i c u l a r luys e d w . i l l b e d i s c u s s ei nd C h a p t eV r .1.3.4. Methaneis alsoseneratedalongwith cr,ude oil andC,-C1h1'drocarbons. Early methaneis stilldepletedin heavyisotope' 'C. ascomparedto thc kcrogenwhichis
6.3 GasGeneratedDuringCatagenesis andMetagenesis of OrganicMatter
TFC] rE0 00
205
Fig. II.6.5. Abundanceof liquid and gaseous hydrocarbons,and hydrogensulfide, in cuttings from a deep well in the Aquitainebasin,France(Le Tran, 1972)
20
t0 60 E5
the sourccfor it. but the differenceis progressively reducedwhenpassingfrom diagenesisto catagencsis and mctagenesis stages:Figure II.6.2. Thus. with increasingevolution of the sediments.the proportion of thermal methane becomesgreaterand the d''C1 becomeslessnegative. Whenthe advanced stageof catagcnesis ( R.,- 1.3 to 2.0) is reached,cracking of C-C bondsbecomesimportantin crudeoil alreaclygeneratedanclalsoin the remainingkerogen.The amountof liquid hydrocarbons decreases, whereasgas increases abruptlv.ashasbeenobservedbv Le Tran (1972)in a deepwell ofihe Aquitainebasin:FigureII.6.5. Gasgenerated by crackingusuallvshowsa lower d''Cthantherelatedoils.duetoagreaterprobabilityforbreaking,:ClrCbonds than rrC r:C bonds (Silverman,ttlO+,ilO;; see above Sect. 5.5). Thermal crackingexperimentscarriedout b]' Sackett(196tt.1978)haveconfirmedthis viewandshowedthat the resultingmethaneis 4%,rto 25%"lowerthan the parent m a t c r i a(l s e ea b o v e S . ect.5.5). Duringmetagenesis. someof the kerogenconstituents arestillableto generate methane.as shown by the experimentsof Jrintgenand Karweil (1966).In particular.the short alkvl chains,which are still presentand would prevent coalescence of the polyaromaticnuclei.are brokenat that stage.The resulting methaneshowsa drrC,closeto that of kerogen:FigureI1.6.2. From observationof the amount and isotopic compositionof methane generatedby thermal crackingduring late cat;genesisand metagenesis in gcologicalconditionswe can suggestthat actual situationsfall betweentwo e x t r e m ec a s e s( F i g .I L 6 . 1 ) : methaneis mainlygeneratedby the crackingof oil; this situationcouldoccur lrequentlywhenkerogenbelongsto typeI or I [, i. e., if it is veryoil-prone:A in F i g .I L 6 . 1 ; methaneis mainly generatedby crackingof the remainingkerogen;this situationcouldoccurfrequentlywhenkerogenis of type IlI, whichis lessoilprone, or when oil has been largelyexpelledout of the sourcerock before metagenesis is reached:B in Fig. II.6.1. Both situationsmavbe foundin gasesanalyzedby Stahl(1975):an exampleof r.rscA wouldbc providedby the gasesof the PermianBasin.Texas,whereasan
206
Formationof Gas
exampleof caseB would be providedby gasesof the NW Europeanbasin, generatedfrom Upper Carboniferous coalsand associated shales.
6.3.2 Nonhydrocarbons gases- mainlvhydrogensulfideandnitrogen- may alsobe Nonhvdrocarbon generatedduringlate catagenesis with and metagenesis and they are associated methaneand light hydrocarbonsin certainbasins,accordingto the t)'pe of sediments and the geological history. a) Hydrogensulfidemay be generatedin largeamountsby thermalcracking from kerogenandfrom liquidsulfur-containing compounds presentin crudeoils: the deep wells drilled in the South Aquitaine basinhave clearlyshownthat hydrogensulfideis generated togetherwith methanebe)'ond120'C:FigureII.6.5 (Le Tran. 1972).Hydrogensulfideis particularlvabundantwhen the organic matteritself(typeII) is rich in sulfur,whichmaybe the casewith carbonateand carbonate-evaporite sequences. On the otherhand.type III kerogenis usuallya poor sourcefor sulfurcompounds. b) Freesulfur,if present.may alsoreactwith hydrocarbons to produceH1S. Sulfatereduction(if presentin sediments), andthe relatedoxidationof hydrocar- beyond 150'C - resultingin bons, mav also occur during metagenesis generationof hydrogensulfideand carbondioxide. (a) or (b), it hasbeenobservedthat the average As a resultof both processes abundance of H.S in naturalgasesreachesa maximumat depthsbeyond3000m ( s e eb e l o wC h a p .I V . 4 . 3 . 2 1 . The existence of H:S togetherwith SOj--containing rocksat greatdepthand high temperatures often resultsin formationof free sulfur.Freesulfurand H2S togethermayform polysulfides whichin turn destroymethaneandgenerateH:S ( W e l t e ,1 9 7 6 ) . c) Nitrogen may also be generatedfrom certaintypes of kerogenduring metagenesis. Lutz et al. (1975)have shownon cuttingsof Westphaliancoal measures, in a deepwell closeto the hugeGroningengasfield,that the nitrogen contentincreases markedlyduringmetagenesis andmay reach607aof the total gasgeneratedat that stage.Experimentalevolutionof variouscoalsseemsto confirmtheview thatthe nitrogenpresentin organicmatteris little affecteduntil an advancedstageof thermalevolutionis reachedand the activationenergies relatedto Nr generationrangefrom,t0 to 70 kcal mol-r (Klein and Jrintgen, 1972).Furthermore.becauseol the smallermoleculardiameterN2 (ca. 3.4 A) should migratemore easilythan CH., (ca. 3.8 A). if diffusionprocesses are involved.The preferentialmigrationof nitrogenmay alsoresultin an isotopic f r a c l i o n a t i o nu .i l h a n e n r i c h m e notf r : N . pyrolysisof oil shales; d) Hydrogenis oftenobservedduringhigh-temperature it may alsobe generated from kerogenin a geological situationwhereadvanced stagesof thermalmaturationare reached.Diffusionof hydrogenmoleculesis, however,relativelyeasy.thusloweringconcentrations.
I I
F L T
6.4 GasOriginatingfrom lnorganicSources
207
6.4 GasOriginatingfrom InorganicSources Somcconstitucnts of naturalgasmavbe producedbv purelvinorganrc processes. i. e.. withoutparticipationof the organicmatterof the sediments. 'Ihe a) atmosphericair dissolvedin shallowwatersmav be traprredwith i n t c r ' t i t i r lu J l ( r s i n t h c p o r c\ p a c co [ \ i ) u n g\ e d i m e n t sO. r \ g c n i s e . r n . u m chd] aerobicmicrobialactivity.Nitrogenand argon might be pieservedin certain cases.However.the argon/nitrogen ratio is frequcntlydiffcrentfrom the ratio in atmospheric air ( 1.2 x 10 :). suggcsting thatothersources of nitrogen and argon are probablvmoreimportant. b) Gasoriginatintfrom magmaticrocks.suchasyolcanicandgeothermal gascs m a l b e a s o u r c eo f C O . , H 1a n ds m a l l ear m o u n tos f C H , . H . S .N . . H e a n cA l r. ln theCaucasusarea.sokolov(197.1)reporte.lcurbondioxideioncentrationsinfrcc and dissolvcdgasesrcaching90 to 99ta. In somcgasaccumulations. CO, is a ntalor conrponentancl is probablvof magmaticorigin. This is the case.for e x a m p l ei .n c c r t a i na r e a so f t h e R o c k yM o u n t a i n(su p t o 9 8 7 rC C ) i, n C o l o r a d o . N e w M e x i c oa n dW . ' l ' e x a s )t.h e P a n u c t r - E h uanroe ro f M c x i c o( 9 6 t ; C O r )a n d p a r to f t h c P a n n o n i abna s i ni n C e n t r aE l u r o p e( M i h a if i c l d9 - 5 CCi O . ) .S o k o l o v (197.1) interpretedtheseoccurrcnces asrcsultingfrom magmatism with igneous intrusionsand/or volcanicactivity.Fluid inclusionsobseryedin magmarrcor metamorphic rocksalsocontainmcthaneandotherIighthvtlrocarbons probablv generatcdbv inorganicsvnthesis. c) Radioactivitfis likelyto be the sourcefor heliumandmostargon.Heliumis producedby disintegrationof the radioisotopes of the uraniumand thorium f a m i l i e s}.5 - U .] n U a n d: r : T h .r e s p e c r i r c l A r .r g o nr e s u l rfsr o md i s i n t c g r a t i oonf '"K. Natural a/c,I5U..rITh potassium uraniumis composcdof 99.3c/clNlJ arul,0.J amountsto 10(l% naturalthorium.andr"K amountsto 0. 12% naturalpotassium. With regardto the averagecontentof naturaluranium.thoriumandpotassium in the earth'scrust.and to the respective half-lifeof theseradio-isotopes. onc can evaluatethe numberof disintegration pcr year in I g of avcragerock (Table IL6.3). whichis directll,rclatedto the amountof gasesgeneratcd. Therefore,with increasingageof the rock. thJcumrilativeamountof gases generatedby radioactivedecayprocesscs increases. This age-dependence has beeneffectivelvobserved.andis shownfbr heliumin FigureI I.6.6basedon data f r m l t l T l ) s r \l j e l d , .I t i sc l c a rt h r r r h cd i . t r i h u t i o L n , lr h i h e l i u mc o n c e n t r a l i (i rnn gasfieldschanges with the agcof the rcservoirrock.The modeof thedistribution in Paleozoic gasfieldscxceeds bv morethantwo ordersof magnitudethe modein Tcrtiarvgasfields. T a b l eI I . 6 . 3 . N u m b eor f d i s i n tegration g I of average rockper miilionvears Potassium Uranium Thorium
29,1x 10'r 1 . , 1 x2 1 0 r l 1 . 5 6x l 0 ' l
Formationof Gas
208
l - i E .l l . h . h . D i s t r i b u t r oonI t h e h e l i u m content in gases from Paleozoic. (TisMesozoic.and Tertiaryreservoirs sot. 1969)
Mesozoic N=]83
Paleozoic N'609
to_5
ro_2
1o''
fle iuln conlenlrn gos
Basins: 6.5 Occurrence and Compositionof Gasin Sedimentary Exampleof WesternEurope moredifficultto Two considerations makethe distributionof gasaccumulations ma)'derivefrom explainthanthedistributionof oil fields.Firstly,gasconstituents of generation differentsources(organic.inorganic)throughdiffcrentprocesses (biogenic,thermogenic,or evenradioactive).Furthermore.thcy migrrte more column.In this connectionit hasto bc easilythan oils throughthe sedimentary becauseof their small molecular rememberedthat lor gaseousconstituents. mayplal'a diameters andtheirrelativelvhighwatersolubility,diffusionprocesses of of gasanddcstruction specialrolewith respectto migrationandalsodispersion the existingfields.Unfortunatelytherearenot yet enoughdataavailableto assess upon the distributionand importanceand influenceof diffusion processes given the composiconcerning occurrence of gases.In generalsomerulescanbe ofthe field, geological situation fractionin relationto the tion ofthe hydrocarbon
6.5Occurrence andComposition of Gasin Sedimentary Basins
Z0g
but the distributionof the non-hvdrocarbon constituents mav changefrom one h a 5 r nl o t n o l h e r . Vrrious basinsof WcsternEurope,ralgingin agefrom Carboniferous to Mio_ Pliocene.offer a wide distributionof naturalgascompositionandorigin.Tables IL.6.,1ro ll.6.h sho\ rhe compositionand origin ofgas accumulations in the pnncrp t grs provrnccs ()fWe\ternEurope:theyareplottedin FiguresIl.6.7and IL6.8. The variousgasfieldscan be groupedoi follo*.. The alpine realnt (Table II.6.4: Schoell 1977. l9g0: Colombo. 1966: - .a) Mattavellict al.. 1983)comprisesmainlvTertiary and pleistocenesediments. wnereDlogenrc gaswasgenerated duringearlvdiagenesis andpreserved in the po Basin (mainly Pliocene)and the Easiern t4ola"sse basin of South Germany (Oligoceneand Miocene). Methane amountsto 9j _99c/a of the gas. higher hydrocarbons are almosrabsent.The high C,/XC"ratio and the low;rrcr a;e in iureementwith the generaischeme.In the po Basin.the immaturecharacter of the l-ateTcrtiarvbedsin attestedby low vitrinitereflectances (( 0.57aR. at 4000 to 60(X)m) and resultsfrom a low geothermalgradientand i shortduration of burial. In older becisol earlvTertiarl'and Mesozoicage,thermogenicgashasbeen qenerated duringcatagenesis in the WesternMolassebasinandin Malossa,Italy, ttheredepthis bevond5(X)0m. The proportionof ethaneantl higherhydrocaibonsis quitc variablc.dependingon the local degreeof catagenesis, whereas Table 11.6.,1.Compositionand origin of the maln gasfieldsin the alpineregionsof WesternEurope Country
N. Italy
Basinor field
Po Basin
S. Germany E. Molasse
N. Italy
W. Molasse Malossa
Pliocene lAg. Reser-f (Pleisrocene.OligoEocene Mesozoicand Triassic rorr Miocene) Miocene Tertiary I lDepth (m) 12003000 900_2000 1800-3000 800_2000 5400 Ct (.'/') > C: (.'/i) c r/-IC'
91 99.1 97.599.3 88 97 ,18-97 0.04-{.3 0.05_0.5 1.5 8 221 0.970-0.999> 0.99 0.900.9ti 0.7-0.9
co:.(/.)
< 0.4 0.1-7
0 . 34 0 . 11 0
0.24 1.5-1tl
\:(?)
< t].-5
t 'Cr(?,)
-. 76to-55 -72to,60 _60to_50 _47to_35
79 19.8
0.ri u.4 0.7 36.2
Origin Biogenic Biogenic Mixed Thermogenic Thermogenic \tageof evolution Diagenesis DiagenesisDiagenesisCatagenesis Catagenesis + Catagenesis
I
Formationof Gas
2t t)
sourcerocks derivedfromJurassic andoriginofsomegasfields Tablc1L6.5. Composition o f W . a n dN . W . E u r o p e Countrvor province
W. Cermany
North Sea
Basinof field
N.W Germany Oil-AssociatedFrigg gasfields
1850
E. Cretaceous L. Jurassic 3200
95 4 0.96
69 4.9 {t.93
i0.8
9.6
Eocene
Jurassic lAsc E. Cretaceous K e \ e r r o tIr (Depth (m) 800 2000 Ct ('/r,\
> C: (."/'\ c1/tc" cor (%) N: (%) HrS (c/.)
65 91 8-30 0.7-0.96
5+84 1338 0 . 5 80 . 8 6 l
< 1
Lacq
15.4 5l to -45
-.11.2
, t L ' C ,( ' . , )
- 5 , 1 1 o- . l l
Origin Stageof evolution
Thermogenic Thermogenic Thermogenic Thermogenic Late CataCatagenesis Catagenesis Late Catagenesis genesis
43.3
Cr/ICn
o ;o
CATIGEIIE$IS
^'""'-{l Il{iiiftil:ffiu*;^ figltil Fig. IL6.7. Relative abundanceand isottlpic conrptlsitionof methanc in gasesfrom W' Eurooe. comoared rvith other locationsin North America and Sibcria Stratigraphicages given denote the agc of the rescrvoit rock. (Data from Boigk et al . 19761Stahl. l97ii: S c h o e l l .1 9 8 U : M e n e n d e z1. 9 7 3 iH d r i t i e re t a l . . 1 9 7 9 )
6.5 Occurrence anclCompositionof Gasin Scdimentary Basins
ll l
TableIL6.6. Compositionand originof somegasfieldsderivedfrom Westphalian sourcerocks(coalmeasures) of N.W. Europe B a s i no r t i e l d
Groningen
N.W. Germanv
Permian CarboniferousPermian{Ag. Rererroir ( R o t l i e g e n) d E. Triassic J t D e p t h( m ) 2 4 5 n 2000-4000 1700 2500 > C: (%) c1/tc"
81 3 0.96
82 97 0 . 56 0.9+0.996
cor (7 )
I
t-t0 I 10 -30to -22
9(%)
N: (%) ,t'tc, (2.)
1,1. ir -36.6
C)riein Stageof evolution
i''[,
-70 -il| |tr--r..''-_
20 95 0.94-0.996 0.5-7 180
-32ro- 19
Thermogenic Late catagenesis to metilgenesis
.30
a
o
UJ
z
uJ
o
uJ
= llf .6.rm||t.0thotoic
Fig.II.6.8. Originandisotopiccom, position of the principal gasesof WesternEurope (Tissot and Bessereau.1982)
d ' ' C , i s t v p i c ao l f t h a t c l o l u t i o ns t a g eA . n i n t e r m c d i a tsei t u i l t i o n o c c u r sr n t h c E o c c n eh e c l o s f t h e E i i s t c r nM o l a s s b e a s i na n di n s o m eM i o c c n ef i e l d so f t h ep o Basin.rrhcre a mixtureof drv gasgcncrateddurin-udiagenesis and *,et -sasfrom e u r l vc a t a g e n e sriess u l t si n a c e r t a i np r o p o r t i o n o f h i g h e rh t ' c l r o c a r b o nasn.da , ) ' C r r n g i n sf r , ) m- h { l l , ) - 5 U 7 , . b) The-Juissicsourceberls,whicharethe majorsourcefor crudeoil in Western Europe(particularlvin the North Sea),havealsogenerateci largeamountsofgas in the North Sca,the AquitaineBasin.andsmallerbut sizableamountsin North G e r m a n v( T a b l cI 1 . 6 . - 5L1e T r a n , 1 9 7 2M : e n e n d e z1. 9 7 3S; r a h l .l 9 7 g :H 6 r i t i e r . 1 9 7 9S; c h o e l l1. 9 8 0 ) . ln the North Sea.the oil-associated gasis accumulated in reservoirsranging from Jurassicto Paleocene age. It has beengeneratedduringcatagenesis: thc
212
Formationof Gas
is important(Cr/.:C": 0 6 to 0 9) and proportionof Cr andhigher h,vdrocarbons -:10%.. -50 The Frigggasfield.wherean import:rnt to of ihe d'rC, is in the range (10 m) probabll'originatcdcluringthe ( oil ring a thin m)ovcrlies gascolumn 170 sourcebedsu'ercburied when thc Jurassic of Mio-Pliocenc. stagc latecatagenesis (C1/IC"= 0.96) The m. It containslessCr andhighcrhydrocarbons to 4000-4000 from 1 5 stageof evolutionof the sourcerock is attestedby vitrinitereflectivities t o 1 . 8 7 e( H € r i t i e re t a l . . 1 9 7 9 ) . A comparablesituationoccursin the Aquitainebasin.wherethe l-acqand and occur presentlvat Meillon gasfieldswere formed during late catagenesis ratio (0.93)and The C]'/IC. respcctively. m depth. 320tH500m and,1000-5000 a n d t h c s t a g eo f c v o l u t i o n ( M e n e n d e z 1 . 9 7 3 ) . ( 4 4 % . ) a r e c o m p a r a b l e t h ed r r C r ran€{c of I to 2 % at in the vitrinite rctlectivities is b! beds attested of the source prescnt.HrS are also carbon dioxide sulfidc and (Robert, 1980). Hydrogen Lacq may resultfrom crackingof both kcrogenand crudeoil previouslvgenerated (manycrudeoils in the Aquitainebasincontaina significantamountof sulfur): or reductionof hydrocarhowever,destructionof methanedue to polysulfides. asa processof H,S generation.The bonsby evaporitecouldalsobe considered lo CO:generation l a t t e rr c a c l i o nw o u l da l s oc o n t r i h u l e dark shalcs.are the and the associatcd coal measures, c) The Westphalian oldestandmajorsourcefor gasin N.W. Europe:southernpart of thc North Sea, Netherlandsand northernCerman.v(Table I1.6.6: Hedemann.1963;Patijn. 196,1a b .; S t a h l .l 9 6 t t :L u t zc t a l . . 1 9 7 5B: o i g kc t a l . . 1 9 7 6S: t a h l .1 9 7 8S: c h o c l l . 'I'he of reservcs importance arc in orderof dccreasing mainreservoirs 1980). Permian (Rotliegend).Early Triassic.and Late Carbonifcrous.The major of a widebeltof Permiansalt(Zcchstein) featureof gastrappingis theoccurrencc s e a l . p r o v i d i n ga n e x c e l l e n t Therewcre two ma.jorphasesof burial:the first onc endedin carly Mesozoic time (Triassicto Earlv Jurassic).the sccondone rcachedits muximumin Duringthis lastburial.thc Cenozoic(3000to 70(X)m on top of Carboniferous). to metilgcnesls. reachedthe stageof latecittitgencsls coalmeasures Westphalian et al Teichmtillcr dcpcndingon thcir gcographicand stratigraphiclocation. beds rangc in the top Carboniferous ( 1979)haveshownthat vitriniteretlectancc from 1.0to 2.61t. Thc bulk of gasnow fountl in thesefieldsis thoughtto have duringthe lattcrphaseof burial.It cannotbe excludcd-horvevcr. beengenerated arc high that someof the gasis cvcn notvbeinggcneratedwhcretemperatures and organic where thc coal and cncrgies necessary activation enoughto rcach yet too mature. matterin shalesis not of methane.as C'/fCn ranges Gascomposittonshowsa strongpredominance jC' -36 Thermalcracking to - 19%". from 0.94to 0.9967..whereasdr rangesfrom is probabli shalcs dark of coal and coaly kerogenprescntin the associated whercC,t vicinity of Groningen' in the for thc high valuesof drrCl: responsible value tln top of retlectance gas only 36.6%.. is 0.96ancldrrC'of methane is .:-C,, and Ebstorf. Wtistrow (late in catagenesis): is only 1.0to 1.7% Carboniferous 2 1 % . . r c t lectance d " C , r e a c h e s ( C r / ; C , , : 0 . 9 9 ) a n d d r y w h e r eg a si s a l m o s t 1976; Teichmtiller (Boigk et al (metagenesis) ' of 2.5% valuesare in the range c t a l . .1 9 7 9 ) .
6.6Distribution in Sedimentary Basins of Gases
213
Otherconstituents of gasare 1 to 107cCOzandN.. whichis foundin variable anountsfrom I to 90%. In Late Carboniferous reservoirs. wheretheextension of verticalmigrationis limited.the highestvalues(10% N.) seemro be associated with the stageof metagenesis. ln Permianreservoirs.the highestliguresare observedin the Wristrowfield,with a varvingcontentof N3reaching80c/r.andin the Germanpart of the North Seawith a maximum907cN] content.Boigket al. (1976)have interpretedthis fact as an cnrichmentof N, due to extensive migration.However,theremay be othersourcesfor nitrogengas.
6.6 Distributionof Gasesin Sedimentary Basins The compositionof hydrocarbongasesin WesternEurope,and alsothe carbon isotoperatio varv in generalagreement with the principlesexpressed in Sections 6 . 2 a n d6 . 3 .a n da l s oi n T a b l eI L 6 . l . F r o mt h e s ee x a m p l e si t, a p p e a rtsh a tt h e distributionof quitc a numberof hydrocarbongases.includingtheir isotopic composition.can be reasonablyinterpretedin termsof stagesof maturation: cliagcnesis. catagenesis. (Fig. IL6.7). The distributionof nonmctagenesis hvdrocarbon gases is morecomplex.astheoriginmavbe differentfrom onebasin to another.and furthermoreit can alsobe affectedby migration.For instance, Hitchon(1963)described in the Albertabasina succession of gaszoneswith high concentrations of N.. CO.. and H]S. respectively, from the shallowaccumulations of the easternflank to the deeperaccumulations close to the Rocky Mountains.ln WesternEuropethe situationis comparableas far as HlS and accumulationdepthsare concerned.N,. however.is found preferentiallyin accumulations derivedfrom advancedmaturationstases.and moreoverfrom importantvertical migration.COl is found ar relativelyshallowdepth (late diagenesis to earlvcatagenesis) in SouthGermany,but alsoin deepaccumulations derivedlrom late catagenesis (Lacq) or even metagenesis (North Germany).Thusthe interpretationof the distributionof nonhydrocarbons hasto be madein the frameof the regionalgeological conditions. A specialcluestionis the possibleexistenceof a lower limit - a floor - to cxplorationfor hvdrocarbongasin sedimentary basins.The deepestgasfields prescntlvknown rangefrom 7 to 8 km approximatcly.Beyondthesedepths. tempcratures may rangefrom ca. 160'to 350'C in sedimentarv basins.At this point.mostsourcebedshavereachedan advanced stageof metagenesis andthere is little chancefor additional methanegeneration,except in some young scdiments wherefastburialis associated with a low geothermal gradient.Thus,in ccneral.the existinefields at such depthswould have been generatedin a shallowersituation.and subsequently buried at greaterdepths.This scheme rcquiresa goodscalanda smoothstructuralhistoryto ensurephysicalpreservation of the accumulation. Thermalstabilitvof mcthane.at thesetemperatures and evenhigher(Hunt. 1975).is suchthat presentand foreseeable drilling depthsdo not reachzones $'hcre methanecan be dcstrovedbecauseof temperature.Destructionof methane.however.is possibleby chemicalreactionsinvolvingfree sulfur or
214
Formationof Gas
sulfatesat elevatedtemperatureand resultingin hydrogensulfideand carbon dioxidegeneration.This may particularlyhappenin carbonateor carbonateevaporiteserieswheresulfurhasnot beentrappedby iron to form iron sulfides. Finally.theremaybe anotherlimitationdueto the lossof reservoircharacterisof tics. especiallvporosity.At great depth. dissolutionand recrvstallization mineralsunder high pressureconditionsbecomeimportantwhen approaching metamorphism andtheycanobliteratethe existingporosity.Thereare.however. situationswherea hydrocarbonsaturationwasreportedto preventcementation in the poreswherehydrocarbons are present.
Summaryand Conclusion There ist a wide variety of natural gasoccurrences,whosecompositionand modesof formation may differ considerably.Characterizationof natural gaseshas to rely on a small number of parameten, as there are not many constituentsin gases.Prominent features are the ratio of methane to total hydrocarbonC1D Cn and the isotopic composition:carbon and hydrogenisotopes. Biogenicmethanewich is only generatedat low temperaturelevels(below75"C) can be recognizedby very low isotopicratios (6 "C1 from -9AV- to - 55%").Pure biogenic gasdoesnot containhigherhydrocarbonsin appreciableproportion.Biogenicmethane is of economicimportancein certainregionsof the world (e.9,, WesternSibeia, Canada). There is a continuousgenerationof wet gasparallel to oil generation.Beyond the main phase of oil generation, due to increasingtemperature, cracking of oil and kerogenbecomesa predominantprocess.Therefore more and more wet and subsequently dry gasis being generated. The thermal stability of methaneis suchthat a lower limit - or gasfloor - is not determindedby high temperaturesas such. but by the depletionof hydrogenin the sourcematerial. Foreseeabledrilling depths,therefore,do not reachthe stagewhere methanebecomesthermallyunstable.However,methanecan be destroyedchemically. for instanceby convenion to H2Sin the presenceof sulfur. (e.g., CO2,H2S)ofnaturalgases mayhaveaninorganic Nonhydrocarbon constituents o sin.
Chapter7 Formationof Petroleum in Relationto Geological Processes. Timinsof Oil andGasGeneration
7.1 GeneralSchemeof PetroleumFormation The historyof petroleumformationis summarizedas a functionof increasing burialof the sourcerock in FigureII.7.l. which represents the abundance and compositionof the hvdrocarbons generated.and in FigureII.7.2. whichshows the correlativeevolutionof kerogens.The depth scalerepresented in Figure IL7.1 is basedon examplesof Mesozoicand Paleozoicsourcerocks.It is only approximate andmayvaryaccording to the natureof theoriginalorganicmatter,
-
--> Hydrocarbons generated
;
E E
;;
E Fig.IL7.1. Generalscheme ofhydrocarbon formationasafunctionofburialofthesource :ock.The evolutionof the hydrocarbon composition is shownin insetsfor threestructural :r pes.Depthsareonlyindicativeandcorrespond to an average on Mesozoic andpaleozoic :irurcerocks.Actualdepthsvaryaccording to the particulargeological conditions: typeof \erogen.burial historv,geothermalgradient.(Modifiedafter Tissotet al.. 1974).This :igurecanbe compared with otherdiagrams proposedby Sokolov(in: Kartsevet al.. l97l) ,nd Hedbere(1974)
Processes in Relationto Geological Formationof Petroleum
216
its burial history and the geothermalgradient.Based on the fundamental in the previouschapters.the generalschemeof oil andgas knowledgepresented asfollows. formationmay be summarized
7.1.1 Diagenesis a) YoungSediments arepresent.Theyareinherited At shallowdepth.smallamountsof hydrocarbons duringthe earlydiagenesis directlyof with minorchanges from livingorganisms, structuresresultingfrom their of the youngsediment.They havecharacteristic fossils.At thisstage.the asgeochemical biogenicorigin,and maybe considered compositionof the kerogenis controlledmainlyby the initial input of organic matterandby the natureandextentof microbialactivityin theuppersedimentary layersof sedimenl. generated at thatstageis methane.ln spccialcases, The onlynewhydrocarbon microbialactivitymay resultin abundantmethanegeneration(biogenicgas).
b) ImmatureStage occurs,asboth time anddepthspan,little transformation Duringan appreciable conditionsand hydrocarbonsand kerogenare metastableunder near-surface to be need a sufficientincreasein temperatureand time for rearrangements initiated. to a sufficientlevel,heteroatomic haveincreased Whendepthandtemperature broken.Oxygeneliminationfrom kerogenis bondsin kerogenareprogressively particularlyimportantduringthisphaseand resultsin CO2andHzO formation. -10\ ''- 0r lLROI,FN F\01 Po,\Ll0A.PPOD
E c o 2 , H 280i 0 c t Nc c N 4 E OL I jF-e RESI0UAL tulATTER 0R6ANlC * lnopolenl c fl]ro orqcs)
-AIoM|C0/C+
Fig.11.7.2.Generalscbemeof kerogen evolutionpresentedon van Krevelen's evolution stadiagram. The successive gesare indicatedand the principalproduringthat time. Resrductsgenerated dual organicmatterdoesnot exhibitan evolutionpath. (Modifiedafter Tissot. 1973)
7.1General Scheme of Petroleum Formation
2lj
The firsl petroleumproductsliberatedby this transformationincludemostlv heteroatomic (N, S, O)-compounds of highmolecularweightparticularlyasphaltenesandresins.A certainamountofgasmightalsobegenerated, especiallyfrom organicmatterof type IIL
7.1.2 Catagenesis a ) P r i n c i p aSl t a g eo f O i l F o r m a t i o n As temperaturecontinuesto increase.more and more bondsare broken.e. g., esterand someC- C bonds.Hydrocarbonmolecules, and particularlyaliphatic chains.are producedfrom kerogenandfrom the previouslvgeneratedN. S. C)compounds. Someof the hvdrocarbons released areCrrto Cr0biogenicmolecules comparableto the geochemical fossilswhich were formerlyentrappedin the kerogenmatrixor linkedby variousbondingsuchasester,ether.etc.However. mostof the new hydrocarbons generated duringthe mainzoneof oil generation havea mediumto low molecularweight.Thcl' haveno characteristic srructureor specificdistribution.contraryto the geochemical fossilswhichare progressively dilutedb),thesenew hydrocarbons. This is the principalstageof oil formation,as describedby Vassoevich et al. (1969).However.liquidoil generation is accompanied by formationof significant amountsof gas.
b) Stageof Condensate and Wet Gas Formationby Cracking As burial and temperaturecontinueto increase,breakingof carbon carbon bonds occursmore and more frequently,and affectsboth the sourcerock hydrocarbons alreadyformed and the remainingkerogen.Light hydrocarbons are generated throughthiscrackingandtheir proportionincreases rapidlyin the sourcerock hydrocarbons and petroleum.Due to the kineticsof formationand the advancedstructureof kerogen,methanebecomesquickly the dominating comooundliberated. The overall transformationoccurringduring catagenesis is equivalentto a processof disproportionation. On the one hand, hydrocarbons of increasing hydrogencontentare generated.The averageatomicratio H/C is 1.5 to 2.0 in curdeoil and is 4.0 in pure methane.On the other hand,the residualkerogen becomes depletedin hydrogenwith an atomicH/C ratioof about0.5by the endof the stageof catagenesis.
7.1,3 Metagenesis: Dry Gas Zone After mostlabilematerialhasbeeneliminatedthroughcatagenesis, a structural reorganization occursin kerogen,with the development of a higherdegreeof ordering. However. in this stage (metagenesis) no significantamounts of
218
Processes in Relationto Geological Formationof Petroleum
hydrocarbonsare generatedfrom kerogenexceptfor some methane'Large andof amountsof methanemayresultfrom crackingofsourcerockhydrocarbons reservoiredliquid petroleum. The floor oi the possibleexistenceof methanein the subsurfaceis contraryto not controlledby its thermalstability(Hunt' 1975).The all other hydrocarbons (up to about550'C)is suchthat stabilityof methane,evenat highertemperatures zoneswheremethanecanbe not reach do presentand foreseeabledrilling depths chemicanbe destroyed however' Methane. of temperature. iestroyedbecause II 6)' Chapter (see above or sulfate presence of sulfur callydueto the of methane, occurrence ln facttheremaybe an economiclimit for commercial of porosity' due to lack especially characteristics, of reservoir dueto desradation
Ratio 7.2 GeneticPotentialandTransformation of the main stepsof evolutionoutlinedabove(Sect 7 1) is a Tbe succession to the thresholdsand the common fact, but the temperaturecorresponding of theorganicmatterand nature dependson the amountof petroleumgenerated alsoon the temperaturehistory,namelytemperatureversustime relationship is Iessclear. The influenceof pressureand potentialcatalysts influence of the kerogen the respective between In an effort to distinguish andmakeuseof two we can define intensity, compositionandof the catagenesis ratio. the transformation potential and genetic terms:the a) The geneticpotential of a given formation representsthe amount of to ' if it is subjected petroleum- oil andgas- thatthe kerogenis ableto generate potential This of time. interval a sufficient during temperature adequate in ofkerogen,whichin turn arerelatedto the dependion the natureandabundance originalorganicinputat the time of sedimentdeposition,andto the conditionsof of the organicmatter in the young microbialdegraditionand rearrangement serliment.Thus, the geneticpotential dependson externalfactors' such as etc, as topography,climate,oceanology,existenceof biologicalassociations potential of a poinaedout by Debyser(1969).It may be notedthat the genetic (or by the by the type of kerogen characterized iormationmay be conveniently quantitative A evolutionpath) and the abundanceof kerogen. corresponding evaluaiionoi the genetii potentialcan be made on the basisof a standard in Part V (Espitali6et al.,1977). pyrolysistechnique,whichis discussed different Thi geneticpbtentialof a petroleumsoutcerock is not essentially there is no pyrolysis' as from th-etotal oil and gasyield of an oil shaleupon oil shale and an rock source fundamentaldifferencebetweena shallowimmature further - providedthe organicrichness is sufficient.Thispointwill be discussed i n C h a p t e IrI . 9 . b) Thetransformationratio is lhe ratio of the petroleum(oil plus gas)actually formed by the kerogento the geneticpotential.i.e ' to the total amountof the petroleumthat the kerogenis capableof generating.The ratio measures
7.3Natureof theOrganic Matter.GasProvinces Versus Oil Provinces
219
extentto whichthe geneticpotentialhasbeeneffectivelyrealized.An evaluation of the transformationratio is providedby the standardtechniqueof pyrolysis. describedin Part V. togetherwith the geneticpotential.The ratio can alsobe approximated.down through the main stageof oil generation(thus before reachingthe zoneof wet gasformation).by usingthe bitumenratio. The transformation ratiodependson the natureof the organicmaterialandon geologicalhistory(mostlythe temperatureversustime history). the subsequent Therefore,the transformation ratio dependsmostlyon internalfactors:geothermal gradient.subsidence, and teclonics.The temperaturecorresponding to the beginningoftheprincipalstage ofoil formationis alsodependent on thenatureof kerogen,but the beginningof the stageofgasformationis lessdependent on this factor,in accordwith the kineticsof crackinsreactions.
7.3 Natureof the OrganicMatter. GasProvinces Versus Oil Provinces 1. The natureof the originalorganicmaterialis an importantfactorthat cannot be disregardcd. LarskayaandZhabrev(1964)pointedout that the vrriou5types ol organicmatter involvedin their Azov-Kubansurveyreacteddifferentlyto burial. When gatheringdata from many sedimentarybasins,it is clear that kerogenmadeof marineor limnic autochthonous organicmatter,includingthe microbialbiomassliving in sediment(type I or II, rich in aliphaticgroups), generatesabundantbitumen: 180 mg/g of organiccarbonin the paris basin. 200mg/gin the Uinta basin.On the contrary.kerogencomprising a largeamount of continentalplant debris(Type III. rich in aromaticgroupsand oxl,genated functions)generateda smalleramountof bitumen:100mg/g of total organic carbonin the Doualabasin. An illustrationof the importanceof the compositionof the organicmatteris shownin FigureI L 7.3,wheretwo shallowkerogens. whichhavenot yet reached the stagcof catagenesis, are compared:one belongsto type ll, i.e.. a "high" evolutionpath in the van Krevelendiagramlthe other.from a "low" evolution path.belongsto typeIIL Upon pyrolysisunderinert atmosphere, the firstone is more than twice as productiveas the second.Furthermore.the relativcabundanceof aliphaticstructuresand carbonyior other groups.as illustratedby IR spectra.resultsin a differentcompositionof the degradationproducts.For example.COr + H:O amountsto about50Eaof theproductsyieldedby type-lll kerogen.as comparedwith onlv 25c/afor typeIl. Thus,the geneticpotentialof thehvdrogen-rich typeII is aboutthreetimesthepotentialof the hydrogen-poor. 2. The frequentlydiscussed questionof oil versusgasprovincesis linkedto the geneticcharacteristics of the organicmatler and alsoto the degreeof thermal maluratlon. a) In general, the organic matter incorporatedin marine or lacustnne sediments is derivedfrom naturalassociations of flora andfauna.andit hasbeen mixed and homogenizedby naturalprocesses (in riversand of sedimentation
Processes Formationof Petroleumin Relation to Geological
220
Type tr PorisBosin L.Toorcion
-
Itr Type DouoloBosin U.Cretoceous go0 13OO1600r4O0r?00 1000
W o v en U m b e cr m - l
type ll' of the petroleumpotentialof two different.kerogens: -o"""r" Fig. II.7.3. Comparison show samples shallow e;in Lelr: Ii spectrarecardedon il'p.-iii. ;;"ti.';;r;;' carbonyl and aromatrc more III type that tvrreIl containsmore uttpnotttmltttiol' and
"f,:HlT#itll':*"*fl :l iffi'"#Ji,i ffi f i::iF*:;:t;3;lf iff.hffi rsT'r) et al ;Fo- ' H'or.'reol'n"io*nrl i\\or ;ttil;i;;;i;;
to the situationcorresponds marinecurrents)and by microbialactivity This ll)' I or lype (kerogen diagram ilhief, .uolution paths"on the van Krevelen "in.o-ni..t.ii" and evolutionof thistypeof organicmatrerduringdiagenesis the thermal on depending uotrtoir and sasseneration' .r,;;#;i;;-;;*rt-in "drv"' i e ' gasis normallv Luriedsediments.inr"e*ample' ffi,i;:;;t;il;; are present where the.sediments ;l*;.t ihe singlehydrocarbon il',il".'i '.wet.'gas,co,ntaining ethane. methane. uuri.J r,')one or severalkilometers, or not wrih oil fields At great depth' gas is found associated ;;;;;;-;.. "dry"' asmethaneis largelypredomlnanr' lccimulationsare again lowlandsor basinsborderingcontrnents tj On the otire,hind' intracontinental matte^r.providedm,o-stlyby higher organic .eleive abundant ;;;;;. ,; humic irimarly.of lignin and terrestrial plants.This organlcmalter consists ' etc resins with- varying subordinateproportionsof waxes' i;;;i;"; in dispersed result ;lltiie organicandmineralcontents'it mav ;;;;;;t;;.;ih" "low evolutionpaths" the to correspond They ."ril""."'",", nr"ter or coal"beds. on the van Krevelendiagram(ti'peIII)'
7.3Natureof the OrganicMatter.GasProvinces VersusOil Provinces
221
The structureof suchorganicmatteris in essence an assemblage of aromatic cycleswith short carbon chains.For inslance,the structureof lignin can be thoughtof a polvmerof substances like conifervlalcohol,shownin FigureL,1.15. Subsequentdegradationproducesmainly light hydrocarbons(particularly methane.by crackingthe shortchains). Therefore,this type of organicmatter mostly generates"dry" gas,which containsCH.,. COz and N,, and is rather similar to gasesfrom coal mines, especially in advanced stagesof catagenesis or metagenesis. The gasfoundin the Netherlands andin the southwestern partof the North Seais thoughtto originate from this type of sourcei.e., coal and associatedcarbonaceous shalesof C a r b o n i f e r o uasg e( P a r i j n .l q b 4 ) . Besidesthe predominantlyaromaticstructurederivedfrom lignin and humic substances. there may be variousproportionsof lipid, chain-likeor cyclic. materialpossiblyinheritedfrom higherplants(waxes,resins),microbialbiomass (membranes. waxes)or evenalgalinput.Theselipidsare ableto generateliquid hydrocarbonsat depth. They are usuallysubordinate.but they may reacha significantproportionof the kerogenin certaindeltaicenvironments. In this case,sourcerockscontainingtype III kerogenand the associated coal are responsible for commercialoil and gasaccumulations, suchas Handil and Bekapaifields. in the Mahakam delta, Indonesia(Durand et al., 1919).A comparable situationmight be foundin someotherdeltaicenvironments. c) In a moregeneralway.it canbe saidthat someorganicmattersarelessable than othersto generateoil in commercialamounts,but any organicmattermay generategas,providedit is buriedto a sufficientdepthduringa longenoughtime interval. The relationbetweenoil andgasoccurrences from the sameorganicmaterial. dependingon the temperaturehistory,is illustratedby the comparisonof the s o u t h e a s t ear n ds o u t h w e s t eS r na h a r ian A l g e r i ai:. e ., t h el l l i z iB a i i na n dA h n e t Mouydir Basin,respectively. The Illizi Basinproduceslargeamountsof oil and associated gas from Silurian and Devoniansourcerocks. while the Ahnef Mouydir Basin producesonly methane.From geologicalreconstitutions. the maxrmumburialdepthto the top of Silurianrocksis at least1000m deepcrin the Ahnet-MouvdirBasin.and furthermorethe geothermalgradientis somewhat higher.The corresponding temperatures are more than 130.Cfor the AhnetMouydirBasinand 100'Cfor the Illizi Basin.If we assumean activationenerAy for crackingreactionsof approximately50 kcal mol 1, the reactionrate is multipliedby more than 100betweenthesetwo temperaturesi. Therefore,the crackingmay be negligiblein the one casc, and completedin the other. Furthermore,this interpretationis in agreementwith the observations on the carbonization of sooresandnollerr.
7 The classical rule of doublingof rate every 10"Cis acceptable for reactionswith an activationenergyin the rangeof 10-20kcal mol i. lt is not applicablefor cracking reactions. wherethe activationenergyis muchhigher.
222
Formationof Petroleumin Relation to GeolosicalProcesses
7 . 4Temperature,Time and Pressure Burial. and thustemperatureincreaseare obviouslyof primaryimportancefor hydrocarbongeneration.However,the exactrolesof temperature,time, and pressurehaveto be distinguished, althoughthey areto someextentinterdependent. Oil and gas are producedfrom the kerogenof the sourcerocksthrougha succession of chemicalreactions.Thesereactionsare governedby the usual kineticsof thechemicalreactions, andthishasbeenverifiedon examples takenin sedimentary basins(e.g., thc Parisbasin,Tissot,1969).Therefore,the transformation ratio dependson temperatureand time. This conclusionis intuitively understood, asoncwouldexpecta differentamountofpetroleumto begenerated from a sourcerock heatedto a giventemperature during1 ascomparedwith 100 million of vears. The reactiontimeis implicitlyaccounted for in thevariousindicesofcatagenesis (vitrinitereflectance, sporesandpollencoloration,etc.).because the changes of theseparametersalso obey kinetic laws. and thus theseindicesintegratethe effectsof temperatureand time. However,geologicaltime has seldombeen explicitlydiscussed. althoughit is of primaryimportance to knowthe timingof oil and gasgenerationand to compareit with the time of the formationof traps (depositionof imperviouscover.folding,faulting,etc.). The respectiveinfluenceof temperatureand time has been observedfrom laboratoryexperiments (Chap.11.5,TableII.5.5) and is confirmedby compariU.Devonian (350 MY)
L.Jurassic (180r!1Y )
[. Tertiary rvY 135 )
U.T€rtiary ( l 0t v YI
E
5
;
a
100
200 m g / g o r g a n ic a r b o n
Fig.IL7.,l. Compareddepthandtemperature of thebeginningof theprincipalzoneof oil formationin severalsourcerocksof differentases(Tissotet al.. 1975)
I
7.5Timingof Oil andGasceneration
223
sonof the actualevolutionof sourcerocksof differentagesin sedimenrarv oastns. T h e r r r n s f o r m a t i orna r i oo i r h e o r g a n i cm a e r i s p l o i r e di n F i g u r eI L 7 . + a s a functionof depth for severalsourcerocksof agesrangingfrom Devonianto Miocene.in placeswherethe geothermalgradientis broadlysimilar.It is clear that the oil-generation threshold,situatedat the top of the principalzoneof oil formation.varieswith the agesof thesourcerock:about-50.Cin the Silurianand Devonianof the easternSahara,60'C in the Lower Jurassicof the parisbasin. 70'C in the Upper Cretaceous and Lower Tertiaryof WestAfrica, l l-5.Cin the Mio-Pliocene of the Los Angelesbasin. From theseobservations it can be assumedthat thereis a time temDerature relationship.eachone of the two parameters beingableto compensate for the othcr. The kineticequationsshowthat the transformation ratio of the organic matteris influencedmoreby temperature thanby time:i e. , the influenceof time is linear. whilc that of tcmperatureis exponential.For this reason.very old organicsediments suchasthe Moscowlignitesof lowerCarboniferous age.buried to depths less than 200 m. have never reacheda more advancedstase of c o l l i f i c aito n ( K r r w e i l .l 9 5 o ) . The exactrole of pressureis difficultto elucidate.becausetemperatureand pressureare not independentfactors.both being linked to burial depth.The consideration of two neighboring areas,like theLos AngelesandVenturabasins, that comprisethe samesedimentary sequence. but havea differentgeothermal uradient.providessomeinformation.The thresholdof oil senerationis 2400m in the Los Angelesbasin,and3600m in the Venrurabasin.Both depthscorrespond to thesametemperature.115'C,despitetheverydifferentpressures (assumed to bc approximatelyproportionalto the respective depths).Therefore,the influenceof pressureis probablysubordinate,comparedto that of temperature. Experimental work hasconfirmedthis opinion(Chap.II.5.6.3).
".-5Timing of Oil and GasGeneration Determinationof the timing of oil and gasgenerationcanbe donein rwo ways, "oth usingthe graphof subsidence in the variouspartsof a sedimentary basin, : e.. the depthversustime curvein a certainnumberof selectedrepresentative ' c a il t , n s . a) lf a sufficientnumberof samples is availablefrom thesourcerockformation. :he bitumenratio or hydrocarbon ratio is plottedagainstmaximumburialdepth :.) constructthe curve of petroleum generation.The main thresholdsare :ctermined- top of theprincipalzoneof oil formation- top of thecrackingand :r\ tormationzone.Then theyare drawnon the variousgraphsof subsidence to 'rndthe time whenthe respective thresholds arecrossedby the depthversustime , rrvc. Oil (or gas)is assumed to be generated in significant amountsasfrom that : ne. h) A more generalwav hasbeenproposedby Tissot(1969)and Tissotet al. ,915). using a mathematicalmodel to simulatethe thermal degradationof
224
Formationof Petroleumin Relation to GeolosicalProcesses
kerogenand the formationof petroleum(oil and gas).The principlesof the simulationare discussed in ChapterV.4. The modelis calibratedfor a givenformationby usinglaboratoryexperiments on kerogenpyrolysis.Datafromextractionofcoresandcuttingssamples (bitumen ratiosandhydrocarbon ratios)arealsousedifthey areavailable,but theyarenot strictlynecessary. Then,the amountof oil andgasgenerated by the formationis computedasa functionof geological time for anylocationin the basinwherethe depth versustime curveand the geothermalgradient(or reflectancemeasurements)areprovided.Thus.a datingofthe phenomena possible with an becomes evaluationof the timc when oil generationbeganin significantamounts.the durationof thisperiod.andthe time of crackingandgasgeneration, if thisis the case. Thc resultsof the timing determination,by either method,can be checked againstgeologicalinformationon existingoil or gasfields(timeof sealing-cover deposition,time of folding.faulting,etc.). Examinationof the resultsobtainedin variousgeological situations showsthat oil generationis neveran abruptphenomenon. It resultsfrom the kineticsof the successive chemicalreactionsinvolvedandlastsan appreciable periodof time.A quickoil generation extendsover5 or 10millionyears,whereas a slowgeneration may coverover 100million yearsor more. Examplesof quickand earlygenerationof oil are foundin sedimentary areas wherethe rate of subsidence has beenvery high and the geothermalgradient abnormallyimportant in relation to the global tectonicphenomena.In the Pannonianbasin of Central Europe, Pliocenesourcerocks have produced commercialoil over a few millionyears.The thickness of the Pliocenebedsmay reachup to 3000m (a subsidence rate over 500 m per million years),and the geothermalgradientreachesin placesmorethan50"C km r. Otherexamplesof ratherquickoil generationare foundin Mio-Pliocene bedsof the circum-Pacific area,suchasIndonesia, Sakhalinor California;againhighratesofsubsidence are involved.andgeothermalactivityis generallyhigh.Similarsituationsmayoccur in rift openingareas,like the Red Sea-Gulfof Suezat presenttime, or thesouth Atlantic rift in Cretaceous time, Examplesof petroleumgenerationover a shortperiodof time (10-2-5 million years).but not necessarily early.arefoundin basins.troughsor grabenswherea thick sedimentationoccurredin a short period of time, burying previously deposited sediments of anyageto greatdepth.In the Ahnet-Mouydirbasinandin part ol the Illizi basin(Algeria),sourcerocksof variousages. the southwestern rangingfrom Silurianto Upper Devonian,were deeplyburied in lower Carboniferoustime andgenerated oil andgasalmostat the sametime.In manyparts of the world a thick sedimentation occurredin Cretaceous time. anddetermined burialandcatagenesis transformation of the sourcerockspreviouslydeposited in platform conditions.In westernCanada,both Devoniansourcerocks of the Leduc area and lower Cretaceoussourcerocks producedoil and gas in late Cretaceous and earll'Tertiarytime. Suchsituationsare usuallyassociated with normalgeothermalgradients. Generationof oil over a very long periodof time usuallyoccursin platform areaswhere subsidencerate was alwavsmoderate.This is the caseof the
I
7.6Compared Timingof Source RockDeposition andPetroleum Generation
225
Paleozoic sourcerocksol northernSahara(HassiMessaoud area),progressively buried in Mesozoicand Tertiary times at a rate ca. 25 m per million yeari. Jurassic sourcerocksof the Parisbasinwereprogressivelv buriedduringJurassic. Cretaceous andlowerTertiaryat variousratesfrom 5 to 15m per millionyears.In both of thcseexamplesoil generationextendedover ca. 100millionycars.
7.6 ComparisonBetweenthe Time of SourceRock Deposition andtheTimeof Petroleum Generation Besidesvariationsin the durationof the mainstageof oil generation. theelapsed time from sedimentation until the rock entersthe principalzoneof oil formation may varv considerablv from a few million yearsto morethan 300millionyears. It has been mcntionedthat many sourcerocksof southernAlberta in the westernCanadianbasin,whetherDevonianor Lower to Middle Cretaceous. producedmost of their petroleumin late Cretaceous and lower Tertiarytime. This meansthat the principalsrageof oil formationbeganca. 300million years after depositionof the Devoniansourcerocks and ca. 40 million yearsafter depositionof the Cretaceoussourcerocks from the ColoradoGroup. Other previouslymentionedexamples,Pliocenesourcerocks(pannonianbasinl.reach the mainstageof oil generationwithin a few millionyearsafterdeposition. An interestingexample.with geological controlof the timing,is providedbv the HassiMessaoudareain thc norrhernSaharabasin(Algeria;Fig. II.7.5). In this basintwo sedimentation cvclesoccurred.Paleozoicshalesand sandstones rangefrom Cambrianto Carboniferous, andtheyincludetheorolificsourcerocrs
w Haoud Berkaoui
'1000
I
E
t,n','y
ffi
Juass,"
HassiMessaoud
I
E E *
Rhourde elBaguel
Canb.o-0rdovicran Buserent t\tarnonconfofmrry
Fig. 11.7.5.Geologicalcross,section in the HassiMessaoudarea(Algeria)showingthe Paleozoicreservoir(Cambro-Ordovician) and sourcerock (Silurian).unconformablv ovcrlainbv Mesozoicseries(Pouletand Roucachd.1969)
226
Formationof Petroleumin Relation to GeologicalProcesses
U.PaleozoicMesozoic Ce -
:
o
a _
3
r000
Fig. 11.7.6.tsurialhistoryof the Silurian sourcerocksin the HassiMessaoud {ea (top) and hydrocarbonsgenerated (bottom). as a function of geological time (modifiedafterTissotet al.. 1975). Quantitiesof oil and gasproducedare computedby using the mathematical model.presented in Paft V
I
I I t ' " - " I I I
aooo
!
G=23.5oC/ lOOOm
E -
Gosqeneroted
o5 I
0ilgeneroied
of Lower Silurianage.The areawasgentlyfoldedin late Carboniferous time. thendeeplyeroded.In particular,the CambrianandOrdoyicianreservorrs were exposedto the atmosphereduring Permian.The secondsedimentation cvcle startedin Triassictime with a thick saltdeposit,followedbv 4000m of various sediments. The oil fieldsarelocateciiustbelowthe unconformitvin theCambroOrdoviciansandstoneand sealedby the salt cleposit.Shouli oil have been generated duringPaleozoic timeandtrappedin the anticlines, it wouldhavebeen weatheredduring all Permiantime, becausethe reservoirwas exposedto the atmosphere for severaltensof million years.Therefore,it may be hl,pothesized thatoilwasgenerated in boththe Paleozoic andthe Mesozoic-Tertiarv cvcles.but that the oil trappedduringthe first cvclewaslostby weathering,andonlv the oil trapped during the secondcycle makes up the presentaccumulations. An alternativehypothesis is that oil wasgenerated only duringMesozoicor Tertiary. oncethe thick saltdepositedprovidedan adequateseal. Consideration ofthe burialcurve,asa functionof time (Fig.II.7.6),showsthat thesourcerocksremainedat moderatedepth(lessthan1500m) duringPaleozoic: thenfoldinganderosionwerefollowedby a thicksedimentation duringMesozoic tlme, resulting in a far more important burial (ca. 4000 m). Use of the mathematical modelshowedthatonly a smallamountof oil wasgenerated during
7.6Compared Timingof Source RockDeposition andPetroleum Generation
22j
the first 300millionvears.The principalstageof oil generation wasreachedonly duringCretaceous. a lime whenthe saltcoverwasalreadyavailable.The results of the mathematical simulationcan be checkedagainsrgeologicaldata on the nearby Rhourdeel Bagueloil field. where the reservoiris also the CambroOrdoviciansandstone belowthe unconformity.The trap is providedthereby a horst. boundedwith north-southfaults, whoseage has been determinedby sedimcntation studies.The faultsoccurredin Lower Cretaceous time. and no structuraltrap of an)'kind wasavailablebeforefaulting.Therefore,the oil field hasnecessarilv beenformedafterLowerCretaceous. in completeagreement with t h e r e s u l t so f t h e m a t h e m a t i csailm u l l r i o nO . t h e ri n s t a n c eoif h t i f o r m a t i o no f petroleumhavebeen reportedby Hedberg(1964)in Venezuela.Canadaand Argentina:again.a sourcerock locatedbelowan unconformityyieldedoil only afterdepositionof a rescrvoirand/ora sealingformationabovetheunconformity. A morcgeneralquestionconcerns thetimeof formationof existingcommercial oil or gasaccumulations. The answerwould requircthe studyof the main oilproducingbasinsby usingthe methodsproposedhere.However.the evaluation of the timing of pctroleumgenerationhas been made for severalbasinsin Europc.Africa andNorth Americawhichareconsidered to be tvDicalof themain g e o t e c l o n si ci t u ailr , n s . In thisregard.a largeproportionof the presentlyexistingoil andgasfieldshas beenformed during the last 100million years.thus in Cretaceous or Tertiary time. and thev may amountto more than 907oof the petroleumreserves. This categorvobviouslyincludesoil and gas fields derivedfrom Cretaceousancl Tertiary sourcerocks: amongthem the importantdetritalseriesdepositedin major dcltaic systemsand along the continentalmarginsare of particular importance.In additionto this.the categoryincludesoil or gasfields,formedin the last 100 million vears from older sourcerocks and, particularly.from Paleozoic formationspreviouslvdepositedin platformbasins.As subsidence was often moderatein this type of basin. the sourcerockswere not buried to a sufficientdepthto reachtheprincipalstageofoil formationduringPaleozoic: two examplesof this situationhave alreadybeen mentioned,the Silurianof the northernSaharaandthe Devonianof theLeducarea.A subsequent deeperburial wasnecessary. andthis frequentlvoccurredin Cretaceous or Tertiarytime, asa part of the major phenomenaof globaltectonicswhichtook placesinceLower L.rctaceous. On thecontrary.Paleozoic sourcerocksdeposited in moremobileareas.where subsidence wasimportant,haveoften beeninvolvedin Paleozoic orogeniesand submittedto intensefoldingand erosion,whichcausedescapeor weatheringof petroleum.Thissituationconcerns mostof Paleozoic bedsfrom westernEurope. However,in certainplacesof the world. hydrocarbons definitelygeneratecJ in Paleozoic time havebeenpreserved. e. g.. in oil and gasfieldslocatedalongthe westernborderof the AppalachianandUral mountains.in the Amadeusbasinof Central Australia.and in gas fields of the Ahnet basin (westernSahara)in Alseria.
228
Formation of Petroleumin Relation to GeolosicalProcesses
SummaryandConclusion The succession of main stepsof evolution of organicmatter (diagenesis,catagenesis and metagenesis)is common to all types of sediment. However, the amount of hydrocarbons,their composition,and the depth ofoil and gasgenerationmay change. The most important parameters are the nature of the organic matter and the temperatureversustime relationship. The occu[ence of gas versusoil provincesdependson the nature of the organic matter and/or the thermal history. In particular, kerogenftom te[estrial plant debris (type I lI) generatescomparativelylessoil than the other typesI or II. However,it could providea goodsourcerock for hydrocarbongasat greaterdepths. The thresholdcorrespondingto the beginningof significantoil generationva eswith the geothermalgradient,the depth andthe durationof burial. The relatedtemperature rangesfrom 50'C in basinsof Paleozoicageto 115"Cin Mio-Pliocenesediments. The timing of oil generationmay alsovary. The p ncipalphaseof oil formationmay follow sourcerock depositionafter a delay rangingftom a few million to 300 million years.In particular,a largenumberof the presentoil fieldshavebeengeneratedduring the Cretaceousand Tertiary, regardlessof the ageof the sourcerock.
Chapter8 Coal and its Relationto Oil and Gas
8.1 GeneralAspectsof Coal Formation When discussing the formationand occurrence of petroleum.it is necessarv to includea brief reviewon relevantaspects ofcoal formation.Both petroleumand coaloriginatepredominantly from organisms of the plant kingdomand both are subjectedto the samegeologicalprocesses of bacterialaction,burial.compac_ tion,.andgeothermalheatingthat conslitutediagenesis and catagenesis. There are, however,also someessentialdifferencesbetweenthe modis of coal and petroleumformation.Basically,thesedifferences centeraroundthefactthatcoal is found at its site of depositionas a solid and relativelypure massiveorganic substance, whereaspetroleumis liquid and migratesreadilyfrom its plaie of origin into porousreservoirrocks. Kerogenis the main precursormaierialof petroleumcompounds.It is finely dispersedand intimitely mixed with the mineralmatrixin petroleumsourcebeds.Most coalsare remnantsof terrestrial higher plants. whereasthe kerogenof petroleumsourcebeds is generally dominatedby phytoplanktonand bacteria.Most acknowledgedpetroleum sourcebedsweredeposited in marineenvironments andmostcoaliformedunder nonmarineconditions. In the following discussionon coal, especiallyon generalaspectsof coal _ formarion,the InternationalHandbookof Coal petrogriphy(196j, 1g7),.1g75) hasbeenconsultedfrequentlv. _.Coal containsa variety oi plant tissuesin differentstatesof preseryation. Tissuesof distinctorigin are microscopically idenlifiableand can irequentlybe relatedto certainpartsof the plant, suchascuticles,woodystructurcs,spores, etc. Togetherwith particlesof lesscertainoriginthey are termedmacerals, and are.the petrographiccomponentsof coal. During and after depositionin sedimentary basins,plant remainsundergoa sequenieof physical,biochemical and chemicalchanges(diagenesis and catagenesis), which resultin a seriesof coals of increasingrank. The seriesbeginswith practicallyunalteredplant materialand peat, and continueswith increasingrank through brown ioal, bituminouscoal and finally to anthracite.peat is consideredto be the first memberin this seriesof coals. A distinctionis made betweenI umic coals andsapropeliccoals.Humic coals, typically.passthrougha peatstagewith accompanying processes of humification after accumulationat the site where the plants grew. The major organic componentof mosthumiccoalsis a lustrousdark brownto blackmaterial.visible to the nakedeveand mainlyderivedfrom the humificationof woodvtissues.In
230
Coalandits Relationto Oil and Gas
lower rank coals,this materialis representedby a group of maceralscalled huminite,and in bituminousand anthracitecoalsby a groupof maceralscalled vitrinite.Humic coalstypicallyare stratified. coalson the otherhandarenot stratifiedmacroscopically andhave Sapropelic Theyare formedfrom relativelyfine-grained organicmudsin a dull appearance. quiet.oxygen-deficient bodiesof shallowwatersuchasponds,lakesandlagoons. They normallydo not passthrougha peat-bogstage,but follow the diagenetic path of organic-richsediments.Like thesesediments,sapropeliccoalscontain (transported) varyingamountsof allochthonous organicandmineralmatter.The (local) algal remainsand varying organicfraction consistsof autochthonous amountsof degradationproductsfrom nearbyexistingpeat swampsor spores from more distantplants.Microscopically, sapropeliccoalscan be subdivided into bogheadand cannelcoals.Bogheadcoalscontainlargeramountsof algal remains.Cannel coalsare characterized by higherconcentrations of spores. There are many transitionsbetweenthe two basictypesof sapropeliccoal. further. Sapropelic coalsarerelativelyrareand.therelore.neednot be discussed Thereare, howeverobvioussimilaritiesbetweenhumicand sapropelic coalson one hand and sourcerock-typesedimentson the other and, therefore,the accumulation,diagenesis and catagenesis of coalsare pertinentto the understandingof petroleumsourcebeds.
8.2 The Formationof Peat 8.2.I GeologicalAspects Largeamountsof plantmaterialmustbe accumulated to facilitate andpreserved the formation of peat. The accumulationof large massesof organicmatter dependson the efficiencyof photosynthesis. High primaryproductivityon land becamepossibleonly after the adventof higher plants with self-supporting skeletons whichrepresent a highlevelofplant evolution.Plantcommunities with trees,shrubsandreed-typevegetation evolvedduringDevonian,andfor thefirst time allowed the productionof the necessary biomassfor the formation of peatbogs. andhencecoal.The florasthatproducepeatdeposits aregenerallyof a richcoastaltype.Inthe Paleozoic theyconsisted mainlyofa Pteridophytic florain lhe Carboniferous coalsof EuropeandNorth Americaanda Glossopteris florain the PermianGondwanacoals(Given, 1972).Late Mesozoicand Tertiarypeat swampssupportedan angiospermflora like that growing in peat-forming environments today. Besidesthe evolutionarystateof plants.thereis a numberof other importanr factorsin peatformation.Of primaryimportancearethe climateandthe tectonic conditionsof the area (Teichmiiller,1962).There is a direct relationbetween ratesof plantgrowthand climaticconditions.With increasing temperatureand humidity. plant growth increases. resultingin a mountingproductionof plant biomass. Tectonicevents,suchastheformationof depressions. grabenstructures or otherwisesubsidingareasare favorablefor the tbrmationof peat.
I 8.2The Formationof Peat
231
Highestgrowth rates are reachedin tropical swampforestssuchas are found togetherwith their associatedforest peits along ihe northeasterncoast of Sumatraandthe southerncoastof Bornio andNew-Guinea. However,degrada_ tron processes that affectdeadplantmaterialare alsomoreintenseandfasterin tnesehumtdandhor climates.Therefore,it mayverywell be that the abundance of vegetablematterin tropicalswampsis generillymorethanolfsetby the more rapid decay and destructionthere. peat and peat-likeaccumulations are at p_resent frequentlyfoundin cordcountriessuchasScandinavia, Scotland.Alaska, Canada.etc. Thus.optimumconditionsfor peatformationandpreservation may also be found in cold countrieswhere the vegetationcan aciumulate and be preservedbeforedecay(Hedberg,1977).In gineral for the accumulatlon and preseruation of thick depositsof peat an equilibriumbetweenthe accumulauon rate of organicmatterand subsidence hasto be maintainedover lonqpenodsof time. Yearlv accumulation ratesof peatsin present-day peat areasaverageabout 1 mm (Teichmiiller.1962;Given and Dickinson.iOi:). f"ut deposis up to severalhundredmetersthick areknown,andbrowncoalieamsmori than 300m in thickness in the Stateof Victoria,Australia,andover400m in WesternCanada attestto uninterruptedpeatformationoverperiodsof about1 millionyears.This i l l u s t r a t eas, s e c o neds s e ni ar l _ r e q u i r e m feoni t h ea c c u m u l iaol no f g r e at q u a n t t t r e s o t o r g a n i cd e b r i sa n dp e a t .I t i s a s p e c i al e l c t o n ic o n d i r i o p n r o v i ; r n gp e r s r \ l a n t . moderatesubsidence withoutthe development of strongrelief.To completethe peat-forming process,it is necessary to hive coverage oTthe accumulating plant debrisby stagnantwaterto preventits oxidationanj destruction. The efficiency of peat-forming processes is relativelylow. Lessthan 10% of plantproductionis accu^mul.ate! The larger part is decomposed eitherduringpeatformation 9s_q9ar. or afterburial(Givenand Dickinson.1973).Nevertheless, the jficiency of peat formationis higherby abouta factorof 10ihan organicaccumulation in aveiage fine-grained marinesediments whichmay ultimatJlybecomesourcebeds. were originallylaid down in basinsof long- Most imporlantcoal occurrences. Iastingsubsidence in coastalor paralic(coastalswamp)environments. ExamplJs arethepeatareasoflndonesia,theTertiarybrowncoiisofwesternGermanyand the Carboniferouscoalsof easternUSA and northwesternEurope, and the Donbas in.USSR. Important coal occurrences also developedaiound huge freshwaterlacustrinebasins,suchas the Carboniferous coalsof Bohemia,tf,e Saardistrictin Germany,MassifCentralin France.andSpain.The maloreras of coalformationare summarized in TableII.g.1.As a consequence of theirdeltaic andcoastaldepositional environments, recentpeatbogs,ai well asrheirancient counterparts, are found associated with a varietyof sedimentary types(Spack_ mannet al., 1966;Hemingway.1968).Theseassociated sedimenis aregenerally fine-grainedclastics(sandstone,mudstone,shale),buf may developcoarsei lithologies(conglomerates) locally.Carbonates aregenerallyminorcomponents. Repetitivesequences with severallithologies, typicallysandstone, shale,coaland somelimestone.are a featureofmost coal-bearing stiata(Westoll,196g).Study o_ftheserhythmicor cyclicsedimentation sequencei hasindicatedthatrecronrc or climaticeventsor both producea relativeiise and fall of sealevelwhich is an importantgeological factorcontrollingthe processfor coalformation.
232
Coalandits Relationto Oil andGas
The sedimentarycyclesmentionedaboverepresentdepositionunder subaerial, fluviatile, intertidal and shallow marine conditions.Becauseof the unstablesedimentary seltingof sucha region,depositionfollowedby erosionand redepositionis common. Rivers may migrale laterallyand erode a partially buriedpeatseamresultingin "wash-outs" whicharethe productsofsucherosion. The peat materialthal waswashedawaymight be transportedto quieterwater and admixedin minor quantitiesto autochthonous algal material,and thus contributeto the formationof a sapropelic coal.It mightalsobe deposited with a highermineralcontentand form part of the kerogenof a sedimentary rockunit. Anotherpart of the erodedpeatmaterialmaybe oxidizedandpermanentlylost. TableII.8.1. Majorerasof coalformation. (Modified afterGiven,1972) Cenozoic
N. America
Europe
Pliocene Miocene Eocene
+ (Alaska)
+
Far East
SouthernHemisphere
+ (Australia) + (Australia)
++
Mesozoic Cretaceous
++
Jurassic Triassic
++ (Japan, tntnu) * *
++
++ (China)
++ (All Gondwanaland)
+ + (Australia) + (Australia)
Paleozoic Permian Carboniferous
++
++
- absent ++ coalsveryabundant, + abundant,
8.2.2 Biochemiculand GeochemicalAspects Almost instantaneously following accumulationof dead plant debris on the ground,especially biochemical processes areinitiated.Mechanical breakageand compactionand geochemical processes alsooccur.At the surfacethesechanges take placeunder oxidizingconditionsand after coverageby additionalplant debris,sealmentand stagnantwater, reducingconditionsprevail.The pH is typicallyneutralor mildly acid at the surface,and becomesincreasingly acidic with deplh of burial (M. Teichmiillerand R. Teichmriller,1975).Thesevarious chemicalenvironments supportdifferentbacterialandfungalpopulations (Table II.8.2).Microbialactivitydecreases rapidlywith depth,but it isuncertainwhether the relativelyslowandlimiteddecomposition of planttissuein peatsis causedb1
8.2The Formationof Peat TableII.8.2. Microbialpopulations in peatprofiles.asrecordedon dilutionplates.Counts of bacteriaare in millions.of otherorganisms (After Given.1972) in thousands. Joe River
Litrle SharkRiver
StreptoDepth.cm Bacteria mycetes Fungi
StreptoDepth.cm Bacteria mycetes Fungi
0 8 25.0 3it 46 0.226 213-22t 0.013
0.8 61 69 190-198 3 1 83 2 6
l 1. 4 2.1 <0.2
239 2.7 1.1
RookervBranch {f 8 32.1 78.1 I l6-124 0.0142 <0.1 178-t{i6 0.0110 <0.2
6.21 0 1.01 <0.I 0.001,1 <0.2 0.0018 0
4.4 l 8 <0.2 0
North HarnevRiver
57 <0.2 <0.2
0 8 70.9 6l- 69 0.857 235-243<10 1
0.1 1.6
13.9 13.4
0
c}temical inhibitionor by nutritionalfactors(GivenandDickinson,1973).Woody tissuesare attackedalmostexclusively by fungi(Moore,1969),particulargroupi of which.preferentiallydecompose lignin and cellulosecomponents. It appears that lignin degradationoccursonly under aerobicconditioni,or at leastunder conditionswhenoxygencan be transferredor dehydrogenation can take place (Flaig. 1972).Cclluloseis ntore easilvremovedby hydrolysis.and thus prel.ercntiallvlostrelatir,eto lignin.Peatstill containsfreecelluloscwhichis absentin soft brown coal (lignite-B). The compositionof the bacterialflora changesmarkedlybetweenoxidizing and reducingenvironments. Bacteriapreferentially attackthe carbohydrate and proteinaceous portion of plant cells.Becausethe.semacromolecules are easilv hydrolyzed.they serveas the major substratefor microbialactivitv.lt is also rmportantthat the biomassof the sedimentarv bacteriaandfungifoffn a porrion of the final organicdeposit.Their metabolitesare not genera'ily lost from the sediment.excepttbr the gaseousproductssuch as CO,. NH]-and CH,,.The presencern deeppeatsof microbiallyderivedcompoundssuchas certainlipid components.carbohydrates. amino sugarsand amino acids.providesindirecr evidencefor microbialcontributions(Givenand Dickinson.1973). Humic substances are the mostcharacreristic productsof peatification. They are definedas that part of the peatthat is solubGin a basiciolvent, suchas an aqueoussolution of sodium or potassiumhydroxide.By definition. humic substances can be extractedin rhisway inclusivelyuntil the stageof hard brown coal (sub-bituminous). Typically,they occur as brown, hydratedgelswith no internalstructure.The formationof humicsubstances is a iwo_stage process.In the first stage.plant materialis mechanically disintegrated and wi'ihihe helpof mrcroorganisms depolymerized into aromatic,phenolicandcarboxylicmoieties. In the secondstage,the polymersof humicsubstances are built up by random
23.1
Coalandits Relation to Oil andGas
repolvmerization and polycondensation of the moleculart1'pes(Flaig, 1972). The changes whichoccurduringthe humificationof plantmaterialarecomplex. and involvenot onlv the modificationof the carbonstructures of molecules, but alsoalterationsof the compositionof the hetero-atomic groupsattachedto the carbonskeletonsby sulfur-,nitrogen-and other specificbacteria.The role of lipidsduringthe formationofpeatis partlyunknown.Higherplantsarerelatively poor in lipids, but certain parts. such as leaves,spores,pollen. fruits and especially resin-bearing tissues(e.g., of conifers)are rich in lipidsandlipid-like substances. Becauselipidsare insolublein water,and not so easilyutilizedby microorganisms. thev are preferentiallyconcentrated during the formationof peat. In summary.two stagesofpeatformationcanbe recognized (Kurbatov,1963): a primaryphaseat andimmediatelybelowthe surface,whichis characterized by rapidoxidation.and a secondary phasein whichslowerconversions occurunder reducingconditions.The extentof decomposition and humificationis largelya functionofthe severityofthe primaryphase.Factorssuchasrateofburial.water tableandclimatecontrolthe durationand conditionsof the primaryphase.and thus control the compositionof the organicmatter that is availablefor the coalificationprocess.There are obvioussimilaritiesbetweenthe diagenesis of organicmatter in soils and sedimentsas describedin ChapterII.2. and the diagenetic stepsduringthe formationof peatandbrowncoals.Specialreference is madeto the natureandrole of humicsubstances in coalandits relationshiD to the maceralgroupshuminite(in brown coal) and vitrinite (in bituminoushard coals)and the importanceof humic substances in the formationof kerogenin sedimentary rocks.
8.3 CoalificationProcess Coalification or carbonification is the process of chemicalandphysicalchangebr biochemicalinterferences(e.g., bacteria),temperature,pressureand time. imposedon the organiccomponentsthat survivedthe peat formarionprocess. Coalificationis convenientlysubdividedinto a biochemicalphase,as long as organisms suchasbacteriaandfungiareactivelyengaged, andinto a subsequent geochemical phasewhen biochemicalprocesses haveceasedto play a role for further coalificationincrease.The well-knowntermspeat, brown coal,bituminous coal and anthraciterepresentdifferentstagesof the coalificationseries. Thesestagesof the coalification processaretermedIevelsof rank whichindicate the maturityofthe coal.Someofthe parameters usedto definerank areshownrn F i g u r eI I . 8 . 1 . Coal rank can be determinedby generalparameterssuchas moistureand volatilemattercontent.reflectance, or by essentially chemicalparameters, such ascarbonor hydrogencontentandcalorificvalue.No onerank indicatoris ideal for all rankranges:moistureandcalorificvaluearetypicallyusedfor browncoals: volatilematterand vitrinitereflectance are usedover the bituminoushard coal range,andcarbonor hydrogencontentor volatilematterdeterminations arebesl
ll.3 Coalification Process
235
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236
Coaland its Relationto Oil and Gas
0etermined on coals lromfour European areas
R
re tu re \
M
.64
@
% r e f l s c t a n c(ei n o i l) andvolatilematterofEuropeancoals.(After Fig.IL8.2. Correlationbetweenreflectance McCartneyand Teichmriller.1972)
Yolatile m a t t e r( d .r . l .) 0
50
1
0
t5
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0
00
l0
t0
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/0
65
70
75
80
85
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Fig. II.8.3. Relationshipbetweenvitrinite reflectanceand different chemicalrant parameters. (After Teichmiiller.1971.takenfrom Stachet al.. 1975)
8.3Coalification Process
Xj
appliedin low volatilebituminoushard coalsand anthracites. Attempts have beenmadeto standardize internationalcoal_runtta.rnr.e.oirelation between rellectanceand volatilemattercontentof coalsfrom different areasrs grvenin FigureI I.8.2.The relationship betweenvitrinitereflecrance andseveralchemical r a n Kp a r a m e t e rl s s h o w ni n F i g u r el l . g , 3 . pear-formjng process actsdifferentiallyon the variousplanftissues, ,^_,i_:l1l,l. ." tlqrd-richparts-such ascuticlesandresinsversuswoodytissues rich l:r_.-liTll" rn celulose and Irgnin,coalificationaffectsdifferentcomponents in different ways. The number of readily distinguishable plant remainsdecreases with Increastng.coatrtrcarron. tsyanalogywith the term mineralappliedto rocks,each mrcroscopically identifiablecomponentofa coalis termeda maceralaccording to the classificatio-n Stopes/Heerlen (Anonymous:InternationalUanOOoolof Coot Perrography1963.1r)7r.1975).The three main g.oup, oro,u...uls are: the remalnsot wood) and humiccomponents variouslytermedhuminiteor vitrinite rank coalsrespectively; the remainsof lipiO_iiln.ericsor ptantr, :i.]:-^:.-l _!lChwaxes, sucn. as rcsrJrs. spores.cnticlesand algal bodies.termed liptinite (or cxinite);andhardercarbon-rich. brittlcparticlei calledinertinite..t.ne inertinite groupcontainsa numberof macerals thai arepresentin all rankrangesandwhich representthe more aromaticand oxidizedc-omponents. Somemaceralsof the inertinitegroup are probabryremnantsof biological oxidation,tbrestfires or even reworkedmaterialfrom older sediments.Huminite in biown coalsand vitrinitein bituminoushard coalsare the mostcommon components. Among the major physicalchanges coalificarion bringsuldut, ur" a reduction . in the bedmoisturecontent,an incieasein density,, a"!..ur" in po.osityandan increasein refractiveindex in later stages.Chemicalchanges rihich occurare condensation, polymerization, aromatizition,ancllossof furictionalgroups,i. e., of functions.containingo, S, ancrN linked to the motecurar siructureof coal. Polymerizationand condensation occur as an extensionof the humification processthat takesplacein lower-rankcoals.The net resultof thesechanges is a contrnuous but nonlinearenrichmentof carbonwith increasing rank. {lq6tt reporrsthal the numberuu"rug" rnoi..urur weightof -. Il^*::::,:-l pvflorne extractsot coalsrisefrom about600at g0% C (on a water_and ash_tiee basis,i. e., w. a.f. or dry andash_free, d. a.f. ) to 1000_l20bin theg6_SS7"C.ange. The generalnalureof the coalmoleculet orlU""n ,ugg"it"J t oii rrorncnemicat (Gwen.1960.1961; CooperandMurchison,1969) anjf?orni-.of.uio"n." 1Crrt, a,ndHirsch, 1960).Modelsderivedfrom thesestudies or" lffuirru,",fin Figure IL E.,1. Chemicalidentification offunctionalgroupsandgeneralinformationon the carbonskeletonareestablished, but thestruituril e*ariplesgivenmustbeseenas attemptsonlyto givethegeneralframeworkof thecoalsiruct"ure. Theyaremarnty intendedto describestructuresof hard brown coal to Uituminouscoal. The r linkedby hr drorromrricl nd mcrhyl(nebridgesrnd I;:1:T^rli:::l'1:.1.c,genera n n g e dw r l h m e t h ) 1 h . y d r o \ ) . c a r b o x l .c a r b o n y la. m i n oa n d ' r , t h e r functionrl groups. The aromaticitvof vit nite varieswith rank, and hasbeenmeasured .. by such d i v e r s el e c h n i q u eass X - r a yd i f f r a c t i o no. p r i c aal n ds p e c t r o s c o p i c examination, and chemicalstudies.Vitrinitestypicallycontain abott 70Vo L.orno,,..u.oon atomsin hardbrowncoalsandover90Zoin anthracites. Liptinitesinitia y travea
Coaland its Relationto Oil and Gas
23il
cr'unoe - YYE?^( ,'\.^c,/\l^\
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I
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-*<1. ->\3r-'
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;'F-:-:--j--=;
onchemical andphysicochemical based natureof coalstructures Fig.II.8.,1a{. General o/oC, mainlyfromX-raydata.(AfterCartzandHirsch.1960|. (a)Vitrinite13,1.5 evidence. data.(AfterGiven.1960.l96l). (c) Vitriniteat C. fromchemical (b) Vitrinite82.0on (After et al ' 1968) Teichmuller levcls. rank various
very low aromaticcontent.but in low-rankbituminouscoal.507ccarbonatoms arefoundin aromaticrings.Thereafterthe aromaticityrisesrapidlyto join thatof vitrinite in anthracites.The inefiinite maceralgroup has 90-100% aromatic carbonat all ranks. The hetero-atomconcentration,as well as the typesof functionalgroup6changewith rank. Hydrogen, oxygen. sulfur, and nitrogen are the maior ofthe oxrsen noncarbonatomsin coalsat all ranks.The typesandconcentrations functionalgroupsare shownin Figure IL8.5, plotted againstrank. Nitrosen pyridylrinss occursasprimary.secondary and tertiaryaminesand heterocyclic thionaphthylandthiophenicrinss andsulfurasthiols.sulfidesandheterocyclic
8.3 CoalificationProcess tI
239 Fig. II.li.5. Relationship betweenoxygen tunctionalgroupsof vitriniteswith increasing ranklevel.(After vanKrevelen. 1963)
- c00H
)c=0
- 0tl
90
% carton--...."---_ (increase in ran|t) Inorganicsulfur.often in the form of pyrite,is an almost ubiquitouscomponent of coals.Similaritiescan be observedbetweenthe structural modelsderivedfor coal (Fig. II.8.4) and for sedimentary rock kerogen(Fig. II.t.gt. tsorhmodels compnsearomaticcentersor n-uclei,crosslinkedby bridges and fringed by
A majordifference is foundin ir," "ri",.-"r bridges. In llr^::::lriq_r:lt:. Kerogen,especray at lower to
mediumlevelsof evolution,the bridgescontain "n9.on the average,longeraliphaticchainsthancan be observedin coal. Tgt" DflogesIn coat are more often cyclicand hydro-aromatic in nature,possibly alternatingwith short chain-tikeIinkagesinciuding h";;;r;;.1; funcrions.In ar3maticity,in coalis higherthai in kerogen.With increasing 9::,:ril,^,1:^.i.-r:lt ranK' the averagestackingnumberof aromaticsheetsincreaies in coalaswell ai ln Kerosen. A vastamountof chemicaland geochemical data has beenaccumulated fbr coal (van Krevelen,1961;M. Teiihm0ller unO n. f"i.t.tifi.r, f!rOZ,1SOS, Stach'sTextbook of Coal petrology,1975),but one of tire most useful and succlnctpresentations is thevanKreve_len_diagram (Fig.ILg.6).Thisdiagramhas been usedthrough-outthis part of the uoot tJoesciibJttre ctrlmiiar evotutionot kerogen(Figs.rr.2.9, rr'4.r1 and IL4.12). It incorporates informarronon tne threemostabundantelementsin coal,andalsodisplaysthe esseniiaAifferences
rr.a.n.\nuooifi-tt " oiug.urn g:9rp:.asinFigure 1rig. 1,","-.":1,*^ll.lll | L ^ . o , s n o w sl h e c o a l l i l c a t i o nt r e n d so f t h e \ a r i o u s
p l a n t c o m p o n e n t s s, u c ha s woody tissues,sporesand cuticlesas they evolve tt rougf, tfr" piut, brown coal,
Coaland its Relationto Oil and Gas
210
HUUINIf E vlrit
ttel
ratio 0/C atomic position plantandcoalmaterials andtheirrespective in the H/C-, Fig.11.8.6.Selected (vanKrevelen-diagram) O/C-diagram
bituminoushard coal, and anthracitestages.The original diagramby van like dehydration,decarKrevelen(1961)showedall simplereactionprocesses all these (Fig.II.8.7).It waseasyto readbecause boxylationanddemethanation highly aliphatic lines. In the reactionprocesses are representedby straight waxesandexines,the predominanttrendis lossof hydrogen.The hydrogen-rich considerableoxygen content of humic substances(humic acids, huminite, rank.The relativelysmallchanges vitrinite)is markedlyreducedwith increasing their highinitialcarbon experienced by the inertinitegroupof maceralsindicates duemainlyto theirhtgherinitialaromaticity,and contentandchemicalinertness possiblydueto the factthat theymaybe reworkedmaterial.The final productof
+
hydroqenalron
idehydroqenalion , generation ol CHa 9 6 9 2 E
I
0.5
1
o/c
t.5 atofiicratio
Fig. II.8.7. Original II/C versusO/C diagram proposed by van Krevelen (1961)showingthe trendsfor elimination of water, carbon dioxide and generationof methanefrom coal
211 Fic. I L 8.13.Evolutionof theelemental composition of coalduringburial.The path of humiccoalsstarts with peat and reachesfi, nally anthracite and metaanthracite.A few cannelcoalsand boghead coalsare alsoshownwith higherI L/('ratios(Durand e t a l . . I 9 8 -)]
Atomic Ratb O/C
the-coalificationprocessis a mcta-anthracite (highrank anthracite)whichhasa highlyaromaticcarbonstructurewith similaritieito a poorlyorderedgraphite. The chemicalchangesin coal duringits evolutionthrougi the differentrank stagescan be comparedwith the evolutionof variouskerJgentypes.It can be deductedfrom a van Krevelendiagram(Fig. II.4. I l) that hrimicioals and typeIll kerogen,rich in terrestrialplantmaterial.aresimilarto eachother.-Ihis is.of course.no surpnse.asthe bulk of the startingmaterialand,to a certainextent, thc environmentof depositionand earlydiag"enesis are alike.Coal and kerogen followthe samegenerairrcndthroughoutthe evolutionaryprocess(Fig. II.glg). typifiedby an increascin carbon(carbonification) anda lcissin functiorialgroups as documentedin thc lossof oxygenfunctions(Figs.II.g.5 and Il.5.g). Wiih respectto oxvgen functions.coal (vitrinites),and type_llI kerogen show a similaritvby the absence of estergroups.TvpeJ and type_Ilkerogen,howeyer. containcstercroups.
ll.4 Coal Petrography Clal petrographfis the studyof the macroscopically, and more importan y, of themicroscopicallv recognizable components oicoal. It describes coil in termsof its maceralcomposition andits rank Ievel.whichis mostconveniently determined bv reflectedlight microscopv.A petrographicmicrocsope is usedto observea highlypolishedsurfaceof a coalblockor grainsin refleciedlight. ln reflectance measurements. the amountofincidentIightwhichis reflectedfiom the surfaceof a maceralis comparedto thelightreflectedfrom a standardofknownreflectance. Measurements areperformedwith monochromatic greenlightat a wavelength of -546 nm. Oil immersionobjectivelensesaretypically-used tu"enhance thecontrast of petrographic components. Meanraudomiefleciancevaluesin oil Rnf % t are
Coalrnd itr Relationro Oil and Gas
generallygiven at lower ranks. i.e.. below 1.2olrreflectance.Becauseofthe increasing anisotropl'of vitrinitewith rank,meanmaximumreflectance (Rumax. 7.), obtainedusinga linearlypolarizedlight beam(ratherthan meanrandom values(Rn 7c). employingno polarizer)shouldbe givenat leastbeyond1.2% meanreflectance. Whilethisprocedureis practicalwhenstudyingcoals.it is also usefulin kerogenstudies.Due to smallparticlesize,however,it may be a very time-consuming procedure.The reflectanceof particlesas small as 3 rrm in diametercanbe measuredroutinelv.
8.4.I M icropetrographicComponents : the Macerals Maceralsof coal are microscopically differe,rtiated on the basisof their reflectance,their shapeand structureand sometimesby suchadditionalmethodsas etching,electronmicroscopy. fluorescence, and luminescence. In a typicalcoal. thrcedifferent maceralgroups,i. e.. liptinite.vitriniteandinertinite(Fig. I I.8.9) canreadilybe distinguished on the basisof reflectance values.Liptiniteshavea relativelylow mediumretlectanccand inertiniteshavethe highestreflectance. Differencein reflectance betweenmaceralgroupsare greaterin browncoaland lignitethan in bituminouscoal and graduallydecrease towardanthracite.The
Fig. IL8.9. Polishedsurfaceof bituminous coal.reflectedlight.oil immersion.The maceralsliptinite (datk), vitrinite (g/e)/ and inemnite (light) can be seen. (Photo Hagemann)
u.,1CloalPetrography
213
1O.O- -
--
Fig. II.ll.10. Reflectance changesof main groupsof maceralsas related to degreeof coalification. (ModifledafterAIpernandLemos de Sousa.I97tl)
2.Or
t.o)r--- -" o.5: o.2 os
orl o.05 50
40
30
20
ro
Volatile matter 70 lwhote coa|
curve__of vitrinite and liptinite reflcctancemergesaround 1.3 1.67r R,, max. (Fig.I L ll. l0). I nertinitesascomp.ared to vitrinite--s remainol higherreflectan ceup t o t h em e t a - a n t h r a cliet ev e l( 3 . 5 r . R , , m a x . ) . T h e l i p t i n i r e c o m p r r n e n t s , h o w e v e i , r c lai n r o m co l t h e i ro r i g n i a lm r r r p h ( ) l o.gS) l { ) r c \ l. e r l e ut i c l e sa.l g a lh ,r t j t e 5r .e s i n s J n d s u b e n n isi r ei d e n t i f i a b l c Fluorescence is a usefultechniquefor identificationof liptinitematerial.An excitationwavelength(emissionpeaksbetween366 and 43-5nm of a mercurv lamp)in the UV-or bluerangeof the spectrumis usedto producea fluor"r."n.., whichis generallyrestrictedto the.littiniticcomponenti.Using this technique, liptiniticcomponents areabreto emitiightfrom blueto red,this;videnceiscailed fluorescence (Alpernet al., 1972).A colorchangefrom blueto greento yellow to orange redrith increasing rank the basisof a rank pararieterthat may be .to _is especially usefulfor lowerrank coalsandcorresponding sporinite_type seormen_ tary organicmater (Ottenjannet ai., 1974).The coloioi sporesin transmittcd light has also been usedas a rank pu,o-.i"r. especially in kerogens(Staptin, 1 9 6 9J; o n e sa n dE d i s o n .1 9 7 9 ) . -fhe inertiniticcomponentsof coalsare generallycharacterized by a high retlectance.It has recentlvbeen suggested(Teichmiiller,1974) th;t highl), reflectingmicrinitesmav be formedfr-omliprinitic 1orhuminiric)suostances at i r b o u t h e r a n k l e v e ln e a rt h e m . r x i m u m o f p e r r o l e u mg e n e r a t i o nI t. h a sb e e n generallvacceptedin the past rhat inertiniticcomponJntsare the productof oxidationby forestfires or by bacterialor atmosphericoxidation. Reworked materialmay alsobe includedamongthe inertinitei. The maceralsof the huminiteor uitriniregroupare physicallyand chemically rntermediate betweenthe liptinite^ andinertinitegroupi. in the Lrowncoalstage huminiteis thoughtto be formedfrom massive ce'ilulaitissues, suchaswoodand bark and from the precipitarionof, and gelificationby, humicsubstances. With increasing rank. the cellularstructures arechanged.Cellwallsswell,andcavities and pores becomemore and more impregnatedand filled with structureless humicsubstances. ln this way, the cellulaicharacleris increasingly obscured. Beginningin hard brown coals,around0.4ck meanreflectance. ihe huminitic
Coaland its Relationto Oil and Gas
211
componentis from hereon andthroughouthigherrankstermedvitrinite.At this thenthe vitrinitecomponents are stagecell structuresmaystill be recognizable, vitrinite is calledcollinite.ln calledtelinite.B_!contrast,massivestructureles retlectedlightunderoil immersion,vitriniteappearsasmediumgrayintermdiate betweenthe brighter inertinite and darker liptinite (Fig. I1.8.9).Macerals becomeprogressively more alike in coalof higherranks.
8.1.2 Applicution of ReflectanceMeusurements fttr SourceRock-TypeSedinents 'I'he
rank or maturitvof a sedimentary rock containingorganicmatter can bc determinedby-measuringthe rcflectanceof finely dispersedsmallhuminiteor vitriniteparticles.ln pafiicular.thisparameterallowsa sedimentto be evaluated with respcctto whctheroil or gasgcnerationhastakenplace(Vassoevich et al., 1969:Teichmr,iller.1971:Dow. 1977).This method is further discussedin C h a p t eV r . l. A number ol peculiarpoints are associated with measuringreflectancein scdimentaryrockswhichdiffer from measuringcoalsamples.ProperidentificaTypetr Kerogen I Z
Aulochihonousvrtnnite n""y"rA "itrinit"
E 1 0
o.5
'r.o
1.5
2.O
F n i n o i l ,5 4 6 n m ( c o )
Typem Kerogen m Aulochlhonousvn,inite vilrinile Z Fecycred
q5
100
r.5
RmInoir.546nln (qo,
histogram Fig. IL8.11. Frequency showingvit nite compositionof a type II and type III kerogenof sourcerocks acknowiedged
8.5 PetrQleum Generatio;r
215
tion of vitrinite,which alsorelieson its specificrangeof reflectance, may be a problem.sincethere are generallyno adjacentmaceralsfor comparison.This may be especiallydifficult becausethere is a continuousreflectanceand morphological sequence betweenvitrinite,andcommonparticlesofthe inertinite group,i. e., semifusinite and fusinite.Identificationofvitrinite, and distinction from fusinitein polishedsectionsof sedimentsis, therefore.at leastpartlv a matter of personalexperience(Jonesand Edison. 1978).Thus. althoughthe relativerank of sediments can be determined.it is not alwayspossibleto asign unequivocallv an absoluterank on the basisof vitrinitereflectance to a particular sample.A commonlvused method to minimize operator bias is to plot a histrogramof all rcflectancemeasurements from a large number of maceral grains(Fig. lt.lt.I l). ln sediments poor in organicparticles.thisis oftcndifficult and time-consuming.Therefore. it is recommended(Hagemann.197.1)to prepareconcentrates of organicparticlesby removingmostof the rock matrix with HCI and/or HF. sometimesfollowed bv heavy liquid treatment.This facilitatespropcr identificationof vitrinite and allowsa statisticallysufficient numberof particlesto be measuredwithin a reasonable time frame. It is also suggested to use a sequenceof samplesin a well to determinethc maturation gradientin a sedimentarv rock sectionratherthan absoluterank ol a particular s a m p l e( D o w , 1 9 7 7 ) . It is lrequentlyobservedthat low rank vitriniteandhuminiteshowa considerablespreadof reflectance values.This spreadprobablyreflectsthc diversityol plant material from which the vitrinite is formed and possiblya ditTercnt diagenetichistory.Anisotropyalsocausesan increasingspreadof reflectance valuesasrank is increased aboveabout 1.5R0(%).
8.5 PetroleumGeneration 8.5.1 Generationof Low Molecular WeightVolatiles Phvsicaland chemicalchangesin coal. due to increasingtemperatureand pressurewith burial, resultin coalificationand producea sequence of coalsof increasing rank. Followingmicrobialand physicilchanges duringthe diagenesis of peat and browncoal.geochemical changes. suchasa decrease in oxygenand hvdrogenand a relativeincrease in carbon,take placeduringthe catagenesis of bituminousand anthracitecoal (Fig. 1I.8.8).This resultsin an eliminationof functionalgroups. increasingaromaticityand a decreasein the chain-like structures in coal.Simplematerialbalanceconsiderations indicatethatthe lossof hydrogen,oxvgen.andalsocarbonresultsin liberationof hvdrogen-andoxl,genrich carboncontainingmolecules duringcoalification processes. In general.two tvpesof reactionare rcsponsiblcfor the liberationof smallermolecules.They are condensationreactionson one hand and the splittingof carbon-carbon bonds,asin cracking.on the other.Therearealsodefunctionalization reactions. suchas decarboxvlation. In condensation reactions,two reactivemolcculesare
Coaland its Relationto Oil and Gas
246
Iiptinite
Micririte
llitrinite
.l.c
85
90
95'AC
85
90
Fig.I1.8.12.Amountof gasliberated coaliflcafromdifferent macerals duringincreasing (After tion.Amountof gaswascalculated on thebasis of elemental analysis of macerals. Jrintgen andKarweil.1966)
linkedto form a productof highermolecularweightthan eitherof the reactants. At the sametime, low molecularweightcompoundssuchas CH. and H.O are liberated.For example,the condensation of aromaticsystemslike benzeneand toluene or benzeneand phenol would producebiphenylsand CHo or H2O respectively. The generationof low molecularweighthydrocarbons, especially methaneand other volatile nonhydrocarboncompounds,such as CO2, is a necessary by-productof increasingcoalification.and is determinedby type of organicmatterpresent,temperature,and time. Many authorshavecontributedto the understanding oftheseprocesses ofcoal maturation.Fundamental paperson the principlesof gasformationin coalwere publishedby Jiintgenand Karweil (1966)and Jiintgenand Klein (1975).It is important that the different maceralgroupsliptinite (exinite),vitrinite and inertiniteexhibitverydifferentdegassing curvesduringthecourseofcoalification according to theirdifferentchemicalmake-up(Fig.I1.8.12).Unfortunately. their data cover only coalsin the low rank bituminousto anthraciterange which contain between85 and 95% C and about 30 to 5Vo volatile matter. This corresponds to vitrinite reflectancedata betweent.0 and 1.0 n,, 1%.1.Some methaneis liberatedfrom liptiniteat relativelylow rank.e. g. , under0.857omean reflectance.This is especiallynoteworthyin the contextof sourcerock-type sediments. Dataon thegeneration ofvolatilesin peatandthe lowerrankcoalsare very scarce.The first volatileproductsliberatedin both coal and kerogenat temperatures below 100'Care mainlyH2O and C02 with smallamountsof CHa ( v a nH e e ke t a l . , 1 9 7 1 ) . A calculatedmodelis presentedin FigureI1.8.13(Jiintgenand Klein, 197-5), wherethe integralamountof volatileproductsduringcoalification is shownasa
8.5 PetroleumGeneration
217
t0%
l0%+
d€creasing volat;le matter
-- cHl <
Hzo
33, IHC l{z t50 r25
CJ
t00 E
1000
-E
2000
F
looc
T .{ ' c / r o -o 50
EI
u
u
lr
2it
J2
10
1g 22t 228
I i m e( m i l l i oyne a r) s m = heatingrati0(temperature increase during subsidence)
grad GT= geothermal ient Fig.IL8.l3. Integralamount ofvolatile compounds generated duringcoalification shown asa functionof geological subsidence, i.e.. temperature andtime.(AfterJiintgen and ] . l c i n .l q T i )
lunctionof rate of subsidence (temperature and time). The modelis calculated accordingto actualgeologicaland geochemical datafrom the Carboniferous in theRuhr areaof Germany.A subsidence rateof 3000m over20millionyears,ano ln lncrease in temperature from 20'C. at thesurfaceto 140"Cat maximumdepth o f b u r i a(J3 0 0 0 mr)e s u l t s i n a h e a t i n g r a t e o f 6 " C / l 0 6 v e a r s o r al .b2oxult0 r r ; C / min.For extrapolation of natu-ral degassing rates.experimental datawith heating :atesin the range from 10 r to l0r"C/min are available.Accordingto thii nodel. the evolutionof volatilesbeginswith H:O. COr and CO beforelarge rmountsof CH] are released.This is in agreementwith observations in nature
Coal and its Relation to Oil and Gas
248
of the Fig. II.8.14. Percentage total generatedgasthat cannot be storedin a coal.eitherin an adsorbedstateor in free form. Storage conditions are 100'C at pressuresof 1000, 100 and 50 atmospheres,respectively. (After Jiintgen and Karweil, 1966)
-
AT IOOoC AND lOOOatm
---
-
ar 1oo"c AND
1 o oa t m
ar loooc AND
5o.tm
suchas bond energiesof functionalgroups and with theoreticalconsiderations and carbon+arbon bonds. The bulk of methanegenerationin coal begins generallyin the rangeof mediumvolatilebituminouscoal (about 1.3 to 1.4Va for vitrinitereflectance). As shownin FigureII.8.12,thisis morecharacteristic andinertinite-type materialsthanfor liptinites.With respectto thisphenomenon. in the rangeof more dataare neededin lower-rankbituminouscoals,especially 48 to 30a/.volatilematteror 0.50to 0.85Rn(o/a),andat still lowerranks.Thisis also true with respectto N2 generation,which has not yet been sufficiently documented. greatlyexceeds the duringcoalification The generationof volatilecompounds storagecapacityfor theseproducts.eitherin an adsorbedstateor in free form. of CHathat cannotbe This is illustratedin FigureII.U.14,wherethe percentage degreeof coalification. retainedin coal is calculatedasa functionof increasing
8.5.2 Generationof HeavierHydrocarbonsand Nonhydrocorbons Aside from volatilesproducedwith increasingcoalification,highermolecular in coal. weightsubstances, similarto thosefoundin petroleum,arealsogenerated be viewed in connecnonhydrocarbons should Theseheavierhydrocarbons and in ChaptersII.5 andII.6 tion with the schemeofoil andgasformationdeveloped and with the comparisonof coalificationstagesand petroleumgeneration publishedby Vassoevich et al. (1969).It wasobservedthat duringcoalification, to a maximumin high volatilebituminous extractionyieldsfrom coal increased
8.5 PetroleumGeneration
1.00
l.l0
219
l,?0
r . 3 0 l.ao
1.50
preference carbon inder0 f n- a l t a n e s
t,t0
F i g .I I . 8 . 1 5 .R a t i o so f t h e odd even predominance of the higher r-alkanes (CPI, n-C1 to n-C1) in Carboniferous coalsof the Saararea.SW Germany. in relationto rank of coai. (After Leyrhaeuserand Welte. 1969)
s:: iF
r.r
s :
' <
.= -
,,
coalsat a level of maturityof 0.8-1.0 R0% vitrinite reflectance (Leythaeuser. 1968;Leythaeuser andWelte, 1969).With furtherincrease in rank.a conunuous decrease in the amountofextractswas observed. Thecomposition ofextractsalso variedregularlywith increasing coalification. Very strikingwasI decrease in the o d d - e v e np r e d o m i n a n c(eC P I ) o f t h e h i g h e rz - a l k a n e s( F i g . I L 8 . 1 5 )a n c la n increasein lower molecularweightcompoundsand aromaticswith increasing c h l 0 r 0 f 0 r m - e r (t rm a cg tl q0 r g c. a r b o)n 0
5
l5
25
2 1 02 r 5
g
= =
1.5
0
5
1
0
40
h y d r o c a r b(om nq s / go r g .carbofl )
Fig.IL8.16. Quantityofextractandhydrocarin a bonsas a functionof \itrinile reflectance seriesof coalsfrom peatto anthracite.(After D u r a n de t a l . . 1 9 7 7 )
250
Coaland its Relrtrontu Oil andCas
rank. Thesedatawerelaterconfirmedby Hood andGutjahr(1972).Hood et al. ( 1 9 7 5 a) n dM u k h o p a d h y aeyt a l . ( 1 9 7 9 ) . The generalschemeof hydrocarbongenerationfrom coalparallelsthat from kerogenin petroleumsourcerocks.Durandet al. (1977)investigated a seriesof coalsof differentrankswith the samegeochemical methodsusedto studyorganic matterin sourcerock-typesediments.He found a closesimilaritybetweenthe phvsicochemical properiiesof coal and kerogenand in their itructural and chemicalevolution brought about by catagenesis. As previouslvnoted, the greatestchemicalandevolutionarysimilarityis observedbetweencoalandt1'peIII kerogen(Fig. IL4.11).The catagenetic changesin coal and in kerogenare basicallya progressive eliminationof sterichindrances.suchas heteroatomic links, aliphaticchainsand saturatedcycles.in order to attain a more stable molecularconfiguration underhighertemperature andpressure conditions.The resultis the well-documented formationof CO1,H1O.H2S,resins.asphaltenes, and hydrocarbons. 'I'here aredetailedstudiesof C,5*-hydrocarbons extractedfronta seriesofcoals betweenpeatand anthracite(HagemannandHollerbach,19801 Hollerbachand Hagemann.1981).It servesas an examplefor the similarityin hydrocarbon in coal and in sediments. The relationshipbetweenthe quantitvof Eleneration extractand the typesof heavierhvdrocarbons from coalsol ditTerentranksis givenin FigureII.8. 16.The maximumyieldof hydrocarbons wil h 0.9R(l coincides (7r) whichis aboutat the samerank levelasobservedby Leythaeuser andWelte ( 1 9 6 9 )A. t l o w r a n k s( 0 . 2 - 0 . 5 %R n ' )h. i g hm o l e c u l rw r e i g h nt - a l k a n ews i r h2 5t o 33 C atoms. having a verv strongoddIredominance. clearlydominatethe distributioncurve.Fromabout0.-5-1 .37oRnthemaximumon the I -alkanecurve shiftstowardC numbersaround20 and the odd predominance is graduallvlost (Fig. Il.8.l7). Thus the generationof hvdrocarbons and nonhvdrocarbons in coal.aswell asin petroleumsourcerocks,is a normalproccssthat accompanies t h e d i a g u n e sai rn dc a t a g e n c soi fso r g a n i cm a r r e r . Analysesof thc saturatesbl combinedgaschromatography and massspectrometrv revealsthat in coalsof the peat and brown coal stagesonly cyclic diterpanes occur.andthat thevdisappear in bituminoushardcoalsaround0.87r h n d t l a g e m a n nl,9 8 l ) . C h a i n - l i k d \ i t r i n i t er c f l c c t a n c(eH o l l e r b a c a eiterpancs (pristaneandphvtanc)appeararound0.,l-qi.peakataboutl.?.Zmcanrcflecta and decrcasctoward anthracitc.The naximum amount of cvclicand acyclic diterpanes in cachgroupof compounds is aboutI t/. ( l0r ppm) oi the toalexrilLct. The behaviorof pentacvclic tritcrpenes with a doublebondin ringC (Fig.L.1.l0) is verysimilarto thatof thc cl,clicditerpanes. Thcyexhibita maximumaround0.,1% meanreflectance. compriseabout I '/c of the extractand disappearin low rank bituminoushard coals.Saturatedpentacvclic triterDanes and steranesexhibita b e h a v i o rs i m i l a rt o c h a i n - l i k cd i i e r p a n i s .T h e v a p p e a ra r o u n d0 . 4 % m c a n reflectancc andmakeup about0.1- 1% of the extract.Unlike acyclicditerpancs, thel' disappearcompletelfin high-rankbituminoushard coal (aroundL6 -./rRn max.)befbrcthe anthracitestageist attained.Thisbehaviorof acyclicandcvclic C r . . - i : o n r e n o i da.n ds t e r o i t Fi a t e n l a t i \ e l )i n t e r p r e t eadsa n r I r i c h m c n t o a n d subscquentreleascfrom insolubleorganic matter (coal or kerogen)during diagcnesis andcatagenesis. An importantcontrollingmechanism seemsto bethe
8.5 PetroleumGeneration
t51
Carbon number 15
20
25
30
Coals of different localities andranks PEEL
GARSDORF SOKOLOV DARMSTADT PEISSENBERG WARNDl WESTERHOLT BLUMENTHAL S I E RD S O RF 1.1 l.s 2.1
BOCHUM HERBEDE
Vnsxaduxr
2.5 2.7 2.9 3.J
MERKSTEIN
15
20
25
30
35
Carbon number F i g I. I . 8 . 1 7x. - a l k a n e d i s t r i b u t i o n c u r v e s a s a f u n cnt iiot enr e o ff lvei tc t a nt cna es e n eosf coalsfrompeatto anthracite. (AfterHollerbach andHagemann. 19lll)
locationand distributionof reactivesites(cspeciallvdouble bondsand OHsroups)on theoriginallinearisoprenoids andtheterpenoidanclsteroicl moleculcs. .\.5.3 Coalas a SourceRock The importanceof coalasa sourcefor gasinitiallyrecognized by thegeochemical uork of Karwcil(1956.1969)andthe geological considirations ofpatiln 1DO+o. b). ln the latterpaperthe roleof the Carboniferous coalsin the formationof gas fieldsin northwestern Europewasdefined.In lateryearsadditionalevidence was accumulated to verifythat Carboniferous coalsare a primarysourceof gasin this region(Stahl.1968;Bartenstein andTeichmriller,1974;Ltutz et a|.,1975).Hence it is well established that coalsare ableto generateandreleasesufficientgasto form largecommercial gasaccumulations. However.evidence for commercial oil
251
Coalandits Relationto D) andGas
accumulations derivedfrom coalis not so abundant.It is knownthat someliquid There are numerous are generatedin coal during catagenesis. hydrocarbons sandlensesetc being reportsof oil shows.smalloil seepsand oil-impregnated with coalsfrom all over the world. closelyassociated with coalsoften belongto the high-waxtype, as the Crude oils associated "Frankenholz"oil from the SaarRegionin Germanyt"La Machine"oil from Central France.and oils from the Midland coal measuresin Great Britain (Hedberg,1968).Crudeoils of the MahakamDelta areaof Indonesia(Durand of oil and andOudin. 1979)alsohaveto be mentionedhere.The closeassociation coalin the OtTicinaareaof Venezuela(Banks.1959)mustbe rankedasanother indicationtbr a liquid hydrocarbonpotentialof coals. Whenconsidering coal.onc usuallythinksin termsof rank andforgetsthatcoal mayconsistof ratherdifferentmaterialwith respectto its basicstructureln this scnsethe liptinitecontentof differentcoalsmayvaryconsiderablyLikewisethe fixed to aromaticnucleimay vary availabilityof chainlikemolecularstructures of liptinitein coalsand chainlike on the amount Depending coal to coal. from evenin molecularstructurestheremight be a potentialfor Iiquidhydrocarbons coals. An examplefor this is the Mahakam Delta, Kalimantan.Indonesia (Durand and Oudin, 1979; Durand et al ' 1983) Other authors stressed of lightoils for the generation the rolc of resiniteandexinitemacerals specifically a n dc o n d e n s a t (essn o w d o n1, 9 8 0 1 l ' h o m a1s9, 8 1 i P o w eel tl a l ' 1 9 8 2 )l .t m u s tb e is concludedtherefore.that the generativepotentialfor higherhydrocarbons potentialof coalhasa definitelypresentin certaintypesof coal.The generative great similarityto type-lll kerogenwhich yieldsgas rather than oil. but may generatecommercialamountsof crudeoil' dependingon the liptinitecontent. For instance.it hasbeenobservedin Australiathat MesozoicandTertiarycoals whereasPermiancoalsare only a sourcefbr havegcneratedoil accumulations, gas (Thomas. lglll). Likcwise, the Miocenc coals of the Mahakam Delta generatedoil and gas. whereasWestphaliancoals of WesternEurope only c/o providegas.evenin the areaswherevitrinitereflectivityis I or lower.As far as are undcrstoodthere seemsto be a lower expulsion migrationmechanisms The limited primary efficiencyin coals especiallyfor heavierhydrocarbons. due to the high is probably oi coal out hydrocarbons migrationof heavier asa massive, generally occurs coal fact that and the of coal. absorptioncapacity phase. solid organic continuous.
Summaryand Conclusion
253
Summaryand Conclusion Coalconsists mainlyof detritusfrom higher(terrestrial) plants.Mostcoalsareformed undernonmarineconditions. AImostimmediately followingaccumulation ofdeadplantdeb sontheground,first biochemical,and later, after burial, geochemicalprocessesare initiated. With continued bu al. theseprocesses progressivelycausecoalificationofthe organicmatter to form thematurational sequence of peal.browncoal.bituminous thardlioal andfinaly anthracite.Thesestagesof the coalificationprocessare termed levelsof rank. Coal rank can be determined by physical parameterssuch as reflectanceand moisture contentor by essentiallychemicalparameters,suchasvolatile matter contenr.carDon or hydrogencontentand calorificvalue. Chemicalchangesin coal durinp its evolution throughthe differentrank stagescan be comparedwith the evoluriin of various kerogen types. The greatest chemical and evolutionary similarities are observed betweencoal and type III kerogen. During coalification.low molecularweight hydrocarbons,especiallymethane,and other volatile nonhydrocarboncompounds,such as carbon dioxide and warer! are generated. The main phaseofmethanegenerationin coalbeginsin the rang€ofmedium volatile bituminous coal(about1.3to 1.47a.eflectance). The formationof volatilecompounds during coalification greatly exceedsthe storage (absorption) capacity for these productsin coal. In additionheavier,nonvolatilehydrocarbonsare formed. However, in contrastto thelargeamountsofmethanegenerated, thepotentialofcoalsto produce higherhydrocarbonsis limited. The generativepotentialofcoal is in this respectsimilar to type III kerogen, which yields mainly gas, but may generate commercial oil accumulations,dependingon the liptinite content.
Chapter9 Oil Shales:A Kerogen-RichSedimentwith PotentialEconomicValue
9.1 Historical century. The first mentionof the oil shaleindustrygoesbackto the seventeenth "found who out a whena BritishPatentno. 330wasissuedin 1694to MartinEale, pitch, of rock" great quantities and oil out of a sort wayto extractandmake of tar (Cane.1967).The first industrialplantwasdevelopedin Autun. Francein 1838. followedby anotherexploitationin Scotland,1850.Sincethen manycountries developedan oil shaleindustry:Australia(1865).Brazil (1881),Ncw Zealand ( 1 9 0 0 )S, w i t z e r l a n(d1 9 1 5 )S, w e d e n( 1 9 2 1 )E, s t o n i a( n o wU S S R )( 1 9 2 1 )S, p a i n (1922),China(1929),SouthAfrica ( 1935).The highestpointof thedevelopment wasreachedduringor immediatelyafterWorld War II. However,all plantsbuilt during or immediatelyafter the war had a small capacity,roughly50 to 200t of oil per day,andtheir costof productionwashigh due to the importanceof labor for mining and crushingthe rock. Around the middleof the century,the low costof the oil shippedfrom the Middle Eastmade made increasing consumption the shaleoil uneconomical, and the exponentially Therefore,all oil shaleplants the amountof shale-oilproductioninsignificant. between1952(Australia)and 1966(Spain)with the closeddown progressively exceptionof two countries:USSR (SSRof Estonia)and China (Manchuria). wherethe miningconditionswereadequateto justifya continuedexploitation. More recently,severalcountriesmadea surveyon the occurrenceof largeoil and the to justify importantinvestments shaledeposits.with sufficientreserves potentialto contributesignificantly to the demandin oil. Braziland the United Statesdevelopedpilot plantson the worldslargestoil shaledeposits:the Green River Shalesin Colorado,Utah. and Wyoming,and the lrati Shalesin Brazil. their oil With energybecomingshort, other countriesare againinvestigating resources. shale The shaleoil producedin the year1972wasestimatedat 10milliont in China. 3 million t in USSR(Estonia)and 50,000t in Brazil(Combaz,1974).
9.2 Definitionof Oil Shales Oil ShaleVersusPetroleumSourceRock or chemicaldefinitionof an oil shale.In fact.an) Thereis actuallyno geological to be amountuponpyrolysisis considered shallowrockyieldingoil in commercial a no i l s h a l e .
petroleum 9.2Definition of Oil Shales. Oil ShaleVersus Source Rock
255
The mineralconstituents of oil shalesvarylargelyaccorcling to types.Someare trueshales, whereclaymineralsarepredominant, but others.like thewell-known Green River Shales,are carbonateswith subordinateamountsof ouartzfeldspar,clayminerals.etc.In fact,thevariousoil shalesminedaroundrheworld during the last century range from shaleto marls and carbonates. The onlv excludedlithologyis sandstone. asthisparticulartypeof sedimentary deposiris not compatiblewith the accumulation and preservation of orsanicmarrer. The organicmatter containedin the oii shalesis mainlv"aninsolublesolid material.kerogen.Thereis no oil andlirtleextractable bitumennaturallvoresent in the rock. Shaleoil is generatedduringpyrolysis,a treatmentthat c;nsisrsof heatingthe rock to ca. 500'C. The kerogenof oil shaleis not distinctfrom the kerogenof petroieumsourcerocks detined in part IL To some extent the pyrolysisIeadingto shaleoil tbrmationfrom kerogenis comparable to the burial of the sourcerocks at depth. that generatesoil by the resultingelevationof temperature. However.thereare differentrequirements for petroleumsourcerocksancloil shalesin respectto organicrichnessand rank oi evolutionof the rock. Manv investigators haveconcludedthat any rock containingabout 0.5a of organic carbon may produceoil or gas, providedit is buried to a sufficientdipth. Hydrocarbons are mobile,sothat migrationmayresultin commercial accumulationsof oil or gas,evenif startingfrom low concentrations of organicmatterin a sourcerock. On the contrarv.an oil shalemusthavea largeamountof organic materialto be ofeconomicinterest.At least,onewouldrequirethat the oil;hale yieldsmoreenergyasshaleoil than it requiresro processtle rock.The average pyrolysistemperatureis 500'C. The energyto be providedfor heatingto thit temperature is approximately 250caloriesper g of rock,andthe calorifi&alueof kerogenis 10,000caloriesper g. Therefore,if the kerogencontentof the shaleis 2.57oby weight,the total calorificvalueis usedfor heitins rhe rock.Belowthe thresholdof 2.5%, the rockcannotbe a sourceof energy(Burger.1973).In fact.a lower limit of 5oZ organic content is frequently used, and it corresponds approximately to an oil yieldof25 I per metrict ofrock, or 6 US gallonspeishort t'. It shouldbe remembered that in the presentsituationsuchlow ratiosarestill uneconomical. The US literaturefrequentlyusesl0 US gallonsper shortt (,12I per metrict) asa lower limit for oil shales. The requirementsregardingthe evolutionstageare quite different for a petroleumsourcerock and an oil shale.A petroleumsourcerock requiresa sufficientburial history and the stageof catagenesis in order to degradea substantial partof kerogenandthusgenerate oil. On thecontrary,if onelooksfor an oil shalewith a largeyielduponpyrolysis.the bestsiluationis a shallowburial. i.e., an immaturestageof kerogenevolutionbeforecatagenesis begins.The rockscontainingorganicmatterwhich havebeenburieclto greatdepih do not representpromisingoil shales,even if a subsequent folding and erosionhas b r o u g h t h e r o c kb a c kl o t h e \ u r f a c e . It canbe saidthat the cquivalentof an oil shale,sufficientlyburied,constitutes a petroleumsourcerock. However.the reverse(i.e.. that the equivalentof a t t l U S g a l l o n : 3 . 7 8 5 1l s; h o r t t o n = 0 . 9 0 7 m e t r itcUt ;S s a l l o n / s h o r t t = 4 . 1 7 I / m e r r i c t .
256
Value Economic withPotential Sediment Oil Shales: A Kerogen-Rich
petroleumsourcerock,shallowlyburied.constitutes an oil shale)is not necessarily true, due to the requirementof richness.
9.3 Compositionof OrganicMatter Opticalexaminationof oil shalekerogensshowsthat someof them are almost entirely made of algal remains,whereassome others are an admixtureof amorphousorganic matter with a variable content of identifiableorganic remnants.The main algaltypesare BotryococciandTasmanaceae algabelongingto theChlorophyceae Botry'ococcus is afreshor brackish-water to the present and formingcolonies.It hasa wide extensionfrom Precambrian of are knownasearlyasthe Precambrian time: severalbedswith Botryococcus Bohemia.Kerogensfrom Autun boghead(Permian.France)and torbanite (Carboniferous. Scotland;Permian.AustraliaandSouthAfrica) areconsidered colonies.andthe Recentcoorongite to be almostentirelymadeof Botryococcas braunii, a similarsituation.The presentBotryococcus from Australiarepresents sourceof the coorongite,showstwo differentstates:a greengrowthstageandan orangerestingstage(Cane.1976). Tosmanitesare consideredas marine algaecloseto the presefitPachysphaera: the kerogenof some oil shaleslike tasmanite(Permian.AustralialJurassicAlaska)consists almostentirelyof thesealgalremains(cysts).They Cretaceous, present in manyotheroil shales,asin the LowerToarcianshalesof the Paris are Basin.France,or in the Lower Silu an shales,alsocontainingGraptolites,from Algeria. Rocksconsistingmainlyof a singletype of organismare anomalousfrom a rockscannotbe point of view.The organicmatterof theseanomalous geological of kerogensusedso far in this book, readilycomparedwith the classification which has been definedon amorphouskerogen.For instance.Ma{well et al. (1968)havestudiedsuchanomalousorganicmatter (i,e., the restingstageof to the sourcematerialof coorongiteand Botryococcus braunii) c
q .- l C o m p L \ \ l t i u n , )O t rglnic Ma cr
1
257
Ker.!€. trom . r shoe r r o i e v o! n o n p o r h s o i o m o r p h o !kse r o g e n (sl , n , m ) Pr nc po f,e d of keroqei5 i e d c n o c c r D o nz d o n p o r i r o i h u m r c c o o l
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rclativelvvariable.The atomicO/C ratio rangesfrom 0.02to 0.20.Nitrogenis muchlessabundantandalsovariesa greatdeal.The atomicN/C ratiorangesfrom 0.5x 10 I ro 5.llx 10 2. A comparisonof the elementalcompositionof kerogenfrom oil shaleswith organicmatterof petroleumsourcerocksshowsthat, accordingto the classification previouslyestablished. the amorphous sapropelic organicmatterof oil shales usuallybelongsto eithertvpeI or typeIL Thereis no oil shalerelatedto typeIIL
25lt
Vaiue Economic withPotential A Kerogen-Rich Sediment Oil Shales:
of type-llI organicmaterial.derivedfrom higherplants. are Concentrations coal. shalesaccompanying usuallyclassifiedas humic coal, or carbonaceous Furthermore.this type of materialgenerallyyieldsa largeamountof gasupon pyrolvsis,but only a limitedamountof oil. suggeststhat type-I kerogen present in the Infrared spectrophotomctry Keroseneshales,GreenRiver shales.torbanite,and bogheads.containslarge amountsof Iong aliphatic chains and little polyaromaticmaterial. This is confirmedby analysisof bitumenextractedfrom Green River samples'which particularlythoseof high containa Iargeproportionof normaland isoalkanes, presentin lrati shales' kerogen molecularweight. On the other hand, type-ll (Messel the RhineValley.Lower shale in oil shales kukersite,andvariousother westernEurope) from Toarcian shales Lower Silurianshalesfrom Algeria, in turn' is This' and naphthenic. aromatic material. containsmore cyclic ofthe naturalbitumenthatincludesa largeamount confirmedby the composition molecules of polycyclicnaphthenicand naphthenoaromatic andstructureof oil shalekerogenmay The reasonfor thedifferentcomposition be found in the differentorganismscontributingto the shales However,the mayalsobe of at the timeof deposition versuspreservation extentof degradation would eliminatemostof the easily importance.Selectivemicrobialdegradation compoundsand preservemainlyIipids.Theselipids.enrichedby hydrolyzable to type l. bacteriallipids.would resultin kerogencomparable
9.4 Conditionsof Deposition in which oil shalesare depositedhavebeen The main geologicalenvironments (1967) and are listedbelow: reviewedby Duncan particularly those of tectonicorigin formed during a) Large lake basins, theseoil shalesare marlsor even orogenicmountainbuilding.Mineralogically. mayincludevolcanictuffsand associated sediments limestones. The argillaceous in tbrmation. The mainoil shalesdeposited River minerals, as in the Green saline suchan environmentarethc GreenRivershalesof Eoceneagein Colorado.Utah basin' and Wyoming(up to 600 m thick), the Triassicbedsin the Stanleyville age.New Brunswick.Canada. Zairc and the Albert Shalesof Mississippian in thefirstplaceIargestableplatformswithrather represent b\ Theshalktwseas thin depositsof oil shales(a tew m to a few tensof m thick)extendingsomerimes of squarekilometers.The mineralphaseis mostly over hundredsof thousands silicaand clay minerals,but carbonatemay also be present.There are widespreaddepositsof black shalesof this type in severalgeologicalsystems: Cambrianof northernSiberiaand northernEurope,Silurianof North Africa' Permianof southernBrazil, Uruguayand Argentina,Jurassic(Toarcianand Kimmeridgian)of westernEurope,etc. Shallowsea depositsalso include those formed in subsidinggeosynclinal buriedto greater basins.However,suchdepositsusuallyhavebeensubsequently depth.and may evenhavebeenfolded.As a result,their oil potentialhasbeen
9.6Pvrolvsis of Oil Shales
259
partlyconsumed througha naturalevolution,andthe remainingorganrcmaterial resultsonlyin_alowergradeoil shale.Only thegeologically you-nger ones,sucnas theMioceneshalesof Sicily,Italy, andCalifornla,ha"ve kepi a suiftcrent potential to be classified amongthe richeroil shales. c\ S.malllokes,bogsund lagoonsassociated with swampsresultin oil shale , deposrrs associated with coalmeasures. This situationis met in permianbedsof westernEurope (St. Hilaire. France)and in Tertiary bedsof China (Fushun, Manchuria).whereoil shaledepositsoverliecoalbeds. A typicalcharacteristic of thevarioustypesof oil shalesisa vervfinelamination of thin (lessthan 1 mm) alternaringlayeriof organicmarrerani minerals. This laminationwitnesses quietsedimentation, where-the mineralsareeitherprecipi_ tated from solution (carbonates)or transportedas very fine detritus (clay minerals.silt)- It suggests a succession of ieasonalor other periodicevents. Lamination also proves the absenceof benthos.A situationof this kind represents a confinedphysicochemical environment,wherethe decayof part of the organicmatter usesup oxygento produceCO]. Thus organlc matter ls preserved from reworkingby benthicfaunaandfrom ierobic miciobialdegrada_ tlon.
9.5 Oil ShaleDensity Densityof the organicmatcrialis low comparedwith density of the mineral grains..The specificgraviryof concentrated kerogenis usualiycu. 0.95 1.05. I heretorc.orl shalerichnessmay be evaluatedby usingdensiiy,provided the mrneraicomposition doesnot changesignificantly within-thesectionconsrdered. Stanfieldet al. (l96tl). from the US bur"ou of Min.r. taue sno*n a gooct correlationbetweenincreasing oil yield and decreasing formationdensityii the GreenRiver shales. Well logspermit in situ evaluationof oil shales(Tixier and Curtis. 1967).A densitylog run in the Piceance basin.Colorado,allowsprecliction of oil yieldirom thc modificdFisherassay. in the 0 40 gallonsp., t ,ung.. with a maxrmumerror mishr ais()be i,f ,,r,meint"r.,,. n. rn increasing lt_, flnic-loeei11 K L r '^, r gli']:l::: ccno n l e n rt \ r e l l e c t c dh r a n i n c r c a s i ntgr a n s itti m e .
9.6 Pyrolysisof Oil Shales The technologyot git s!3t9 processing is basedupon pvrolysisof the rock at a temperaturearound500.C. The fundamentalaspectsof kerogen degradation upon heatingin industrialprocesses are the sameis thosedescr-ibed in"partII in respectto experimentalevolution of kerogen in the laboratory, by using thermogravimetric and relatedtechniques.The variousbondsof it. [".og"i macromolecule are broken, liberatingsmall moleculesof liquid and gase"ous hydrocarbons, and alsonitrogen,sulturand o*yg"n.ornpnrn,js.However,due
260
withPotential Economic Value Oil Shales: A Kerogen-Rich Sediment
the kineticsof theexperimeotal to the shalequicklyreachinga hightemperature, and industrialreactionsare somewhatdifferentfrom thoseunder naturalsubsurfaceconditions.In particularthe industrialproductsincludeusuallya large proportionof olefins.sulfurand nitrogencompoundsin the liquid phase;gases may containabundanthydrogensulfideand ammonia.The standardprocedure for laboratoryassays of oil shalesis the modifiedFischerassay. havebeenused(Burger.1973): Variousretortingprocesses process. a) Externalheatingthroughthe wall of the retort.The Pumpherston fbr a century; it was was of most techniques usedin Scotlandsince1860, theorigin (Scotland. France. Spain. with various amendments widely used in Europe of the retortingunitswerelow, and energeticbalances Sweden).The capacities werepoor. b) Combustioninsidethe retortingunit resultsin a betterenergeticbalance, is the low calorificvalueof the gasdilutedby nitrogenand but the disadvantage comhustionproducts.Valorizationof hydrogensulfideandammoniais difficult. andChina(Fushun). Thistechniqueis widelyusedin the USSR(Pintschprocess) In the USA. the experimentalGas Combustionprocessof the US Bureauof unitsnow underinvestigation. Minesis the originof severalprocessing c) Anothermethodusedis the circulationof externallyheatedgasthroughthe especially if the residualcarbonofthe shale.The energetic balanceis satisfactory, retortedshaleis burntto recoveralsotheseresidualcalories.The gasproduccdis of highcalorificvalue.Thistechniquehasbeenusedin France(GrandeParoissc) and is now usedin Brazil(Petrosix). d) Heatingthe shaleby hot solidsis a new typeof processthat insuresa good encrgetichalanceand a high calorificvalueof the gas.The UTT processin the in the USSRuseshot shaleashesasa calorificvehicle.whereastheToscoprocess is more the technology USA usesceramicballs externallyheated.However. processes. complexthan for the other e) Finally.powderedoil shalesare usedin the USSR (Estonia)to burn and providecaloriesin electricplants.
9.7 Oil Yield; Compositionof ShaleOil of kerogenin oil shale The oil yield upon pyrolysisdependson thc abundance (usuallydeterminedbv the organiccarboncontentof the rock).on the natureof history,in kerogen,and on the naturalevolutionof the rock (dueto geological particularto depthof burial). The valuesof oil yieldsand the ratio of organic matter convertedto oil (computedon the basis of the amount of carbon c o n v e r t e da)r es h o w ni n T a b l e1 1 . 9 . l . Oil shalescontainingtype-l kerogen.with H/C atomicratio over 1.5usually of organicmatterinto shaleoil: GreenRivershales showthe highestconversion (Colorado)7lJ%;Keroseneshales(Australia)66%. Oil shalescontainingtypeII lowerH/C atomicratio,extendovera widerange kerogen.with a comparatively of organicmatterconversionratio: from only 26o/ain Swedento 66elcin USSR historyof kukersite.The naturalevolutionof organicmatterduringgeological
9.8 Oil ShaleDistributionandReserves
261
the sedimentmavinfluencethe oil yield of the shales.Burial due to subsidence, andthe associated elevationof temperature, mayhavealreadyconvertedpart of the organicmatter,evenif subsequent foldinganderosionhasbroughtthe shale up to surfaceagain. This is probably the causeof the comparativelylow conversionrate of Permianand Carboniferous oil shalesof westernEurope: Autun and St. Hilaire45-55%;Scotland56%: andlowerPaleozoic oil shalesof Sweden 267a. Shaleoil densitiesare usuallyhigherthan thoseof the averagecrudeoil: they rangefrom 0.88to 0.98.andthe mostfrequentvaluesare0.91{1.94.Oilsderived from tvpe-l kerogen,whichhasa highcontentof aliphaticchains,aresomewhat lightcr(oil from Keroseneshales0.89,torbanite0.88)thanoil derivedfrom typeII kerogen,whichhasa higherproportionof cyclicstructures (oil from kukersite 0.97.lrom the lowerToarcianshalesof westernEurope0.93-{.94).Viscosityand cloudpointsareof industrialimportancein respectto pipe-linetransportation of shaleoil. The cloudpoint is frequentlyrelatedto the contentof highmolecular wcightalkanes(paraffin).lt is particularlyhighfor shaleoil derivedfrom several tvpe-Ikerogens,like the GreenRiver shales(cloudpoint 25-30"C,viscosityca. 25 centistokes at 37.8'C). It is comparatively low for oil derivedfrom type-ll kerogen.which containslessheavyparaffins,for examplethe lower Toarcian s h a l eos f w e s t e r nE u r o p e( c l o u dp o i n t- l 5 ' C . v i r c o s i l 5v i e n t i s t o k east 3 7 . 8 . C ) . Distillationcurvesol shaleoil showa largeproportionof mediumfractionsthat distif below300'C: Green River shales30 to 10Vc.IowerToarcianof western Europe50%. This mediumdistillatecontainsabout30,50% olefins.which are virtually absent in natural crude oil. Another impo ant differenceis the abundanceof nitrogen-containing cycliccompounds.The nitrogencontentof shaleoil rangesfrom 0.5% in Keroseneshaleoil to 1.Bto2.1Voin GreenRiver shaleoil. Thcsefiguresare considerably higherthan thosemeasuredin natural crudeoils,wherethe averagevalueis 0.097o.An exceptionis the kukersiteoil thatcontainsonly0.1% of nitrogen.Sulfurcompounds arealsoabundantin many shaleoils.but the totalsulfurcontentis morecomparable to thevaluesmeasured in naturalcrudeoilsfrom manyproducingcountries.particularly from theMiddle t asl.
9.8 Oil ShaleDistributionand Reserves (TablesII.9.l andIL9.2) Lower Paleozoicoil shalesweremostlydepositedin the shallowseasbordering the Scandinavian-East Canadianprecambrianshield: central-eastern United StatesandCanada,Sweden,USSR(Estonia).Theyareonly moderatelyrich, as manyof them havebcensubjectedto a moderatethermalevolutionfor a very longperiodof time. However,kukersileand associated deposits,of Ordovician age, are amongthe richestoil shales.They are situatedbetweenTallinn and Leningrad(USSR)andconsistof relativelythin (2.5-3m) calcareous oil shaleat shallowdepth (5 100m). Kukersitecontainsup to 40% organicmatterwith a
262
Oil Shales:A Kerosen-Rich Sedimentwith PotentialEconomicValue
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withPotential Economic Value Oil Shales: A Kerogen-Rich Sediment
ratio of 66Vointo oil plusgas.They havebeenusedto produceshale conversion oil and chemicals,domesticand industrialgas,and electricityby burningthe of the areawould be 21 billeanerbedsin electricpowerplants.Total reserves lion t of oil shale,or approximately3.5 billion t of shaleoil. Other oil shale depositsare locatedin the Siberianpart of the USSR.
Tablc II.9.2. Approximateestimationof the main oil shale reserves.(After Burger, 1973,World Energy Conference, 1980.and others)
27r,000
North America USA Canada SouthAmerica: Brazil Northernand WesternEurope Ital), USSR(includingSiberia) Jordan Marocco Zafte China Thailand Othercountries
26.1,000 7,000 127.000 1.00t1 5.600 56.000 7,800 7,100 l6,000 4.100 2,000 ca. 2,000 Totalca. 500,000
Millionm' potentialoil
(SilurianandDevonian)oil shalesweredepositedin shallow MiddlePaleozoic season the large shelvescoveringNorth Africa (Libya and Algeria) and the easternandcentralpart of the UnitedStates.ln Africa. the richestbedsare the blackshaleslocatedin the transgressive lowerSilurian.In the UnitedStates,the The mainextensionof the Devonianblackshalesrepresentmostof the reserves. Alabama,and depositsis over Indiana,Kentuckyand Ohio, but Tennessee, otherstatescontributeto a total of 10billion t of potentialshaleoil. UpperPaleozoic oil shalesoccurwidelyoverthe Gondwanaarea.The world's secondlargestdepositis the Irati shaleof Permianagein southernBrazil.In the mainarea,nearSaoMateusdo Sul, Parana,the depositscomprisetwo horizons (respectively 3.20m thick with an oil yield of 97c, and 6.50m thick with an oil The total reservesare yield of 6.47c).separatedby 8.6 m of barrensediments. equivalents occurin Uruguayand estimated to be l2il billiont of oil. Stratigraphic Argcntlna. oil shales are located in Australia Other rich Permian-Carboniferous (Keroscncshales,tasmanite)andSouthAfrica (Ermelo).TasmaniteandErmelo to be remains shalecontainmostlyvellowbodiesolorganicmaterial,considered of algalcolonies.
q.8 Oil ShaleDistributionand Reserves
265
In the northernhemisphere manysmalldepositsof oil shale,oftenassociated with coal,occurin tectonicor post-orogenic basinsof Carboniferous or Pelmian age.alongthe hercynianbeltofwesternEuropeandtheAppalachians: Scotland, France,Spain,EasternCanada(Albert shale),EasternUnited States.Other depositsare locatedin the northwestern UnitedStates. TriassicIacustrine oil shalesareextensively reportedin CentralAfrica (Zaire), wherethev irreassociated with limestones andyolcanics. Their oil yield is high, and the total reservesare estimatedto be 16 billion t of potentialoil. Some Triassicoil shalesare alsoknownfrom Switzerland, AustriaandItaly. Jurassicblackshalesare widesrrread in westernEurooe.Thev includeseveral bedsof variousrichness from Lowerto UpperJurassic. The moit frequentis the LowerToarcian(Posidonia shales.or Liassice),whichoccursovera wideareain Francc,Luxemburgand Germany,and to a smallerextentin Spainand Great Britain. The oil contentis rather low (t4o/i). but the largearealdistribution makesit an importantpotentialreserve. Other Jurassicto Cretaceous oil shalesare known in northernand eastern Asia.wherethevare associated with coal.andin Alaska,wheremarinedeposits includetasmanite(with Tasmanites algae),cannelcoal,and a shalycoalnamed ''whaleblubber" rock. Low-gradeCretaceous oil shalesoccurin SouthCentralCanadaandwestern UnitedStates.An association of marineblackshaleswith phosnhorite andchert e x t e n do s y e rs e v e r acl o u n t r i eosf t h e M i d d l eE a s l( S v r i l .i s r a e l J. o r d a n ) . The main Tertiarl' oil shalesare the lacustrineGreen River shales,which constitutethc biggestreserveofthat typein the world.estimated to be morethan 250billion t of potentialoil. The depositof Eoceneageextendsover four main basins:the Piceance basin,Colorado,whichis considered to containtwo thirdsof the rcservesithe Uinta basin,Utah, wherethe GreenRiver shalesare outcropping alongthe borders.and in the GreenRiver basinand the Washakiebasin, Wyoming.In the Uinta basin.the GreenRivershalesareburiedto a greatdepth (up to 6000m) in the northernpart of the basin,wherethe shalesact assource rock for Altamont and othcr oil fields, Lacustrinedepositsof Tertiaryagealsoincludethe ParaibaValley(Taubate1'remembe) in Brazil.and the Aleksinacshalesin Yugoslavia. Othernonmarine oil shalesarc associated with coal bedsin China(Fushun,Manchuria).Several small depositsoccur in local basinsor grabenscausedby Tertiarv orogenic movementsin Europe,SouthAmcricaand WesternUnitedStates. Marinedepositsof Tertiaryblackshalesare knownfrom Sicily.ltaly, where the reserves are evaluatedat ca. 5 billion t of potentialoil. and from California and SouthernUSSR.
Oil Shales:A Kerogen-RichSedimentwith PotentialEconomicValue
SummaryandConclusion Oil shalescontainlargequantitiesof insolubleorgaric matter,kerJgen,whichyieldsoil upon pyrolysisat temperaturesof about 500'C. The rock containsno producibleoil, andusuallylittle extractablebitumen.The mineralftactionofthe rock mayincludeclay minerals.carbonatesand silica. The origin of the organicmatter is frequentlymarineor freshwateralgae,but other planktonic organismsand also bacterialbiomassmay contribute significantly.When comparedwith the classificationestablishedfor petroleumsourcerocks,kerogenofthe oil shalesbelongsto type I or II. Oil shaleshave been depositedin lake basins,shallowseas,bogsand lagoonsfrom late Precambrianto Tertiary. Shaleoil containsa significantproportionof unsaturatedolefinswhich are absentin natural crude oils. Sulfur and nitrogenare also abundantin shaleoil.
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Tissier,M. Oudin. J. L.: Characteristics of naturallyoccurringandpollutanthydrocarbons in marine sediments.In: Proc. 1973Joint Conf. Preventionand Control of Oil Spills. Am. Petr. Inst. 205-214.\973 Tissot,B.: Premidresdonn€essurlesm€canismes et la cin€tiquede la formationdu p€trole dansless6diments. Simulationd'un schemar6actionnel surordinateur.Rev. Inst.Fr. P6tr.24, 470-501(1969) Tissot,B.: Versl'€valuation quantitative du pdtroleform6danslesbassins s€dimentaires. RevueAssoc.Fr. Tech.Petr.222,27-31(1973) Tissot,B., Bessereau, G.: Geochimiedesgaznaturelset originedesgisements de gazen Europeoccidentale. Rev. Inst.Fr. P..tr.37,63-77(1982\ Tissot,B.. Vandenbroucke, M.: Geochemistry andpyrolysis of oil shales.In: Geochemistry and Chemistryof Oil Shales.Miknis,F. P., McKay,J. F. (eds.).Am. Chem.Soc. Symposium Series230, 1983.pp. l-11 Tissot. B.. Califet-Debyser. Y., Deroo, G., Oudin, J. L.: Origin and evolutionof hydrocarbons in EarlyToarcianshales.Am. Assoc.Petr.Geol.B\rll. 55,12,2177-2193 (1e71) Tissot.8.. Deroo. G., Espitalid.J.: Etude compar6ede l'€poquede formation et d expulsiondu p€troledans diversesprovincesg6ologiques. Proc. 9th World Petr. Congr.2, 159 169(1975) Tissot.B., Deroo,G., Herbin,J. P.: Organicmatterin Cretaceous sediments of theNorth Atlantic:cont butionto sedimentology andpaleogeography. In: DeepDrillingResults in the AtlanticOcean:ContinentalMarginsandPaleoenvironment. Talwian,M., Hay, W., Ryan,W. B. F. MauriceEwingSeries,3.Am. ceoph. Union, 1979,pp.362-374 Tissot,B., Deroo, G.. Hood, A.: Geochemical studyof the Uinta Basin:formationof petroleumfrom the GreenRiver formation.Geochim.Cosmochim.Acta 42, 1469 l48s (1978) Tissot,B., Durand,B., Espitali6,J., Combaz,A.: Influenceof the natureanddiagenesis of organicmatterin formationof petroleum.Am. Assoc.Petr.Geol.Bull. 58, 499 506 (1e74) Tissot,B., Espitali6,J.: L'evolutionthermiquede la matiereorganiquedess€diments: applicationd une simulationmathematique. Rev. Inst.Fr. P€tr.30,'143771(19'75) Tissot,B., Espitalie,J., Deroo,G., Tempere,C., Jonathan,D.: Originandmigrationof hydrocarbonsin the EasternSahara(Algeria). In: Advancesin OrganicGeochemistry 1973.Tissot.B., Bienner,F. (eds.).Paris:Technip,1974,pp.315-334 Tissot, B., Oudin, J. L., Pelet, R.r C tdres d'origineet d'€volutiondes p€troles. Applicationd l'€tudegdochimique desbassins s€dimentaires. In: Advancesin Organic Geochemistry1971.von Gaertner,H. R., Wehner,H. (eds.).London-NewYork: PergamonPress,1972,pp.113134 Tissot.B., Pelet,R.: Nouvellesdonn€essurlesmdcanismes de gendseet de migrationdu petrole,Simulation mathematique et application a la prospection. Proc.SthWorld Petr. C o n g r . 2 , 3 54 6 ( 1 9 7 1 ) Tissot.B. . Pelet,R., Roucach6, J., Combaz,A. I Utilisationdesalcanescommefossiles geochimiques indicateursdesenvironnements g€ologiques. In: Advancesin Organic Geochemistry 1975.Campos.R.. Goni, J. (eds.).Mad d, 1971,pp. 1\7-154 Tissot.8.. Demaison,G.. Masson,P., Delteil.J. R., Combaz,A. I Paleoenvrronmenr anq petroleumpotentialof MiddleCretaceous blackshalesin AtlanticBasins.Am. Assoc. Pet. Geol. Bull. 64, 2051-2063 (1980) f ixier.M. P., Curtis,M. R.: Oil shaleyieldpredicted from welllogs.Proc.7thWorld Petr. Congr.3, 7l!715 (1967) Tornabene, T. G.. Langworthy.T. A.: Diphytanyland dibiphytanyl glyceroletherlipids of methanogenic Archaebacteria. Science 203,5l-53 (1979)
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Trask,P. D., Wu, C. C.: Doespetroleumform in sediments at time of deposition.Am. (1930) Assoc.Petr. Geol. Bull. 14, 1451-1463 Treibs, A.: Chlorophyllund Hemindedvatein bituminrisenGesteinen,Erdolen und Erdwachsenund Asphalten.Ann. Chem. 510, 42-62 (1934) of a novelseriesof Trendel,J. M., Restle,A., Connan,J., Albrecht,P.: Identification (C2a-C27) terpenehydrocarbons in sediments andpetroleum.J. C. S. Chem. tetracyclic Commun.304 306(1982) Vallentyne,J. R.: Geochemistry of carbohydrates. In: OrganicGeochemistry. Breger,I. A. (ed.).Oxford:PergamonPress,1963,pp.465 502 Vandenbroucke, M.: Structureof kerogens asseenby investigations on solubleextracts. In: Kerogen.Durand,B. (ed.). Paris:Technip,1980,pp. 415443 Vandenbroucke, M., Albrecht. P.. Durand, B.: Geochemical studieson the organic matterfrom the DoualaBasin(Cameroon).IIl. Comparison with the Early Toarcian shales,ParisBasin, France.Geochim. Cosmochim.Acta 40, 1241-1249(1916) Vassoevich. (in N. B.: Terminologyusedfor designating stagesand stepsof lithogenesis Russian).In: Geologyand Geochemistry. Chaps.1-7. Leningrad:Gostoptekhizdat. 1957 Vassoevich. N. B.. Akramkhodzhaev, A. M., Geodekyan,A. A.: Principalzoneof oil formation.In: Advancesin OrganicGeochemistry 1973.Tissot,B., Bienner,F. (eds.). Paris:Technip,1974.pp. 309 314 Vassoevich, N. B., Korchagina,Yu. I., Lopatin,N. V., Chernyshev, V. V.: Principal phaseof oil formation:Moscow:Univ. Vestnik6, 3-37 (1969)(in Russian);English (1969) translation:Int. Geol. Rev. 1910,12,11,1216t1296 Vassoevich, N. B., Korchagina, Yu. L, Lopatin,N. V., Chernyshev, V. V., Tschernikow. K. A.: Die Hauptphase der Erddlbildung.Z. al]'gew. Geol. 15, 611622 (1969) Vassoevich, N. 8., Visotskiy,I. V., Guseva,A. N., Olenin,V. B.: Hydrocarbons in the (1967) sedimentary mantleof the earth.Proc.7th World Petr.Congr.2, 37-',1'5 Vitorovic, D.: Structureelucidationof kerogenby chemicalmethods.In: Kerogen. Durand,B. (ed.).Paris:Technip,1980,pp.301 338 Vitorovie.D., Djuricii, M. V.. Ilia, B.: New structuralinformationobtainedby stepwise oxidationof kerogenfrom the Aleksinac(Yugoslavia) shale.In: Advancesin Organic pp. 179-189 Geochemistry 1973.Tissot,8.. Bienner,F. (eds.).Paris:Technip,197,1, Vitorovii, D., Krsmanovii,V. D., Pfend, P.: Eine Untersuchung der Strukturdes AleksinacerOlschiefer-Kerogens mittels verschiedener chemischerMethoden.In: Advancesin OrganicGeochemistry 1975.Campos,R.. Goni, J. (eds.).Madrid,1977. pp.7l1 731 Vogler. E. A.. Hayes,J. M.: Carbonisotopiccompositions of carboxylgroupsof bio1979.Douglas,A. G.. synthesized fatty acids.In: Advancesin OrganicGeochemistry Maxwell,J. R. (eds.).1980.pp.697-104 Wakeham,S. G.: Synchronous Uuorescence spectroscopy and its applicationto indigenousandpetroleum-derived hydrocarbons in lacustrine sediments. Env. Sci.Technol. rr,212-216 (1977) Wakeham.S. C., Schaffner, polycyclic C., Giger,W.: Diagenetic aromatichydrocarbons in Recent Sediments:Structuralinformationobtainedby high performanceliquid chromatography. Inr Advancesin OrganicGeochemistry,1979.Douglas.A. G.. Maxwell.J. R. (eds.).Oxford:PergamonPress.1980.pp.353-363 Wakeham,S. G., Schaffner.C.. Giger,W.: Polycyclic aromatichydrocarbons in recent lakesediments I. Compounds havinganthropogenic oriSins.Geochim.Cosmochim. Acta 44, 40H13 (1980) wakeham.S. G.. Schaffner.C.. Giger,W.: Polycyclic in recent aromatichydrocarbons
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lake sediments- ll. Compoundsderivedfrom biogenicprecursorsduring early diagenesis. Geochim.Cosmochim. Acta 44,415429 (1980) Waples,D. W., Haug.P.. Welte.D. H.: Occurrence of regularC2.isoprenoidhydrocarbon in Tertiarysediments representing a lagoonal-type, salineenvironment. Geochim. Cosmochim.Acta 38,381 387(1974) Welte,D. H.: Nichtflrichtige Kohlenwasserstoffe in Kernproben desDevonsund Karbons der BohrungMiinsterland1. Forschr.Geol. Rheinl.Westfal.12,559-568(1964) Welte,D. H.: Kohlenwasserstoffgenese in Sedimentgesteinent Untersuchungen riberden thermischen Abbauvon Kerogenunterbesonderer Beriicksichtigung der rr-Paraffinbild u n g .G e o l .R u n d s c h5.5 - 1 ,1 3 1 - 1 4 4( 1 9 6 6 ) Welte, D. H.t Zur Entwicklungsgeschichte von Erddlen aufgrundgeochemisch-geologischerUntersuchungen. Erddl u. Kohle. Erdgas,Petrochem. 20,65 77 (196'1) Welte.D. H.: OrganicGeochemistrv ofcarbon.In: Handbookof Geochemistry. BerlinHeidelberg-New York: Springer.IIi1. 6L1-6L30.1969a Welte. D. H. : Determination of 1rC,/rrC isotoperatiosof individualhighcr ri-paraffins from differentpetroleums.In: Advancesin OrganicGeochemistry 1968.Schenck.P. A.. Havenaar.L (eds.).Oxford: PergamonPress.1969b,pp.269-277 Welte..D. I{.: Petroleumexplorationandorganicgeochemistr}. J. Geochem.Explor.l, |1 136(19'72\ Welte. D.: Grenzbedingungen fiir Kohlenwasserstoffbildung und -umbildung.Erdoel E r d g a sZ e i t s c h r . 9 21,1 3 1 1 5( 1 9 7 6 ) Welte. D. H.. Ebhardt. G.: Distributionof long chain r-paraffinsand fatty acidsin sediments from the PersianGulf. Geochim.Cosmochim. (1968) Acta 32, ,165-466 Welte.D. H., Waples.D.: Uber die Bevorzugung geradzahliger n-Alkanein Sedimentgesteinen. Naturwissenschaften 60, 51G517(1973) Westoll,T. S.: Sedimcntary rhythmsin coal-bearing strata.In: Coal and Coal-Bearing Strata.Murchison.D. G.. Westoll,T. S. (eds.).Edinburgh-London: Oliverand Boyd. pp. 71 103 196f1. Whitehead. E. V.: Molecularevidence for thebiogenesis ofpetroleumandnaturalgas. In: Proc.Symp.Hydrogeochem. Biogeochem. Washington: The ClarkeCo., 1973.Vol.II, pp. 158-211 World EnergyConferencer Surveyof EnergyResources, Munich,1980.357p Yen. T. F.: Presentstatusof the structureof petroleumheavyendsand its significance to varioustechnicalapplications. Am. Chem. Soc. Div. Petr. Chem. (Preprint).F. I02 ll4 (1q72) Ycn. T. F.. Erdman.G. J.. Pollack,S. S.: lnvestigation of rhe structureof petroleum asphaltenes by X-ray diffraction.Analyt. Chem.33, 15871594(1961) Youngblood.W. W., Blumer,M.: Polycyclic aromatichydrocarbons in the environmenl: homologous seriesin soilsand recentmarinesediments. Geochim.Cosmochim. Acta (1975) 39. 1303-13r1 Zobell.C. E.. Anderson.D. A.: Verticaldistributionof bacteriain marinesediments. A m . A s s .P e t r .G e o l .B u l l . 2 0 , 2 5 8 - 2 6(91 9 3 6 )
PartIII of Oil andGas The MisrationandAccumulation
Chapter1
An Introductionto MigrationandAccumulation of Oil andGas
Petroleumaccumulations aregenerallyfoundin relativelycoarse-grained porous andpermeablerocksthatcontainlittle or no insolubleorganicmatter.It is highly improbablethatthe hugequantitiesof petroleumfoundin theserockscouldhave originatedin them from solid organicmatterof which now no trace remarns. Rather, as discussedin previoussections,it appearsthal fluid petroleum compoundsare generatedin appreciablequantitiesonly throughgeothermal actionon highmolecularweightorganickerogenusuallyfoundin abundance only in fine-grained sedimentary rocksandthatusuallysomeinsolubleorganicresidue remainsin the rock at leastthroughthe oil-generating stage.Hence,it can be concludedthat the placeof originof oil andgasis normallynot identicalwith the locationswhereit is foundin economically producibleconditions,andthat it has hadto migrateto its presentreservoirs from its placeoforigin. Thismigrationof petroleum.and the subsequent formationof commercialaccumulations, will be d i s c u s s ci n d t h i sp a r to f t h eb o o k . The relcaseof petroleumcompounds from solidorganicparticles(kerogen)in sourcebedsand their transportwithin and throughthe capillariesand narrow pores of a fine-grainedsourcebed has been termed primary migrationby numerousworkers.The oil expelledfrom a sourcebed passesthroughwider poresol more permeableporousrock units.This is calledsecondary migration. Petroleumcompoundsmay migratethroughone ore more carrier bedswith similarpermeabilities and porositiesas reservoirrocks before trappedby an impermeable or a very low permeabilitybarrier.Oil and gasaccumulations are thus formed. Sincepracticallyall poresin the subsurface are water-saturated, movementof petroleumcompoundswithin the networkof capillaries and pores hasto takeplacein the presence of the aqueousporefluid. Suchmovementmay be due to activewater flow or may occurindependently of the aqueousphase, eitherby displacement or by diffusion.Theremay be a singlephase(oil andgas dissolvedin water) or a multiphase(separatewater and hydrocarbonphases) tluid system. The distinctionbetweenprimary and secondarymigrationwas initially not basedon the recognitionof different migrationprocesses, but only on its occurrence in poresof differentsizesandlithologyandpossiblvin a differentstate of distribution.The loss of hydrocarbonsout of a trap is frequentlvcalled dismigration. Petroleumcompoundsare generated from finelydisseminated organicmatter in sourcebeds.thus.the firstappearance of petroleumis alsoin a dispersed form. Processes of primaryandespecially secondary migration,whichfinallyleadto the
of Oil and Gas An Introductionto Migrationand Accumulation
294
SEA- LEVEI.
SECONDARY
I
t-=\
@\
\\>\
SECONOAR'I MIGRATION
ofprimary presentation schematic of oil andgasacculumations: Fig.UL1.l. Formation l: Initial evolution of basin stage in ttt" initial andadvanced ;'J ;;;;;;;ctation secondarl primary and of stage Advanced /1: migration ;d secondary ;;t;;i;rt.d of accumulation andformation mipration mechanisms must' therefore'encompass formationof petroleumaccumulations compounds petroleum dispersed concentrate " that 07 and gruui,i", oi gas and oil, tjie latter generallybetween irr"'tp*iir. porewatersThisiswhygas O.qe.rnr1 ut..6nsiderablyl6werthatthoseofsaline reservoirrocksof suitable wheri highs tttuttural ln io,ino ;;oJiy ;f;i;";b
n"1*r'i :il: @n"11"_"1',r.'1v 1':":"#:"f .::;;i:lilix',liili'ilJTiilT::i\ rocksuchasan evaPonteor a sna a structural is known as ;i u'e.olig"ur high' such as an anticline ;;til; and Pinch-outsof trapssuchassandlenses' i.iroLurn ,*p. oth'er typeJof 11-e.!s petroleum as stratigraphic L"r" p.t*."lfa and poioustocL units, are known andporosrtydetermlne in permeability_ traos.ln all thesesituations,the changes of a gasalcumulation A schematicpresentation ;J"i;;il;;;;'"iit"ol". Figure in given s.econoury.ig.utlJn and some oil trap types are ;.#;'il I I I . 1 . I. in detail' migrationarenot ye.tunderstood of primaryandsecondary Processes and relationship' permeability Basic data on pore geometry' poiotiti and is there Likewise' in uuii.d a"nsesourcerocksarerare ilil;;;;id;*uL.
Summary andConclusion
2g5
little information on movementand distributionof petroleumcompoundsinside the poresof sourcerocks.Therefore,it shoutdbi realizedthat ihe following discussionon petroleum migration is largely theoretical and should not bi considereddefinitive.The theoreticalconceptsshouldbe quantifiedto determine their effectson the migrationmechanismifor a better understanding of the mlgranonpnenomena.
SummaryandConclusion The releaseof petroleum compoundsfrom kerogen,and their transport within and through the capillariesand narrow poresof a fine-grainedsourcerock are defined as pdmary migration. The movementof petroleum after expulsionfron a sourcerock, through the wider poresof more permeableand porouscarrier and reservoirrocks, is calledsecondarymigration. Poresin the subsurfacearc normally water-saturatedand henceany movementof petrol€umcompoundstakesplacein the presenceof aqueouspore fluid. The specific gravitiesof gasand oil are considerablylower thall thoseof salinewaters,This is why gasand oil are found in structuralhighswherereservoirrocksof suitableporosity and permeabilityare coveredby denseand relativelyimpermeablecap rocks.
Chapter2 Physicochemical Aspectsof PrimaryMigration
2.1 Temperatureand Pressure Most petroleumand gasaccumulations are found betweenthe surfaceand to a depthof about6000to 7000m. The physicalandchemicalconditionsthatprevail in sourceandreservoirrockschangewith depthofburial.Mostpronounced is the increaseof temperatureandpressure, The increasein temperature with burialdepthis a consequence of the transfer of thermal energyfrom the interior of the earth to the surfacewhere it is dissipated.Differentgeothermalgradients("C km r.1are observed,depending upon the overall thermal conductivityof the rock strata.regionalheat flow conditions,and subsurfacewater movement.A world averagegeothermal gradientis considered to be 25"Ckm ' (LeeandUyeda,196-5). Generallystable partsof the earthcrustwith crystallinerockshavelower geothermalgradients than mobile orogeniczones,with the exceptionof "arc-trench-gaps" of active marginswheregradientsmay be aslow as l0'C km I. The \ariationin geothermal gradientsin sedimentarybasinsis typicallyin the rangeof 15"C km-' to 50 "C km r, althoughgradients aslow as5'C km r andashighasTT"Ckm I, and in specialsituationsup to 90"C km ', havebeenobserved.For example,a very l o w g r a d i e not f a b o u t5 " C k m ' w a sf o u n di n a 1 4 , 5 8 5 -wf ie l li n A n d r o sI s l a n di n -r the Bahamas(Levorsen.1954)anda gradientof 7b.9"Ckm wasrecordedin a well drilledin the upperRhine Grabenin southwestGermanynearthe Landau oilfield(Doeblet al.. 1974).In the Waliooilfieldof the SalawatiBasin.Irian Jaya (Indonesia)gradientsas high as 90"C km I havebeenreported(Redmondand Koesoemadinata, 1976). Temperatureprofiles,or graphicpresentations of temperatureversusdepth, show that geothermalgradientsare not alwayslinear, but that there are irregularitiescausedmainly by varialionsin thermalconductivityof different lithologicunits, by proximity to the surface,and by subsurfacewater flow. Examplesof suchtemperatureprofilesare givenin FiguresIII.2.1 and IIL2.2. The increase in temperature with depthis greaterfor higherheatflowsandlower thermalconductivities. Heatflow is theamountof heat(calories) flowingthrough a unit area(cm:)in a certaintime (seconds). Thus,heatflow is expressed in [cal cm 's-']. Thermalconductivity(i) is a physicalquantitythat definesthe amount of heatthatflowsthrougha specificmediumovera certaindistance duringagiven time increment.if there is a temperaturegradient.Thus,there are threemain factorscontrollinggeothcrmalgradients; a difference in primaryheatflux related to global tectonicphenomena,differencesin the thermalconductivityof the
1.1 Temperature and Pressure
297
0roundraler Stntilra0tyLittolo0t
m;ilF I i
l"'I
I
I
t '.aourFEr t -
i l *l " "
i
f:3.aourFER frLid3
i
f
-B ! n r F n d n . i h
9!v
I
I
I
1 e.*-""'
Frs- III.2.1. Example of sub_surface remperatureprotile. Ground \r,aterflow and 'llcrenrc:. i n . j h : l I u ] c o n d u c r r r i ro) f r o r : l t 1 p e . influence the temperature-depth ' . l , r r r , ' n \ h r (nM ( ) J i t i c d a f t e rK a p p c l m c ) e rl q. h l ,
!tratieraphiccolumn,and sub-surface fluiti flow. The primarygeothermalflux. ,ommonlvobservedin old. stablecratonrcparts of the crusttendsto be low. ancl rr \oung. orogenicareasand activerifting areas.tendsto be high (Lee and L v e d a .1 9 6 )- .5 Rock unitswith poor thermalconductivities act as barriersto heat flow. and :.rusea risein temperature whencomparedto av".ug"g.oai"nta.ihis phenum.rop trf saltdiapirs,especiay iittiy are covereo by shales. ,,:1i":,:iil:T:--on Kr,cK.satr r\ an e\tremelygoodthermal,conductor (,t : 11 14)andthermalenergy .\ !'asrrvtransmjttedfrom greater-depth towardthe surface.shalesarerelativery
tl1r,1al = 11,t11 lnaucrors(.i. 3_7)andconseque",rt,;;;;;i';;;svcannor De .:r\srpatcd to the surface
asrapidly.This causeisaitdiapin to cieaiegeothermal --rghs on theirto-ps. Averagethermalconductivities of a numUeioiinct t1,pes are : ' t e di n F i e u r eI I I . 2 . 3 Thermaiconductivity is not only affectedby mineralogical . composluon.but .i:o dependsro a greatextenton porosity u",f p"r..rliliiy. u"nj *f,"tf,". tt. Irrresare sat[ratedwith water or gas.Circulati,cn of watei hu, u u".u ,,rong subsurface femperatures,eirheru. u *r..ing or'.onhngagent. In _l5l:. zones, :l :r.rben risinghydrothermal_like water\ from d".p_i.ut.J iru.,ur".on.. :,rr be combinedwith the tectonicallyinducedprirnu.u frigt h.u, itux (Rhine i:aben.. CentralVikingGrabenofthe NorttrSea,etc.1. Ueu,i_nrf., Ou",o *u,.. .rr.ulationis now consideredto be a relatively..rn,iron f.uirr" ii sedimentarv
298
Aspectsof PrimaryMigration Physicochemical
EITNAP|}LATTl) I t I P E n A T U nPt n 0 f l t t ( rch!netic) 0f flfl.t satRI
T e m p e r a(t'uCr )e profileof wellSaar1 in SWGermany(Modified temperature Fig.III.2.2.Extrapolated 1974) Haenel. and afterKappelmeyer
basins.Thus it would not be restrictedto grabenzonesonly. Thisphenomenon might explain observedheat anomalieson top of structuraltraps. such as anticlines. recordedin wellsdrilledfor oil andgasarearound270' Thehighestremperatures to 280'C, asobsenedin two very deepwells(6500and 7800rn) in SouthTexas I and the Gulf coast.The geothermalgiadientin the 6500m well was42oCkm and in the 7800m well 35'C km-' (Landes.1967).
2.I Temperature and Pressure
299 Fig. tll.2.-1. Averagethermalconducti, vitiesof selected rock t)pes.(Aftcr Kappelmeverand Haenel.1974)
T-Doronit€
-
l0
A.hydrite
I
15
20
cal./cm's,deg
Judgingfrom oil occurrcnces all over the world. andfrom the datapresented abovc.it appearsthat mostoil sourcebedshavenot beencxposedto tcmperaturesmuchhigherthan 100"Cduringthe timeofgencrationandmigration.Light oils.condensates andgases.however.areprobablygenerated lrom sourcebedsat highertempcratures (15tI-180'C).whichin cxceptional casesmavrcachan uppcr limit of about 250"C.Temperaturcsprevailingduring secondarymigratronin carrierrocksand rcservoirrocksare similarto thoseencountcred in gencrrtion and primarl' migration.especiallyif sourccrocks and reservoirrocks are in JUXraposlnon. Another phvsicalparamcterincrcasing with depthis pressure.prcssurerises with increasingovcrburdenunder the gravityload of overlyingsediments. As long as the poresof the sedimcntarvcolumnare interconnectcd and ultimatcly opento the surfaceandthereis sufficientpermeability, the tluidsinsidethe porc spaceare undernormalhvdrostaticpressure. The gravitvloadcauses sediments to compactand to loseporosity.This rcductionin porositvis onlv possibleif a commensurate portion of the relativelyincompressiblc. aqueousporc fluid is expelledandallowedto moveawav.If therearc sedimentarv layerswithstronglv rcducedpermeabilitv.andif the gravityloadcontinuesto increase, movementof the porc lluid beneathsuchimpermeable layersis impeded.Consequentlv. thc pore fluid is confinedin the sedimentand compactionceasesor is at least retarded.Under suchconditions,previouslynormztlhydrostatic pressures in thc pore fluid increaseand the sedimentbecomesoverpressured. Ultimately. p(trostatic pressure (svnonvmous with lithostaticor geostatic pressure) is attained \\'henthe pore fluid carriesthe entire load of the overlvingwater-saturated sedimentcolumn.Interstitialfluid pressurcs in sedimcntary rockscan.thereforc. varv betweennormal hydrostaticand higherpressures. A plot of the normal hvdrostatic pressure gradientandthe petrostatic or geostatic pressure gradientis s h o w ni n F i g u r e1 1 1 . 2 . 4 .
300
Aspectsof Primary Migratior Physicochemical
( barandPSI) Pressure
gradients andpetrostatic Fig.IIL2.4. Plotof hydrostatic The hydrostaticpressuregradientis between I and 1 09 bar per 10 m (0.,133-d.47 psi ft 1).dependingon the salinityof the porewater'A representagradi;t is 2.3 bar per 10 m (1 psi ft .r)' however' petrostatictive auerage' in tireoverburdinbulk specificgravityare common,and geologicall-i" variations 6000 importantin somecontext(Chapmin.1973) At depthsbetween1000'and with watcr' pores saturated m. migratingpctroleumandgasmustmovethrough from rangingbetwcen100and 1400bar and at temperatures at fluid pressures about50'C to about250'C burial with increasing There is alsoa gencralincreasein pore water salinity rm-r and 250 mg l-'m ' I mg 70 depth. GradientsTor salinityvary beiween (Engelhardt.1973),but pore water salinitiesmay also varl' dependingon fluid convection.and geologicalconfiguraiitnJtogy.temperature,su-bsurface andmayreach rangefrom freshwaterto saltsaturation tion. P"oiewatir salinities r. comsandstones conditions' pressure Under normal valuescloseto 300g I is in shales water pore Morever, shales adjacent than monlyhavehighersalinities as bicarbonate and in sulfate enriched and geneiallydepietedin chloride Porewatersalinitiesare frequentlyabnormallylow in EnInpu..dto sandstones. hiphoressurezonesand in areasof meteoricwaterinflux
2.2 Compaction
301
2.2 Compaction 2.2.1 Compaction,Porosityof ClasticSedimentsand Abnormally H igh Pressures Compaction in sediments resultsin an increase in bulk densityandlossofporosity with increasing effectivepressure, temperature andtime.The rateofcompaction is largelygovernedby materialpropertiesof a sediment(both physiial and chemical)and by the rate at which liquid pore fluid can be expelied.Because compactioncausesfluid flow throughsedimentary rocks,it is commonlyconsi_ deredto be an importantfactorin petroleummigration.This kind of fluid flow. however.wouldbe restrictedin carbonate-type rocksandfrom thispointof view, carbonates and argillaceous sediments behavedifferently.The transportof fluid throughinterconnected poresof a rock is dependenton permeability,which vanesconsiderably, althoughno rock is absolutelyimpermeable. The dynamics of fluid flow is commonlydeterminedfrom Darcy,sLaw. Hubbert(1940)gavea correctmathematical expression of Darcy'sLaw, K- e A 6 A
a:
lt
-
ot
where. ,4 = crosssectionalareaofthe rockthroughwhichtheflow is measured (cm2), K : intrinsicpermeability(darcy), O : volumeflux per unit time (cmrs-1), 4 = d y n a m i cv i s c o s i loyf t h e f l u i d( c e n t i p o i s e ) , p = d e n s i t yo f t h e t l u i d( g c m ' ) . - 6 A = hydrodynamic gradientalongthe flow path,/. converielcein.geologicalcontext,permeabilityis usuallyexpressed in .For millidarcies(md = 10-r darcy)or evenmicrodarcies (ad : 10 6darcy).Darcy's law is valid for laminarflow, whereinertialforcesare negligiblecompared'to viscousforces.During viscousflow, thereis an interactionbetweenthe liouid movingthroughthe porousrock and the surfaceor the inner pore spaceof the rock. For clasticsediments,plots of porosityversusdepth show a more or less exponentialrelationship.There is initially a very rapid loss in porosiryat relativelyshallowsdepths,andwith furtherincreasing overburdenpressure, the rate of lossin porositydiminishes.Semilogarithmic plots of the depth-porosity relationship resultin a straightline, the slopeof whichvarieswithincertainlimits from basin to basin. This basic fact about compactionof sedimentswas recognizedby Athy (1930)and Hedberg(1926.1936).They derivedempirical equationsfor the porosity-depth relationshipof sediments, wherethe depthof burialor overburdenpressurewasplacedin the exponent.FigureIII.2.5 shows suchporosity-depth relationships, asmeasuredon samplesfrom threedifferent basins:Teftiary sedimentsof the Gulf coast(Dickinson,1953),Teftiary sedi-
Aspectsof Primar)'Migration Physicochemical
5
0.20
l 00 0 0
tl volume ot pores volum€
totol
rock
0 e p t h( m e t e rasn df e e)t
fromthreedillerent for samples asdetermined Dcpth-porosit) Frs.111.2.5. ':e.limenlsrelJttonship Venezuelal Eastern of sediment-s Tertiary coast; thc Culf ot lo"ri^, i"tti"t1 195'1) Levorsen' (Modified after of Oklahoma sediments Paleozoic
ments of Eastern Venezuela(Hedberg, 1936) and Paleozoicsedimentsof shownon a theoretical Oklahoma(Athy.1930)HubbertandRubey(1959)have basisthat the efttctivc compressivestressequalsthe maximumdifferential minusthe internalfluid df the overburdenpressure pressure. whichis composed plessure. ' little or no sandor Compactionmaybe delayedin thick shalesequences.with of porewaters escape the ('Chop*un.1972) ln suchsequences silt inteicalations maybuild pressures highfluid cannotkeeppacewith iubsidence'anclabnormally may escape of fluid Impedance shales. uo. e.oeciallvin the centralzoneof thick evaporttes impermeable as such seals aiso be causedby unusuallyeffective of enottr", rrnpn,tontcauseoi abnormallyhigh pressuresis the generation shales in organic-rich hydrocarbons weight methaneand other low molecular however.aretransitory,andindicatethat 1CedU.rg,1974).Abnormalpressures, pressures, therefore'occurmore Abnormal eouilibriim hasnot beenattained in older Mesozoicor than sequences sedimentary frequentlyin youngTertiary evenPaleozoicbasins. for porosityreduction asa primary-cause -ompressivestressis wiclelyaccepted gone undisputed' not have effects these but pressures. and abnormallyhigh cannotcause alone overburden by compression that Bradley(1975)haslrgued extra load is the Accordingly' depths present drilling at pressures abnormal transferredto the sedimentbelowthJseal and not to the interstitialpore fluid deviating sedimentPressurcs within the sealcd-offportionof a water-saturatcd temperature to Bradley( 1975).aremainlycausedby according from hyclrostatic.
2 . 2C o m p a i o n
30J
PSI
1cn3r9l
\a
E00 d e n s i t y{g/cmJ)
B :
.*" =l
5000
100
T e m p e r a t(u' C r )e T e m p e r a t(u" rF)e Fig.IIL2.6. Pressure-temperature-density diagram forwaterundergeological conditions. r,i5.C km r and onrhisdiagram geothermal are gradients of 18"C k; lip-:r:.q9:d 10'LKm ror hydrostatrca water.(AfterBarker,1972; ) pressured Magara,1974)
changes. Thisis in linewith observations andconsiderations by Barker(1972)and \lagara (1974), who discussthe effect of water expansionwith increising tcmperature,and its influenceon fluid pressuresin buried sediments.A pressure-temperature-density diagramfor water under geologicalconditions, accordingto (1972) and Magara (,1974),is givin in'Figur e 1II.2.6. _Barker \up_erimposed. this diagram are geothermalg.idi.nts of "1g.C km-r, -o_n 'C 15 km ' andJ6_'Ckm I for hydrostatically pressured water.Fromthisdiagram rt can be deducedthat for commongeothermalgradients,thereis a continuous :rpansionof watcr with increasingdepth. The water expansionis greaterat riqher geothermalgradientsas the lines for thesegradientsinterceprwarer
Aspectsof Primary Migration Physicochemical
304
Fig. I11.2.7.Relationship betweenspecificvolume of water and increasing depth of burial for different geothermalgradients. (After Magara.1974)
A
i - F7/ /
I jY
10000
0 e p t h( t e e t ) isodensitylines at shorterintervals.This relationshipbetweensPecificwater volume (i.e., water expansion)and increasingdepth of burial for different gradientsis ihown in FigureIII.2.7. Magara(197'1)pointedout that geothermal ihe e*pansionof waterdueto risingtemperatureis an additionalcausefor fluid The directionof fluid movementwouldbe from hot misrationin the sub-surface. to iold areas,from cleepto shallowand from a basincenterto the edges'Thus, fluicl flow follows the same generaldirectionsas fluid temoerature-intluced " n o r m a l -c o m p a c t i o n . o i e m e n t c a u s e db y hasalmost thatfor manyyearsthe effectandroleof temperature It is surprising itsspecific water. of behavior The compaction. whenconsidering beenneglected temperature-related' are all as a solvent' capacity volume. its viscosityand Bradley (1975)remarksthai porosityloss in nature is causedby a group of dependenton reactivity'temperature,surface interreiatedchemicalprocesses "lithification" It refers to all processes these calls pressure. He area. and whichbecomecemented'to wherebyporosityis reducedin sandstones processes recrystallize' which to limestones' and are compacted. ihales.which
1.2Compaction
305
2.2.2 Compactionof Carbonates In the contextof this discussion carbonates are referredto assedimentarv rocks containingmore than 50% by weightof carbonatemineralsand lessthan 507o detrital minerals(claysand quartz). Some carbonatesourcerocks may be composed almostentirelyout ofcarbonatematerial(lessthan5Zoclaymineials.l. Carbonatesare chemicallymore reactivethan silicatesand, hence.during compactionbehavedifferentlythan clasticsediments. The porosityreductionin clasticsedimentsis more stronglyinfluencedby mechanical and physicalrear_ rangementsof mineral grains than generallyencounteredin carbonatesor evaporites.Chemicalprocesses dominatethe porosityand permeabilityreduction of carbonates. The initial or primary porosityof fine-grainedcarbonatesedimentscorre_ spondsto porositesobservedin clasticsediments.There is experimentaland cmpiricalevidencethat at lower overburdenpressures, up to a depthof about 100m to 300m, compaction occursasin clasticsediments. With increasinsdeDth of burial, chemicaldiagenesis becomesmore important.It stabilizestlie rock fabric. mainll' due to solutionand cementationprocesses and is much more pronouncedin carbonates than in clasticsediments. Recrystallization of carbonate-typesourcerocks is an important processwith respectto petroleum generationand migration. With increasingburial, recrystallization convefis initially fine-grainedsedimentsinto coarse-grained rocks. In this way. finely disseminated bitumenand other foreignmaterialsuchasclaymineralsbecome concentrated at grainboundaries andin intergranular spaces. Thisis anrmporranr stepin bitumenconcentration in carbonate rocks,On the otherhand.it is known that in reservoirrocks.organicmaterialmay hinderchemicaldiagenesis of the mineralrock skcleton.It is not clearhow thisaffectssourcerocks. There is evidencefor very earlv lithificationof ccrtaincarbonatesediments. Ginsburg(1957)pointedout thatin carbonatcmudsmostof thewatermaybelost $ithin the first meter of burial. Hollmann (1962)describesthe underwater consolidation of limestones.Dcepll' buriedcarbonaterocksfrequentlytio not :how signsof advancedgravitational compaction,asit is demonstrated b1,intact macrofossils and microfossils. and sometimes by relativelyhighporosityvalues. Carbonaterocks.in additionto primarvporosity,may developa pronounced recondarvporositvalter depositionas a consequence of dissolutionprocesses. Secondary porosityresultsfrom modificationof primarvporosityand,therefore, thetwo frequentlycannotbe clearlydistinguished (Aoyagi,1973).It shouldalso bemcntionedherethatdolomitization porositvbecause oftenincreases themolar rolumeof CaMg(CO1)2 is smallerrhan rhevolumeof CaCOl.For a moredetailed treatmentol compactionand diagenesis of carbonatesediments,the readeris referrcdto Engelhardt'sbook (1977)TheOrigin of Sediments and Sedimentary Rocks. Finally,it shouldbe emphasized that thereare many argillaceous limestones t e . g . , m a r l s )a n d m a n y c a l c a r e o ussh a l e sT . h e s eh y b r i " ds e d i m e n t ns a v e a mineralogical compositionintermediatebetweenthoseof Durecarbonates and ilavs.Manvof thesehvbridsediments haveveryhighorganiC contentsandappear ro be petroleumsourcerocks(Hunt, 1967).
Aspectsof Primary Migratlon Physicochemical
and InternalSurfaceAreas 2.2.3 PoreDiameters
:,':'l::$:;""';"$'il:l;Hil':;: 3::i1i':.';T,:l'::i,'J"TJi:TL3'j,ifi scale is a very on i microscopic pore system "*m:ru';x;l*::*:,'ltLiil"T;ffi sediments.The sedimentary
:.""#'J'xil:"il1 ijff"?'ff gT* lu:m*tllil'li';'xxlii""i1",1 T:1'"'::,::!* *i[**m*"';'::;n:*:;*lT:T:1';:;i:l]'jx!:ii: 11"rury il:*:"H:il:]"ffi tsIf,':',1":'1"j; i',",'i',:;i:"'1":itT:::::
ijill...*;tili"t'ffi ;*t***::*'*':u.!1* in % shaleporositY r0
20
30
( barandPSr) fluid pressure 600
o 2s to
tool
a
200
0
25 !0
rooI
T e m p e r a t(u" C r)e depth of of variousphysicalparameterswith increasing Fig. I1I.2.8. Interrelationship sedlments burialfor shale{YPe
2.3Fluids
3o1
= 10 e m - 10 A) in a depthrangeof about2000m, and continueto decrease with increasingdepth. Pore diametersof this sizeare of specialinterestwith respectto petroleummigration.Because poresare usuallyflat at depth,it canbe assumedthat moleculessuchas asphaltenes exhibitineeffectivediametersof about50 A cannotmovethrougha pore sysremwith pleudo-porediametersof 50 A or smaller. In clasticsedimentsand to a greaterextentin carbonates, thereis a general tendencyfor individualgrainsizeto increase with increasing depthof burial.In clasticsediments, dueto both theseeffectsandthereductionin porespace,there is a distincttrend towarddiminishingspecificsurfaceareaswith depths.Shales havesurfaceareasfrom about10-60;' g- r betweendepthsof 500and3500m. In sandyshalesthesesurfaceareasare reducedand sandstones haveevensmaller specificsurfaceareasof about5 m2g-r {Dreier, 1968:Heling,19707. This conceptof porosityreductionand changes of pore diameterswith depth was originallydevelopedwhen studyingpure sandstones and shales.Source rocks,however,containingvariousamountsof organicmatter would show a much more complexdepth porosityrelationship.It is known that coal, for instance,containsa macro-and a microporesystem.Whereasthe macropore systemis relatedto depthof burial.the microporesystemis mainlyrelatedto the ranklevelofcoal.In the samesensetheconversion of solidkerogenparticlesinto liquidor gaseous compounds is maturity-related andcreatesporosity.Thisnewly createdporosityand the accompanying changes in specificsurfaceareasare not static,but dependon theexpulsionofhydrocarbons andthe reorganization ofthe kerogenstructure.Robin(1975)for instancehasmeasured internalsurfaceareas in kerogenfrom fine-grainedsediments.With increasingthermal evolution thevrangefrom lessthan 10mr g-rorganicmatterat 1000m to about35 m2g-rat :1000m depth.
2.3 Fluids 2.3.1 Stateand Behaviorof PoreWater Mineral surfaceshave attractivevan der Waals' forces that causephysical adsorptionof liquidsor gases.The extentof adsorptiondependsuponthe nature ofthe mineralsandofthe molecules beingadsorbed. andis a functionofpressure and temperature.Physicaladsorptionmay attract several monolayersof molecules to a surfacein contrastto chemicaladsorption(chemisorption) which accomodates onlyonemonolayer.Severalmonolayers of watermolecules maybe physicallyadsorpedon clay mineralsurfaces. This water is not only physically adsorbed,but there are hydrogenbondsbetweenthe electronegative oxygen atoms of the mineral surfacesand the water molecules.Water molecules themselves, if not vaporized,are also hydrogen-bonded to eachother. Water boundto claysurfaces hasa higherdegreeof orderingandmaybe comparedto ice crystals,and is immobilized.Ice hasa more openstructure,and consequently a lowerdensitythanliquidwater.Therefore.the specificvolumeof bountlwateron
308
Migration of Primary Aspects Ph)'sicochemical
claysshouldbe higherthan that of freeporewater(l-ow' 1976)With increasing becomesmallerandsmaller' overburdenanddicreasingporosity,porediameters The surfaceto volumeraiio of the poresis changedand freerater is expelled preferentiallv.The ratio of free water to bound water is thus continuously in bulk water' ieduceclalongwith a decrease At preseni.little is known about the state of water in sourcerock-type attachedto sedimentsat depth. Severalmonolayersof water may be firmly Clay conditions temperature and pressure capillary walls dependingon water of adsorbed (or more) monolayer\ -in.ruiogirt. considerthree to iive under su'rfaceconditionsto be a reasonableassumption With increasing thenetworkof boundice-likewatershouldcollapselt anclpressure. temperature of watershouldbe left in direct however.that abouttwo monolayers is expected. (Low' 1976)where petroleumisdepths at even nroximitv to clav minerals of the existence consequcncesof important ( pointed out 1975) !.n".ut.i. Dickqv an advanced itructuredpnr" tui"t on'primaryoil migrationin sourcerocksin to be about stageof compaction.lf a fiat porehetweenclayparliclcsts assumc..l r s l w a t e r{ a h r ) uot A ) o n e a c hs i d e ' r o u l dl e a v e J o ' A r n h e i q l r .t \ ( ) m o n o l a l e r o openings Suchporewidthsarecompatiblewith pseudo-pore little mobile-water. observedin shalesat about3000m depth. 2.3.2 ChangingChemicalCompositionof PetroleumCompounds andstructureof kerogen' thecomposition In ChaptersIl.'1andIL5, we discussed of this a consequence As anclits euolutionwith increasingdepth of burial petroleum ot varletv a pressure' and evolutionunder increasingtemperature and phvsical lumpounds.i. e.. bitumeiwith difTeringstructuresand.chemical the kerogen' of evolution -state.of the to properties.is generatedAccording and relative absolute in difTerent compounds iitfcrent typei of petrolcum different all thesc Furthermore' migration primary for quantitiesaie auaitat,te as documentedb-vthe by migiationprocesses-, Jo-fnuna. can bc transpuitecl asfoundin of compounds types same the contain that oils greri varietyof crule b i t u m e ni n s t t u . with Potentlalsource Bitumento organiccarbonratiosin shallowsediments depththisratro increasing per g With mg 30 to'10 around are rockcharacteristis reachesmaximaat about 150to 180mg per g or more Thereafter'the ratios decrcaseas the principalphaseof oil formation is passed Simultaneously' of the bitumenchanges throughoutthe phasesof oiiformation.the composition andto a lesserdegreeso does increases The a-mountof saturatedhydrocarbons (N' S' O compounds) Heterocompounds the amountoi aromatichydrocarbons. with maturation.Low molecularweighthydrocarbons in concentration decrease depth Beyondthe principalphaseof with increasing generated arepreferentiallv oil formation. when crackingbecomesthe most prominent transformation in average resultin a decrease reaction.mainlvgasis generatedThesechanges depthand increasing with molecularweigirianddiameterof bitumenmolecules with decreases also formed maturation.The averagepolaritvof the molecules The changed is behavior adsorption depthand.hence.theirwatersolubilityand
2.'1Possible Modesof PrimaryMigration
309
amountsof low boiling point aromaticsincrease,and theseare more watersolublethan otherhydrocarbons of comparable molecularweight.Thesedifferenceshave to be consideredwhen discussing the principalmodesof primary migrationwith increasing depth. It is alsoimportantthat the kerogenitselfchanges. becomingmorearomatic with depthand more ordered.and havinglessand lessheteroatomic functional groups.
2.4 PossibleModesof PrimarvMisration The typeof petroleumcompounds thataretransported duringmigrationthrough the narrow pores and capillariesof water saturatedsourcerocks range in molecularweightfrom methane(CHo)(mol. wt. : 16)to suchheavycompounds suchas asphaltenes (mol. wt. up to 5000and more). Consequently. at normal pressure andtemperature. thesecompounds maybe gaseous, liquidor solid.The effectivemoleculardiameterof petroleumcompoundsrangefrom 3.8 A of CH1 to about5(f 100A in asphaltenes. A numberof relevantmoleculardiameters are l i s t e di n T a b l eI I I . 2 . 1 . The transportationof petroleumconstituents can occur in differentways. dependingon the stateof distributionin the sourcebed environment.From a theoreticalpoint of view, therecan be separateoil or gasphases.individualoil dropletsor gas bubbles.colloidaland micellarsolutions,or true molecular solutions.Eachof thesepossiblemodesof transportation is favoredby particular physicaland chemicalconditionswithin a sourcebed.The transferof massmav occureitherby bulk flow or by diffusion.Transportof petroleumconsrituents i; an),oneof the abovemodcsrequiressomekind of energv.The two mostcommon drivingforcespresentin sourcerocksto causebulk flow are temperatureand pressure.Thus the movementof petroleumcompoundsin bulk flow is either directlvor indirectlycausedby the presence of pressure gradients andprobablyto a lesserextentb,vtemperaturegradients.In additionto bulk flow, theremay be diffusion.Thc flux of petroleumconstituents by diffusionis proportionalto their Tablc IIL2.1. Approximate, effective molecular diameters of selected petroleum compoundsand some rcferencemolcculesin A Molecule
Effcctivediameterin A
Molecule
He H2
2.O 2.3 2.9 3.2
CH,I benzene /l-alkanes cyclo-hexane complex rrng structures asphaltenemolecules
Hro
coz N2
I nm : 10 e m :
l0 A; seealsoFigureIIl.2.!i.
Effective diametcr in A
3.8 4.8 5.,1 10- 30 50-100
310
Aspectsof PrimaryMigration Physicochemical
from concentrationgradient The direction of net transport due-to diffusion is to potential higher to lowei concentrationsor from regionsof higher chemical sub-surface in the re;ions of lower chemicalpotential For example,methane is teids to diffuse from warm regionsto cooler regions Transport by diffusion the smaller the molecule, also controlled by the molecularsizeof the diffusing by molecule the ea;ier its diffusion. Therefore the importance of transport to restricted practically is diffusionof hydrocarboncompoundsin the subsurface 10 carbon than less with species gaseoushydrocarbonsand other low molecular aroms. Two factors are very important for the transportationof petroleum and gas throughthe pore systemof iub-sudacerocks.Theseare whetherthe fluid system andwhethertheporewallsareoil-wetor water-wet' is moiophasicor polyphasic, fluids or a fluid and a gas, the boundary immiscible Wheneverthere are iwo properties unlike eitherfluid' This boundary phases have betweenthesedifferent on the contactsurface'is known as acts which force and the is calledan interface immisciblephasesthat between tension the interfacial It is interfacialtension. system The other fluid polyphasic of a behavior the flow determines largely irnfioitant parameter controlling fluid movementis the wettability of the surrounding p-orewalls with respectto the different fluid phases' e*oig ttt" possible modes of primary migration -there are two major categoriei: one which requires the ictive movement of pore waters' such as glob"ules and bubblesdrivin by water,micellaror molecularsolution;theother of pore water flow' suchas irodes of migrationcan take placeindependently diffusion and hydrocarbonphasemovement' 2.4.1 Hydrocarbon Globulesor Bubbles The transportation of oil and gas in form of individual globules or bubbles through the narrow, water-saturatedporesof compactedsourcerocks involves of capillarypressuresbecausethere are two immisciblephases the ph"enomenon and an interface.ns tong ai gloUulesor bubblesare of the samesizeor smaller pore than the pore diameters;theie is no restrictionto movement lf effective be must pressures particles, capillary gas oil or than openingsare smaller move' can bubbles globules or overcomebefore it Before a globui-eor bubble can be forced through a smallerpore diameter' phases' has to be distorted. The interfacial tension between the two fluid however,resiststhis distortion.The critical entry pressureor capillarypressureto pores force the bubblesor globulesof the nonwettingfluid phasethrough the At diameters must be overcome. iapillary pressureis higher for small pore
dvnecm-2 h"iori r,'t 'io" than240'000'000 ;;; ;i,;;;;;";;r;'i00 globule oil spherical to movea be required i2;d b.t wouldtheoretically of pore water and oil througha pore, assumtngan tnrerfacialiensionbetweenthe ln ' 30 dy"necm iHobson, 1973).However, due to the strain anisotropy a more like and-shaped .o*p*ting .ttute., the oil particlesshouldbe elongated a move such to needed spinlte ttra'na sphere.Thus, the pressuredifferential oirticle should be lower than calculatedabove'
2.,1Possible Modesof PrimaryMigration
311
High pressure differentials couldtheoretically be reachedlocallyin sourcebeds during advancedcatagenesis and metagenesis at great depthsof burial. The generationof low molecularweighthydrocarbons from the polymerickerogen causes a greatincrease in molarvolumes,andhighpressure centersor pocketsare createdinside sourcerocks. Furthermore,a large part of the pore spaceis probablyoccupiedby immobilewater.fixed on mineralsurfaces and the actual pore spaceis considerablvreduced.leavingonly a small volume in which generatedliquid or gaseous hydrocarbons canexpand.From this point of view. highpressure differentials asdiscussed above,couldbereachcd.Beyonda certain pressure,however the mechanicalstrengthof the rocks is surpassed.and fracturesdevelopwhichrelievethe pressureand inducemigration. It is concludedthat the movementof oil globulesin a waterphasecontributes littleto primarvmigrationof petroleumin dense-compacted sourcerockswithout rock fracturing.Becausegas has a somewhatgreatercapillarydisplacement prcssurethan oil. it is evenmoreunlikelvthat the transportof gasin the form of huhblep s l a r sr m aj o r r o l ed u r i n gp r i m a r ym i g r a t i o n .
2.4.2 Colloidaland VicallurSulurions The transportof petroleumcompoundsthroughthe networkof poresin source rocksascolloidalandmicellarsolutionshasbeensuggested by Baker( 1959,1967) and Meinschein(19-59).More recentlv. Cordell (1972, 1973\ re-evaluated colloidalsolutionsasa possiblemechanism for primarymigration.The colloidal sizerangeis generalll'considered to extendfrom aboutl0 to 10.000A. Colloids mayconsistof smallparticleshavingthesameindividualstructureor theymaybe tbrmedbv aggregates of smallermolecules of differenttypes.In additionto this, singlehigh molecularweightmolecules (e.g., polymers)maybe largeenoughto fall in the colloidsizerange.Colloidalparticlescan normallybe removedfrom solutionsby a membranefiltrationprocess. Polarorganicmolecules, havinghydrophobicandhydrophilicparts,mayform orderedaggregates calledmicelles.Thesemicellesmay contain100 or more molecules with the hydrophobicpartson the insideand thc hydrophilicpartson the outside.Crudeoils and sediments are knownto containpolar surfaceactive moleculessuch as naphthenicacids. Louis (1967)reportedmore than 17a naphthenicacidsin somecrudeoils of the USSR. Seifertand Howells(1969) reportedup to 2.57r carboxylicacidsin a Californiacrude oil, being highly interfacially activein the presence ofaqueousalkali.Someofthesesurfaceactive compounds aredefinitelyof primaryorigin(Seifert,1975),but it is suspected that nonspecificnaphthenicacids may be producedby oxidation of reservoired petroleum. The principal reason for advocatingmicellar and colloidal hydrocarbon solutionsasa primarymigrationmechanism is thatit seemsto be the only wayto solubilizepracticallywater-insoluble petroleumhydrocarbons in aqueouspore solutionsat relativelvlow temperatures. Baker(1962,1967)proposedsolubilization bv soap micellesas the main mechanismfor transportof petroleum compounds within reservoirs andtraps.Accordingto Baker(1962),the distribu-
3\2
Aspectsof PrimaryMigration Physicochemical
the tion of hydrocarbonsin petroleumreflectsvariationsin the kind and sizeof to suitable dissolvelonic micelles micellesin which hydroiarbonsselectively A' of about60 wouldhavemediandiameters solubilizepetroleumhydrocarbons comparison direct A A in diameter' 5000 about be would micelles while neutral pore betweenthe postulatedsizeof crude oil solubilizingmicellesand average would migration diametersin ihalesshows,however,that this kind of primary definitelybe limiteddown to a depthof about2000m (Fig IIl 2 8) Duringthis are availablein source polarcompounds depttranrttime interval,surface-active carbonates' argillaceous sourcebeds, ,o.k, in gr"u,", quantity.In calcareous preserved to be possibly could pores wherelarger calcireousshales, marlsor e"uen for micelle hindrance chemical greaterdepth. there would be anothersevere irrigration.'Manyorganicacids,which are most importantsurfaceactivecomtendencyto form insoluble have-a poindsfor natuialhldrocarbonsolubilization. Hence,in aqueouspore Mgr+ and Caz+ as saltswith divalentiations. such wouldbe broken'and micelles existing possibly solutionsrich in Ca:* andM92*' salts or magnesium calcium as insoluble piecipitated the organicacidswouldbe in shales and in pore diameters very small of barrier physicil In aidition to the micelle for obstacle yet another be may there hin,ltun"". additionto the cherni"ul chargedionic migration.There is probablyan electricalrepulsionof negatively. Finally' 1973) (Cordell' minerals of clay surfaces charged by negativeiy miielles it shoulclbe po'intedout thit manytimesmoresurfaceactivecompounds(50-to areneededin orderto solubilizean equal 100-fold),thit havenot beenobserved. amount of hydrocarbons.Furthermore,polar surfaceactivecompoundsare preferentially adsorbedon clayminerals. in colloidalandmicellar All in all. iolubilizationandtranspo of hydrocarbons form doesnot seemto be a very iikely processduringprimary migration lf' then only in relativelyshallowsourcerocks' it wereof importance, nevertheless. were during the initial phaseof petroleumformation lf micellarprocesses S-' in N-' be enriched also should respoisiblefor petioleumaciumulation,oils oils' Crude solutions micellar form whicharemorelikelyto O-nolarcompounds, howeuer.are mostlydepletedin thesepolar compoundsascomparedto source rock bitumen.Only ceriainshallowoilJ are rich in heavyandpolarcompounds'
2.4.3 MolecularSolution Molecularsolutionis anotherpossibletransportingmechanismof petroleum compoundsduring primary migration.The solubility of the very different petroleum.nInpo,indtof low andhighmolecularweight,ofsaturatedacycllcor is very differentin cyclic hydrocarbons.of aromaticsand heterocompounds petroleum selected some of solubilities aqueous lists III.2.2 *ater. Table watermore are hydrocarbons weight molecular general low In compounds. Low boilingaromatlcssuchas solu6lethan highmolecularweighthydrocarbons. toluene(solubility: Lenzenelsoluiility: 1.740ppm) and its simplederivative. 554ppm) are amongthe moit solublecommonpetroleumhydrocarbonsPolar than siuchasorganicacidsor alcoholsaremorewater-soluble heterocompounds, judging from Yet with the samecarbonnumber' hydrocarbon the corresponding
2.4Possible Modesof PrimaryMigration
313
the compositionof rock extractsand crudeoils, thesecompoundsare generally mobilizedin proportionsother than their solubilitiesin water would suggest. Thosecompounds. like saturatedhydrocarbons whichareamongtheieastwatersoluble,aregenerallythe moststronglvenrichedin crudeoils,ascomparedto the extractsobtainedfrom theirsourcesediments. Thisis especially truefor the most insolubleseriesof compoundson an equal-carbon-number basis,the n-alkanes, whichare notoriouslyenrichedin petroleumwhichhasnot beenbiodegraded. Whereasthe aboveargumentabouthydrocarbon solubilities wasreferringto pure compoundsin distilledwater,McAuliffe (1980)equilibratedwholc crude oils with sea water. and other salinewaters.He found the comoositions of hvdrocarbons dissolvedin thesewatersto be againvastlvdifferentfrom rhoseot the crudeoils. The solubilityin aqueoussolutionsof varioussaturatedand unsaturated hydrocarbons up to ninecarbonatomswasalsostudiedb1'McAuliffe(1966)and thatof long-chain n-alkanesby PeakeandHodgson( 1966)andFranks( 1966).In thescinvestigations it wasfoundthat a breakin the solubilitvof n -alkanes occurs aroundcarbonnumber 10. In 198UMcAulilfe contirmedand extendedthese f i n d i n g s( F i g .I l l . 2 . 9 ) . The solubilit)'dccreases with increasingcarbonnumber.but the hng-chain hvdrocarbon molecules are relativelymoresolublethanwouldbe expectedfrom an extrapolation of the solubilities ol the lowermembersof thc series.It hasbeen suggested that this unexpected solubilityof the highermembersmay be due to micelleformation(PeakeandHodgson,1966).The solubilities measured ranged from about100to lessthan0. I ppm in the Co-C,urangeandbelow0.01ppm in the C,, C.,,,range (McAuliffe, 1980).In caseof the highert-alkanes.milliporc filtration(50nm = 500A poresize)reducedthen -alkanecontentasmuchas97% of the accomodatcd n-alkanes(Peakeand Hodgson,1966). A comprehensive treatmentof the molecularsolutionof hydrocarbons during primarymigration,waspresented by Price(1973.1976).An increasing solubrlity of petroleumandpetroleumcompounds in distilledwateroccurswith increasing temperature. The solubilities increase graduallyto approximately 100.C,wherea moredrasticincrease is observed(Fig.III.2.10).The solubilities ofoils arein the rangeof severalup to 100ppm below100'C,but increase up to 100ppm or more between 100 and 200'C. Two different solubilityrates wcrc found in the temperaturerangefrom 25 to 180'C. The more insolublehigherboilingpoint compounds incrcasetheir solubilitywith increasing temperatureat a far greater ratethan the moresolublelowerboilingcompounds. Valuabledatawere alsopresented(Price.1973)on the solubilitvdifferences betwcenvariousclasses of petroleumcompounds at elevatedtemperature. lt was found,asexpected,thatthepresence of heteroatoms (N, S, O) on a hydrocarbon moleculedrasticallyincreases its solubilityin water.The solubilityof aromatic nucleiis muchmoreincreased with risingwatertemperature thanthesolubilityof cyclo alkanenuclei, which are only slightlymore solublethan iso-alkaneor n-alkanestructures.Salinityincreases werefound to considerably decrease the aqueoussolubilitiesof hydrocarbons. Price(1973)concludedfrom his findings that only a smallpercentageof the world's petroleumcould haveundergonc primarvmigrationby molecularsolution.Thisconclusion wasreachedmainlybv
314
Aspectsof PrimaryMigration Physicochemical
TableIII.2'2.Aqueoussolubilitiesofselectedpetroleumcompoundsat25"Cinppm (wt./wt.) Compound
Price( 1973,1976)
Methane Ethane Propane rl-Butane lr-Pentane n-Hexane fi-Heptane fi-Octane '1-Nonane
n-Paraffins
0.6 39.5 I 0.20 9.41 ! 0.04 2.21 ! 0.012 0 . 4 3 11 0.007 o.r22 ! lsoparaffins 0.2 19.1 1 2.3-Dimeth,vlbutane 0.2 t 1 3 . 0 2.2-Dimeth,vlbutane 0.05 1.41 L 2.,1-Dimethylpentane 0.02 5.25 t 2.3-Dimeth)'lpentane 0.02 t 1 . 1 4 2.2.4-TrimethylPentane lsobutane 1.0 41i.0 a lsopentane 0.02 ! 2.54 2 Methylhexane 0.0211 0.192! 3-MethYlhcPtane 0.011 0 . 1 1 5t 4-Methyloctane Bicycloparaffin 0.031 0 . 8 8 9I decane Bicyclo[,1.4.0] Napthoaromatic 2.'7 u8.9 i Indan Cycloparaffins 2.{) 160.0 ! Cyclopentane 1.0 41.8 1 Methylcyclopentanc 2.O1 .L 0.10 ,-Propvlcyclopentane I .l .3-Trimenthyl3 . ' 7 3X 0 . 1 7 cyclopentane 0.ll 66.5 a Cyclohexane o.2 16.0 i Methylcyclohexane .1-Dimcthyll.trans 0.17 3 . 1 t . 1j c,vclohcxane I .l .3-Trimeth,vl0.05 1;7'7! cvcl(])nexane Aromatics 1'740.0 t 17.0 Benzene 554.0 l 15.0 Toluene 2.o 134.0 ! ar-Xyiene '1.0 16'7.O a .r-Xylene 1.0 t 1 5 7 . 0 P-X]'lene 1,2.4-Trimethyl1.2 51.9 X benzene
McAuliffe( te66) 24.1 l1 60.4 r 62.4 t 61.4 X 38.5 ! 9.5 ! 2.93 ! 0.66 I 0.220 !
1.3 2.r 2.1 2.1) 1.2 O.20 0.06 o.o21
4.06 !
O.29
2.2 I 1t.12 48.9 ! 2.1. 41.8 l: 1.6
156.0 1 42.O t
9.t) 1.6
55.0 ! 1.1.0 !
2.3 1.2
1780 515
t 45 X 11
175
t u
57
! 1
2.,1Possitle Modesof primaryMigration
315
TableIIL2.2 (continued) Compound
Price11u73.1e76.;
1.2..1.5-Tetramerhyibenzene Ethylbenzene Isopropylbenzene Isobutylbenzene Thiophenc 2-Ethvlthiophene 2.7-Dimethvlquinolinc Indole Indoline
McAuliffe(1966)
3.48 1 0.28 131.0 a 1..1 152 .18.3 ! 1.2 5 0 10.1 t 0..1 S u l f u ra n dn r t r o g ecno m p o u n d s 3015 a t4 )
q
)
+
t8 t 5
1
t'195 ! 127 3558 ! t'l1 10,800r 700
10,ooo Lege.'d
1,OOO
ar 25"c
100
ar 25 0c
b 1 0 --
p
r
DrelhylnaohthaterEs
o.1 o.o1
o.oo16
12 16 20 24 Carbon Numb€r
Fig.IIL2.9. Solubilitics of normalalkanes andaromatics in water.(AfrerMcAuliffe. r9lJ0)
comparingn -alkanedistributionpatternsof differentcrudeoils with thesolubility pattern_of the rr,alkaneseries(Fig,_IIl2.9). asinvestigated bv McAuliffe (1966. 1980)F , r a n k s( 1 9 6 6 )a n d p r i c e ( 1 9 7 3 ) A . b r c a ki n ' t h e , f o i . * t , " n p f l r , n g a-alkane abundance in petroleumversuscarbonnumber(Fig.Ill.2. I I) was takcn i r sa n i n d i c u l i o nf o r t r a n s p o r t a t i ohny m o l e c u l a.ro t u r i o n i l t e p , , r r t , o n u t rh. n r e a R\ 4 t t h r e s p e c t o c a r b o nn u m h c rs h o u l dh e o a o n r " q r a n . ao f s p e c i f i c tempcrature andpressure conditions.lrregulart -alkanedistributionparrernsare
A'pecl\ of Primarl Migration Phvsrcochemrcal
316
oil
E
l0o
t71
75
T e m p e r a t(u" rCe) water of wholeandtoppcdcrudeoilsasa functionof increasing Fig. IIL2.l0. Solubilities (After Pricc.1973.1976) temperature.
u,
2l t.r
--BAGUEL MESSAOUDS.€. - _ . M E S S A O U DN . W . . .. ...EL GASSI
l0
-
15 16 17 18 19 20
Ca rb o nn u m b e r
nf Fig. IIL2.1l. Distributioo n-alkancs in four different crudeoils. (After Price. 1973. 1976)
of a flush-typemigration'whichwouldhavetakenplaceat eviclence considcred l o w c rt e m p c r r l u r e{sh e l o wl 2 l r ' C l f r o ml e r sm l l u r c ' ' r u r c eh e d ' . that carbondioxideandhydrocarbon Bral' and Foster(1980)havesuggested tocks are dissolvedin pore water in source gua*hiah are generatedtogether wouldmobilizethe Iiquid gases These dissolved !oncurrentlywih oil generation. waterexpelledduring any with rock the source leave could so they hydrocarbons in the light interpreted be can also experiments these compaction.However, dioxide carbon help of with the recovery oil enhanced gainld from resultsof in waters is of h1'drocarbons miscibility no increasecl case injection. In this to solubilizafion due are achieved production gains in hydrocarbon The observed. of phaseand decrease of carbondioxile in oil with swellingof the hydrocarbon permeabilin the relative viscosityanclinterfacialtension.This in turn increases
2.,1Possible Modesof PrimaryMigration
311
tbr oil and thus favorsthe movementof a hydrocarbonphasein an aqueous environment. KvenvoldenandClaypool(1980)presented an examplewherecarbondioxide from a submarineseepin NortonSound,Alaska,carrieda minoramountof gasand gasoline-range hydrocarbons.(The CO, in this case was supposedto originatefrom basementrocksand was believedto be of geothermalorigin.) Becauseindividualcyclicand branched-chain moleculesin the gasolinerange were much more abundantthan straight-chain hydrocarbons Kvenvoldenand Claypooldeducedthat the hydrocarbonmixture was immature.Thus carbon dioxide may have acted as an extractionagentwhen migratingthroughthe sedimentary column. The influenceof pressureon hydrocarbonsotubilityundergeological conditionshasnot beenstudiedin detailexceptfor methane(Bonham,1978).It canbc deduced,however.thatthereshouldbe an increase in solubilityamongthelighter gaseous hydrocarbons with increasing pressure. The subsequent releaseof these hydrocarbons from solutioncould be facilitatedby a pressuredrop between sourcerocksand rerervoirrocks. It doesnot seemunreasonable to assume thatmolecularsolutionin Dore$ aters is a potentialmechanismfor primary migration.This mechanism,however. seemsto functionbestfor the lighterhydrocarbon fractions,because theyarethe mostwater-soluble. The differentsolubilitybehaviorofn -aikanes with morethan l0 C-atomsin aqueoussolutions(Fig. IIL2.9) seemsto be associated with the formationof n-alkaneaggregates, not unlikeorderedmicelles.Suchaggregates, however,canbe filteredout by the narrowpores(100A andsmaller)prevailingin shalesbelow2000m. Finally,it hasto be remembered that in a typicalmedium crudeoil, asgivenby Bestougeff(1967),and asfoundsimilarlyin manypartsof the world, the distributionof hydrocarbonclasses (Fig. IIL2.12) is not at all as
TableIII.2.3. Comparison in compositionof lighthydrocarbons between crudeoil andcorresponding sourcerock extract.(After Martin et al.. 1 9 6 )3 Hvdrocarbon Pentane.n + iso Hexane.r] + iso Heptane.n + iso Cyclopentane Methylcyclopentane Ethylcyclopentane lsomersof dimethylcyclopentane Cyclohexane Methylcyclohexane Benzene Toluene
Crudeoil (vol.-7.) 14.4 18.4 25.9 0.9 0.6 15.3 3.8 1,2.6
o.2 1.2 100
Sourcerock (vol.-%)
2.9 3.6 5.6 0.2 1.5 0.'7 4.1 1.t) 11 . 5 10 58 100
Aspectsof Primary Migration Physicochemical
cl'-c2s c26-ca5ca6-c60 cs-cl' qt-c* ct1-ct7 ;ffi-ahms I
n-allanes
i I
.
-
i-allanes
V 75
't00% in a typicalmedium classes compound petroleum of various Fis.lll.2.l2. Distribution 1967) crideoil. (AfterBestougeff, presenl' solubilitydistributionof the compounds predicted "A;;;;i;*1;by the aqueous in water' there shouldbe ihe soiubilitiesof hydrocarbonclasses and andmuchfeweriso-alkanes unJ nupttti'"no-aromatics ;;;;;;i.;;;;io. tne on example an excellent Martinet al. (1g63)presented in crude_oil. n-_-a(anes oil in a crude of light hydrocarbors lf**pr".y 0.,;.en the aclualcomposition t he t o c o n t r a r l i i . c o r r e t p o n . . l i n s . s o urroccek ( T ; b l e l l l 2 l ) ;;;;;;J1. toluene and suchasbenzene hydrocarbons ."pJ.t"a "nrl.tt..nt in water-soluble
ir"" "h i;i;iii
Transportacompounds inthese i.zj' tn..tuato-ltisdePleted
to be an important iinn in u,lu"ou. solution. therefore,does not appear petrolii,lion in primarymigrationoof mechanism to.ur"an-'ainlyon the concept m on discussron The foregoing be transportedwhen a given ttrai'tvarn.'uruon,dissolvedin pore waterswould movingbv bulk flow' activelv contuining*ater would be ;;1;'.;il;;;rion solutionwouldbe molecular in Anotherpossibilityof transporto?hydrocarbons diffusion. 2.4.4 Diffusion-ControlledProcesses
i e ' thereis no the aqueousphasewould be stationa.ry' In diffusionprocesses moleculeswould move needfor flow of water,whereasdissolvedhydrocarbon to "t t'i*r'.r conceniration(high chemicalpotential) il:,#;;;;;;;;.'ii"..t lerms' general in Thus, olacesof lower concentra'on1r6wchemicalpotential). gasaccumulations destrovs and disseminates tirat ;';;;;.;tt Siff*io"n ;;ft r a t h e rt h a nc o n l r i b u t i ntgo a c c u m u l a t i opnr o c e s s e s '
2.4 Possible Modesof PrimaryMigration
319
TableIll.2 4. EffectivediffusioncoefficientsD (cmrs-r) for diffusionof a-alkanesthrough tbewater-saturated porespaceof sourcerock-typeshales.basedon in-situmeasuremenrs in shallowcoreholes.(After Leythaeuser et al., l9g2)
CH. C,H^ C,H, rso-Crll n-C.H$ n-Cl\Hr: n-C6Hrr n CiHr6 n,C l:l D 2 . l 2 x l 0 " 1 . l t x l 0 6 5 . 7 7 x l 0I 3 . 7 5 x 1 0 ' t . 0 l x l 0 r 1 . 5 7 x 1 0 ' l t 2 0 x l 0 " . 1 . 3 t x l 0 r 608x10,
In specialgeologicalsettings,however,Leythaeuser et al. (19g0.19g2)have . shownthat diffusionof dissolvedhvdrocarbonmoleculesin fact represenrs an effectiveprocessfor primarymigrationof gasbut not for oil. Basei on newly determinedeffectivediffusioncoefficients for low molecularweighthydrocar_ bons(C1 C,u).whicharelistedin TableI1I.2.4.a modelwassetup t; calculatc the amount.ofhvdrocarbons escapingbv ditfusiontrom sourcerocis with geologic time. The role of diffusionwasconceptualized as that of an initial proiessfor hydrocarbontransportationwithin the sourcerock unit itself. Transportation dlstancesare thought to be short and alwaysfollowing local concentration gradients. As shownschematically in FigurcIIL2.13.suchionditionsexrstwithin the sourcerock near the contactto over- or underlyingporouscarrierrocks, aroundfracturesor faults,or in proximityto interberjded iiltrtone:i. E€
Ee
p ffi fi
gran sourcerock * Frschrcs C*rir rocr d fan $tstone
Predornhant cas Migratbn lrlechfiism :
r+ +
by diflueidl oth€f
Fig. IIL2.13. Schematicillustrationof rhe most likely sites and directionof light hydrocarbon diffusionin shalesourcerocks: ,,l. towardcontactwith carrierrocks. B. towardfracturezone, Cr towardfault, D. towardrnterbedded srlt\tonelens Within-carrier rock itself a transportmechanismother than diffusion /E) prevails. (Modifiedafter Leythaeuser et al.. 1982)
320
Aspectsof PrimaryMigration Physicochemical
haveclearlyshownthatmoleculardiffusionby itself Thesemodelcalculations ofsuchquantitiesof gaswith geologictime,which canaccountfor transportation fields.For example,it wascalculaledthat a in the order of commercial-size are gas-prone Jurassic shalesourcerock 1000km2x 200m volumeof a high-mature transportin 540,000yearsthe unit in WesternCanadahasyieldedby diffusivecumulativeamountof 10' kg (1.5 x 10'STP m') of methane.The samestudy of the diffusioncoefficientvalue revealedthat. due to an exponentialdecrease fractionationeffects with increasingmolecularcarbonnumber,compositional throughsourcerocks. occurduringinitial periodsof diffusionof hydrocarbons This resultsin a changein reservoirgas compositionwith geologictime. In in a particular.it wasshownthat the compositionof the gaswhichaccumulates reservoirby transportationfrom a sourcerock. wherediffusionwasthe initial processof primary migration,is controlledby three principalfactors:The concentrationof initially generatedhydrocarbonsin the sourcerock. their in diffusionrates.andthe sourcerock thickness. relativedifferences
2.4.5 HydrocarbonPhaseMigratnn primarymigrationby molecularsolutionin an aqueouspore When considering fluid or by a separateoil or gasphase,problemsof pore diametersand absolute cannotbe neglected.The limitingfactorsand and relativerock permeabilities and wettability.for the i.e., interfacialtension.capillarypressures restrictions, globulesand bubbles movementof large molecules,molecularaggregates, sourcerockswith narrowporethroatshavebeenreviewed throughfine-grained wouldnot occurif The mainobstacle,however,whichis highcapillarypressures, to existin the sourcerock at the a continuousorganicphasecouldbe assumed stageof oil expulsion. The possiblemovementof petroleum.asa separate.moreor lesscontinuous by Dickey (1975).He oil phaseduring primarymigration,hasbeen discussed pointedout that the saturationof oil in termsof totalporefluid in maturesource thatmostof thepore rocksmaybecomeverylarge,ashighas507o,if it is assumed permeabilityand of relative wateris structuredandeffectivelysolid.The concept whenthe unimportant equilibriumsaturationwith respectto eitherfluid becomes a movable as cannot be considered pore walls and water is largelyfixed to the than waterrather are oil-wet surfaces phase.In addition.if someof the interior Finelower oil saturations phase at even wet, oil can move as a continuous coated with particles part clay with of their and grainedrocksrich in kerogen surfaces interior oil-wettable some to contain organicmattershouldbe expected belowwhichno Therefore.the equilibriumsaturationin sourcerock-typeshales, reservolr known under would be expected lower than may be oil can flow, of permit primary migration would that conditions conditions.Sub-surface and petroleumasa continuousoil phase,shouldbe highbitumenconcentrations iittle mobilewaterin a potentialsourcerock.Hence,deeplyburiedsourcebedsat ' ofoilformationandat a relativelyhighdegreeof compaction themaximumphase oil continuous offer the greatestopportunityfor primarymigrationasa separate
2.4Possible Modesof PrimaryMigration
321
phase.The drivingforce for fluid movementundersuchconditionswould be a pressuregradient. Meissner(1978)followingthis line of thought,presentedan examplefor oil phasemigrationfrom the Willistonbasin.He foundwhenstudyingthi organicrich Bakken shalein the Willistonbasinthat at peak hydrocarbongeneranon stagea continuous oil phasewasin existence in thatshale.Thiswasinferredfrom the fact that water wasneverrecoveredduringdrill stemtestingof the mature Bakkenshale.Furthermore,the matureshalewasove.p.essured and electrical r e s i si vt i t yw a se x t r e m e l h vigh. A similar alternativehypothesisvisualizeskerogen as forming a three_ dimensional organicnetworkin the rock,whichis wettablewith oil andfunctions as the transportavenue(McAuliffe. 1980).Effectsof this kind. combinedwith pressure-generating processesinside a sourcerock, would certainlvsreatlv facilitateprimarvmigrationasa separateoil phase. A comparableeffect would be achievedin an1,kind of sourcerock bv microfracturing dueto overpressuring porefluids.The microfracof theenclosed tureswouldthenfunctionasconducrors. The basicidea.originallrintroducedbv S n a r s k r{ 1 4 6 2 )a n d T i s s o ta n d P e l e rt l q T l ) . i s r h a t a l a i g e i n c r e a . ei n p o r e pressuremay be sufficienteitherto overcomethe capillarypressureor evento exceedthe mechanical strengthof the rock andinducemicrocracking. The main causes for pressurebuild-upare (Durand,1982): - thermalexpansionof water (Barker, 1972), - specificvolumeincrease of organicmatterby generation of gaseous andliquid hydrocarbons frorn kerogen(Snarsky1962;Tissotand pelet.l97l). - partialtransferof the geostaticstressfield from the solid rock matrix to the enclosedpore fluids,resultingin an overallincreasein pore pressures. This transferis the resuhof conversionof part of the solid terogento liquid or gaseous compounds(du Rouchet,1981). The formationof microfractures by localcentersof highfluid or gaspressure is restrictedto relativelydeeplyburied,compacted, low permeabilityrockssuchas shalesor tight carbonates. Thus.this processis of similarimportancein source r o c k sw i t h d i f f e r e n ll i t h o l o g i e ist t h e y c o n t a i ns u f f i c i e nbr r g a n i cm a t r e rl o generateliquid or gaseous low molecularweightcompoundsandlorare brought to sufficienttemperaturefor thermalpressuring of water.They shouldalsobe impermeableenoughto allow pressurebuild-up.This mechanism.unlikemost others.doesnot necessarily demanda waterdrivefor hydrocarbon movement. Accordingto Snarsky( 1962),themechanical strengthof a rockis exceeded and fracturingoccursif the internalfluid pressure in a rockor in localpressurc cenrers insidethe poresis greaterthan by i factorof 1.,12ro 2.40ovei the hydrostatic pressurein the immediatesurroundings. Internalporepressures that exceedthe normal hydrostaticand even the petrostaticpressureare possible,whenever massive generationof gasandoil from kerogentakesplace.Thisprocess mayalso help to form abnormallyhigh-pressured shalesasdiscussed previously.Snarsky (1962)calculated that at a depthlevelof 3100m, the internalfluid pressurein a rock may rise to at least750 bar, which is higherby a factor of 2.5 than the hydrostaticpressureand higherby a factorof 1.1 than the petrostaticpressure
Physicochemical Aspectsof PrimaryMigration
322
of about at that depth. Tissot and Pelet (1971)report internalgas pressures "petrostatic pressure"of 440bar 540bar in a blockof shalethat waskeptundera during an experiment.Microfracturingand a subsequentpressurerelease study. resultedin this experimental theoverallvolumeexpansion of RecentlyUngereret al. (1983)havecalculated for its importance genesis and also evaluated organicmatterduringhydrocarbon primary migration.They concludedthat volumeexpansionof organicmatter aloneprobablyplaysa minor role in the expulsionof oil. The volumeexpansion was estimatednot to exceed15oloof the initial volume during catagenesis. Volume expansionmay, however.have an impo ant influenceat the end of when dry gas is formed. Under these and during metagenesis catagenesis of the rock matrixcanoccur. microfracluring circumstances pressure release, subsequent build-up,microfracturing, It is clearthatpressure process.The expansionof fluid or gas,and finallytransport.is a discontinuous processmustbe repeatedmany timesin the sourcerock unit duringgeological meaningfulgasor oil movement.After a microfractime in order to accomplish ture has been opened,the enclosedfluid or gasexpandsand the fractureis then buildsup A new pressure immediatelyhealcdby the overburdenpressure. andthe processbeginsagain(Fig. IIL2.1a). af ter hydrocarbon generalion
.@
) -
E Ll
4'F.
pores completely saluraled wath water pores invaded by hydrocarbons
&E -./ -
organic maltet fraclwe direction of HC movement
rockduringhydrocarinvasion in asource andhydrocarbon Fig.III.2.l1. Microfracturing (AfterUngerer et al.. l9ll3) fromkerogen. bongeneration
A pressurebuild-up inside the pore network of a sourcerock must not are not too of the rock. If capillarypressures causemicrofracturing necessarily high either becausepore throatsare not too narrow, or becausethere is a networkof kerogen,then continuousorganicphaseand/ora three-dimensional gasandoil canslowlybe drivenout ofthe sourcerockswithoutrockfracturing.In movementof gasandoil would be a continuous sucha casethe pressure-driven it is logicalto acceptpressure-driven process. From the foregoingconsiderations
Summaryand Conclusion
hvdrocarbon phasemovementwithoutor with microfracturing as an important mechanism of primarymigration.Transportation ofpetroleumiompoundsunder theseconditiols may take placein continuousgasor oil phasesincl therefore chromatographic effectshaveto be expected.We will seein the nextchapterthat geologicaland geochemical constraints practicallysingleout a pressure_driven hvdrocarbon phasemovementasthe dominatingprimaiymigrationmechanism.
Surnmaryand Conclusion P€trolelm and ias accumulationsare found betweenthe surface and depth levelsof abou: 6000to 7000m and deeper.Thus, petroleumana g", rnortrnou"it rough po.es saturatedwith water, at fluid pressuresrangingbetween100bar and 14@ bar and at temperaturesftom about 50"C to about 2S0.C. Compactionin sedimentsresultsin ar increasein bulk densityand loss porosity of with increasingdepth, temperatureand time. At shallowdepths there is rapid lossin increasing overburden pressure the iate of loss rn porosrty fj::::l:-T_1 :]]hor mrnlsnesgreat ty, Becausecompactioncausesfluid flow throughsedimentaryrocks ii commonlyconsideredasan important factor in petroleumirigration. This kind of :as ruto ltow. however, would be rest cted in carbonate-typeiocks due to early compiction. there is aho a marked regutar de"r"or" ,n por" IlIi..1T"; AlP"tt"_t cramete_rs. depths(belowj000 m) in detritaisediments they reachvalues _greater arounqtu to tJ A. whichis in the rangeof the dimensions of the molecules of certain petroleum constituents.Adsorbed, nonmobile water may make these pores even smaller. From a theoreticalpoint of view, primary rnigrationcan occur as discrere,more or . lesscontinuousoil or gasphases,individuai oil l;oplet, o. g", UutUi"r, lolloidal and micellarsolutions,or tlue molecularsolutions. Physicochemical, geochemicaland geologicalconsiderationsmake individual drop_ . lets and bubblesand colloidal and rnicellai solutionshighly unlikeiy aian effective meansof transportduring primary migration. Molecularioiution in iore watersdoes not seem uueasonableas potential mechanismfor primary migration, especiallyfor the lighter, most solublehydrocarbonfractions. il9"es not appearrobean importantmechanismin primarymigrarion ^" |.:"::tl:r:. ol orl. However, there are indicationsthat primary migration in molecular solutions playsan important role for gasin the form of diffusion-'controlled proceises. form of primary migration during the main'phasesot o ^^1.^l::1lrO"""nt lanO gas) rormatronseemsto be a pressure_driven, discretehydrocarbonphasemovemenr. "l l.r": molecularweighthydrqcarbonsftom polyrnerickerogencauses ]l:-9,.i:*t]"" an rncreaseIn specrhcvolumes.Furthermore,high fluid pressuresmay by ciused by a combinationofthe effectsof the.matexpansionof pore water, rapid biriil, anOparrial transfer of the geostaticstressfield from the solid rock mat;ix;o the enctoseO pore fluids ln this way pressurecentersare createdin a sourcerock that cause a more or less contlnuouspressure-driven hydrocarbonphasemovement.In relativelyimpermeable source rocks, these pressurecente$ may even induce microscalerock iracturing. Pressurebuild-up, microfracturing,subsequentpressurerelease,expansion ol fluid or
Aspectsof P mary Migration Physicochemical gas,and finally transportare a discontinuousprocess.This processmust be repeated many times in sourcerocks in order to accomplishmeaningfulgasor oil movement A pressure-driven,discretehydrocarbonphasemovementcan function.inall kinds of sourcerocks,independentlyof lithology lt is consider€dasthe dominatingprimary migrationmechanism.
Chapter3 Geologicaland Geochemical Aspectsof PrimaryMigration
Observedfacts with respectto primary migration of petroleum can either be relatedto time anddepthof migration,or to chemicaldifferences, or similarities betweenthe compositionof sourcerock bitumenand relatedcrudeoils.
3.1 Time and Depth of PrimaryMigration Petroleumgeologistshave long distinguished betweenearly and late primary migrationmainly on the basisof the availabilityof compactionwaters.Early migrationis considered to takeplaceduringthe first 1500m of subsidence, when sediments losemuchof their porosity,yieldinga largevolumeof water. The questionof early migration of oil before the onset of the main phaseof hydrocarbonformationfrom kerogen,is vital to petroleumexploration,especiallywith respectto the vastoffshoreareaswith youngTertiaryandPleistocene sediments.There is little questionthat the majority of the world petroleum reserves werelbrmed in and migratedout of sourcerocksat temperatures of at least50 to 70"C in a minimum depth range of 1000to 1500m. In shallow sediments, however,betweenthe surfaceto about1500m burialdepth,thereare only minor amountsof hydrocarbons and other petroleum-related compounds present,mostof which are directlyinheritedfrom organisms, or producedas a resultof earlydiagenesis. A majorexceptionis veryearlymethaneproductionby microorganisms. The questionis: can oil or gasbe formedin largequantitiesin loung, immaturesourcesedimentsin the first 1500m of burial when early sedimentdewateringis mosteffective? KidwellandHunt ( 1958)reportedan oil-likeconcentration of hydrocarbons in sedimentsless than 10,000yearsold from the delta of the Orinoco River, Venezuela.Oil and free gaswerefound in a lenticularsandbody, at a depthof about 35 m. The hydrocarbonconcentration,consistingpredominantlyof aromatics, wasaround160ppm. Gray claybedsenclosing the sandlens,had an averageof 55 ppm hydrocarbons. Other interbeddedsandswere open to the surfaceand did not containsuch high hydrocarbonconcentrations. Detailed pressure studieson pore fluidsindicatedfluid movementfrom the mudstowards the continuoussands.The movingfluidscontainedlessthan 16 ppm hydrocarbons.From thesefindingsit wasconcluded that therewasmigrationof hydrocarbons,eitherin molecularsolutionor in an extremelydilutecolloidaldispersion and that the hydrocarbons were filteredout of the movingstreamof water by
I
326
Geological andGeochemical Aspectsof PrimaryMigration
capillaryactionin the lenticularsand.No suchfilteringoccurredin the open sands. Embryonic,noncommercial hydrocarbon accumulations in deepseasediments arereportedby Mclver (1975).Findingsfrom two locations, theChallenger Knoll in the Gulf of Mexicoand the ShatskyRisein the WesternPacificOceanlend supportto the idea that bitumensand petroleumhydrocarbons can begin to migratevery early. Sedimentsfrom the ChallengerKnoll 136m belowthe sea floor andin a waterdepthof3572m containedan immature,somewhatunusual, high-sulfur(5%) crudeoil with a highspecificgravityandmodestvolumesof gas andgasoline.Chemicallythisoil is similarto typicalTertiaryoils,producedfrom caprock reservoirs at two saltdomesin Texas.Anotheroil seepor oil saturation in sediments from one of the SigsbeeKnollsin the Gulf of Mexico (Gealyand Davies.1969)is also worthy of mention.When referringto theseoilshowsin youngdeepseasediments in the Gulf of Mexico,it hasto beremembered thatdue to sedimentcompaction,pore watercanmovepastthe saltandupwardthrough the sedimentsand over the dome,displacingthe originalinterstitialpore fluids (Lehner.1969;Perry.1970).The oil saturationreportedin someyoung,deepsea sedimentscould, therefore,have been derived from deeper,more mature sediments. The ShatskyRise hydrocarbons were found in Pleistocene sediments,4m belowthe seafloor in 4282m of water.Detailedgeochemical analyses suggest that it is a stringerof migratedimmaturebitumen.About onethird of theextract consisledof nitrogen-sulfur--oxygen compoundsand asphaltenes. The relative enrichmentof totalbitumenandheavyhydrocarbons overneighboring sediments is greaterthanin the Orinocodeltaembryonicaccumulation reportedby Kidwell and Hunt (1958). At severallocationsin the Gulf of Mexico. the concentrations of the low molecularweighthydrocarbons. methane,ethaneandpropaneweremeasured in the watercolumnat depthsbetweenthe surfaceand37,12 m (Franket al.. 1970). Hydrocarbonconcentrations rangedfrom 1.2 x 10-6to 125x 10 5 ml I I sea water.The mol ratiosof methaneto ethanepluspropanewerelow, especially at the bottom closeto the sediment-water intedacewherc valuesof about 10-20 Cr:C, + Cr weremeasured.Suchlow ratiossuggcsta thermalcracking-derived hydrocarbonsourcefrom an actualseep.becausebacterialprocesses typically produceC,:C2+ Cr ratiosin the thousand. The oil andgasfieldsin NagaokaPlain,Japan,producemainlyfrom volcanic and pyroclasticrocksenclosedin MiocenethroughPleistocene mudstonesand siltstones.An unconformityseparates the Miocenefrom the underlyingbasement.Magara(1961t) hasmadea verydetailedstudyon compaction,porosities, permeabilitiesand pressureconditionsof thesesedimentsand on pore fluid movement.Convincingevidenceis presented thathydrocarbon migrationcaused by differentialcompactionoccurredbetweenenclosingmudstonesourcebeds and the volcanicreservoirs. In the areaof the Sekiharaand Katagaifields,there was apparentl),downwardmigrationfrom the PlioceneNishiyamamudstone source beds into the volcanicreservoir.In the Sekihara-Katagai area the presumedsourcebedsof the oil are buriedto about1000to 1500m. It appears logicalto seethishydrocarbon generation in connection with a highheatflow, as
- - TimeandDepthof PrimaryMigration
32.7
...Lggested by Klemme(1975).In this area,temperatureestimates for this burial .irpth are up to about75'C. Temperatureand age(pliocene)reportedfor the ,,urce sediments that they are in the earlystageof petroleumgeneration.The :ekiharaandKatagaioilshavea specificgravityof about0.87g cm-3.Thus,there r circumstantial geologicaland hydrodynamic evidencethat therewasprimary nrgrationof hydrocarbons out of a relativelyimmaturehighporosityllVlOo" 1 nudstone, apparentlyin connectionwith pore water moviment inducedby i!rmpactlon.
Youngdeltaareasare frequentlyrich in oil occurrences at relativelyshallow Jepth lcvels(800 1500m) and it is srill dispuredwhetheror not thia oil is of .hallow origin, or derivedfrom deeper-seated sourcerocks. However.there .eemsto be evidencethat at leastsomeof theseoils,eventhosefoundin isolated ..tndlenses,may be derivedfrom deeper,morematuresourcebeds.This is the :3\e. for instance,in the MahakamDelta (Durand, 1979),the Niger Delta E\ amvet al., 1978;EkweozorandOkoye,1980)andrheGulf Coast(Millikenet il. . l98l ; Gallowayet al., 1982).The migrationavenues in suchcasesarethought :o befracturesassociated with deep-reaching growthfaults(WeberandDaukoiu, 1975:Price,1976)or piercements. productionin the areaof the Gulf Pleistocene rf Mexico has frequentlybeen cited as evidencefor earl! Aenerationand :risration.This area,however,is a "diapiricjungle" of shaleintiusionsandsalt :rercementsassociated with extensivefaultingand folding (Woodburyet al., -97-j),whichoffermanymigrationavenues for deeper.morematuresourcebeds. The mere presenceof oil in immature reseruoirs.especiallyin structurally :(rmplexareas,cannot be consideredcompellingevidencefor its origin in m m a l u r es o u r c eh e d s . Prim,qr,y migrationoca.tring during themainphaseof hydrocarbongeneration, tcnerallvat depthsbetween1500and 3000m to 3500m. is documentedbv the majoritvof oil accumulations of Cenozoic.Mesozoic.andpaleozoicagearound rheworldandshouldbe accepted asa fact.The mechanisms of primarymigration thatprevailduringthis stageof sourcerock maturation.are now understoodin ieneralterms:it is conceived asa pressure-driven hydrocarbon phasemovement. The term lateprimurymigration,asopposedto earlyprimar)'migntion, was )risinallyusedto describemigrationafterthe first 1500m of subsiclence. which ;oincideswith the beginningof the mainphaseof hydrocarbon generation. I n the lisht of the new findingson primary migrationmechanisms the term '.late" primarymigrationhasalmostlostitsoriginalmeaning.asthisstageof migrationis nrobabll'the mostimportant,if not the only one. With increasing depththe decrease in porosityandwatercontentof sedimenraryrocksis paralleledby a strongdecrease in permeability. Thisis a reasonwhy late primary migration from compactedlow permeabilitysourcebeds was tormerlvquestionedby manypetroleumgeologists. Thereare, however,examples where the geologicalsettingsuggests that late primary migrationfrom mduratedlow permeabilitysourcebedsmust havetaken place.The giant oil iieldsof HassiMessaoud in theeasternpartofthe AlgerianSiharaproduiesfrom a Cambriansandstone reservoirin a largedome-likestructure(Fig. IL6.7). The realis providedby impermeable Triassicshalesandevaporites whichunconformably overlie the Cambriansandstonereservoirand Lower paleozoicshales.
328
Aspectsof PrimaryMigration Geological andGeochemical
Silurianblackshaleshavebeenassumedby Balducchiand Pommier(1970)for geologicalreasonsto be the main sourcefor the Hassi Messaoudoil. This assumptionwas later verified by geochemicalsourcerock--crudeoil correlation (Tissotet al., 1975;Welteet al., 1975).Duringa firstcycleofsubsidence, analyses the Silurianblack shalewas overlainby some 1000to 1500m of Paleozoic Sinceboth the Siluriansourceandthe in the areasof HassiMessaoud. sediments uplift Cambrianreservoirhavebeenexposedto the surfaceduringpre-Triassic wouldhave subsidence duringthe first, Paleozoic anderosion.anyoil generated been lost to the surface.Only the oil generatedduring the secondcyclesof whichreacheda depthof 3700m duringthe Mesozoic,is now found subsidence, reservoir(Tissotet al, 1975). in the Cambriansandstone primary migrationof gaswasmore widelyaccepted The late generationand petroleum geologists than that of oil. One exampleworth mentioning, amoung Thereis abundant however,is the hugeGroningengasfield in the Netherlands, that the gas in the geologicaland geochemical evidencewhich demonstrates Permiansandstonereservoirsof the Groningenareawasgeneratedfrom Late coal shalesandinterbedded includingorganic-rich coalmeasures. Carboniferous were buriedbetween seams,during Mesozoictime, when thesecoal measures 4000and 6000m (Lutz et al., 1975).At thesedepth levels,dense,compacted watercannot andmovingcompaction shalesandcoalsarerelativelyimpermeable to be an effectiveprimarymigrationmechanism. be considered Hedberg(1964)cited severalother examplesof late primarymigration.He pointedout that the evidencefor latemigrationliesin caseswherethe supposed prior to the depositionof sourcemust havebeencompactedand consolidated werecreated.Some reservoirrocksor beforefold- or fault-trappingstructures examplesof late primarymigrationhe citedare the hugeQuiriquireoil feld of EasternVenezuelaand the San Pedro field of Northern Argentina. In the Quiriquirefield, productionoccursmainlyfrom the PlioceneQuiriquireFormaunconformtion, an alluvialfan seriesof continentaloriginwhichwasdeposited age.The oil is thoughtto ablyon truncated,foldedbedsof Mioceneto Cretaceous have been generatedin theseTertiary or Cretaceousbedsbelow the unconformity. ln the San Pedro field the oil is reservoiredin Permo-Carboniferous whichdid not form until lateTertiarytime.The sources sandstones on structures to Reed( 1964),areDevonianin age.Unfortunately,in both of thisoil, according these cases,there is no detailed geochemicalevidencefor the sourcerock-oil correlation. for primarymigrationduringthemainphaseofhydrocarA specialmechanism oI claydehydrabon generation,wasseenby someauthorsin the phenomenon tion. Clay dehydration is the releaseof bound water during the alteration of smectite(montmorillonite)to illite, mainlyunderthe influenceof temperature. This phenomenon wasdescribedand analyzedby Burst (1959),Weaver(1959), (1969,1970),and Heling andTeichmiiller(1974).It was Dunoyerde Segonzac with respectto petroleumgenerationandmigrationby Powers(1967) discussed and Burst (1969). The main conclusionsof Powers (1967) were that the organicmud development of a shalesourcerock requiresa smectite-containing and its subsequent alterationto illite with deepburial,andthat abnormallyhigh of the watersdesorbedfrom fluid oressures mav be causedbv a volumeincrease
-i.1Time and Depthof PrimarvMigration
329
lmectiteduringthe changeto illite. Burst (1969)expandedon this dehydration phenomenon of swellingclavsandpointedout that in the Gulf Coastareathereis a coincidence of productivehorizonsandthe depthlevelswhichhe interpretedas a second-stage.(interlayer water)claydehydration.He suggested thatlarge_scale \\ aterreleasedueto dehydrationandthe subsequent fluid movementin i sour.. rock-typesedimentcontainingpetroleumhydiocarbonsmay initiate primary mlgratlon. Maeara(1975)re-evaluating the daraof powers(1967)andBursr(1969)added newinformationon claydehi'dration,abnormalpressures andpetroleummigra_ tion. He emphasized that smectitedehydrationcannotcauseabnormallyhieh prcssures. The relcaseof largeamountsof boundwaterwouldresultin a vblurie decreasc, sinceboundwaterhasa lower densitythan free water.Furthermore, \lagara (1975)statedcorrectllithat smectitedehydrationalone.without good drainage.mav not be sufficientto initiateprimary petroleummigration.The :tudicsof Dunoyerdc Segonzac ( 1970)andHelingandTeichmiillerllgT4)show that there is a verv gradualreorganization in the mixecl-laver smectite_illite seqxence. In particular,the pure illite stageis not reachedevenat 4000m depth anda temperaturcof 160"Cin the Logbabawell, Cameroon.The samers truear c'omparablv hightemperatures in the RhineGraben.Thus.claydehydrationand the releaseof additionalwateris gradual,anddoesnot o..u, in o narrowdepth interval.The kineticsof claydehydrationoccur.thcrefore,in the sametime and depthrangesasthe phenomenon of compaction.Consequently, the mechanism ofclay dehydrationdoesnot seemlo be moreeffectircfor primaryhydrocarbon migrationduringthe main phaseof petroleumgenerationthan normalcompac_ tlon. The presence of expandable claymineralshasfrequentlybeenconsidered to be a prerequisite for an effectivesourcerock whichis ableto expeloil by primary migration.In the USA, thereis a coincidence betweenoil-producing stritigraphii horizonsfromPaleozoic throughCenozoicageandthedistributionofexpJndible clal's (Weaver, 1960).The presenceof expandableclays and the primarv migrationof oil. however,are not necessarily related.For example.the lower Paleozoic blackshales,sourcebedsfor the HassiMessaoud oil in Algeria.do not contain,andalmostcertainlynevercontained, expandable clayminerals,because t h ec l a vf r a c l i o no f t h e s es h a l ecso nt a i n so nl y k a o l i nr t e ,p r i m a i yi l l i t ea n dc h l o r i t e (Millot. 1964).The coincidenceof oil occurrences ind expandableclays as observedby Weaver(1960).canalsobe explainedin termsofa betterpreserva_ tion of organicmatterdue to expandable claysor catalvticeffects.Likewise,no expandable clayshavebecnfoundin anyof the sourcebedsin the oil_producing Willistonbasinof North America(Barker,1976).
t f I
3.1TimeandDepthof PrimaryMigration
32g
smectiteduringthe changeto illite. Burst (1969)expandedon this dehydration phenomenon of swellingclaysandpointedout thatin rheGulf Coastareathereis a coincidence of productivehorizonsandthe depthIevelswhichhe interpretedas a second-stage (interlayerwater)claydehydration. He suggested thatlarge-scale waterreleasedueto dehvdrationandthe subsequent fluid movementin a source rock-typesedimentcontainingpetroleumhydrocarbons may initiate primary mlgratlon. Magara( 1975) re-evaluating the dataofPowers(1967)andBurst(1969)adcled new informationon claydehvdration.abnormalpressures andpetroleummigration. He cmphasized that smectitedehydrationcannotcauseabnormallyhigh pressures. Thc releaseof largeamountsof boundwaterwouldresultin a volume decrease, sinceboundwaterhasa lower densitythan free water.Furthermore. Magara(1975)statedcorrectlythat smectitedehydrationalone.without good dralnage.may not be sufficientto initiateprimarv petroleummigration.The studiesof Dunoyerde Segonzac ( 1970)andHelingandTeichmiiller(1974)show that there is a verv gradualreorganization in the mixcd-laversmcctite_illite seqlence.In particular.the pure illite stageis not reachedcvenat 4000m depth anda temperature of 160"Cin the Logbabawell, Cameroon.The sameis tru; ar comparablyhightempcratures in the RhincGraben.Thus,claydehydration and the releaseof additionalwateris gradual,and doesnot occurin a narrowdeplh interval.The kineticsof claydehydrationoccur.therefore,in the sametimeand depthrangesasthe phenomenon of compaction.Consequently. the mechanism of claydehydrationdoesnot seemto be moreeffectivefor primaryhydrocarbon migrationduringthe mainphaseof petroleumgenerationthan normarcompaction. The presence ofexpandable claymineralshasfrequentl]'beenconsidered to be a prerequisite for an effectivesourcerock whichis ableto expeloil by primary mrgration.In the USA, thereis a coincidence betweenoil-producing stratigraphic horizonsfrom Paleozoic throughCenozoicageandtherlistribution ofexpandable clays (Weaver, 1960).The presenceof expandableclays and the primary migrationof oil, however,are not necessarilv related.For example.thc lower Paleozoic blackshales,sourcebed: for the HassiMessaoud oil in Alseria.do not contain,andalmostcertainlynevercontained, expandable claymineials.because thc clayfractionof theseshalescontainsonly kaolinite,primaryillite andchlorite (Millot. 1964).The coincidenceof oil occurrences and expandableclays as observedbv Weaver( 1960).canalsobe explainedin termsof a betterpreservation of organicmatterdue to expandable claysor catalyticeffects.Likiwise, no expandable clavshavebeenfoundin anyof thc sourcebedsin the oil-producing Willistonbasinof North America(Barker.1976).
Geologicaland GeochemicalAspectsof Primary Migratic.::
330
3.2 Changesin Compositionof SourceRock BitumenVersus CrudeOil in chemicalcompo:iThreedifferentaspects arerelevantwith respectto changes oil: tion betweensourccrock bitumento reservoired - hydrocarbondistributionin the contact zone betweensource rock and reservolrrock. - grosschemicalcompositionof crudeoils versussourcerock bitumen. the phenomenon of oil-sourcerock correlation. Very fittledataareavailableon detailedinvestigations of sourcerocksirtactual are knownandare contactwithcutoil-t'illedreservoirrock.Severalsuchexamples cited hcrc becauseit is this kind of informationthat is necdedto solvethe problemsof primarymigration. takenat regular1-minteIvalsacrossthetransitionzoncresenoir Coresamples rock-sourcerock. havebeenanalyzcdfrom a well in the Parisbasin(Vandenbroucke.1972).The well penetratedthe lower Jurassicorganic-richToarcian Domerianreservoirrock.The mincralosical marl sourcerockandthe calcareous compositionof the core samplesof both sourceand reservoirrocks.and their organiccontentincludingtotal amountof organiccarbon,extractablcasphaltenes. resins.aromaticand saturatedhydrocarbons.and the distributiont-'i hvdrocarbons were investigated. The sourcerock sampleswere found to be The increasingly depletedin total extractas the reservoirrock is approached. maximumdeDletionobservedwasin the orderof about10cl.of thc totalcxtracl especialh and wasnoticableovera distanceof about4 m. Lesspolarcompounds. lost to the reservoirroc\ thoseof lower molecularweight,werepreferentially Thus. the movementof petroleumcompoundstiom the sourcerock to the reservoirrock is apparentlycontrolledlargelyby adsorptionand desorption phcnomcnaalongthe migrationpaths. Another investigationof this kind has been presentcdby Tissotand Pelel ( 1971).who studicdthc transitionzoncbetweensourcerockandreservoirrockin a detrital shale*sandseriesof Devonianage in the Algerian Sahara(Table
r r r . 3)..1 of extractsacrossthe transitionzonesourcr TableIII.3.l. Abundanceand composition series.Algcria.Sahara.(Afte. Tissotan.i rock in a Devonianshale-sand rock-reservoir P e l e t l.9 7 l ) Proximity to reservotrln m
ExtractlC".," (mg g )
Hydrocarbonsu a/. in extract
Asphalteneso qL in extract
2 I 7 10.5 l4
12 86 90 t12 I l8
54 6l 63 63
12.2 ll.2 1.5 5.7
tr4
5.u
' Eachvaluerepresents the averageof threeor four measurements
-: I Changesin Compositionof SourceRock BitumenVersusCrudeOil
331
Aeain a graduaidepletionin extractablebitumenwasobservetlas the sandv :.'iervoirwasapproached. but in this casethe zoneof depletionrerchedabout -rr m into the sourcerock. The depletionwasstronge\tclosertto the reservolr , \ h e r ct h e s o u r c cr o c kh a dl o s ta b o u r4 U q o f i t s o r i g i n abl i r u m e nc o n t c n tT . he "rrumenremainingin the sourcerock nearestthe reservoirwas moststronslv :nrrched.inhigh molecularweight heteroatomiccompoundsand Lleplete.lin rrdrocarbons.A vcry similardepletionof sourcerock hydrocarbons in contact '.rithrcservoirrockswasreported by Neruchevand Kovacheva(1965). Corresponding compositional fractionation effectsdueto exDulsion havebeen . . h . . r r c d .h ( , w ( \ ' e ra. l s oo n i r m o l e e u l alre r e l . i n t u o c o r e h o l e s Deneatins iequencesof interbeddedmature shalesourcerocks and reservoir,nnds oi S p i t s b e r g el snl a n d( L e v t h a e u s e tr a l . . l g l l l a . b : M a c k e n z ieet a t . . l 9 g 3 ) .T h i n :halesinterbeddedin sandsand the edgesol thick shaleunits are depletedin petrolcum-range hvdrocarbons to a suchhigherdegreethan the centeriof thick :halcunits.Furthermore.it waspossiblcin thisstudyto obtaininformationon the quantitiesof the hvdrocarbons expelled. Figurelll.3.l showsthe comparisonof the Cri_-saturated hvdrocarbondata tronl two sourcerock shalesfrom the srmc sequcnce:In the ;(,nterof a thick F i e .I I I . 3 .I a c . r r - A l k a ndce p l e tronin sourccrock dueto prima r\' mrgratlon.i. c.. prelerential e\pulslon of Iower molecular rveightrr-aikanes.(After Levth a c u s cer t a l . . I 9 8 3b )
t I
t
t l
Li
332
of PrimaryMigration Geological andGeochemical Aspects
sourcerock-typeshaleunit at 127.0m depththe originallygeneratcdbimodal hydrocarbondistributionis presented.while the thin shaleat 62.5 m showsa distribution(Fig.IIL3.1a).Sincekerogenquality hcav1,-end biasedhydrocarbon et al., 1983b),this and maturit_v are identicalfor both samples(Leythaeuser compositional differencemustresultfrom expulsion,i.e., the thin shaleduring the courseof primarl' migrationpreferentiallylost the lower molecularweight hydrocarbons. In additionlt.lthat pristanewasexpelledto a lesserdegreethan of r-alkaneswith a comparablecarbonnumber.Subtractingthe concentration hydrocarbonsof the depletedshale from the hydrocarbonmixture of the andcompositional unmodifiedshale,leadsto informationon the concentration profile of the hydrocarbonsexpelledfrom the thin sourcerock shale(Fig. as a lIL3.1b). By expressing the concentrationof the expelledhydrocarbons percentage the expulsionefficiencyis of the originallygeneratedhydrocarbons obtained(Fig. lll.3.lc). lt rangesfbr rr-alkanes betweenabout 10% and 80c/r depending on carbonnumber.Theseexpulsionefficiencies shouldnot beapplied to sourcerocksin generalbecausethe examplepresentedhereis a specialcase wherea thin sOurcer(rckis sandwiched betweenpclrousreservoirstriltapermitting optimumhydrocarbondrainageconditions. of the hydrocarbons In the samestudyit couldbe shownthat the composition with a pronounced impregnating the interbeddedreservoirsandsis in agreement beingexpelledfrom the shale,i. e.. low molecular fractionationof hydrocarbons weighthvdrocarbons migratepreterentiallyfrom sourceto reservoir.Thus. in in the this scquencethe compositionof the hydrocarbonproductaccumulating ratherthanby reservoirappearsto be controlledprimarilyby physicalprocesses the type and the maturityof the organicmatterin the sourcerock. crudeoilsandsource between A comparision of thc grosschemicalcomposition and rock bitumenshowsthat mostoils are enrichedin saturatedhvdrocarbons
rt|cl Irtractof sourcB
0il
RA - RG.in. ..d
S = S.tur.t.d
..ph.lt.n..
HC
betweencrudeoilsand in termsof grosschemicalcompositiorr Fig. III-3.2. Comparison (After Tissotand Pelct. and carbonatesequence. sourcerock bitumensin a shalelsand 1 9 7 )1
l.l Evaluation of Geological andGeochemical Aspects of primaryMigration
333
d e p l e t e di n p o l a rN . S . O c o m p o u n d(sF i g .I I L 3 . 2 ) .T h i se n r i c h m e nhta sb e e n trequentlyobserved(Hunt andJamieson,1956:Bray andEvans.1965:Tissotand Pelct.l97l). This is in agreementwith the observedadsorptionbehaviorofthe diflerentgroupsof compounds. Anv mechanism of importancein primarymigrationalsohasto accountfor the geochemicalcorrelationbetweencrud.eoils antl sourcerock bitumen.correlation aspectswill be treatedin great detail in part V. It has been established that hvdrocarbondistributionamonghomologous seriesandsomeindividualhvdro_ carbonratios practicallyunchangedduringmigration(Williams,io7+; -remain Welteet al., 1975; Deroo,I 976). The numerousexamples of successful oil/source rockcorrelations basedon individualhydrocarbon ratiosarea strongargumentin favorof a hvdrocarbonphasemovemcnr.
3.3 Evaluationof Geologicaland Geochemical Aspectsof PrimaryMigration Theobservation of cmbrvonic,noncommercial hvdrocarbon accumulations. such as occur in the Orinoco River delta (Kidwell and Hunt, 195g)and in young secliments of the deepseaof the Gulf of Mexicoandthe WesternpacificC)cean (Mclver. 1975)is not in contradictionwith currentgeochemical thinking.The hydrocarbons andbitumensfoundin theseoccurrences areenrichedin aromatics and high molecularweight polar hcterocompounds, or both. This would be expectcdif migrationoccurredfrom relativel!immaturcsourcematerialandwith compactionwater as the main transportationvehicle.The majoritv of such intmaturc.embrlonicpetroleumoccurrences. however,areprobablveventuallv lostto thc surfacebccause of the lackof suitablesealsin the firstfcw hundredsof metersof burial and the flushingactionof upwardmovingcompactionwatcr. Autochthonous.immaturebitumenapparentlyoccursnot onlv in Rccentand sub-Recent mudsand clavs.but alsoin sands(Palacas et al.. 1972)itnd ls never found in producibleconcentrations. This materialshoulcinot be confusedwith hvdrocarbonsgencratedjn more mature source rocks which subsequentlv migrateto shallowerdepth.Geochemical analvses canhelpdistinguish between thermallvimmatureand maturchvdrocarbonassemblages. Primarymigrationof petroleumhvdrocarbons in voung.immature.unconsolid a t c ds e d i m c n tsse e m s t b o er a r c ,b u t c a n n obt ee n t i r c l y r u l e d o uAt .n e x a m p l e o f commercialquantitiesof oil andgas.derivedfrom sediments buriedto approximatel),1(XX) to 1500m. at a relativclvearlvstateof compaction(Nagaokaplain, Japan)hasbeenpresented.In this caseavailableinformationpointsto primary migrationin connectionwith porc water movementinducedbv compaction. Commercialpetroleumoccurrenccs derired irom \uch shallowondvuung,ouaa" sediments apparentlvare rare. The Naeaokaexample.however.is acceptedas evidenccol commercialoil poolsresultingfrom earlvmigrationwith compacrion water as a main causefor primary migration.The scarcitvof such young pctroleumaccumulations originatingfrom relativell,immaturesourcebedsii
of PrimaryMigration Aspects andGeochemical Geological
331
primarilydue to the lack of traps and sealsat this stageof burial' and by the generalinsolubilityof petroleumcompoundsin low temperaturecomfaction iuater.The compositio;of migratedpetroleumat this stageshouldretlectthe thermalimmatuiity of the sourcebedsand the solubilitiesof the transported compounds.lt should contain significantquantities.of relativelyunaltered ' polar.highmolecularweightN ' S' suchaswater-soluble molecules. biociremical enrichedin aromaticsA diagramhydrocarbons boiling O compoundsand low primarvmigration' concerning considerations other and these maticsummaryof of compaction by means primary migration Early I11.3.3. in Ficure is Dresented prevents Lithification sediments argillaceous-silty to detrital. wateris restricted early excludes and therefore sediments. young carbonate in most compaction soluble even if sufficient primary migrationin shailowcarbonatesequences' organrcmatterls Present. of biogenicgasis a specialaspcctof iha n..urran.. nf shallowaccumulations Prominent in youngsediments. of hydrocarbons the migrationandaccumulation C h a n c fe0s; m o d e0 f p r i m a l tm i g r a t i 0dne p e n d i0nng dilferentparameters
./l dissolv.d i. w.t.r (nic.ll.r
----.--
\
as disq.t. liquid oil Ph.3e
.v.il.bilitY
..di..r.tG g.s.ou.
Ph..G
gGn...tion PGttol.!m
0 1 2 30 | 2 3 0 r 2 3 0 1 23
z
*R=" *'1'./
,',JfiJil;.".
F .9
ar]
t
t'oo! o o o o o o o o o o o/ -oo o oo
-']
0 =none chances: | =small 2 =good 3 =srcellent sketchdepictingpossiblemodesof primarymigration\\'ith Fig. 111.3.3.Diagrammatic burial in,Jreasing
1.3Evaluation of Geological andGeochemical Aspects of primarvMigration
335
examplcs of thiskind arenumerous smallgasfieldsin theplioceneandpleistocene sediments of the Po Vallel'.Otherexamples mightbe foundat theeasternsidcof t h e W e s t e r nC a n a d ab a s i n . e . g . . t h e g a s f i e l d M e c l i c i n eH a t ( 3 1 0 m ) . The modeof primarymigrationin suchcaseshasnot beeninvestigated. Compactionand the resultingmovementof pore water diminisheswith i n c r e a s i ndge p t ho f b u r i a l .T h e c h a n c e fso r p r i m a r \ m j g r a t i o no f p e t r o l e u m compounds dissolved in andmovedby compaction waterareprobablygreatestat a depthof about 1000m, whenshalesstill showsufficientporositvforidditional compaction,and yet are buried decp enoughro reachthe onset of thermal of hydrocarbons eeneration from kcrogen.At depthsbelow2000m the porositv c u r v e fso r s h a l e(sF i g sI.I L 2 . 5 .I I I . 2 . 8 )t e n dt o w a r dt h em o r el i n e a r c g i o nu. , h e r e thercis little lurther porosityreductionwith continuedincreascin burial. Thc overallcfficiencvof primarymigrationin molecularor micellarsolutionin conncctionwiti activell,movingcompaction waterscilnnotbe high ascompared to othcr possiblemodesof primarv migration.Thc specialrole of diffusion_ controlledprocesses duringprimarv migrutionof dissolveLl gaswill be treatcd s c p a r a t e ltvo w a r dt h e e n d o f t h i sc h a p t c r T . h e s o l u h i l i t i ccs, l p e t r o l e u mc o m poundsrn porewatersarevcrv low (TableIIL2.2) cvenunderthe mostfavorable conditionsWith vervfew exccptions, solubilities are in the rangeof sevcralppm to about10(lppm at highertemperatures (> 100"C)andgreaterdeDths.Evenrhe most optimisticmaterialbalancecrlcularitrns shou,thir solubiliiiesof hvdrocarbonsat temperatures of lessthan l2-5.C.corresponding to depthr t,r ihour 30(X) m. aretoo low to accountfor oil accumulations in productivepitroleumpro_ rrnccssuchas the EasternSahara.Californiaor the LouisianaGulf Coast.If a sourcerockvolumeof 100x 100km and 100m thick is assumed. the cumulative rmount of watercxpel,led dueto compactionbetween1000anci3000m depthof b u r i a l .i s a b o u t 2x l 0 r r m r . T o a c c o u n t f o r o ai l c c u m u l a t i oinnst h er a n g eo f 2 0 0 millionto 2 billion t. whichis a reasonable fiqurelor oil in placein thosebasins. c r u i l eo i l s o l u b i l i t vi n w a t e rw o u l t lh a r e r , ) b e b e t u , c e nl a ) 0 0a n d 1 0 . 0 0 D 0 Dm. Hvtlrocarbonsolubiliticsof thisrangeare highlvunlikclv.At greaterclcpthsnn.J h i g h e rt e m p e r a t u r c st h. e s o l u b i l i t i em s i s h t b e h i g h e r .b u t i n s u c hc a s e st h e amount of compactionwater is not sufficicnt.BetweendcDthsof 3000and - 5 0 0m 0 . o n l v a - 5 %p o r ev o l u m er e d u c t i o cna nb e e x p c c t e (dF i g .l l l . 2 . 8 ) .M a s s balancecalculations in thc WilljstonbasinandLos Angelesbasinwerealsoused by Jones( 1980)in orderto pointoul hr)u ineffecri\(, \o-lutionmigrrttonanclhow i m p o 1 1 ; 1i rnc1L r n l i n u o u o si l p h a . cm i g r i r t i r rnnt u { th c . Finall)'.the compositionof most crude oils does not reflect the solubilitv d i s t r i b u t i o nosf t h ev a r i o u cs l a s s cosf p e t r o l e u m c o m p o u n desi t h e ri n m i c e l l l r o .i inmolecularaqueoussolution.Micellarsoluri,rnasaneftcctiremcansotDnmilrv migrationis contra-indicatcd bv additionalfactors.suchasthe lackof suffi.ient amountsof surfaccactivesolubilizers. micellcstoo largcto passthroughpore throats.and chemicaland electricalhindrances. Thus. althoughtncre mirv Dc \ o m cc i r r l \p r i m t r ym i g r l l i o ni n u q u c o us\ o l u t i o ni n c o n n e c t i ouni t h c , l m p , , i t r , - , n waters.especiallv with the moresolublelow molecularweishthvdrocarbons and s o m ep o l a rh i g hm o l e c u l awr e i g h N t . S . O c r . r m p o u n di tsi s. i r c e b t i o nr a t h e tr h a n t h er u l e .
336
Aspects of PrimaryMigration andGeochemical Geological
we cannow evaluatethe relativeimportanceof At thispoint of the discussion primarymigrationwith or withoutwatermovementasfollows: quantitieswhenh1'drocarCompactionwatersare only availablein necessary phaseof hydrocarbon the main yet generated; during been bons have not generationfew or no compactionwatersare availablc. - The relative permeabilitvconceptrequiresfor effectivemovemcnt of a hydrocarbonphasea minimumratio of the oil/watersaturation.Thiscanonly be achievedoncethe major part of watcr hasbeenexpelled. Any water phase transportwould require an alterationof thc chemical compositionof reservoircdpetroleumsin a mannercontraryto that observed shouldbe enrichedandnon-polarcompounds in nature.i. e., polarcompounds oil/sourcerock correlationbased depleted:furthermore,the fact of successful phasemovement on hydrocarboncompositionalsofavorshydrocarbon - Gencrationand migrationof hydrocarbons are comparablein compacting carbonatesourcerocks. clasticand non-compacting Basedon thesetbur arguments we concludethatthemajorityof oiloccurrences can only be explainedb1'a hydrocarbonphaseprimarymigrationmechanism. Expulsionin an aqueousphase may play a rolc only under some special circumstanccs. It is thereforeexpectedthat at depthlevelsof about 1500m and deeper.the predominantmodeof primarymigrationshiftsfrom earlysolutionmigrationto a form of oil phasemigration.The more or lesscontinuousoil phaseis graduallv phaseat depthsgreaterthanabout3500m. superseded by a gaseous hydrocarbon The beginningof late primary migration as a separatehvdrocarbonphase coincides with the mainphaseof oil formationandcontinuesuntil thegeneration potentialfor liquid oil is exhausted,and mainly gaseoush1'drocarbons are controllcdnot only formed.The onsetof lateprimarymigrationis consequentlv by the depth porosityrelationship,but by the generativepotentialof a given sourcerock aswell. Thus, it cannotstartuntil maturitiesequivalentto vitrinire reflectancevaluesof about 0.5 to 0.6% are attainedand favorabletypesof organicmatterarepresent.Severalfactorscontrolthe formationof a discreteoil phaseand its movementthroughthc sourcebed. Among theseare the total of porosity.the ratio of fixed to mobile pore water. the averagecross-section pores.the amountand type of kerogen(organiccarbon)and the presenceof a three-dimensional kerogennetwork.and finally the amountof bitumcngcnerated-
At depthsof about3000m and deeper,porositiesin sourcebedsare below part ofthe porevolumeis occupicdby immobilefixed l0oZ.wherea considerable porewater,andbitumenconcentrations, ascomparedto freeporewater.aretoo of a separate oil highto be accomodated in the aqueousporefluid. The existence generation, phasein sourcebedsat peakof hydrocarbon therefore.seemsto be a necessity. Due to the continuedgenerationof bitumcnand gaswith burial,this separate oil phaseis pushedawayfrom the kerogenthroughthemainlywatcr-wet micropores. Thistvpeof primarymigration.is at its mosteffectivelcveltbrsource rockswith high kerogenand bitumencontent.The movementof a separateoil networkof until a continuous phasemustovercomemountingcapillarypressures
-i -i Evaluation of Geological andGeochemical Aspects of primarvMigration
33.1
bitumenoccupiesthe centerpore space.At a certainpoint, pore diameters will becomctoo smallandoil particlestoo largeto be forcedthroughwater-filled pore rhroatsagainstextremelyhighcapillarypressure. Continuedformationofqasand other low molecularweighthvdrocarbons. plus the thermalexpansionof pore \! atersandpartialtransmission of geostatic stress,keepthe pressurerisinguntil themcchanical strengthof therockis exceeded. At thispointmicrofissures open. the hvdrocarbonphaseexpandsand coalesces further. and an incrementof migrationis accomplished. It is thusvisualizedthat a pressure-driven phasemovementasa hydrocarbon p r i n c i p am l echanism o f p r i m a r vm i g r a t i o np r e v a i l sd u r r n gt h c m a i np h a s eo f hVdrocarbon formation.Thisprocess canprobablypersistwithoutrockfracturing up to a certaindepth.Bevondthatdepthwith the onsetol massive gasgencrarion. rround 3000to,l(XX)m in denserocks.microfracturing will be initiareo. B e l o w t h e s ed e p t h l e v e l sa n d r t l e m p e r a t u r . *" h o u e 1 5 0 . C t h e l i q u i d prcssurizing phaseis ultimatel),replacedby a gascous phase.Zhuzeet al. (1963) suggcsted that normalcrudeoils aswell as low molccularweishthvdrocarbons rrrerlrsrolred rnd rrartrportcdas a cumpressr.'d gl.e.,uspho.ei The| cirlculated that onc billion mr of gas (adjustedf6r atmosphericpressureconditions)at
00
.Ct
$1* i.s/
t000
_
900
\
800
r'i-/
.v/
= E
600
9r o
500
/,,
/ty' t t'/./
500
600
700
800 900
( bar) Pressure
Fig. II l.-1.,1.Solubilitv of petroleum in gas at varioustemperatures as a function of increasing pressure. (Alter Zhuzeet al.. 1963)
338
Aspects of Primary Migration Geological andGeochemical
pressures of,l(X)to 800bar and at temperatures between70 and 200'C, could transporta quantityof petroleumin the order of 100,000to 800.000t. The relationship betweenpetroleumsolubilityin gasat varioustemperatures is shown in FigureIII. 3.4.Numerousfieldsthroughoutthe worldcontainingwet gasesor condensates are cvidencefor this kind of migrationmechanism. Furthermore,it can be expectedthat some of the deep overpressured shale sourcerocks. occurringin Tertiarv deltas.suchas the Mahakamdelta, Indonesia,containa singlehvdrocarbonfluid phase,due to the largeproportionof gasand the high solubility of medium range hydrocarbonsin gas. under such pressureand temDerature. This situationwould resultin a gas-phase migration,until the pressurehas decreased. The remainingamountof mediumhydrocarbons in gaswoulddepend on the prcssuredrop. Anothcrpossibilityfor gasasa transportvehiclefor higher hvdrocarbons is the percolationof the rock columnwith a gasphaseoriginating form a dceplvsituatedsource.Thisgascouldextractheavierhydrocarbons lorm overlvingsourcerocks.This may be the origin of the so-calledshallowearlymature condensates. It is clear that under the conditionsdiscussedabove. asphaltenes and other heavvmolecules cannotbe transported.
3..1Conclusions and Suggestions on PrimaryMigration In summary.it is concluded thatthereis no reasonto assume thatonemechanism ofprimarymigrationisresponsibleforallpetroleumaccumulations(Fig.lll.3.3). Thc predominantmechanismof primary migrationmay changewith different subsurface conditionsprimarill,relatedto increasing dcpthof burial.ln shallow sediments downto depthsof aboutI (X)0to 1500m. solutionmigrationseemsto be favoredascomparedto an oilphasemigration.Solutionmigrationmayalsoplava role at greaterdepthsat highertemperatures. but onlv involvingcertainlight hvdrocarbons.Solution migration alone does not seem to produce major deposits.cxccptunderexceptional geological conditions.asin shallowgasfields o f e a r l vd i a g e n e t iocr i g i n . The dominatingand most effectiveform of primarymigrationsccmsto bc hvtlrocarbon-phase migration.first in form of a liquid oilphaseand at greater depth as a gaseous(or eventuall.vsupercritical)phase.Hydrocarbon-phase migration is mainly accomplishcdb1'a pressurc-driven movementthrough prclcrentiallvoil-wetporesor a networkof kerogenor by microfracturing of the rock due to an internal pressurebuild-up. Hydrocarbon-phase migrationis probablyprevalentfrom mediumto greatdepthsanddoesnot necessarily exclude a minorcontributionfrom solutionmigration.Hydrocarbon-phase migrationis in data. agrecmcntwith both the empiricalandfactualgeological andgeochemical In thiscasewaterflow is not neededasa drivingforcefor migration.andin fact permitsthe release waterhindcrsratherthanaidsmovemcnt.Microfracturing of hydrocarbonsfrom compacted.dense.relativelf impermeablesourcerocks. Therefore.carbonates aswell asshalescanexpelpetroleumin a similarmanner. Therearewell-documented carbonatesourcerocks(Hedberg.1964),suchasthe
.l.JConclusions andSugqestions on Prirnarv Migration
339
La Luna Formationof WesternVenezuelawhichis thin-beddedcarbonaceousbituminouslimestoneol Crctaceousage.or the dark-coloreddenselimcstone betwecnrecfsandcalcarcnitcs in the DevonianSwanHills Formationof Alberta and the entirc Middle East oil province.Thesedense.consolidated carbonate \ourcerocks.apparentlv canyieldoil withoutcompactiOn through,.hvdrocarbon". phase pressure-m igration Similarlv compacted.deepl)' buried shalescan releasehvdrocarbons.as in the caseof the Siluriansourcerocks of Hassi \lcssaoudin Algcria. the Tertiary-Cretaceous of the Quiriquirefield in Venezuelaand the Carboniferous coal measures of Groninsen.Netherlandsand the \'tississippian-Dcvonian Bakken Shrle in rhc WillisronBasinof North Dakota ( D o w . 1 9 7 , 1W1i l l i a m s1. 9 7 4 ) . -fhis is in agrcementwith thc observations that the maturationof organic matterand the gencrationof hvdrocarbons. andalsothe occurrence of oil fields lrrc comparablein siliceous.clastic,and carbonatesequences. Hvdrocarbonphasepressurc-ntigration of oil would also be similarin clasticand carbonate \cdiments.All othcr primarvmigrationmechanisms, however.involvinglarger quantitiesof watcr. would requirea differentkind of primarv migrationtbr c a r b o n a t eosn l y . Another phenomcnon.conflictingwith primarymigrationof oil in aqueous solution.but in perfectagreemcntwith hvdrocarbon-phase migration.is that mostcrudeoils are enrichedin saturatedhydrocarbons and depletedin polar compoundsas comparedto sourcerock bitumen.As in establishcd chromatocraphictheorvutilizedin laboratories aroundthe world, similarsolventsprefer c n t i a l l ve x t r a c ta n d t r a n s p o r ct o m p o u n d os f a s i m i l a rc h e m i c anl a t u r c .I n n o n p o l a(ra l k a n )es o l v e n t st b. r c x a m p l ep, o l a rc o m p o u n dt se n dt o b el e f tb e h i n d . -l'here is experimcntaland geologicalcvidencefor the prefcrcntialselective adsorption of differcntorganicmolecularclasses on clavsurfaces. The prcliously c i t c d i n v c s t i g a t i o nbsv N c r u c h e va n d K o v a c h e v (a1 9 6 - 5 b ) .l V a n d e n b r o u c k c ( 1 9 7 2a) n db v T i s s o a t n dP e l e t( l 9 7 l ) a n dL c v t h a e u s e rt a l . ( l 9 l i 3 a .b ) p r o v i d e additionalevidenccfor suchchromatographic adsorptionphenomcnain source rocks. 'Ihe proposedmodelof a pressurc-driven movementof an csscntiallvhvdrocarbonphaseasa majormcchanism ofprimarvmigration.rvouldalsoexplainwhv an et-iective sourccrock must containa certainminimum amount of organic mattcr.If thereis too littleorganicmatter.thc ratioof freelvmovableporcwatcr to bitumenor gasmay be too high and thusunfavorable for hvdrocarbon-phase migrationevenif thc porositvis lou . This irrgumen t hr,uerer. .li,esnotrcvcisablv meanthat rocksconsisting solelvof organicmatter.like coal.shouldbe the most productivesourccrocks.Hvdrocarbon-phase migrationis alsoin agrccmentwith the accomplishment of successful geochemical correlationsbetweencrudeoils andsclurcclocks. Besidesthe previouslvdiscussed rolc of diffusionas an initial processfor primarymigrationof gas.diffusioncanalsoplaya role in destroying reservoired qasaccumulations by lossthroughthe caprock.Leythaeuser et al. (1982)have alsocvaluatedthe latter process.The rate of dissipation wascalculatedfor the H a r l i n g c n g a s f i ei nl dH o l l a n dw . h i c hh a sr c s c r v e s ol .l g 3x l 0 eS T Pm ro f m e t h a n c in-place.B1' diffusion of methanethrough the ,100m thick shale caprock
340
Aspectsof PrimaryMigration andGeochemical Geological
continuingfor 4.5millionyearsthisfieldis reducedby onehalf of its originalsize gasfields Basedon thesedata aboutthe rateof destructionof commercial-sized only a have gas accumulations proposed that was with geologictime the concept have may either gas fields Existing geologic time scale limit;d life in termsof the persist through to be able may or they ages. young accumulation geologically by additional periodsof geologiatime only if therewascontinuedreplenishment gas accumulations certain latter sense In the rocks. source gen".uiion in the gu. ian be consideredas dynamic systemsreachingsome kind of steady-state equilibriumbetweengaslossand gasgenerationfrom the sourcerocks' Sourcerocks have been mainly viewedwith respectto their physicaland proceschemicalproperties.althoughthey are the productsof sedimentological basins historyof individualgeological ses,andcomprisepart of the sedimentary featuresand tectoniceventsare alsoof importancein Thus, sedimintological petroleummigration.Sourcebedsareoftenfinelylaminatedwith an organic-rich mav layer altematingwith organicpoor layers Suchgeometricarrangements scale some a larger provide preferentialavenuesfor primary migration On iou.ce t"qu.n"e.. especiallyclastics'may contain interbeddedsand and silt for drainagethanthick. uniformshale layers.Thiswould be moreadvantageous sequences. Tectonic events mav causeabnormallyhigh strain in sedimentaryunlts. This may aid to increaseinternalfluid especiallybv horizontalcompression. In activetectoniczonessuchas fracturing. rock prissureswhicheventuallycause East' repeatedearthquakesmav Middle and the parts of California iertain movemenl. hydrocarbon fracturing and contributeto
Summaryand Conclusion The predominantmechanismof primary migrationmay changewith different subsurface conditionsprimarily related to increasingdepth of burial. In shallowsediments down to deDthsof about 1000 to 1500m, before the onset of the main phaseof hydrocarbonformation, solutionmigrationseemsto be favoredascomparedto an oilp'hasemigration. Solution migration may also play a role at greaterdepthsat higher However.solutionmigrationalonedoesnot seemto producema.ior temperatures. deposits. ihe dominating and most effective form of primary migration is a hydrocarbonphasemigration. in relativelyimpermeable.densesourcerocks it is accomplishedby microfracluringof the rock. Hydrocarbon-phasemigration is in agreementwith the empiricaland factual geologicaland geochernicaldata Water flow is not neededasa driving force for migration. Well-documentedexamplessupport the conclusionthat hydroiarbons can bi releasedfrom such compacted,dense,relatively impermeable source rocks. The CretaceousLa Luna limestonesof Western Venezuela,dense limestonesbetweenreefsin the DevonianSwanHills Formation of Alberta. Silurian Bakken sourcerocks of Hassi Messaoudin Algeria and the Mississippian-Devonian Shalein the WillistonBasinof North Dakotacanservehereasexamples' movement'diffusionof dissolvedhydrocarFor gases,next to a hydrocarbon-phase bon moleculesis an impo ant processof primary migration,especiallyin deepbasins' It followsfrom the previousdiscussionthat distancescoveredby primary mrgratron are commonlyin the order of metersor tens ol meten.
Chapter4 Secondarv Misration and Accumulation
Secondarymigrationis defined as the movementof petroleumcompounds throughmorepermeable andporouscarrierbedsandreservoirrocks,asopposed to primarylmigrationthroughdense.lesspermeableand poroussourcerocks. Secondary migrationterminates in hydrocarbon pools.but tectoniceventssuchas folding.faultingor upliftingmavcauscredistribution of filledoil or gaspoolsand thusinitiatean additionalphaseof secondary migration.When this rcsultsin a new accumulation it is sometimes called remigrationor tertiarymigration. Oil andgasaccumulations generallvoccurin the structurallvhighestavailable part of a trap. This is becauseoil (spec.gr. 0.7-1.0g cm r) and gas (spec. gr. < 0.001g cm ') havelowerdensities thanthe surrounding aqueousporefluid (spcc.gr. L()_l .2 g cm r) andrisebv buovancythroughthe water-saturated pore spacc.The maindrivingforcein secondary migrationis buovancy.Oil andgaswill form a pool whenevertheir further.generallvupwardmovementis retardedby a Iesspermeablelaverof rock.Thisempiricalobservation is almostasold astheoil industry.and it is cxemplifiedin thousands of accumulations aroundthe world. The formationofoil andgaspoolsrequiresa decrease in the poreopeningsizcto preventthe continuationof a two- or multi-phase fluid flow. Thus.the termination or continuationof secondarvmigrationis determinedby the interplay bctweenthe drivingtbrcethat causes the movementof hydrocarbon dropletsor slugs.and the capillarvpressuresthat resistthis movement.As long as the aqucousporefluid in the sub-surface is morc or lessstationary.the only driving forccfor movcmentof a discretehvdrocarbon phaseduringsecondarl' migration is buoyancv.Wc know.horvever. thatthereis waterflow in thc sub-surface dueto hvdrodvnamic gradients.It is importantto distinguish betweenhvdrostatic. noflow or equilibriumstatc. and hvdrodynamicconditions.water flow due to hvdraulichead gradicnts.Hubbert (1940, 1953)describesthe principlesof hvdrodvnamicconditions.Water flow under hvdrodynamicgradientsmodifies thc buovantriseof oil and gas. Threeparamctcrscontrolsecondarv migrationandthe subsequent formation of oil andgaspoofs.Thesearethc buov-ant riseof oil and gasin water-saturated porcrusrocks. cupillarypressuresthat determinemultiphaseflow and. as an importantmodifvinginfluence.hvdrodvnamic fluid flow. Secondarymigration resultsin the formationof hvdrocarbonpoolsand may causehvdrocarbons to scepout at the surface.
Chapter4 SecondaryMisration and Accumulation
Sccondarymigration is defined as the movementof petroleumcompounds throughmorepermeable andporouscarrierbedsandreservoirrocks,asopposed to primar! migrationthroughdense.lesspermeableand poroussourcerocks. pools.but tectoniceventssuchas Secondarv migrationterminates in hydrocarbon folding.faultingor upliftingmaycauseredistribution ol filledoil or gaspoolsand thusinitiatcan additionalphaseof secondary migration.When this resultsin a new accumulation it is sometimes calledremigrationor tertiaryrnigration. Oil andgasaccumulations gcnerallyoccurin the structurallyhighestavailable part of a trap. Ttris is becauseoil (spcc.gr. 0.7 1.0 g cm r) and gas (spec. gr. < 0.001g cm ') havelowerdensities thanthe surrounding aqueousporefluid (spec.gr. 1.0-1.2 g cm ') andriseby buoyancythroughthe watcr-saturated pore space.The maindrivingtbrcein secondary migrationis buoyancy.Oil andgaswill form a pool whenevertheir furthcr.gencrallyupwardmovementis retardedby a lcsspermeablelaverof rock.Thisempiricalobservation is almostasold astheoil industrv.and it is exemplifiedin thousands of accumulations aroundthe world. The formationof oil andgaspoolsrequiresa decreirse in the poreopeningsizeto prevcntthe continuationof a two- or multi-phase fluid flow. Thus,the termination or continuationof secondarymigrationis determinedby the interplay bctwccnthc drivingtbrcethat causesthe movementof hydrocarbon dropletsor slugs.and thc capillar! pressuresthat resistthis movement.As long as the aqueouspore fluid in the sub-surface is moreor lessstationary.the only driving tirrcefor movementof a discretehvdrocarbon phaseduringsccondary migration is buoyancv.We know.however.thatthereis waterflow in the sub-surface dueto gradients.It is importantto distinguish hydrodvnamic betweenhvdrostatic. nollow or equilibriumstate. and hydrodynamicconditions.water flow due to hydraulichcad gradients.Hubbert (1940. 1953)describesthe principlesof hvclrodvnamic conditions.Water tlow under hvdrodynamicgradientsmodifies the buo-"-ant riscof oil and gas. Threeparametcrs tbrmation controlsccondarymigrationand the subsequent of oil and gaspools.Theseare the buolantriseof oil andgasin water-saturated porous rocks. cupillar,t:pressuresthat determine multiphaseflow and. as an importantmodifyinginfluence.hltdrodynamic fluid flow. Secondarymigration to rcsultsin the formationof hydrocarbonpoolsand may causehydrocarbons scepout at the surface.
312
Migrationand Accumulation Secondary
4.1 The BuoyantRiseof Oil and GasVersusCapillaryPressures In the previouschapterit wasconcludedthat our presentknowledgeindicates of primarymigrationrsmost thatthe mosteffectiveandpredominantmechanism solution migrationis less phase and that transport probably a hydrocarbon greater importanceonly in certain is of important.Primarymigrationin solution phase petroleumgeneration.at of the main geological settings.beforeand after migrationin its initial secondary great In any case, depths. relativelyshallowor primary migration. prcdominant mode of the stagesis influencedby sourcebedsasa low-porosity leavethe dense,fine-grained. As hydrocarbons phascandenterthe largerporesof a carrierbedor reservoir discretehydrocarbon o n irsstate form.depending rock,larger globulesof oil or gasshouldimmediately of dispersion.Larger bodiesof oil may move upwardby buoyancy.but trny to flow due to highcrsurlace thereis more resistance dropletsmay not because cnergiesper unit volume in smaller bodies of oil. This phenomenonwas recognizedand introducedinto the literatureof secondarymigrationbv Athy (1930).Hydrocarbons, however.which lcavea sourcebed in a dissolvedstate but. tbllow waterflow througha carrierbedor reservoir cannotriseb-vbuoyanc,v. rock. When finally releasedfrom solutionby a decreasern temperatureor pressureor due to increasein pore water salinity,they havc a fate similarto phase. which leavea sourcebed asa separate hydrocarbons. ) ave . o b s o n( 1 9 5 , 1a)n d H o b s o na n dT i r a t s o o( 1 9 7 5 h G u s s o w( 1 9 5 4 .1 9 6 8 )H pressure displacement elaboratedon the principlesof buoyantriseandcapillarv and practicalexamplesttf this for a discretehydrocarbonphase.Refinements (1975)and the following (1968) and Berg prescnted by Poulct principlewere work of thesc authors. heavilv on the relics discussion The interfacialtensionbetweentwo immisciblephases(liquid-liquidor gasasa forccthatactsat the interfaceto producea pressure liquid)canbe envisaged pressurcdifferenceacrossthe interfaceis anotherform. across it. The difference capillarypressure.The work done by thc interfacial of thc term or meaning. tensionresults.for example . in a changcin the surfaccareaof a drop of oil when immcrsedin water.The drop will tcnd to assumcthc smallestpossiblesurface area.which ideallyis a sphcrc.The interfacialtensionbetweenoil and water througha pore resistsdistortionof thc sphericaldropletand retardsits passage to throatwith a diametersmallerthanthesizcof the droplet.The fbrcenecessary squeezethe dropletthroughthe pore is alsocalledcapillarypressurc.or more in rock poresare the causetor correctlyinjcctionpressurc.Capillarypressures arehigherfor smallerporc diameh1'drocarbon entrapment.Capillarypressures ters.and assumespecificvaluesfor a certainrock. Under hydrostaticconditions,buoyantforcescoulCbecomehigh enoughto migration Buoyantforces overcomecapillarvpressure,whichresistssecondary with the densitydifferencebetweenporewaterandoil (9,,-9,,)andwith incretrse behaveanalogously. hydrocarbons increasing heightof the oil column.Gaseous theydevelopstrongerbuoyantforces Oil but because the-vhavelowerdensities. an equilibrium trappedin a reservoirundcr hydrostaticconditionsrePresents prcssures in the and capillarv statebetweenbuoyantforcestrying to moveoil
.1.ITheBuoyant Riseof Oil andGasVersus pressures Capillary
343
sealingcap rock rhat resistthis movement.The equation [fq. (l)] for this equilibriumsituationis
)- '. ,l t \,,
I \ ,, )
- 2" 8' (Q.-AJ
(1)
wnere. 1, = intcrfacialtensionbetweenoil andwaterin dynecm-r, r : radiusof poresin cm, ;,, = heightof oil columnin cm, g - acccleration of gravitvin cm s :. 9 , , = d e n s i t yo f w a t e ri n g c m - l 9,, = densityof oil in g cm r. This cquationor derivatives of it can be called..buovancv equation... 'buoyani In additionto the heightof the oil column:n. rhe forccis definedby , the acceleration of gravitv(g = 98t cm s r1 an.l the densitydifferencebetween watcr and oil (9,, .o,) in g cm r. The capillarypressureis determrned bv the interfacialtcnsionbetwcenoit andwatcr(i) in iyne cm-r andthe radius cmj 1in of p'c]I9throatsin the sealingbarrierrock (r,) as comparedto the reservoir lh" rock radii (r,,). It followsthatthe maximumheightof an oil columnwhichcanbe heldin place, and which is alsocallcdcriticalhaight(2.), is givenas follows(Hobson, 195,1; B e r g .1 9 7 5 ) :
''="(+-i)''(p'-e')
(2)
The secondarymigrationof oil througha water-wetcarrierbed or reservoir rockhasbeen.described by Hobson(1954).An undistorted oil globuleat restin a rock is in equilibriumwith the surrounding porewater.In such-acase.the radius of the rockpore (rr,)mustbe cqualto. or l-aige.than,that of the oil globule. The pressure differenceacrossthe oil-waterinre;faccis energetically equatable with the capillarypressurc.It causes the pressure within rhe o"ilgtobule io be greater thanin the surrounding water.The piessure/p) (in dynecm1:)insidetheg'lobule equalstwicethe intcrfacialtensiony dividedby the ridius oftire globule(r);thus .'| p = 2; (Berg. 1975).If the buoyantforceis greatenough to forcethe globule upwardthrougha porethroat,theglobulemustbe distorted.The above equation showsthat the excesspressureinside the globuleincreasesas the radiusof curyaturedecreases. The pressure al.theupperendof a distortedglobulein a pore throatis, therefore,greaterrhanin th. poii The ci'pillarypressure 1flg. III.4.1.1. opposesthe buoyantforce untir the radii of curvatureinsidethe dlstortedoir globuleare equalat the lowerand the upperend. Oncethe globule hasreached thisstageand movedhalfwaythroughthe throat,it canriseiy buoyancy. If the curvatureis smallerat the lower end than at the upperend, capillarypressure gradrentand buoyantforce act in the,u." up*uri directionand secondary migrationis furtherfacilitated.
Secondary Migrationand Accumulation
-t.l.l
Fig. III.,1.1. Transport of an oil globule throughpore throatsin a water-wetsuh-suropposes faceenvitonment.Capillarypressure the buovantforceuntil thc radiusof curvature insidcthe distortedoil globuleis equalat its lower and upperends.(Modifiedafter Berg. 1975)
2v 2x rl rn
2t
2r
t
f
This sameeffect ma! be of importanceacrossa boundarl'bctweena finerock. i.c.. betweena dcnsesourccrock and a grainedand a coarse-grained porouscarrierbedor a rcservoirrock.As soonasoil particlesreachthe boundary carrierbcd. andarc not betwccna line-graincdsourcerock anda coarse-graincd of rock. they ntc driveninto thc coarsebcd by capillary encloscdbv eithert-"-pc forces.Thc transferreducesthe total surfaceareaand the curvatureof the oil piuticles. The buovant force acting on a distortedoil globule or a gas bubble is determinedprimarilyby the dcnsitydifferencebetweenthe oil or the gasandthe surroundingwater.The densitydilferenccbetweennormaloils and porewaters 'for gas The otherimportant rangefrom0. I to 0.3g cm-r andirpproach1.0g cm parameterinfluencingbuoyantforcc.andtheonly variablein a givensuh-surface situation.is the height of the oil or gascolumn [seealso Eq (2)] All other parametersincludingpore and throat diametersof the rock and densityand pressure andtemperature intcrfacialtensionof thefluidsundergivensub-surface buoyant andcapillary conditionsare fixed.Therefore,the equilibriumbetween oil or gas of new forccscan only becomeunbalancedthrough the accretion when the buoyant vertical height. materialto form largerbodieswith greater will movc (1)]. the oil droplet pressure capillary force is greaterthan the IEq. would mean this this equation. pore throat. Rearranging throughthe :,, 8 (0"-4,,)>
i I
) v |
\r'
I \
_ _l
(3)
rtl
Migration and Secondary 4.2 Hydrodynamics conditionsprevailin hvdrostatic thatessentially To thispointit hasbecnassumed water carrierbedsandreservoirrocks.Thereis. however'frequentlysub-surface flow. and the influcnceof hydrodynamicconditionson secondarymrgratron cannot be neglected.Anv flow of water in an aquifer. depcndingupon its migrationof hydrocarbonsWater direction.coultlhinderor facilitatesecondary
.1.1Hvdrodvnamics and Secondary Migration
-t4)
Fig. IIL.1.2. Transportof oil globulethrougha pore throat rn a water-wetenvironmentunder hydrodvnamicconditions.Upward flow of water helps buovancvto overcomeopposingcapillarvpressure(Nlodifiedafter Berg. 1975)
upw.rd
llow
oi w.t.r
tlo$ is relatcd to hvdrodvnamic gradients. If the gradicnt is in an upward dircction. it aids buovtnt forces in moving pctroleunt. tf the hvdrodvnamic gratlient is in a tlownward direction. the buoyant tirrceshave to be greater thrn required under hvdrostatic conditions. in order to balance or ovcrcome the opposingflow pressures.In terms of accumulation.an oil column hcld in placein a reservoir against a barrier rock exhibiting a certain capillary pressurc can be hiqhcr if there is additional pressurefrom clownward-flowingwatcr which helpsto h r t l l n r ' et h c h u " \ i l n t [ ( ' r c e \ , ' l o i l . H."drodvnamic gradients could be an important mechanism for secondary migration. especiallvduring the initial phase. The final direction of secondary migration of oil and gas would be dependent on the relative magnitudes and dircctions of hvdrodlnamic and buoyant forccs. A crucial point in the interplal, betwcen these forces is the magnitude of the hvdrodl'namic gradicnt along a stringerof oil. Consider first the theoreticalsituation of a vertical or a horizontal stringer of oil. and then the morc probable situation of an inclined stringer.The first caserefers to an oil stringer of height 2,,.and densitv q,,, in water of density o,,. with a vertical positive hydrodynamicgradient.m, in the water, that resultsin an upward flow (Fig. I I I.,1.2).This upward flow of water parallelto the oilstringer with thc height ;,, assistsbuovant force. If this stringer of oil is held in place b1, capillarv prcssure.the prcvious equation can be rewritten as follows:
.r(+
l:
., t (o,,-q,,) + 2,,. m.
(1)
Thisequationmustbe comparedwith Equation( I ). It mcansthat thecapillary (expression pressure on left sideof thc cquation)rcsultingfrom thesmallcrradius r, in a porethroatat the upperendof the oil stringcr.balanccs thc buoyanttbrcc plusthe tbrceresultingfrom the upwardflow of watcr(cxprcssions on rightsidc of the equation) . I f thereis downwardwater flow. m is a negativequantityandthe hcightof the oil stringer;,,mustincrease accordingll' aslongasthe oil stringeris h e l di n p l a c e . Considernow a horizontalstringerof oil alongthe top of a horizontalcarrier bed.In thiscasc.buoyanttbrceis negligible.andthe drivingmechanism canonly be derivedfrom horizontalwaterflow (Fig.IIL'1.3).The hydrodvnamic gradient. rn, along the length./, of the oil stringerdeterminesits movementunder thc
MigrationandAccumulatlon Secondary
346
Horizontal Fig. IlL1.3. transportof a stringerof oil under hydrodynamiccondigrations.The hydrodynamic dient. m, alongthe length.1, of the oil stringerdetermines its horizontaldispositionin the carrierbed
conditionsin the carrierbed (HobsonandTiratsoo.1975). As buovancynowcan be neclected.the equationcan be writtenasfollows: l . m --
t l
- l /. ,t l _
\ r'
1 r
-l
(5)
I
r" I
The expressionat the left-handside ol the equationis the driving force. on the right-handsideof resultingfrom the horizontalwaterflow.The expression exertedb)' the capillary the equationis alrcaclyfamiliar.sinceit is thc resistitnce prcssureto preventthe movcmentof the oil stringer.Thc smallerradius.r,. in a pore throat. is now at the leadingend of the stringer.and pointsin the same directionasthc tlow of water. Finally.considera stringerof oil. inclinedat an angle.@. The flow of water headsat its lowerand occursalongthe stringer.andthe differencein hydrostatic to in addition the beightof the oil upperends(Fig. III.4.4) mustbe considered (Berg.1975).The changein the columnheld in placeonly by capillarypressures gradient.lf proportional to the hvdrodynamic is heightof the oil columntrapped If it is in a shorter. will be thc oil column direction. the flow of wateris in an up-dip ( moditied 1)] can be The basic equation will be longer. down-dipdirection,it IEq. (Poulet, 196t1): follows in thiscaseas t l
1. [S (s,,-s.,.)sin6/ t m] = ) r t _
\ r,
1:/- '
1 r
_l rt I
(bl
Fig. It1..1..1.Transportof inclined stringerof oil under h_vdrodynamic in hvdroThe difference conditions. staticheadi/Xr-Xjl at the lowerand upper end of the oil stringer.the heightof the oil colum12,,/.andthe flow of water determinethe nlovementof the oil st nger
-1.3Geolocicaland Geochemical Implications of Secondary Migration
317
.1.3Geologicaland Geochemical Implicationsof Secondary Migration As previouslvdiscussed, the principalfactorsgoverningsecondary migrationofa petroleumphasearebuovancyandhydrodynamic waterflow. Capillarypressures resultingfrom narrowwaterwet-porethroatsopposethis movement.The final distributionand occurrenceof petroleum,once it has been generatedand released from a sourcebed,is thereforecontrolledmainlyby thesethreefactors: buoyancy.hydrodynamics and capillarypressures. Whetherprimarymigration occursmainly in a hvdrocarbonphaseor lessprobablyin aqueoussolutionis crucialto the initialstages of secondary migration,but lesssoin thefinalstageand thc formation of pools. Ultimately oil and gas must appear as a discrete hydrocarbonphasebeforethey canaccumulate in a trap. It is difficultto assess the exactpoint when hydrocarbons beginto coalesce massivelyinto largerbodiesof oil or gns.ln productivebasins,suchas in the EasternSahara.Tcxas.or SouthernCalitbrnia,oil stainsarefrequentlyobserved in the top part of thc rcservoirbedsevenat somedistance from producingfields. Oil-fieldstudieshaveshownthat thereis alwaysan irreducibleamountof oil left behindin the reservoirrock after productionis completed.This suggests that migrationasa massiveoil phasedoesnot normallyoccurover greatdistances in theorderof 1(X)km. unlessspecialgeological conditions exist.for example.along the unconformitybetweenCretaceous and Paleozoic bedsor in the Cretaceous sands themselvesin Albcrta. Western Canada. and orobablv aide in the emplacemcn u tl t h e h u g eA t h a h a s ehae a v ;o i l a c c u m u l a t i o n . It is vcrv difficultto quantifythe relativeimportanceof buoyancyor hydrodynamicforceson secondary migration.It seemsclear,however,that in the initial stagesof secondarvmigration,oil particlesare on the averagesmallerand in a more dispersedstatethan duringthe final stage.Initially microscopic and submicroscopic oil particlesshouldbe relativelymoreabundantin the aqueouspore fluid thanlargeroil particles.In general.vervfinelydispersed oil dropletswill not strictlvfollow the law of buoyancyand will be more easilymovedby relativcly u'eakhvdrodvnamicwater flow. The ovcrall influenceof hydrodvnamicuater movementis, therefore.morc importantduringthc initial stagesof secondary migrationthan duringthe linal stages. Hydrodynamicphenomcnaare crucialfactorsin the understanding of subsurfaceflow networkleadingto the formationof oil pools.As a consequence, an increasing numbcrof papersarebeingdcvotedto thissubject.The aim is a better undcrstanding of hydrodynamic flow patternsin hydrocarbon-prone basins,such as in the PcrmianBasinof thc United States(McNeal. 1965).in the Western Canada . 9 6 9 a1. 9 6 9 b 1 H i t c h oa n dH o r n . 1 9 7 , 1o)r s e d i m c n t a rbva s i n( H i t c h o n 1 in thc AlgerianSaharaof NorthernAfrica (Chiarelli.1973).Theseinvestigations pursuea reconstruction of pastandpresenthydrodvnamic flow patternsbasedon actualsalinityandfluid pressurcdataandthe regionalgeological evolutionof the basin.lt is logicalandnecessary thatsuchstudiesbe linkedwith investigations on the regionalhvdrocarbon generative potentialandmaturityof sourcerocks.This meansthat paleo-hydrodynamic patternsneedto be elaboratedfurther.
1 I
I I
I
Migrationand Accumulation Secondarv
3,lll
jur enile hasin
basin intermediate
senile basin
Fig. IIL,I.S. Three main types of . e d i m ( n t a) rb r r s i' nc l l s r r [ r cldc c o rd i n gt o hldrodlnamic conditions:juvenile bar i n s . i . e . . h e f o r er n v r r : i t rhn) m c t c o r i c waterslintermediatebasinsduringinvasion by meteoricwaters;senilebasrns, afterinvasionby meteoricwaters.(After Coustauet al.. 1975)
r-::i::::::-
l\
t
I
L
l\\ Ll\
l\ t\
\ Lt
_l
!.nd
./t
nydrcdynani.llow
-,'
nydtodynanic llos doelo.rc.s3 hydroslalic head (arle3ian)
Pr'3sur'
due to compaclion
Modelshaverecentlybeendevelopedthat link the generalmovementof pore fluid in sedimentarvbasinsto parameterssuch as type and rate of sediment deposition.porosity.pore fluid pressureand temperaturc.Two suchmodels ( M a g a r a1 . 9 7 6S : h a r p j.r . a n dD o m e n i c o1. 9 7 6o) n T e r t i a r yb e d si n t h e G u l f o f Mexicoconcludethat thereexistsa compaction-induced horizontalmovenentof towardthe edgesof the basinor porefluid throughmorepermeablcsandyla-'-ers is to permeablefaults.Verticalmovementof fluidsacrosslithologicalscqucnccs comparativelylessimportant.Furthermore,the temperaturedistributionis a sensitiveindicatorof tluid movement.which is largelyresponsiblefor heat transtcr. g to hydrodvnamic sedime ntarvbasinsaccordin Coustauet al. ( I 97,5 ) classified conditionsandrelatedthisclassification to theirpctroleumpotentiallrom organic before geochemical Threemaintypesofbasinsaredistinguished: considerations. ( I ). during(2) andafter(3) the invasionby meteoric(fresh)waters(Fig.I I L,1.5). young.with compaction-induced ccntrifugal. | . Juv,enile baslrrs, not neccssarily l a t e r aw l a t e rm o v e m e n tE. x a m p l e sN: i g e r i aG . u l f o f M c x i c o .D o u a l ab a s i n . North Sea.NortheastSahara(betweensalt deposits)amongothers.Petroleum interestin the basinsis verv strong. 2. Intermediate with centripetalwatermovcmcnt.artesianpropertiesand baslrr.r freshrvaterinvasions.Examples:PersianGulf. East Sahara(lllizi-Tinrhert basins).ParisBasin.CentralTunisia.Sahara(below thc salt) and others. Petrolcumintercstin suchbasinsvariesfrom very strongto modcratc. 3. Senilebaslrrswith hydrostaticconditionsand generallyinvadedby meteoric waters.Exirmples:NorthwestAquitainebasin.partsof North Spanishbasin. Thereis little or no petroleuminterest. Thc papcr by Coustauet al. (1975)accountsfor the fact that hydrodynamic flow of water is initialll of great importanceto secondarymigration and formationof pools.Howevcr.if it is too strongor laststoo long.hydrodynamic
.l.l Geological andGeochemical Implications of Secondarv Migration
349
waterflow retardsthe formationof pools,or evendestroysexistingaccumulationsbv dismigration. As oil particlesgrow in size.thev becomemore subjectto buovancv.Larger particlesgrow at the expenseof smallerparticles.and attempt to attain the smallestpossiblesurfacearea with respectto volume.Upward movementof petroleumparticlesor gas bv buoyrncl starts wheneveiopposingcapillary pressures are low and thereis an insufficientcounter-flowof water.Becauseof buovancv.oil and gasglobulesalwaysseekthe highestpoint in a carrierbedor reservoirrock. Their movementfollowsthe directionof lowestresistance. This mav resultin a very tortuouspath. generallyupward.more or lessnormal to bcdding(Gussow.1968).As thevmove.oil particlesmergeinto globules, patches and stringers.This increasestheir buoyanttbrce and assiststheir movement throughnarrowerpore openings.enablingthem to movethroughfiner-grained rocks.Eventuallya situationmay be reachedwhereat somccrestalpoint pore diameters get too small.andcapillaryprcssures canno ktngerbe oreriome.This could bc the start of an accumulationwhich mav finallv attain ec()nomie dimensions or remainmerelva smalllocalconcentration of hvdrocarbons. P o u l e(t 1 9 6 8a) s s u m eadc e r t a i ng r r i ns i z e( m e c l i arna d i ir r f g i a i n 0s . 5m m )i n a n inclinedsandstone (dip: t(X)m over-lt)km). a ccrriiindensitltlifterencebetween o i l ( 0 . l l 7 5 g c mr ) a n dw a t e r( 1 . 0 7g c m ' . ;r n c la c e r t a i ni n t c i f a c i ar le n s i o n y ( 4 0 . , 1 dvnc cm ' at 40'C). and calculatedthc lengthof a continuousoil stringerthat couldbe formedandheldin place.Theseconditionsaresimilarto thoseobserved i n t h e D e v o n i a nr c s e r v o iirn t h e I l l i z ib a s i ni n t h e A l s e r i a nS a h a r a . a ) U n d e rh v d r o s t a tci co n c l i t i o ntsh.ec r i t i c ahl e i g h t o f t h eo i l c o l um n ( : , , )w o u l d be 3-5cm. and at a greatervaluethe stringeru,ouldmove.The lengthL of the oil stringeris calculatedfrom the angleof inclination(@)of thc sandstone andthe h e i g h(t: , , )o f t h eo i lc o l u m na c c o r d i nt go r h cc q u r t i o nL = A stationar-"-oil s u ta phaseof 140m longcouldbetbrmedalongthetop of rheinclinedsandstone. With increasing dcpth.i. c.. increasing temperature and pressure. only minorchanges with respectto the heightof thc oil coiumnwouldbe expected, unlessgasisatlded to thc oil. Oil water surfacetensionI wouldincrease with pressurc. but would be approxrmatelv compensated bv a decrcase at highertempcratures. b) U nderhvdrodvnamic conditions.however.the lengthof thc oil phasewould be changed.In a horizontalreservoir.similarto that described above.a Drcssure r c m I w o u l db e gradicntof0.5dvnecm r c q u i r e dt o m o v et h i so i l s t r i n g i r u i t h a lengthof 140m. If the rescrvoirhad a permeabilirv of I darcl rnd a porositvof 2 5% . a p r e s s u rger a d i e notf 0 . 5d v n ec m r c m L u r i u l d c a u s e u r t c r o i a v i s c t x i t v of I centipoiseto tlow about60 cm per year.This flow is of the same()rdcrot magnitudctl,picallvobservedin artesianaquifers. Similarcalculations canbe madefor the movcmentof gusin thesamereservorr ( P o u l e t1 . 9 6 1 3A) t. a s h a l l o wd e p t ho f b u r i a l .r h cc r i t i c ahi c i g h ro t a g a sc o l u m n heldin placewould be about I 7 cm, assurning a surfacetensionof 50 dvnecm At greaterdepth. the densitvof the gas increasesslowlv.but tnc gas-warer surfacetensiondecreases stronglywith increasing temperature andpressurear a rate of about l0 dvnescm ' for 100bar. and about 3 dvne cm I for 30.C. { } d c p t h .t h t ' c r i t i c shl c i g h o T h u s .u n d e rh v d r o s t a t icco n d i t i o nast , 1 0 0 m t f a gas
350
( Plioc€n€ - Pl6istocene
U.
:*--!
Oligoc€ne
0
1
2
3
El producing int€.vsl - oil pool
F i g . I I L l . 6 . G e o l o g i c a lL r n s \ s e c l i o nthrough Quiriquire oil field. Venezuela. (After Silverman. 1965)
columnwouldbe -5cm. ascomparedto an oil columnof 35cm. Thc solutionof oil in the gaswouldchangethe heightof the gascolumn.but only to a minorcxtent. From the foregoingit is deducedthat a delicatebalanceis established whena bubbleorstringerofoilorgasisheldinacarrierbed.Anytectoniceventsuchasa fold. regionaltilt, uplift or subsidence, or a suddenearthquakc.maychangethis delicatebalancc.Tectoniceventsare. therefore.also an imDortantmcansof influencingsecondary migrationand the arrangement of fl{)$ patterns. Moving oil particlesare in steadvcontactwith the surroundingpore wilter. Polar oil moleculesare thereforepreferentiallyconcentratedat the oil-pore water interface.Thesemoleculesand other, more watcr-solublc hydrocarbons suchaslow boilingaromatics, areexchanged betweentheoil andthewaterphase. Althoughthe watersolubilitvof thesccompoundsis verl' lorv.they will. nevertheless.bc preferentiallylostto the waterduringsccondary migration.Dcpcnding on the wettabilitvof the internalsurfacesof the carrieror reservoirrock. petroleumcompoundswill occasionally be adsorbedand the more polar compoundswill be prcfcrcntiallyleft behind.Silverman( 1965)reviewedgeochemical changesobservedin crudeoils duringsecondary ntigrationand prcscntcdnew evidenceby analyzingoil samples.taken in an up-dip direction from the Q u i r i q u i r eo i l f i e l d ( F i g .I I L . 1 . 6 i)n V e n c z u e l (aT a b l el l l . 4 . l ) . T h e s ec h e m i c a l changesin crude oils. causedby secondarymigration.can be summarrzed as follows:alongthepathof migrationthcreis an increase in nonpolarhydrocarbons and a decreasein contentof asphaltencs. resins.porphvrinsand other nonhydrocarbons.Water washingmay causea loss in low boiling. more soluble. hvdrocarbons. Furthermore.a slightdecrease in the rrC/rrC-isotope ratiocanbe observed(Table Ill.4.l). causedprimarilyby the preferentiallossof aromatic hydrocarbons. The influenceof secondarymigration on the chemistryof gasesis less u n d e r s t o o (dM a y e t a l . . l 9 6 l l ; S t a h le t a l . . 1 9 7 - 5S: t a h l .1 9 7 6 ) T . he changes observedto dateare difficultto explainbecause they are apparentlynot always consistent.The parametersinvestigatedare r:rC/rlC-isotope ratios, nltrogen content.and ')N/''N-isotoperatios and amount and distributionof "hcavier
.1..1 Terminationof Secondary Migrationand Accumulationof Oil and Gas
-J) I
TableIII.4.1. Quiriquirefieldcrude-oilanalyses. (After Silverman,1965) Well
Q-59It
Q-278
Q-2411
Producing interval (depth in feet)
3885,.1070
2665-2855
2()702570
Relativeparaffinicity
0.78
0.81
t.o2
Relative concentrations of carbonyl (C=O groups)
0.il3
0.40
0.27
Sulfur content (wt.9/..)
1.26
0.78
0.70
Relative conccntrations of vanadium porphyrins
19.6
10.6
lo.3
I 'ct rc ( d i n t i r )P D B
2' 7.r
27.3
2'7 .s
hydrocarbons"(C5C). The difficulty in explainingdifferencesin chemical compositionof gasesalonga presumedmigrationpathmay in part be dueto the relativelysmallnumberof well-documented casehistories.primarilv.however. the problemcenterson the great varietyand ambiguityof factori which can influencethechemicalcomposition of gas.Amongtheief;ctorsarediffusionand effusionprocesses, solutionand adsorptionphenomena,and temperatureand pressureeffectsand finally the greatabundance and mobilityof gasmolecules.
4.4 Terminationof Secondary Migrationand Accumulationof Oil and Gas The_end of secondary migrationandthe final stagein theformationof oil andgas poolsis the concentration of oil andgasin thehighestavailablepart of a trap. Ii is necessary that the total accessible pore volume in the trappingzone is of a magnitudesufficientto accommodate quantitiesof commercialinterest.Oil or gasmay be trappedin any rock of suitableporosity,regardless of lithology.The cap-rockor seal.by virtueof its generaldecrease in pore diameters,musrexert capillarypressureswhich are great enoughto stop the passageof oil or gas particles.Any suchsealwould stop hydrocarbonpirticlei of alertain size,no matter,whether the drivingforceof secondary migrationis buoyancyor hydrodynamicflow. Underhydrostatic conditions,oil or gaswouldsimplyrise,according to the principlesof buoyancy.to the highestavailablepart of the trap withoui grossmovementof the aqueousphase.
352
Secondary MigrationandAccumulatron Fig. 111.1.7.Displacement and separationof oil and gasin an anticlinal reservoirunder the increasing actionof flowingwater. (Modified afterHobsonand Tiratsoo.1975)
Under hvdrodynamicconditions.however.the moving watcr must move throughthe barrierrockor sealwhichfiltersout thc hydrocarbons. Alternativclr-. water can cscapethroughthe reservoirrock. onceit haspassedthe crestalor other part of a trap where hydrocarbons are unloaded.Under hydrodynamic conditions.it is not alwaysnecessary that oil or gasoccurin thecrestalpart or the culminationof a trap. The force of the moving water has a directionand magnitudcthatmayholdandtrapa h1'drocarbon phaseat othersuitablepositions of the reservoirrock. A barrier effect may be causedbv a changeeither in lithologyduc to finer grainsize.bv moreintensecementation. or bv a changein dip (e.g.. at hingelines).It may alsobe a down-diphydrodvnamic gradientthat holdshydrocarbons in placeagainstthe forceof buoyancv. An examplefor displacement of oil andgasin an anticlinal.structuralreservoir is givenin FigureIII.'1.7.Under hydrostaticconditionsand in a reservoirwith properties.gas-'oilandoil watercontactswill be horizontal.At uniformphvsical low ratesof water flow the oil-water contactmay be inclined,but the gas oil contactremainshorizontal.At higherflow ratesthe oil will be displaced further and the oil-watercontactincreases in steepness and, thus.oil and gasbeginto becomeseparated.A gas-oil contactremainshorizontalbut the gas water contactis lesssteepthan the oil-watercontact(Hobsonand Tiratsoo,1975). Besideshydrodynamics. capillaryeffectsin reservoirswith inhomogeneous pore sizedistributionsmav be anothercausefor irrcgularhydrocarbonwater contacts.lf the averagegrain size diminishesacrossan oil-filled sandstone reservoir,a curvedoil-water contactmay parallelthis change.In line-grained rock,waterriseshigherdueto capillarvattractionthanin the coarse-grained part of the reservoir. Oil andgasfoundin a pool reflectin composition whathasbeencollectedfrom the drainagearea of secondarymigration.A huge anticlinalstructureunder hvdrostaticconditions,for example.collectsoil and gas.that risesby buoyancv independent of their source.As oil andgasrise,theyaccreteandbeginto tbrm a continuoushvdrocarbonphasenetwork throughoutthe porc systemof the reservoirrock. The water in the pore svstemis displacedby the moving hydrocarbons accordingto the principlesof buoyancyandcapillarypressures. As longasthe mineralsurfaces in a reservoirrock arewater-wet.whichis generally thecase.a certainminimumamountof wateris lefi behindin thereservoirrockas
'1..1 Terminrtion of Secondarv Migration andAccumulation of Oil andGas
J53
a water film coatingthe mineralsurfaces.This wateris termedthe irreclucible mrnimumamount of connatewater, which is found in all hvdrocarbon-filled reservoirs. llthe poresizedistributionis inhomogencous in a reiervoir.theremay be certalnparts where oil or gas cannotenter, becauseof excessive caoillarv pressures. Thus, even in an oil-filled reservoirthere may be isulutedwater_ saturatedpocketsdevoidof hydrocarbons. Sucha situationoccurscommonlvin carbonates and in detritalreservoirrockswith secondary cementation. The degreeof oil or gassaturationof a reservoir,like secondarvmlsranon. dependslargelvon the dualismbetweenthoseforceswhichmovehrdroJarbons. buovancvand h)'drodynamic flow cf water,and the resistingforce of capillarv pressures. A t)picalcaseshowinga crosssectionof theoil or watersaturation with respectto totalporosityin a reservoiris givenin FigureI I L.1.g. U nderhyclrostatic conditions.a maximumoil saturationis reachedin the upperpartol the reservoir whichvarjcsaccordingto the propertiesof the reservoirandthc containedfluids. The rclative oil saturationis usuallvlowcr in finer-grainedreservoirrocks bccause the ratioof porevolumclo enclosing surfrccis leisfavtrrable. andthercis Inorc watcr adhcringto mineralsurfhces. Saturationis alsolowerwith heavier. more viscousoils. A transitionzone with incrcasinglvlower oil saturation s e p a r a t et hs e o i l a c c u m u l a t i of rno m r h cz o n eo t c o m p l e t $ i r r e r s a t u r a t i o(nF i g . III..1.ll).This transitionzone hasa lower oil saturaiionand it is a distinctand separatezone.Buovantforcesare not sufficientto injectoil into smallerpores rvhichdemandhigherentrvpressures to be filled because of a lackin oil_column hcieht. Exact data on transitionzones:lre rare. In many reservoirsthere is a p p a r e n t llvi t t l ei n d i c a t i o on f s u c ha r r a n s i r i ozno n eo f a p p i c c i a h lteh i c k n e s s . The cas oil ratio at the time of emplaccmenr clepcntls mainl) on hvdrocarbon availabilityat the rime of pool lbrmation.If moregasthanoil is availablein thc dralnagearea.the rcsultingaccumulation is most likely to havea separategas srndgrain r0n,0l irreducible conn,lotater
transilion aone ront of complcta rateasaturrtion ,tltl
50 (Xof por, volomel ratersaturation
Fig.III..l.lt. Cross-section throughan oil-filledreservoir showingthe relative distributionof oil and water approachingthe oilwatercontactzone.(After Maver-Gurr.1976)
35.1
Secondary Migration andAccumulation
cap.lf. however,mainlyoilwascollectedby the anticline,therewill be no gascap with gasunderthe reservoirconditions. andthe oil mightevenbe undersaturated The relativedistributionof oil andgas andtemperature. i. e.. at a ccrtainpressure in a pool.therefore,dependson manyfactorssuchasthe supplyol oil or gas.the the kind of oil phasein placc(i. e.. light oil reservoirpressureand temperature. of the sealing gas heavy oil) andthe relativepermeability dissolve more than can pool. bubblepoint Hencc, the so-called content of the with respect to the caprock conceptwhichhasbeenwidelyusedfor datingthe time of or saturation-pressure oil is is not valid.This conceptrequiresthat only gas-saturated oil accumulation emplacedin a reservoirby migration.This is a rarecase.and it is unjustificdto assumethat oil and gaswouldalwaysbe at a stateof equilibriumat the timeand in a reservoir.Sucha casemal-.cxist.but it underthe conditionsof emplacement would be purelyaccidental.
Migration 4.5 Distances of Secondary Widely differingopinionshave bccn expressedabout distancesof sccondarv migrationhasbeeninferred.suchas a short-range migration.In someinstances. in the isolatedsandlcnsesin the Tertiarvof the RhineGrabenand the pinnacle secondarymrgratron reefsin the Devonianof WesternCanada.Long-distance Middle and for man-"" heavy oils in Canada for the Athabasca hasbeensuggested virtuallv and evide nce instances. howeve r. there is limited Eastoil tields.I n many no proot. that thereshouldbe a relationAn approximatcbalancecalculationsuggests andthe drainagearea.If it isassumed shipbetween thesizeof anoil accumulation that a reseNoircontains20% of the rockvolumeoccupiedby oil. then a ratioof that thisoilhas about100kg of oil per t of rock is present.If it is furtherassumed been generatedfrom a sourcc bed of comparablethicknesscontaining2% ratio of kerogento oil of about207r,.then organicmatterand a transformation 4 kg of oil is generatedper ton of rock. If the efficicncyof primary approximately per ton migration(expulsionefficicncy)is about25% , then 1 kg of oil is released of sourcerock. Under theseconditions.the ratio of the areaof thc oil pool, as comparedto thedrainageareaof thesourcerock is in theorder of aboutl : 100.In ratherthan areasthiswouldmeana ratioof 1:10.Thus.an oil termsof distances field 3 km in diameterwould have a drainageradiusof about 15 km. l'he distributionof oil fieldsin productivebasinswith sufficientavailabletrapsshows are reirsonable. that theseassumptions Giant oil fields (Halbouty. 1970)with over 100million t of oil in placearc of of suchan enormousmassof oil or gasb-'-processes known.The accumulation migrationdemandsa very largedrainageareaand a correspondingly secondary for suchoil fieldsshowthat' even B alancecalculations largevolumeof sediments. of migrationmustcoverdistanccs secondary underthemostfavorableconditions, square several hundred at leastseveraltensof kilometers,or drainageareasof kilometers.An oil depositextendingovet severalhundredsquarekilometers. suchas the Athabascatar sandin Canada,requiressecondarymigrationover
-1.5Distances of Secondarv Migration
355
A N ' I T H E T I C FA U L T
M.ior boond..y l.uh r.(rt.ck.. prin.ry conduit lor digr.tio..
R..arvonr rirh .olun^. .r th. .titt poirt. in
Minor.ccumul.rion.
Sldr
ars I flilll]ll]lllTllllll
orL
wrrrn li:ui.r-irl
Fig. III.,1.9. Sectionthroughgrowth fault in the Niger delta showingthe positionof variouskinds of oil accumulations and possibleavenuesof migrationbetweenpools. (After Weberand Daukoru.t97-5)
distancesof the order of 100 km or even more. Such distancesare not unreasonablc. providedprivilegedmigrationavenuesare availablesuchas the major Crctaceous-Paleozoic unconformityin WesternCanada. Secondarv migrationoververticaI djstances, morethanthethickness of a single rese-rvoirrock. is only possiblethrough faults. fracture svstems.and other prcferredavenues suchasdikes.thrustplains.andmud volcanoes. Thiscanlead to thc supcrposition of scveralproductivereservoirs, asin deltaareassuchasthe Gulf Coastand thc Niger delta, wheregrowth faultsare probablvavenuesof r c r t i c um l i t r a t i o {nF i F .I t l . J . q ) H . o u e v e ri .r i r n o 1u n 1 l r t r t . 1t h a tg r o u r hf n u l t s are conduitsfor vertical migration(Hedberg, 1977).Under such conditions individualaccumulations in multiple-zoneoil fields may represent a dl,namic -be rather.than a sratic svstem (Philippi, 1977):pools mav simultaneously rcceivingand losinghvdrocarbons. Anothcraspectof a dvnamicsituationwith respectto petroleumaccumulations -migrarionasdescri'bed is a process.called separation by Silverman(1965).It may be inducedby a naturalruptureof a sealingcaprockby faultingor fracturing,and causesa redistributionof the hydrocarboncontentin a reseivoir.Becausethis separation-migration process resultsin largedifferences in hydrocarbon composition, morepronouncedthan thosecausedby ordinarysecondary migration,it is brieflvdiscussed alongwith petroleumalterations(part IV).
356
SecondaryMigration and Accumulatlon
Summaryand Conclusion andreservoirrocks' Seconda n migration,themovement of oil andgasthroughcarrier I hesearethe parameters three by pools is controlled una-,ir"r,jtr"iu.", formation of buovantnseofoilandgaslnwater.satulatedporousrocks,capillaryplessurgsthat ."riiprtt." flJnu.ano. asan importantmodifyinginfluence'hydrodynamic i"iJr.*f are statlonary'l e ' fluid flow. As long as the aqueouspore fluids in the subsurtace migration is secondary for force driving only the conditions, under hvdrostatii condihvdrodvnamic under i, our.t no* in the subsurfaie'i e ' li[;. ;;;;;";i. Capillary flow water this by modified gas be may """i. inl u""vr"i rise of oil and in narro* ro.k po,.s are the causefor hydrocarbonentrapment' oressures ' haveto be diitoned ;ii;i;;;;;t *s bubble
Chapter5 ReservoirRocksand Traps,the Sitesof Oil and GasPools
Petroleumis ultimatelycollectedthroughsecondarymigrationin permeable, porousreservoirrocksin the positionof a trap. Any permeableandporousrock may act as a reservoirfor oil and gas.They may be detritalor ciasticrocks, generallvof siliceous material.or chemically or biochemically precipitated rocks, usuallycarbonates.It is not uncommonthat petroleumis found ln fractured shales.Occasionalll'. igneousand meramorphiirocksare hosrsfor commercral quantitiesof petroleum,when favorahlylocaredin proximitl, to petroleum_ bearingsedimentarysequences. The fundamentalchaiacteristic of a traDis its upwardconvexshapeof a porousreservoirrock in combinationuith a more denseand relativelvimpermeable sealingcaprock above.The ultimateshapeof the convexltymay be angular,curved, or a combinationof both. The onlv rmportantgeometricparameteris that it mustbe closedin verticalandhorizontil planeswithout signiticantleaks to form an inverted container.The strike contoursof thisinvertedcontaineron a structuralmapmustencircleclosedareas comprisingwhat is termedclosureareaor closureofi trap. A rareexceptionto thisrule is true hydrodynamic trapprng. Causesfor the formation of traps are numerous.Thev mav be due k) depositional tcaturcssuchasa sand-body embeddedin andsealedby shalesin a transgressive sequence. or a porousreef rock buriedby denselimestones and shales.Suchtrapsarecommonlvrcferredto asstrategraphic traps.Trapsformed b,Vtectonlceventssuchas foldingor faultingare referredto as structuraltraps. Numerousand quiteclaborateclassifications of oil-fieldtrapshaveenvolved.In the.contcxtof oil migrationand accumulation. however.suchcomplexclassifi_ calronsare not nccessarybecausemost traps can be describedin common geological tcrms.and in rcalitymanytrapsbeai teaturesrhatmakethemdilTicult to classifv. For a detaileddiscussion on geologicaland petrologicalaspectsof reservoir rocks and traps. such books as Geologl.of petroleumby Levorsen(1967). PetroleumGeologyby Chapman(1973)and lntroducrionto petroleumGeolosy bv Hobson and Tiratsoo (1975) may be consulted.The subsequentbriif discussion on reservoirrocks and trapsfollowsto a considerable extentthese referenccs.
4
J I
I I
II
358
ReservoirRocksandTraDs.the Sitesof Oil and GasPools
0 ,1 0,1
I
l0
100
1000
10000
(millidarcy) Permeability Fig. IIL5.l. Relationshipbetweenporositiesand permeabilities for sandstones of differentgeological agesin NW Germany.(After Fnchtbauer. 197'l)
5.1 ReservoirRocks The two mostessential elementsof a reservoirrock are porosityandpermeabilit)'.The rockmustcontainporesor voidsto storepetroleumandtheseporesmust be interconnected. The rock must be permeableto fluidsand gases.A pumice rock, tbr example,althoughhavingvery high porosity.is not a good rcscrvoir rock because poresare not interconnected. Porediameters alsomustbe abovea certainminimumsizebecause the ratioof totalporevolumeto innersurfacearea mustnot be too low. If the pore sizeis too small,the capillaryattractionol the mineralgrainswill holdthetluidsin theporespaceof therockandfluidscouldnot be producedbecausethe absoluteand relativepermeabilities are too low. Most clasticreseryoirrocks have grain diametersin the range of 0.05 to 0.25mm, resultingin averagepore radii in sandstone rescrvoirsbctwccn20 and 2004. A certainrelationship canbe observedbetweenporosityandpermeability in clasticrocks.An increasein porosityis paralleledby an increase in permeabilit)'.This trend is demonstrated in FigureIIL5.1 for sandstones of different geological agesin northwestGermany. Porositiesin reservoirrocks usuallyrangefrom 5 to 304/o.The porositvof carbonaterocks is frequentlysomewhatlessthan that of sandstones. but the permcabilityof carbonates maybe higher.A roughfieldappraisal of porositiesin percentand permeabilities in millidarcyof the mostcommonreservoirrocksis g i v e ni n T a b l eI I I . 5 . l ( a f t e rL e v o r s e n1. 9 6 7 ) . The majority of petroleumaccumulations are found in clasticor detrital (highlysiliceous. reservoirrocks,includingsandstones and siltstones comprised mainly of quartz). arkoses(derivedfrom granites.mainlv containingquartz.
i . l R e s e v o iRr o c k s
359
TableIII.5.1. Appraisalof porosities andpermeabiliriesof commonreservoirrocks(Levorsen.1967) Porositv%
Appraisal
0 - s
negligible
5 t0
poor
10 15
fair
t-5-t0
good
2(|25
very good
Permeability in mili darcv
1.0- 10 10 - 100 l()U lout)
feldspnrand mica). and gravwackes(sandstones with particlesof dark. finegrainedigneousrocks).Clasticrocksprobablvcomprisemorethan60% of all oil occurrences. and an additional30% is found in carbonatereservoirs.It is significant. however.thar morethan40% of the so-called giantoil andgasfields arc found in carbonates. The enormouspetroleumaccumulations in tlie carbo_ natesof the PersianGulf distortsucha classilication. The remainingl0% of all petroleumoccurrences are found in fracturedshales,igneousanclmctamorphic rockse . tc. Two differentkindsof porositycanbe identifiedin sedimentary rocks,primarv or Intergranular and seconclarv porositv.Sandstones commonlyhale a primury
20
I
J I I I
p o r o s i t (y% I 10
I
@c-1.."",o," t0
@
@
:',"*rc
r..,i".y
m i c a c c o u sE i r t y s . n d s t o n 6 . - c..,-"ous
sandstone6
""na"ron."
2000 ? E
3000
Fig. IIL5.2. Relationshipbetween porositv and depth of burial for sandstones of different geological age.(Afier Frichtbauer,197.1)
I
JbU
Reservoir RocksandTraps.theSitesof Oil andGasPools
porositywhich is largelydependenton the packingcharacteristics and on the variationin sizeandshapeof the grains.Well-sonedsandswith nearlyspherical grains.suchascertainbeachor dunesands,makeexcellentreservoirs with high primarvporosity.The Rotliegenddunesandsof the giantgasfield of Groningen in the Netherlandsare an examplefor the outstanding qualityof suchreservoir rocks.If too muchfine-grained materialis present,reservoirqualitydrops.With increasing overburdenand diagenesis. sandstones tend to loseporosity,mainly dueto recrystallization andcemcntation. This decrease in porositywith increasing depth of burial is shown in Figure Ill.5.2 (after Frichtbauer,1974).The secondary growthof quartzon sandgrainsis impededwhen sandstones are oil impregnatcd. Philippet al. (1963)usedthisphenomenon to estimatethe timeof formationof an oil pool. The degreeof cluartzdiagenesis was determinedby estimatingthe percentagcof quartz grainswith secondaryeuhedralquartz overgrowthin water-saturated sandstones free of hydrocarbons. This wasthen plottedagainstmaximumdepthof burial.Oil-impregnated reservoirsandstones werecomparedto this depth-quartzdiagenesis plot. From the degreeof quartz diagenesis of the oil-impregnated reservoirrocksthe depthandtime of emplacement of oil wasderived.
5.2 Traps A trap is a geologicalfeaturewhichenablesmigratingpetroleumto accumulate and be preservedfor a certain time interval.Traps occur in fundamentally differentformsand canenclosevery differentvolumesof porespaceand hence petroleum.The maximumtotal holdingcapacityor closedcolumeof a trap is the volumebetweenits highestpoint and the "spillingplane"or outflowIevelat the bottom. Traps are rarely completelyfull, i.e., the petroleumcolumn rarely extendsdown to the point at which it would spill out of the trap. Furthermore, trapsareneverfull in the sensethatall availableporespacein thereservoirrockis occupiedby petroleum.Thereis alwaysa certainresidualamountof water,which cannotbe displacedby migratingpetroleum. Traps can be formed either by tectonicactivity such as faults, folds, etc. (structuraltraps)or from sedimentary depositional patterns(stratigraphic traps). The majorityof knownoil fieldsis foundin structuraltraps,but structuraltraps areeasierto locateby currentgeophysical andgeological explorationtechniques. Stratigraphic trapshavebecomea primetargetfor explorationonlyin morerecent years.Sometimesthere are combinedtectonicand depositionalcausesfor the tbrmationof a trap. Then thesetrapsare calledcombinationtraps. Anticlinalstructures arethe mostcommontypeof traps.Oneof the largestoilproducinganticlinesis the Kirkuk lield in Iraq. The productiveareaextendsin threeculminations over a lengthof nearly30 miles.The "GoldenLane" fieldsin Mexicoalsooccuraslocal"highs"alongan archextendingsome50 miles.Most oil-producinganticlines.however,are only a few milesin lenght,and are often associated with somefaulting.
5 . 2T r a p s
361
? I ? rr'
t 1{ I
datum: top of -3000-
Fig. IIL-5..1.Depth conk)ur llnes of Wiimington Anticline. California. ( A f t c r M a v u g a1. 9 7 0 )
Ranger zone
d€pth in f e.t
-.rsarl
b€tow sea- tev6l
The giantWilmingtonoil ficld in the LosAngclesbasinof southernCalifbrnia mavbe citedasanexampleof a structuralanticlinaltrap.The LosAngelesbasinis oneof thc mostprolificoil-producing basinsin the world in tcrmsol the ratioof p o o l e do i l t o t o t a l r o c k v o l u m e .T h e W i l m i n g t o ns t r u c t u r e( F i g s .I l l . - 5 . 3a n d l l l . 5 . 4 )i s a b r o a d a . s s v m e t rai cn t i c l i n a e b o u tl 2 m i l e sl o n ga n d4 i i l e s w i c l eI.t i s dissected by a seriesof transverse normalfaultswhich separatethc producing sandand sandstone reservoirsinto manv differentpools(Mayuga.l97t)).Scali are providedbv claystones andshales.Oil propertiesin the variousfault blocks differ considerabl)'. illustratingthat the aicumulationof hydrocarbons apparentlyoccurredindependentlv in eachof the blocks.The differentoil propeiiies alsosuggest that the faultsprovideeffectivebarriersto communication between adjacentfault blocks.Therearesevenmajorproducingzonesrangingin agefrom late Mioceneto earlvPliocene.The upperpart of the-Wilmingto-n structurewas erodedduringthe earlv Pliocene.and subsequent submergence resultedin the depositionof an additional18002000 ft of almost horizontalplioceneand Pleistocene.sediments on top of thc anticline(Fig. III.5.,l). The entircWilmington oil field is estimatcdto containabout 3 billion barrelsof produceableoil ( M a y u g a1. 9 7 0 ) .
st'
ltE - 4 k m
2000 F
U!conformitv
3000 t000 5000 6000
o m
oil beaiing
Fig. 1t1.5.,1.GeologicalcrosssectionthroughWilmingtonAnticline.California.(After Mavuga.197t1)
362
Reservoir RocksandTraps.theSitesof Oil andGasPools
A differenttvpe of structuraltrap is associated with so-calledgrowthfaults whichplay an importantrole in suchprolificareasasthe NigerDelta regionand the faultsremain the Gulf Coast,USA. The term growthfault is usedbecause activeaftertheir formation.andallowfastersedimentation on the down-thrown side.Thc cnhanceddepositionof sedimentsacrossthe growthfault initiatesa rotationalmovementwhich tilts the bedstoward the fault-creatinganticlinal "rollover" structuresalongthe laults. The subsequent discussion on oil fieldsandtrapsin the NigerDelta is mainly derivedfrom WeberandDaukoru( 1975).The majorityofthe approximatelv 150 oil fields in the Niger Delta have at least one fault which influencesthe accumulation in additionto the majorgrowthfault.The faultzonescanbesealing or nonsealing. The sealingcapacitvor conductivitvof a fault depcndson thc throw. and on the amountof shalcor sandcaughtup or otherwiseincorporated faultsis into the faultzonc.Evidcncctbr vcrticalconductivity of ma.jorboundar_'providedbv the fact that in most casesthe fault intersectionuith the upper bedding plane of a reservoir (Fig. IIL1.9) acts as thc spill point of the accumulation. It is possiblethat thesespillpointsare alsothe cntry pointsof the oil from the faultzoneinto the reservoir.The growthfaultsarein mostinst nces sealingon the up-thrownside of the fault zone. The small dip closureof the structuraltraps and the fact that faults with throws of 150 m or more are considered aspotentialverticalleaks.is thoughtto be the causefor therelatively shortoil columnsin the Niger Delta,whichrarelyexceed50 m. Therc arc numerousoil fieldswhereoil is trappedin monoclines againstseals providedby lault planesabovethe monocline.The Pechelbronn oil ficld in thc Tertiary of the Rhine-Grabenand the oil fields in CretaceousWoodbine sandstones alongthe fault zoneof Mexiain easternTexasare examplesof such
Sectior through salt-dome field
for Fig. III.5.5. Somepossibilities with salt plugs. oil poolsassociated (ModifiedafterLevorsen.1967)
-i.2Traps
7
3 Mil€s
363
1
5
6
0
1
2
3
1
5
0
Mit., tscar.s 6re approrin.r€)
Fig.I11.5.6. TheBellCreekoil fieldin Montana. A geological profilethrough thepowder RiverBasin,showing theoil fieldlocation andthegeological setingof thechanncl and barriersandrvhichactasreservoirs. (AfterMcGregor andBiggs.1970) fault trap.s.It shouldbe pointedout again,however.that fault zonesare nor alwayssealing.but arc sometimesnonsealing.Fault zonesmay evenprovide migrationavenues.ils in thc NigerDelta or otherareaswith deepreachingfault s)stems. The associtttion oJ oil au:umulutionswith salt/ome-,is anotherexamplcol structuraltrapsformedby tectonicevents.Oil accumulations in tecronlcIeatures causedb) the upwardmovementof salt from deeplyburiedevaporitebedsare knownin severalpartsof the world, includingnorthernGermanvand the North Sea.Romania.WestAfrica andthe Gult Coa.st rcgir_rn ol USA and Mexico.The r e l a t i o n s h ibpe t w e e nt h e o i l a c c u m u l a t i o na sn d t h e s a l ti s o n l v s t r u c t u r aal n d thcre are manvwaysto providesuirahletrapsin association \\ rih saltintrusion. The trapsmaybe formedalongthe foldedoi faultedflanksof the saltpluqor on top. of the plug whcre archingand/or faulting is producedin the ovlrlving sediments. Somepossibilities tbr oil poolsin association with saltplugsareshown i n F i g u r eI I I . 5 . 5 .
36,1
Reservoir RocksandTraps,theSitesof Oil andGasPools
The formation ofs trutigraphictrapsrsrelatedto sedimentdepositionor erosion andis thusdistinguished from formationof structuraltraps,whichoriginatefrom tectonicevents.It is clear, however,that tectonismultimatelycontrolsmany geologicalprocesses suchas depositionand erosionof sediments. Stratigraphic trapscan be relatedto facieschanges. diagenesis of sediments andto unconformitiesin the sedimentary rock column(Rittenhouse. 1972).Most basinscontain the prerequisites for the creationof stratigraphic traps.Typicalexamplesare barriersandbars,deltaicdistributarychannelsandstones andcarbonatereefsof differentforms and sizes.Suchbarrierand channelsantlsand carbonatereef structures, frequentlyembeddedin fine-grained organic-rich sedimentary sequences.are idealtrapsfor migratingpetroleum. The giant Bell Creekoil field in Montana,USA (Fig. III.5.6), servesas an examplefor bsrrier and channelsantl reservoirs.The geologyof the Bell Creek field is describedby McGregor and Biggs (1970).The Lower Cretaceous stratigraphictrap at the northeasternedge of the PowderRiver basinis not controlledby tectoniceventsotherthanregionaldip. The MuddySandstone trap
ll0RTll AtBEnTA EASlli (s*,-"
:
:
I
.
. h o w i n e, , n . . . d 3 p . . . r . , . i i o n . h i p I
i
.t
!
:
I "g 1 ,".
i''
g z;f -z at
!
:
ffi
t;* Fig. III.5.7. Geologicalcross-section throughthe North Alberta Basin,showingthe temporaland spatialdistributionof the Devonianreefs.(After Balsset al.. 1970)
Summary andConclusion
365
wasformed near the intersectionof littoral marine bars and a delta systemwith distributarychannelsands.Profilesthrough such channeland barrier sands enclosedin shalesare alsogivenin FigureIII.5.6. Fossilorganic reefsiorm very prolific stratigraphictraps in many parts of the world. for examplelhe Middle East,Libya, southwestern USA and in Alberta, WesternCanada.Reefsare constructed by variousgroupsof organisms, suchas corals,algae,echinoderms, mollusksandforaminifera.Theymaybe interpreted as "fringingreefs","barrier reefs","atolls" or "pinnaclereefs,',dependingon their environmentof growth and shape.Accordingly they vary in length, width and thickness.As an example of reef traps the giant oil fields of the Mittdle Devonianreefsofthe Rainbowareain Alberta, are briefly discussed from a paper by Barsset al. (1970).The growthof the RainbowMemberreefsin the Black CreekBasinof northwestern AlbertawasinitiatedduringUpperKeg Rivertime when banks of reef-constructing organismsstartedto flourish (Fig. III.5.7). Structuralcontrol of reef growth, if it did exist, was subtle.The reefsin the Rainbow sub-basin,atolls and pinnaclereefs,have vertical reliefs up to 820 ft. The reefs are envelopedby fine-grainedoff-reef sediments,consistingof salt, interbeddedanhydrites,dolomites and shales.An evaporitecover providesan excellentsealfor petroleumentrapment.Reserves from the RainbowMember poolsare estimatedto be in excessof 1.3billion barrelof oil-in-olace. Finally,it shouldbe pointedour that migratingpetroleumcan be capturedby all kindsof traps,by structural,stratigraphic or combination traps,only aslongas they are in the path of migration.
Summaryand Conclusion Petroleumis ultimately collectedthrough secondarymigration in permeableporous reservoirrocksin the positionofa trap. It is characte zedbyitsupwardconvexshapein combinationwith a porous reservoirrock with a relatively impermeablesealingcap rock above. Generally one distinguishesbetweenstratigraphicand structuraltraps. Stratigraphictraps are mainly causedby depositionalfeatures,and structumltraps by tectonicevents.Thesecausesfor the formationof a trap may alsobecornbined.Typical examplesfor stratigraphictraps are barrier sand bars, deltaic distributory channel sandstonesand carbonatereefs.Typical examplesfor structuraltraps are anticlines, fault traps, and traps associatedwith salt domes. The majodty ofpetroleum accumuiations arefound in clasticreservoirrocks.suchas sandstonesand siltstones.Next important reservoirrocks ar€ carbonates.Fractured shales,igneousand metamorphicrocks play only a minor role. The maximumtotal holdingcapacityor closedvolumeof a trap is the volumebetweenits highestpoint and the spilling plane or outflow level at the bottom. Porositiesin reservoirrocks usually rangefrom 5 to 30%. Traps are never full in the sensethat all availablepore spacers occupiedby petroleum. There is alwaysa certain residualamount of water, which cannot be displacedby migratingpetroleum.
to PartIII References
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36'7
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ti']
Srutgc ri:,Schwe,tr.ruun,r.t . v..rugru".'rii""oi""g.
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370
Referencesto Part III
of montmorillonite dehydration ascauseof abnormalpressure Magara,K.: Re-evaluation (1975) and hydrocarbon migration.Am. Assoc.Pet.Geol. Bu11.59,292-302 - Directionsand from clasticsediments duringcompaction Magara,K.: Waterexpulsion volumes.Am. Assoc.Pet.Geol. 8u11.60.5,13 553(1976) Martin. R. L.. Winters. J. C., Williams,J. A.: Compositionof crude oils by gas geological distribution.6th world Pet. chromatography: significance of hydrocarbon Congr.Sec.V, paper13(1963) von May. F.. Freund,W.. Mtiller. E. P.: Modellversuche riber lsotopenfraktionierung Erdgaskomponenten wehrendder Migration.Z. angew.Geologie14,37G380(1968) Mayer-Giirr.A.r PetroleumEngineering.In: Geolog-vof Petroleum.Beckmann.H. (ed.).Stuttgart:Enke. 1976 Mayuga.M. N.: Geologyand Developmentof Californias Giant WilmingtonOil Field. (1970) Am. Assoc.Pet.Geol..158-184 Geologyof GiantPetroleum Fields.Memoir1,1. McAuliffe.C.: Solubilityin waterof paraffin.cycloparaffin. olefin.acetyiene. cycloolefin (1966) and aromatichydrocarbons. J. Phvs.Chem.70,1261-1215 McAuliffe,C. D.: Oil and gasmigration:chemicaland physicalconstraints. In: AAPG Studiesin Geolog)'No. 10. Problemsof PetroleumMigration.Roberts.lll. w. H.. Cordell.J. R. (eds.).1980.pp. 89 107 McGregor.A. A.. Biggs.C. A.: Bell CreekField.Montana:a Rich Stratigraphic Trap. Geologyof Giant PetroleumFields.Halbouty.M. T. (ed.). Memoir 1,1.Am. Assoc. Pet.Geol..Tulsa.128 116(1970) Mclver. R. D.: HydrocarbonOccurrences fron JoidesDeepSeaDrillingProjects.Proc. 9th World Pet. Congr..Tokyo. II London:AppliedSciencePubl.. 1975.Vol. II. pp. 269-280 cnvironments. McNeal,R. P.: Hydrodynamics of the PermianBasin.Fluidsin subsurface commem.vol. Am. Assoc.Pet. Young.A., Galley,J. E. (eds.).Memoir4. semicent. G e o l . . 3 0 8 - 3 2(61 9 6 5 ) Meinschein. W. G.: Originof petroleum.Am. Assoc.Pet.Geol.Bull.43,925 943(1959) Meissner.F. F.r Petroleumgeologyof th€ BakkenFormation,Willistonbasin.North DakotaandMontana:WillistonBasinSymposium, MontanaGeological Society.1978, pp.207 221 Milliken,K. L.. Land, L. S.. Loucks,R. G.: Historyof burialdiagenesis determinated from isotopicgeochemistry. Frio Formation.BrazoriaCounty,Texas.Am. Assoc.Pet. Geol. Bull. 65, 1397 1413(1981) Millot, G.: Geologyof Clays.Engl. transl.by Farrand,w. R., Paquet,H. New YorkHeidelberg-Berlin: Springer.1970:Paris:Masonet Cie., London:Chapman& Hall, 1964 Nagumo.S.: Compactionof sedimentary rocks a consideration by the theoryof porous media.EarthquakeRes.Inst. (Jpn)43,339 348(1965) gdologiques I. S.: De l'influencedesconditions surla teneur Neruchev.S. G.. Kovacheva. en huile des rochesmdres(in Russian).Dokl. Akad. Nauk. S.S.S.R. 162,913-914
(1e6s) Palacas,J. G.. Love. A. H., Gerrild. P. N.: Hydrocarbons in estuarinesedimentsof Choctawhatchee Bay, Florida.and their implications for genesisof petroleum.Am. A s s o cP . e t .G e o l .B u l l . 5 6 ,1 4 0 2 - 1 4 1( 1 8972) on systems. L Exploratoryinvestigations Peake.E.. Hodgson,C. W.; Alkanesin aqueous in natural the accommodation of C1;C11r-alkanesin distilledwaterand occurrence w a t e rs v s t e m sJ . A m . O i l C h e m .S o c . 4 3 . 2 1 5 - 2 2( 129 6 6 ) Perry.D.: Earlydiagenesis of sediments andtheir interstitialfluidsfrom the continental slope,northernGulf of Mexico.Trans.GulfCoastAssoc.Geol. Soc.20,219-227(1970)
References to PartIII Philipp.W.. Drong, H. J., Fiichtbauer,H.. Haddenhorst. H. G., Jankowskv.W.: The historyof migrationin rhe Gifthorn trough (NW Germany).proc. brh fiorld pel. Congr.Irrankfurt.SectionI, paper19.pDr.457_..17g (1963i -. Philippi.G. T.: On the depth,time and mechanism of origin of the heavyro medrumgravitynaphthenic crudeoils.ceochim. Cosmochim. Acia 41,33-52(1977) Poulet,tr4.:Probldmespos€spar la migration.secondaire du petroleet sa miseen place danslesgisements. Rev. Inst. Fr. P6r.23, 159-173(1968) Powers.M.C.: Fluid,release mechanrsms rn compactingmarinc mudrocksantl their rmportance in oil exploration.Am. Assoc.pet. Geol. Bull.5l, 12,10_1254 (1967) Price.L. C.: The solubilitvof hydrocarbons and petroleumin water as appliedto the primar]'migrationof pctroleum.Ph. D. thesisUniv. California.Riversrde. I97l Pricc. L. C.: Aqueoussolubilitt of_p9trofum as appliedto its origin and primarv m r g r a t r o nA.m . A s s o cP . e t .G e o l .B u l l . 6 0 , 2 l 3 2 1 1 / l 9 j 6 \ R c d m o n dJ. . L . . K o e s o e m a d i n aR t a. .p . . $ a l i o o i l f i e l da n dr h eM i o c e n ec a r b o n a t eo sf Salawatibasin.lrian-Jrva.Indonesia. -ithConv.Indones.pet.Assoc.JakartaJune.7 9 (rq76) Reed.L. C.: SanPedroOil Fieid.provinceof Salta.NorthernArgentina.Am. Assoc.pet. Geol. Bull. 30, 591-605( 196,1) Rittenho^use. C. : Stratigraphic-trap classification. In: Stratigraphic Oil and GasFields Classification. ExplorationMethods.andCaseHistorier.-fing,n. E. (ed. Jointpubl. ). Am. Assoc.Pet.Geol.andSoc.Explor.ceophvs.AApG Mernoirt6. jEG Spec. lubl. No. 1,1.Tulsa.l+2u (1972) Robin. P.: Caracterisatton des k6rogdneset de leur 6volution par specrroscople infrarouge.Disserr.Uni\'. Cath. Louvain.Groupede physico_Chimie Min. Cotulyr"
(re75)
Ro.i:l:l.J du: Stressfields,a keyto oil migration.Am. Assoc.p€t. Geol.Bull. 65,7,1_85 (198t) Seevers. D. O.: Personal (1969) communication Seitert,W. K. : Carboxylicacidsin petroleunr andsedimenrs. lnrFort5chrittederChemie organischer Naturstoffe.IIerz. W.. Grisebach.H.. Kirbv. G. W. (eds.).Wien-New York: Springer.1975.Vol. 32 pp. l-.19 Seifert.W. K.. Howells.W. G. : Inrerfaciallv activeacidsin a Clalifornia crudeoil. Analyt. Chem.41. .55f562(1969) Sharp.Jr.. J. M.. Domenico.p. A.: En-erg), transportin thick sequences of compacting sediment.Geol. Soc.Am. Bull. 87,390-.100 (1976) Silr'erman.S. R.: Migrationand segregation of oil and gas. In: Fluidsin Sub_surface E v i r o n m c n t sY.o u n g A . . . G a J l e lJ. . E . ( c d s . )M . e m o i r l . A m . A s s o cp. c t .G e o l . . 5 3 6 5( 1 9 6 5 ) Snarskl'.A. N.: Die primiireMigrationdesErdrils.Freiberger Forschungsh. C 123,63_73 (1962) Stahl.W J.: Carbonand NitrogcnIsotopesin HvdrocarbonResearch and Exploration. ,lth Eur. Coll. Geochronol.Closmochronol. lsotopeGeol..Amsterdam(1976) Stail. W. J.. Boigk. H.. Wollanke.G.: Carbon and NitrogenIsotopetJata ot Upper Carboniferou,s andRotliegend NaturalGasesfrom North Giermany andTheir Relationship tu the M:l_:t],f rhe Organic SourceMarerial. In: Advancesin Organic ^of 1q75.Campos. R.. Goni. J. (eds.). EmpresaNacionalAdaro de 9e,\-hcmisrn Investigacioneq Mineras.S. A. Madrid(Spain).1977.pp. 5:O-:SO Tissot. Il.. Deroo. G.. Espiralid.J.: Etude comparacde l.ipoquc de rormarroner d expulsiondu pdtroledans diversesprovincesgdologiques. pioc. 9th Worid pct. Congr..Tokl-o.London:Applic<1 Sciencepubl.. tCZS.Vnf. II. DD.159_169
312
Referencesto Part III
et de migrationdu de genase Tissot,B., Pelet.R.: Nouvellesdonndessurlesmdcanismes SthWorldPet prospection. Proc la et application i p6trole,simulationmathdmatique C o n g . .M o s c o wP. D 1 ( 4 ) ( 1 9 7 1 ) Ungerer. P.. Behar. E., Discamps.D.: Tentativecalculationof the overallvolume datal eipansionof organicmatter during hydrocarbongenesisfrom geochemistry implicationsfor primary migration.ln: Advancesin OrganicGeochemistrl'.lglJl JohnWiley. 1983.pp 129 135 Bjoroy. M. et al. (eds.).Chichester: desextraits primaire:variationdecomposition M.: Etudede la nrigration Vandenbroucke, In: Advancesin OrganicCeochemistr! rochemdre/r6servoir' de rochei un passage PergamonPress.1972.pp. 547 565 1972.Oxford-Braunschweig: Weaver.C. E.: The clay petrologyof sedimentsln: Clavs.Clay Minerals Proc Nat' Conf. ClaysClay Min.6. 1959.154-187 usesof claymineralsin searchfor oil. Am. Assoc Pet.Geol Bull Weaver.C. E.: Possible 44.r505l5l8 (r960) gth Weber.K. J.. Daukoru.E.: PetroleumGeologyofthe NigerDelta.Proc. Worid Pet' Congr..Tokyo. London:AppliedSciencePubl . 1975.Vol ll. pp 209 221 . :CorrelaW e l t e .D . H . . H a g e m a n nt l.. w . . H o l l e r b a c hA.. . L e y t h a e u s eDr . . S t a h l W London: Congr..Tokyo. 9thWorldPet Proc petroleum rock. and source tionbetween A p p l i e dS c i e n cP e u b l . .1 9 7 5V. o l . l l . p p . 1 7 9 - 1 9 1 of oil t1'pesin WillistonBasin Am. Assoc.Pet.Geol' Williams.J. A.: Characterization (1971) Bull. 58, 12,131252 Woodbury.H. O.. Murray Jr.. I. B . Pickford.P J . Akers. W H.; Pliocencand andTexasAm.Assoc Pet shelf.Louisiana outercontinental Pleistocene depocenters. (1973) Geol. Bull. 57, 2428-2,139 du comportement G. S.r Surlesloisgen€rales G. N.. Ushakova, Zhuze.T. P..Jushkevich. gazopdtroliferes aux grandesprofondeurs(in Russian).Dokl. Akad des systdmes N a u k .S .S .S .R . r 5 2 ,7 l 3 7 1 6 ( 1 9 6 3 )
Part M The Compositionand Classification of Crude Oils and the Influenceof GeolosicalFactors
ChapterI Compositionof CrudeOils
l.l PetroleumVersusSourceRock Bitumen Petroleumoriginates from the bitumenofsourcerocks.Migration,however, and especialll' primarymigration.is thecauseofconsiderable cfiangeirncomposition bil"-:". .omparedto perroleum.On,r," on" r,onJ.l,ili,'a srnal amount i. :.h:l ol the lotal dispersedbitumen is mobilizedand transferreJinr,, .urr,", o. reseryolrrocks. and an cven smalleramountis accurnulated in oil fields.In producineareasthe ratioof rescryoired oil to dispenedt itr*"n *ng"r; rro, f, I O to l: 10.000'.On the otherhand.suchclrainage ii s.t".rir;, ;; ;;;;" by thegross comparrson betweenthc bitumenpresentin sourcerocksand the correspondrng crudeoil in reservoirs. The heavieitan,lmostpola. m, e*i"..'iit. orplort"n.r. arc strongl) adsorbcdon the sourcerock and can hardll,be expelledinto the rescrvoir.Therefore.thecommondistributionofp"trot"u, aon-.t,tuents lncrude orl parallelsthe adsorptivebchaviorof theseconstitucnii. i."..'rn. leastpolar saturatedhvdrocarbonsare most frequent.then follow aromaticsand ben_ zothiophenes. and reastabunciantare t'hemostpolar oncr,""ri-""r,r" adsorbed a n c t h e l e a s t s o l u b laes p h a l t c n eEs r. e n _ u i r h iang i r * s i r u c t u r at fv p eI t e 1 s 1 1 s /1-alkancs. light rnolecules secmto he flvored compareito t au,u un"r. The rcsult of this chemicalseparationi, ,f,u, ,fi. .of .."lLi'Jnrnpost,on ot crudcoii hvdrocarbons from diffcrentfieldsi, rnor" un,f,r,r-it n-ntr,. .ornpnri_ tion of the corrcsponding bitumenhydrocarbons.
I .2 AnalyticalProcedures for CrudeOil Characterization Ad€quatc samplingol crude oils is essential fbr their characterrzatron. A satisfactory procedurervouldbe to obtain , ,ingt.-ptui" .u_fL* of petrolcum underpressureconditionsas thel,arc in the reservoir. This requiresthe useof pressure v e s s e l sa n d a r a t h e rs o p h i s t i c a t er d n d c o s t l t , , u r p i i n gp r o c e d u r e . Thereforegeoche mists.usuay deaiwitt sa-pt.s ot tainei ,rttl! *"rt r"uoun,t., atmosphencpressure:hencc.the light hydrocarbons are completetyor at least partlv lost as a functionof their ind]vidualboiling poini p."f" "".t Roucachd ,
"\'erages computed,over a source rocksequence, butdrainage
mavbe Ile-:"^tlill.:.".: more elttclent over certain areas or horizons ciose to permeable beds. and tnefficient somcwnereelse
of CrudeOils ComPosition
376
that the well-headsamplingprocedureis representa(1970)havedemonstrated comprising10carbonatomsor more.In orderto relate tiveonly for the molecules andcrudeoil propertiesto a standardstate'not influencedby all concentrations rangingfrom 160 samplingcontlitions,the crudemaybe distilledat temperatures to 210'C accordingto the variouslaboratories. The compositionof the light distillatedependson samplingconditions.andis of the Iightend andcomposition only regardedasan indicationof the abundance the mainpart of a crude of the oil; the heavierfraction,whichusuallycomprises oil. canbe usedreliablyto describeits chemicalcompositionand to comparert with othercrudeoils. However, it should be kept in mind that the most abundantindividual in mostcrudeoils. arefrequentlyin the lightfraction.For instance. hydrocarbons amongaromatic Similarly, the r-alkane distributiondecreaseafter n-decane. are weight alkylbenzenes low molecular and other molecules, benzene,toluene, very abundantconstituents. ma1'be usedto describethe compositionof crude Threedifferentapproaches ( F i g . I V . 1 . l ) : oils and heterocoma) Isolationand individualdeterminationof hydrocarbons Project6. In thisstudythe PoncaCity. poundswasthe targetof the API Research Oklahoma.crude oil was used to determinethe hydrocarbons.and in API ResearchProject48 the Wasson.Texas,crudeoil was usedto dcterminethe sulfurcompounds(Rossiniand Mair. 1959;Smith.196E).
ldonlificotion
EETTEE t
t
t
r t
t
I
lsololionof individuol consliluats
r*-t l'-l
Densily :*98'C Viscositt Cfror"cCfnr"c Cloud pornl
r-l
a
(lenperolures shornrcferlo poinl) boilinq
ond
FFl
elc
***P
ldenlificatb quonlilolira distribuln
E- oc'l
trFl (.'-r
GC
lts GCtts
f-7'{-l |
0l
I
* olsooppljcoble lo sourcerocl exkdcl Fig. lV.l.1.
Principles of the main analytical procedures for crude oil characteazation
1.2Analvtical Procedures for CrudeOil Characterization
37.1
This valuablework resultedin the identifrcation of about350hvdrocarbons, 200 sulfur compoundsand some other nonhydrocarbons. Most of them are compoundsof relativelylow molecularweight,comprisinglessthan l5 carbon atoms. However. they representa large part of-usuai crude oils. as 169 compoundscover16Vcof the ponca City crucle,and just 10_20compounds accountfor 5(170a/coI the light gasolinefraction from various crude oils (Bestougeff,1967). Identificationof individualmolecules hasbeencarrieclout moretrequentlvon thc low molecularweightfractions.In additionto Apl Research projeci6,Mirtin et al. (1963)andPouletand Roucachd(1970)havedeterminedthe individual Co andC, compourds.andpart of the CsandC, compounds. on severalcrudeoilsof variousorigin.More recently.ErdmanandMorris(1974)reportedthe rndividual concentrations of about 100components up to |1_decane. b ) D i s t i i l a t i o on f p e t r o l e u m u s i n gw i d en , n o r r o * f r a c r i o n1s 1 5i n t h er o u t i n e ,, U. S. Bureauof Minesprocedure)is associated with somechemicalor ohvsical measurements on the fractions,e.g., densitv.viscositv.sulfurcontent,etc. This proccdureis directlyrelatedto the processes usedin refiningcrudeorls,andit is dcsignedto give valuableinformationaboutthe bestusag"e of a new crudein industrialplants. refractiveindexandotherphysicalpropertiesdependon the .Density,viscositl,. relatlveamountsof the variousgroupsof chemicalcompounds (alkanes. cycloal_ kanes..aromatics.thiophenederivatives,etc.) in a given fraction..l.hus. it is possibleto ded_uce from thesephvsicalpropertiescert;in aspects of the chemical compositionof a crudeoil. In addition.paraffin-naphtheni_aromatic (p_N A) analvses canbc madefrom someof the distillationfrictions.To takerntoaccount somereguiaritiesof crudeoil composition.a correlationindcx (CI) has been q J o q g s e db y S m i t h ( 1 9 4 0 )o f t h e U . S . B u r e a uo f M i n e s ;t h c c o n s e c u u v e distillation. fractions are representcdbv a numberthat is a functionof specific gravitvand boilingpoint. The CI valueis calculatedbv the formula: CI=
JR6JO + l7l.7G {-s6.8 :
with K : ,i.rugc Uoitingpoint of rhc fraction(Kelvin). C - specificgravitvof the fractionat 60-60"F. The correlationindex can be plotted againstthe numberof the successrve distillationfractions.and in this way tvpicil curvesare obtained(Fig. IV.l.2). The correlationindexis zcrofor linearalkanes.increases for cvcloaikanes. andis highfor aromatics.The indexis quite usefulfor two componentsvstem e. g.. - but lessvaluablefor the usualthrec componenr alkanesplus cyckralkanes systems(Smith.196E). The distillationprocedurehasbeenwidelvused.especiallv by the U. S.Bureau . of Mines.A largenumberof crudeoil analyses (around10.000) is availablein that form rechniques used ersewhere and reporied in the literature .otherdistill.rion i n c l u d ef o r i n s t a n c eE n g l e r .I T K ( U . S . S . R . )a n d T B p ( T r u e B o i l i n gp o i n t . accordingto ASTM specitication D 2892).
tt ',
I
37tl
of CrudeOils Composition
i " :
I Fig. IV.1.2. Examplesof correlationindexasa functionof distillationfractionnumber. Left: thtee typical U.S. crude oils; nghtr the main oil typesof Paleozoicand Triassicoils from Sahara,Algeria. (After U.S. Bureauof Mines Rcp. Invest.47.10:Louis, 1967)
c) Methodsbasedon quantitative separation of thevariousstructuraltypesand on determination of the moleculardistributionwithin eachtype,are mostuseful purposes.In particularOudin (1l970). for geochemical Durandet al. ( 1970).and Castex(197,1) haveproposeda completesequence of procedures resultingin a quantitativeevaluationof the chemicalstructureof crudeoils. Crudeoils. aswell as bitumensextractedfrom sourcerocks,arc dividedinto fractionscorresponding to the main structuraltypes.Asphaltenesare prccipitatedbv fi-hexane.thensaturatedh1'drocarbons, mono-.di-. polyaromatlcs and resinsare scparatcdbl usingliquid chromatography accordingto the scheme shownin Figure lV.l.3. Insteadof conventionalliquid chromatography on a column.mediumpressure liquidchromatography or thinandhighperformance Iayerchromatography may be used(Huc et al., 1976;Suatoniand Swab,19761 Radkcct al., 1980).Within eachstructuralgroup,distributionby carbonnumber andidentification ofspecificcompounds areobtainedby gaschromatography and massspectrometry. The mainadvantage of this procedureis to avoidany distillationwhichcould destroyor altersomelabileconstituents of petroleum.Furthermore,the method is alsosuitablefor smallamountsof bitumenextractedfrom sourcerocks( 100mg bitumenif usingcolumnliquidchromatography anda fewmilligramsif usingthinlayerchromatography), thus allowinga soundbasisfor oil-sourcerock correlatlon. The resultspresented belowarebasedmostlyon thethird typeofprocedure(c) describedabovc.
1.3Main Groupsof Compounds in CrudeOils
379
ry
0rslrllatron
tr.ction hoiling.bovc2l0oC
thaentory a"atfs6 t nn4red spe.tt0photoneltl ucteotn00nettt t.taror.e
L q u r dc h r 0 m a l 0 a r a p h y o na u m r n a / s[ c a ! e lc o L r m t / kh Io/et. htanotogt opr)')
, -60s .h.onota4/oph/ I ttonc t1rtrnro, dele or l tloke lhatanettr dcte.tot)
I
tlenentot/ ord//s's Intt 0t ed spe. n opnol 1tucl./ tuc/r0r na9nerc t.t0n0nce
N l oe c u l a r s r e ! e5 A t at ureo 1ddu.hba /
: ' _ v _____J_ tt-attrnes tsoatkrnes -JI ||
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_
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/
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60s thtanotagrcrhr I F/ane tortzotton deterlot, /i0nelhotonettl detectatJ
*"':;";:{'"'
Fig. IV.1.3. Flow chart of crude oil analysis: separation of main struclural types of molecules. and subsequent quantitative analysis of the fractions by gas chromatography and mass spectrometry. The sequence of analytical steps is also'uied for source rock brtumen,thus allowing the correlation of source rcrks to crude oils
L3 Main Groupsof Compoundsin CrudeOils The grosscompositionof a crudcoil (Fig. IV. 1.,1)canbe definedby thecontent ot: - .saturated h-ydrocnrbons, comprisingnormalandbranchedalkanes(parafTins). and cycloalkanes (naphthenes). aromatich:-drocorbons, includingpurearomatics. cvcloalkanoaromatic (naph_ thenoaromatic) molecules. andusuallvcvclicsulfurcompounds. The Iatterire
380
Composition of CrudeOils AROMATICHC
636 Crudeoils
,-'- iTiijii,"li'",""",,
8o I.IC SATURATED
60
ao
2oltsocoupourttDs
( FESINS.ASPMLTENES )
of 636crudeoils:saturated Fig.IV.1.4. Ternarydiagram showing thegross composition (wt. 7. in thefraction aromatichydrocarbons, andresinsplusasphaltenes hydrocarbons, boiling above2lO"C); for constructionof isofrequency contoursa triangleunit of 1 0 : 1 0 : 1w0a su s e d most frequentlvbenzothiophene derivatives.Their total abundancecan be roughl)'evaluatedthroughthe sulfurcontentof the aromaticfraction. resinsandasphalteres, madeof thehighmolecularweightpolycyclicfractionof crudeoils comprisingN, S and O atoms.Asphaltenes are insolublein light alkanesand thusprecipitatewith n-hexane.Resinsare moresoluble,but are likcwisevery polar and are stronglvretainedon silicagel whcn pertbrming liquid chromatography. unlessa polarsolventis usedasthe mobilephase. as all crudeoils consistof thesefour Theseparameters are not independent, groupsof components. lf oneof thesegroupsis missing.theotherthreegroups.of plusaromaticsplusrcsinsandasphaltenes course.amountto 1007 . assaturates introduces are unity. This fact automatically a certaindegreeof correlation groups Furthermore, theconccntrations of betweenthese andtheirsubdivisions. severalh1'drocarbon tvpes or N. S. O compoundsshow a high degreeof covariance. asa resultof a commonorigin,or commonchemicalaffinities.This problemwill be discussed later (Sect.1.10). In the following. variousdistributionswith respectto compoundswill be treatmentof a largenumberolcrudeoilswith the discussed. basedon a statistical helpof a computer.
1.3Main Groupsof Compounds in CrudeOils
381
q. oI the fraction boiting above 11 T_a!f-e_JV Gross compositionof crude oils (wt. 210"C).(Analysesby IFP) Average value of: (number of samples)
Normal produciblc crude oils (s17)
All crude oils includingtan (636)
Disseminated bitumen (10s7)
SaturatedHC Aromatic HC Resins+ asphalt. Aromatic sulfur (7. of aromatic fraction)a
57.2 28.6 t4.2 2.O7
-s3.3
29.2 t9.7 5t . l r.85
24.2 18.5
" Number of samples for aromatic sulfur: 230 and 88. resDectivelv.
The average valuesof themainclasses of compountls areshownin TableIV. 1.1 for 636reservoired crudeoils and alsofor disseminated bitumensin 1057source rocksof variousagesand origins.Thc 636samplesof petroleumincluclesome heavvunproducibleoils and tars from trrs sanrjs.The valuesfor -517normallv producibleoils (of the 636)are alsoshownseparately. asmanyof the tar" are in fact degradedoils.whosecompositionhasbein alteied. Saturatedhydrocarbonsare usuallythe most importantof thc rnree matn c o n s t i t u e nltiss t e di n T a b l eI V . l . l . T h e m a i ne x c e p t i o nasr c d e g r a d eodi l s .t h a t mayhavepartlyor completelylosttheir alkanes.anda few immiture oilsrich in heavyconstituents and naphthcnoaromatics. The clistribution of the saturated HC contentshowstwo modesand a shoulder(Fig. IV.1.5): - a firstmaximumcorresponds to about607. of saturated hydrocarbons in crude oil. and is mostlyrelatedto ',paraffinic-naphthenic', oils, a secondmaximumat 1(1457ris relatedto a morearomatictypeof oil and is separated from thc previousmr)deb! a minimumcorresponding to 50%. a shoulderaround20 259arepresents the degradedheivy oils and tars that havelost part or all of their alkanes. Aromaticand naphthenoaromatic hydrocarbons are usuallythe secondmost importantgroupof constituents. Theircontenl in crudeoilsvariesfrom20to 45c/r by weightof the fractionboilingabove2l0"C,for87c/ct of theoilsl t0% of the oils containlessthan 20c/caromatics.and 3 c/.containmore than457.. Resinsandasphaltenes usuallyrangefrom0 to ,10% of non-degraded crudeoils dependingon genetictypeandthermalmaturarion.Theircontentis usuallvhish in shallowimmaturepetroleumand decreases with increasins deprhandiubslequent cracking.In heavyoils and tars resulringfrom alterition by microbial activity.waterwashingandoxidation,their contentrangesfrom 25 to 607c.due to an eliminationor a degradation of hydrocarbons (Fig. IV.1.4). The role of the high resinsand asphaltenes contentin heavyoils. and its influenceon specificgravityand viscosirvis tliscussed in ChapteriV.6.
I
of CrudeOils Composition
382
, 53.35 Averoge Slondord deviolion,l9.05
636 Tololnumberof somples, N u m b eorf c l o s s e s2 ,5
Nunbarof Fr.quency Closses t (%ofs{l0roi€d sompl.s oil) HCincrude !
-0.168 Asyrnnetry, K u r t o s i-s0, . 5 5 2
Frequancy
. tat
. t !rla+d2
oils Heovy,degroded . !!r
oils Aromolic 29
50
Poroffinic ond poroff oils inic-nophlh.nic
r00 in crudeoil (fractionboilingabove hydrocarbons Fig.IV.1.5. Distributionof saturated by IFP) oils(analyses of normalandheavydegraded 21b'C),on 636samples When comparingcrude oils and sourcerock bitumens'it is evident that (mosilysaturatedones)areenrichedin crudeoils'whilstresinsand hydrocarbons dueto their strongadsorptionin source impoverished areconversely aiphaltenes r o c k sa n dl o u s t r l u b i l i tcyh a r a c t e r i s t i c s
in CrudeOils 1.4 PrincipalTypesof Hydrocarbons L1.I General Hydrocarbonsprcsentin crude oils are normal. branchedor cyclicsaturated of thesebasic and variousassociations aromatichydrocarbons hydrocarbons. absent are essentially as olefins, such hvdrocarbons. unsaturated types.Other of alkanes in terms considered frequentlv is of hydrocarbons The composition be it should Horvevcr andaromatics (or naphthenes) cycloalkanes (or paratTin.s) , is a rule Mostcycloalkanes of thesebasicstructures thatassociation iemembered and aromaticsbear normal or branchedchainsimoleculescomprisingboth naphthenicand aromaticcyclesplus chainsaccountfor most of the heavier hvdrocarbons. of separationand additionalquantitative As a resultof the usualprocedures any moleculecontainingat leastone aromaticcyclels quotedas measurements, "aromatic",even if severalsaturatedcyclesand chainsare associated to the naphcomprising aromaticcycles.In the samemannel' a saturatedmolecule the Therefore' or naphthene thenicringi andchainsis countedasa cycloalkane
1.:1Principal Tvpesof Hydrocarbons in CrudeOil
38j
alkanecontent.as computed,is restrictedto moleculescomprisingonly chains andit is underestimated in respectto the totalamountof saturitedchains.On the contrary.the aromaticcontent,ascomputed.is overestimated. comparedto the real amountof aromaticcvcles. The respectivecontentof alkanes,cycloalkanes and aromatics- with the . aboverestriction- are shownon FigureIV.1.6 for 541crudeoils.The average valuesof thesetypesof compoundare Iistedin TableIV.1.2for the threeclasses of crudeoils and bitumenpreviouslyconsidered. Whenlookingat a greatnumberof crudeoils,the average valuefor eachof the . three differenttypesof hydrocarbon(alkanes,cycloalkinesand aromatics) in thesecrudeoilsis similar.For all threetypesofhydrocarbon,it rangesfrom 3dto 357o(TableIV.1.2). However,the distributionof cycloalkanes is rathernarrow comparedto the others.Cycloalkanes vary from 20 to 40o/cof hydrocarbons in almost80% of crudeoils. Only 9Tchavelesscycloalkanes and ljolo havemore; the-latterincludea largeproportionof clegraclei oils,wherea relativeincrease of cycloalkanes and aromaticconstituents resultsfrom the selectivedestructionof alkanes.
5 4 1 C r u d eo i l s
AROMATIC IIC
--r-
sorrequ.ncY c 0 n r o u r s{ p e rc e n rI
H i g hw o r I
I
crud.oils
I
N. ISO-ALr(AN ES (PARAFFINS}
CYCLGATXAN ES { NAPHTHENES)
Fig. IV.1.6. Temary diagramshowingthe compositionof hydrocarbonsin the fraction boiling above 21O'C: n * iso-alkanes(or paraffins),cyctoalkanes(or naphthenes),and aromatics.for541 crudeoils analyzedby IFp. For constructionof isofrequencycontours seealsoFisureIV.1.4
of CrudeOils ComPosition
384
'/c of hydrocarbons). Table IV.1.2. Average composition of hydrocarbons (wt. (AnalysesbY IFP) Average value of: (number of samples)
Normal producible crudeoils (517)
All crude oils includingtars (s41)
Disseminated bitumen (668)
n + isoAlkanes Cycloalkanes Aromatics Saturated:aromaticsu Alkanes: saturatedu
33.3 31.9 34.5 2.8 0.49
31.'7 32.1 36.2 2.'l 0.48
27-'1 29.3 43.0 1.8 0.47
u Averagevalueof the ratio. showstwo modesthat jn fact correDistributionof aromatichydrocarbons spond to the bimodal distributionpreviouslyobsened with respectto the in total crudeoils. This is the resultof of saturatedhydrocarbons concentration of crudeoils: minor constituents are usually and asphaltenes resins fact that the "paraffinic-naphthenic" oils' a first maximumaround30% is mostlyrelatedto a/o by a minimumat 40 from. separated - a secondmaximumaround457. whichis relatedto a morearomatictypeof oil ' and also to degradedoils where the increaseof the aromaticcontent is correlativeto the alkanedegradation oilsresultin The distributionof alkanesis alsobimodal.Paraffinic-naphthenic is clearly population A second a singlc broad maximum around 30 35%. than 157c lower generally and outliried.with an alkanccontentlowerthan2070. by partly removed been to degradetloils whosealkaneshave It corresponds so little altered' been has content The iromaticandcvcloalkane bioclegra^dation. orlgrn thc same oil from crude normalanddegraded that the pointsrepresenting in Figure ioin alonga line issuedfrom the alkanepoleof the triangulardiagram
tv.1.6.
1 . 4 . 2n - A l k a n e(sF i g s l.V . l . 7 I V . l . 9 ) All linearn-alkanesfrom C1to C41.anda few bevondC$ havebeenidentifiedin crudeoils.They usuallyamountto l5-20o/cof crudeoil, but their contentcanhe verylow. asin ireavydegradedoils' or ashighas35To The low molecularweight r1-heptane n-aikanes(C5-C1)mayreachthe highestvaluesfor singlecompounds: (Smith, 1968) Texas oil. Ordovician' crude Fasken (vol of 7"1 4.4 % for accounts atoms carbon of number with the regularly decreases the abundance BeyondC'n, oils. in mostcrude for high cloud High molecularweight n-alkanes(> n-C:,,)are responsible of a to the.appearance point! of certaincrudeiils. The clouclpoint corresponds wax a high have cloud of wax crvstalswhen the oil is chilled Oils of this type
L J P r r n e r p1 a r| p e so f H l d r o c a r b o ni n s C r u d eO i l
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f{ f x j i i i '
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Fig lV l T Examples of normar andbranched alkanes andarkenes foundln crudeoirs in c o n t e n ta. l t h o u g ht h e i n d u s t r i a. l. w a x . ' o r . . p a r a f fw a x . . m a vc o n t a l na l s oi s o _ alkanesandalkyl-cvcloalkanes. For jn51o n.. Bestougeff( I rlbTjhasfoundonlv26 and 16% n-alkanesin paraffinwaxesof HassiMesiaoudonj eut", crudc,rils. respectively. ri-Alkanessomctimcsshowa slightpredominance of odd_or even_ numberedrnolecules in certainimmaturecrudeoils. suchas the Utnta Basinor some Metliterrancancrude oils. The significance of this particularpoint is d i s c u s s ei nd C h a p t e Ir L 3 . The. amount of a-altanes (Fig. IV.l.9) is largelv dependenton genetic conditiols.andespecially on the natureof the origin"at organicmatter.High_wax crudeoilsandthosederivedfrom terrestrialorganicmatteiusuallycontain-a large proportionof t-alkanes.whereasmarineor mixedorganicmut",ialyield, rnni. cvcliccompounds.During catagenesis of sourcerock-organic matter.thereis a qeneraltendencytowardsan increase of n-alkaneconceitrationwith increasing maturation.resultingin a seriesof crudeoils of similarorigin,but of increasin! t-alkane content. On the other hancl,microbialdegrad'ation ot reservoired
386
Compositionof Crude Oils
+" t ^ ^ I E >
1.5
ae 05
C-olomsper molecule----------------
Field
L l t n l oB o sn S l o l eL n e DorLUs N o r l hS m y e r J o h nC r € € k
Location
Age
Liloh
Eocene
Konscs o 0klahorn
0rdovician 0rdovclon
TolalC4Cabn-all in crudeoil l v o l u m% eI ?01 312 165 160 212 247
in differenttypesof crudeoils.(After Martinet al., Fig.IV.1.8. Distributionof n-alkanes 1963)
crude oils resultsin a removal of n-alkanes.Those crude oils are among thoscrepresented at the left hand sideof FigureIV.1.9, with a smallamount of r -alkanes.
L 1 . 3 I s o a l k a n e( sF i g s I. V . l . 7 a n d| V . 1 . 1 0 ) Many branchedalkanescontaining10carbonatomsor lesshavebeenidentified. BeyondC1n,the seriesof isoprenoids up to C25,andonly a few otherisoalkanes. suchassqualane.havedefinitelybeenidentified.This situationresultsfrom the of higher very large numberof possibleisomers.and the smallconcentration molecularweightisoalkanes. The highestindividualconcentration ofisoalkanes is foundin the C6-C,range. and/or-heptane,and it may reachmore namely2-methyl-or 3-methyl-hexane
1.,1PrincipalTypesof Hydrocarbons in CrudeOil
387
I 35 E 34 32 E
2S 26 24 22
18 t6
12
a 6
z 2
4
6
A
12 i4 t6 rB zo 22 24 26 2e 3b 32 34 :e re qb lz qq qe as sb % n - A l k a n e s/ s o t u r o l e dh y d r o c o r b o n s
Fig. IV.1.9. Frequencydistributionof z-alkanecontentsof the saturatedhydrocarbon fraction:232 crudeoilsand sourcerock bitumensof variousorigin(Tissotet al., 1977)
I E
: ;
I9 ;
a
20 30 n - H e r o n e( v ou m e% )
-..+
n - H e o r o n (ev o l u m € . d
Fig. IV.1.10. Relationshipbctween C6and C?hydrocarbonsin crude oils; there is a close correlation between n-hexane and /1-heptanein crude oils ofdiffcrent agesand geographic origins. There is, however, no correlation between methylcyclohexane and z-hipiane (Smith, 1968)
of CrudeOils Composition
388
of the than 1olcof the crudeoil. An importantcontribulionof the understanding normal and isoalkanedistributionhas beenprovidedby Martin et al. (1963). They carriedout detailedanalysisof the light fractionsof 18 crude oils from abundance of the variousoriginandages.Theynotedthattheorderof decreasing five hexaneisomersis nearlythe samein all crudeoils.The sameis true for the wasfoundby Poulet nineheptaneisomers.anda comparable orderof abundance (Table lV. 1.3).Themost andRoucach6 on lowerPaleozoic crudeoils,andothers frequentconfigurationin the CsC, range.is (Smith.1968)one teftiarycarbon with two tertiarycarbonatomsis atom(2-methylor 3-mcthyl):the configuration lessabundant.Other types (one quaternarycarbonatom, or more than two tertiaryatoms)arc usuallyverv rare. (2-methyl.and The strongpredominance of normal.iso- and anteisoalkanes 3-methyl),amongC1,-C3 alknnes.canbe explained.On the onehand.the major sourceof long alkyl chainsare alkanes.acidsand alcoholsfrom waxesof higher plantsandmicroorganisms. whichmostlycomprisethescstructuralarrangement. preferentiallv resultsin r, On the other hand.the crackingof heaviermolecules on Table iso.anteisoalkanes. andaromatics.Furthermore.thc resultspresented IV.1.3 showthat isomerization equilibriumof hcptanesis not reached.evenin lower Paleozoicoils. The heptanescannotbe consideredas an isolatedsystem and isomerization reactionsare probablyslowerthanotherreactionsBcneriiting heptanesfrom heaviermolecules.Under conditionsof advancedmdturdtion. crackingreactionsremovingheptanesmight alsobe fasterthan isomerization. In addition,Smith (196U)notedin 7ll crudeoils of variousoriginsand agesa betweenthe abundances of the variousC.-. Cr,-and Crseriesof relationships saturatedhydrocarbons:the more similar the structure,the better is the correlation.For instancex-pentaneand n-hexancare stronglydependent.and the same is true for n-hexaneand n-heptane.If isoalkanesare considered, with moleculcsstructuralli'closeto the linearalkanesshowa strongrelationship of heptaneisomersin crudeoils.(After Tissot,1966) TabletV.1.3. Abundance Isomerizationequilibrium of heptanes(wt. %) 29li K Vapor Liquid phase phase
n-heptane 2-methyl hexane 3 methyl hexane 3-ethyl pentane 2,2 dimetbyl pentane 2,3 dimethyl pentane 2,4 dimetbyl pentane 3,3 dimethyl pentane 2,2,3 trimethyl butane
1.25 9 5.1 0.4s 32 25 9.9 11.,{ 5.9 100.0
400 K Vapor Liquid phase phase
2.25 ,{.3 tl.2 15.4 6.u5 I t.l 0.6 1.3 21.9 t6.7 30 28.8 rJ.15 8..1 11.3 10.,1 4.75 3..1 r00.0 100.0
l{.i)
Abundance in crude oils (wt. '/., liquid phase)
Averageof 18 crudesafter Martin et al. (1963) ) 5.5
15.6s 13.8 12.15 t9.2 1.,15 2.6 13.2 0.6 6.1 29.5 6.8 1.1 9.8 0.4 2.9 0.1 100.0
100.0
Averageof HassiMessaoudcrudes tnonophasic samples) 52.0 16.0 22.1 2.1 0.,1 ,1.11 2.0
100.0
1.4Principal Typesof Hydrocarbons in CrudeOil
3g9
them,e.g., methylpentane with n-hexaneetc.On the contrary,whencomparing two very differentstructuressuch as linear and cyclic,e.g., n_heptaneani methylcyclohexane, thereis no correlation(Fig.IV.1.i0). This situationcan be explainedby the fact that low to medium molecular weighthydrocarbons are not inheritedbiogenicmolecules.but are generated throughthermaldegradationand crackingof C-C bondsof either kirogen or largerbitumenmoleculesalreadyformed.The succession of chemicalreaitions resultingin the variouslight alkanes.for instance,is alike. and the more the structures of two alkanesresembleeachother,the greateris the chancethat the reactionpathsarethe same.Thereforeparallelreactions andcomparable kinetics resultin rather constantratiosof closelyrelatedmolecules.On the contrary, ditlerentstructures suchascyclohexane and n-hexaneare likelv to derivefrom diflerentpartsof the kerogenmacromolecule, througha differenrsuccessron of n nb e e x p e c t ebde f w e i nt h e m . r c a c l i ' ) n sa.n dn r rq 9 1 r s l 2 1 i g ca - l 9 l " C , , li s o a l k a n easr e p a r r i c u l a r lai b u n d a nirn c r u d eo i l s . F o r e x a m p l e 1.6-dimethyloctane and 2-methyl-3-ethvlheptane amountto 0.55and 0.6,1clr of PoncaCity crude oil, respectivelv.This predominance.comparedto other decanes.is probabl;rdue to their origin from monoterpenes of higherplants ( F i g .I I . 3 . l 2 ) . In the mediumrangeof molecularweight.the most remarkablemolecules belongto the seriesof isoprenoids (Fig. IL3.13).Thev areisoalkanes from Ceto C.5with one methvlbranchon everyfourth carbonatom.Their geneticoriginis discus-sed in Part IL They are of particularinterestin ,".peci to pet.oleum genesis. They frequentlvamountto about I % of a crudeoil. The mostabundant arepristane(tetrameth)'lpentadecane C,r) andphytane(tetramethylhexadecane C1). Togetherthevaverageup to 557. of all acyclicisoprenoids. pristaneis often more abundantthan phytane,with an averageratio of 1.35. The pristane predominancemay reach 4 to 10 in some high-waxparaffiniccrude oils of nonmarineorigin from the Gippsland,Cooper and Bowen-SuratBasins, Australia(PowellandMcKirdy, 1975).Otherirnportantisoprenoids areoflenC,o and C,r, whereasC,, and C22are practicallyabsent,due to an unlikelymodeof cleavageof the precursor.Someirregularisoprenoids (cf. Chap. ILj; are also presentin certaincrudeoils, suchas squalane(C1) and lycopane(Ca). In additionto the seriesof isoprenoids. gaschromatography of the branchedcyclicfractionofsaturatedhydrocarbons showstheexistence in somecrudeoilsof homologousseriesof isoalkanes in the C1nC-,urange.High molecularweight isoalkanes andanteisoalkanes (2-methyland3-methyl.Fig.IV.1.7)arepresentin paraffinwax.The1"are especially abundantin certaincrudeoils,for examplethe Uinta basinoilsderivedfrom nonmarinesourcebeds,or Indonesian oilsderived from a paralicenvironment(Tissotet al.,1971). 1.4.1 Alkenes(Fig. IV.I .7) Unsaturated chainsare relativelyunstable.and thusvery unusualin crudeoils. However,very small quantitiesof n-hexene,n-hepteneand n-ocrenewere identified b y P u t s c h e(r1 9 5 2 )i n a P e n n s v l v a nci ar u d i o i l . O n t h e o t h e rh a n d ,
of CrudeOils ComPosition
390
havebeenrecentlyreported or hopenes, polycycliccycloalkenes, suchassterenes in ancientsediments.They have also been found in certainyoung crudeoils (Hollerbach,1976).However,this type of compoundhasnot beeninvestigated very activelyuntil now. (Fig.IV.1.1l) 1.4.5 CycloaLkanes of lowmolecularweight(( Cr6) andtheirderivatives cyclohexane, Cyclopentane, is are importantconstituentsof petroleum.In particular,methylcyclohexane
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I J
1..1PrincipalTvpesof Hyclrocarbons in CrudeOil Normol crude oil (900m) B es h l L o k e ?
:;l'l[--l
391
Heovy degroded oil Pelcon(400m) ?
s ",1^'P'r;".* ) ''*F;l:uud
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.,;uc,o,|nssr I
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:
I !
H i g hr o xc r u doei l Deplh.l680m +
c y c l o o l h o n e Is
{N'.be'ofrhqs) I
Highror crude oil Deplh.4800m +
ss
- 3
Ss .lN PS
.E
as {Numbo c rl r i n g s )
{ N u m b e rronl g s )
I"ig.IV. I .l 2. Distributionof saturated hydrocarbons asfoundby ringanalysis. Additional data on normal and isoalkanesare from separationof normal and iso-cvclicfractionwith the help of 5 A molecularsieves,followedhy quanrirarive GLC analysis. The analysis clcarly showsthe cffectsof biodegradationand thermal alteration.(Data on Alberta oils from Derooet al., 1977;dataon WestAfricanoilsfrom Claretet al., 1977)
of CrudeOils Composition
392
Table IV.1.4. Compositionof cycloalkanes(wt. o/c).(Analysesby IFP) Average value of: (number of samples)
Mono + dicyclics Tricyclics Tetra + pentacyclics
(2ee)
All crude oils includinglars (34'7)
Disseminated bitumen (448)
53.9 20.1 21.9
52.3 20.9 25.5
39.9 21.3 35.6
Normal producible crude oils
frequentlythe mostabundantof them, and it reaches2.4ahof total crudefrom is alsoimportant,and both Conroe.Texas(Smith. i968). Methylcyclopentane methyl derivativesare usually more abundantthan the respectiveparent with less Most of alkylcycloalkanes molecules.cyclopentane and cyclohexane. andonly a or cyclohexane. than l0 carbonatomsarederivatives of cvclopentane Peculiarstructureswith few are bicyclic,e. g., bicyclooctane or bicyclononane. - havebeen - i. e., cyclobutane andcycloheptane differentring arrangements alsoreportedin minuteamounts. Cycloalkanes of the mediumto heavyfraction(C'n-Crs)are usuallymadeby arrangementsof onc to five 5- or 6-memberedrings (Table IV.1.4). The ("ring analysis") is usually hydrocarbons distributionby ringnumberof saturated accordingto the methodof Hood and O'Neal obtainedb.vmassspectrometry. (1959):the interpretation"'isbasedon the fragmentpeaksobtainedwith an ionizationintensityof ca. 70 eV. Examplesof distributionby ring analysisin n o r m a la n dd e g r a d e cdr u d eo i l sa r es h o w ni n F i g u r eI V . l . l 2 . Mono- and dicyclicsgenerallyamountto 50 55c/cof the total cycloalkanes >C1p.Thoseof high molecularweightusuallycontainone long chain(linearor slightlybranched)and severalshortmethylor ethyl chains(Hood et al.. 1959). regularlvasa decreases The abundance of the variousmono-anddicycloalkanes functionof molecularweight(i. e.. numberof carbonatoms). >C 1n.Someof them Tricycloalkanes averageonlv 20% of thetotalnaphthenes enanthrene. of the perhydroph are likelv to presentthe structuralarrangement regularlyasa functionof decreases The abundance of the varioustricycloalkanes molecularweight.beyond20 carbonatoms. or adamantane(Fig. A very peculiar compound is the tricyclodecane. I V . 1 . 1 1 ) ;i t h a s b e e n i d e n t i f i e di n a v e r y s m a l l a m o u n ti n t h e H o d o n i n . crudeoil (LandaandMachacek.1933)andfrom the PoncaCit-v Czechoslovakia. crude oil (Mair et al.. 1959).Sincethen. Petrov et al. (1974)have found hydrocarbons ol the adamantaneseriesin many crude oils. where they may amountto 10-157cof the tricvcloalkanes. ion, whichmay 10 The nrethodgivesthe numberof ringsper stablemassspectrometric differ from the numberof ringsper molecule.On the other hand.if the methodis polycyclics e.g . appliedto bitumenfrom immaturesourcerocks.unsaturated from saturatedbeforethe sterenes might be abundantand shouldbe separated analvsis is run
1..1 Principal Typesof Hydrocarbons in CrudeOil
t9J
and pentacycloalkanes average257. of the total naphthenes>C,u. __TetraTheir structurei.sdirectll, related..toihe tetracyclicsteroids inc pcntacvclic triterpenes(F'ig.II.3.l6). The distributionof tetracyclicsteranes is mainlvfrom l7 to 30 carbonatoms.whilstrhe distributionof pentacyclic trit".p"""r' fr"r'iii maximumfrom 27 to 32 carbonatoms,but sometimes extendsup io 35 carbon atoms.Their origin andoccurrence, asgeochemical fossils,havebeendiscussed in Part II. Someof them havebeenide;tified by gaschromatographv andmass spectrometry, anda few havebeenisolatedin crysta ineform (fiilli ei al. 1970). . They show a high opticalrotationand are responsible to. ri.xt ot the optic;l activitvof fossilhydrocarbons. In general,tetraandpentacyclic cycloalkanes are mostabundantin youngand immaturecrudeoils.alio rich in resins. asphaltenes andpolyaromatics. Althoughthe total cycloalkane contentof sourcerocksanclpetroleumis very similar.monoanddicycrics are moreabundantin crudeorr.,tr,r" in,.u.r.,o.-t., the contrarvis true for tetra-and pentacvclic molecules. L 1 . 6 A r o m a t i c s( F i g .I V . l . t 3 ) True aromaticsare moleculescontainingonly aromaticringsand chains. They usuallvincludeone to four or five conclensed aromaticrings-and a smallnumber of shortchains.Compoundsbelongingto this basict1,pe-have beenidentified: benzene (1 ring).naphthalene (2 rings).phenanthrene ind anthracene (3 rings), pvrenc. benzanthracene and chrysene(4 rings).A secondfundamentalti,pe consistsof one five-memberedring in addiiion to the six_membered rings: fluorene(3 rings).benzotluorene andfluoranthenc(4 rings),andbenzofluoranthene(-5rings).The molecularmir-ss formulaof nrornntiL,i, C"H." wherep n. v a r i e s . w i tthh e n u m b e ro f r i n g s( F i g .I V . l . l 1 ) . B c n z e n e( p = 6). naphthalene = (p l2). and phenanthrene (p : lg) t1,pes are the mostaiun.lant.The major components in eacht1'peare usuallynot the parentcompound.but the alkvtaied derivativescomprisingone to three additionarcarbonatoms.r,.,, iorton." tt" majorconstitucntrllf the alk1,l-benzene CnH,n..., tvpe is frequentlvtoluene.which mavreachl.S9Zofcrudeoil.andsomerimesxtlenilthet.,ialo,m.p_xylenesmay a m o u ntto 1 . 3? i .o l c r u d eo i l ) .w h e r e abse n z c niesu s u a l l lyc s sr b un j a n t( u pt o I i . of crudeoil). The samesituationis rrue for the naphtLalene tvpe C.H." ,.. the d i s t r i b u t i o on f w h i c h( F i g .I V . 1 . l - i ) h a si t r m a x r m ' ma r C , . o r C , . . iir tri_ 1Oi_ mcthl'lnaphthalene). andfor thephenanthrene typeC'H1".,r,with a maximumat c',0or.C,, (di- or tri-methylphenanthrene). Beyontrthe.semaximaall distributionsdecrease rapidiy.due to the scarcityof long chains.Higher_boiling alkyl_ benzenes,naphthalenes.etc. which ociur at iorver conce'ntration t1,-picaily c o n t a i nt w o t r rt h r e cm e t h tI g r o u p sa n do n el o n gc h a i n( H o o de t a t . . lOSl1. r ne prcterencetor mono_di_andtri-methylderivatives couldbe explainedif naphthalene andphenanthrene sericsu eremosrlygenerated bv fragmentation of s t e r o i d so r t e r p e n o i d sH. o w e v e r .i n d i v i d u am l e t " h ydl e r i v a t i v ee. . g . , m e t h y l _ phenanthrene. arenot clearlyrelatedto biologicalprecursormolecule.s (Radkeet a l . .1 9 U 2 ) . Whenseveralstructuralt),pesarepossible,asfor molecules with thrceor more Jromatrcrngs. one of them is favoredin crudeoils. Thusalkylanthracenes are
of CrudeOils Composition
394
I "-i''-!,-1", -C -.,- O--...-B:1r". \"'
,,rc...:a,.c... ,
V\?z\ I ll ,/'\,,'\Z\
,r-,zv\ (_14,
,r,_/.\ I ll \.,^\,,.
cleHtl ----> 0imelhyl-
Crl Hrr l--->T,rneihylnophtholen€
| ->
+
crcHn Mrrhymuorene
croHrz
T€lrohydrorophlholene (letrolrnl
3i =n
.J' ,-'r.\',!l I
ll
I
>("
" ..
-------ili,l".o'.nto,oDh€nonlhr.ne
X z\ /..i.l:"li;," rrrn'rhYlPicenc
dY
(naphthenoaromatics) Fig. IV.1.13. Examplesof aromaticsandcycloalkanoaromatics foundin crudeoils are largelypredominantThis presentin smallamount.and alkylphenanthrenes series of the phenanthrene origin with the suggested iactwoulclbe in agreement traces of (1975) reported et al (1975). and McKay Blu."t Youngblood,tid rings in aromatic to eight with four hydrocarbons p-olynuclear aromatic higher is composed which as diphenyl' types such aromatic condensed to-rritrueft.Non bridge'arealsoknownin of two benzeneringsjoinedby a singlecarbon-carbon mrnoramounts. (Fig.IV.l .13) 1.1.7 Naphthenoaromatics of the highboilingfraction Thesecompoundsare usuallythe major constituents arornaticrings'fused They includeone or severalcondensed of hydrocaibons. ringsandchains.The mostfrequentstructuraltypes,compnsrng with naphthenic up to five rings,ari presentedin FigureIV l.14. The distributionof the total fractionby numberof aromaticcyclesin the aiomaticand naphthenoaromatic
L,l PrincipalTypesof Hydrocarbons in CrudeOil Mas6 lornul!
Monoar0maltcs
0itt0mal|cs
Triaromatics
395 Soltur!romatic Dolec!los
+ c nH z n - e
9.j" ci H2n_roS
c nH z n e
c nB 2 n - r o
c nH a n - r z
\AsJiy'"
a
c nH z n - r q
c nH z n - r o
cnHzn,re
i
nE.
cnB2n,2o
E
aromctics
D
Nophthenocromorics D
s u t i u ro r o m ' t i c( r h i o p h € nd€c)r i v o t v e s
Fig IV-1.1.1. Examples of aromatic. naphthenoaromatic andsulfuraromatrc (thiophene) derivatives corresponding to themolecular massformulaC"Hlnn.In manycases. several structures arepossible andonlythemostfrequent or mostprobibleis sh.,run moleculeis shown in 'l'able IV.1.5. Mono_ and di_aromatics are especially abundant, to polyaromatics, paraffinic_naphthenic in crude oils from _compared detritalsedimentarl' series. Naphthenoaroma_tic-s are panicularlyabundant,ascomparedto pure aroma_ . tlcs. ln voung or shallowimmaturecrude oils. Thc aromatictypes become p r e d o m i n a nall t e ra n i m p o r t a ntlh e r m ael r o l u t i o n
of CrudeOils ComPosition
396 lJinlo B0sin
Conodo Weslern
Bosin NorlhAquiioine
Eocene MissrssLppron lJ Creloceous (Pembin.,l0O0ml i'lorinollonicsl,259:rrl iRedWosh'1650ml
i t o r c o n s' 4 0 0 mI m o s l l ys l e r o d s
I r'Y?
09='
l0
I E
@n
20
I
r50 I
dr!e"70ri 0Dnenes
@n
----_----J Numberof corbonolonlspel molecule
andsulturaromatic ofaromatics'naphthenoaromatics Fig.IV.1.15. Typicaldistributions group of compounds ' this Among formulir CnIIln mass the 'l to corrisponding de?ivatives Polyalkylnaphthalenes are diaromatics and molecules tetracyclic a;e monoaro$atics aronatics (alkyldibcnzoaromaticsof the same mass are In fact sulfur-containing : usuallv abundances n-2 Maximum with m is C.H:'' Theirtrueformula ,nS. ihiophenes). of cachtype.The resultsarc obtained "." i"...J.a t", the di- or tri-methylJerivatives (10 eV) usingparentpeaks Lellr two massspeclrometr,v from low-ionization-voltage andhighPembina.u. Cretaceous. from oil a low-suifur crudeoilsfrom wcsterncinada: fll om Dcroocl .le7-1.)Rigltrahigh( tr ir n Data Mississippr E. armattan . fronr H sulfuroil *ax ..ude oil from Red Wash. Utah. antl a bitumenfrom the Jutassicof the North (ca' el.iiui"" Basin.France The latter i\ partrculrrll-rich tn monoaromlticsteroids cI c:") Naphthenoaromatics may show various structural arrangements Bicyclic (1 aromatic. 1 saturatedclcli) inclane.tetralin (tetrahydronaphthalene)and their methyl clerivativesare uiuallv abundant and have been determined in several common' crude oils. Tricyclic tetrahydrophenanthreneand derivativesare also which are moleculcs pentacyclic and Of particular importance aic thi tctracyclic form' thesaturated beside^s mostly relate.l to steroid and triterpenoid structures: rings' discussedin Section 1.'1.5.they have been identified with I 2' or 3 aromatic
1.,1PrincipalTypesof Hydrocarbons in CrudeOil
391
beingnaphthenic.rings (Fig. Il.3.t6). The distributionof tetracyclic lle,:1her naphthenoaromalics. accordingto the numberof carbonatoms,may showtwo d i s t i n cm t a x i m ao, r o n l yo n e ( F i g .I V . 1 . l 5 ) : - a first maximumaround20 carbonatomsprobably resultingfrom dealkylation of morecomplexmolecules, a secondmaximumaround27to 30carbonatomscorresponding to the original structureof the biogenicsteroids. Theirconcentrationmay be importantin naphthenicor paraffinic_naphthenic crudeoils. All togetherthe averageamountoi tetra pluspentacyclic molecules - naphth^enic, - is over l07c by weightof the aromaticor naphthenoaromatic c r u d eo i l f r a c t i o nb o i l i n ga b o v e2 1 0 . C( T a b l eI V . 1 . 6 ) . Table lV- 1.5. Average compositionof the aromatic fraction of crude oils and bitumen extracted from source rocks (wt. %.ol the k)tal aromatic fraction boiling above 210.C). (Analysesby IFp) Average value of: (number of samples)
Crude oils (121)
Monoaromatics Diaromatics Triaromatics Tetra- and polyaromatics
33.0 23.4 12.9 7.3 16.6
80.6
23.1
19.4
100.0
100.0
Total aromatics + naphthenoarom. Thiophene derivatives (benzo-, dibenzothiophenes) Total
Disscminated Dttumen (108) 26.3 21.1 llt.'7 11.5
Table IV.1.6.. Tetra- and pentacyclicmoleculesin crude oils and bitumens(wt. q. of the crud€_oilfraction boiling above210.C; wr. o/oof bitumenextractedby CHCI3).(Analyses bv IFP) Averagevalueof tetra + pentacyclicsin:
Crude oils wt. a/t
Naphthenicfraction Aromatic + naphthenoaromatic fraction Total
Disseminated brtumen
Number of samples
wl. o/t
Number ot samples
'1 .3
299
5.6
.148
1.4
121
:1.I
108
-,
9.7
Compositionof Crude Oils
398
1.5 SulfurCompounds The averagesulfur contentof crudeoils, basedon 9347samplesis 0.65% by weight. The distribution(Fig. IV.1.16) is bimodal with a minimum at 17o separating: - low sulfur crude oils containingless than 17o sulfur and comprisingthe majorityof the samples(about7500), - high sulfur crude oils containingmore than 7Vosulfur and grouping a smaller n u m b e ro f c r u d e s( a b o u t1 8 0 0 ) . However,aspointedout by Smith(1968),the picturewouldbe quitedifferent basis.As crude on a productionplusreserves if the diagramwerebe established andthe WestTexasPermianbasinaccount oilsfrom the Middle East.Venezuela for a largepart of the world productionplus reserves,and are generallyhighsulfurcrudeoils.the secondmaximum (> 1% S) maybecomeasimportantas.or evenmore than, the first one (< I % S). Sulfuris the third mostabundantatomicconstituentof crudeoils,following carbonandhydrogen.It is presentin mediumaswellasin heavyfractionsofcrude Closses ( % Si nc r u doei l )
Numberof somple!Frequency
Frequency
s
i
12y.
0
-_!
. __J
001 .o
Lowsulfur' crude oilsi
0.1 -=
. Hiqhsulfur c r u doei l s
10
; Fig. IV.l.16. Distributionof sulfur in 9315crude oils of variousagesand origins. (Analysesby U. S. Bureauof MinesandIFP)
1.5SulfurCompounds
399
={.ilii'
-ot-
H
H
l
H
I
\./ l + - h i o c y c l o h e x o r e
a-n'--n \-^.4.;
alrYY
,/s\.,. > 2-Thiobutone
r_H
I I "7-"-ti"
F
--- 2-Bulonethiol
l
N-C-S-C-C-H t t l H H H
"-i
l-
Meihylbenzothiophene cH3 cHa
D i m € t h y l d i b e n z oo iphh e n e
Fig.IV. l. I7. Examples of sulturcompounds rncrudeoils oils. In particularthc sulfur contentis not specificallyrclated to the heaviest fractionsof crude oils, as is the casefor nitrogen.in the low and mcdium molecularweight range(up to C.r) sultur is asslociatcd only wirn carbonand hl,drogen.In the heavicrfractions.ofcrude,rils.rt is frequenil; incorporated in l r r y c . p r r l l s l c l im s o l e c u l ecso m p r i s i nN g . S . O ( r e eS e c .i . x ; . S u t t u rc o m p o u n di \r l e n t i f i eidn t h ei i g h ta n dm e d i u mf r a c i i o no f c r u d e o i l s( u p t o C . , ; )b e l o n gr o f o u r m a i nc l a s s eosf u n e q u ailm p o r t a n c(eF i g . I V .t . l 7 ) : a) thiols.alsocalledmercaptans b) sulfides c) disulfides d) thiophenederivatives. sulfurcompounds areparticularlvabundantin immarurecruoe .Nonthiophenic o l s , w h e r e atsh r o p h e n iscu l f u ra b u n d a n ci cn c r e a s ewsi t h m a t u r a t i o n (Ho et al.. t971)
400
of CrudeOils Composition
ofan Thiolscanbe thoughtofasderivedfrom hydrogensulfide,by substitution isoalkanethiols' Normal and atom alkyl or cycloalkylradicalto an hydrogen havebeenfound in petroleum(Smith' and cyclohexanethiol cyclopentanethiol Mostthiolshavea low molecular reported 1966).Aromaticthiolshavenot been most abundantin Wassoncrude oil. The (less carbon atoms). than 8 weight by weightof the crude. only to 0.0053c/o and amounts Texas.is ethanethiol of sulfideby substitution from hydrogen as derived thought of Sulfidescanbe thetwo hydrogenatomsby alkylgroups.Theyareusuallyof verylow abundance which The major compoundsof this type in Wassoncrudeate the 2-thiabutane amountsto 0.0022Vcby weight of the crude, and variousthiacyclopentane bv one aromatic Alkylaryl sulfidesare substituted derivatives(up to 0.002570). ring and one alkyl group. of Disulfideshavea similarstructureassulfides,but the sulfurbridgcconsists two sulfuratomsinsteadof one: a few of them havebeenrdentifiedin the low to dithiahexane). molecularrange(dithiabutane ring. comprising 5-membered Thiophenecan be describedasan unsaturated the (e.g., chromatography) one sulfurandfour carbonatoms.In manyrespects very is generally thiophenering behaveslike a benzenering. Thiopheneitself and benzonaphthothiophenes dibenzothiophenes scarce,but benzothiophenes, are (comprisingone thiophenering and 1, 2, or 3 benzenerings.respectively) in small present are oils. They of all high-sulfurcrude importantconstituents amountin low-sulfurcrudeoils.In Wasson.Texas.crudeoil' the mostabundant whichamounts moleculeof thistypeidentifiedsofar is 2-methylbenzothiophene. c/o seem to befairly dibenzothiophenes crude. Although to 0.0025 by weightof the tt) givc an moment possible at the is not oils. it abundantin certaincrude evaluation. individual ratio dibenzothiophene Ho et al. (1974)haveshownthat the benzothiophene: maturationof crudeoil Dibenzothiophenes with increasing normallydecreases are frequentlythe major group of thiopheniccompoundsin matureor altered thedistributionby molecularweiSht.theparent high-sulfurcrudeoils.Regarding moleculeis generallylessabundantthan the substitutedderivatives.and the to moleculescomprisingtwo or three maximumcontent usuallycorresponds (Fig. additional carbon atoms. i.e., dimethyl-, trimethyl-dibenzothiophene An exceptionis the IV.l.1-5).The same is true for benzonaphthothiophene. whichsometimes coversa widerangeof molecudistributionof benzothiophene, lar weights.Moleculescomprisinga saturatedcyclein additionto aromaticand with one thiopheniccyclesare also known, particularlythe dibenzothiophene additionalsaturatedcycle. Thiophenederivativesare particularlyabundantin crude oils with a high For 121crudeoils from various contentof aromatics,resinsand asphaltenes. origins.total thiophenederivativesaveragemore lhan 207c of the aromatic f r a c t i o nb o i l i n ga b o v e2 1 0 ' C( T a b l eI V . 1 . 5 ) .
I 5 SulfurCompounds
l0t
TableIV.l.7. Nitrogencontentaccording to distillalion fractions of Wilmingron, California, crude oil ( s m i t h ,1 9 6 8 ) Boiling range ('C)
Nitrogen (wt. %)
upto 130 r 30-250 150 300 -r00-350 150 400 .100-500 Residuum
0.00 o.0021 0.028 0.10 0.25 0.48 o.9'7
L6 NitrogenCompounds (Fig.fV.t.18) \itrogen contentis usuallvmuchlower than sulfurcontentin crudeoils: about 907. of thecrudescontainlessthan0.27cof nitrogcn.The average valueon crude oils is 0.0947cby weight.The largestcontentsare known in cirtatn cru
0.25%) from normal, nitrogen_poor fetroleum. Degradedasphaltsare enrichedin nitrogen,as the rclati"vepirt of resinsand asph.altcne-s is increased,comparedwith normal crude oils:^nitrogencontent r c a c h e1 s.2,1% i n W h i t e r o c kC s a n y o na s p h a l(tU t a h ) . T h c - m a i np a r t o f n i t r o g e ni s f o u n di n h i g hm o l c c u l awr e i g h a r n dh i g hb o i l i n g pointfractions. shownbv Table IV.1.7. Theschigh motcular wergtltcom_ _as poundsa_rein fact polvcvclicaromilticstructurescontainingmr)rc or lessat randomdifferentheteroatoms (N. S. O). However.a ccrtainnumberof low to mediummolecularrvcightspccificnitrogencomp,rund:irreknownfrom naphtha andgasoil fractions:about25 to 30.ri of them usuallvconsists of basicnitiogen compounds: pl,ridine,quinoline.The rcst is madeof nonbasicmolecules.eig.. carbazoles and indoles(Seifert. 1969).More recentl).Schmitteret al. (l9gi) investigated the basicfraction of crude oils and showedthat brsic nitrogen compoundsare mainlv monoaza-arenes with a singleN_atom:quinolines;ncl benzoquinolincs derivativcs.In particular.the tricyalicfractioncontainsseveral dimethyl and trimethvl - benzoquinolines. which are usuallvthe maior e o m p o u n dorI t h i sg r r r u p . Porphyrins containfour p1'rrolenucleianda metal,mostlvvanadiumor nickel. _. They will be discussed in Section1.9asorganometallic complexes.
of CrudeOils Composition
\ \.. "..-.2'....2"...* ,_!_,
|J
i
t
l
" '...2c'r.,rc\",,' ll
I .,.'::",rd'r
i
E l
/y-\
-0,-l | * Zc'r "
-fY^r .$'i/
.:i
a,v.ct' _>r.ihrrerad,nr
-
i
l| \'^nt
l-0"'o'"c
B.uemorn
i
- o'- .z\____-,\+ c.'b.Io'' u\Nly' ! i | "-lr-"-!.-n-1.r".-1..r*
2
l'....2'.r.-,,.'.r:.,2ts
: t
I
l
,
I
I
! | ".t"rl="-.-lr-.-l(l "-",-GD'i*!,h'',' u0'..on. r al i l-lr ".'--i.'-fi-"'..2''r" 6 E| H o "3 l t
oqD
................ D'Denro'doi
l " t I
ac x !
> oc'eso'
€l
3l'\ 7'
il a!(" o', El..[- n
H""'r;i:r:.r*'
o l
t -1"
| .-+4)'tH
r?'?'iil'11i'llf'
in crudeoils ,g. IV.1.18. Examplcsof nitrogenandoxygencompounds
l.1lIligh MolecularWeightN. S. O Compounds: ResinsandAsphaitenes
403
1.7Oxygen Compounds (Fig.IV.1.t8) The most importantgroupsof oxygen_containing compoundsare acids.which seemIo be a commonconstituentin youngand immature crudcoils.Saturateci fatt' acidsfrom C, to C;,1,and also iso"prcnoid acidshave been identified. Naphthenic acidshavebeenknownfor a longtimeto be importantconstituents in s o m en a p h t h c n iacn da s p h a l t iccr u d eo i l sf r o mU S S R( G r o i n i l . B i b i _ E i b aet .t c ). , C a l i [ o r n i a :c ] c l o p c n t a n ea n t l c v c l . , h e x r n c - c u r n o x vai rcci d s )Le 1 9.,r. z ucyctopen eta,and i-L l] | acettcacidsCr_C,n.Someof the naphthenic aclcsprescntln h c a v vc r u d eo i l sa r c c e r t a i n l y . o r i g i ncaoln s t i t u e not sf t h eo i l ( c . g . . t h o s ew i t h a hopaneskeleton resultfrom a delrajati,l"n'olihio.iginalc.ucle ). Someotherm.a-y bv chemicalor biochemical oxioatron. S-eifbrtand.Howclls(1969)estimatedthe amountof carboxvlrc acrdsup to ^ 2.5ti,bv \\,eight. in a voungand immatureCalifornia.ru.t. oii.-ifr." separarerl anclidentifieda largenumbcrof r.urboxvlic acidsbclongingto manvhvdrocarbon s t r u c t u r at lv p e s .N a p h t h c n iac n d n a p h r h e n , u r o m r ttilcp e s a r c r h e m o s ta b u n _ dant.tolbwcd bv poll,aromatic andhiterocyclict1,pes (Stifcrtand.feeter.1970). Of particularinterestis the idcnrilicarion of-steroijcarbo*1ti."c,Urs (-130 ppmtbr thefull)'_saturitted tvpe).someof rhcmhcingan indicationof animatcontribution to the formation of petroleum(Seiferr.1973).In spite of the considcrable ,.hi..l.tssot compound.four steroidacidsweredefiniterytttentified ::Tll:Il,l..t l n o l n e t rs l r u c l u r ( . p r o \ be rns l n l h e s r s . The m:st ubiquitousgroupof oxygencompounds in crucleoilsis probablyrhe group of pentacvclicacidswith a hopaneikeieton. Thev are considereiby Ourissonet al. (1979)to bc clerivedfrom bacterialIipiO, *.fl as the Ci ''bacteriohopane tetrol" and otherconstituents of bacterialmembrancs. Severalphenols(Seifertand Howells. 1969),un,l purtl.ufuriv . cresols.are abundantin the acidiclractionof crudeoils.OtheroxygJn .ornfornO, t uu" t,""n in minor amounrs:kerones,alk1,lor cyclici icluaing'nuo.. non.,, onO l.!:lufi.,g o tDenloIulirns.
LU High MolecularWeightN, S, O Compounds: ResinsandAsphaltenes High molecularweightconstituents of crudeoirs usua|v containN, S and o compounds.They are referredto as resinsand asphaltencs. Asphaltenesand mostresinsare complexstructuralarrangements martcof polycvcl'ic aromatlcor naphthenoaromatic nuclei.with chainsa;d hererodtoms (b, irt.Sj..l.heyconsti_ tute the heavyendsof petroreum.and theyhaveto be considereiasthe natural high molecularweightend memhersof rirc aromaticunJ pt, nr it . no-uro-rti. weightincreascrheyondca. t o0. ihe fron.,o,r,ryot ::li:^,Y^L:l]1.^rolccular rne motecute contarning oneor moreheteroatoms (O, N or S) is higi. Therefore. thereis practicallyno purearomaticor naphthenoaiomatlc molecuiern tfrefreauy end of petroleum.and all constituents containO, N or S atoms.occaslonally substitutingfor carbonatomsin thc cvclicstructure.They alsocontainmetals ( S e c t1. . 9 ) .
of CrudeOils ComPosition
404
basedon the separation are usuallvdistinguished Resinsand asphaltenes procedure.However,thereareseveralwaysof preparingresinsandasphaltenes. Precipitationby propancseparates rcsultingin slightlydifferentcompositions. byn-pentane from the restofcrudeoil. thenprecipitation resinsandasphaltenes (insoluble)Alternaor r-heptaneseparatcsresins(soluble)from asphaltenes are readilyprecipitatedfrom crudeoil by r-hexaneor n-heptivcly asphaltenes on by Iiquidchromatography from hydrocarbons tane.then resinsare separated which can. to some polarity are two factors size and alumina.[n fact, mo]ecular mav for eachother(Long, 1979).Thus.differentcompounds extent.compensate and also from crudc oils. procedure from different precipitated with the same be s y n t h e t ioc i l s .s u c ha ss h a l eo i l o r c o a ll i q u i d s( B o c k r a t he t a l . , 1 9 7 9 ) . plusresinsgenerall,vamountto lessthan l0 % in paraffinic oilsand Asphaltcnes oils; they may-.rcach l0 to '107c in less than 20% in paraffinic-naphthenic amountto only 0 to 20% oils.Among them.asphnltenes aromatic-intermediate oils.Heavydcgradedoils.suchasthe Athabasca of thosenormal.non-degraded Resinsand or Orinocodeposits.maycontain25 to 607cresinsplusasphaltenes. asphaltenes are also prescnt in sourcerock bitumen, where thev are more from sourcerock abundantthanin tlprielatedcrudeoils.Resinsandasphaltcnes petroleumcompounds, but not identical bitumenare closeto the corresponding from bitumenand of resinsandasphaltenes TableIV. 1.8 showsthecompositions from crudeoils. from petroleumhasbeen and resinsprecipitated The structureof asphaltenes investigatedby using severalphysicaltechniques.such as X-ray diffraction, (Yen, 1961. 1912.1919). and infraredspectroscopy nuclearmagneticresonance asfollowsby summarized Thiswork resultedin a structuralmodelof asphaltenes ( 1 9 7 2 ) I V . 1 . 1 9 : a n ds h o w ni n F i g u r e Yen polyaromatic sheel.Aromaticcyclesare a) The basicunit is a pericondensed number of longchains.Sulturis groups and a small by methyl mostlysubstituted groups,nitrogenin quinoline groups and usuallyfound in benzothiophene oxygenin etherbonds.All of them representa defectof the aromaticstructure. b) lndividualaromaticsheetsareusuallystacked(ca.5 Iayers)to formpartlc/es Molccularassociaassociation. or intramolecular bv intermolecular or crystrrllites aromaticsheets tion resultsfrom;r-z bondsbetweenDericondensed
'Iablc
(wt. % ) of resinsend asphahenes IV. L8. Averagccomposition
O + S HiC + N alomic rJ3.0 10.0 2.0
0.5
?.0
1.5
t t . 5 t .t 6
Crudeoils
Sourccrock brlumen
8 . I 2 . r ) 5.0
Ll
Asphaltcncs
ul.,1
R c s i n (sl 5 l )
?8.0 9.2 1.2
2.8
0.9
r0., t.-r7
A\ph.lltene\(175)
71.1
1.0
t.7
l:.t
1.5
7.6
LllJ
1.8High MolecularWeightN, S, O Compounds: ResinsandAsphaltenes
405
A Crystollile I C h o i nB u n d t e C Porlicle D Micelle E weokLink f G o po n d H o l e G Introctusler H lnterctuster lResin J s'nole Lover K P€koporphyrin M u"ror
F i g .I V . 1 . 1 9 .M a c r o s t r u c t uor e f asphaltenesprecipitated from crude oils. (Accordingto Yen. 1912)
c) Severalcrystallitesmayfrom aggregatesor micellesof differentsize.The size andthe molecularweightvarvwith the analyticalmethodapplied:the molecular weightofthe individualsheetsis ca. 500 1000.andthe sizeoftheir polyaromatic nucleusis 8- l5 A. The molecular* eightof a particleresultingfromthestackingis 1000-10.000, andtheir averagethicknessis I:-20 A. Valuesof molecularweight reaching50,000or morehavebeenmeasured on aggregates of Darticles. The size of thelatterdependson the numberof particles. +tL!o I beingan average value. d) Resinsincludeminor amountsof free acids,estersor ethers.The bulk of reslnsis, moreover,likely to containmolecules comparable to. but lessaromatic than. asphaltcnes. The numberand sizeof aromaiicnucleiwould be reduced comparedto asphaltcnes, thusloweringthe possibilityof intermolecular bonds. Therefore scparatedresins probably comprisea single sheet and show a comparatively lower molecularweight(500to 1200).Their lighterconsriruents maystill comprisesometrue polyaromatics. However,mostof the resinsarerich in heteroatomsand particuiarlyoxygen. Resinsare not stable compounds especiallv whenexposedto air andsunlight.andtheyprobablyevolvefurtherand go on to form asphaltenes. Whenheat-treated. thevproduce.bv dismutationand c r a c k i n gh. )d r r r c a r b o nasn r la r p h a l t e n e s . Someauthorsconsiderthatthe modeldeveloped by yen basedon precipitatecl asphaltenes is alsoa validrepresentation of asphaltenes in crudeoils.Someothers areof the opinionthat aggregates or evenparticlesdo not existandarea resultof asphaltenes being separatedin the laboratoryfrom the other constituents of crudeoil. The microstructure of asphaltenes (e.g.. the sheetand generallythe particle described by Yen) is widelyacceptcd.However.therearesomediscussron about the intramolecular forcesandparticularlythe role of cross-linking by oxygenor sulfur bridges.Most of oxygenand part of sulfur are engagedin cross-linking, or in functionalgroups:they are removedby thermaltreatment.On the contrarv. nitrogenand the remainingpart of sulfur are engagedin heterocycles
406
Composition of CrudeOils
of the polyaromaticnuclei;upon heatingthey are found in the residualcoke (Moschopedis et al., 1978;Speightet al., 1979). The problemof the macrostruclure (micellesor aggregates), and the evaluation of the molecularweightare clearlyconsequences of the physicalstateof asphaltenes in their environment.Laboratorymeasurements of the molecular weightof previouslyseparatedasphaltenes are dependenton the natureof the solvent.the asphaltene concentration, andthe temperature (Moschopedis et al.. 1976).Asphaltenesform aggregates evenin dilute solution:in solventsof low polarity, such as benzene,the asphaltenes from Athabascatar sandshave a molecularweight of 4000 to 7000; in solventsof higher polarity. such as nitrobenzene, their molecularweightis lowerthan 2000at 120.C(Moschopedis et al., 1976).Recently,Boduszynski et al. (1980),usingchemicalfractionation of a vacuumdistillationresidue(called"asphalt")followedby infraredanalyses and field ionizationmassspectrometry (FIMS), expressed the view that particlesof very high molecularweight(beyond5000)do not existin crudeoil. In crude oils. asphaltenes are probablydispersedby the action of resins (Pfeifferand Saal.1940;Speightet a|..1979;Speight,1980).Resin-asphaltene interactions, especially throughhydrogen-bonding, arepreferredto asphalteneasphalteneassociation and thus they keep theseparticlesin suspension. The overall structurewould be of micellartype as proposedby pfeiffer and Saal ( 1940):FigureIV. 1.20.In thisscheme,the coreof the micelleis occupiedbv one . ,. -.-)
/-'i
^<
| ;j:;t:5).{i}j :;...
iifi.rt?;:qi"iii;
t*'ri,;#?)k"i" f"n
sullo
a€ > (1 '0V\ -o u-*ro"
'is\ iJ:t31\s^-o
.,loi 0,,,, o o-n, 3 J S
{z\JPp)7?17' Fia?"--'tc-'.i 'j- o,-;r'!i€ah.l ? . i ' - < o ( J ^ uo \ ) @ \ o j * S O\ ' * 0 : l € r:jl':ii;ii:,lJ:1|'",i;"1 J Asphattenes
rO Resins o Aromatichydrocarbons ^- Paraffinic and,/or naphthenic hydrocarbons
Fig.IV.1.20aandb. Possible micellar structureof asphaltenes and resins in crude oils. (Adapted,with changes.from Pfeiffer and Saal, 1940).(a) Asphaltenes are fully dispersedwhenthe crudeoilcontainsa sufficicnt amount of resins and aromatichydrocarbons. (b) Asphaltene-asphaltene association occurs in crudeoils wherethereis a shortage of resinsand aromatichydrocarbons,as cornparedto the abunthisisthecase, danceof asphaltenesr for instance.in heavvdegradedoils
l.il HighMolecular WeightN, S,O Compounds: Resins andAsphaltenes
407
"molecules" or severalasphaltene and it is surroundedby interactingresins.In turn. the resinsare surroundedby aromatichydrocarbonswhich ensurea progressive transitionto the bulk of crudeoil wheresaturatedhvdrocarbons are usuallypredominant.This interpretationmay not be conflictingwith the model developedby Yen, asthe coreof the micellecouldbe a particl! or crystallite. The sizeof the micellesformedby asphaltenes surrounded by interacting resins in theirnaturalenvironmenti. e., normalcrudeoil or heavyoil, is likelyto playan importantrole in the viscosityof theseoils. When the oil containsa sufiicient amountof resinsandaromatichydrocarbons, the asphaltenes arefully dispersetl. On thecontrary,if thereis a shortage of thesemolecules, theremaybeinteraction betweenasphaltenes and formation of large-sizedaggregates. This situation wouldresultin a higherviscosity ofcrudeoil, andeventuallya gel typestructure. This consideration ii heavydegraded _mayexplainthe high viscosityobservecl oils..wherelow molecularweightaromaticsand possiblysomeof the resinsare lost b)' water washingand/orbiodegradation. and thuj becomeinsufficientro peptizeasphaltenes. In coal liquids,producedby hyclrogenation of coal,Tewariet al. (1979)have , shown that interactionsby hydrogen-bondinginvolve phenolic OH and N-containing bases.In crudeoils.theremay alsobeinteractionof phenolicOH with ketones,quinones,etc. presentin the heavyendsof Detroleum. High viscositydue to the asphaltene ahundance and physicalstructureis the mainhindrancefor productionof heavyoils.Furthermore.asphaltenes are also considered asresponsible for waterin oil emulsions observedwhencertaincrude oils are produced. Pyrolysisof asphaltenes has providedvaluableinformationon rnerr srruc_ ture and also on their possiblerole in petroleumgeneration.The behavior during pyrolysiscan be comparedto that of kerogen.Up to 9j.^ay_!1t1enes 300-350"C.oxygenfrom carbonylor carboiylegroup is eliminated,wheieas somelight hydrocarbons are generated.Beyondthatiemperaturethe yield of methane,higherhydrocarbons andalsohvdrogensulfideincreases andreaches a maxrmumin the 410-470'Crange.About 507. of AthabascaasDhaltenes are convertedto volatiles.whereasthe residuumbecomesinsolublein benzene andhighlvcondensed: H/C : 0.45at 600.Cversus1.2,1 in the originalasphaltene ( M o s c h o p e dei sr a l . . 1 9 7 8 ) . Furthermore.Rubinsteinct al (1979)havecomparedthe productsvieldedby p1'rolvsis of PrudhoeBav asphaltenes with thosenaturallypresentin the related crudcoil. Theyhaveidentified.besides olefinsresultingfiom cracking.a number of constituents comparable to thoseof prudhoeBay cride oil: n, andiso-alkanes, polycl'clicnaphthenes,aromatics.Arefjev et al. (19S0)made a comparable observatron on asphaltenes from degradedoils of the USSR and showedthat p)'rolysisproductscan be usedfor reconstruction of the degradedparaffinic fraction.More recentlyBehar (1983)pyrolyzedthe asphalteiefractionof ten different crude oils and found a remarkablesimilariiy betweenthe relative abundance and distributionof the mainconstituents in pyrolysisproductsandin the relatedcrudeoils. A comparableinformationis providedby Chappe(19g2),who submitted kerogensand asphaltenes separatedfrom crudeoiii to a selectivecleavageof
40tl
of CrudeOils Composition
etherbonds.In both cases,he identifiedglyceroletherscomprisingC2nandCan isoprenoidchainstypicalof the membranes of Archaebacteria. All thesedatapointto a comparable kerogen structureof the unitsconstituting and asphaltenes. In fact. asphalteneconstituentsin sourcerocks should be consideredas being small fragmentsof kerogenliberatedby eliminationof heteroatomic or otherbonds.Thusthe overallstructure(nucleibearingchainsor functionalgroups,linked by chainsor heteroatomic bonds)might be preserved from the kerogento the heavyendsofcrudeoils.Progressive ofthe aromatization cyclicnucleiwouldfavorthe stackingof pericondensed sheets.ln thisrespect.it canbe observedthat the structureproposedby Yen for asphaltenes is reasonably comparable to the one proposedin Part II of this book for kerogen. Furthermore,the study of severaltypical successions of source-rocks as a functionof burialsuggests the existence of evolutionpathsof the asphaltenes in sourcerockssomewhatcomparable to the evolutionpalhsof kerogens(Castexin progresTissot,1981):With increasing burial,S/C,O/C and H/C ratiosdecrease sively,resultingin a morearomaticandcondensed asphaltene bearingfewerand fewerchainsor functionalgroups.A consequence is likelyto be the formationof somehydrocarbons. in additionto thosegenerateddirectlyfrom kerogen.This hypothesis is supportedby the experimental evidences reportedabove. The role of resinsand asphaltenes in migrationis not completelyclear.Table in source-rock IV.1. 1 showsthat the averageamountof theseheavyconstituents In reservoired crudeoils, bitumenapproximately equalsthat of hydrocarbons. the ratio rs only I to 6. In particularthe ratio of the averagevaluesresins* asphaltenes to aromatics decreases from 2.6in sourccrocksto 0.5in normal,nonin source degradedoils.Adsorptionof the heavvconstituents on mineralsurfaces rocksis partiallyresponsible tbr thatsituation.However.the previousconsiderathat each tionson the mechanism of asphaltene solubilization by resinssuggest petroleummight be able to mobilizc out of the sourcerock the amount of whichmaydispersed by resins.andin turn the amountof resinswhich asphaltene (Tissot.1981).Suchhypothcsis may be solubilizedby aromatichydrocarbons would contributeto explainthe strongcorrelationbetweenthe abundances of aromaticsand heavv constituentsin crude oils reportedbelow (Sect. 1.10). Obviouslythis interpretationwould not apply to degradedoils. which may andresins.asdegradation occurrcd containa muchlargcramountof asphaltenes in the reservoiraftermigration.
1.9 Organometallic Compounds Crudeoilscontainmetals.particularlynickelandvanadium.in variableamounts crudesfrom Algeriaandthe USA. up to from lessthan 1 ppm in somePaleozoic ppm ppm vanadium nickel in Boscancrudefrom Venezuela. The 1200 and 150 Canada. measured on 64 crude oils frorn the United States. averagecontent Venezuela.North and West Africa. the Middle East.USSRand Australiaare vanadiumand nickelcon63 ppm vanadiumand 18 ppm nickel.The respective tent, accordingto their origin,is shownin FigureIV.1.21. Both are at a low level
1.9Organometallic Compounds
409
E E
r r 0 n , l r o lX, u r o i l Votgo,Urcl W . C o n o( ldr oi s sl o L C r e r . )
---
-----T.
500 Craroeoui
Nrckel tppml
Fig.IV.1.21.Vanadium andnickelconrents of 175crudcoilsfrcurvarious oriein.The rlarlerl areacontains 7-5 crudeoilsof paraffinic. naphthenic. or paraffinic-naphthcirc tr pr-.i from Devonian andCretaceous of Alberta.paleozoic of pennsvlvania andC)klahoma. Tertiarv of GulfCoast.Merozoic andPrrleolenc Ii parrsandNorthAquitaine basins ancl liomtheRhineGrabenrnd NorthSea.Tcrtianof pannonian basin.pileozoic of Algeria and Libva.LorverCretaceous of WestAfrica.Tcrtiaryof Australia.paleozoic and Mesozoic of Bolivia.etc. a few ppm - in many oils from detritic sand-shales series:paleozoicof North Africa, Cretaceous of Alberta (Coloradoandpost-Colorado groups)Cretaceousand Tertiaryof WestAfrica, Tertiaryof Australia.In theselow-sulfur crudeoils nickelis frequentlyprevalentovervanadium.The abundance of metal is variable.but generallyhigher in sulfur-richcrude oils. where vanadiumis frequentlyprevalentover nickel: Uppcr Paleozoicof Ural-Volea.lowermost Cretaceous of Alberta (Mannvillegroupandassociated oilsalongihe pre-Cretaceousunconformity).Crctaceousof Mexico, Cretaceousand Tertiary of the
410
Composition of CrudeOils
Middle East,Tertiary of Venezuela.The highestvaluesof metalsare reached a m o n gc r u d eo i l sf r o m V e n e z u e l a . content. There is a good correlationbetweenmetals,sulfur,and asphaltene asmetalsareusuallyin the resinandasphaltene Thisfactis quiteunderstandable correlation,degradedoils,which fractions.As a resultof the metal asphaltene oils containalsomoremetalsthan the nondegraded are enrichedin asphaltenes, of similarorigin. Other metalslike iron. zinc, copper,lead, arsenic,molybdenum.cobalt, manganese.chromium, etc. have also been reported.They have not been and it is difficult to evaluateif their occurrenceis analyzedsystematically. commonor not. However,vanadiumand nickelseemdefinitelyto be the most abundantmetals.A variablepart of them is incorporatedin porphyrincomplexes,but the structuralaffiliationof the remainingpart is largelyunknown. Porphyrinswere first identifiedin petroleumby Treibs (1934).They are by a tetrapvrrolicnucleus,probablyinheritedfrom chlorophyllor characterized hemin(Fig. IL3.20).Besidesnitrogen,theycontainoxygenanda metalwhichis usuallyvanadium(asvanadylV:O) or nickel;iron andcopperarealsoreported. Porphyrinsvarv in content from tracesto 400 ppm in crude oils. They are generallyassociated with the resinfractionof crudeoils.Whenpurified,theyare red or brown. Their massspectrashow a distributionof molecularweights porphyrins.The possibleorigin of these suggesting a seriesof alkyl-substituted haveshownthat in ChapterIL3.12. Laboratoryexperiments seriesis discussed porphyrinsare labile at higher temperatures.This could explain why old content, paraffiniccrudeoils,with a low densityand a low resinand asphaltene containonly tracesof porphyrins.In addition,the relativelylow evolutionary for the low stateof plantsduring early Paleozoictimes might be responsible porphyrincontentof Cambrianand Ordoviciancrudeoils (Louis,1967). Althoughporphyrinsaccount Metalsoccurnot only in porphyrincomplexes. for 100a/c of vanadiumin Baxtervillecrudeoil, they accountfor only 314loin Boscan,Venezuela.crude,and for only 217ctn La Luna, Venezuela,crude: ErdmanandHarju ( 1963).The balanceof metals,i. e., 0 to 70c/"of vanadiumin otherlhan freeporphyrins. crudeoils, is includedin structuralassociations They may includeporphyrinmoleculesinsertedbetweentwo sheetsof the asphalteneparticleand linked by z-vanadiumbonds(Tynan and Yen, 1967). Alternativelymetalmaybe complexedin a porphyrinnucleus,the latterbearing complexsubstituents. In the heavyend of crudeoils thereis frequentlya goodcorrelationbetween the sulfur content and the vanadiumcontent. These two elementscan be of the heavy duringcertainstepsof hydrotreatment eliminatedsimultaneously to assumethat vanadiumis fractionof crudeoils. It thereforeseemsreasonable mainlyboundin chemicalstructuresin whichsulfurplaysan importantrole, as nitrogendoesin porphyrins.
I .10Covariance Analysisof Main CrudeOil Constituents
411
1.10 Covariance Analysisof Main CrudeOil Constituents The concentrations of the variouscrude oil constituents are not independent paramcters. Someof themshowa highdegreeofcovariance. The reasontor one kind of dependence is obvious.It is relatedto the grosscomposition of crudeoil. AS:
saturated+ aromatics+ resins+ asphaltcnes - 1. Howeyer.anotherkind of dependence canbe observedwhcndealingwith the conccntrations of morespecificclasses of compounds: benzothiophenes. monocvcloalkanes. etc . or with otherchemicalandphysicar parameters suchassurfur content.viscositv.density.etc. TablesIV.I9antiIV.r.r0showtheline.rcorrelationcoefficientsofimportant parameters.suchas the abundanccof the main classes of compoundsand the sulfurcontcnt.on J:[9crudeoils front all tvpesof resenoirsrocksclcpositcd in v a n o u sc n v l r o n m e n t s . Their dcpthsrangcfrom nearsurfaceto morethan.1000 m. andthe agesofthe producingformation range from Cambrianto pliocenc.l.here are two marn groupsof interrclatedparameters: the first group consistsol sulfur contentand aromaticconstituents (asDhal_ tenes.resins.thiophenedcrivativesand aromatichvdrocarbons)l - the secondgroup consistsof paraffins. mono and dicl,cloalkanes and alkylhenzenes. Thereis a strongpositivecorrelationamongthe parameters of the first group (average g - 0.8).Thel'are alsocorrelatedwith somephysicalpropertiessuchas densitl,andrriscosity. Thevcorrelate,howevcr.negatively withihe concentration of saturatedhydrocarbons in crudeoil. The correlationbetweenthe secondgroup of parametersis weakerbut still definite.Theseparameters are alsocorrelatecl to tiredegreeof maturationof the oil. namel)'to depthof reservoir. Only the paramcters of the first groupwill be discussed herc.The cxplanation fbr the relationshipbetwcensulfur. the aromaticconstituentsand the heavy constituents._ resinsplusasphaltenes, maybc foundin theoriginof sulfurin crude oils. and in thc modeof asphaltene solubilization in crudeo]ls. Sulfuris not a major constituentof biogcnicmoleculcs.whereit rankswell bchindoxvgcnandnitrogen.Therefore,it hasto be introrlucedinto the orsanic matter,either into kerogenduringearll' diagenesis in youngsedimentsor'into crudeoils. The time andconditionsof sulfurintroductionare discussed in ChaptersIL2, IIL,I and IV.5. The obscrvedrelationbetweensulfurand aromaticsubstances may be interpretedin two ways: a) Sulfuris p^referentially fixed by molecules containingan aromaticcvcle.The scarcityof aromaticcvclesavailablein Recentsediments(only somepoly_ aromatics)would not favor this hypothesis.Nevertheless humic acidsof Recentsediments alread),containa fair amountof sulfur.
412
of CrudeOils Composition
Table IV.l.9. Linear correlation coefficientof thc nrain classesof compoundsand sulfur content of 3.19crude oils. (Analysesby tFP) Pcrceniol crudeoil Saru- Ar(nnalic Rcsins +Asph rarcd HC H('
I
0.ll{r
'l sulhtr in crude oil
9i hydrocarhrns
Paral frns
Naph lhenes
A(! malis
Total
Herry
deri
>Cr
0.91
0.79
0.ll-1
0.33
0.91
0.70
0.62
056
o 63
0.7.r
0.-10
0.9-r
0.6l
0.53
0.71i t; sultur in crudc
-0 72 -1).2{)
0.51
0.1t0 0.56
050
o.12
0.79
0.lll
0.55
0. 19
0 8I
()..11
{) .15
{) .10 - o.-59
o.3tt
oll
Par Naphl
ti.7+
0.b2 0.21
Thioph Heavy
>C,.
Table IV.1.10. Linear correlation coefficient of some classesof compounds (349 crude oils). (Analyses by IFP) Paraffins
1+2 ring naphtbenes
Alkylbenz.
Depth
0.85
0.59
0.38
0..15
Paraffins
0.49
0.60
1 + 2r i n g naphthenes
0.46
0.44
Saturated HC Saturated HC
Alkylbenz. Depth
0.35
1.10Covariance Analysisof Main CrudeOil Constituents
4lj
Table IV.l.1l. Correlationcoefficientof sulfur content,depth of producinghorizonand physicalpropertiesof crude oils (2000 samples).(Data from U.S. Bureauof Mines, IFp and others) Depth
Specific gravity
Viscosity'
0.121
Pour point
Sulfu/ (in crudeoil)
Deptb Specific gravity
0.393
Viscosit)/
o.32r
Pour point
0.015
0.090
Suffur" (in crudeoil)
0.29|,
0.661
0.070
a lt shouldbenotedthatsulfurandviscosity haveapproximately alognormal distribution. Therefore, for computation of thecorrelation coefficients, thelogariihmic valuesofsulfur andviscosity haveto be used. b) Sulfuris incorporatedin organicmattcrasan agentof aromatization. Sulfur would combinewith unsaturatedchainsand promotecyclization,or with saturatedor partlyunsaturated cyclesandpromotetheir aromatization. This secondhypothesis fits bestwith observedfacts. Furthermore.thewaytheasphaltenes aresolubilized in crudeoilsby resinsand aromatics.and thus transportedduring migration (see above Sect. 1.g) is probablyresponsible for thestrongcorrelationbetweenaromatics andresinsplus asphaltene. irrespective of their originalabundancein the bitumenof souicerocks. Another aspectof the sameproblem is the dependence of some physical properties(density.viscosity)on the sulfur content of crude oils. Density, viscositvand sulfurare often the only informationavailableon crudeoils which were routinelycollectedman)' yearsago. The correlationis shownin Table IV- 1.1I using2000crudeoils of variousorigin.age.and reservoirtypes. ln fact. densityand viscosityof a crudeoil dependmostlvon the contentof heavyconstituentsofpetroleum,resinsandasphaltenes.Aspreriouslyseen,the abundanceof resinsand asphaltenes is. in turn, closell,relatedto the sulfur content.That the relationship is indirectis expressed by the fact that the sulfurdensityof sulfur-viscosity correlations(Table IV.1.ll) are not as good as the sulfur-resins* asphaltenes correlation(Table IV.1.9). Furthermore,a high contentof resins+ asphaltencs is not the only causeof high viscosity.In highsulfurcrudeoils (S > 1%), whereresinsandasphaltenes aregenerallyabunclint and dominateany other causeof viscosityincrease,the correlationbetween density.viscosity,and sulfuris better. Amongthe physicalparameters, thereis onewhichis completelyindependent from the othersmentionedsofar. the pourpoint. Pourpoint.therefore,ii ableto
414
Composition of Crudeoils
provideinformationwhichcannotbe derivedotherwise.It denotesthe content of high molecularweightn-alkanesin crudeoils. naphthenes. and Other constituents of crudeoils, like tetra-and pentacyclic two the polyaromatics, correlate,to a certaindegree,with the aforementioned groupsof parameters. As a resultit canbe saidthat generally: - crudeoils rich in aromaticscontainmore tetra-and pentacyclic naphthenes, polyaromatics than the proportionally derivatives other oils, but andthiophene l e s ra l k v l b e n z e naens do t h e rm o n o a r o m a t i c s : crude oils poor in aromatics,when immature,containa certainamountof naphthenes.In a more advancedstageof evolution,paraffins(normal or branched)arepredominant. mono-anddicyclicnaphthenes andalkylbenzenes are abundant.
Summaryand Conclusion The main groups of compoundsin crude oils are saturatedhydrocarbons,aromatic hrdrocarbons. resinsand asohaltene:. Saturatedhydrocarbonsare usuallythe major group, exceptin degradedheavyoils. They contain normal plus isoalkanes(paraffins)and cycloalkanes(naphthenes).The relative abundanceof paraffinsand naphthenesis rather comparablein many crude oils. with the exceptionof paraffinicoils and degradedheavyoils. Aromatic hydrocarbonscomprisepure aromatics,naphthenoaromatics (condensed aromatic and saturatedcycles)and benzothiophenederivatives(containingheterocycleswith sulfur). Resinsandasphaltenes arehigh molecularweightpolycyclicmoleculescontainingN, S. and O atoms. The basicunit of their structureis a condensedpolyaromaticsheet. Severalsheetsare usuallystackedto form particles,which may in turn form aggregates or micelles. Thereis a strongpositivecorrelationbetweenthe concentrations ofaromatics,resins plus asDhaltenes. and the sulfur content.
Chapter2 Classification of CrudeOils
2.1 General Variouscrudeoil classifications havebeenproposedby geochemists andpetro_ leum refiners.The purposeof theseis u.ry diff...nt,-aid alsothe physical or chemicalparameterswhich have been used in the classifications. petroleum refinersaremostlyinterested in the amountof thesuccessive distillatron tractions (e.g.. gasoline.naphtha, kerosene.gas oil, lubricating distillate) and the chemical.composition or physicalpropertiesof thesefractlons(vrscosrty, ctoud test. etc.). Geokrgistsand geochemists are more interestedin identifyingand characterizing the crudeoils,to relatethemto sourcerocksandto measuretheir gradeof evolution.Therefore,theyrely on the chemicalandstructural informa_ tion of crude oil constituents, especiallyon moleculeswhich are supposedto conveygeneticinformation.In that respeclmolecules at relativelylowconcenrra_ t i o n s s, u c ha sh i g hm o l e c u l awr e i g h n t - a l k a n e s .t e r o i dasn d, . r p " n . . . m a yb eo f greatlnterest. The typeof informationon crudeoil composition variesa greatdeal.A large numberof distillationresults- Hempel.Engler,ITK, anclotier methods_aie available.. often includingsomeadditionalmeasurements on the fractrons,e.g., density.viscosity,refractionindex,etc.For example.about10,000analyses ha-ve beencarriedout accordingto the U.S. Burcauof Minesroutinemeth;d. They include mostly crude oils from the United Statesand canada, with several hundredsfrom othercountries. Analysesbasedon the contentof the variousstructuraltypesin crude oils (alkanes,cvcloalkanes. aromatics.N. S. O compounds). and ihe drstributionof molecules within eachtypc, havebeen_ madein the last l5 years.However,lhey are probablyIessnumerousthan distillationdata.andthc iesultsare frequently unpublished.. The proposedbelowusingthis type of geochemical .classification parametcris based mainlyon the analvses carriedout by the InstitutFranEais du P6trolcon more than 600crudeoils of variousoriginsandages.
416
of CrudeOils Classification
2.2 Historical proposedsincethe beginningof An excellentreviewof the main classifications the centuryhasbeenmadeby Smith(1968). A well-knownand up to now frequentlyusedclassification of crudeoils has beenset up at the U.S. Bureauof Minesby Smith(1927)and Lane and Garton (1935).Thisclassification is basedon distillationandthe specificgravitiesof two key fractionsof distillation(TableIV.2.1). It wassuccessfully appliedto intercompare crudeoilsfrom certainprovincesin the UnitedStates.However,with the progress in modernorganicgeochemistry, and the higherlevelof informationderivedfrom it, it becameobviousthat the classificationwas ambiguous.On the one hand, rather different chemical compositionof oils could result in the same groupingof these oils in the classification. On the other hand, differentgeologicalsituationscould not be accountedfor, except changesin depth. Since the U.S. Bureau of Mines classification canhardlybe developedanyfurtherwith respectto morecomposiparameters tionalinformationon oils,a newclassification basedon geochemical is proposedin the followingSectionIV.2.3. (1950).Creanga(1961) havebeenproposedby Sachanen Otherclassifications and Radchenko(1965).In addition,in refinerypracticethereis a classification basedon refractiveindex,densityandmolecularweight(n.d. M.). Sinceall these classifications have not been extensivelyapplied in the past, they are not discussed here. of crudeoils.(Accordingto LaneandGarton,1935) TableIV.2.1. Classification Specific gravity
KeyfractionI 250-275'C (Atm.pressure)
Keyfraction2 275-300'C (40mm Hg)
KeyfractionI 250 270'C (Atm. pressure)
Keyfraction2 275-300"C (40mm Hg)
< 0.8251 < 0.825r < 0.8251 0.8256to 0.8597 0.8256to 0.8597 0.8256to 0.8597 >o.8602 >o.8602 >_0.8602
< 0.8762 to 0.9334 0.876? >_o.9340 \< 0.8762 0.8767 to 0.933.1 >o.9340 >o.9340 0.8767to 0.9334 < 0.8762
paraffinic paraffinic paraffinic intermediate intermediate intermediate naphthenic naphthenic naphlhenic
paraffinic intermediate naphthenic paraffinic intermediate naphthenic naphthenic intermediate paraffinic
2.3 Basisof ProposedClassification of CrudeOils The newlyproposedclassification is basedprimarilyon the contentof thevarious structuraltypes of hydrocarbons:alkanes.cycloalkanes(naphthenes),and aromaticsplus N. S. O compounds(resinsand asphaltenes). It alsotakesinto
2..1Classification of CrudeOils
4t7
accountthe sulfur content. lt should be noted that the contentof alkanes. naphthenes and aromaticsis consideredhere as a resultof analyticai methods p r e s e n t liyn u s e .i . e . : - all datarefer to the portionof petroleum boilingabove210.Cat atmospheric pressure; alkane(paraffin)conrentincludesn- and isoalkanes. but not the alkyl chains substituted on cyclicnuclei; - cycloalkanes( naphthenes) include all moleculescontainingone or more . saturatedcvclesbut no aromaticcycle; aromatics includeall molecules containing at leastonearomaticcycle:thesame moleculesmay includecondensed saturitedcyclesand chaini substitutedon tnc nucleus. In FiguresIV.l..l and IV.1.6, some550crudeoils were plottedrn triangular diagramsshowingthcir relativecomposition with respecttoihe wholecrudeoils (saturates. aromatics. N. S. O compJunds) andwith respectto hydrocarbons only (n+isoalkanes. cvcloalkanes, aromatics).From theseplots,it iJobvrousthat the coveo r n l ) - p d ror f t h c t r i a n g l eisn f o r mo f e l o n g a t ecdk r u d s . O n t h ec r u d eo i l :ils oragr. m (tsrg.r v. r .l). the cloudstartsat the poleof saturates anclbendsoverto the N, S,.O compounds.On the hydrocarbonaiugrurn iii'g. iV.t.o.ytt . .luuO startsat the poic of n+isoalkanes andbendsover to the aroiraticnydrocarbons. This is.-of course,anotherexpressionof the correlation ^riri* ,lir.rrr.O in S c c t i o nI V . l . 1 0 . This pccuiiardistributionof crudc oils and the gcochemical consequences derivedfrom it arc in essence the basisfor our prop,-i.O.ro..iii...-o,ion. As a first parameterfbr the classification. we'strrltcunsl.leitt e concentration of saturatesin crudeoil. which is negativelvcorrelatcdwith aromatics.resins, asphaltenes. and sullur content.The distributionof ,uturui.J lf,r,,r.orOun. in crudeoilshasbeendiscussed in Chap.IV.l andis shownin Figure'IV.1.S. There. two modesareclearl),separated by a minimumat 507c;thiscui is usecl -or to separate trvo.different populationsof crudc oils. one more pa.atTinic paratfinic_ naphthcnic.the other more .romatic or asphartic.Th; 507ominimum of the sarurates corrcsponds to the l7e minimumof the distributionof sulfurin crude oils(Fig lv l . l6 ). as shownbv a cross-correlation of thosetwo paramerers. In additionro thismajorsubdir, ision.anotherimporiant.ur .in"t " lntroau."d a m o n et h e c r u d eo i l s .b a s e do n t h e a l k l n e c o n c i n t r a t i o n . T h c d i s t r i b u t i o on f n*isoalkancs(paraffins)showsa minimum at about 10%, whlch separates anotherclassof crudeoils. Heavvoils,generallydegrrded. ,"or.r"n, this class: thc l"\\ l:')ntenrol ri . iroalkane,i. mairilyrluc to hiod.era.liriin-.-
2.4 Classification of CrudeOils (TableIy.2.2 andFig.IV.2.1) a) Accordingto the previousconsideration. crude oils w l be consideredas *paraffinic" "naphthenic" or if thetotalcontentof saturatedhydrocarbons isover 50% of a particularcrudeoil. For practicalpurposes, andfutuie use,n connection
of CrudeOils Clxssrfication
418 Table IV.2.2. Proposedclassifiaationof crude oils Concentrationin crudeoil > 210'c S:saturates ArA:aromatics + resins + asphaltenes
AA < 507c
Number of Sulfur content samplesper class in crudeoil (approximate) (Total:541)
p:paraffins N: naphthenes
P>N and P>40%
s > 50,/.
Crudeoil tlP
P 14oo/c and N ( 40olr N>P and N > 40olr
100
Paraffinic
Paraffinicnaphthenic
Naphthenic
126
P > to.n intermediate
>t% s _<507. AA 2 5O%,
,ll
"<,s%i;lTilj P<10 Aromattc N)25"/'nuph,h"ni"
generally S< 17.
36
justifiedb;' minor cutsare additbnalboundaries cnvironments. with geological areshownin Thesetwo boundaries andcvcloalkanes. at:10%alktrnes established "paraffinic"oils from intermediatc"paraffinicFigure IV.2.1. They separate "naphthenic"oils. naphthenic"oils and thoscfrom as"aromatic"if the total contentin saturated b) Crudeoilswill be considcred h y d r o c a r b o ni ssl e s st h a n5 0 % . i . c . . t h e t o t a lc o n t e n it n a r o n r a t i c rsc. s i n sa n d on the is morc than 50%. Accordingto the previousconsideration asphaltenes oils. separatesaromatic-intermediate" alkanecontent.an additionalboundar-'l 0 w i t hm o r et h a n1 0 %n * i s o a l k a n e sa.n dh e a v -dve g r a d eodi l s .w i t hl e s st h a n t i . The lattcr classis dividcdfor practicalpurposeinto twtl subclasses - ' a r o m a t i c - a s p h a l ot iicl s" w i t h l e s st h a n2 5 % n a p h t h c n e s . - " aromatic-n aphthcnic"oils with morethan 259/rnaphthenes ar()mrticto a differenccof the sulfur c(ltntent: This minor cut corresponds oils asphalticoils are high-sulfuroils, whereasaromatic-naphthenic usuallv containlessthan 17r sulfur. in of crudeoil, and the approximateequivalence into classes The subdivision Figure Figure IV.2.1. IV 2.2 and in Table respcctto sulfur content.is shown tV.2.2showsthatlow-sulfurcrudeoilsmostlybelongto theparaffinic.paratTinic-
2.5 Characteristics of the principalClasses of CrudeOils
5 4 1 C r u d eo i l s
rpahaFrrril-
419
AROMATICIiC }NSO COMPOUNDS
ffiH,iffi5t
Fig. lV.2.t. Ternarv di.rgra m showingthecomposition of thesixclasses of crudeoilsfrom 5 4 1o i l f i e l d s naphthenicor naphthenicclasses.On the contrary, high-sulfurcrude oils mostly belong to the aromatic inrermediate classand _'ii th?u ,i. J"grrO.a _ to tt. aromatic-asphalticclass.
2.5 Characteristics of the principalClasses of CrudeOils ..Theparuffiyicc/assiscomprised of lightcrudeoils.somebeingfluid, andsome ,l r g n - \ n a x . h t g h - p o u r - p o icnrtu d eo i l s .T h e v i s c o s i toyf t h e s eh i g h _ p o u r _ p o i n t o i l s i \ h i g h .d u er o a h i g hc o n t e Ino f a _ a t k a n e>sb . n . A t s t i g htry : l l : : T , ' : tem-peratures rp.*,lrre erevated (.]5 50.C), however,the viscositybecomes normal. e,Plrattrnrcctasscomprises someoils from the paleozoicof North Africa, lf ,, unlreo Slatesand SouthAmerica,someof the lower Cretaceous oils from the basinsborderingthe SouthAtlanticOcean(beneaththe massiue saltsequence in WestAfrica:the {agellan basinin SouthAmerica.etc.),rnJror* f..tiury olt, from WestAfrica, Libya,Indonesia. andthe pannonian'br.in ln C.nt.uteurope. Among^them.somehigh-waxcrudeoils occurin the nonmarine Lower Creta_ ceousof the Magellanbasin.the Crelaceous andTertiaryof WestAtrica, andthe
j
420
Classification of CrudeOils
541 Crudeoils
AROMATICI€ iNSO COMPOUNDS L.Cr.ttc.oot hrrvt oilr Athrb.rcr,.lc.. Jorr!sic,Cr.t!c.oos S .A q r i r r i t r .
..
,r,"ra'r,rl;;;r" Middli ts!t
L . C r c t r c e o u{rM r n n v i l l .I rorn!l oik.Alt.ria
h o ! v yo i l s , l V . A f r i . r
txltneon ol non deg6ded
]tr c',4'.lrs
1.LlLl.l,s.1% .-
-<'::f;i:lli s < loln
N+ISO.AUGNES ( PARAFfli€ )
CYCLO-ALKAN ES (MFT'IHENES)
Fig. IV.2.2. Ternarydiagramshowingthe occurrence of high-and low-sulfuroils.with respectto the crudeoil classification
Tertiary of lndonesia.Most of the oils associated with the Green River and Flagstaffformationsof Uinta Basin,Utah. belongto the high-waxtype. Specificgravityis usuallybelow0.85.The amountof resinsplusasphaltenes is below 10%. Viscosityis generallylow exceptwhenn-alkanesof high molecular weightare abundant.Aromaticcontentis subordinate and mostlycomposedof mono- and diaromatics,frequentlyincludingmonoaromaticsteroids.Benzothiophenesare very scarce,sulfurcontentis Iow to verylow. 2. The c/rissof paraffinic-naphthenicoils has a moderateresins-plus-asphaltenescontent(usually5 to l5olc)and a low sulfurcontent(0 to 17c).Aromatics amountto 25 to 407oof the hydrocarbons. Benzo-and dibenzothiophenes are moderatelyabundant. Densityandviscosityare usuallyhigherthanin the paraffinicclassbut remain moderate.The paraffinic-naphthenic classincludesmanycrudeoils from Devonian and Cretaceous(mostly from Coloradoand post-Coloradogroups)of Alberta,from the Paleozoic of North AfricaandUnitedStates exccptfromthe Permo-Carboniferous of the Permianbasin.WestTexas.The classalsoincludes Jurassic andCretaceous oilsof the Paris.North AquitaineandNorth Seabasins.
2.5Characteristics of the PrincipalClasses of CrudeOils
4ZI
Cretaceous and Tertiaryoils of WestAfrica, (abovethe massivesaltsequence), Cretaceous and Lower Tertiaryoils from North Africa, and someTeriiaryoils fromlndonesia. 3. There are only a few crude ollsin the naphthenicc/ass.Someimmatureoils can be mentionedhere.suchas someoils from the Jurassicand Cretaceous of SouthAmerica.However.the classincludesmainlydegradedoilswhichusually containlessthan20% n*isoalkanes.Theyoriginatefrom biochemical alteration of paraffinicor paraffinic-naphthenic oils and they usuallyhave a low sulfur content.althoughtheyaredegraded. Examplesarefoundin the Gulf Coastarea. in the North Sea.and in USSR. 1. The aromatic-intermediate classis comprisedof crude oils which are often heavy.Resinsandasphaltenes amountto ca. 1U-30%andsometimes more.and the sulfurcontentis above17c. Aromaticsamountto 40 to 70o/cof hydrocarbons. The contentof monoaromatics, and cspeciallythose of steroid type, is relatively low. Thiophene cierivatives (benzo-anddibenzothiophene) areabundant(2530% ofthe aromatics and more). Specificgravity is usuallyhigh (more than 0.85). This class includesmostcrudeoilsfrom Jurassic andCretaceous of the MiddleEast(Saudi Arabia. Qatar,Kuwait. Irak, Syria,Turkey),from the Permo-Carboniferous of the Permianbasin(West Texas),from the Mississippian, Jurassicand Lower (Mannville)of Alberta, someJurassic-Cretaceous Cretaceous heavyoils of the SouthAquitainebasin,someUpper Cretaceous-Tertiary oils of W. Africa, and someoils from Venezuela.California,and the Mediterranean(Spain.Sicily, Greece). 5. and 6. The c/assesof aromatic-naphthenic snd aromatic-asphalticoils are mostlyrepresented by alteredcrudeoils.Duringbiodegradation, alkanesarefirst removedfrom crudeoil. This resultsin a shift of the crudeoils awayfrom the alkanepoleasshownin FigureIV.2.3. Later.a moreadvanced degradation may involveremovalof monocycloalkanes and oxidation.Thenthe positionof an oil i n t h e d i a g r a mi s a l s os h i f l e dl o w a r d st h e a r o m a t i p c ole. Therefore.most aromatic-naphthenic and aromitic-asphaltic oils are heavy, viscousoils resultingoriginallyfrom degradation of paraffinic,paraffinic-naphthenic. or aromatic-intermediate oils. The resin-Dlus-asDhaltene content is usualfl'above25% andmay reachtrOoi.Houever.tie relaiivecontentofresins andasphaltenes. andthe amountof sulfur,mayvaryaccordingto the typeof the originalcrudeoil. a) The aromatic-naphthenic c/assis mainly derived from paraffinic or paraffinic-naphthenic oils. The Lower Cretaceous oils of WestAfrica, whenaltered, containmostly resins.and keep a low sulfur content:the heavy oils of the Emeraudefield containca. 25c/cof resins,but the sulfurcontentis only 0.4 to 0.87a(Clareteta|.,1977).The sameistrue for tar-likeoil in the LowerJurassic sandsof Malagasy(Bemolangatar sands),with a sulfurcontentbelow1o/oanda resin:asphahene ratio of 2 or more. b) The c/ass of aromatic-asphalticoils includes a few true aromatic oils, apparentlynondegraded, from Venezuela andWestAfrica.However,it ismainly comprisedof heavy, viscous.or even solid oils, resultingfrom alterationof aromatic-intermediate (particularlyhigh sulfur)crudeoils.
422
Classification of CrudeOils
The resultis usuallyan asphalticoil, or tar, whosesulfurcontentis above1alc and may reachup to 9c/oin extremecases.Their resinand asphaltene contentrs veryhigh,ca.30to 60%. with a lowerresin:asphaltene ratiothanin thearomaticnaphthenicoils.Most degradedtar sandsfrom WesternCanadafall in thisclass: Athabascawith ca. 57asulfurcontentin the MacMurravarea.PeaceRiver,and Wabasca.Heavyasphalticoils from Venezuelaand the SouthAquitainebasin (France)alsobelongto the samegroup.
2.6 ConcludingRemarks The six classes of crudeoils definedaboveare verv unevenlvpopulated.Most normal(nondcgraded) crudcoils belongto threeclasses onlv. Thev are: " "aromatic-intermedlate oils.whichusuali), containmorethan 17csulfur.Thev are frequentll' generatedin marine sediments,depositedin a reducing environment: 5 4 , | C r u d eo i l s
AROMATrc HC + NSO COMPOUNoS
M a i n p a t ho l d l g r . d a t i o n
.r $:11::;"3il:il*'"-'',
Hith rultur,d.rr.d.d. lri.brrc.,W.t.rc. .ic..-
l l a i n p a l ho l b i o d . g r a d a t r o n. l y
,
t1
6 e n s , a l r a n do l t h e r m a l
I i.rirtiptirn.nd mrnnrill, nornrloil.
_
L o wr l | l l o . , d . t r . d . d C r . l r c . o o . , W . r tA l r i c r
N'ISO.AIXANES ( PARAFFINS )
CYCLGATXAIiI ES (NAP$HENES)
Fig. IV.2.3. Ternary diagramshowingthe main trends of alterationand thermal malurationof crudeoi)s.Biodegradation is exemplified by the alterationof the paraffinic oilsfrom the LowerCretaceous of WestAfrica.A morecompletedegradation. including oxidation,is shownfor the aromaticintermediate oilsfrom the Mississippian irndLower Cretaceous(Mannville)oils of WesternCanada.The extremeend of degradationis reachedfor the Athabascaheavvoilsor tars
Summary andConclusion
423
- "paraffinic-naphthenic"and ,,paraffinic" oils, which usuallvcontain lessthan 1% sulfur.They arefrequentlygenerated in deltaicor coasialsediments of the continentalmargins.or in nonmarinesourcebeds. The aboveevaluationis basedon the frequencyof differentoils investigated. However,if we now considerthetotalamountof knownproductionandresirves, the relativeimportanceof the classes is changecl. By far the mostimportantclasses. with reipectfo quantity.are the aromatic_ asphaltic, thenthe aromatic-naphthenic andthe aromitic-intirmediare. The firsr two comprisethe enormousreservesof heavyoils and tar sands(Athabasca, Arctic Islandsof Canada,EasternVenezuela,Malagasyetc.). The aromatic_ intermediateclassis represented by the largeaccumulations oi crudeoils in the Middle East. The otherextremeis the classof naphthenic oils,whichis comparatively rare. Evolutionand aherationaffectsthe compositionof a crudeoil 1Euun,"t ul., 1971)and thus mav progressively changeits classification. If we refer to the ternarydiagramof FigureIY .2.3, thermalevolutionis denotedbv a tliminurion of aromaticand heavy constituents.and an increaseof paraffins.In advanced stages, the compositionapproaches the paraffinpole. that would otherwisebe reachedonly by gas(methane).Thus a paraffinii-naphthenic oil. at shallowor mediumdepth,may be changedinto a paraffinicoil ai greatdepth.Alterationis primarilydenotedby a shift away from the paraffinpote, aue to biochemical degradationof alkanes.Then an additionalincreaseif resinsand asnhaltenes m a yr e s u l ft r o m o x i d a r i o n . Throughalteration.normal paraffinic-naphthenic oils mav be changedinto aromatic-naphthenic. Aromatic intermediateoils may also be changedto the aromatic-naphthenic classor evento aromatic-asDhaltic heavvoils.
SummaryandConclusion The classificationof crude oils is basedon the content of,t+isoalkanes (paraffins), cycloalkanes(naphthenes)and aromaticcompounds(aromatichydrocarbons,resrns, asphaltenes). The main classesof normal crude oils are the followine: - paraffinic oils,,containingmostly normal and isoalkines, and lessthan 1ZoS; - paraffinic-naphthenicoils, cor.tainingboth linear and cycloalkanes,and lessthan lVaS: - aromatic-intermediate oih, containinglessthan 50Zoofsaturatedhydrocarbons,and usuallymore than 1% S. Evolution and alterationchangethe compositionofcrude oils. For instance,thermal evolutionof a paraffinic-naphthenic oil may resultin a paraffinicoil. Alteration results in generalin hearyoils of aromatic-naphthenic or aromatic-asphaltic classes: - p.arffinic and paraffinic-naphthenicoils are usually degradedinto aromatic-naph_ thenic oils, with a moderatesulfur content (lessthan 1%); - aromatic-intermediate oils arc usuallydegradedinto aromatic-asphalric oils, with a highsulfurcontent(morethan 1%).
Chapter3 Geochemical Fossils in CrudeOilsandSediments as Indicatorsof DepositionalEnvironmentand Geological History
of FossilMolecules 3.1 Significance fossilsarebiologicalmarkersthatcanconveyinformationaboutthe Geochemical in sediments. typesof organisms contributingto the organicmatterincorporated correlation,and/orreconstitution of Thus,theycanbe usedfor characterization, are the depositional environment,in the samemannerasmacro-or microfossils thatasidefrom commonlyusedby geologists. However,it shouldbe remembered geochemical fossils,alsoother molecules may be usedfor correlation,provided they are characteristic enough. In addition,a comparisonbetweenthe originalbiogenicmoleculeand the moleculefoundin sourcerocksor crudeoilsmaygivevaluableinformationabout the historyexperiencedby the molecule:microbialactivityor chemicalrearrangementat the time of deposition,and subsequent chemicalreactionsduring catagenesls. The mostcommonusesof geochemical fossilsmay be listedasfollows: (oil-oil andoil sourcerock); a) ascorrelationparameters b) for reconstitution of depositional environment; duringdiagenesis andcatagenesis; c) for elucidation of chemicaltransformations d) for detectionof contamination with foreignmaterialin marineor freshwater Recentsediments. The problem of oil-oil and oil-sourcerock correlationswill be treated (Chap.V.2). asit is not specifically fossilsbut relatedto geochemical separately includesother markers. The quality of informationprovided by geochemicalfossilsin terms of depositional environmentdependson threefactors: - their stateof conseruation, which may or may not allow one to link them to precursormolecule; their biochemical - the distributionof thisbiochemical precursor(parentmolecule)in the present animaland/orplantkingdom; - the assumption that the distributionwascomparablein ancientorganisms. dependsmostlyon the structureof Conservation of the originalcharacteristics the originalbiogenicmolecule.Relativelyinert molecules,suchas n-alkanes, are affected suffer little chemicalalteration,and their relative abundances On especially by dilution,when new alkanesare generatedduringcatagenesis. the contrary, acids, alcohols,ketones,etc. may suffer severalalterations,
3.1Significance of FossilMolecules
425
dependingon diageneticand catagenetic conditions:lossof functionalgroups. alkylation,dealkylation, reduction,aromatization. etc.As a result.themolecules found in ancientsedimentsmay rangefrom unchanged biogenicmolecules.for examplen-alkanes.and lessfrequentlyacidsor alcohols.throughcompounds verycloseto theoriginalmolecule,suchassteranes andtriterpanes, to molecules k e c p i n go n l y t h ec l c l i cs k e l e r o nl i.k e a r t r m a t iscr e r o i d s . Generallv.onecanexpectgeochemical fossilsto be usefulfor correlation.For reconstitutionof paleo-environments. however.the originalstructuraldetails have to be preserved.For instance,the carbonskeletonof sreroidsmav be derivedfrom a wide rangeof specificsteroids.knownfrom a largenumberof animalsor plants. so that the occurrenceof moleculesretainingonly the tetracvclicnuclcuscannotbe linkedto a particularenvironmentof deposition. The informationdeducedfrom the occurrenceof fossilmoleculesis alsoa functionof the distributionof the precursorin living organisms.On one hand. biogenicmolecules,typicalfor certainorganismsor for a classof organisms. provide informationon the fossil biologicalassociation.For instance.ocldcarbon-numbered high molccularweightn-alkanesand relatedwaxesfound in sedimentsare tvpicalproductsof terrestrialhigherplants.On the other hand, vervcommonmolecules occurringin manykindsof organisms providelittleor no informationon thc paleogeographv. facies,and environmental conditions.For example.C15-C2u n-alkaneswithoutodd-carbon-number predominance may be generatedby all typesof kerogenduringcatagenesis. Therefore.the interestin geochemical fossilsdependson the distributionof biochcmicalprecursorsin presentlvlivingorganisms. It shouldbc emphasized, however. that the distributionof different precursorsin contemporaneous organisms is still insufficiently known.Moststructuraldeterminations ofbioqenic molecules carriedout until now havebeenmadefor the neecls of the food or drus industrier. Thus.onlv the organisms. or evencertainpartsor organisms usedin industrial processes havebcenanall'zcd:tobaccoleaves.grainsof cereals.essential oils of higherplants.whilstthe restof the plant is practicallvunknown.Furthermore, the lipids.hvdrocarbons. etc.svnthesized or accumulated by prokaryoticorganisms.andparticularlybacteria,are almostunknownwith regardto thc numerous existingspecies.Nevertheless, the bactcrialbiomasscan in somecasesbe verv significant comparedto thetotalorganicinput.An exampleof thestillincomplete knowledgeon the occurrence precursors of biochemical of eeochemical fossilsin livingorganisms is pentacvclic tritcrpenoids oi the hopanesleries. Theywerefirst consideredas characteristics of lower terrestrialplants,as they were mostly knownfrom ferns.mosses, and otherplants.Later theywerecliscovered in one anaerobicbacteria.then in severalotherprokaryoticorganisms stemmingfrom manydifferentenvironments (Ensmingeret al., 1974:van Dorsselaer. 1975).In fact. since then they have been found in many ancientsedimentsincluding pctroleumsourcerocks,oil shales,coals,andalsoin crudeoils,provingthatthey are very wide-spread in geological conditions(van Dorsselaer. 1975). A new tool for relatingfossilmoleculesto existingbiogeniccompoundsis stereochemistry. Ackman et al. (1972),Cox et al. (1972)and Maxwell et al. (1973)demonstratedthe interestof stereochemistry of acyclicisoprenoidsto
126
Ilistory Fossils astrndicators of Geological Geochemical
clarify this relationship.Along the sameline, Ensmingeret al. (1974)distinguishedthe importanceof the geneticheritageand the subsequent evolutionin the stereochemistry of pentacyclic molecules of the hopanetype.
FossilsasIndicatorsof GeologicalEnvironments 3.2 Geochemical of conditionsof depositionis fossilsfor reconstitution The useof geochemical relativelynew.However.thisfield of applicationlooksquitepromising.andnew developments can be forecast.Fossilmoleculesof typical biologicalorigin. presentin the bitumenextractcdfrom rocks.and suchmoleculesoccurringin migration As far aspetroleumis concerned, interesting. crudeoils areespecially fossilswhich are high processes havcoften loweredthe contentof geochemical molecularweight and/orpolycyclicmolecules.The resultis that characteristic moleculesare less frequentlyidentifiedin crude oils than in the extract of sedimentary organicmatter. of a single The interpretationmaybe based.in somecases.on the occurrence practice is rather dangerous. compound.However.so far this and characteristic biogenic in the knowlcdgeof the distributionof contemporaneous as advances present-day the interpretation. moleculesare rapid,and may alter fossilsarelikely to provideknowledgeon the major associations Geochemical The thrce main of organismscontributingto the organicmatter of sediments. in termsof quantitativeimportanceol the biomass.arethe typesof associations. with kerogentypesil, III andI or lII. respectivelv: following,roughlyassociated a) Marine organic matter: phytoplanktonis the major contributor. with subordinatezooplankton,and somctimesa significantcontributionof benthic are: algae.The principalbiogenicmolecules - rr-alkanes and n-fatty acidsof the mediummolecularweightrangc,C,1 C;n, synthesizcd assuchby of C'5andC1itt-alkancs. with a frequcntpredominance acids; from C,n and C',1 by decarboxylation algae.and probablyalsodcrived i s o p r e n o i d s : C'. C',1 - abundantstcroids(particularlyC;r cholestane) and somecarotcnoids: - ubiquitoustriterpenoidsparticularlvof thc hopanessericsalso occur, thc closeto unity or higher. ratio is relativelyhigh.generalJy stcrane/hopane b) Contfuental organicmatter,mainlymadeup of the debrisof highcrplants,is or in peatbogs. sediments. in dcltaicor other mainlyland-derived incorporated sometimeswithout important degradation.Biogenic hydrocarbonsinclude mostly odd n-alkanesof high molecularweight (C2.-C11)"1some tricyclic diterpenesin the cycloalkaneand mostlyin thc aromaticfraction.and some Whensteranes ubiquitoushopanes.However,cyclicmaterialis onlvsubordinate. The predominate Cll cholestane. over andstigmastane are present.Cr,rsitostane in marineorganicmatter; 11 Thc alkanecontentis usuallymuchhigherin continentalthan lts mattersuperlmposes continental of material contribute, where both types thus. in the ClrCrr range patternof x-alkanedistribution.especiaLly
.1.2Geochcmical Fossilsas Indicatorsof GeologicalEnvjronments Welllog
Total orgaoic carbon
€
5
I Rosisliviry I m?/m )J I {ohms
0r9.C , + , - J \
Eirumen
,uf€il
5660ppm
Saturated HC
Alkanes
;N
I J,1f.,,t,,,1 t,l---_l
E,ol-----__l ,ol--_l I o.8f^.r"{H;-:*:,3 l € ^
Amqdrs
tt60ppm
4
l30ppm o
E
i'll
02)
;lu I ':Fl I
i,lf.,,rfJ 0 l ? 3 4 5 6
I
If
It
3
| ,lf,rr.,, , 'T5 ---------------+
A
,"Tffid
----------.----.
I|t i
n i n gn u m b e r
F i g .I V . 3 . 1 a a n d b .C o r r p a r i s oonf b i t u n r e nf rso m i n t e r b e d d eodr g a n i rci c hs h a i e(s2 5 / . 1 organtccarbon) andorganicleanshales(ca.0.-5_1 organiccartonlin UpperCreto"euu,uf thc coasrllbasin.cabon (adaptedfrom Claretet al'. 1971), r;i"'i;rganrcconrenr \howsthc altcrnatjonoforganicrichandlcanshalcs(b)the aJil; suiioa""..u. nitf,a r"atorrur" proportionalto the amountof cxrracrable bitumen.big""i. laa" ,h "ia'/brrro,,r7 .nnto,n. hrghmtrtecular weight.and branciedaikanes.bur iew rsoprenoids. l.i),]-::lll"l.)..t Inr\ organtcnrrtrer ir tlerr,,edmostivfrom terrestrial higherplants.Organicrich shale l.trriddle a(J np) cortainsmorecyciicmatenal.especially steranes. manvisoprenoros. and lowcr motecularweightrr-alkaies.Tlre organic ;";; i;;;;;';;ganic_r,.r, .r,ar" is mosttvmarine and autochthonous. A,ote.the totll amount of ,_ritunaa is used as rcference(100?i ) for x -alkancsandisoprenoiddistribution
Distribution 0istriburionol & n-alkanes ol saturatedHC isoplenoids
15
+
f--l--l
30
25
30[-_]
l
tilhlilil
2A
l l l
!0 0
GaschrornatograDhY iso+ cyclo-alkanes lraction
10
!:E.^-..."1 l : ! ; n ' . q . , m It c v , t ot u ll;;
0
; =
t5 ->
25
15
25
'f-l - l l '],^l
..n 4
1
1
", I ,,] i
'ffi +
Carbonatoms per molecule
bedsof Fig. IV.3.2a-d. Principalsourcesof organicmatter in the Paleocene-Eocene Uinta Basin (Utah). (After Tissotet al.. 1911.1978.)(a) Mostly algae(rC'5, nC,7' higherplants(nC2sto nC::); (b) higherplants.with andsubordinate steranes, B-caiotane) (d) (c) higherplant materialreworkedby microorganisms: algaeand microorganisms; the activity; microbial resultingfrom m-ostly aliphaticchains(n, iso.and anteiso-alkanes) plantmaterialpartlyof alkanesaie probablyof mixedorigin,partlyremainsof degraded N;lei the total amountof r-alkanesis usedas reference(100%) for microorganisms. n-alkaneandisoprenoiddistribution
3.2 Geochemical Fossilsas Indicatorsof Geological Environments
429
+ I
I
] "
I I .9
*o"n"
*
.! ,o e
,'*"'*
!, -.o{'""^"'"
{
Kerogen
V
01
o?
0/C otomic rotio-_-Mari[G
ton man[a
t .?11il,1"1,"",".*, i s =
.9
-
,# -
{
C r u d eo i l
l{umbrrof corbon _-____} par|notacula oloms
Fig-IV.3.3, n-Alkaneandisoprenoid distributionin marineandnonmarine sourcerocks, andrelatedcrudeoiisfrom MagellanBasin.Nonmarinebitumensand crudeoilscontain morehighmolecularweightn-alkanes andlessisoprenoids thando marincbitumensand crudcoils. of kerogenandevolutionpathareshownon a van _fop:elementalcomposition Krevelendiagram.n-Alkaneand isoprenoidscaieasin FigureIV.3.2 sterane/hopanc.ratio is usually low to verv low. Isoprenoids are mooerately abundant and dominated by pristane. In some casesthe pristane: phytane ratio mav reach:{ to 10 (Brooks, 1970;powell and McKirdv. 1975). c) Microbial organic mafter is abundant in some lacustrineor Daraltcenviron_ m e n t s w h c r e p l a n t m a t e r i a l i s h c a r i l ; d e g r a d e dd u r i n g a l t e r n a t i n gp e r i o d s of subaeria_l microbial activitv and water flooding. The rilated organic matter is enriched in lipids, that may be either the remainsof the degraded-plantmaterial, -Biogenetic or may be reworked, or synthesizedbl microorganisms. hydrocar-
,130
Ilistor\ Fossils asIndicators of Geological Geochemical
bonsare dominatedby long chainrt-, iso- (2-methyl)and anteiso-(3-meth1'l) alkanes.sometimes extendingup to C*uor C1,.normalll'withoutan) appreciable are scarcc.excepthopanes predominance. Isoprenoids andcvclichydrocarbons environmentof thc rclated The geological whicharesynthesizcd by prokar)'otcs. (1968). waxvoils hasbcenstudiedby Hedberg fossilsare a witncssol the main orgxnicct)nIt is clear that geochemical Thus. for instlncc.or8anic tributors.whctheror not thcy are autochthonous. in a sediment laiddownrn a marlne ma1' be containcd terrestrial origin mattcrof in deltasareas.in matter deposited particular. terrestrial organic In cnvironment. can bc identifiedasof terrestrial marinebrackishor nonmarineenvitonments. origin;microbialbiomassgrownon land-plantdebriscan likewisebe identified as such. u,hethcrit is transportedinto marine. lacustrine.or lagoonal-type sediments. arepresented Examplesol an idcntificationof thc majororganiccontributions i n F i g u r e sI V . - l . l t o I V . 3 . 3 .T h c m a r i n e / c o n t i n c n dt ai sl c r i m i n a t i oi sn c l e a r l r of llc shorvnin an cxamplelrom Gabon. WestAfrica. The Uppcr Cretaceous i z i l ef o r n a t i o n s - u h i c h a r c s l s h l l c s A n g u i l l ea n cA M a n d j ii n c l u d cas s e r i e o in thisarea(Clarctet al . 197'1)The the sourccrocksof the oil ficldsdiscovered sequcnceshowsa finclv stratifiedscdimcntar)'unit.bcingalternastratigraphic 'I'his s i t u a t i o rne s u l tfsr o ma t i v e l l l o r va n dh i g hi n o r g a n i c a r b o n( F i g .I V . 3 . I a ) . organic duplexorigin of the organicmaterial.A smallamountof land-derived the horizons for sourcc thisis the only-. matteris prcscntthroughoutthc sequence: c o n t a i n i nogn l v0 . 5? i o r g a n i c a r b o no. r l e s sa. n d 1 0 0t o 5 0 0p p m b i t u m e nT h e continentalorigin is cienotedb) a total high alkanccontent.and amongthesea with odd prefercnce.and a lou dominanceof high molecularweightrt-alkanes. ( F ' i g . l b ) . l n a d d i t i o nt o t h i sc o n t i n u o ursn p u t s, o m e I V . 3 . c o n t e not f i s o p r c n o i d s of plankmatter.especially organic incorporatcd autochthonous horizonshave horizons result. these As a anoxic conditions. due to tcmporary tonic origin. contain morc organiccarbon (0.5 5%) and bitumen (500-6000ppm), with are (especially sterancs)and isoprenoids cycloalkanes differentcharacteristics: are prefcrentiallvof low to mediummolecularweight abundant;rr-alkanes GreenRiverformationin Uinta Anothercxampleis providedbv the lacustrine levelsof burial Samplestakcn at compartrblc Basin,Utah (Tissotet al.. 1971]). depth show a progressivcchangewith increasingage. which is tentativelY (Fig. IV 3 2): microbialdcgradation interpretcdto be the resultof an increasing of the upperbeds(cquivalentto Mahoganvledge)showvarirluscontributions s 1 ,a n dC 1 1/.- c a r o t a n eh: i g h e cr o n t i n e n t a l p l a n t s : a l g a l i p i c l sn: o r m a al l k a n eC and C'r. C'o sterancs:and possiblyhigher aninlals:C1. r-alkanesC..=C:1:. s t e r a n efsr o m b i l ea c i d s( a ) : - in thc middleto lowerpartof the GreenRiverformation.algallipids(bothC'.. C'. and carotanc)are first degraded(b). thcn odd n-alkanesClaC1; from of microbialorigin whilstiso-andanteiso-alkanes higherplantsalsodecrease. startto appear(c)i is moreadvanced: in the underlyingFlagstaffmembcr.microbialdegradation othcr microbes.or from from bacterjaand there.thebiomassmostlyoriginates up to Cu)without heavy r-alkanes plant lipidsreworkcdbv micro-organisms:
3.2Geochemical Fossils asIndicators of Geotogical Environments
431
anycarbon-number predominance. seriesof iso-andanteiso-alkanes; thereare no longerany steroids(d). The associatedcrude oils retain the same characteristics: Redwashor D u c h c s noei i sr e s e m b lteh eu p p e rh e t j .w i r ha l g a a l n dh i g h e fri a n t c . , n t r r b u r r o n ; A r r a m o na l n d o t h e ro r l \ a \ \ o c i t t e du i t h t h e l o w e rh e d sm o s t l ; c o m p r i , ,hei g h molecularweightnormai,iso-andanteiso_alkanes. Anotherexampleis found in the Magellanbasinat the extreme end of South America.The Springhilformation,of earlvCretaceous age,hasbeendeposited in a marine basinand in small adjacentiontinentaldeiressions. Continental orcanrcmaterial(probablyreworkedbv micro_organisms) exhibitsa highalkane contcnt.especiall)r of high molecularweighta_alkanes with somcocldpredominance.and a low contentof isoprenoidi.On the other hand. marrneorganrc matter showsa lower alkanecontent.with a preferencefor low ro meorum molecularweight. and an abundancein isoprenoids. The differentassociated crudeoilsstill retainthe equivalenthydrocarbon pattern(Fig.IV.3.3).Thusit is relativelveasy to recognizethe origin of the organicmiterial. cven where continental-derivecl organicmatteris incorporated into a scdimentdcpostted in a m a n n ee n v i r o n m e nat .n dc o n t a i n i nsgo m ed e f i n i t em a r i n em i c r o f o s s r l s . However.morematurecrudeoilsusuallvcontaina minoramount ofgeochem_ ical fossils.diluted by abundanlmole,cules issucdf(.rm kcrogenupon thermal catagenesis. An exceptionis the uintr basin.whererhecharacieristic ,-, iso-ancr anteiso-alkanes persistdown to -5000m and more. This situaiion is due to favorablecircumstances: the compositionof the plant and microbialbiomassis suchthat the corresponding kerogen(typeI) is mostlybuilt from acrds.alcohols. etc. \ith long aliphaticchains.Thus, hydrocarbons yieldedduringcatagenesrs resemblethe originalbiogenicchemical.structures. i.e.. normaland isoalkanes. Apart from the-main typesof association, specificgeochemilalfossilsmay providea morereiinedinterpretation in termsof plantoi animalgroups,ctimate ctc,.Albrechr andOurisson(1969)foundin theEoceneMesselsha'ie ofthe Rhine Valleya high concentration of the pentacvclic triterpenealcohol,rsoarborinol. preserv^ed for 50 million years.Todaythis alcoholis knownto occurrn troplcal plantsfrom Indonesia.living in a climaterather comparable to the assumed climateof the Rhine-GrabenareaduringEocenetime. according to geological reconstitutions. Seifert( 1973)studiedthe srereochemistry of four individualsteroidacids(C.: and C1) in the Midway'Sunsetcrudeoil from the ptiocene oi California.The r r t i o s . o fr h e . c i \ : t r a ncso n f i g u r a t i oonf t h e A ; B r i n g sa l l o w e d h i m t o i n f e ra n i r n r m act o n t n b u t r otno r h eo r - q a n m i c a t t e r .t h r o u g hb i l c a c i d sT. h e r e l a t e dd a t a **".rcr.t aborc { Fig._ Ir.3.l8). Huangand Meinschein(1g76_Ig79) l:ll.: T:,"-,f or\cussed t hestgntttcdnceof sterolsascnvironmental indicators; whereas Brassell et al. (1980)consideredthe variousclassesof lipid components as potentral indrcators of terrcstrial,marine(non-bacterial) ani bacterialinputs. However.it-shouldbe pointedout thatthemostwidespread biologicalmarkers are inheritedfrom the membranesof baclerialiving in the sediment. suchas pentacvclic hopancs.or long isoprenoidchainsfrom Archaebacteria. l he latter are particularlvobservedin the asphaltenefractionof crude oils anclin the
Histor] of Geological aslndicators Fossils Geochemical
,132
in a kerogenof the relatedsourcerock,wherethcyareprotectedby incorporation most frequent polymericnetwork (Chap. 11.3.6).lt is not surprisingthat the identifiablcbiologicalremnantsare issuedfrom the most resistantpart the the throughout membrane of the lastmemberof the chainof livingorganisms bacteria are methanogenic cycleof organiccarbon:bacteria.ln thisconnection, they are activewithin the uppersectionofthe oi particulir importance.because column. sedimentarv is that remnantsof the organisms of theseconsiderations The consequence by the effectof microbial be obliterated living in the basinof depositionmay possibleto conclude makes fossils geochcmical of specific activity.The presence of deposition' environment present in the were that certaintypesof organisms of these the absence proof of as a interpreted be cannot However.thcii absence organrsms.
FossilsasIndicatorsof Early Diagenests 3.3 Geochemical In some cases.geochemicalfossilsmay provide some intormatlon on the conditionsprevailingin the freshlydepositedsediment:reducphysicochemical environment.acidicconditions,etc. ing and hydrogenating bistributions showinga strong predominanceof even tt-alkanesare less f r e q u e n(tc a .1 : 1 0 t)h a nt h eo d dp r e d o m i n a n c( Te a b l el l . 3 . 3 l T i s s oet t a l . 1 9 7 7 ) '
T|lni!io eoteocene
{
. I
{
15 20 25 --.-.-------_---
30
&rbonoloms9ermolacole
Spoin cr€1oc€ous
Graaca t,rocene
Fig. 1V.3.4. Examplesof the predomin i r n c eo l ( \ c n r - l r l k a n c as n d p h r t l r n ei t o prenoid in bitunrenextractedfrom sedimentandin crudeoils.This predominance or er,1P('rile u \ u r l l ! , r c c u r \r n c a r b o n d l e seriesdepositedin reducingand hydrogenating environment.r-Alkane and lsoprenoidscaleasin FigureIV.3.2
J.J Ccochentical Fossilsas lndicator! of Thcrmal Mirturation
.1-ll
The phenontenon oftcnconcernsall r-alkanesfrom Cl6to Cr,r.Liring organisrns usualld v o n o t s v n t h c s i zecv e nn - a l k a n e sb.u te v e n _ n u m b e racccild s a n da l c o h o l s are importantconstituents of fatsand vcgetalwaxes.[n a normalenvironment. oxidationo{ alcoholsinto acjds.and then a subsequent decarboxvlation of the l i r l l c rg c n c n r t c,\r d dn - l l k a n c s .I n v e r r r e d u c i n gh. v d r , r g e n a t ienngr i r , , n m e n t r . r r Ir l c r r h o l os r a c J d ps r o d u c ces r " n , r _ i l L i n e r l. n a s i m i l r ru u v p h l r o l :t :Ldr rr :)]rls" \nu c df r , r m 1 ; ' r " . 1 , 1 . . nn,o f c h l o r o p h vm l r a vt r a n s f o r m a r o n gt w od i t i e i e n t w a y s :i n n o r m a o l r o x i r l a t i negn , , i r o n m e norx. i d a i i o nt o p h l , r a n racc i d( C 1 , ) and subsequent decarbox,r'lation generatcs pristanc(C1e):in very reducingenliron_ m e n t .r c d u c t i o np r o d u c epsh y t a n e( C 1 , :F i g .I L 3 . l , l ) .I n f a c it h e evenpredomi_ nanccof normalalkanesis almostalwaysassociatecl with the predominance of p h v t a n co v e rp r i s t a n e ( W e l t ea n dW a p l e s1. 9 7 3 ) . Examplesof r-alkanesand isoprenoidrlisrrihutionfrom strongly reclucing paleoe.nvironrnent are prcsentcdin Figure IV.3.4. Suchdistributirnsusualli ( , c c u ri n c t r h ( ) n a t Je n d r r r C \ . t p o r r l ree r i e s ;p l l e r r c c n e a n d E o c e n cc a r h , , n a t . r r r L r mI un t \ r a U . l t e o c e nm e ur l sa n de v a p o r i t ef rso mt h c R h i n eG r a b e nc. r u d eo i l s o r i g i n a t efdr o mt h eM i o c c n eo f t h eM e d i t e r r a n e aSne a A : m p o s t aS. p a i nG : reecc ( W e l t ca n d W a p l e s 1 . 9 7 3A : l b a i g e sa n dT o r r a d a s1. 9 7 4t;j c m b i c t re t a l . I 9 7 6 ; T i s s o ct t a l . . 1 9 7 7 ) . S t c r o l s .l i k e p h l t o l . m a l b e c o n v e r t e da l o n gs e v c r a p l a t h w a y se. i t h c r t o \ t c r . r n c(\) rl o i r r r r m r t i c A s . s u g g e s t i oi snm a d ct h a th i g hs t e r a n e : a r o m asttiec r o i d r r n o s . l r er t s o c o r r e l r t e Jt o \ e r v r e d u c i n g p a l e o e n v i r o n m e nSt st r.u c t u r ar le a r _ r a n g e m c n It n s v o r v r r sgt e r a n e s .t e r e n easn da r o m a t i sc t e r o i dcsa n b ei n d u c e d bv s r t ( \ p r e s c nirn c l m r n c r ; r {l sR u h i n s r c e i n r J l . . I , r 7 5 ra. n d m l i 1 :' l' l\ lr:d1e.ri cl l c \ a n t r n l o r m a t i ( 'onn , . l i l r g e n c : i s . nr
3.zl Geochemical Fossilsas Indicatorsof ThermalMaturatron C e r t a i nm o l c c u l c sa. l t h o u g ho b v i o u s l vh a v i n ga b i o g c n i cs t r u c t u r e . are not krown as suchin livingorqanisms.For instanc-e C,,,ani C," isoprenoids havea c h a r a c t e r i sst itcr u c t u r e i . e . . o n em e t h t .gl r o u po n e v c r vf o u r t hc a r b o n a t o m_ whichis not likelvto be generated by an inoiganicpr,,..rr. H,,*,.u"r. thevitrenot knorvnin livingorqanismsrtr Reccntscdimcnts.and thereforett., t,,.r. a t,a generatedrlurinq diagcnesisor catagenesis from existingbiogenicprecursor mole'culcs. i. c. . mostprobablvfrom phvtvtchainsattacheci to ch-lorophvll. Such geochemical fbssilsma\ act ls a witncssof thesubsequent rearrangements ot the o r c a n i cm a t t c ro c c u r r i n d q u r j n gt h e r m am i aturation. A wcll-knorvnillustrationof the aboveconsiderations is thc drstributionof hcavvn-alkanas.Odd a-alkanesof biogenicorigin are progressivelv dilutedby n.-alkanes without predominance. generatedduiing catage-nesis from kerogen. Thesealkanesare formedbv rearrangement of lon! aliphlticchainsoriginaiing tiom acidsand alcoholsof wax esteri.The evolutiin oi tfr. oJJfrcouminrncc ( C P I )i s c o m m o n l vu s c da sa n i n c l e xo f m a t u r a t i o (nC h a n . V. l.t. Acvchcisoprenoitls are alsoof rntcrestin this conneciion,as thev are rather t v p i c a al n de a s i l vi d e n t i f i a b t eB.r o o k sc t a l . ( 1 9 6 9 )C . o n n a n( 1 9 7 . +a)n c ip o w e l l
43.1
Histor\ of Geological asIndicators Fossils Geochemical
and McKird,v(1975)rvorkerion coalsand crudc oils from Australiaand Nes Zealand.ln coalsand comparablesedimentsderivedfrom contincntalor[anic matter.theyconsiderthat oxidationof phylol into phytanicacidoccursto a large generatton extentduringtheinitialaerobicstagcof plantdeca.vThusprogressive of diagenesis stage late the acid during of phvtanic decarboxvlation of pristaneb--v pristane:ph\ of the an increase in wouldresult of catagenesis ani the earlystage -l.tigit.tt uilu.t or. recordedin the coal sequcnceBroun and tane ratio. itt. coalshavemocleratepristane:phytaneratios(l to 7)' whereas subbituminous high volatilebituminouscoalshavehighervalues(7 to l5) . are the crackingzoneis reachedand.isoprenoids increases. Vhen catagenesis C'' and C'o' decarboxylation prevalent over become affected:cra&ing reactions be ma\ compounds the latter of content the Thus. are ienerated isoprenoids catagenesls. ftrr advanced usedas an indicator of fossilmoleculesmay suchas stereochemistrv More specificcharacteristics pristane'hasbeen isoprenoid' acyclic Cr! of the configuration The alsobe used. such as the immature sediments. (197U). ln shallow etil. Patience stucliedb.v whereasa present aloneis pristane) (6R. 10S isomer meso the shale. Messcl rn more present is mixtureof threeisomers(6R. 10S;6R. 10Rl65' 10Spristanes) lsomers other to mature shalesand crude oils. The ratio of meso-pristane stagc' becomcsl:1 towardthe end of the diagenesis occur in Rccent and ancientsediments' of the hopaneseries'fhe1' Triterpanes belong to different isomertcsettes includingcoals. and in crude oils ( F i g .l V . 3 . 5 ) : - (l1n.2l/J) H-hopano e r c r ph o p a n e . ( 1 7B . 2 l c l H - h o P a noer B n h o P a n e . - ( I 7 B . 2 r p \ H - h o P a noer P Ph o P a n c . pi3hopanc-typc tritcrFene\lrnd ahvayssvnthcsize Howcver.livingorganisms havencverbcen triterpencs hopanc-typc ry'l subordinate sometimes /iu t.vpe).but isomcrtzasubsequent by generlted is cp series Thusthe detcctedin organisms. of catagcnesrs' stages early and the diagcnesis during tion of thellp.-orBc series. occursin positionC-22of the C'1 ttaces.anothciisomerization in -or. ",tu"n."i-Ihui scriesmal' be usedas an hopane thc of stcreochemistr! to Cr. hopancs. (vanDorsselaer'1975:Ensminger' indicatorofthe maturationof organicimatter ' 1978 1980)Thesechemical Molclowan and 1977: Seiiert al.. iuii, int'ning",.t or pollutionin Reccnt oil migration crude of for detection bc used alsi iossilsma-,sedlments. deservespccialintercst'particularl!the a;;";;t of configurationin ster(tnes FigureIV 3'6 With in C,-6,,, steranes(Mackenzicet al ' 19t10): 205/20R-ratio burialdepttrtheconfigurationat Cll0 chglq:s from a 20R predomlincreirsing around nanceat ihallow depthto equalimountsof 20R and20Sconfigurations of equilibrium The i. e.. wellinto thecatagenesiszone thepeakof oil generation. the than slowly the'C-20 ste;eochemistryin steranesis reached more or thatof C-l7 isoprenoids' centersin long-chain *.*t.t"rni.,ty nt the asvmetric can be usedto in steranes "",1 C-ZZin hopanes.Thus the 20S/20R+ 20Sratio and (diagenesis) zone immatule measurethe evblutionof sourcerocksover the
l.-1Geochcrnical Fossilsas Indicatorsof ThermalMaturation
Structure
0ccu(ence
Ssfles
PP-Hopafe
.135
Irample ol t h e E o c e n es h a l e s i n t h o n h i n ec r a b e n Locally m a xr n u mb u l a t d e o l h
200m
1800m
Fig. 1V.3.5. Slereochemistr) of the hopaneseries.The f/- arrdfrl_hopanes ()!!ur rn Recentand in ancientimmaturesediments. suchas the Eo.ana ihola, of Messel.W. Germanv.\\herethe maximumburialdepthis ca.20llm. In the RhineGraben.the santc Eoceneshaleshavebeenburiedto 1800m at Stockstadt andthereonlyrf_hopancoccurs. beinsthe more stableform. (After van Dorsselaer. 1975) part of the principal zone of oil generation; in some cases,it can also oe used to assessthe dcgree of maturity of crude oils. The conversionof monoaromatic to triaromatic steroidshas also been shown b y M a c k e n z i ee t a l . ( 1 9 8 1 )t o c o v e r t h e s a m e r a n g e o l t h e r m a l m a t u r a t i o n .I n a d d i t i o nt o t h i s .s e i f e r ta n d M o r d o ua n t r q 7 x) s u g g e s t erdh a rt h e e r r e n to f c r a c k i n g in thc side chain of the monoaromatic stcroids could be useclas a maturation p a r a m e t c r .w h e r e a sM a c k e n z i ee t a l . ( l 9 i l l ) u t i l i z e d m o n o _ a n d t r i a r o m a t i c steroids-Cracking of the sidc chain seemsto occur mainll. in the catagenesiszone. Porphvrins also changc with increasincmaturation. The convcrsionof DpEp to ctio porphvrins has bcen obscrved fbr a long timc. although the mcchanismis n o t v c t c l e a r . M a c k e n z i ce t a l . ( l q x { t ) u . e . l i h . . D p E p c t i - or a t i o f o r v a n a c l v l
,^r{--Y
c-20 ...(^,^-
.A afr208
co-208
+
increasing maturalion
lt^'-Y a'i''r
aJf
z0s
Fig.IV.3.6. Changes of contieuration at C-20in tbc steranes during burial:with increasing maturation the preclominanceof 20R is changedto equalamountsof 20R and 20Sconfiguration
.l-]b
Geochemical FossilsasIndicatorsof GeolosicalHiston
ialdepth
Pristae
llofanes
meso/toral. ,4/toral
V=0 Poryhyrins
Sleranes
20S/20R+20S
0PtP/etio
\
1r
LateDiagcnesas
\
\'/
//
500
\ 2500
50
50
of burialdePthin the prramelers asa function Fig.IV.3.7.Change\ of somemolecular et al.. l9lt0.1981) Lowerloarcianshales fromMackenzie of thePrrisBasin.(AdaDted porphl,rinsand showedit to dccreascwith increasing burialdepthover the late diagenesis zones. anclcatagenesis maturittion appliedto characterize A comparison of the molccularparameters study of their is presentedin Figure 1V.3.7accordingto the comprehensivc distributionin the Lower Toarcianshalesof the ParisBasinby Mackenzieet al. ( 1 9 8 0 .1 9 8 1 ) O . n t h i s e x a m p l c i.t i s c l e a rt h a t s t e r e o c h e m i s torfy l o n g - c h a i n zonc, whereas isoprenoidsand hopanescan only be uscd over the diagenesis andconvcrsionof at C-20.aromatization of steranes stereochemistr-v of steranes up to the peakofoil porphyrinscovertherangeof latcdiagcnesis andcatagenesis, changesare generation.However,thcscdifferentchemicaland stereochcmical likely to occurwith differentkinctics.Therefore.the situationin other basins. history.may be somewhatdifferent.some with a diftercnt time-temperature possibly more sensitiveto temperaturethan others. For conversionsbeing (1982). studyingsamplesfrom the Pannonianbasinin instance,Mackenzieet al. suggest that aromatization of monoaromaticsteroidsis more CentralEurope, a fast elevation of tempcrature than isomerizationat C-20 of accelerated by steranes. of the differentscalesuntil more Thusit is difficultto orovideintercalibration between geologicalexampleshave been studicd. Also the correspondence etc., suchasburialdepth,vitrinitereflectance, molccularandotherparameters, in FigureIV.3.7, oncedifferentbasins might be more scatteredthan suggested are plotted.
3.5 Presentand FutureDeveloomentin the Use of Geochemical Fossils lnvestigationfor geochemical tbssilshas developedrecentlyto a considerable (GC) and mass extent due to the association of gas-liquidchromatography GC-MS.Much (MS). andto the recentavailabilityof computerized spcctromctry which mll occur in work is relatcd to polvcvclicterpenoidsand steroids.
Summary andConclusion
$j
sedimentsas saturated.unsaturated, and aromatichydrocarbons, or as acids, alcohols.ketones.etc.,manyof them with differentstereochemistrv. However. thegreatest careis necessary beforea specificidentification is made,andbeforea conclusionis drawn with respectto the biologicalprecursor.For instance. Mulheirnand Ryback(1975).workingon stereochemistry of steranes, pointed out that in somecasesa completeidentificationof stereochemisrrv cannotDe achievedby usingonly GC-MS. In particular,configurationat the C_14atom, which is.of piylogeneticimportancein C,, and C,, steranes. requiredisolation and study of opticalrotatorydispersionand high resolutionproton magnetic resonance spectroscopy. Sincethen,morepowerfulresolutionin gaschromatog_ raphy.might help to solvethis problem in a simplerway (Maxweilet al.. 19g0!. Althoughidentification of geochemical fossilsoftenrequiresa greatanalvtical e ll o r t .l h e i rs t u d yi sg e n e r a l lrle w a r d i n gT.h e ya l r e a d p y r o v i d ea p - r r w c r ttu, rI o lt o correlatevariouscrude oils or oil showsdiscoveredin sedimenrarv basinsbv petroleumexploration.Once the variousoil typeshave been separated. geo_ chemicalfossilsare againusedto tie thesecrudi oils to their respecnve source r o c k s( C h a p .V . 2 . 1 . As pointedout. geochemical fossilsmay alsobe useclto characterize facies, environments of deposition.and diageneticaspects.Even the thermalhistnry may be understoodbetterwith their help. Furthermore,the future use of geoihemicalfossilsis promising,as these moleculescan hopefullybe taken as indicatorsfor verv chjracteristics bioenvi_ ronments,suchastropicalforestor salinelagoon.In thai respect. geochemists are heavilydependent on the progress of the natural-product chemistito accumulate dataon the occurrences of characteristic molecuies in livingplanrsand animals. The introductionof stereochemistry inrothedeterminrrionolseochemical fossils i s o I g r e a ta s s i s l a n ci nec o r r e l a r i nlgi r i n g a n df o s s im l olecules] Comparableinterestshouldbe attachedto the nonhvdrocarbon, polar frac_ tions of bitumensand crudeoils. They may includesomcgeochcmical fossils. retainingtheir originalfunctionaigroup\ (acids.alcohols).Someof thesehave alreadl'beendetermined.particularlvin the samerangeof molecularweightas the hydrocarbons. However.analyticalprocetlures becomedefectivewhenN. S. O content,sizeof the molecules,and polvmerization increase. In additionto theseuses,analy.isoi geochemical fossilsin ancientsediments andexperimental evolutionin recentmudsareof greatinterestto environmcntal and pollutionstudies.Thev obviouslvprovideinformationon rhe chemicaland biologicaldegradation of organicmolecules (Eslinton. in modernenvironments l q 7 .)r.
Summaryand Conclusion Geochemicalfossilsare biologicalmarkersthat frequentlyconvevgeneticinformation aboutthe tvpesoforganisms contributing to theorganicmatterof sediments. Theyare usedfor correlations(oil-oil and oil-source rock), for reconstitutionof depositional environments,and alsoas indicatorsfor diagenesisand catagenesis.
438
GeochemicalFossilsas lndicatorsof GeoloeicalHistorv
Identificationof the major sourcesof organicmate al (marineautochthonousand terest al plants),and of the importanceof reworkingby microbes,canbe achievedby usingfossilhydrocarbons.More specificgeochemicallossilssuchasalcohols,acids.etc. may provide a more refinedinterpretationin termsof plant or animalgroups.climate, etc. Normal or reducingconditionsof diagenesismay alsobe characterizedby structural rearangementinvolving biogenicmolecules.Other changeswith respectto structure or distribution of certain types of hydrocarbons(r-alkanes, isoprenoids,triterpanes, steranes,or porphyrins)also reflect the intensityof catagenesis.
Chapter4 GeologicalControl of PetroleumType
-1.1GeneralandGeochemical Regularities of Composition Rccularities in crudeoil lnd gascomposition havcbeenobserved firr a longtime S o m eo f t h o s cs e e mt o h a v ca r c g i o n asl i g n i f i c a n cee..g . . g e o l o g i s tksn L r $t h i r r crudeoilsfrom the Middle Eastarc generallvrich in sulfur.whilcoilsfrom North Africa havea lon sulfurcontent.Other regularities seemto be more[cneral:for l n s t a n c cl .n s c v e r apl l a c e so l t h c w o r l d .i t h a sb e c no b s e r v e d thutojl (gn511y d e c r c a s eusi t h d c p t h :m o r eg e n e r a l l lg. c o c h e m i sat sn dr c f i n e r sh a r e , r b s c r v c j t h a tc r u d eo i l d e n s i t vs. u l f u rc o n t e n ta. n c vl i s c o s i tayr ec l o s e l vr e l a t e d . Thc prescntcomposition o[ a crudeOilresultsfrom man1, factors:naturcof thc o r i g i n aol r g a n i cm a t t e r .t e m p c r a t u rhci s t o r v m . i g r a t i o ns. u b s e q u e n e tv o l u t t o n and altcration.etc. Someof thcsefactors.suchas the originaloiganicmatcrial and thc particulargeologicalhistorv.tend to make crudeoils ditTerent:some others.for examplemigrationor alterationprocesses. tend to makecrudeoils morc similar.As a resultof suchcomplexand sometimescontradictorvinflu_ ences.it is oftendifficultto dctecta singlephenomenon clearlyresponsiblc for rhc observeddistributionof crudeoil consrrtuents. Howevcr.trvomain tvpesof obscrvations can be distinguished: a ) G c n e r aR l cgularities Ccrtain rcgulariticsare wcll known amongcrudeoils. Thc strongcorrelations observed betweentheabundance of certr in hvdrocrrbons.or cllsseiof hvtlrocar_ bons.clementssuchassulfur.and physicalpropertiesof the crudeoils(dcnsitv. vrscosrt\'. etc.) arean expression of thoscregularities. Thesecorrelations arevalid rcqardless of the t)pe of depositionor geological historyof the basinandshould be considercdas factsof generalsignificance. resultingfrom purelv chemical c o n s i d e r a t i o nl 'sh.e v h a v eb e e nd i s c u s s ei nd C h a p t e IrV . l . b) Gcochemica Rle g u l a r i t i e s Theseoccuron a local. rcgional.or more generalbasis.and are expresscd bv jn compositionof oils accordingto thc overallgeological changes serring: - thc cnvironmcntof dcposition:marinc versusnonmarincorgantc ntatter. J c t r i t i . . , ' \r ' i r r h t , n asle( , . l i r n c n l l r t i o n .
GeologicalControl of PetroleumType
440
thermal evolution: depth history, time elapsedsince sourcerock burial' geothermalgradients, alteration of petroleum in reservoir: biodegradation.oxidation, water washing.
Relatedto the Environmentof Regularities 4.2 Geochemical Deposition arederivedfrom thesourceof theorganicmatcrial. Someof theoil characteristics of the uquaticbasinof dcposition. and someothersfrom the physicochemistrv in the voungsedimcnt. includingactivityof microorganisms 1.2.1 Marine VersusNonmarineOrganicMqtter(Fig.|v.1.1) the marinr'or It should be first pointed out that. in organicgeochemistr)'. continentalcharactcrhasto be relatedto the originof the predominrntorganic
541Crudeoils
AROMATIC HC +NSO COMPOUNDS
M€inficld ol crudr oilsdorivsd lrom marinoolgrnicnatlor l J . c r . l . c . o r .i L T . r i i r r y t { . A t r i c a + W . s i ' rA n lricr
Jur.atic,Cr!ioc.out Middl. [.rt
Fgu P . r a rB i r + l o r r h S . r
0 . r o n i . n ,C r . t r c . o | l .
N.ISO.AIXANES (PARAFRiTS)
ES CYCLO.ALXAN (iTAn.llHENES)
Fig. IV.1.l. Tcrnarydiagramof crudeoil composition.sho$'ingthe principalfield of of crudeoilsfrom marineand nonmarincorigin occurrencc
4.2Geochemical Regularities Related to theEnvironment ofDeposition
441
material.In all cases.this materialhasbeendepositedbelowaquaticmedia,as thisis a conditionfor a satisfactory conservation. organismslivinsin thewateror o n t h e b o t t o m o f a l a k e o r s e a g e n e r a l l yc o n t r i b u t er o t h e ; r g a n l c m a t t e r incorporatedin the sediment.They includemostlyphyto-andzooplanktonand bacteria.but macroscopicalgaemay be, in some circumstances, important contributors.However.the land-derivedorganicmattercan also be piedomi_ nant. comparedwith other sclurces of organicmaterial.especiallyin paralic basins,either intracontin ental or in thc sea borderingland. and in ieltaic environments. The lattcrsometimes cxrendfar into the opensea.asin the caseof the Ganges.Nigeror Amazondeltas.whosedctriticmaierialis sDreadover the c o n l i n e n t a' hl c l l : r n ds l o p e a . n de v e nr e a c h etsh e a b y s s apll i r i n s . ',t)Marineorganicr?atuer usuallycorresponds to kerogentypeIl and resultsin crudeoils of paraffinic-naphthenic or aromatic-intermediate iype (Fig. IV.1.1). Thc amountof saruratedhvdrocarbons (Fig. IV. 1.5)is ca. 3t}_7b % in crudeoil. and.,10-75q.relatedto hl,drocarbons. Isdprenoidisoalkanes. comparedwith fl-alkanes.are rather abundant at shallow or medium depths. Saturated hvdrocarbons of sterancand triterpanetvpe are often p....nf in shalktwand immaturecrudcoils.anddccreasc u,ithdepihanclthermalevolution.Aromatrcs amountto 25 to 60% of hydrocarbons. a valuesignificantly higherthan in crude oilsderivedfrom nonmarincorganicmaterial.The sameis truelor sulfurcontent. whichmay reachhighvaluesin certaintypesof marinecrudeoils:thisparticular aspectwill be discussedin Section4.3.3. Resinsand asphaltenes are also relativelvabundantin youngand immaturecrudeoils. This t1'peincludesmost crudeoils from Jurassicand Cretaceous of Middle East.Devonianof Alberta, Cretaceous (abovethe salt series)of WestAfrica, Mcsozoicfrom Paris.AquitaineandNorth Seabasins,andMesozotcof Mexico. b) Nonmarineorganicmatterdcpositedin a deltaic.marginalor openmarine envlronmentusunllycorresponds to type-Ill kcrogen.In rare casis (internal basins).. nonmarinematerialmayresultin type-lkerogen.Whenit predomrnantes, tcrrestnalorganicmattergenerates crudeoils of paraffinicor sometimes paraf_ finic-naphthenic type. The amountof saturatedhydrocarbons is ca. 60 907ain crudcoil. and7ll907c in hydrocarbons. Normalandisoalkanes, andmono_and dicyclicnaphthcnes are abundant.polvcyclicnaphthenes, and particularlythose ot steranetype.arc rare.Totalaromatics aresignificantly lowerthan in crutleoils derivcdfrom marincorganicmatter:thevamountto l0io 30% of hvclrocarbons andcompriscmostlymono-and di-aromatics. Sulfurcontentis lou trr r err,lou. usuallvlessthan 0.5% . Resinand asphaltene contentis usuailybelow 10% but m l r . i n s o m ec a s c r r. c l c h l t t ' ; 1 n, o u n * a n di m m a t u r ec r u d eo i l . , . The nonmarinetype includesmost crudeoils from the lower Cretaceous of West Africa, Brazil. South Argentinaand Chili (Magellanbasin).the lower Tertiaryof the Uinrabasin(Utah),theTertiaryof pannonianbasinand theRhine Graben.and part of the oils from Nigeria,the Gulf Coast.and severalTertiary basinsof Indonesia.High-waxcrudeoils(Hedberg,196g)belongto thistype,and are discussed in Section,1.2.2. c) Thegeochemicalsignit'icance of the differencebetweenthe more Daraffinic oils of nonmarineorigin and the more aromaticoils of marineorigin hasto he
112.
Geological Controlof Petroleum Tvpc
interpretedbv consideration of thc variousorganicinputsinto thc scdiment.and the subsequent (Tissot alterationof the organicmatterduringearlvdiagencsis eta]..1979). Continentalorganicmaterialis mainiycomposedof plants.They are rich in celluloscand lignin (higherplants)and containsubordinateamountsof other carbohydratcs.protcins and lipids. The latter include particularlvaliphatic hvdrocarbons of high molecularweight.the closelvrelatedwaxesandfattv acids of mediumchain lengthfrom fats. The plant materialmay be either directlv transportedto the basinof sedimentation. or reworkcdandincorporatedin soil humic acids. thcn transportedas such. ln many casesthe environmentof depositionis marine.andthe organicmatteris associated with abundantdetrital minerals.forminga thick deltaicdeposit.Alternatively.it ma) be transported by turbiditvcurrentsand extcndmuchfartheron the continentalslope.On the onc handthe fastburial.andon the otherthe deepwater.preventa strongreworking b.vmicroorganisms at thatstage.Suchsituations arefoundin manybasinsof Late Cretaceous-Tcrtiary agelocatedon continentalmargins.Undertheseconditions. the occurrenceof anoxic-reducing environments. with sulfatereductionin the bottomwaterlayer.is unlikelv.Thus,the possibilityof introductionof sulfurin t h c s e d i m e n ta. n ds u b s e q u e n ti lnyk e r o g e ni .s r e s t r i c t chdc r c . T h e r e s u l t i nkgc r o g c n( t 1 p cI I I ) c o m p r i s em s a i n l la' p o l v a r o m a tni ce t w o r ka, s lignin and soil hunic acidsare aromaticpolvmerscontainingox1'genand some s h o r ta l k v lc h a i n sH . o w c v c r t. h i sm a t e r i ails d e g r a d a b o l en l yw i t h d i f f i c u l t ya n d constitutes a ratherinert part of kerogenuntil an advanced stageof catagenesis is reached.rvhengasis generatedby crackingthe shortalkl'l chains.Thc subordinatc lipid fractionof kerogen.derivedlrom wax cstcrsand fats.yiclds.trough decarboxvlation. andruptureof C-C bonds.alkylchainsof variouslengths.As a rcsult kerogenmatleof continentalmatcrialmay generatc.when subjectedto thermalcatagcncsis. first alkaneswith few cvclicmolecules.then gas. Sulfur c o n t c n its l o w . Marirrcorganicmaterialcontainsabundantproteins.carbohvdrates andlipids. Thc latter inclurlepolvcvclicnaphthcnicmatcrialsuchassterolsfrom algaeand marineanimals.and triterpenesof the hopanescricsfrom prokarvotcs(bluegrccnalgaeandbacteria).Thc lipidsalsoincludesomemolecules appropriate for cvclization.likc unsaturated fattv acidsfrom fa1s.Furthcrmorc.situlltlonswhere m a r i n ea u t o c h t h o n o uo sr g a n i cm a t t e ri s a b u n d a n t lp! r c s e r v c idn t h c s c d i m e n t usuallvcorrespond to anoxicconditionsin the bottomrvaterla1'er(Tissotct al.. 1979.1980)rvheresulfur is cxtractedfrom sulfateof the seawaterb1'actionof anaerobicbactcria.Sulfur mav be incorporatedin scdimcnt.thcn rcact with orqanicmattcr and possiblfinclucecvclizationand aromatization. At this stage.thereis alsoan importantcontributionfront anaerobicbacteria which arc vcry activc in these sediments.as algal material is rather easily degradable. In particular.membranes olmethanogenic bacteria(Archcbacteria) are a sourccof isoprenoidchainsof variouslcngths.identifiedin kerogenbv M i c h a e l i sa n d A l b r c c h t( 1 9 7 9 )a n d C h a p p ee t a l . ( 1 9 7 9 )F . r o m t h c s cv a r i o u s sources.t)'pe-ll kerogenmadeof autochthonous marineand bacterialorganic mattercontainsabundantcvclicmaterialandisoprenoidchains.Whensubjectcd to catagenesis, it mav releasemore biogenicpolycyclicnaphthenes and morc
-l.l Geochentical Rcgularities Relatedto the Environmentof Deposition
.lt-l
d i a g e n e t ia c r o n l a t i cm a t e r i a l( a r o m a t i c sr.e s i n s .a s p h a l t e n e st h) a n r v i l l t h e n t t n m a r i noer g a n i cm a t t e r .I t m a t a l s or i e l dm o r cs u l i u rc o m D ( r u n d s . l n k n n sr ' l ( c , ' l o l r . t h c t l i r t r i h u t i otl , l m a r i n c: r n dc r r n t i n e n tul lr g t n i cm a l l e r canbe summarized asfollows.The predominancc of thc rathereasilfdegradable manncorganicmatteroccursrvhercconditionsarefavorablefor its preservirtion and contincntalrunotTis limited becauscof phvsiographical . climaticor other reasons..Thisis the casein upwcllingareas.where productionexceedsthe potentialfor de-eradation: arongthc westcrncoastof continentssuchas peru or Northwestand SouthwestAlrica: the marine characteris enhancedwhere tectonicsand/orclimatccreatcerosionalconditionsimproperfor pedogenesis. c.g.. along the Northwcsterncoast of Africa, bordered'bv th; Sahara.A predominance of marineorganicmatter is alsoproridedb1u idispreadtransgres_ sionscrearingshallowepicontinental seaswiih a high blotugicatproducti'vit1,. c . g . J u r a s s rocf \ \ c - \ t e r nE u r o p e .f i n a l l v . i r s i l l c d - ; r h r r r . . l _ h a * ignc o m e t i r l i r \ ( , n n ipr n r ) \ lec r r d i t i o n ri . a n o l h etrr p i c a,l i t u a t i r rlnr r 0 r i n um r r i n i o r g l r n i i mattcr. prcsentl' obscrveci in thc Black Sea.This situationwasverv nrobablv morecomntonovercertaingcological periods:middleCrctaceous hlaik shalel'oi t h e A t l a n t i cB a s i n s( T i s s net t a l . . l 9 l J 0 a ) n dc a r b o n a tpel a t f o r mo f t h e M i d d l e |,AST.
Nonmarincorqanicmatterpredominatcs wherea Iargedrainagesvstemrcsults i n a n a b u n d a nst u p p l vf r o m t h e c o n t i n e n tI.n t u r n . t h i ss i t u a t i J ni s d e t e r m i n e d partlr-bv the locationof humid climaticbeltsand partlv bv the phvsiographv of t h ec o n t i n e n t sA. h i g h l vo x y g e n a t ewda t e rc o l u m ni n t h c b i s i no i d c p o s i t i o n ma1 cnhancethc nonmarinccharacterby creatingfavorableconditions for uesrrucrron o f t h ea u t o c h t h o n opulsa n k t o nT. h ep r c d o m i n a n cocf n o n _ m a r i noer g a n t m c itrrer rs prescntlvassociated with the cxistenceof largc river svstcms.such as the 'l-his Missssippi.Amazon.Nigcr etc. type of sedimentltionreprcsents ntostof the Tcrtiar_\'' progradingdeltls. aktng the presentstablcmargins.Nonmarinc organicmattcralsooccursin somesvn-riftsediments of thc passi,emlrgins.c. g. . c a r l y C r e t a c c o uos f W . A f r i c a a n d i n i n t r a c o ni nt e n t a ll o \ \ l a n d si n d l a k e s ( T e r t i a r vo f t h eU i n t aB a s i ns. o m c ' l - e r t i afrivl l e db a s i nisn C h i n aa n dC r c t a c c o u s i n t e r n abl a s i n isn A f r i c a) . 1.2.2 High-Wat CrutleOil.s H i e h - w a cx r u d eo i l sr e p r c s c nats p e c i a l c a os fet h en o n n t a r i ncer u d eo r l s .I h e v are l i k e l vt o b c d e r i v c df r o mc o n t i n e n t a o lr g a n i m c a t t c r r v h i cwha ss t r o n g l ,r* * , l i k " d bl microorganisms. Celluloseis rathei easilydcgradedby bactcria.Lignin is degrldableon11,in the presenccof oxvgen.bv assiciationof funei and bactcria urrrkin-qsubsequentlv. This situationcan be achievedin shaliowparalrcor lacustrinebasinsrvhercthe sedimentsare periodicallvexposedto sub_aerial dcgradationand water flooding.Furthermoie.fats arc rndrc degradablcthan waxes.Thus-the main components preserved from the tcrrestrial-orqanrc input a r el o n q - c h a inn- a l k a n easn dw a xe s t e r fsr o mh i g h e p r l a n t sa. n csl o i lh u m i ca c i d s . w h i c ha r c a r e l a t i v e l ivn e r tp a r to f k e r o g e nt.n a O O i i i o nt h. e m i c r o t i a lb i o m a s s amountsto an importantpart of thc organicntatter:its Iipiclfractionc0mprises
,+,14
Geological Clontrolof PetroleumTvpe
0istribution of saturated HC
0istribulion ol n-alkanes & rs0pren0t0s 1-
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. S um a t r a , I n d o n e s i a y { ' T et' , a ' 800m
. n ' ' s o. l 2 3 4 5 6
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=
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. U i n t aB a s i n
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n ' , s oI 2 3 4 5 6
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- t ] l
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, W.Europe I' J u ' a s s ' ,
l l
I I ltrrr" 3iq
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123.456
AlkanesCycloalk B a n gn u m .
hvdrocarbons Fig.IV.J.2. Examplcsof high-waxcrudcoils.Lellr distributionof saturated *ith respectto structurrlt)pes: ,'lgll: distributionof /l-alkanesand isoprenoids as a functr()n of the numberof carbonatonrsper molccule.Na)ler the Iotalamountofr-rlkitnes is usedasreference(100', ) lor l,-alkaneandisoprenoiddistribution
.1.2Geochemical Regularities Related to theEnvironment of Deposition
445
mostlvaliphaticchains(n. iso.anteiso)from the microbialwaxes.without oddor even predominanceand also constituents(isoprenoids.hopancs etc.) from bacterialmembranes. The kerogenresultingfrom thcseenvironments is mainll,composedof waxes and a relativelyinert humic fraction,in spiteof the fact that th'erewas a high terrestrialinput with celluloseand lignin.Thereforcit mavgiveriseto high-wix laredke.ro-gen usuallybelongsro type III. direto a sizeable pro_ :r^u-d.,: :ii) Jhe.re l o r l r ( ) no t h u m t cm a t c r i r l .I n s o m ec a s e se..g . t h e U l n t aB a s i nk. e r o g e ni s m a d e essentiaily of longaliphaticchainsand belongsto tvDeI. The relatcdcrudeoils containa highproportion-ofnormal.iso-ancl anteiso_ alkanesandbclongtotheparaffinictypelnig.fV.+.t.y.Long_chainn_alkanesare abundantand may showeitherno predominance or a sliglitodd predomrnance derivedfrom the contributionof higher plants.The hi'ghparafTin contentis responsible for the high pour point and the waxv cha.aitei of the oirs.Somc cxamplesare presentedin FigureIV.;1.2. Thesecrude oils are commonlvknown as high_waxoils. Thev nave oeen reviewedby Hedberg(1968):theyarefounclmainll,in shale-sanclston e scquences and are frequentlvassociated with coalor highlycnrbonaceous struta.the whole beingdeposited in a continental. pararicor ne-arshore marincenvironment.I hese crudeoils are normalll low in sulfur.Thev can be traccclback to the fbllowing geological situations: a) Rift valleys,during the first stageof oceanicopening.are occupted by intermittentswamps.lakes.or narrow seassurroundedb1,continentril crlges d e l i v e r i n tge r r e s t r i aolr g a n i cm a t e r i r l .T h i ss i r u a t i o n I a v o r sa l t c r n a r e s r a g eos I aerobicbiodegradation andflooding.Thistvpeof sedimentation occurredduring Meyrzoic.and particularlvcluringlower iietaceous. in the areaot thc South Atlantic Ocean. [t is rcsponsiblcfor high-waxcruclcoils from Brazil. South A r g e n t i n at.h e M a s c l l a nb a s i n( C h i l i )a n dG a m b a( G a b o n ) . b ) P a r i r l ircr o u u h h r o r d t ' r i ncgo n t i n e n tosr c o r d i l l e r aosf i s r a n dosc c u r nf o r d e d . b e l t sh e l o r eo r d u r i n gt h c m a j r r o r r o g e n isct a g c sA. s a p r e s e n t _ deaxva m p l et .h e t r o u g h sb o r d e r i n g s o m ei s l a n d o s f I n d o n e s im a a v b e c i t e d .T h e o r g a n r m c atrer r r c c u m u l a t iinngt h r ' s e t r o u s h si sp r o v i d e d m o s t l vb v t e r r e s t r i ap l a n t sm o r eo r t c s s o(-{riidcdh\ mrcr(,()rqantsm\. There.high_wax crudcoilsareassctciatcd rvithcoal h e d s .E r r m p l e sr r e t i \ u n dr l o n g a l p i n cf o l d e db e l t s :T e r t i a r v o f I n d i a .B u r r n a . l v l a r i l \ ! u .I n d o n e s r J; r.n t lV e n c z u c l(aO f f i c r n lBr l s i n t . c ) C o n t i n e n t al ol u ' l a n dw s i t h n o n m a r i n sec d i m c n t a t i omna vr c c e r v a e similar r u p p l r ' . t )r f) r { a n i cm i r e r . T h e vv i e l dt h c s a m et v p eo f c r u d c t i l : C r e t a c e o uosf w \ l r m t n ga n d( i ) r o r i i L r oP:rl c o c e n e -E o c en c o f t h e u i n t a B a s i n u . t a hl M e s o z o i c a n dT e r t i n r yo f C h i n a ;C r e t a c e o uosf i n t e r n abl a s i n isn A f r i c a( S u d a n ) . 1.2.3 ClasticSeriesVersusCarbonates- High SutfurCrurlc Oils Pure carbonatesusuallycontain little organic matter. However. carbonate sequcnces mav well includesourccrockswhich are rich in organicmatterand which are usuallvmadeof marlsor argillaceous limestones. Ii suchcarbonate source r o c k s . t h e c l a ) ' c o n t e n t i s f r e q u e n t ll (vIc_a3.0 %o r m o r e .W e s h a lrl e f . etro
416
Type Geological Controlof Petroleum
and sequences" as"oil from carbonate oilshavingoriginatedfrom suchsituations '. or "clasticsequences from sand-shale comparethemto the crudeoilsgenerated but crudeoils rich in sulfuraremore All crudeoils containsulfurcompounds, (TableIV.'1.1).The than in a clasticsequence frequentin a carbonatesequence comparedwith 0.51% in clastics. averagesulfurcontentis 0.867cin carbonates, includetheoils and0.659'ifor all crudcoils.Oilsproducedfrom clasticsequences derivedtiom nonmarineorganicmatter.whoscsulfurcontentis low, asdiscussed in Section4.2.1. However. there is still an obviousdifferencebetweenthe and anditscontentin marinecarbonates averagesulfurcontentin marincclastics marls. Thc distributionof sulfur in all typesof crudeoils hasbeenshownin Figure by a minimumcorresponding IV. t. 16.It is bimodal.andthe modesareseparated and thoscfrom to ca. I % of sulfur.When crudeoils from carbonatesequences areconsidered separately. thesametypeof bimodaldistribution clasticscquences is foundin both groups(Fig. IV..1.3).However,thesecondmodc.corresponding sequences. fronrcarbonate to highsulfuroils.includesabout257r of the samples whereasit includesonlv half of that pcrcentagein crude oils from clastic the sulfurcontcntma! Furthermore.in crudeoils from carbonates. sequences. r e a c hu p t o 5 l i b y w e i g h itn n o n d e g r a d eodi l s ,a n du p t o l 0 % i n h c a v v d e g r a d e d oils or tars.Thcsefiguresare seldomreachedin oils from clasticsequences. Aromaticsusuallyamountto 40 to 60% in highsulfurcrudes.includingbcnzo(10-25?i of the crudeoil). Complexmoleculesol high antl dibenzo-thiophenes m o l e c u l aw r e i g h tc o n t a i n i n g N . S . O . i . e . . r e s i n sa n d a s p h a l t e n easr, e a l s o crudeoilsbelongmostlyto the abundantin the samecrudcoils.Thus,high-sulfur oil types(Fig. 1V.2.2). aromatic-intermediatc or to the aromatic-asphaltic Thereforethc highsulfur of livingorganisms. Sulfuris not a majorconstituent diagenesis. contentobservedin crudeoils hasto be gainedlaterfrom elsewhcre: chemicalreactionwith influencedby microbialactivityat the timeof deposition. crudeoil. or degradation of reservoired evaporites occurringduringcatagenesis, conditions of sulfur is to be found in the environmental ln mostcases. the origin in PartII, thefine at the timeof the sourcerock deposition.As alreadydiscussed Table IV.4.1. Avcragc sulfur content and specific gravity of crude oils from clasticand carbonatesequences.(Data from U.S. Bureau of Mines. lFP, and othcrs)
Reservoirs
Sulfur (wt. 7 )
Specific gravity
Numbcrof samples
Carbonate Clastic All types
0.86 0.51 0.65
0.8,1,1 0.11.17 0.8.17
2464 52 8 1 934'1
Fig. IV.1.3. Distributionof sulfur in crudeoils from carbonatcand clasticsequences (gcometric thanin The secondmode.above1-c? classes). . is moreimportantin carbonate (Datr from U.S. Bureauof Mines.IFP andothers) clasticsequences.
.1.2Geochemical Regularities Relaledto the Environmentof Deposjtion
447
Distributionof sulfur Closses ( % ofs incrldeoil)
Numter_ ol. irequencY s0mpres
Frequency
?
* (j
i
10%
C or b o n o t e s eq u e n c e
s
s ooi 005J= I ; 0llP
--l-n5le ^t: - 1 3
5lE I
10r
t S
s
ii
0.01 002 005 0,1 0.2 05 I
2 5 10
F i g .I V . . 1 . 3
0
7
utoslrc s e qu e n c e
140/.
,148
T],pe Controlof Petroleum Geological
depositedin marinebasinswith a normaloxygen muds (shalesor carbonates) quickly a closed environmentwhereinterstitialwater is sepacontent become by sea water. Oxygenconfinedin thatspaceis exhausted ratedfrom thc overlving rapidll establishcd. conditions are aerobic activitv. and anaerobic microbial bacteria(e.g., Desulfovibriodesulfuri' Under theseconditionssulfate-reducing in confined cars) extractthe sulfatcavailable.andreduceit to H:S. Furthermore. seasor lagoons.the bottom water layermay becomedevoidof oxygen.ln this caselargc quantitiesof HlS ma-"-be produceddirectly from the sulfatesof scawater. Sulfur is not incorporatedin the bactcrialcell. In clay muds.where iron is to form iron sulfides. usualltabundant.it is likelr-that sulfurreadilyrecombincs However.in carbonatcmuds. where iron is much lessabundant.sulfur may combinewith residualorganicmattcr remain as free sulfur and progressivelv Kerogcnoriginatingfrom thiscnvironmentbelongsto t)'peII duringdiagenesis. In addition.sulfurmavact high-sulfurcrudeoilsuponcatagcncsis. andma1_,-ield 'l'his as an agent of aromatization.$'hen incorporatedinto organicmatter. woulclexplainthc strongcorrelationobscrvcdbetweensulfurand a hypothcsis h i g ha r o m a t i c i tovf o i l s( a r o m a t i c sr e. s i n sa. s p h a l t e n e s ) . of Massiveincorporationof sulfur into sediment.in confinedenvironments ol sulfur and subsequent incorporation tvpe sedimcntation. carbonatc-cvaporite into organic matter during diagenesis.is probablvthe causeof high-sulfur evolutionof organicmatter.suchasthe petroleums generatcd duringsubsequent aromatic-intcrme diatecrudeoils of thc Middle Eastandof the SouthAquitaine with a basins.On thc contrary.other ver),reducingenvironmentsassociated crudeoils. For clasticsedimcntationresultin low-sulfurparaffinic-naphthenic example.the crudeoilsderivedfrom lowerSiluriansourcerocksof North Africa. and the Lou'cr to Middle Jurassicof the Parisbasin are low-sulfurcrudes. weredepositedunderanoxicconditions. althoughthe sediments sedimentationma\ Semi-closedseasor lagoonswith carbonate-evaporitc situations.However.theyareparticularlyfrequent developin variousgeological of the in relationto mobilcbeltsalongactivemarginswherefrequentmovements sea seas.suchasthe Meditcrranean crustdeterminctheformationof semi-closed duringNeogene.or the presentBlackSca.In fact.someof the major areaswith of thc foldedbelt thatoriginatedfrom high-sulfuroilsoccurin thc ncighborhood t h e o l d ' T c t h 1 s " g e o s v n c l i n e : t h e M i d d l e E a s t ( A r a b i a . G u l f Sl rtaakt e.Ssv. r i a ) . , r e e c eS. i c i l yS. p a i n )A. c o m p a r a bsl ei t u a t i o n M e d i t e r r a n e a( 1n' u r k e yA. l b a n i aG occursin the Carribean(Maracaibo.Colombia.Cuba.Mexico). not relatedto thc main foldedbelts, Othcr tlpes of confincdenvironments. mav resultin similarsituationsasfar as the fateof organicmattcris concerned basinsof andTertiarvof Egypt,in someJurassic For instance.in the Cretaceous Germany.in the Upper Jurassicof France(Aquitaineand Parisbasins).in the of the SouthEmba area(USSR). Jurassicand Cretaccous has becn A somewhatdifferentinterpretationin terms of paleogeographv geographic distribution of (1965). the who compared proposcdbl Radchenko (from to geological svstems Cambro-Silurian several high-sulfurcrude oils in (1962). lt by Strakhov zones. as reconstructed paleoclimateic Neogene)with thc arid zones located occur in thc crude oils tvpically that high-sulfur wasconcludcd
.1.2Geochemical Regularities Relatedto the Environmentof Deposition
449
TableIV.:t.2. Correlations in low_andhigh_sulfur crudeoils.(Datafrom U.S.Bureau Mines, IFP, and others) of Specific gravity Specific gravity Viscosity (iog)
0.9lt)
Pour point
0.3.52
Sulfur (log)
ll.'t15
Viscosity (k'g)
Pourpoint
Sulfur (krg)
0.726
0 .t 0 2
o.129
0.135
0.27-5
0..113
0.671
0.05IJ
T 3
0.112
Low-sulfurcrudes:S < l E ( 1 5 8 . 1 samptes) High-sulfurcrudes:S > I % (,l l,l samptes)
-
High sulfur crudes
sourhern rropical beltswherccarbonate-cvaporite scdimenta_ li^,I:^1,]1,n.rn.onU tronrslrcouent. lncorporatlonof sulfur into.pc.troleummav also occur at depth. cluring catagencsis, althoughit.seen,ls to trelessfrequentihanin.orporuiiunur ,ultu, into organtcmatterduringdiacenesis. Orr (197i) sho$,edthat t'hermalmaturationof oil with sulfatepresentin."high temperaturcreservoirs mayinvor'e nonmrcrobial rcductionof sulfate.with sulfrrrization of the oil ona ti..oiiun ot hvdrogen sulfide.On rhe contrarv.rhermatmaturatio" *ith";; ;rii;;;; w,.,utarerult in a continuousdecreaseof the sulfur content.with prel.ercntial removalof non_ t h i o p h e n iscu l f u rc o m p o u n c(l H s o et al.. I97.1). Microbialdegradationof crude-oir may alsoresurt . in a sulfurenrichmcnt.iis benzothiophenc derivativesand high rnolecul;;,;;il,;r;;;;mpounds are partrcularlvrcsistantto bacteriatdegradatio (see n Chirp.f\|-Sl. "".t hencerhev areselectivelv concenrrared. Furtheinore.ir,ni.rorriri,i.,*if i;;rl;;;;;;;; sulfideor elementalsulfur. thesemav react with altered..r.t. -rru n,t to produce
.:.pounds.A pcculiar irtc.ation isrepJ.iJ
i" ., ",. 1rr+1i" 1.1::t:1Itu. some hrgh-gravitv oirsandcondensates from westernCanada:h_v
olysluliotb.etween density. viscoiity.. and.,irru...-r*ifirui."tv
+.:,).nou, porntalsoindicares somecorrelation withOcnsity anJviriniuuin,i" i,igt_."1f". ,ll1.i:,i.:.atthough nonewasapparent whentookingat'cruJe orlsin rorat. illll
I n e s co h \ e r \ a r r r r n ;\ r r ep r r r b a h l td u e t o t h e f a c t t h a t t h e a b u n d a n c eo t r c s r n si t n d asphaltenes(containinga large proportron of sulfur) is the main cituseor'il'ittion
450
Geological Controlof Petroleum Tvpe
of density,viscosity,and pour point in high-sulfurcrude oils. Other typesof compoundexcrtinginlluenceon viscosityor pour point (e.g.. high molecular weightalkanes)are subordinate in thesecases.
4.3 Geochemical Regularities in Relationto ThermalEvolution crude Whatcvcrthc gcneticcharacteristics of a sourcerock andof the associated oil mav be thev all experience the samegeneraltrendof evolution: - at shallowdepth.duringthe phaseof diagenesis. the sourcerockis immature. molecules moreor The availablehydrocarbons andassociated arein mostcases less directly inheritcd from living organisms(geochemicalfossils).Their abundanceis generallytoo small to providecommercialoil accumulations. generated Smallamountsof heteroatomic compoundsare alsoprogressively of heavyoils, from kerogen.and the wholemay form someaccumulations - beyonda certainthreshold,catagenesis beginsand the principalzoneof oil formationis rcachcd:theretbrethe major part of pooledcrudeoil hasto be to the expectedwithin the catagcncsis zonc,aroundthe depthcorresponding maximumof oil gcneration. with increasing burial.crackingof carbonchainsbecomesimportantin source are rocks: crackingmay atTectreservoiredoil as well: light hydrocarbons progressively favored,and the GOR increases in the pools, - at great depth. onll light hydrocarbons are remain and gas accumulations formed:gasis thc only commercialdeposittbund at greatdepth Thesesuccessive stepsof evolutionmaybe moreor lessproductive.andoccur of organic at difttrent burial depth. accordingto the geneticcharacteristics m a t t e r .t h e g e o t h e r m agl r a d i e n ta. n d t h e t i m e . I n p a r t i c u l a rs. o m et y p e so f organicmatterare lesscapableof generatingoil (sccPart II). However.when both oil andgasareformed.the variousstepssuccced eachotherasa functionof the temperaturehistor)'. A comparisonof oil and gitsoccurrences with temperaturchistorl shouldbe versustime curvefor the various bascdon the rcconstitution of the temperature parts of thc drainagcarca of eachfield. In fact suchdetailedinformationis availableonlv in a limitcdnumbcrof cases.If we wantto considera largcnumbcr dataon a worldwidebasis.we arc of oil or gasfields.in orderto obtainstatistical restrictedto the useof presentdepth,and ageof the producingseries.We are awarethat the presentdepth is somctimcsverv difTerentfrom the maxlmum clepthof burial. especiallvfor Paleozoicreservoirs.and the age is not a good approximationfor the time eltrpsedsince thc formation has been buried. obscrved.This Thereforesomescattering hasto be expectedon the distributions effect may be eliminatcdto a largeextentif we restrictthe studyto Tertiary basins.wherethe maximumdepth of burial is frequentlythe prescntone. The situationwould bc still bcttcr if we considera sinsleTertiarvbasin.
1.3 Geochemical Regularities in Relationto ThermalEvolution
.t5t
1..1.I Di.stribution ot' Depth.s
The distributionof theproducingdepthsof theoil fieldsreflectsthegeneralcurve of petroleum g e n e r a t i o(nF i g .I V . 4 . 4 ) .A t s h a l l o w d e p t h t, h c n u m b e o r f o i lf i e l d s is small;it increases downwards.reachesa maximum.then decrcases acajnat qlet!9r g:qths. as mosrdeepdiscoveries are gasfields.The averrgedeith tor 1 2 . 0 l to i i l f i e l d so f a n ya g ca n dl o c a l i t yi n t h e w o r l di s 1 4 6 5m . T h i i i s p r o b a b l v influencedto some extent bv the large number of small and shallowpoois producingin the UnitedStatei,andby the paleozoicficlds.whosepresentdepth is otienshallowerthanthe maximumburialdepth.However.Table1V.4.3shows that the averagcdepthof 2609Tertiarvoil fieldsis 1552m. More rcstrictedpopulations(classified by geologicalprovinceand age)show an averagedepthfronr I 195m (231crudeoilsfrom the Tirtiarv of thepannonian a n dV i e n n ab a s i n st)o 1 9 5 9m ( l { } . 1 c8r u r t eo i l sl r o n rt h c T e r r i ; r )o f G u l f C o a s t ) . This rangecan be explainedfirstlvb)' the geothermalconditionsand the burial histor]".then in other casesb! the type of organicmatter or the migration processes. In particular.the geothermalgradientis high in thc Viennaand the P a n n o n i ab n a s i n s( a v e r a g d e e p t h1 1 9 -m 5 ) . w h e r ei t m a v r e a c h- 5 0 . Ck m 1 i n severalareas.It is still ratherhigh in som,..Werr Africir i,.rsin.(avcrageocptn 1362m). suchas thc Doualabasin.wherea reconstitution of a paleograclicnt. computcdbv a mathematical model usingvitrinitc evolution,givesvaluesca. 50'C km r; suchhigh valucsare probablvrelateclto Crctaceo;stimc. On the contrarv.thc geothermalgradientis comparatively low in the Gulf Coastarea (average depth19-59 m): from 22ro 2,1"Ckm I in Louisianato 29tO33.C km r in Texas. The observcddistributions are usuallvfar fron a normaltvpc.whengrouping oil ficldsof variousoriginsand agcs.However.thev becomccloscrto a normat distributionif we sclectsomepopulations from thc iamc producinsscnesin the samegeological provincc.For instancethe hvpothcsis of ti normalirstributroni: acceptable on a 0.{)1probabilityleverf'r the crctaceous-r'ertiarv fieldsof the M i d d l eE a s t( a v e r a gdee p t h :l 9 0 l m ) o n t h c o n e h n t l . r n d h r r r h e - f c r t i a r vo f V e n e z u e l*a C o l o m b i a* T r i n i c l a o d n t h eo t h e rh a n d( a v e r a gdee p t h :l 5 l - t m ) . 1.-1.2(.hungeol Conpositiortttith Depth The.chanqc of composition with depthwasfirstreportedb1,Barton( l9-j,l)on the crudeoils of thc Gulf Coast.He noteda progrcssive decrease of dcnsrtvand an .fhe increascin parafTinic contentu,ithincreasing dcpthol theproducintinteivll. s a m ep h c n o m c n o n r v a so b s e r v e db 1 ' H u n t ( 1 9 5 3 )f r o m t h e T e n s l e e po i l s o f Wvoming.but it couldnot bc gencralized to all crudeoilsfrom this area.asthe deptheffectdoesnot obliteratemajordifferences resultinqfrom diftercntsource r o c k sa n dd o c sn o l a c c o u nflt ' r r r r i a t i o n .o f g e o t h e r m adlr t u . -.Since then. comparablesituationshave been recordcdfrom manv piaces. FigureIV.,l.5 sho\\'sthe avcragedensitvof crucleoils in respectto depthrank. The changcof compositionwith depth resultsmainlv from the progressive crackingof carbon chains,that causesthe contentof light hydrocarbons to
+)l
GeologicalControlof PetroleumType l 0
5
1
0
r I
t 0rigin' l{um.ol somples-
Vl d s I A l l T e f i r at ri e I |
TerlaryCalifornia
?Anq
c
b
Terliary. GulJCoast
288
1038 d
F i g .l V . . 1 . J ad . D e p t hd i s t r i b u t i oonf o i l - p r o d u c i nhgo r i z o n(sd a t af r o mU . S .B u r c a u o f lge i (b) 2609 Mincs.IFP antlothers)r(a) 120l8accumulations from Cambrianto Pliocene accunlulation\ of Tertiarvagel (c) 2llErccumulationsin theTertilrv of Californial (d ) 1(138 in the Tertiarvof the Gulf Coast accunlulations
Table IV..1.3.Averagedepthsof oil fields.(Bascdon I 2018datafrom U.S.Burcauof Mines.IFP, and others) Classesof oil fields
Number of fields
Average depth (m)
Standard deviation of depth distributi()n
(-) All samples Tertiary Tertiary of Pannonian and Viennx basins CrctNccous-Tertiary of West Africa Tertirry of California 'I'ertiarv of Venezuelit Columbia. Trinidad Devonian of Alberta Cretaceous-Tefiiarv of Middle East 'lertiary of Gulf Coast
1 2 0 Il J 2609
1.165 l5-52
903 lt9l
231
I 195
6lJ2
.1.15 2itll
t362 1509
9f 211
l51.3 1630
ll10 7t 2
115 1{)3ll
l90l I r)59
9-13 820
'766 901
increase.However. the correlationcoefficientbetweendepth and densitv, computedon the samesetof crudeoils(all agesandorigins)is onlv - 0.,10.duc to thc varictvof genetictvpesof sourcerocksand crudeoils. and due to rlifferent geothermalconditions.It is somcwhathigher(o = -0.:16)in Tertiaryficlds.The depth densitvrelationis betterif wc onll' considerindividualTertiarr provinces with a simplegeologicalhistory.and wherepresentand maximumburialdepth
.1.-lGeochemical Regularities in Relationto ThermalEvolution
.153
Sp.cificaroriiy (9/cm3)
rrr 1t--TIr l ' l
0 800
I
0900
0 800
0 800
tr--l
l J l{ ' f l t | tl
f ft ll t [ |fsriT'lI
l_l!-l t
0 851
1
'oagg'
-l , t qes| | t'r'"'y I o r i-o i n - l A t d I Tertrary I I Teniary I 6 l o c a r r o n s C o l u m b i a G u t , C o a s r & | catltornra I | | | I I ] j Venezuela I
.lHgi""i-lt^u I | 'oro I
tr
a"
,
_
l
Fig. I\'.1.5. Chanseof spccificgravitl of crutleoils as a functionof deDth.A\erape specilicgra!itl is calculated overfour depthintervals.(Adaptedfrom Goubin.l,r7t: rl.1ra f r o mU . S . B u r e a uo f M i n e s .I F P a n do t h e r s )
arencarlvthe same:Tertiary+ Cretaceous of WcstAfrica (o = -0.56): Tertiar.v of California (q - -t).54). The generaltrend of thermalmaturationis shownin the tcrnarvdiasrantof c r u d eo i l s c o m p o s i t i o nF. i g u r eI V . 2 . 3 .T h c c o r r e l a t i o no f t h e r r * i s o a l k a n e contcntwith clcpthincreascs lrom o : +0.'15if we considerthe abundanceol a l k a n e isn c r u r l eo i l . t o o = + 0 . 6 0i f w e c o n s i d etrh e i ra b u n d a n cien s a t u r a t c d h l d r o c a r b o n sT.h e c o r r e l a t i o ins s t i l l b e t t c rf o r r r - a l k a n eosn l v .T h u s .v a r i o u s p a r a m e t e russ i n gt h c r r - : r l k a n e s : s l l u r i rht reddr o c u r h orni r t i , rh r v e b c c nu s e dt o d i a g n o s tch c l e v e lo f m a t u r i t vo 1 a c r u d eo i l . W e c a n m e n t i o n .f b r i n s t a n c c . ternarvdiagrams(nornral.branchecl andcyclOalkanes) plottcdeitherfor thc total saturatedfraction.or for the Iightfractioncomprising six or sevencarbonatoms onlv. A relatedindex of crude oil rlaturitl was Drtposcclbv Jonathanct al. ( 1 9 7 5 )b. a s e coln t h e r r - h e x a n c : m c r h v l c v c l . , p ernat iur n . ,.T .: h c i \ h o w e dt h a tt h e sameratio. when measuredon bitumenof the sourcerocks.correlates rvithlhe vitrinitc reflectance. Sulfurcontentol crudeoils generallvdecreascs with depth.possiblvclueto c r a c k i na g n de l i r n i n a t i oonf s u l l u ra sH , S( P r i n z l ear n dP a p e .1 9 6 ; l )T. a b l el V . - 1 . . 1 s h o w st h c a v e r a gseu l f u rc o n t e nitn r e l r t i o nt o d e p t h T . h e r er sa c l c a rd e c r c a soel the average sulfurcontentwith depth.from 0.80%in shallowcrudcoils(lessthan t l 0 0m ) t o 0 . 3 3 %i n d e e po n e s( d e e p e tr h a n2 2 0 0m ) . T h e d i f f e r e n bt e h a v i o r o f c l a s t i ca n d c a r b o n a tsee r i e si s e m p h a s i z e( dF i g .I V . . 1 . 6 )a. sc a r b o n a rsec r i e s show a strongersulfur depth correlation(g : 0..16)than clasticsedimcnts (O = 0.27).The assumption that sulfuris eliminatedfrom the licluidphascas hydrogensulfideis backedby the distributionof hvdrogensulfideconrentin n a t u r agl a s .t h a tr e a c h easm a x i m u ma t g r c a td e p t h .b e l o w3 0 ( Xm) ( F i g .I V . 1 . 7 ) .
GeologicalControl of petroleum Type
45,1
Table IV..1.4.Sulfur contentin crudeoils accordingto depth classes. (Datafrom U.S.Bureauof Mines,IFP andothcrs) Depth
Average sulfur content (%)
Number of samples
All samples
0.654
931'7
.0.803 .0.753 .0..132 .0.330
2085 2540 1938 1,158
0- 800m 800 1500m 1500-2200m > 2200m
Sulfur conr.n' 'n crudc oit (weighi "h) ---------------0
05
0
05
E
r500
I I
0 r i g i n A l l a g e& s localions [um.ot 9347 Samples
Carbonate seq!ences 2462
Fig.IV.1.6.Change of sr.rlfur content in crudeoilsasa function of depth.Average values of sulfurarecalculated (DatafromU.S.Bureau overfourdepthintervals. of Mines.IFp andothers)
In addition.Ho et al. (1974)notedthat nonthiophenic compoundsarepreferentially rcmovedbv thermalmaturation:thiopheniccompoundsare affectedbv a decrease of the bcnzothiophene:dibenzothiophene ratio.
4.3.3 Changeof Compositionwith Age The ageof thc reservoirrock is a ratherpoor indicationof the time that hasto be considered fbr the kineticsof evolution:on one hand.the ageof the sourcerock mavbe differentfrom the ageof the reservoir;on the other.the time a sediment wasat the appropriatetemperature, i. e.. deepenough.is not dependenton the stratigraphic ageof the rock. in a strictsense.
,1.3Geochemical Regularities in Relationto ThermalEvolution -Averoqe o
01
455
H2S contentin qos(weighl%) __|> ol
a4
E 3000
: o
4000 5000
t,'t A r c r : r ! rr r r l r r r . , .hl t , l r , , r crnu l t r d .(o l l c n l r n n : r l r r rgr r. rl .i r \ . r t I n . t t , , nr ) l ( r c_ l r,llnlt q.l :. .. :r { r \ ,l,n l L .r r r r r r l r zI c, rr U . l . S .B r r r s . , ,r 1r\ 4 r n u rl .l _ p , l( l , , l i s r . J
T h e r c l i r r cv a r i l t i o n s o f i l e n s i t v .g a s : o i lr a t i o . c t c . . a s a f u n c t i o no l a g e s h o w on Ir.a broad tendencl . rvhenconsideringstatisticallv a great number of oil fields. toward a decreaseof densitv and an incre-ase in light irydio.nrt on, *ith in.r"o.ing age. Our statisticsarc not girod enough if we restrict t'he iniuestisntion to a smallcr aren. e.9.. a sedrmentarvbasin. Variations due to the succeisionof difTerent organic deposits mav be so predominant that the time eftcct-is not abre to o b l i t e r a t et h e m . The distribution of sulfur content.withrespectto age of the producrngscles rs w o r t h s p e c i a lc o n s i d c r a t i o nI.t i s s h o w n i n F i g u r c i V . - 1 . g .i o m p u t e d o n 9 3 1 5 s a m p l c st.h e g c o m c t r i cm e a nd e c r c a s etsr o m p a l c o z r . r(i S c - { J . J l" l ).to Mesozoic (S"= {).29"r_)ro Cenozoic oils (S - 0.1992 ). The shapesuf tf,. OiitriOution, ur. differenr: all.thrce groups of crude oils _ paleozoic. M.ror;i; ;;; Ccnozoic_ show a principal mode around 0.2 9l sulfur. but paleozoic and Mesozoicoils have bimodal distributions. with the second mode around JZ. reparateO trv a mininrumrtl';.Inlhclrlr(rca\e.high_.,ullurcrurlc,,lf..-mpri.lill,,ju;;'"i the total number of crudes. On the contrar),. Tertiary oils show only a regular decreasein frequencv with increasingsulfur iontent. nnd onl1.t 2.," of the crudes contains I -47or more of sulfur. It is n()t clear whether thc occurrenceof conlinetl
.ll'n rc-cvap,)rite sedimenration u,asactuay morefrequenrin il.:1.s-:ll (e.g..Permian ufper rateoz')tc basinof Texasandbasins borclering Rocky "r.U.llg;r-Urut prorince)andin Mesozoic (c.g..MiJaler_ast) thanin Yll:,1:.1'\l I erttarv. I h e l t c t t h i t tm l j o r d e l t a i cs e r i e w s i t hl o w _ s u l f uc ir u d e cs o n r a r a n large numberof the prorlueingfieldsrn the Tertiarymav be a rcason for theobserved
O l S rt t D U l r )( n
,156
Geological Controlof PetroleumType
f,-
trequency ---f-----lz Paleoroic 4 5 5 2s a m p l e s
0.1
Frequency 5
0
Mesouoic pl e s 2 0 9 6s a m
=
Frequency 0
5
10%
0,01 0.1
F i g .I V . . 1 . 8 a - c
Summaryand Conclusion
r57
1.3.4 Combinetllnfluenceof Depth and Age on petroleumComposttton Tcmperatureand time are the major parametersin the kineticsof petroleum qeneration.Consideration of both depthand ageof the producingseriesallows one to cstimatestatisticall),. for a large numberof geologicaliituations,the etTects of theseparameters. However,depthantlagealoneaie a poorevaluation o t t c m p e r a t u raen dt i m eo f e v o l u t i o nl.t i s c l e a rt h a td e p t h( i . e . . t c m p e r a t u r e ) shouldbe more importantthan age.ashasbeenshownin part IL Within a givenbasin.useof relativelynarrowageclasscs providesa general pictureof the evolutionof the variouscrudeoils.un,l it ,hnruiin particuiarthat the variationsin primary organic material remain. in nanv cAses. .original predominantover the influencesexertedby evolution.For instancethe Missis_ srppianto Lower cretaceouscrudeoils in Alberta are statisticallv heavierand c o n t a i nm , , r es u l f u rt h i r nb o t hy o u n g eC r r e t l c e o uusn dD e v o n i r no i l \ . u h a t e e \r t h e d e p t hm a yb e .
4.4 ConcludingRemarkson CrudeOil Resularities Oil compositionis primarilyinfluencedby thc natureandevolutionol thesourcc rock.The mainfieldsof occurrence of the varioustvpesof crudeoilsareshownin F'igurelV.,l.l. However.oil compositionis sometimes influencedbv arrerarrons occurringafterpetroleumhasbeenpooled.Thus.the originalprimarvcomposr_ tion of crudeoil, bearingthe imprintof sourcerockfacieiandevolution.miv bc obscured.Thesealterationsof pooledpetroleumare treatedin thc following chapter.
Summaryand Conclusion Marineorganic matterusuallygenerated paraffinic-naphthenic or aromatic-intermedi_ ate crudeoils. Terrestrialorganicmatter, deriv€dfrom plants,generatescrudeoiis of paraffinicor sometimesparaffinic-naphthenictypes.High_wax-crude oils represenra particular case of the paraffinic classrthe organic maiter is strongly reworked bv bacteriaand enrichedin long chainlipids. High-sulfurcrudeoilscontainmor€than 17csulfur.Thevoccurmorefrequenuy!n carbonate-evaporiteseriesthan in clastic series.Sutfur ii generallyextractedftom sulfateduringsedimentation, and subsequently recombines iither with iron rn clastic sedes.or with organicmatterin carbonate-evaporite series.
Fig. IV.,l.8a-c. Distributionof sulfur in paleozoic.Mesozoicand Tertiarvcrude oils (geometricclasses). The absenceof the secondmode(highsulfur)amongirude oils of Tertiaryagemay be causedby the predominance (Data of clastiiseriesovercarbonates. from U.S. Bureauof Mines.IFP andotnerst
Controlof PetroleumType Geological A generaltrend of evolutionis observedwith increasingdepth andage:a decreasein are dueto in light alkanes.Thesechanges densityand sulfurcontent.and an increase thermalevolution.However,va ationsdue to the originalcompositionof organic that time or deptheffectis not ableto mattermayin certaincasesbe sopredominant, obliteratethe originalpatternof composition.
Chapter5 PetroleumAlteration
Petroleumis a verv complex mixture oforganic compoundsand hasa high cnergv content: it is thermodvnamicallvmetastableunder gcological conditions. petr-o_ leum aftcr being pooled in a ieservoir is therefbie suiccptible to alreration. Altcntion to the importancc of_alterationwas directeclb1, a number of papcrs ( W i l l i a m sa n d W i n t e r s . I 9 6 9 ; E v a n se t a l . . I 9 7 l : B a i l e y i t al.. 1973a).'wirich inrestigrteh d c l r v i c er r u d e , ' i l \ , , 1W e s t c r nC a n a d a . 'I'he geochemicaleffects of alteration. howevcr. had bcen recognlzecl long before bv Russian petroleum scientists(Andreev et al.. l9-5g: Kartsev ct al.. 1959).ln.a more sencral wav it has been known much krnger. for Instanccin the form of the so-callcd tar mats at the oil_water contact oimanv oil tields (e. g..
B u r g rn) . Alteration of pooled petroleumis now observedamongoil accumulations aroundthe worlci.The ctruscs olalterationarenumerous. Thiy mavbe relatedto t h e r e l a l i \ ei n s t r h i l i t \o f l e t r o l e u mr n d ( ) r t o t h c f a c tt h : r tt r , i p . . i . . o n r h eo n e n a n do p e ns v s t e m \a. n d o n t h e o t h e rm a v c h a n g et h e i r l e v e lo f b u r i a l dueto further subsidence or erosion.The composition-ofpooled petroleumcan be altered.bvchemicalor physicalprocessei.An exampleof chemical alteration would be thermalmaturationor microbialdegradationof the reservorred oil, whilephysicaI alterarionmight be via the prefer-ential lossof lightcompoundsbv drtrusron.,)r the additionof new compoundsto the reservoirduc io further migration. The distinctionbetweenciemical antl physicalprocesses is not stressedhere. becausethe two are often interrelaiedand may eveD occur simultaneouslv. In prcviouschapterswe havelearnedthat variationsin crudeoil composition are to a certalnextentinhcritedfrom diffcrentsourcerocks.For instancc. coaly materialin generalyieldsmoregaseous compounds. whilchigh_wax crudcoilsare commonlvassociated with sourcemarerialcontaininghigh proportionsof lipids of tcrrestrialhigherplantsand of microhialorganirims.H igh-'.u lfur crudeoils frequcntll are rclatedto carbonate-tvpe sourccrrcks. Asidefrom the lnfluence of the sourccrock facies.the stateof maturit),of the sourcematerial is alsoof importance.However,alterationof the reseivoiredpetroleummay atfcct the composition of a crudeoil to a greaterextcntthandoesihc character of thc source material.In other words. crude oil altcrationtends to obscurethe original characterof the oil, and thereforeaffectscrude oil correlationstudies. ancl furthermorcinflucncesthe qualitvandeconomicvalucof pctroleum. A comprehensive treatmentof the most important iltcration processes, lhermal altcration.deasphalting, biodegradation and water washinshas been
PetroleumAlteration
160
presentedin a seriesof papersby Bailey,L,vans,Rogersandcoworkers(Evans etal..l97l:Rogersetal.,l972;Baileyetal..l973a.b:Baileyetal.,1974;Roge e t a l . ,1 9 7 4 ) .
5.1 ThermalAlteration Thermal alterationof pooled petroleum,as with the maturationof kerogen, For a givengeothermal proceedsunderthe influenceof heatin the subsurface. at a gradient,it increases with increasing depthof burial and time of residence certain temperature.Thus, thermal alterationor maturationeffectscan be depthandrising predictedfrom a time-temperatur relationship. With increasing to becomespecifically temperature, thereis a tendencyfor crudeoilsin reservoirs amountof low molccularweighthydrocarbons lighterandcontainan increasing A moredetailedexaminaat the expenseof high rnolecularweightconstituents. in the reservoirtemperature tion of changingcrudeoil propertieswith increasing tn WesternClanada in FigureIV.5.l. Thereis a lineartncrease basinis presented of compounds containinglcssthan l-5carbonatoms.Thishappensat the expense gases,especiallymethane.Increase C1.1 heavy constituents.Simultaneouslv in moremature At clevatedtemperatures, exponentially in relativeabundance. by alongwith pyrobitumengenerated zones.only methaneis foundin reservoirs of pyrobitu(Evanset al.. l97l). In reservoirs, the presence crackingprocesses (belowC,.). andthe absence of the intermediate oil menpluslight hydrocarbons reactions during thermal constituents are an indicationfor disproportionation alteration.From oils of intermediatemolccularweight,the ultimatestableend residue productsarelow molecularweightmethane.andan insolublecarbon-rich of very high molecularweight.In this way, the systemfollowsthe inevitable towardmorestablemolecules. thermodynamic trend bv adjustingcontinuously
Gas
Cl5 * tiquids
50
('C and"f) datlem!erature Present inthe reservoirtemperature in crudeoilswith increasing Fig.tV.5.l. Chemicalchanges WesternCanadaBasin.(After Evanset al.. 1971)
5.2Deasphalting
461
The whole proccsscan be viewedas a disproportionation reaction.It can be deducedthat there must be a hydrogentransferfrom donor to acceptor,i. e.. from "aromatic-typestructures".which becomemore condensed. to aliphatic molecules.In the NSO fraction of the oils. this processis accompanied by decarboxvlation. dehvdrationand desulfuration yieldingcarbondioxide,water and hvdrogensulfide. It hasto be realized,however,that it is difficultto distinguish betweeneffects of thermalalterationof pooledpetroleumand the effectsof differentstagesof maturityof a sourcerock at the time of oil release.Sometimes the distinctionis possiblewith the help of solidresiduesin the reservoir.
5.2 Deasphalting Another relativelycommon alterationis deasphalting.the precipitationof asphaltenes from heavyto mediumcrudeoilsby the dissolution in the oil oflarge amountsof gasand/orotherlight hvdrocarbons in the rangefrom Cr to C6.The precipitation ofasphaltenes out ofoils by low-boilinghydrocarbons is soeffective thatit is routinelvusedin laboratories andrefineries for separation ofasphaltenes from othercrudeoil constituen ts.Deasphalting occursasa naturalprocess among heavyto mediumoils wheneverconsiderable amountsof hydrocarbongasare eithergenerated in largerquantitiesin a pool dueto thermalalterationof the oil or dueto gasinjectionfrom outsideasa rcsultof secondary migration.However, it rs necessarv for this gas to be dissolvedin the oil beforeit can precipitate asphaltenes. being ineffective.if it accumulates in a separategas cap. Gas deasphalting is difficult to distinguishfrom thermalmaturation,becauseboth processes olien occurconcomitantlv andthe netchangein oil composition goesin the samedirection.i. e., oils becomelighter. In the reef-bearing areaof WesternCanada.thermalalterationand alsogas d e a s p h a l t i ncga n b e o b s e r v e (dE v a n se t a l . , l 9 7 l l B a i l e ye t a l . . 1 9 7 4 )A. m o n g deeplyburied reservoirsin Upper Devonianreefs.it is quite commonto find prccipitatedasphaltenes in someof the reef porosity.In this rcgionof Western
0
50
1 0 0r n
Fig. IV.5.2. Generalized geologicalcross-section of the WestcrnCanadaseqrmenrarv basin.(ModifiedafterGussow.1962)
PetroleumAlteration
462
t 1 '1)/'
. . ^o^l'
3-'r'a^ ct
.
(EG RIVER OILS SEAVER LA(E
30
OIIS
3l
"API
specilic 0ratity gravityof KegRiver gasroil ratio(GOR)andspecific between Fig.IV.5.3. Correlation (AfterEvans et al.. 1971) basin. Canada sedimentarv Lakeoilsin Western andBeaverhill hasbeencausedby. Canada,it hasbeenarguedthat onl-vlimited deasphalting with massive hasonly beenassociated deasphalting maturation.but extensive 8as deasphalting hasproducedoils whichregionallvare injection.Suchgas-induced anomalously light, suchasDevonianBeaverhillLakeand Keg River oils (Evans West-Eastcrosssectionof this regionof geological et al.. 1971).A generalized WesternCanadais shownin FigureIV.5.2.The reefsrestupona platform.which extendsdowndiptowardthc westinto an organicfaciesof highthermalmaturity zonea regionalupdipflow of gasoccurredthrough From thishighertemperature ln rescrcontinuousporousstrata.and causedthe precipitationof asphaltenes voirs locatedin the migrationpath of the gas. The amount of asphaltenes precipitatedis corrclatablewith the amountof gas dissolvedin the oils The solutiongas:oilratio (GOR) for theseoils is a measureof the amountot gas injectedinto the reservoir,whilethe specificgravityof the oil is a measureof its thecorrelationbetweenthe GOR and asphaltene content.FigureIV.5.3presents the specificgravitvof Keg River andBeaverhillLakeoils.whicharc undersaturatedwith respectto gas.Therefrom.it canbe deducedthat therels an Inverse relationship bctweenthe gascontentand the asphaltenes. thephenomewith deasphalting. alsodiscuss Evanset al. ( 1971). in connection lt is observedin oil pttolswith a large non of the so-calledgravitysegregration. verticalextent.but wherethe temperaturedifferencebetweenthe top and the bottomof the poolis small.In suchpoolsthe crudeoil maybecomeprogressively heavierwith increasingdepth.which is of coursethe reverseof what might be containmuch expectcdfrom thermalmaturation.The top oilsin thesereseruoirs the solutionof more gasin solutionthan oil in deeperhorizons.mainlybecause volume pressure gascausesoil to increasein volume.When consideringthe
5.3Biodegradation andWaterWashing
163
relationship on a nearlvisothermalbasis.the followingcanbe deducedtheoreti_ c a l l y :w o r k d o n eb v a l a r g e rq u a n t i r vo t s o l u t i o ng a . a g a i n r It l o r v e p r ressure roughlvequalsthe work donebv a smallerquantitvof gasat a higherpreisure.I n otherwords.the quantityof gasat the top of the reservoirshouldbe higherthan t h eq u a n t i t vo f g a si n j e c t a b laet t h eb o t t o mT . h u s .r h eb u l kd e n s i t oy f t h J l i q ui do i l w i l l b e l i g h t e ra n dt h e G O R h i g h e ra t t h er o po t r h er e s e r v t r iIrn. a d c l i t i o nm. o r c asphaltenes will be precipitared at the top thanat thc bottom.Thisexplanation bv E v a n se t a i . ( 1 9 7 1 ) oaf n i n v e r s egdr a v i t lr' e l a t i o n s hi np l a r g ev c r t i c a,l, i lp o o l si . . in mostcasescertainlymoreplausiblethana processof gravitysegregati;n.It is hardto beljcvethat. if gravitysegregation is involved.lieavl,compounds(e.g.. asphaltenes) of an oil wouldsettleselectively throughthe tortuousporestructure of a reservoirand would reachthe bottomzone. A distinctionbetweengasdeasphalting and thermalmaturationis possiblc. B a i l e ve t a l . ( 1 9 7 4 )p o i n t o u t t h a t r h e r m a lm a t u r a t i o ni s r e g i o n a lw . hereas dea..phaltid nu g e t o g a r i n J e c t i , ri n ' m , ) r el o c r l i z e da. n d n e i g h b i ' r i nrgc s e r r o i r r ma),produceoil ratherthan condensates or gascondensates. Furthermore.lt ls suggested that the maturationstareof organicmatter(kerogen)in the rockscan be mcasured.ln purc carbonaterocks. the virtual absenceof kenrgenmav p r e v e nst u c ha n e s t i m a t eI .n t h c s ec a s e tsh e n a t u r eo f t h c b i t u m e n - l i k,er r q r n i c m a t t e ri t s e l f c a n b e u s e dt o d e t e r m i nteh em a t u r a t i osnt a t eB . i t u m e n - l i km c utrerwhichhasbeensubjectedlo severethermalmaturationand hasreachecl a high m a t u r i t vs t a g e( m e t a g c n e t i ci s) .l e s st h a n2 7 . s o l u b l ei n C S .a n dt h e H C r u r i oo t the insolublefractionis below0.5-1. Valucslessrhanthesc, indicrre.accorttrnq to M c A l a r va n d R o g c r s( 1 9 7 1 )t,h a t r h e b i t u m e n - l i koer g a n i cm a t t e r1 : r r p h a l r i n t , prccipitate)is producedbv thermalmetagenesis. whereashighcrvaluessuggest d e a s p h a l t i nFgu. r r h e r m o r ci t. h a sb e e nd e m o n s t r a t e( R d o g c i se t a t . . l 9 7 4 l i h a t " r e s e r v o ibr i t u m e n s "( a s p h a l t e n e p r e c i p i t a t e sf o) r m e db l , d e a s p h a l t i n gh .a v e carbonisotoperatioslike the originaloil. whercasresiduesforniedbv thermal a l t e r a t i oin. e . p 1 ' r o b i t u m ehna v es i s n i U c a n thl yc a y i erra t i o s I. t i s i t r r u c rtl h l t r h e c r i r c k r n I ocl i t r h , r nc l r r h , r hn o n J st i u r i n gt h e r m ual l t e r ; r t i , , I l f r t , ( l u ( $ i \ , , l , , n i c i r l l \ light methane.Simultaneousll'.a high nrolecularweicht residucis procluceil. enricheci in thc heavicrisotooc.
.5.3Biodegradation and Water Washing T h e m i c r o b i a la l t e r a t i o no f c r u d e o i l . i . c . . b i o d e g r a d a t i r ) n a .n d t h c a l t e r a t i o nd u c t o w a t e r w a s h i n g .i . e . . t h e r e m o v a lo f w a t c r - s o l u b l e c o m p o u n r l sa. r c c o n t m o n l v observed in oil pools. located in areas invaded bv surfacc-tlerivctl.metcorii tormation waters. Both alteration processesare not necessarilvcouplecl.but in e s s e n c ct h e v a r e f r c q u e n t l yo b s e r v e di n c o m b i n a t i o n .I ' h i si s n o t s u r p r i s i n ga. s both processesare initiated b\' the action of moving sub-surfacewatcr. In caseof biodegradation. meteoric witters arc thought to carrv dissolved oxvgen and microorganismsinto the reseruoir and bring them in contact with the ojl-water interface.Biodegradation of crudc oil is a selectiveutilization of certain tvpes of
464
Petroleum Alteration
It is apparentlystartedunder aerobiccondihydrocarbons by microorganisms. with washing. formation waters undersaturated tions. In the caseof water soluble apparently extract oil-water interface, movingalongthe hydrocarbons, hydrocarbonsselectively,and thus changethe chemicalcompositionof the remainingoil. The different effectsof biodegradationupon crudeoils are well documentedin theliterature(Evanset al., 1971; Baileyet al., 1973a; Derooet al.. 1974;Connan in the"tar arecommonandconclusive et al., 1975).The effectsof biodegradation mats" that are so widely developedat the oil-water contactsof producingoil fields, e.g.. the Burgan field in Kuwait. Biodegradationby aerobicand/or resultsin partial or total removalof n-alkanes'of anaerobicmicroorganisms and aromatics. slightlybranchedalkanesand possiblyof low-ringcycloalkanes and differentdurationleavecrudeoils Different intensitiesof biodegradation exhibitinga differentdcgreeof alteration.Bacteriaintroducedinto an oil pool with oxygen-richmeteoricwaters,apparentlyutilizethis dissolvedoxygenand condiUnderanaerobic preferentially certaintypesof hydrocarbons. metabolize tions,the oxygensupplyof bacteriais probablyderivedfrom dissolvedsulfate by bacteriaseemsto occurroughlyin removalof hydrocarbons ions.The selective the followingsequence:n-alkanes(belownC2.),isoprenoidalkanes,low-ring that in cycloalkanes andaromatics.Recently,someevidencehasbeenpresented as compared are removed, tetracyclic steranes even casesof severedegradation trendof crude triterpanes(Alimi, 1977;Reed,1977).The genegal to pentacyclic Figure IV.2.3. For diagram in in the triangular can be shown oil biodegradation Africa oils from West crude of the Cretaceous the chemical composition example, is shiftedaway from the n*iso-alkanecorner, showinga depletionin these compounds. age from crudeoils of Tertiary to Cretaceous Philippi (1977),investigating California,EastTexasand Louisiana,USA, presentedevidencethat microbial dueto selective opticalactivityamongcrudeoil hydrocarbons degradation causes microbialdigestionof opticalantipodespresentin the primaryparaffiniccrude oils. The optical activity was observedin narrow boiling point fractionsof boilingbetween80 and325"C. Thisboilingrangeis well saturatedhydrocarbons below the molecularweight rangeof tetracyclicand pentacyclicsteranesand triterpanes.which are known frequentlyto exhibitopticalactivityas a feature by The opticalactivity,as described inheritedfrom their primarybiosvnthesis. Philippi (1977). however, is interpretedas a secondaryfeature causedby of previouslynonrotating,racemicmixturesof hydrocarbons. biodegradation for the observedoptical responsible The chemicalstructureof the hydrocarbons of activityis not known.A verystrongargumentin supportof thisinterpretation in the activity microbialdigestionof opticalantipodesis the fact that the optical boiling range80-325'Cin five differentbasinswasonly observedat reservoir below66'C (150"F).Growth of bacteriamay be inhibitedabove temperatures thistemperaturerange. The fate of the microbialcell materialwith respectto the compositionof that most crudeoilsis at presentunclear.Philippi(1977)speculates biodedraded by probably digested is biodegradation formed during microbialcell material stable oil-soluble part of the more and that generations of microbes subsequent
-5.3Biodegraciation andWaterWashing
465 compounds maydissolvein thecrude,oil.Baileyet al. ( 1973b), whileconfirming jll:rgh.1lh]*.,:r)' experimenrs wrth bacterialcutruresbiodegradarron etfectsas ,h" tield. susgestthat protein-beari"s b".i.;;"].;;ir";ay ausmenrthe :::ll l"
tH;i;1.::iil::: ;:.1i.",,:T,Tl';fff ;*:lfii:.fl;il:lli"*,1*t,i:l
gradation of hvdroiarbons andincrease l" pr"p"rii."l" .;",r! ",,, Underanacrobic conditi.ns. certaint_vpc's oi.ba.i.,i"',i"." ,rr"res to sarisfv thcir demandfor oxvsen.rn: ;hisacrivitvrnal_ ::9,:"u.i:Ir.";;;;;;;;;,",n forrns(Rog11: et ar..ie72).depending upon rneseochcmicat :::::l-,]'jll9':", envrronment. In somesaltdomesassociated wiifr UioO-egiaOerr oil. elcmcntal sulfurhasbeenformedr in metal_rich """;,-;;;";;;.;ii;'01-$ii."on.,,n.. ,.uu",.. areproduced in association withaltercU crurte <_,it lConnan anJd.geuuf , f SZO): in oth-er.situations. partof sulfurextracted fo.rnr;ulfut";;),;;; wrth hydrocar_ Doitsto producesulfur_rich
oils and tars. microbialdcgradationof.crudeoils in connectionwilh invasionof ,..^Yl:i-"". meteorrcwatcrsis ampil'documented. in the literature, t;.;rr", and etfectsof troter wushitlgare lesswell described. The main ..r.,,." fir"iii, lack of fielcl
biorregradation' nnjwure, j,Ti,i;f"r"::lsrcs *a,hing. ffxlltJlJ:J:?lii':
warer witshiru nor,',rry,r,,,"rJ'iiil?J"';J;;iltr?J.TT;ff il:Xffi:,j;,J:1";
crudeoil and. althoughthe two.processes might occurindependentlv. they rre
.o,l paraltcr to "u.r,nir,.,.ilr,";;:r;;"* resurts :3i::^l,ll l" soluble :1,:, In a rcrnoval oi more hydrocarbons
fromcrudeoils.iti" inliuiouofrlotut,itit., of hvdrocarbons (Tablelil.2.2) ar. , gnoa."*rr" to. iiii. ,ur".ptiuifityto In gcnerat.tighthydrticarbon, "*rn"*'"",ifv :il::,.ilhl"S dissotved anci s e t e c t t v efl e) m ( \ e \ d fr()m
toiling aromaticslir"rr n,
Dool( -tr"nrll"troleum than heavicrhydrocarbonsLow-
aioo.-g.,.rur;;on;;;;;i_",,iiill;,lilJili;.xll,iilii,li,.1 and wherethey.areaccessible for surface_deriveo ::^!1 jld1.:. warers.Oils seeprng our ar the surfaceare alwaysbiodegraded ,;;;;;;;;;". oxidizedby inorganicoxidation.Volatilecompouno, o.Jtort ty "uuporliiln. cnnr"qr"nrty. atways heavy.enriched r" Nsir .i,rnp,i*lr'",iiru :::l^:f:.are ", a tar_like appearance. An excellenrexampleofproqressive crudeoil alterationis presented by Deroo ct r1. ( le74). basedon crubeJil samples frurn "u.,._ eiiirtlin tre western Canadasedimentarv basin.1.here,systematic changes huu. La"not r"ruaO,going rn an upward direction from normal, una^lteredllf,l"'j".p". p"ols toward "il: in more sha ow posirion.and rinatty ro seuerJyilte.ea neauyoils :::.li:l the sudace(cssthan 10um deep.). The lutr", h;;y ;ii;, or betrer,rhe :..e:l-:.r-ose sandstone rmpregnated with thcse.hea, u n it. i, t nu*n o, a,t JUur., to. ,unA.-J.n. commonoriginof theseeasrernAtbertooirs onottrei,o.iginoilirniio.,ryp.""iou. to alterationhad beenestablished on the basis"f .y;i;""il;;;;:';romarrcs and thiopheniccompounds.The warers associated *irh,th;;;;;;;ional alterarion sequence of crudeoils containprogressivel).,. lessdissolved.sotiasirnOicatlng ttre rncreasinginfluenceof mereo;icwarer. ihree diff*; ;;;: of crude oil degradarion weredescribedin this_srudy by D.roo ;i;i'Aoz+i"ur.ornpur.o to theunattered pooled oits.
FigurerV..5.4 ,i"*, il;;l
,,t tioli'"e.r.turion tn
PetroleumAlteration
466
30
ffi
?o
ffixL' o
o
-r1=
EoGERToN
,
HEAVY i: ,ll[],L orL -, rr^lJlJ*LlJ^l,J.'til,l, I h., ; q{-L ryoPo _-lllL -i-
o
30
-m
HEAVY oll-
20 o
Type b
o
l HEAVY olt
l
30 ?o
_$[lL o
Type c
o
no n,orio"e.JLno ,op,"noos E
n ollones
I
lsoprenoids
m
O t h € ,o l k o n eIsl
Cycookones
of a-alkanes.isoprenoidand other branchetlalkanesancl Fig. tV.5.1. Biodegratlation basin.(After Derooet al.. 197'1) clclo alkanesin the WesternCanadasedimentar,l
these three stagcs with respect 10 rr-alkanes.isoprenoids and other branched within the fractittn of saturated hydrocarbons.The alkanes. and c-vckr-alkanes Bellshill Lake oil is a representativcof unaltered oils. rvith n-alkanes clearly dominating ovcr all other saturated hldrocarbon compounds. The Edgertttn heavl' oil from thc Mannville Formation is characterizeclby a loss of rl-alkanes, and hence a relativc increasein isoprenoids.pristaneand phytane is observcd.In the Flat Lake oil from the Colony Formation no a-alkanesare lctt, and pristanc ancl phy-taneare thc most abunllant chain-like alkanes. Finally. in the scvcrel.v'' altered Pelican heavy oil front the Wabiskaw Formation. even pristane and ph)tane have becn completely removed b1' microorganisms. During all this sequenceof degradation.the distribution of cvcloalkaneswith I to 5 rings remlins p r a c t i c a l l vu n c h a n g e d . In western Canada therc is. akrng with increasingbacterial degradation. an increasein sulfur contcnt of crude oils. It is not vet clear whether sulfur is addedto
5.3 Biodegradation and WaterWashing
167
0 R t G l i l A0tl t spec.0L o.eoo.e5 sulphur- 1.5'/,
%,x) "r,;
GAS
<
l n c r e a si ne s o e c i l iocr a Y i t y
F i g .I \ ' . 5 . 5 . G c n c r asl c h e m oe t n t a i nc r u d eo i l a l t e r a l i opDr o c c \ s e (sA. l t c r B a i l c \c t a l . . l97l)
t h e c r u d eo i l s c l u ct o t h c a c t i o no f b a c t e r i a .o r u h c t h e r t h e s u l f u rc o m p o u n d si n c r u d eo i l s a r e i u s t s c l c c t i \ e l vl e f t o v e r . a n d t h u sr c l a t i v e l ye n r i c h e da f t c r b a c t c r i a havc climinated certain parts of thc hlilrocarbon fraction. -l he general trcnd of degradation. including bioilegradation. rvatcr washins a n d i n o r g a n i co x i d i l t i o n .i s s h o \ \ ' ni n F i g u r c I V . 2 . 3 f o r t h e c o m p o s i t i o n asl h i f to f t h c o i l s f r o m e a s t e r nA l b c r t a . C a n a d a .T h c y s t a r t a s n o r m a l o i l s o f t h e A l b e r t a svncline.and move toward thc aronatic-t\*So-cornerin thc triangular plot. I n s u m m a r v . t h c u l t i m a t e c o m p o s i t i o no l p e t r o l e u m m a v b e i n f l u c n c c c l -l'he stronglv bv alteration subsccluentto the process o1 accumulation. three maior alteration proccsscsarc thernal maturation. dcasphaltingand degradation lssociatedwith the action of surface-derivedformation u'aters.A generitlscheme o f t h c s ea l t c r a t i o np r o c e s s e (sF i g . I V . 5 . - 5 . ) h a sb e e n p r c s c n t c r b l y B a i l , : rc r i r l . ( 1 9 7 , 1 )A. n o r i g i n a lc r u c l co i l o f r e l a t i v e l l ' l o w m a t u r i t v u o u l d t h u s r e s p o n dt o thermal mrturation or deasphaltingbl bccoming lighter. It can be cicstrolcd and c o n v e r t c di n t o g a sa n t la h e a v vi n s o l u b l cr c s i d u cb y s e v e r et h e r m a lm a t u r a t i o n .I f this oil is dcgraded bv the action of formation \\'aters. it increasesin spccific gravitv and hcavy NSO compouncls:its economicvalue is dirninished.Aside lronr the ahcad) dcscribed alteration. thcrc arc still other processesthat mlv affcct 'l-here poolcd petroleum. mav bc compositional changcs inciucccib) selcctivc Iossesfrom the hvdrocarbon mixture via leakagcs. Thc leakage may be an intratbrmational featurc ol thc scalingcap rock. relatedto its grosspcrrncubility. o r i t m a v b c t h c c o n s e q u e n coef a n a t u r a lr u p t u r c o f a s e a l i n gc a p r o c k . d u e t Ou tectonic cvcnt such as faulting. Cascsof diffusional lossesand the dependence u p o n t h c p e r m e a b i l i t vo f s e a l sh a v c b c c n d c s c r i b e db v P h i l i p pe t a l . ( 1 9 6 3 )a n d S m i t hc t a l . ( 1 9 7 1) . H o r v e v e rt.h e e x i s t e n c o c f g a sf i e l d s .f o r m e c il n P a l e o z o itci m e ( e . g . . i n t h e A p p a l a c h i a nb a s i n . U S A ) . i s p r o o f o f t h c l o w e f f i c i e n c vo l t h i s p r o c e s si n g e n e r a l .
PetroleumAlteration
468
.
i
OIIGINAL
-tt
EXIREME PTESSUIE ANO lEMPENATURE
effectson chemical stagesof separation-migration Fig. 1V.5.6. Principleconsecutivc wasa slngle compositionof pooledpetrolum.In this schemethe originalaccumulation ohascfluid. (After Silverman.1965)
of the Whereaslosscsbl diffusionare gradualand incrcasewith closeness leakages' due to changes gcological duration. and to the surface. reservoir inducedby tectonicevents.may be moresevereandmoreor lessinstantaneous canbe expected of pooledpetroleums in chemicalcomposition Profoundchanges as dcscribedby Silverman(1965).For separationby separation-migration. fluid systems compositionalchanges.single-phase rnigrationwith consecutive havefirst to be convertedinto two-phasesystems,for instance,by a pressure depicted release.due to faulting.In FigureIV.5.6, this processis schematically prefcrentialmigrationof the more mobilevapor phase The with a subsequent from the liquid,whichstaysbehind.As the vapor vaporphaseis thusseparated andtemperature the reducedpressure migralesinto shallowertrappingpositions, conditionsmay causethe vapor to revert to a two-phaseaccumulationby maybe continued,andthus,in an upward Thisprocess retrogradecondensation. lighter hydrocarboncontentmay be found. direction,traps with successively in nature Thereare no recordsaboutthe frequencyof separation-migration
Sumrnarvand Conclusion
469
Summaryand Conclusion The ultimatecompositionof petroleummay be influencedstronglvbv alteratlon atter accumulation. Crudeoil alterationtendsro bringaboutchanges i-nihi character olthe oil influencing its quality and economic valui and adveisely affecting crude oil corelation studies.The most important alterationprocesses are thermal maturation, deasphaltingand degradation. Thermal maturation typically occursto pooled crude oils when increasing burial producesa rise in.reservoir temperature.With increasingtemperature and time of reslqence. crudeottshecomelighter,dueto crackingof heaviercompounds, andtheir gas content mcreases.Another relatively common alteration is deasphalting.the precipitationol asphaltenes from heavyto mediumcrudeoils by the diss;lution in the 9il_9flarge amountsof gaseousand/or other light hydrocarbons.Gas deasphaltingis difficult to distinguishfrom thermal maturatio;, be;auseoils alsobecomelighter. Biodegradationis the microbial alterationof crude oils. It is selectiveut ization of types of hydrocarbonsby microorganisms. Meteoricwarersare rhoughtto carry ::ltall the microorganisms into the reservorr.The selectiveremovalof hydroiarbonsby bacteria.seems to occur roughly in the sequence,n-alkanes,isoprenoidalkanes,low ring cyclo-alkanesand aromatics.Other iypes ol degradaiionoccur througn warer washing,oxidation.and evaporation.
Chapter6 HeavvOils andTar Sands
The terrns"heavvoils" and "tar sands"are trivial nameswhicharc not precisely manner.The termsheavvoilsandtar definedin a physical.chemicalor geological featuresas observedb_"exploration sandsarc derivedfrom phenomenological geologistsand reservoirengineersin the field and by refincrs.Although it is generally-. understood thatthc bitumcn-likeproductin tar sandsis anextra-heavy betwecna heavyoil oil. it hasto be realizcdthat thereis no clear-cutrlifference and the bitumcn-likcproductin tar sands. "heavyoil" Thereis of coursea fundamental differcnccbctwcentheexpression andtheterm"titrsand.Heav-voilsareapetroleumproductthatma\rrccurina rockthatcontains hostrock.Thc tcrm tar sanddenominates a sandysedimentary a b i t u m e n - l i k e .x t r a - h e a vovi l i n r e l a t i v e llva r g cq u a n t i t i c sA.m o n gr e f i n e ras n d resultingfrttm the chcmicalcngincersthe term tar is often usedfor a substance destructive distillationof organicmatter.It is clcarthatthe termtar in thisbookis not usedin this lattcr mcaning.but asa naturallyoccurringpetroleumproduct. tf H c a v l o i l sa n d e x t r a - h c a voyi l so f t a r s a n d sa r e .i n n r o s tc a s c st.h e r e s u l o productsthatoccurin petroleunrdegradution in rescrvoirs: theyarethusresiclual p o r o u sr o c k s( s a n d s t o n ccsa. r b o n a t e e s .t c . ) .w h e r ep e t r o l c u mh a se n t e r e db y migration.accumulatcd and becamedegradetl.Thesedegradcdproductsshow l i t t l eo r n o m o b i l i t y e . v e na t s u b s u r f a cceo n c l i t i o nds u . et o t h e i rh i g hv i s c o s i t y . -Iherefore thev are freqLtcntllnot produciblebv conventionaltechniques. Sometimcsa small anlount of hcavv oil can be produccd.with a verv low eflicienclof the primar_,reco\crvproccss(a few per cent of the oil in-place). Verl hear'1oils can onlv bc produccdbv mining the rescrvoirrock ttr using of the s o p h i s t i c a t emde t h o i l so f e n h a n c c dr c c o v e r v D . ue to the importance to the rcscrvcs of oils anrl tar sands as compared knorvn rcscrvcsof helrr'_v'. prescntl}, to research and oil. efforts are dcvoted convenlionalcrudc {reat for production.transportand refiningof heavt crudc dcvclopmcntof pmcesscs oi l s .
6.1 Definitions o f h c a v yo i l s A s i n d i c a t ebdc t o r ct h c r ci s n o f o r m a lo r w i d e l va c c e p t cdde f i n i t i o n that. beyonda viscosityof l(X) Holever. it is gencralll-considered andtar sancls. centipoises at reservoircontlitions.the oil is difficult to produceby conrmon to a densityof approximatelv techniques. The limit for productioncorresponds
6.I Definitions
171
2 0 ' A P I ( j . e . . 0 . 9 3s p e c i f i g c r a v i t y ) a, l t h o u g hr h e r ci s o n l y a g r o s sc o r r e l a t i o n betweendensityand viscosityof heavyoils,aswe shallsee'beliw(Sect.6.3). Someauthorsdifferentiatebetweenheavyoil accumulations andtar sandsbv u s i n gt h e f o l l c r w i negr i r e r i a ; H eav,v"oi I ac'<:umuIatio tts - viscositv:100to 10.000ccntipoises at reservoirconditions; s p e c i f igcr a v i t v0: . 9 3t o 1 . 0 0g c m r 1 1 0 . - 1 2t.o 2 0 . A p l ) a p p r o x i m a t e l y . The heavyoilsarestillmobilein rcservoirconditions. butthcvmavbeproduced h r c r r n v e n l i r r n l l t e c h n i qounelr) w i r hl o w p r o d u c r i orno r ea n db . e r a l l" i f i . , . n . u . Tar sands(extra-heavy oils) - viscositv:> 10.000centipoises at reservoircondjtions; - s p e c i f igcr a v i t y > : 1 . 0 0g c m - r ( < 1 0 . 1 2 .A p I ) . The oil has no mobilitv at reservoirconditions,and cannotbe oroducedbv c (' n \c n li t l n . l t e c h n i q u c . . This classification. althoughconvenicntto the reservoircngineer.is not appropriatefor the geologistor gcochemist. Viscosityis a temperature-depend c n t p a r a m e t c r( S e c t .6 . 3 . b e l o w ) .e s p c c i a l l ivn e x t r a _ h e a voyi l s . s u c h a s Athabasca oils.whcreviscosityis reducedby threcorclersof magnitudebetween 2 0 " a n d 8 0 " C .T h u s .m o s to f t h c W C a n a t l ad e p , ' s i t (sr e s c r r o i tre m p e r a t u r c 10"-20'C) would be classified astar sands.and mostof the Venezueladeposits (reservorrtemperature50"-80'C) would be hcavvoils. althoughsomeof these oils from both countrieshave comparrhlcvi.,cositlat stand-urd remperarure conditions. Extra-heavvoils can occur in tar sandsand carbonatereservoirsas well ( c a r b o n a t tcr i a n g l ei n A l b e r t a ) ,a l t h o u g ht a r s a n d sa r e q u a n t i t a t i v e lm y ore rmportant.Thc organiccontentof tar sandsis calledcxtra-hcavv oil or bitumcn. i i n da l s ot a ro r a s p h a l tA. l t h o u g ht h er c r mb r r um e ni sc o m p a t i b l e w i t h eg e n e r a l definition of bitumcn (thc extractablefraction of toial organie mattcr in s e d i m e n tCsh: a p I. L : 1 . I ) . i t i s b v n o m e a n s s p e c i f i c f o r t a r s a n d s . a s a n r . , , h e a v v o i l o r c o n v e n t i o no a il l w o u l da l s os a t i s t yt h ed e f i n i t i o nA. s p h a l st h o u t dh e r r , r i r j c d . a s t h i s t c r m i s c o m m o n l yu' s c d b v r c t i n e r st o d e s i g i r a rlen l s p h a l t c n e _ r i c h prccipitateobtainedwith a fl.-cut (instcadof usinefi -pcntaneor |l -heprane Thc ). tcrm tar is alsousedto designate a productfrom distillationof organiamarrer.as indicatedabove.Thus it seemspreferableto usethe term .'cxtra-heavv oils... l n t h ef o l l o w i n cs e c t i o nrsh et e r n th e a v vo i l sw i l l b e u s e dg e n c r i c a l livn.c l u d i n q cxtra-hcavyoils from tar sandsunlessotherwisestated. Onc particulartvpc of crudcoil. altboughviscousor solidin surfaceconditions shoulclnot be confusedwith heavvoils: theseare wax!.oils.whichare normal. non-degraded oils (Chap. IV..1.2.2);their high viscositvis due to the creat abundance of long-chain'l-alkanes.Thesemav be solidat surfacerempcrature. but thcy are usuallvdissolvedin other constituents of crude oil in reservoir conditions.Waxv oils usuallvshow a low contentof asphaltenes. The specific gravityof waxvoils is lowerthan that of heavvoils. Heavv oils mav be compareduith other conr entional-.ril-su bstltutes.such as coal liquids and shaleoil. AII rhree show a high specificgravitr,,closeto
172
Heavv0ils andTarsands
1.0 g cm r. a largecontentof sulfur anclnitrogen.and a hydrogendeficiencv. Thus.comparable problemsaremetin refiningthescproducts.Coalliquids.however. are closerto heavyoils than shaleoil as they containa large proportion of asphaltene-like material(althoughprobablynot identical)and arehighlv viscous.Shaleoil containsmore alkanesthan the otherand alsoalkenes.which arespecificofa hightemperature crackingofthekerogen;theyarefrequentlyless viscous.exceptsomewaxyshaleoils.
6.2 Compositionof HeavyOils Like all typesof petroleunr,heavyoils are madeof hydrocarbons, resinsand asphaltcnes. Howevcr,theproportionsof thesevariousconstituents arediffercnt i n h e a v vo i l s( T a b l eI V . 6 .l ) a n di n c o n v e n t i o noaill s( T a b l eI V . l . I ) . H c a v yo i l s containlesshvdrocarbons. especiallv alkanes,moresulfuraromaticcompounds. resinsandasphaltenes. In turn, thisdifferencecontrolsthe specificchemicaland physicalpropertiesof heavyoils. a) Saturatedhydrocarbons usuallvamountto lessthan 25%. with an average value of 16%. comparedto 57c/c in normal crude oils: heavv oils are particularlydepletedin l+iso alkaneswhichare commonlylessthan5% I b) aromatichvdrocarbons and bcnzothiophene derivatives frequentlvrcpresent 25 35% of the heavvoils.with an average of 307r: thistigureis comparable to TableIV.6.l. Grosscomposition of someheawoils H\ Llrucarhon\(q r. ^, )
Number of Sample \
Asphaltcnes Resins (wt.%) ( w t . . 7)
Totrl
Aromrtic
SJtur.lted
Athabasca
23.3
28.6
llr l
32.2
15.9
15
Wabascr
2l.6
30.6
1'7 .'7
-12.I
15.6
7
PeaceRiver
.13.7
23.2
2 8 r.
20.s
7. 6
-1
Cold Lake
20.t)
21t.0
51..1 30.5
20.9
7
E. Venezuela TarSands Heavvoils
22.l 12.6
37.6 32..1
10.3 26.(l 55.0 36.4
14.3 1ti.6
I 5
Average on '16heavv oils
22.9
10.6
46.5
30..1
16.1
.16
85.8
2i1.6
57.2
517
Loc.rtions
oilsof various ongln ( T a b l cI V . l . l )
1,1.2
6.2Composition of tleavyOils
4: 3
the averagevalueof 29% in normalcrudeoils; howeverthe proportionof benzothiophene derivativesis comparatively moreimportantin hiavy oils; c) resinsplusasphaltenes rangefrom 25to70Va:lhey commonlyamounrto more than ,107c of the heavyoils from W. Canadaand E. Venezuela,wherethey average 5,17o: thesefigureshaveto becomparedwithan average valueof 147o in convcntionaloils; rcsinsare frequentlyin the 25 357c iange. whereas asphaltenes mayvaryfrom 10to 507. of the heavyoils.Theasohaltenes/resins r a t i oi s l o wi n c a s t e r n V e n e z u e ldae p t , s i t1sa rer a g e0 . 3t t )0 . 7) . i n t e r m e d i a itne Athabasca.Wabascaand Cold Lake tleposits(0.-lto 1.0),whereasit ma1 occasionallv reachhigh values.suchas2 in the peaceRiver accumulatron. The H/C ratio of the resinsand asphaltcnes lrom heavvoils is comparable to thatofthe sameconstituents in normalcrudeoils.Ijowever.the largeabundancc o l r e s i na s n da s p h a l t e nceasu s eash rr l r . ) gned c f i c i c n ( o) f h e a v lo i l s ,a sc o m p a r e d to normalcrudeoils. In WesternCanada.the atomicH/C ratio of the heavilv dcgradcd A t h a b a s coai l i s 1 . , 1 t5o 1 . 5 0 t. o b e c o m p a r e w d i t h H / C - 1 . 7i n r h e L l o v d m i n s t eori l . w h i c hi s s o m e w h abr i o t l e g r a d e a dn . t l L 8 i n n o r m a lc r u d e s . Similartiguresare measuredon the heavyoils from EasternVcnezuela. The sulfur.nitrogenand metalcontentsof the hcavvoils deDendnot onlv on t h c b u l k c o r n p o . i t i , r(n\ t t u r n l c da n t la r o m a t i ch l d r , , c r r r h o nrre. s i n lsn d a s p h : r l tencs). which is controllcd by the degreeof dcgradation.but also on the a b u n d a n cocf t h c c l e m e n t(sS ,N , V . N i ) i n t h eo r i g i n a ln. o n - d e g r a d e c rdu d eo i l . The.rufurcontentof theheavvoilsis,on theaverage. higherthanin therelated normal crude oils, as the benzothiophene dcrivatives.resin and asphaltene fractionsare moreabundantin heavyandextra-heavy oils. In WesternCanada, for instance.thercis a stronglinearcorrelationbetweenthe sulfurcontentof the heavyoilsfrom LowerCretaceous reservoirs andtheir percentage of aromatic+ resin+ asphaltene fractions(Fig. IV.6.1). In EasternVenezuela, a comparable
6! 70 Fig. IV.6.1. Sulfurcontentof someheavyoils from W. Canadaas a function of combined aromatic.resinand asphaltene contents(Deroo e t a l . .1 9 7 7 )
4'71
Heavy(JilsandTarsands
gradationis observedfrom normalcrudeoils containingl7o S or lcss.to heavy oilswith '10-507oresinsplusasphaltenes and3-5 % sulfur.andfinallvextra-heavy oilswith 50 70% resinsplusasphaltenes and5-10% S. Sulfurconcentrations of up to 5-6% are rather commonlvobservedin heavv oil provinces.such as Athabascaand other krcationsin W. Canada.or Boscanin Vcnczucla.Higher amountsol sulfur are lessfrequent.but concentrations up to l0% havebeen reported.It is not yet clearwhetherthissulfurenrichmentis onlv a concentration process(bv selective destruction ol thecompounds conrliningn.rsulfur).or a nct uptakeof sulfurfrom the environment. llowever. sonrcparaffiniclow-sulfurcrudcoils.oncedegraded.mavresultin relativelylorv-sulfurheavyoils becausebenzothiophene derivativesare scarce andthc sulfurcontentof resinsandasphaltenes is low: thc Crctaccous heavvoils o f E m e r a u d eC. o n g ow i t h o n l y 0 . . 1t o 0 . 8 % S ( C I a r e e t t a . . 1 9 7 7 )t:h e l - o w c r T e r t i a r yA s p h a l R t i d g ea n dS u n n v s i dt e a rs a n d sU. t a h .w i t h0 . 5% S .t h eP l i o c e n c D e r n ah e a v vo i l i n R u m a n i aw i t h 0 . 7 t i S . The nitrogencontcntof heavyoils alsodependson the resinplus asphaltene content.andpossiblymoreon the composition of the originaloil: Athabasca tar sands,althoughheavilvdegraded. containonlv{).,1?iN. whereas somcCalifornia h e a v vo i i sr c a c h0 . 8 ? . a n dt h e W h i t e r o c kC s a n y o na s p h a l tU. t a h ,L 2 , 1 % . The occurrcnceof neruls(V. Ni) is alsorelatedto the resinand asphaltene contentas thesefractionscontainboth porphyrin-boundand non-porphvrinboundmetals.Their abundance. howevcr,is againinfluencedby theoriginaltvpe of crude oil which is probablyan importantfactor. For instance,the highest concentration of metals( 1200ppm V, 150ppm Ni) is reportedfrom the Boscan. WesternVenezuela. heavycrudeoil whichis neitherthe heaviest crudc(11.API ) nor the mostviscous(.100centipoises at 80'C in rescrvoirconditions).But the related normal oils (20 30" API) from WesternVenezuelaalreadycontain 100 300ppm V and 1(130ppm Ni. The Athabascatar sands.which are extraheavvoils. containonlv 300ppm V and 80 ppm Ni, as they are derivedfrom normal l-owcr Cretaceous oils containingonly l0-20 ppm V and ca. 5 ppm Ni ( F i g .I V . 1 . 2 1 ) . In termsof crudeoil classiJicutiott (sceabove.Chap.IV.2). heavyoilsbelongto the aromatic-naphthenic and aromatic-asphaltic classes: their distributionbetweenthe two classes dependson the originaltypeof oit, andon the natureand e x t e n to f d e g r a d a t i o(nF i g s .I V . 2 . 3a n dI V . 6 . 2 ) : - aromaticintermediateconventionaloils. when desraded.are convcrtedto aromatic-asphaltic heavvoils: for insrancethc Mirsissippian and Mannville (EarlvCretaceous) normaloilsof the Albertasynclinebelongto the aromaticintermediateclass.and the relatedheavl'oils and tar sandsfrom Athabasca, etc. bebng to the aromatic-asphaltic class; paraffinicor paraflinic-naphthenic conventional oils are convertcdby biodegradationinto aromatic-naphthenic heavyoils;for instance,the Early Cretaceousoilstiom WestAfrica are changedby biodegradation into the aromaticnaphthenicheavyoil of Emcraude.If degradation is moreadvancedand also incJudes inorganicoxidationprocesses. the sametypesof conventional oilscan be further alteredinto aromatic-asphaltic heavv oils. such as the Orinoco HeavyOil Belt in EasternVenezuela.
6.1 SpecificGravitvand Viscosity.
175
AromaricHC
ljig lV 6.2. r'crnarl'criagram sh.wing the {rosscompositionof someheavvorls from rariousorigins
6.3 SpecificGravityand Viscosity Phvsicalproperties.ofhcavvoils are largelycontrolledbv the abundance and phvsicalstateof resinsand asphaltenes. o l h c a v lo i l si r h i g ha r r dr a n g c sl r o mn q l g c m ( 2 0 . A p t ) r . r , !,eclli fruyirr 1 0 6g c m ' ( 2" A P I ) . H o w e v e rt,h el a r g e sarc c u m u r a t i ocnosr ; e \ p o n d totarsands w i t ha s p e c i l igc r a v i t yc l o s et o t . 0 g c m r ( 6 " - l 2 . A p l ) : A t h a b a s c a ( 6 . _ l 0 " A p l ) . W a b a s c (a1 0 " - 1 3 "A P I ) . C o l d L a k e ( 1 0 " _ 1 2A . p l ) i n W e s t c r nC a n a d aC ; erro N e g r o - M o r i c h a l - J o(b8o. 1 2 " A p l ) i n E a s t e r nV e n e z u e l aA: s p h a l t R i d g ea n d S u n n v s i d(e8 " - r 2 " A p r ) . i n u t a h . e r c .S o m eh c a v yo i r s- n o t c l a s s i f i e d a st a r sands-_ may reach comparablyhigh specificgravities,but they are still produciblebecauscthe in-situvis-cosity is low, clueto a high reser\oir tempcra_ ture. For instance.the Boscanfielclin WcsternVenezu-ela showsa t I" Apl gravity. is only 400 centipoises at reseruoirtemperatureof g0.C -b-ut.viscositv ( d e p t h 2: 3 0 0m ) . Vlsrcsityis probablythe mostimportantphysicalparameterof heav), oils,as ^ tar as productionand transportare concerned.Like specificgravitv, viscositvis intluencedbv the abundanceof resjnsand asphaltenes. Visiositv.t u*.r... i, a l s oi n f l u e n c ebdy t h ep h v s i c aslt a l eo t a s p h a l t e nicnsc r u d co,i l s .i . e . , t h es i z ea n d structureof the miccllesforrncdbv interactionwith resinsand aromatics. This a,spect is discussed in ChapterIV.l.g. ln heavydcgradedoils, there may be a shortaseof resils and aromaticsand a surplusoi asphaltcnes due to in-situ degradation. oncethc oil hasmigrrt.-dint,r thi resenuir. Thusasphaltencs areno n r o r el u l h d i s p c r s c d . a tnhJr r m r r rl o r r n1 , r * " - . r a dJ g A r e B i r t Icn\ . t r r r n .l h i \
\ r I l U ' | n r \ r e \ p r ' n \ l h l e l o r a n i l l c r c a s eo l \ i \ C o s t t v .
476
Heavl C)ilsand Tar sunds
on viscosityis the rcasonfor thestrong The influenceof rcsinsandasphaltenes viscositv-density correlation(linearcorrelationcoefficientQ = + 0.823in Table diagram IV.1.11).Thus a continuoustrend is observedin the viscosity-density (Fig. IV.6.3): at 50'C viscosityincreases progressively from ca. 100ccntipoises oil in a centipoises for a 6'-8'API extra-heavy for a 20" API heavyoil to 100.000 tar sand.However.thereis somescatteraroundthatgeneraltrcnd,astheremav be additionalcausesof densityor viscosityvariations.other than thc variations relatedto the resinplus asphaltene content.In particularthe scattersccmsto increase towardlow tempcratureandhighspecificgravity(lou'API): rt reservoir temperatureof 10 15'C. the viscositvof W. Canadaheavvoils in the rangeof 8'l0'APImaysplitovertwoordersofmagnitude.ie.,5.l0rto5.I0'cenripoises is knownfor a Viscosityis highlydependcnton temperatureithisrelationship long time bv refinerswho use a specialchart of predictingthe viscosityof petroleumproductsat differenttemperatures. Furthermore.it hasbeenobserved that crudeoils.includingheavyoils,broadlyfollow a similartrend(Fig.IV.6.4). The main featureshownhy the diagramis a strongreductionof the viscositv with an increase in temperature.Furthernore,the moreviscousandthe heavier whengoingfrom theoil is, the moreimportantwill bethe reduction.For instance, 20"C to 75'C the viscositvof an "Arabianlight" crudeis reducedby factorof3 whereasthe viscosityof the Lloydminsterheavvoil (15'API) is reducedby a oil (8' API) is reducedbv a factorof 30 and that of an Athabascaextra-heavy factorof 1000. The strongviscositydecreasewith temperature.as seenfor the heavyoils, oilsbased meansthata designation of the varioustypesof heavyandcxtra-heavy for geological or geochemical on viscositv in reservoirconditionsis not applicable
? l
B 3 l
F
I
b
lr
.=
.2 > l
Fig. 1V.6.3. Relationshipbetween specificgravity(or API density)and viscosit)' of someheavyoils.R + A designatesthe resinsplus asphaltenesconlent
6.4 Originand Occurrence of HeavyOils
477 Fig. iV.6.4 Changesof viscositvof someheavyoilsasa functionoftemper_ r l u r e ( k i n e m a t j vc i s c o s i t iys l h e r a l i o absoluteviscosity/specific gravity. at a giventemperature)
' - - \ ---
ly.Catrada Venezuota
T e n p e r a t uIr' e Cl
studres:somcheavy7'-10. ApI crudesfrom EasternVenezuelashowviscosity curvesintermediatebetweenthc 8. ApI Athabascaand the 10"_12.Apl Colcl Lakecurves.However,the two Canadianheavyoilshavereservoirremperarures of 1t)' 15'C, whereasthe Venezuelan reservoirs exhibithighertemperatures of about 50"-60'C. Thereforetheir in-situviscosities are in ihe rangeof 105_107 centipoises.and 10r 10acentipoisesrespectively;thus the Canadiandeposits ,'heavy wouldbecalled"tar sands''andthe Venezuelan oils",althoughtheyirein certaln respectscomparable.If we now wish to decide to use a standard temperaturereferencefor viscosity,suchas50"C (122.F),FigureIV.6.3 shows what could be a proper definitionfor heavyoils and extra-ieavyoils (or tar sands). 'lhe significancc of the abruptdecrease in viscosityof extra-heavy oils with temperaturehas slill to be investigated. Such a dramaticchangesuggests a rearrangement of the macrostructure of the high molecularweightaggregates, i.e.. a changein the physicalstateofasphaltenes in crudeoit lfissot, tltf;. A final remarkconcernsthe variabilityol physicalpropcrtiei,suchasspecific gravityand viscositvwithin the hugedepositsof WesternCanadaand Eistern Venezuela. The volurneof theseaccumulations is enormous(in the rangeof 40to lJ00x I0qm3of rcservoirfor eachdeposit).andthe heavyoilso, extra-iheavy oils havepracticallvno mobility. It is thus obviousthat there mav be substjntial changes in chemicalcomposition. densityandviscosity withineachaccumulation. This.maybe the reasonwhysomanydifferentvaluesofdensity,viscosrry, erc.are cited in the literutureconcerningIarge-size accumulations, iuch as Athabasca, P c a c eR i v e r .o r t h e O r i n o c oB e l l .
6.4 Origin and Occumence of HeavvOils Although some accumulationsof immature, non-degraded,heavy oils are known, they probablyamount to lessthan 1% of th; world reservesof this
HeavY OilsandTarsands
478
(resins category.Theseimmatureheavvoilscontainabundantheterocompounds resultingfrom an early breakdownof kerogen' with the and asphaltenes) heavyoils associatcd highsulfurandnitrogencontent.Theydifferfrom degraded by the occurrenccof n-alkaneswhichhavenot beenbacteriallydestroyed.This area. situationis found. for instance.in somecrudeoils of the Mediterranean of theseheavyimmatureoils mayoccur conditionsfor accumulation Favourable where a particularsourcerock also acts as h reservoir,or where migrrltion condition;are particularlyeasy(intenibfracturation,karst.etc.). Most heavy oils originatefrom nortlal, fluid crude oils which have been degradedin the reservoir\by one or severalof the following subsequently waterwashing,lossof volatiles.inorganicoxidation processes: biodegradation, of thelightendsof the resultin a decrease (seeabove.Scct.5.3).l heseprocesses andan weight alkylbenzenes. crudeoil. andalsoof the alkanesandlow molecular polyaromatics' reslns derivatives. increase of the moreresistantbenzothiophene from extracted sulfur conditions. at certain Furthermore. and asphaltenes. the sullur increase to react with hydrocarbons may sulfateby anaerobicbacteria c ( , n l e nlln d l h e a h u n t l a n cuef h e a v !c o n s t i t u c n l \ . 'l'he The result of this type of alterationis a heavy. highly viscousoil. dcgradationis commonly associatedwith the invasion of surface-derived. with meteoricformationwaters.Thus the extentof degradationis associated parameters suchasdepth.proximityto aerialcontact.and salinityol formation of heavy waters.Excellentexamplesarc providedby the two major Provinces oils. i.e.. WesternCanadaand EasternVenezuela. heavyoil of the Alberta basinshowingthe Early Clretaceous A cross-scction corrcsponding depositsof WcsternCanaciais prescntedin Figure1V.6.5.l'he '[ of the oilsareshownin FigureIV 5.4. he normalcrude of composition changes oils of the Earll Cretaceousin the Albcrta synclincare reprcsentedby the with a water BellshillLakc. a 26" API normallyproduciblecrudeoil. associated r. salinitv> 80 g I Closerto thc outcrops.the Llovdminsterareacontainsheavv with water at 25'C) associated centipoises oils (densityl6' API: viscosity2(XX) " r and800()0 I I API salinitiesof 5(I-80g I : then thc G)ld Lakc depositrcaches
w
shield Precambrian
Malorunconlrmity 0 0
50 l00Kn 50
t00Miles
of WesternCanadabasinshowingheavy-oilsands' cross-section Fig. IV. 6.5. Schematic (Adaptedwith gravity of the variousaccumulations. API rangc of Nambersindicatcthe from Demaison.1977) changes
6.1Originancl(lccurrence of I leavtOils
479
centipoises at l5'C. associated with watcrsalinitieslowerthan 50 g I r. Finally, the outcroppingAthabascatar sands contain an extra-heavyoil (densitv 6-8 " API..viscositv 2. 10"centipoises at 10"C) associated with low salinitywaters ( < 2 { )g l ' ) . Derooet al. (1971.1971)haveshownthat the mainsourcerocksof thenormal crudcoil entrappedin EarlyCretaceous sandsare the associated shales.Due to the closerelationship andthc progressive changefrom thesenormaloilsto heavv oils. they infcrredthat the samesourcewasan importantcontributorto heavy oils. However,thcre may be somecontributionalso from thc Paleozoicand Jurassic shaleswhichare presentbasinward.Hacqucbard(in Derooct al.. 1977) hasshownthat catagenesis of the Cretaccous sourcebedsis reachedbasinward from all major hcavyoii accumulations. If this generalconceptis acceptedthe distancesof secondarvmigrationhave to be in the rangeof tens to scveral h u n d r e do f k i l o m e t e r sD: e m a i s o n( 1 9 7 7 )T . h i s a u t h o rp o i n t e do u t r h a t b o t h stratigraphic features.suchassandstone distribution.andstructuralfactors.such a st h e P e a c eR i v e ra r c h .h a v ei n f l u e n c eedn t r a p m e notf t h eo i l s .T h u sa n o r m a l . nredium-gravitv oil hasprobablymigratedinto the sandslone reservoirs: later.or simultaneouslv. the oils came into contactwith meteoricwatersenteringthe 'Ihis rcsctvoirsfrom the outcrops.and were subscquently degraded. view is suPportedby geochcmicalinvestigations. which have proved a step by step degradation.ciecrclsinrbasinrvardanrl also with the incrcasingsalinity of f o r m a t i o nw a t e r s( D e r o oe t a l . , 1 9 7 ; 11.9 7 7 ) . In EasternVcnczuela.the gcncralsituationis rathersimilarto that in Western ( ' a n a d a( F i g . I V . 6 . 6 ) : a f o r c d c e pb a s i n .w i t h a h e a v i l vf o l d e da n d f a u l t e c i northernflank and a gentlvdippingsouthernllank. containsnormaland heavy oils in Cretiiceous (OtTicina) and Oligocenc-Miocene sandstones. The trapsare fornreilbv a contbinationof normal faults and sitndstonepinchouts.The oil a c c u m u l a t i olnosc i r t c a d t d e p t h sg r c a t etrh a n1 5 0 0m a r en o r m a lm . e d i u ntto l i g h t ( : 5 . { 0 " A P I ) . l o w , s u l f u cr r u d e s( ( l % S ) w i r h a l o w c o n t e n to f r e s i n sa n d asphaltenes: southwarcls. rvhenapproachingthe border of the basin.thc oils ( T e r n b l a d oN r .I e r e vb) e c o n tpc r o g r e s s i v cslhy a l l o w e( rc a .1 2 0 ( ! 1 - 5 0 m0) . h e a v i c r
25"4t r0{25' <1o"Apl oficina {proi,cred,
0-.______4__-__!0Km q 25 sPMires Fig-1V.6.6. Schcmatic cross-section of EasternVenezuela basinshowingheavy-oilsands (Orinoco l]eavv C)ilBelt). Nrorrbers indicatethe raneeof API gravityof the various (Adrpted $i1h changes accunrulatioos from Denaison.1977)
480
HeavyOilsandTar sands
(with 2G407cresinsplusasphaltenes. andbiodegraded a densityof l0-25" API and 1-3Vcsulfur)l finally they gradeinto the OrinocoHeavy Oil Belt (Cerro Negro,El Pao,Hamaca.etc.).a giganticaccumulation madeofheavilydegraded oils:more than507ois madeup of resinsand asphaltenes, API densityis 10'or lower.sulfurreaches+576 or evenmore. The sourcerocks for normal oils are found in the Cretaceousand OligoMiocenebeds.However,it is not yet established whichonewasthe majorsource for the heavyoils. Demaison(1977)locatedthe maturationareaof the source rocksandshowed,thatit is basinward andcompletely outsidetheareaofheavyoil occurrences. He calculateda minimumdistanceof migrationof about 100km. Once migratedinto the reservoir,the normal, medium-gravity oil cameinto contactwith meteoricwatersenteringthe sandstones from the outcrops.in the Orinocoarea.The oil wassubsequently degradedand the locationscloseto the outcropswere more heavilyaltered.Furthermoreit wasobservedby Hedberg (1947)andothersthat, in somelocations,light waxycrudesin the upperpart of the Oligo-Miocenebedsoverlieheavyoils in the lower part. Demaison(1977) suggested thatthissituationis explainedby an easieraccess of meteoricwatersin the moresandylower paft of the stratigraphic column.
6.5 World Reserves and GeologicalSetting The presentestimateof the worldreserves of heavyoilsis shownin TableIV.6.2. All figuresare relatedto oil in-placeas the conceptof recoverable reserues is continuouslychanged,due to the advancement of technologies for producing thesenonconventional crudeoils. The tual figure- 450to 1000billion mr - is comparable to the total ln-l1dce quantitvof conventional discovered crudeoil. whichcanbe estimatedto ca.600 billion mr (includingthe oil alreadyexploited,but excludingnon-discovered potentialrcsources; assuming a recoverable/in-place oil ratio of0.25). The recoverable fractionof the hcavyoilswill dependon the developments of technologics for production.For instance.out of the 220billion m' of heavyoils presentin the Crctaccoussandsof WesternCanada.ca. 6 billion m1 could p r o h a b l lh e r c c o v e r c d b v \ u r f a c em i n i n ga n dc a . - 1 0h i l l i o nm - h v r n - s i t us t e a m f l o o d i n gi,. e . , 2 5 7 co f t h e t o t a ld e p o s i tT. h e b a l a n c ec.a . 1 7 0b i l l i o nm 1 .i 5n o t recoverableby existingtechnologies.but what fraction could be added by dcvelopingnew methodsis yet unknown. The moststrikingfeaturewith respectto occurrence of heavvoilsandtar sands is the abnormallyhigh quantityof in-placeoil in the major deposits. This aspect wasdiscussed by Demaison(1977).In particular,the evaluations of the in-place oil in the singleAthabascaamountto 140billionmr, i. e., threetimesthein-place oil of thelargestof all conventional oilfields.Ghawar.SaudiArabia.Comparable situations probablyoccurwith respectto the OrinocoHeavyOil Belt. but precise definitionof the individualdepositshasstill to be improved.The total in-place reservesof the Middle East province(ca. 300billion mr), however.are in the
6.5 World Reserves and GeologicalSerhng T a b l el V 6 2 P r i n c i p ai nl - p l a crec 5 c r ! e s oht e a v y o i l \ o f r h ew o r l d . {oapreo$ . l t h c h i r n g ef.r. o m D e m a i \ o nI.9 7 7 :B o y d e l a T o u re t a l . 1981 Country
Age of reservoir Volumein_place (billionmr)
\ e n e z u e l aO r i n o c o
Oligo-Miocene 150ro 500 Cretaceous
Cunadl
Athabasca Cold Lake Wabasca PcaceRiver
Eariy Cretaceous
Canada
Carbtxate triangle ( A I b e r t )a
Paleozoic
U .S .S .R .
Melekess (Volta,Ural) Others 'far Triangle LIintaBasin Others
Permian
U . S ,A .
Others
220
0 to 200
20
Paleozoic
-1.3
Permian Eocene
2.5 L 7 5.5 50to 70
fotal
ca. 450to 1000
rhein-place heavy
oitsof westernCanada (creta::::"^:1:l ljjlli,-l_",t.'.,", u":l*n *Ti:,:"'l;:r"o:l]1,":l ven€zu;ra (Is0 t. lui uilii-.tt. T_.t,T immediately u.ir",*nf a'"" jT:".::-*ions
tr,"."'iili""'
pr.."i"*, H,l,:. e^t",n v"n.,uJr,; g"*dl: ;" ;;il i;i,, oij'ii,'J'""ii lyl*i: "r.:it. I::l::,1 in a.relativelysmallareaof the basin?Why ";.
..;"1;;;;;i 1:fTllil: andprobabty partsof rheOrin,i.on.ttl ,o r,ug","u.n (A,habasca ::'..:ll,]1lir"r u h c nc o m p a r e tdo giant the
oil fields? Thesefeaturesrequirein fact severalconditions: unusualexpulsionfrom the maturesourcerocks and efficientsealing, Iongdistancemigration. - wide drainageareaof some individualdeposits. Demaison(1977)pointedout thar.,foreland.,, or ,,foredeep,,basins (Halbouty, 1970;Klemme, 1975;Ballv. 1975)werean excellent situationin thoserespects. They freq.uentlyoffer a largevolumeof mature sourcero"t.. if,in, ,,r. .*or"na. homoc]inal.slope.^with a tremendous drainage u."u. unO'ii,"u."ur..n.. ::l\i'11: ot malor u-nconformity surfacesasconductors1e.g..itrepr._Ci",u."ou, un"on_ tormityin WesternCanada)fayor.along_dlstance migration,especially from the clceppart of the basin.In fact al rhreemajor oir provincesmentionedabove
,182
tleav_v Oils itndTar sands
(Middle East. Western Canada.EasternVenezucla)fit into that schen]e. and also the Volga-Ural basin (USSR). where the Melckess clepositsranks immediatelr b e h i n d V c n e z u e l aa n d C a n a d a( T a b l e I V . 6 . 2 ) . A wide drainage area fbr an individual deposit is iln essentiitlcondition for g a t h e r i n ga h u g e a m o u n t o f o i l i n t o a s i n g l ea c c u m u l a t i o nD . e m a i s o n( 1 9 7 7 ) pointed out that a paleodeltasvstemcomprisingfar-reaching,efficiertlv interfingering carrier sandstoncsis lhe iileal sctting in that respccl. A combinurion of foredeep basin and paleodeltas!stcm is very probabh, the cause for thc major hcavv oil accumulationsdiscussetlhcre. F i n a l i v .d e g r a d a t i o ni t s c l f s e e m st o b e a n i n t p o r t a n t .i l n o r t h e n t o s ti m p o r t a n t . factor for trapping verv larqe quantitiesof oil. The generallavout of thc foredeep basinsjs also favorablc to the eDtr\'of mctcoric $'aters.lront thc outcrops. alonp u n c o n t o r m i t vs u r f a c e sb. a s a ls a n d s t o n e se.t c . I n t h e a b s c n c eo f d e g r a d a t i o nt.h c b u l k o f t h e n o r m a l f l u i d o i l s * h e n m i g r a r i n go n a w i d c . f e a t u r e l e shso m o c l i n a l slopc would reach thc outcr()ps iind would tre lost. In,\itu ilegraclationrvould s u p r e s sm o b i l i t vo f t h e o i l a n d p r c v e n ti t f r o n l c s c a p i n st:h i s i s i n s u r e dn o t o n l v b r a n " a s p h a l tp l u g " b u t r n o r ee f l i c i c n t l yb r i n c r e a s i n gv i s c t ) s i t o vfthu hulk,rt'the d c p o s i t .u n t i l i t c a nn o l o n g e rn r o v e .T h u s .i n a s e n s ch c a v vo i l sp r o v i d ct h e i r o u n s e a l . T h i s p a r t i c u l a r a s p c c t \ \ ' o u l d c x ; r l a i n w h v n o s i n g l c a c c u m u l a l i o no l c o n v e n t i o n aol i l c o r n p a r a b l e i n s i z et o A t h a b a s c ao r O r i n o c u c l e p o s i tiss k n o w n . D e g r a d a t i o nt n d a b s e n c eo f n r o b i l i vr * o u l t 1b e a r c q u i s i t ef o r s u c hh u g cd c p o s i t s .
6.6 Valorizationof Heavl'Oils 6.6.I Produ(tion Heavv oils still mobile under rcservoirconditionscan onlv be producedbv conventional methodsat a k)wrate.with a vervlow efficicnc\': therecovervfactor is only a few percent.Extra heavvoils. immobileunder rcscrvoirconditions. cannotbc producedbv convcntionaltechniques.'l'hus. ex-situminingmethods and in-situenhancedrecovervtechnologies are ncccsslrvto mobilizethe major 'l'he fractionof heavvoil rcscrves. methodsare usualiybascdon the ilrastic viscositvreductionin responscto heatingup thc heavvoil. Athabascaoil sandsare mined. in open pits, in two locationsnear Fort M c M u r r a y ,A l b c r t a . ' l h e s a n di s s u b s e q u e n tsl )c' p a r a t e fdr o m t h e h c a v yo i l bv usingan alkalinehot water process.The total productionis presentlyca. E m i l l i o nm ' y e a r ' . Heavvoils with viscositics rangingtrom llX) to 25000centipoiscs il rcservorr conditionsare produceci bv cnhancedreco!crvtechniques. usingin,situ.cvclicor c o n t i n u o u ss.t e a mi n j e c t i o na. t a n a n n u a rl a t eo f c a . 2 5 m i l l i o nn r ' ( m a i n l r i n Californiaand W. Venezuela).Steam(tempcratureca. 300'C. prcssureca. l0() bar) is injcctedthroughconventional wells;the heat is partlvtransferredto the heavvoil (includingthe heat of stcamcondensation) and the viscositvsubsequcntlvdecreascs. Then oil can bc pumpedeitherthroughseparitteproducinS w c l l so r t h r o u g ht h c i n j e c t i o nw c l l . b v p e r i o d i c a l lavl t e r n a t i n ign j e c t i o na n d
6.6Valorization of HeavvOils
483
production.The processseemsto be presentlyapplicableat depthsshallower than 1(XX) m. providedpermeabilities are sufficient.It is not clearwhetherthe processwould be also applicableto extraheavyoils, with an in-situviscosity beyondI 00.(XX) ccntipoises. A iarge-scale projectin the ColdLakeareahasbeen announced: the in-situviscosityof thisdcpositseemsto be somewhatlowerthan in Athabasca.Wasbasca and PeaceRiver deposits. In-situcombustionis anotherprocessfor reducingoil viscositvand makingit producible.In this case.air is injectedthroughconventionalwells and flows toward productionwells. Once ignition is started at the injection well the combustionfront movesslowlvin thc directionof producingwells.Aheadof this fronl. oil is pvrolvzedandcracked:it vieldsa lighteroil. whichis produced,anda hcavv.cokc-likeresjduervhichremainsin place.The latter is burnt when the combustionfront reachcsit. and providesthe energynecessarv for the process. 'Ihis t c c h n o l o givs p r e s e n t l uv s c di n R u m a n i at,h e U n i t e dS t a t e sa n d C a n a d a , u ith an annualproductionca. l.-5millionmr. andseemsto be applicable downto I 5 ( Xm ) . l - h c a m o u n (t ) th c a v yo i l h u r n ta sc o k er v o u l dh e 1 i f 2 5 ? o f t h e i n - p l a c e oil. Simuitaneous injectionof water is alsosuggested, in order to combinethe advitnlaces of conrbustion andstcaminjectionmethods. Othermethods.suchasinjectionof carbondioxideinto heavyoils.or solvents, x r c s t i l le x p cirm c n t a l .
6.6.2 Relining V a l o r i z a t i o no f t h e h c a v v o i l s c a u s e ss r r m ed i f f i e r r l t i e isn r e f i n i n g .d u c r o t h e i r specificcharacteristics:
h i g hs u l f u r .n i t r o g e na. n dm e t a l( V . N i ) c o n t e n tt:h e vm a ya c t a sp , . ) i s ( )tnos heterogcnous catalvstsusedin rcfining processes; furthermore.sulfur and nitrogencausecnvironmentalproblems. - low H,tClratio. ascomparedto conventional oil. hiehasphaltcne content,whichis responsible for thehighviscosity of theheavy fucls.and is alsoa causeof instabilit)'of somerelinedproducts. The increascof the H/C ratio may be obtainedbv carbon eliminarionor hvdrogenadcljtion.A heaw. carbon-rich.residuecanbe removedby deasphalting (rvith thc use of purc solvents.or a Cl\-fraction)or coking (heatingand 'l crackingthc oil). hen.hydrogenation treatnentsmayfurtherincrease the H/C r : r l i , )l.n d r t l s ,rre m o r c' u l [ u r a n J n i t r o g c n .
Summaryand Conclusion Hearv oils and extra-heavv oilsof tar sandsare. in mostcases.the resultof crudeoil degradation in reservoirs, wherepetroleumhasenteredbv migration,accumulated and subsequently becomedegraded. This latterphenomenon is commonlyassociated with
,18,1
HeavvOils andTar sands
the invasionof the reservoirby meteo c waters and it may include biodegradation, water washing,lossof volatiles,and oxidation. Heavy oils contain lesshydrocarbons,especiallyalkanes,than normal crude oils. and more asphaltenes, resinsand sulfur-aromaticcompounds.Asphaltenesplus resins range from 25Vato 707oof heavy oil. In terms of crude oil classification,heavy oils belongto the aromatic-naphthenicand aromatic-asphaltic classes. Physicalpropertiesofheavyoils are largelycontrolledbythe abundanceandphysical stateof asphaltenesand resins:specificgravity rangesfrom 0.93g cm i to 1.06g cm r (20' ta 2' API). Viscosity rangesfrom 100 to 100,000centipoisesand it is highly dependenton temperature. World reserves of heavyoil in-placeamountto 0.5- 1.0trillionmr, with very large depositsin Westem Canada(Athabasca)and EasternVenezuela(Orinoco belt).
References to PartIV
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4U6
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Mair. B. J.: Terpenoids.fatty acidsand alcoholsas sourcematerialsfor petroleum hvdrocarbons. (1964) Ceochim.Closmochim. Acta 28, 1303-1321 Mair. B. J.. Ronen.Z.: Clomposition of the branchedparaffin-cvcloparaffin portionof petroleum1,10-lll0"C. J. Chem.Eng. Data 12,432-436(1967) Mair. B. J.. Ronen.2.. Eisenbraun.E. J.. florodsky.A. G.: Tcrpenoidprecursors of hvdrocarbons from the sasolinerangeofpetroleum.Science154,1339 1311(1966) M a i r .B . J . . S h a m a i e n g aMr . . K r o u s k o pN. . C . . R o s s i n F i . . D . : I s o l a t i oonf a d a m a n t a n e f r o m p e t r o l e u mA. n a l .C h e m . 3 l , 2 0 8 22 0 8 3( 1 9 5 9 ) Martin. R. L.. Winters. J. C.. Williams.J. A.: Compositionof crude oils bv gas chromatographv: geoiogical significance of hydrocarhon distribution.6rh World pet. CongrS . e c .V . 2 - l i 2 6 0( 1 9 6 3 a ) Martin.R. L.. Winters.J. C.. Williams,J. A.: Distributionof n-paraffins in crudeoilsand their implications to originof petroleum.Nature(London)199,1190 1193(1963b) M a x w e l lJ. . R . . C o x .R . E . . E g l i n l o nc. . . P i l l i n g e C r . . T . . A c k m a nR . . G . . H o o p e rS , .N . : Stereochemical studiesof acvclicisoprenoidcompounds.Il Role of Chlorophvllin derivationof isoprenoidtype acidsin a lacustrinesediment.Geochim.Cosmochim. Acta 37, 297(197-l) Maxwell.J. R.. Mackenzie. A. S.. Volkman.J. K.: Configuration at C-2,lrn sreranes and sterols.Nature286,69+697 (l9ll0) McAlarv. J. D.. Roger!.M. A.: Reservoirbitumens- anotheraid in the geochemical c v a l u a t i o n o f a b a s i n ( a b sG. )e. o l .S o c .A m . A b s . P r o e r a r nls, 3 . 9 3 3 9 1( l 9 7 l ) M c K a v .J .F . . C o g s w e lTl .. E . . W e b e r .J .I i . . L a t h a m D . . R . i A n a l v s i os f a c i d si n h i g h , b , ' i l i n sl 1 r ' r r o l e urm l r . r i l l a t r .F. l l e l5 4 . 5 t ]h l ( l q - 5 ) Michaelis.W.. Albrecht.P.: Molecuiarfossilsof Archaebacteria in kerogen.Naturwissenschaften 66, 42{}-121( 1919) Moschopedis.S. E.. Speight.J. G.r Invesligationof hvdrogenbondingby oxvren functionsin Athabascahitumen.Fuel55, 18'7-192(19761 Moschopedis. S. 8.. Fryer.J. F.. Speight.J. G.: lnvestigation of asphaltene molecular weights.Fucl 55,227-.231 ( 1976) Moschopedis. S. E.. Parkash.S.. Speight,J. G.: Thermaldecomposition of asphaltenes. (1978) Fuel57. 43J-43,1 Mulheirn.L. J.. Ryback.G.: Stereochemistrv of somesteranes from geological sources. Nature(London)256,301 302(1975) Orr. W- I-.: Changesin sulfur contentand isotopicratiosof sulfur duringpetroleum maturation'Studv of Big t{orn basinPaleozoic oils. Am. Assoc.Pet. Geol. Bujl. 58, ( r97'l) 2295231rJ (lutlin. J- t-.: Analvseg6ochimique de la matiarcorganiqueextraitedesrochessidirrren laircs-L Composds cxtractibles au chloroforme.Re\'.Inst.Iir. Pet.25,I 15(1970) Ourisson. G.. AIbrccht.P.. Rohmcr.M.: Thehopan
490
Rcferences 1oPartI\'
of origrnof the heavyto mediumPhilippi.G. 1.: On the depth.time and mechanism gravit;-naphthenic Acta 41, 33 52 (1977) crudeoils.Geochim.Cosmochim. sur la composition Poulet. M.. Roucachd.J.: lnUuencedu mode d'ichantillonnage in OrganicGeochemistrv chimiquedcsfractionslegCres d unehuilebrute.In: Advances l 9 6 6 . H o b s o n . C . D . . S p e e r s . G . C . ( e d s . ) . L o nPdeor g na : m oPnr e s s1. 9 7 0 . p p1. 5 5 - 1 7 9 Poweil.T. G.. McKirdy. D. M.: Crude oil compositionin Australiaand Papua-New G u i n e aA . m . A s s o cP . e t .G e o l .B u l l .5 9 , I l 7 G l 1 9 7( 1 9 7 5 ) Verhaltender Prinzler.H. w.. Pape.D.: Zur Entstehungund zum postgenetischen desErdrils.Erdol u. Kohle 17,539 5,15(1964) organischen Schwefelverbindungen Putscher. R. E..:Isolationof olefinsfrom B radfordcrudeoil. Anal.Chem.24. l55l 1558
(1es2) regularities in the distributionofthc oil-bearing regions Radchenko. O. A.: Geochemical of the world (in Russian).Leningrad:Ne
Refcrencr:s h) Prrt lV
.l9l
Seifert.W. K.. Galiegos. E. J.. Teeter.R. M.: proofofstructureof steroidcarboxvlic acids in a Californiapetroleumbv deuteriumlabelling.synthesis and massspeclromclrv. J. A m . ( h e m .S n ( .9 4 .) h x o 5 x x 7{ l q r 2 ) Seifcrt.W. K.. Howclls.W. G.: Inrerfaciallv activeacidsin a Californiacrudeoil. Isolation of carboxrlicacidsand phenols.Anal. Chem.41, 554-562(1969) Seifert.W. K.. Teeter.R. M.: Identificarion of folvcy(licaromaricandhererocvclic crude oil carboxvlicacids.Anal. Chem.42,7507580970) Silverman.S. R.: Migrationand seeregatron of oil lnd gar. In: Fluidsin Subsurface E n v i r o n m c n tY s .o u n g .A . . G a l l e y J. . E . ( e d s . ) M . e m o i r , l .A m . A s s o c p . e t r .G e o l . T u l s a . 5 3 - 6(51 9 6 5 ) Silvcrman.S. R.: Carbonisotopicevidence for the roleof lipidsin petroleum.J. Am. Oil C h e m .S o c .4 4 , 6 9 16 9 5( 1 9 6 7 ) Smith.Il. M.: Correlationindexto aid in interpreting crudeoil analyses. U.S. Bureauof Mincs.Tech.Paper6ltl (1940) Smith.H. M.: Crudeoil: qualitativeandquantitarive aspects. The petroleumWorld.U. S. B u r c r uo f M i n e sI n l C r r c .x 2 8 h( I t l h hI Smith.H. M.: Qualirativeandquantitative aspects of crudeoil composition. U. S. Bureau of MinesBull. 642( l96ll) S m i t h J. . [ . . E r d m a nJ. . G . . M o r r i s .D . A . : M i g r a t i o nA. c c u m u l a t i oann dR e t e n t i oonf Petrolcumin the Earth. Proc. lllh WorlJ pet. Congr. Moscow.Lonclon:Applied S c i c n cP e u b l . .1 9 7 1 . 2D . D .l 3 2 6 Smith.N.-A. C.: The inte'rpretation of crudeoil analvses. U.S. Bureauof Mines.Rept. Invest.21i06(1927) Speisht.J. G.: The chemistryand technologvof petroleum.MarcelDekker Inc.. New York. 1980.pp. .19E S p e i c h tJ. . G . . M o s c h o p e d iS s . E : Someobservations on the molecular,.nature'of petroleumasphaltenes. PreprintsD i v . P e t .C h e m . .A m . C h e m .S o c . 2 4( , 1 ) . 9 1 ( ! 9 2 - l (t979) Strukhov.N M.: Principies of Lithogenesis. MoscowrIzdat.Akad. NaukSSSR.1962(in Russian ). Transl.Consult.Bur. andEdinburghandLondon:Oliver& Boyd.I. 1967:II. 1969 Suatoni.J. Cl..Swab.R. E.: Preparative hydrocarbon compoundtypeoi analysis bv high pertormance liquidchromatographv. J. Chromatog. 14,53-5 (1976) 537 'Iewari. K. C.. Li. N. C.: C)nmolecularinteractions involvingcoal-derived asphaltenes. P r e p r i n t sD. i v . P e t .C h e m . .A m . C h e m .S o c . 2 4( 4 ) . 9 g 2 _ t 8 9f 1 9 7 9 ) Tissot.B. i Probldmes g6ochimiques dc llrgenesrct de la migratrondu pdtrole.Rev. Inst. F r . P € t . 2 1 ,1 6 2 11 6 7 1( 1 9 6 6 ) Tissot.B.: Connaissances actuelles surlesproduitslourdsdu petrole.RevueInst.Franqais du Pdtrole36, 429+16 (1981) Tissot.8.. Oudin. J. L.. Pelet. R.: Critaresd origine et d'evolutiondes p€troles. Applicationi I dtudeg€ochimique desbassins s€dimentaires. In: Advancesin Orqanic Geochemistrv 1971. vonGaertner.H. R.. Wehner.H. (eds.).Oxford:pergamonpress. 1 9 7 2p. p . t l 3 1 3 4 Tissot.B.. Pelct.R.. Roucach€. J.. Combaz.A.: Utilisationdesalcanescommefossiles giochimiquesindicateurs desenvironnements g€ologiques. In: Advancesin Organic Geochemistrv 1975.Campos.R.. Goni, J. (eds.).Madrid. 1977 Tissot.B.. Deroo. G.. Hood. A.: Geochemical studyof the Uinta Basin:formationof petroleumfromtheGreenRiverformation. Geochim.Cosmochim. Acta42.l469 l:lg5 (le78) Tissot.B.. Deroo.G.. Herbin.J. P.: Organicmatterin Cretaceous sediments of theNorth Atlantic:contributionto sedimentology andpaleogeography. In: DeepDrillingResults
492
Referencesto Part lV
Talwain,M . Hay. in the AtlanticOcean:ContinentalMarginsandPaleoenvironment W.. Ryan.W. B. F. (eds.).MauriceEwing Series,3.Am. Geoph.Union. 1979,pp' 362-314 and Tissot,B., Demaison,C., Masson,P., Delteil,J. R., Combaz'A.: Paleoenvironment petroleumpotential of Middle Cretaceousblack shalesin Atlantic Basins.Am Assoc' (1980) Pet.Geol. Bull. 64' 2051-2063 Gesteinen'Erdtilen ErdTreibs,A.: Chlorophyll-und Hiiminderivatein bitumindsen wachsen und Asphalten.Ann. Chem 510,42-62(1934) Tynan,E. C., Yen, t. F.: Am. Chem.Soc.Div Pet.Chem ReprintsA-89 (1967) J. GeochemExplor' I' Welte,D. H.: Petroleumexplorationandorganicgeochemistry. t17-136 (1972) |1-Alkanein Sedimentgeradzahliger Welte,D. H.. Waples,D.: Uber die Bevorzugung (1973) 60' 51G517 gesteinen. Naturwissenschaften of petroleumandnaturalgasln: forthe biogenesis Whitehead.E. V.; Molecularevidence Tokyo. 1970.Ingerson'E. (ed ) WashingBiogeochem. Proc.Symp.Hydrogeochem. ton: The ClarkeCo., 1973,pp. 158-211 Williams.J. A.. Winters,J. C.: Microbialalterationof crudeoil in thereservoir'In: Symp in Geol.Envir. Am. Chem Soc.Nat. Meet. Preprints14.22 3l Pet.Transformations (1e69) 1980Munich.Sept 8 12' 1980. SuryeyofEnergyResources World i,nergyConference. 357pp. Yen. T. F.: Presentstatusof the structureof petroleumheavyendsand its significanceto varioustechnicalapplications.Am. Chem. Soc Meet. Reprints F 102 111(1972) Yen, T. F.: Structuraldifferencebetweenpetroleumand coal-derivedasphaltenes' Preprints,Div. Pet.Chem..Am. Chem.Soc.24(4),901-909(1979) of the structureot petroleum Yen. T. F.. Erdman.G. J.. Pollack.S. S.: Investigation (1961) by X-ray diffraction.Analyt. Chem.33' 1587-1594 asphaltene inthe eni ironment: aromatichydrocarbons Youngblood.W. W.. Blumer.M.: Polycyclic CosmochimActa Geochim sediments. ma ne and recent in soils series homologous 39. 1303-1314 [975)
Part V Oil and GasExploration: Applicationof the Principlesof Petroleum Generationand Misration
ChapterI
Identification of SourceRocks
The derivationof petroleumfrom nonreservoir rocksis a well-established fact. Rocksthat are, or may become,or havebeenableto generatepetroleumare commonly called source rocks. The presenceof insolubleorganic matter (kerogen)is a primaryrequisitefor an activeor a potentialsourcerock. A crucialtestin the recognitionof a petroleumsourcebedis the determination of its contentof organicmatrer,both soluble(bitumen)andinsoluble(kerogen). A secondimportantstepin source-rock identification is the determination of the typeof kerogenandthe composition of the solventextractable hydrocarbons and nonhydrocarbons. Finally, from optical and/or physicochemical properties, evolutionarystagesof kerogencan be determined.This conceptis commonly referredto asthe "maturityofsourcerocks".A combination of parameters allows theidentification of thecontentandtypeof kerogen.andthelevelof maturityofa sourcerock. The term sourcerock is appliedirrespective of whetherthe organicmatteris matureor immature.Sourcerock qualirvis definedin termsof amountandtype of kerogenandbitumenandits stageof maturity.However,whileit is possibleio recognize whethera sourcerock is immature.and thuscannotyet haveyielded oil, it is yet ver1,difficult,exccptin spccialcases. to find out whetheror not oil has mlgratedout ot a maturesourcerock.
I . I Amount of OrganicMatter The kerogencontcntof a sedimentis normallydeterminedby combustionof the organiccarbonto CO. in an oxygenatmosphere aftercarbonatecarbonhasbeen removedby acid treatment.Such organiccarbon analysesare conveniently performedon finelygroundrock samplesin a combustion apparatusutilizingan inductionfurnaceand a thermalconductivitycell to measurethe evolvedCO,. However,thisprocedurereallydetermines only organiccarboncontent.andn;t the total organicmatter or kerogen.ln somelaboratoriesextractableorganic matter (bitumen)is removedprior to determinationof organiccarbon.The fractionoforganiccarboncorrespondingtobitumenisusualllnotmorethan0.1 or 0.2 of the total. However,lessscatteringis commonlyobservedon organic carbonvaluesdeterminedon bitumen-freesamples.Although organiccaibon analysesare relativelysimple, it has repeatedlybeen observeclthat values obtainedfor the samesamplesin differentlaboratoriesmay differ by about
Identificationof SourceRocks
496
Table V.1.1. Conve$ionfactorsfor computationof total organicmatter from organiccarboncontent Type of kerogen Coal
Stage
Diagenesis
1.25
1.34
1.48
1.57
End of catagenesis
t.20
f.i9
1.1u
1.12
for otherelements(H, O. N ' S) presentln kerogen' t 0.-l7rC,,.,.To compensate hasto bc multipliedby a conversionfactor. carbon the valuefciundfor orgnnic of kerogendependson type and level of composition becausethe elementai factorsrangingfrom conversion ( determined 1958) and Hunt Forsman evolution. organicmatter nonmetamorphosed to 1.40 for up rocks metamorphosed 1.07for be derived can factors on conversion information More detailed oxygen. rich in from TabiJV. 1.I with respectto t1'peof organicn]atterandstageof maturation. Conversionfactorsfor certainpeatsmay be ashigh as2 0. in sourcerocksarepresented Averagevaluesof organiccarbonfor shale-type TableI1.3.2.They are generallyin the rangeof 2%. Averagevaluesof organic sourcerocksare in the rangeof 0 67o' carbonfor carbonate-type Not only the amountbut alsothe t)'peof organicmatteris critical To taketwo extremeexamples.a rock with 2 7.' C,,,, composedmainly of inertiniteand rederivedanthiaciteis a muchpoorersouicerockthanonecontaining0.5% C,,,-q Anotherratherfrequent of algalmaterial.suchasis foundin form of Tasmanites. of0.57r C,,,0represents wherethe background situaiionis met in rocksecluences to 5 7cC,',,. derivedmainly 2 a terrestrialinput.whereasparticularlayerscontain in FigurelV 3 l ' is shorvn this kind plankton.An exampleof from autochthonous rocks has been petroleum source for The lower limit of organic carbon some (1958) investigated Ronov manner' primarilyin an empirical established provinces' and nonoil from oil environments ages and of different 26,000samples in oilprovinces sediments rockshale-type The criticaflowerlimit for nonreservoir contentin oil carbon organic The average carbon organic turnedout to be 0.57r times ashigh asin three be almost was found to platform basinsof the Russian carbon is 0.5% organic limit of lower province. This oil areasoutsidethe rock source generally acknowledged from findings recent with more consistent for detrital Iimit lower a I % C,,,, prefer to use however' units,Somelaboratories. sourcerocks. wascarriedout studyof organicmatterin limeslones A rathercomprehensive partsof the from many rocks ancient 1'100 of about ( Analyses 1962). by Gehman world showedthat ancientlimestonescontainmuch lessorganicmatter than was0 207r , and The averageorganiccarboncontentof limestones ancientshales. that of shales0.94%. The averagehydrocarboncontent'however,was about by 100 oom antl similar for ancientlimestonesand shales The observations
1.2Typeof Organic Matter
49'/
carbonateGehman(1962)and corroborativedata on generallyacknowledged tvpe sourcerocksleadto the conclusionlhal0.3c/corganiccarbonis the lower limit for carbonate-tvpe sourcebeds.Averagevaluesof organiccarbonfor these carbonate-type sourccrocks are considerablyhigher and range above0.67c ( s e ea l s oT a b l eI 1 . 3 . 2 ) . Theserninimumvaluesof organiccarbonfor potentialsourcerocks.with0.37o for carbonatesand 0.5% for shales,shouldbe consideredonly as necessary backgroundvalues, rathcr than as positive indicationsfor a source rock. not only becauschydrocarNevertheless, minimumvaluesare of significance. a bonsin sourcerocksare generatedfrom insolublekerogen,but alsobecause criticallevelof hydrocarbons hasto be reachedbeforeexpulsionfrom a source rock is possible.Prior to expulsion.the specificadsorptioncapacityof a source rockfor hydrocarbons hasto be satisfiedand,moreover,sufficienthydrocarbons for movementof a pressure-driven hydrocarbonphasehave to be available. Finally. it shouldbe pointedout that this minimumorganiccarbonvalue as a sourcerock criterion no longer appliesto rocks in a very advancedstageof stagewhenonlydry gasis generated. At thisstage maturity.i. e.. in a metagenetic 0.3 or 0.5% organiccarbonmay merelyindicatea residualamountof organic matterthat initiallymay havebeenmorethan twiceashigh.
1.2 Type of OrganicMatter A distinctionbetweenvarioustypes of sedimentarykerogenis essentialfor proper sourcerock appraisal.becauscdifferenttypesof organicmatter have diflerent hydrocarbonpotentials.The differencesarisefrom variationsin the chemicalstructureof the organicmatter. zooplanktonandhigherplantshavebeen Remainsof bacteria.phvtoplankton. recognizedas main contributorsto kerogen in sediments.Maior chemical ditllrencesin grosscompositionexist betweenorganismsliving in an aquatic environmentand those living in subaerial(nonaquatic)environment.This that while terrestrialplants need distinctionstems from the generalization structuralsupportfrom polymerssuchaslignin.aquaticplants.supportedby the surroundingwater. need none. Therefore.the distinctionbetweenkerogen derivedfrom aquatic(planktonic)organismsand terrestrialhigherplantsis of of transportanddepositionandthe importance.Furthermore.the environments oforganic resultingmodeof preservation alsoinfluencethechemicalcomposition matter.Thesefactors,however.are asyet difficultto detect. Type and qualityof kerogencan be differentiatedand evaluatedby optical andphysicochemical methods.Opticalmethodson onehandallowa microscopic of details,suchasmixing visualization of the kerogenand thusan appreciation only Mostopticaltechniques, however.recognize from differentorganicsources. a fractionof the total kerogenof a sediment,a fractionwhichis not necessarily methodsmonitorthe total representative. On the other hand.physicochemical different organicmatter usuallyin form of bulk analyseswithout recognizing have specific commonlyapplied or sources.Thc varioustechniques components
498
Identificationof SourceRocks
merits and limitationswith respectto applicability.Therefore,as a rule they shouldnot be usedseparately and exclusively. but ratherin combination. L2.1 OpticalMicroscopicMethods Opticalrnicroscopv canonly be usedto describethatpartofthe organicmatterof a sedimentoccurringin particleslarge enough to be visible in an optical microscope(about I prmor larger). However,also some finer "amorphous" organicmattercan be identified.usingtransmittedlight or fluorescence techniques. Definitive texts on optical microscopvare availablefor the specialist ( O t t c n j a n ne t a l . . 1 9 7 , 1T:e i c h m r i l l e r1, 9 7 4 A ; l p e r n ( e d . ) , 1 9 7 5 ;S t a c he r a l . , 1975).Generallythreedifferenttechniques areappliedfor microscopic studiesof soutcerocks: l. investigation in reflectedlight of a polishedsurfaceof wholerock pieces, 2. investigation in rcflcctedlight of a polishedconcentrate of organicparticles. isolatedfrom a rock (organicparticlesare concentrated by meansof acid macerationand/ordensityflotation). 3. investigation in transmitted lightoforganicparticles. isolatedfrom a rock,and strewnon a glassside. To undertakeall threetypesof studiesis necessarily time-consuming. In the pastusuall)'in anylaboratorvonly oneapproachhasbeenused.The application, however,of two techniques seemsadvisable because the differenttechniques, to a certainextent.yield complementary information.Resultsstemmingfrom a TableV.1.2. Approximate equivalence of varioustermsusedin kerogendescription.(AfterCornford.1977) Provenance
Terminologies Algal
Amorphous
Liptinite
Amorph.
Type II
Herbaceous (fibrous) '.-,-woody
Vitrinite
(plant
Humic
st,uy:)-,_ --<.' ' i F
Type I
Type III
LOaty
(angularto sub-angular fragments)
Inertinite
Residual
1.2Tvpeof OrganicMatter
49g
grven technique are internally consistent.although interrelating the ter_ minologiesof the different techniquesis still in piog.".s. Table V.1,2 lists approxlmateequrvalence of varioustermsusedin kerogendescription. It seems that specialists concernedwith microscopicstudiesof kerogenare currently propagatinga new geneticallyorientednomenclature for sejimentaryorganii particles.However. for historicalreasonsand in order not to prolifirate nomenclature. it is probablybest,for the time being,to adoptcoalpetrographic termsbasedon the maceralconceptto describediipersediedimentaryorginic m a t t e r .T h e m a i n g r o u p so f m a c e r a l si n c o a l , i . e . . l i p t i n i t e .v i t r i n i t e . and inertinite haye been describedin ChapterII.g. A shori description of their charactcristic appearance in scdimentscan be givenasfollows: a) Liptinite (c.g.. algae.resins.sporesetc.):identifieclbv characteristic shapel high transrnittance and intcnsefluorcscence at lo*er ie'crs ot milturit\ and lorvretlectance comparcdto the other maccralgrouns. b ) F l u m i n i t eV. i t r i n i t e :a n g u l a r t os u b r n g u l apra r t i ; l e ss. o m e r i m essh o w r n g cell slructure.Identifiedbv moderatctransmittance. intermecliate reflectance. antl usualll'abscnceof fluorescence. c ) I n e r t i n i t e :a n g u l a r .h i g h r c f l e c t a n c eo.f t e n e x h i b i t i n gc e l l u l a ro u t l i n e or granulartexturc.No fluorcscence. opaquein transmittedlight. d) Other particlessuchassolidbituminitej.and variousmicroiossrrs. Reflectecl /lgftr microscopl,allowsthoseparticlesto bc studiedthat take a polish. Thus particlesof huminite. vitrinite and inertinitegroups are easily ide^ntified (Stachet al., 1975).Amorphousor structureless orga-nic -part malterrsmore difficult to identify in reflectedlight. However,a certain of it. which is derivedfrom Iiptiniticmatter.canbe recognized usingincidentUV excitationto producefluorescence. In reflcctedlight the rank of coalsis dcterminedby measurement of the retlectance of thehuminiticor vitriniticparticles(seeChap.II.g). Theapplication to the evaluationof the maturityof petroleumsourcerocksis discu.ssed later (Sect 1.3..1)Next to_maruritymeasurement. the main useof reflectedlight in studyingkerogenis the differentiationof humic (sometimes alsocalredherbaceous.rvoodyor coaly)organicmatter.The two maceralgroupsof vitriniteand inertiniteare easilyidentifiedby virtue of their reflectanc-e. shape.anrlrnternal structure.Vitrinite is of a mediumgrey appearance, whereasinertinitesshowa much brighterreflectance.The distinctionbetweenthosetwo tvpesof humic matteris of importance,because inertinitesdo not exhibitany oil potential.and 'has perhapsa limitedpotentialtbr gas.whereasvitrinitematerial somepotential for oil. and a definitegaspotential. Anotherimportantobservation. only possiblein reflectedlight,is the recognition of reworkedopaqueorganicmatter. derivedfrom old-ersedimentsind redeposited. Erodedandredeposited coalfragmentsarereadilyrecognized when exhibitingtypicallamination,assocalledbi- or trimaceritepariiclesin whichtwo or three maceralgroupsare combined(Fig. V.l.l). Other reworkedorganic particles.not stemmingfrom coalseams.mav showoxidationrimson thc outer sideof the particledue to intensecontactwiih oxvgenduringreworking.These oxidationrimsin reflectedlightshowin mostcasesa brighterreflectance. tn most
of SourceRocks ldentificarion
t n r ( l l e c l e ' l l c h t :l a r g er c d c p l l ' i t ecdt ' a ll t a c m r n tl T r i m a c c l l l e i Fiu V I I A ler.r!.en r € r e ' Ja n d i n t r t i r r l e r r r ' i ' ' K e r L J c e n t , ' ! . , ; . ; ; " h ' r t " . . n . : " * . ' " ' r i r r i n i r ep a r t i e i e s Gerntanl Rcflectedhght'oilimmerEmsland sandstone. coicentrateof Carb;iferous sion.(Photollagemann)
due to weathering(oxidaFie. V. t.2. An altercdtnertrnrteparticle.showingdarkening r i o - n tR. e f l e c t el,ijg h t .o i l i m m e r ' i o nt P h o l oH a g e m a n n )
cases.howcver.oxidatlonrlmshaveirlowelleflcctance(Ha8cmann.1977:Fig. by lower and redder liptinite V.1.2). Redepositedmatter can also be identified Finally. if higher-rank vitrinire. the ihan expccted{or the rank of n,ror;.n.. can be found vitrinite of population matter is redeposited' a higher reflectance
1.2 Type of OrganicMatter
t
501
:1,
\ ^ :
.
,
,
a.
t
a
l
a
,, J
t.'
l
-
.
Fig. V.1.3. A kerogenin transmittedlight: largeround objecrsare algal bodies.Smaller round objectsare palynomorphs.Angular opaquepardcle.,iare vitrinitic plant remains. Kerogei conccntrateof Upper Liassicshale(pliensbachian). Luxemburg.Transmitted light. (PhotoHagemann)
$.!'-
,:4. \, r:-- -r _ l
-
-
.
€
I
u
---
I
.x-5
=b
t
l"l t t , ,rll:,;.$ I
Fig.V.1.4.A kerogen in transmitted lighttsmallangular darkobjects arevitriniticplant particles. Thegrc) tansparent objectin upperrightcorneris afragment of cuticleI the/or1g d.arkperforated objectlnlowerleftcornerisfusinite.Kerogen conientrate of UpperLiassic shale(Pliensbachian). Luxemburg. Transmitted light.(photoHagemann) Particles of higherreflectance are alsoobservedin contemporaneous peats.They are obviouslynot reworkedand couldbe the productof selective oxidation. For studiesin transmitted light,a similarmicroscope is employedto that used for the observation of rockthin sections or biologicalpreparations. Thisapproach
Identificationof SourceRocks
(Algae).Asinglespotona to identifyliptinites Fig.V.l.5a andb. Useof fluorescence whitelightand isshown(a)in reflected polished shalefromProbiitipJugoslavia. Miocene (middle'teJi) particles large liptinite Three fluorescence. (b) underUV excitation shoNing in (b) by theirbrightfluorescence underwhitelightin (a).areidentified not apparent (/opri8ftl)isbestidentified in particle A huminite arealgalbodies. These iiptiniteparticles Hagemann) (a)underwhitelight.(Photos
to the study of kerogenusingtransmittedlight is derivedfrom palynological examinations(Combaz. 1961, 1980).Strewn grain concentratesof organic and/or rocksafteracidmac-eration from crushedsedimentary particles.recovered densityflotation,are used.The type of organicmatteris identified,usingthe and shapeof particles,and in somecasestheir fluorescenceThe translucency techniqueis excellentfor the identificationof the structured transmitted-light particlesof the importantmaceralgroup of liptinites'such as algal remains'
1.2Typeof Organic Matter
503
spores.pollenor cuticles(Fig. V.1.3 and V.1.4). "Amorphous"organicmatter, withoutclearoutlines.of no distinctshape,and withoutidentifiablestructureis alsoreadilyobserved.Itis moderatelytransparent andofa yellowishto brown,or dark-browncolor.This typeof organicmatteris frequentlytermed,,sapropelic". The amorphousorganicmatteris frequentlyconsideredas beingderivedfrom planktonandhavinga goodpetroleumpotential.Althoughthisis partlycorrect. it hasto be keptin mindthatthereareotheroriginsfor amorphous organicmatter which do not necessarily havc a good petroleumpotential.Precipitationor adsorptionof dissolvedor colloidalorganicmattersuchashumicacidsresultsin structureless. amorphousorganic matter having a Iow petroleumpotential. Furthcrmore.intensemicrobialreworkingof organicdebrismay also destroy originalmorphological structures of variousbiologicalorigins.Thisdiscussion is presentedin ChapterII.4.8 and illustratedin FigureIL4.l6. Basedon thesc considerations. it is concludedthat identificationof amorphousorganicmatter, by usingtransmittedlight. is by no meansa proof of a goodpetroleumpotential. Additional informationhas to be obtainedfrom fluorescence studiesand/or chemicalanalyses.In contrastto the amorphousorganicmatteris the opaque matterof the vitriniteandinertinitegroups.whichis not easilyidentifiablesince onlyoutlinesaregenerallyvisiblc.The opaqueorganicmatteris classified usually as"humic". althoughfibrousplantremainshavebeenclassified as,,herbaceous" ( T a b l eV . 1 . 2 ) . The resultsof transmittedlight studiesmainlygiveinformationon the typeof o r g a n i cp a r t i c l eps r c s e n t R : o g e r s( 1 9 7 9 )M . a s r a ne t a l . ( 1 9 8 1 )I.n f o r m a t i o o nn maturitvcanalsobe obtained.Thismatteris discussed laterin moredetail(Sect.
1.3.r).
Fluores<:ence studiescanbe carriedout in eithera transmitted or reflectedlight mode. Ultravioletor blue light is usedto excitethe organicmatter and the fluorescence spectrumis observed.Fluorescence canbe usedin threewaysrnrne studv of sourcerocks. Firstlv. sinceit is diagnosticof the liptinite group of macerals.it is particularlvuscfulto identifyamorphousliptinitic matter (Fig. V. I .5a. b). Thercis oftenno othergoodmethodto visualizethe finelydispersed particlcsof algal or microbialorigin. It has also been proposedrhat oil-like components tcrmecl"exsudatinite".in the nomenclature of microscopv. can be seenbv usingtluorcscence (Teichmiiller.1974).The seconduseof fluorescence is in determiningthe levelof maturitvof kerogenbv eslimatingthe intensityand c o l o ro f f l u o r e s c e n c( S e e c t .1 . 3 . 1 )T. h e t h i r d u s eo f f l u o r e s c e n cae q. u a n t i t a t i v e measurcment of thc fluorescence spectrum,usinga photometricmicroscope, has not yet beenfully explored.It is alsousedfor maturitystudies. Followingthe previousdiscussion, a generalschemeof preferredmicroscopic techniques for identificationof typesof kerogenin sourcerocksis givenin Table
v.1.3.
The tablesummarizes somecommonterms.usedfor typesof organicmatter. mostfrequentlyfoundin sourcerock-typesediments andrelatestheseto various microscopic techniques (reflected.transmittedand fluorescent light). Microscopvcannotdescribeparticlesof kerogensmallerthan about I trrm. Amorphousorganicmatter composedof massesof smallerparticlescan be recognized withoutfurtherdifferentiation by employingfluorescence techniques.
Identificationof SourceRocks
504
Table V.1.3. Schemefor subdividingorganicmatter in sourcerocksby microscopy,based on preferredmicroscopictechniques Reflectedlight technique Identificationby Reflectance
Particlesinspected
'^b
X E
iDtr
Liptinite: Resinite Sporinite Cutinite Alginite Liptodetrinite
Low
Huminite: Vitrinite: Telinite Collinite htertinite: Fusinite Semifusinite Sclerotinite Macrinite
Medium
Internal structure: shape morphology
E E
High
Fluorescentlight technique Identificationby
Transmittedlight Algae, resin bodies, pollen, spores, cuticles
Alginite, resinite, sporinite,cutinite
lnternal struclure, shape,morphology color of fluorescence
Distinction between fluorescent (liptinitic)and nonfluorescent (humic) amorphous organlcmatter
No distinctoutline or shapecolor of fluorescence
of the particulateorganicmatter,the juxtapositionof particles In concentrates and good flat polish allow accurate identification of the various macerals Polished whole rock preparations give a more representativeview of the compositionof the oiganic matter and relay informationon the mode of depositionof organicmatter. When compaiingthe schemefor subdividingorganicmatter (seealsoTable V.1.2) in tranimittedand reflectedlight with the typesof kerogenidentifiedby
1.2Typeof OrganicMatter
505
TableV.1.3 (continued) Tmnsmitted light technique Identificationby
Particles inspected Translucency
Internal structure, shape, morphology, color
Algae, resin bodies,ponen, spores,cuticles
Translucent
Structured humic mafter: herbaceous, woody,coaly
Weakly translucent - opaque
Shape
Amorphous matter
Generally translucent
No distinct outline or shape
physicochemical means,the followingcanbe concluded:Zype-1kerogenusually consistslargely of structuredalgal material (Botryococcus,Tasmanites),easily recognizable in a microscope(Fig. V.1.6a, b); it may alsoconsistof structured algal remainsmixed with amorphousmatter. The latter possiblystemsfrom intensemicrobialreworkingof variouskindsof organicmatter. Tvpe-ll kerogen usually consistsof a large proportion of sapropelicorganic matterwith an amorphous appearance plusidentifiableparticles.Thoseparticles includealgalor otherplanktonicremnantsandsomeland-derived plantmaterial, suchas pollen(Fig. V.1.7). Humic materialmay alsobe a minor componentof tvpc-ll kerogen.Type-lll kercgenusuallyconsistsof humic,opaqueand coalv m a t e r i a(lF i g .V . 1 . 8 ) .H o w e v e rs, o m eo f t h e a m o r p h o um s a t e r i aal l s oh a st o b e groupedwith tvpe-lll kerogen.Resldr.ral kerogenmaycomprisccoaly,inertinitic, and alsosomcamorphousmaterial. Technicaldetailsof microscopicstudiesin reflectedlight: reflectancemeasurementsand maceraldeterminations canbe madeon HCI-treateddensityconcentrates(separatedby heavyliquids)of particulateorganicmatter embeddedin "epoxy" resin and polished,or alternativelyon whole rock surface.The concentration procedureisolates typicallybetween20 to 60oZofthe totalorganic carbonin the form of organicparticles.Nearly completerecoveryof organic matter. as comparedto total organiccarbon,is only possiblewith HCI and subsequent HF treatmenton finely groundrock. This latter methodis usedfor kerogenisolation.The reflectedJight technique for observing kerogenusesa coal petrographicmicroscope.The microscopeis equippedwith oil immersion objectivelensesfor reflectedlight,a photometeranddevices for fluorescence and transmitted-lisht studies.
Identificationof SourceRocks
506
{'.
'.' h I . t\-:
-"aJt;'i.
'tt. - 'L..
LG l ' ? - lf-
Fig. V.1.6a and b. Organicmatter of the Probistipoil shaleof Mioceneage trom is shownin white tepresentstyfe I kerogen.In (a) a kerogenconcentrate Ju-goslavia The Botrvococcus the algae of are colonies objects aark light. The iirge tra'nsmined partly algal of matter, organic amorphous unstiuctured particles represent light smaller origin. In-(b) the samekerogenpreparationis shownin reflectedlight usingultraviolet verifiesthe liptinitic nature "*cltation io'prnducefluoresience Strongbright fluorescence is now visible The nonstructure (a). internal in Some shown of the algal material p3rticle.at the bottomrightofthe figureis pyritizedalgalmaterial fluoresceitelongated alSal thanthe structured lessfluorescence organicmattershowsgenerirlly The amorphous (Photos Hagemann) material. Rcflectance is measured with polarized light of 546 nm wavelength' uslng microscopeswith a photometer head Standardsare used, which give a specific reflectanie in oil. The effectivemeasuringapertureis usually3 pm or greater'The effectivc magnification should be greater than 400 Fluorescenceobservation is
1.2Type of OrganicMatter
507
Fig.V.1.7. Tvpe-llkerogen.composed ofsapropelic organicmatter(amorphous) mixed wrth small algaeand sporcs.and somc huminiticparticles(angulat,dark fragntents). Sapropclic matterappearsto be verv faint duc to the low degreeof coalification of this sumple.Kerogen concentratcof Sinemuriirnshale. Luxcmburg.Transmittcdlieht. (PhotoHagcmann )
Fig.V.l.lt. Type-lllkerogen composed plantparticles of terrestrial of huminiticand inertinitic nature.Someparticles showdistincr cellular structure. KeroAen concentrate of Pliocene shale fromtheBlackSea.Whitereflected lightorlimmersion. l photoHagemann) generallycarriedout with a 100-wattmercurylamp(wavelength of light usedfor cxcitatnn: emissionpeaks 365-435nm) with a diachronicilluminator and appropriatebarrierfilters.
Identificationof SourceRocks
Y I
1.2.2 ChemicalMethodsBasedon KerogenAnalysis the maintypesof organicmatteraremostly Chemicalmethodsfor characterizing from the mineralfraction,or not separated' basedon kerogen,eitherseparated Additionally,the studyof extractablebitumencan alsoprovideinformationon (e.g., elementaryanalysis the typeof organicmatter.Mostchemicalparameters areinfluencedby the type abundance) or naphthene of kerogen,IRabsorbance, Thus, it is necessaryto matter. the organic and also by the maturationof typeversusdegreeof organic the original of influence the respective distinguish measurea single which methods by achieved maturation.This cannot be be basedonly on can (CR:Cr). distinction The ratio carbon parameter,like the analysis.or as elemental parameters, such independent several methodsusing methods. of different a crossplot basedon in ChapterIl '1arethe basis The fundamentalpropertiesof kerogendescribed organicmatter'Theirstudl'requiresa preliminaryeliminationof for subdividing the mineralfractionby acid(HCl. HF) treatment.The efficiencyol thistreatment from onerockto another'Pyriteis the majormineralpresent variesconsiderably and its residualabundancemay rangefrom 0 to more in kerogenconcentrates cannotbe usedlor analysis thata concentrate than507/c.It is generallyconsidered if it containsmore than 30% pyriteor other minerals.A typicaldifficultyis the are coatedby a film of occurrenceof framboids,where pyrite microcrystals matter(Sect.II.4.3). organic -Elemental of kerogen,whenplottedon a van Krevelendiagram(Fig' analysis IL4.11) providesan immediatereferenceto the threemain typesof kerogenas difficultin thewet becomes in thisbook.However,thecharacterization described where reflectance). vitrinite level of (beyond the 1.3-1.5E antl dry gaszones alike become manypropertiesof the differentkerogens Iifiared spectrophotometryprovides information on the occurrence and oi the variousfunctionalgroupsin kerogenand alsoparaffinicityor abundance aromaticity(Fig. II.a.7). In particularthe absorptionbandsprovidea comparative evaluationof the petroleum potential of different source rocks This intensityof the absorptionbandsrelatedto evaluationis basedon thi respective and to polyaromaticnuclei (source of hydrocarbons) groups CH2, CHl aliphatic ( i n c r tp a r to f k e r o g e nF; i g .1 1 . 73 ) . Eleitron spin resonance(ESR) applied to kerogen may also provide some information;bout the type of organicmatter.Howeverthe main influenceon ESR resultsfrom thermalevolution,andthereforethistechniqueis treatedlater ( S e c t1. . 3 . 2 ) . in kerogenalsodependon theoriginandchemical Carbonisotopedistributions thatterrestrialorganicmatteris structureof organicmatter.It hasbeenobserved r2C, as comparedto madne pl-qlltolt] th.e enrichedin tfie lighter isotope. the d''C of land(seeSect.L2 5). In ancientsedim-ents differenceis 5 to 1-0%" derived kerogenis usually3 to 5%. lower than the drrc of marine-derived kerogen. refleclance ' stagesofmaturation,e g.' beyond1.57. vit-rinite In veryadvanced the because difficult, ofthe originaltypeoforganicmatterbecomes identification similar' of the differentkerogensbecomeprogressively chemicalcompositions
1.2Typeof OrganicMatter
509
However, electron microdiffraclion is still able to differentiate between the originallypesl, II or III of the organicmatter.The discriminative parameteris the size of aggregatesproducedin kerogen by heating at 1000'C. These aggregates appear as cloudswhere individualaromaticplateletsand stacks becomeparallel.The sizeof the aggregates is 80 A or lessin typeIII, ca.200A in type II, and 500or even1000A in rype-Ikerogen(Oberlinet a|.,1.974). | .2.3 PyrolysisMethod Basedon WholeRock Samples The maindifficultyin applyingchemicalmethodson kerogento routinework is the rather lengthyand costlycharacterdue to the preliminarystepof kerogen isolation.This represents a restrictionon the numberof kerogenconcentrates availablefor analysis.A quick methodis neededto providethe sametype of informationfrom the originalcoreor cuttings,andrequiringno specialpreparation other than grindingthe rock. The data obtainedfrom the studyof many kerogenshave enabledEspitalidet al. (1977 ) to developa standardpyrolysis methodofsourcerockcharacterization andevaluation(Rock-Eval).The method usesa specialpyrolysisdevicesketchedin FigureV.1.9.The sample(ca.100mg) is progressively heatedto 550"C under an inert atmosphere,usinga special temperatureprogram(Fig.V.1.10).Duringthe assay,the hydrocarbons already
THERII,IAt CONDUCIIVIIY DITECTON
H2A Itap
CO2 rtap
Fig. V.1.9. Principleof the Rock-Evalpyrolysrs deviceof Espitalidet al. (19'7'l)
510
Identificationof SourceRocks
" Anoyss cycle
Fig. V.1.10. Cycleof analysis and exampleof record obtained by the pyrolysis method of Espitalieet al. (1977).Application to petroleumexploration
fxcmp e ol recoro
O a l o r9 o r s h o w 3
OilI gos poleniiol Srr52 (lqlron oirock)
T y p eo f o r 9 .m o l 1 6 r 5 2 / o r qC H y d r o q e n
Molurolion
A p pi c o l i o n to petroleum exploroiion
T r o n s l o / m ool n r o l o S1/51+52 P€.k lemeeralure Tm!!(Cl
presentin the rock in a free or adsorbedstateare first volatilizedat a moderate (S,) is measuredby a flame temperature.The amountof thesehydrocarbons ionizationdetector(FID). Then, pyrolysisof kerogenresultsin generating hydrocarbonsand hydrocarbon-likecompounds(S2) and oxygen-containing volatiles,i. e.. carbondioxide(S3)andwater.The volatilecompounds generated are splitinto two streamspassingrespectively througha FID (detector)measuring 52and a thermalconductivitydetectormeasuring53.An adequatetemperature programallowsa goodseparationof S1and 52peakson the FID detector. window However.the measurement of 53is Iimitedto a convenient temperature in orderto includethe mainstageof CO2generation from kerogen,andto avoid other sourcesof CO, (suchasdecomposition of carbonates, and particularlyof siderite.whichis the mostlabilecarbonate). A fourth parameteris the temperature T-,, corresponding to the maximumof hydrocarbongenerationduring
1.2Type of OrganicMatter
511 < o 300
!? 2
=
T-
ai
----------------
25
50
---r 0IyGtN |N0EX(mgC02,/gorqC )
HyoR0GtN tNoEXimqNC,/g orqC) ^ Sahara lLibva.Alsena )
L Sahara I Ltbva.Alget le )
Fig.V. L I L Correlation of hydrogen andoxygen indices measured by pyrolysis of rock with H/C and O/C ratios.respectively. measured by elemental analysis of kerogen (Espitalie et al.. 1977) pyrolysis.However,it is usedmostlyfor evaluationof the maturationstage, whichis treatedlater in Section1.3.2.FigureV.1.10showsa typicalrecordwith indicationof S,, S2,S1and ?",,,,,. The tlpe of the kenryen is characterizedby two indiccs: the hytlrogenintlex (S.iorganiccarbon)and the oxygenlndex(Sj/organiccarbon).The indicesare independcntof the abundance of organicmatterand are stronglyrelatedto the clcmentalcompositionof kerogen.In particularthere is a good correlation betweenhvdrogenindexandH/C ratio.andbetweenoxygenindexandO/Cratio, respectively (Fig. V.1.11).Thus the two indicescan be plottedin placeof the normalvan Krevelendiagram,and interpretedin the sameway. Rockscontaining typicalkerogensof typesI. II and III havebeensubjectedto pyrolysis,and their hvdrogcnand oxvgenindicesare plottedin FigureV.I.12. The rnreemarn evolutionpathsare generallydistinct,and canbe readilyrecognized. A semi-quantitative evaluationof the geneticpotentialcan be achievedby usingpvrolysis.The quantitySr represents the fractionof the originalgenetic potentialwhich hasbeeneffectivelytransformedinto hydrocarbons. The quantitv Sr represents the other fractionof the geneticpotential,i.e., the residual potentialwhich hasnot yet beenusedto generalehydrocarbons. Thus S, * S1, expresse.d in kg hydrocarbonsper ton of rock. is an evaluationof the genetic p0tenttal'-. l2 This evaluationaccounts for two aspects. abundance andtypeof the organicmatter
Identificationof SourceRocks
512
ofthe source Fig.V.1.12. Classification rock types by using hydrogenand oxygen indices. This diagram is readily comparableto the van Krevelendiagram plottedfrom elementalanalysisof kerogen(Espitali6et a!.,1977)
,l
'rt
z
I
(5
Lower Toa.ctan, Patis Basln S Ilu I ta h - D evon i a n, Alge I ta Ltbva Uppet Cretaceous Douala Aastn
II
u
.ll
.i[,* 'l+t" ------|
u 50
roo
r50
0 X Y G EI N O E(Xm gC 0 2 l go r gC )
Genetic polenliol
E
Y
(kg HC/lon of rock)
10 20 50100200 0 100 200
A
300 400 500
B
600 700
c
800 900
D
1000
I
I
1100 1200
F
1!00 1400 r500 r600 1800 1900
F
Fig. V.1.13. Example of a well log of the geneticpotential.Geneticpotentialis plottedas a functionof depth and allowsa quantitative evaluationof the sourcerocks.Units,4, C' and potential.UnitsB andD Ehaveno hydrocarbon sourcerocks.Unit Fshows aregoodto excellent to decreasing a fair potentialfor hydrocarbons, low potential at the bottom of unit F. Some individuallayershavea muchhigherpotential
1.2Typeof Organic Matter
513
The followingclassification of the sourcerocksis suggested: - lowerthan 2 kg t r (2000ppm): no oil sourceroci, somepotentialfor gas. - from 2 to 6 kg t I (2000to 6000ppm): moderatesourcerock, above6 kg t ' (6000ppm): goodsourcerock. Exceptionally, somevaluesashighas100or 200kg t I maybe observed. Such rockscontainabundantorganicmatterof type I or II andmayprovideeitheran excellentsourcerock, if the burialdepthis sufficient,or an oil shale,if the burial depth is shallow.FigureV.1.13 showsa well log of geneticpotentialplotted versusdeDth. Other iechniquesof pyrolysishavebeenproposedto distinguish the typesof organicmatterand to evaluatethe sourcerock potential.However.their main limitationist the difficultyto distinguishan immaturerock with a row geneuc potentialfrom a morematurerock with an originallyhighgeneticpotential.The previouslydescribcdmethodof Espitalidet al. ( 1977)easilyallowsthisdistinction by plottingoxygenindexversushydrogenindex.Without the oxygeninclex,the interpretation remainsdubious,unlessan independent evaluationof thematuration is carriedout (e.g., vitrinitereflectance). An exampleis the '.carhonratio" (C*:C.), whichwasoriginallvproposeLl hy G ranschandEisma1l ei0) to evaluate the degreeof maturationof a sourcerock;it is discussed in SectionV. 1.3.2.Later lt wasusedasa tool for characterizing the typesof organicmatter.The residual carbonCq. after pyrolysisat 900'C under inert atmosphereis relatedto the a b u n d a n coef p o l y a r o m a t n i cu c l e iT . h e v o l a t i l ef r a c t i o nC 1 m i n u sC x ( C 1b e i n g the total organiccarbon) is relatedto the abundanceof aliphaticchainsand oxygencontainingfunctionalgroupswhicharelostd uringpyroi;sis.Thusthe low C^:C, valuestruly correspondto immaturesourcerockscontainingaromaticpoor kerogenof type I or II. Their geneticpotentialis high.In contrastthe high Cp:C'.valucsmav be obtainedeither from the samegood sourcerock after m a t u r a t i o no,r f r o mr o c k sc o n t a i n i nagr o m a t i c - r i ct yhp " - i l l k . t o g " n .e v e na t l o w stagesot maturation.Thus the interpretationremainsambiguous.unlessit is h lr l r n oht c r m e t h , ' d . complsleL Pvrolysis-fluorescence is a quick testwhichcan be easilyperformedat a well site(HeacockandHood. 1970).A weighedamountof coreor cuttingsis heatedin a testtube.Bitumenwhichhascondensed alongthe cold partsof the testtubeis then dissolvedin a few millilitersof solvent.The fluoresience of rhe solutionis measuredby using a simple. filter-type UV fluorescenceequipment.The fluorescence intensityPF showsa goodcorrelationwith the parameterS, of the Rock-Evalpyrolysistechnique.Thisis quiteunderstandable ls the samevolatile fraction(hydrocarbons andrelatedcompounds) is involvedin both assays. Thus, PF_can be usedfor a quickevaluationof the residualgeneticpotential.Again,the lackof anothcrindependent parametermakesthe true distinctionof the kerosen typesdifficult.A crossplot of PF versustotal organiccarbonmay proride some informationin this respect.
514
Identificationof SourceRocks
I .2.4 ChemicalIndicatorsBasedon Bitumen fossilswhichcarryinformationabout Extractablebitumenincludesgeochemical the originof the organicmatter.Theycanbe usedto identifythe typeof organic matter(e.g., typeII mostlymarineandtypeIII mostlyterrestrial)and,in turn,to makesomeappraisalof the sourcerockpotential.However,therearelimitations of bitumenamountto only a smallpart First,the freemolecules to thisreasoning. of of the total organicmatter.Furthermore.the contentof the variousclasses one vary widely from compounds(e.g., n-alkanes,acids,sterols,etc.) may n-alkaneswith an sourccof organicmatterto another.For cxample,long-chain plants, whereasthey only in higher odclnumberof carbonatomsare important 907o marine containing marine sediment are ncgligiblein plankton. Thus a bearsa usually from the continent plant debris brought planktonplusl07r higher considered. are n-alkanes weight if high molecular fingerprint. tvpicalterrestrial Anothersourceof difficultyis the mobilityof bitumen,i. e., it maymigratealsoin andnot only in typicalreservoirrocks.Suchaccumttsourccrock-typesediments lation is markedby abnormallyhigh bitumento organiccarbonratios(above the originalpatternof 200mg:gof organiccarbon).Under suchcircumstances fossilmoleculesmay be changedby an input of bitumenwith a differentdistribution. of thetype Fromtheseconsidcrations it isgenerallybetterto basetheevaluation oforganicmatterandof thc sourcerockpotentialondataobtainedfromkerogen. Kerogenusuallyamountsto 807aor more of the total organicmatter,and it is the major unableto migrate.Thus,informationbasedon kerogenencompasses of the rock under part of the organicmatter, and is definitelyrepresentative consideration. in ChapterII.3. fossilsis described of geochemical The natureandoccurrence in Chapter is discussed in termsof geologicalenvironments Their significance IV.3. For example.the sourcesof organicmaterialare clearlyoutlinedin the GabonandMagellanbasinsby the distributionofsaturatedhydrocarbonsln the of organicmatter,at a coastalbasinof Gabon(Fig.IV.3.1),thereis a background This in sediments. permanently incorporated levelof about0.57oorganiccarbon, potential is low. However. petroleum origin and its organicmatteris of terrestrial somelayerscontaina largerproportionof organicmatter(from 1 to -5%)with a "effective"sourcerocksof highpetroleumpotential.In fact,theselayersarethe of the oil fields,althoughtheymayamountto lessthanhalf of the totalthickness theformation.Inthe MagellanBasinofsouthernmostAmerica(Fig.IV 3.3).the Springhillformationhas been depositedin severalsmall basins,separatedby from marineto nonmarine,dependstructuralhighs.The environmentchanges is marine,the ing on the localconditions.In someareas,wherethesedimentation organicmatteris mainlyderivedfrom plankton.Therethe petroleumpotentialis high, and the crude oils belongto the normal paraffinicor naphthenictypes. is nonmarine,and the organicmatter is mainly of Elsewhere,sedimentation terrestrialorigin,with a highproportionof longparaffiniccarbonchainsderived lower and from plant waxes.The relatedpetroleumpotentialis comparatively crudeoils belongto the high-waxtype.
1.3Maturationof the OrganicMatter
515
1.3 Maturationof the Organicmatter Thermal evolution of the source rock, during diagenesis,catagenesis and metagenesis, changes manyphysicalor chemicalproperties of theorganicmatter. Thesepropertiesmaybe considered asindicatorsfor maturation.Theparameters most commonly used in petroleumexplorationare optical examinationof kerogen.physicochemical analysisof kerogen,and chemicalanalysisof extractable bitumen.Hood and Castano(1974)comparedthe varioustechniquesof measuringthe thermalevolutionand proposeda numericalscalefor it called LOM (Levelof OrganicMetamorphism; Hood et al., 1975). l..l.l OpttculItrtliatort ol ll4oturity Examinationin transmittedor reflectedlight. with or without fluorescence. providesdifferent t1,pesof intbrmationon the thermal evolutionof organic matter{secalsoSect.V.1.2.1).The main advanrage of opticaltechniques is to discriminate and locateorganicmatterof differentoriginsand to measuretheir respcctiverank of cvolution. For example,a contributionof older kerogen tragments,rcworketlfrom ancientrocks.can be identifiedbv its higherlevelof rcflectanceor carbonization. l'he proportirrnol reworkedkerogenmay not be accounted for bv purelvchemicaltechniques, whjchmeasurebulk propertiesof the mixture of contemporancous and reworkedorganicmatter. Furthermore, theremight be someadvantage in bcating the measured particles. a) Carbonization of Palynomorphs Observationof palvnologicalconcentratesin transmitted/rgrt is the basis for severalscalesof maturation.They userhe progressive changesof color and/or structureof the spores,pollen and other microfossils. The color is originally yellow.thenbecomes orangeor brown-yellow(diagenesis), brown(catagenesis), and finally black (metagenesis). Alteration of the structuralfearuresoccurs mostlyduringcatagenesis andmetagenesis. For instance, sporesandpollenat the levelof metagcnesis are no longersuitablefor stratigraphic investigations. The originalobservations of sporesand pollencarbonization were madefifty years agoin coal.then in othertypesof rocks.A reviewanda systematic investigation of the carbonizationof sporesand pollen grains,basedon observalions and experimental work, wasmadeby Gutjahr(1966).He useda colorscale,andalsoa standardprocedureto measurethe absorptionof light. The authoremphasized that differenttypesof sporeor pollengrainsshowdifferenlabsorptionvaluesat low levelsof maturation.This fact is due ro the original diffirence in wall thickness,and possiblyto differences in chemicalcomposition. With increasing levelsof maturation.differencesbecomelesspronouncedand disappearin thi wet gasand dry gaszones. Correia(1967)andStaplin(1969)proposedtwo scalesin whichboth colorand structurealterationwereused;theyarerespectively namedstateof preservation
516
ldentification of SourceRocks
3 I
Fig. V.1.14. Histogramshowing th€ distributionof the reflectance of organic matter, measuredby reflected light on a polishedsection.The various constituents- liPtinite. vitriniteand inertinite- have different ranges of reflectance. (Middle Liassicshale, Luxemburg; modified from Hagemann,1974)
tl"m qsAl
,7-
Viirinil. 38% |
E
t\
/K
[1"""r',rr"rr*]
!v7
'----*,""",,",,"",""""': "" ""'i*,'" I andthermalalterationindex.Fivestagesare definedby color of palynomorphs, and alterationof shapeand ornamentation.Stage 1 refers to fresh yellow to barelyidentifiableremnantsof thezone material.whereasstage5 corresponds F i g .V . 1 . 3 2 ) . o f m e t a m o r p h i s(m
b) Vitrinite Reflectance Reflectanceof coal maceralsmeasuredin reflected/rglrl has long been used to evaluatecoal ranks. A relationshiphas been establishedbetweenhuminitecoalranks(Alpern' 1969; andotherpropertiescharacterizing vitrinitereflectance Therefore, vitrinite 1972) and Teichmiiller, Teichmiiller, 1971; McCartney to definecoaltfication parameters best of the as one reflectanceis now considered stapes. havebeenextendedto particlesof disseminated Reflectancemeasurements et al in shalesand other rocks:Vassoevich (kerogen) occurring organicmatter (td69), Lopatin (1969),Teichmnller(1971),Hood and Castano(1974).Dow are estab(IOZZ;.Uirtog.urnsshowingthe frequencydistributionof reflectance to valuescorresponding lished.They usuallyshowseveralgroupsof reflectance or maceralsof the kerogenconcentrate.In the same the variousconstituents, from liptiniteparticlesto vitriniteandfinallyto increases sample.the reflectance increasewith ineriinite (Fig. V.1.1a). Both liptinite and vitrinite reflectances thermalevolution(Fig.V.1.15).However,only huminiteor vitriniteparticlesare vitriniteis themain scale,because to thecoalification generallyusedfor reference of vitrinite is maceral'ofhumic coals.Thereforethe meanrandomreflectance Dreferredto otherparticles.However,thereare situationswhereseveralgroups of uir.init" particlis, with different reflectance,are present.It is frequently consideredthat only the group of particlesshowingthe lowest reflectancetruly material Other vitriniteparticles represents the situationof the autochthonous -with higher reflectance are consideredto be reworked:Figure II.tt ll' Although this practiceis frequentlyjustified, there are geologicalsituations
1.3Maturationof the OrsanicManer
51'1 Fig. V.1.15. Comparedincreaseof the huminite/vitriniteand exinite/liptinitemaceralswith burialin a Logbabawell (Douala Basin,Cameroon).The standarddeviation o is shownfor eachsample,and also the level where fluorescence ends (Alpern et al., 1978)
. BmVitrinite ^ Rn Erinite
whereseveralgroupsof vitriniteare presentand reworkingis unlikely.Thus,it canbe considered that, accordingto the sourcematerialand conditionsof early diagenesis, theremaybe differenttypesofvitrinite,whichmayreactin different waysto thermalevolution. From correlationof huminite-vitrinitereflectance with other Darameters of sourcerock maturation,and alsowith the occurrence of oil and gasfields,the followingstages (for whichR, is themeanreflectance canbe distinguished in oil): a ) R. < 0.5lo 0.7%: diagenesis stage,sourcerock isimmature, D,I 0 . 5 l o 0 . 7% < R , ,< c a . 1 . 3 7 o : c a t a g e n e s i s s t a g e , m a i n z o inl g e eo nf e r a t r o n ; alsoreferredto asoil window, ca. I.3Va< R,,< 27o:catagenesis stage,zoneof wet gasandcondensate, d ) R,,> 2Vc:metagenesis stage;methaneremainsasthe only hydrocarbon(dry saszone). There are no sharpboundariesof the oil zone, as oil represents a complex assemblage of a wide varietyof componentswhich are generatedat different rates. Furthermore,kerogenhas no uniform structure.Instead,the relative abundanceof chemicalbondsthat differ in strengthvarieswith kerogentype. A high relativeabundanceof weakerbonds,such as some heteroatomicbonds frequentin type-ll kerogens,meansthat onset of oil generationoccursat a relativelyearlystageof the maturationprocess.On the contrary,a highrelative abundanceof strongerbonds, such as C-C bonds of the aliphaticnetwork constituting the bulk of type-Ikerogen,meansthatonsetof oil generation occurs at a relativelylate slageof the maturationprocess. This effectis strongerin the lowerthan in the higherlevelof maturation.For example(Fig. V.1.16)the beginningof the oil zonecorresponds to a reflectance of about0.5% in the ParisBasin(kerogentypeII) ascomparedwith 0.l7o inrhe Uinta Basin(kerogentype I). The maximumor peakof the oil generarion curve (Fig.V.1.33)occursat about0.97oreflectance in the DoualaBasin(kerogentype III) and about1.1V0in the Uinta Basin(kerogentypeI), whereasit seemstobe reachedat 0.87ain the ParisBasin(kerogentypeII).
518
Identificationof SourceRocks Fig. V.1.16. Approximate boundariesof the oil and gaszonesin terms of vitrinite reflectance. Boundaries may change slightly according to the time-temperature relationship,and also to the mixing of various sources of organicmatter Condensoteond wei qos zone
The beginningof the wet gaszonelikewisehasno sharpboundary.It generally occursbetweenl.2o/oandI.4Vo.However,the lowerboundaryat 2Vohasmore generalvalue,becausesimilaritiesin chemicalcompositionof kerogenbecome strongerat that stage.This convergenceis shownby the H/C and O/C ratiosofthe varioustypesof kerogen(Fig. II.5.1), and also by other physicalanalyses of kerogen. Huminite-vitrinitereflectanceis the most widely usedopticaltechniquefor determiningthe maturityof a sourcerock. It is probablythe bestopticaltool presentlyavailablefor that purpose.However,there are severallimitationsto this technique,whichshouldbe kept in mind for a properinterpretarion: I . Oneof the parameters usedfor identification of syngenetic vitriniteparticles is its reflectance.This reflectance,however,is subsequently measuredas a maturityindicatoron the vitrinite particles,which havebeenselected,among others.on the basisof their reflectance. 2. Vitrinite is abundantin type-III kerogen,and it can be usedexcellently. Vitrinite is only moderatelyabundant,or evenscarceJ in type-II kerogen,and mostlyabsentin typeI: thissituationcouldbe an importantlimitationfor the use (Alpern,1980).Furthermore,marineor lacustrine of reflectance (types kerogens I and II) may alsocontaingelifiedparticlesresemblingvitrinite althoughtheir chemicalcompositionand the evolulionof reflectance may be different(Alpern et al., 1978).For instance,Alpern (1980)foundin the lowerToarcianshalesof the Parisbasin, besidesa predominantmaceralof the liptinite group, three gelifiedmacerals with differentreflectances. Two of themaredefinitelysyngenetic, with an averagereflectance of 0.25and 0.6% respectively and their relative abundance changes with the depositional environment, from the coastlineto the o p e ns e a :F i g u r eV . 1 . 1 7 . 3. The reflectanceintervals shown above for the oil and gas zonesare only approximate.There havebeen many discussions about the boundariesof the variouszones,in termsof reflectance: Dow (1977),Herouxet al. (1979),Alpern
| ..1Maturationof the OrganicMa[er
@
21. t5 t0.
G)
d ' a
40f 30 25. 20. l5: t0f
o
519 Fig. V.1.17. Reflectance of gelifiedmacerals in the Lower Toarcianshalesof the eastem Paris Basin. ln addition to a predominant maceralof the liptinitegroup(reflectance 0.10 to tl.l5 7. not represented here).two populations of syngeneticgelified macerals(average R",0.25 and 0.67orespectively) are present. The depositionalenvironmentchangesfrom coastal(A) to opensea(C). Locations.4, B, C are different,but closeto locationsG2, G6, and GlO shownin FigureV.1.25. (After Alpern. 1980)
Rn%
(1980).Robert (1980),Teichmiiller(1982).In fact the observations reported aboveand alsorheoreticalconsiderations (Tissotand Espitali6,1975)indicate that the rateof transformation of theorganicmatterduringburialdependson the chemicalcomposition.Obviouslythe vitrinitemaceralhasa compositionrather comparable to type-lll kerogen.Its chemicalreactionat increasing temperature is similarto that observedfor type-lll kerogen.There is no reasonfor different kerogen,like typesI andII. to havethesamerateof transformation asfortype-III kerogenor vitrinite.Thus,differentsourcerockswith the samethermalhistory shouldhavereachedthe samelevelof vitrinitereflectance. but not necessarily the samestageof evolution.in termsof oil or gasgenerationzones.Furthermore,it shouldbe realizedthat differentthermalhistories,e. g., heatingrates,mavresult in the samelevel of vitrinite reflectance but in differentamountsand typesof hydrocarbonsgenerated.This is again due to the differencein the chemical structurebetweenvitriniteandthe mainhydrocarbon generatingkerogens,and also becauseof the differencein the kineticsof the hydrocarbon-generating proccsses. The rar& of the associated humiccoalis anotherasDect whichhasbeenusedfor studyingpetroleumgeneration.It is closclyrelatedro vitrinitereflectance, asthe rank is determinedeither by this techniqueor by other techniques,such as volatilematteror carboncontentwhich can be correlatedwith vitrinitereflectance.Coal rank has beenusedby the previouslycited authorsusingvitrinite retlectance and alsoby Brooks(1970),Hacquebardand Donaldson(1970)and D e m a i s o (n1 9 7 5 ) . c) Fluorescence Fluorescence of variousliptiniteconstituents is inducedby blueor UV light.The visiblelightemittedby kerogenin response to excitationmaybe characterized by its intensitvandcolorspectrum(vanGijzel, 1971;Alpernet al., 1972;Ottenjann
Identificationof SourceRocks
520
.!
t.t L 500 fluorrscmcemari n Imar $orhitc
0
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splctnr $ori.nt o
3 :;tH
{
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andtwo parameters vitrinitereflectance between relationship Fig.V.1.18. Observed in a seriesof coals(Teichmiillerand of thesporinitemacerals fluorescence characterizing Durand,1983)
is intensein shallowimmaturesamplesanddecreases et al., 1974).Fluorescence at the It hascompletelydisappeared and mostof catagenesis. duringdiagenesis changed:the endof the oil zone.Furthermorethe colorspectrumis progressively light movesfrom yellowtowardred with wavelengthrangeof the fluorescence These changesvary quantitativelyfor the different increasingcatagenesis. liptinite macerals(sporinite,cutinite,fluodnite) but remain qualitativelythe same(Teichmiillerand Durand, 1983).Furthermorea good relationshipwas andpyrolyvitrinitereflectance of coalmacerals, observedbetweenfluorescence sisdata:FigureV.1.18.Ottenjannet al. (1974)proposedthe ratio 0 : intensity to measurethischange. at 650nm:intensityat 500nm of sporinitefluorescence is presentlygettingto be a routine tool, but more work is still Fluorescence and However,reflectance of the observations. neededto clarifyvariousaspects be comand may scale pafts of the maturation fluorescencecover different intensityoccurs in fluorescence plementary.In particular,mostof the decrease varies little, i.e.' the low and reflectance is where vitrinite over the range generation zone. the oil beginningof
1.3.2 PyrolysisMethodsfor MeasuringMaturit! Many methodsutilizingsometype of pyrolysishavebeenproposedin order to the rank of evolutionof the organicmatter.Only the mostrecent characterize in detail,whereasthe othersarebrieflyreviewed.In general, onesare discussed pyrolysisis conductedunder an inert atmosphere(nitrogen,helium) with a pieseiectedrateofheating(approximatelylG-50'Cminr).Thepyrolysisyields three main groupsof compoundsequivalentto Sr, 52, and 31 of the method the typesof organicmatter: describedin Section1.2.3for characterizing
l . l V a t u r a l i otnr ft h eO r g a n iM c atrer
521
hydrocarbonsalready present in the rock (S1) which are volatilized by moderarehearingar 200to 250"C, hydrocarbons andrelatedcompounds (S2)generated at highertemperatures by pyrolysisof insolublekerogen, - carbondioxide(S.1) and water. pyrolysismethodof Espitalie et al. egjj), aspresentedin Section1.2.3, ..The uUlzes the threeparameters51. 52 and S,. Other methodsmeasure only the parametersS1and/orS.. In addition,the temperature?..,,.is recorded at the maximumof^hydro,carbon generationduringpyrolysis(Fig. V.1.10). Barker (1974).Claypootand Reed (1976jand-Espitaiieet at. (1977)have . shown that two indicesare of great interest in chiracterizingthe rank of the ratio Sr:(Sr+ 52) and the temperaruref.,, (Figis V.1.10and :y?lylo": v . l . l y l . l n a b s e n coet m r g r a t i o n t h e S 1 : ( S*1 5 2 )r a t i oi s a n e v a l u a t i oonf t h e transformation ratiodefinedin ChapterII.7. The continuous increase ofthis ratio asa.tlnctionof depth(Fig. v. r.19) makesit a valuabreindexof maturatron. ln additionto measuring maturation.the valuesS1andS,:(S1* S.)canbe usedfor a quantitotive evaluarron of the hydrocarbons generated. The resultis convenrenflv
I A 800h
.' il\
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",
I
I
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ts rooo
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t l
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i
iA 2r40h
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r\ .11\
o o,r 0,2 o,3 o3 0,5 ---------------+ TffANSF0BMATt0iJRATt0
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450
460
TEMPERATURI IOC)
Fig.V.1.19. Characterization ofsourcerockrnaturityby pyrolysis methods.Transforma_ tion ratio and/orpeaktemperature T,", may be usedai iniicatorsof thermalevorution. Theseparameters are definedon FigureV.i.10. (Modifiedatt", erpitaii! et at..t9j7l
ldentificationof SourceRocks
522
per per ton rock and in gramshydrocarbons in gramshydrocarbons expressed cannotbemade However,suchevaluation kilogramorganicmatter.respectively. due to the possibilityof minute accumulations from isolatedmeasurements, (microreservoirs) occurringin sourcerocks, which would alter the figures ratioSr:(SI + Therefore,it is necessary to plot thevariationof thetransformation high anomalously by their S) versusdepthin orderto identifysuchaccumulations valuescomparedwith the averagecurve. progressively, but the numericalscale The temperatureI.o, also increases has to be calibrated Thus the T-,, scale changeswith the rate of heating. indexis not procedure. This temperature laboratory accordingto the individual affectedby migrationphenomena.Furthermore.thereis a good correlationof indexI.u, thetwo indicesSr:(S1+ S2)and I,,u.in a givenseries.The temperature can also be corrclatcdwith other maturationparameters,such as vitrinite reflectance.Figure V.1.20. for instance,showsthe very good relationship for type-llI kerogenandhumiccoals The betwccnT.o, andvitrinitereflectance temperatureindex I,,,,,.is consideredto be amongthe most reliableonesfor characterizing thermalevolution.It is particularlyvaluablein the caseof marine or lacustrinekerogen(typesI and Il) wherevitriniteis often scarceor absent. whetherthesematurationindices51:(S1+ S2)and I,,,, are The cluestions influencedby the typeof kerogenmaybesolvedby useof the Rock-Evalpyrolysis availabilityof hydrogen methodof Espitalidet al. (1977).Due to simultaneous and oxygenindices.the identificationof the kerogentypesis carriedout at the * 52)isfairly sametime.From suchstudiesit hasbeenfoundthat the ratio 51:(S1 and metagenesis. independentof the type of organicmatterduringcatagenesis The temperatureI.o. is influencedby the type of organicmatter during the It is lower in the terrestrial diageneticstageand the beginningof catagenesis. lacustrine typesI andII However, kerogenof typeIII andhigherin the marineor typesin the peak different kerogen the T.,,,valuesare almostequivalentfor the gas zone. generation and later in the zoneof oil Oldermethodsfor determiningmaturity,suchasthe carbonratio.areaffected by the kerogentype.For example,GranschandEisma(1970)comparedthe total organiccarbonCr with the residualcarbonCp after pyrolysisat 900'(Ca is
E
t.0
r.5
2.0
Vitrinire rellectanceBm l%)
2.5
Fig. V.1.20. Relationship inbetweenthetemperature dex T,u* and vitrinite reflectancefor type-III kerogenandhumiccoals.(After Teichmiiller and Durand, 1983)
1 . 3M a t u r a t i o on f l h e O r g a n i cM a e r
523
approximatelyequivalentto Cr minusthe carbonof S). They usedthe ratio Cp:Ct as an indicator of increasingmaturation. However, the changeof sedimentation from onebedcontaininga morealiphatickerogen(typeI or II) to anotherbedcontaininga morearomatickerogen(typeIll) is alsomarkedby an increaseof the Cp:C1 ratio and can be misinterpretedas an increasi of maturation.Bordenaveet al. (1970)useddifferentconditionsof pyrolysisand calculated the ratio of the volatilecarbon(i. e.. carbonof S,) to the totalorganic carbon.The problemsof interpretationare rathercomparable. LeplatandNoel (1974)usegaschromatography to analyzethe varioushydrocarbons comprising the peakS1and then they computeratiosof differenthydrocarbons. Another method of pvrolysisis based on the detailed analysisby gas chromatographv of the hydrocarbons comprising the peakS, (i. e., hydrocarbons naturallypresentin the rock;Jonathanet al.. 1975).Althoughpvroll'sisis usedas a tool. theaim is thc compositional analysis of thebitumenandthc methodwill be d i s c u s s ei nd S e c t i o n1 . 3 . 4 .
1.J.3 ChemicalIndicatorsoJ Maturitv Basedon Kerogen Most kerogen propertiesevolve during the rearrangementof the kerogen structurewith increasing burial.The maturationrangeis differentfor thevarious physicalor chemicalparameters. andit corresponds to part or all of the thermal history. For instance,oxygen content is mostly affectedduring diagenesis whereashydrogencontentchanges mostlyin the catagenesis zoneandcrystalline organization(as seenby electronmicrodiffraction) appearsin the metagenesis zoneonly. However,many of thesetechniques haveto be consideredmore as researchrather than routine tools, due to the time and care neededin their application. analyslsof kerogen,plottedon a van Krevelendiagram,givesa - .Theelemental fairly good indicationon the maturationsrage:Mclver (1967),Durand ancl Espitalid(1973),Laplante (1974),Tissot et al. (i974). The relationshipof elementalanall,sisto a classical maturationindex- huminite-vitrinitereflectance- is shownin FigureII.5.1. From thisgraph,it is obviousthat a complete elementalanalysis. or at leastC. H. O, is needed.Due to theobliquetrendofthe iso-maturitylines.the knowledgeof the carboncontenlalone,or evenH/C is not suflicient.For example.an atomicratio H/C - 0.8 is observedat the endof the principalzoneof oil formationin type-Il kerogenwhereasit is alsoobservedin immaturesamplesof type III which are approachingthe beginningof the oil generation. The approximate boundaries for the threemaintypesof kerogenare shownin TableV.1.4.Fromthistableandalsofrom FigureI1.5.1, it appears that the H/C ratio is a betterindicatorof the maturationof kerogentypesI or II and the O/C ratio in the caseof kerogentypeIIL However,the two ratiosshouldbe interpretedtogether. Among physicalanalysisof kerogen,electronspin resonance (ESR) hasbeen proposedasa routinemelhodfor measuring organicmaturityor evenpaleotemperatures(Pusey,1973).The observationof an ESR signalin kerogenresults from the occurrenceof unpairedelectrons- i. e., free radicals- in kerogen.
Identificationof SourceRocks Table V.1.4. Average value of the atomic compositionof the basickerogentlpes I, II, III at th€ beginningofthemain zonesof hydrocarbongeneration.Intermediatecomposition of keroeenresultsin intermediatevalues Beginningof the following zonesof hydrocarbon generallon:
Averase value of the atomic ratios
oit Wetgas Dry gas
1 . 4 51 . 2 5 0 . 8 0.05 0.08 0.18 o.'7 0.'7 0.6 0.05 0.05 0.08 0.5 0.5 0.5 0.05 0.05 0.06
Kerogenrype
Free radicalsappearin kerogenas a result of splittingbonds.For instance, crackingof an alkyl chainsubstitutedon a polyaromaticnucleusresultsin two fragmentswhich are free radicals,eachwith an unpairedelectron.The free radicalsexistuntil they recombine,eitherwith a hydrogenatomor with another alkyl chain.Thishappensmoreeasilyfor the alkylfragmentthanfor thekerogen fragment.Marchandet al. (1969)studiedthe ESR signalof kerogen:theyhave susceptibility shownthat the paramagnetic lp is proportionalto the numberof free radicals,and that 1, increaseswith thermalevolution.Pusey(1973)proposed using the "spin density" (number of free radicalsper g kerogen) to measure organicmaturityand paleotemperature. Durand eI al. (1977)investigatedsystematicallythe ESR signalon different types of kerogen and coal ranging from the stage of diagenesisinto the The generaltrendof variationis the samefor all typesof kerogen. metagenesis. with depthandtemperature;itreachesa maximumat a The signalfirst increases level correspondingto approximatelyZVo vitrinite reflectanceor more and al oflr is related greaterdepthsit decreases again(Fig.V.1.21).The initialincrease nucleiandit is on polyaromatic to the eliminationof the alkyl chainssubstituted decrease ofthe signal The subsequent correlativewith oil andwet gasgeneration. may be due to coalescenceof the polyaromaticnuclei to form larger sheets.In to the dry gaszone. termsof hydrocarbonpotential,it corresponds of a maximumin the;p curve(Fig.V.1.21)is a first sourceof The occurrence uncertaintyin appraisingthe maturityof a sourcerock by ESR. In fact, a given ESR value may correspondto two different stagesof evolution (one abovethe maximumand the otherbelowit). A secondlimitationstemsfrom the different behaviorof the kerogentype. At a certainstageof thermalevolution,the valueof thesignalis largerin type-IIIkerogen,comparedto typeI or II (Fig.V.1.21).This fact may be explainedin terms of stability of the free radicals:free radicalsare nucleithantheyareon alkylchains.Thus,they morestableon rigidpolyaromatic type-IIIkerogenthantheyare moreabundantin aromatic-rich arecomparatively oforganiccomposition typesI or II. Therefore,certainchanges in aromatic-poor
1.3 Maturation of the Orsanic Matter
525
s E
Fig. V.1.21. Influenceof kerogentype and thermalevolutionon the ESR signal. Paramagneticsusceptibilitylh of kerogentype III (Douala Basin)is plotted as a function of depth. Evolution of paramagneticsusceptibilityof kerogen type II is plotted for compadsonon the basisof vitrinite reflectance.(Adapted from Durandet al. 1977)
3 E
e
."i -J ;
'Xo.tosuemCGsperq- ---t bforgonrccorbon
may_bemisinterpretedin terms of increaseor decreaseof paleotemperature.A third sourceof uncertaintyis the time effecton recombinationof free radicals. At higher agethere is a greaterchancefor free radicalsto recombine, Finally, ESR may be consideredas a valid method for measuringorganic maturity during the catagenesis stagein basinswheretwo conditionsareveritied: (a) the typeof organicsedimentation remainsapproximately constant(e.g., Gulf Coast)and (b) rhe stageof metagenesis (i.e.','2Vovitriniie reflectance) is nor reached.It can be pointedout that on certainshales,like thoseof the Douala basin,a directuseof naturalrockgivessatisfactory results(Duraid et al., 1977). However,a preliminaryconcentration of kerogenis generallvreaulred. Other physicaltechniqueslike infrared speitophitometry, thirmal analysis, nuclear magneticresonanceand electron microd.iffractionmay be applied to characterize kerogenmaturation.Nevertheless, they ure .oniid"red more as researchtools that can be used in some instancesto solve specificexploration problems.
1.3.4 ChemicalIndicatorsof Maturity Basedon Bitumen Abundanceandchemicalcomposition of petroleumcompounds (hydrocarbons, resrnsano asphattenes) presentin sourcerocks dependon the nature of the original organicmatter, and the degreeof thermal mituration. Thus, numerous methods have been developed using the amount or the composition of the extractable bitumento characterize the stageof thermalevolution.However,it shouldbe pointed out that migrationof hydrocarbonsmay affecttheir abundance
526
Identification of SourceRocks
sourcerocksmayacqulrea Certainshalebeds,siltor carbonate andcomposition. etc.). recrystallization, slightporosityand permeability(due to microfractures, preferential accumulaThen they can be affectedby a short-rangemigrationwith tion of hydrocarbons,especiallythoseof low molecularweight.This situationcan usually be detectedby abnormally high transformationratio (obtained by the carbonratio. pyrolysismethod)or bitumen:organic
a) Abundanceof Bitumen The first step of the analyticalprocessis usuallysolventextractionfollowed by hydrocarbon separation and weighing. The obundanceof bitumen may be expressedas bitumen ratio, i.e., total extract:totalorganic carbon, or as organiccarbon.The generaltrendof hydrocarbon ratio,i. e., hydrocarbons:total variation of theseratios with increasingthermal evolution hasbeen discussedin ChapterIL7 and is shownin FigureII.7.1. If a sufficientnumberof samplesis availablefrom a sourcerock formation at different depths,the valuesof bitumen or hydrocarbonratiosare plottedversusburialdepth,and the generationcurve for the Uinta, Parisand can be drawn.This type of graphhasbeenestablished in FigureII.5.12.On thisplot, the immaturezone, Doualabasinsandis presented the principalzoneof oil formation,and the zoneof gasformationcanbe traced convertedin termsof burialdepth. and subsequently Equivalentproceduresmay be usedto obtain the abundanceof hydrocarbons andweighing,whichrequiretime solventevaporation withoutproperseparation, and care. One suchprocedureusesquick extractionfollowed by IR spectroscopy of the solventcontainingthe hydrocarbonfraction,utilizingthe CH-bandat is directly of hydrocarbons about2930cm I (Welteet al., 1981).The abundance procedure is basedon Another measurement. evaluatedfrom the absorbance (for between instance heating by moderate volatilization of hydrocarbons the pyrolysis provided by the particular first signal S1 the of the rock. ln 200-250'C) method describedaboveshowsa very good correlationwith the extractable volatilizedis limited to althoughthe rangeof the hydrocarbons hydrocarbons, aboutC1n. There are severallimitationsto determiningthe evolutionstagefrom the from variousdepths First,a seriesof samples amountof bitumenor hydrocarbon. is not always available.Then, there may be changesof kerogen type or geothermalgradientoccurringin the basin.In turn, thesechangesaffect the amountof bitumenand makeit difficultto tracea regularcurveof hydrocarbon generation.Changesof kerogentype can be accountedfor if the total organic organiccarbon(signal52of the carbonis replacedby the amountof pyrolyzable pyrolysis). Thiswasdoneby Tissotet al. (1978)in Uinta Basinstudies.Then,the hydrocarbon ratio becomes rather comparable to the transformation ratio migrationand minuteaccumulain Section1.3.2.Finally,short-range discussed tion within the sourcerock may alsoalterthe amountof bitumen.
1.3Maturationof the OrganicMatter
52.1
b) Compositionof Hydrocarbons: General For the.above reasonsJmany aulhors use for maturity studiesthe composuton ratherthanthe abundance of hydrocarbons. The basicionceptappears clearlyon the right sideof FigureII.7.1. Originally,the only hydrocarbonipresentarethe geochemical fossilswith specificdisrributions. With increasing temperarure. new hydrocarbons aregenerated;their distributionis wideanddois not favorsoecific compounds. The progressive changeof the hydrocarbon distributionobseivedin sourcerocks reflectsthe dilution of the geochemicalfossilsby the newly generatedhydrocarbons. Thus indicesreflectingthesecompositionalchangei may be used to evaluatesourcerock maturation.Manv indiceshave been proposed,basedon variousclassesof hydrocarbons. Thiir validityobviously dependson the uniformity of distributionof the selectedcomDoundsin the varioustypes of Recent sediments.If the compositionof a cirtain classof hydrocarbons is the samein Recentsedimentsfiom anv depositronal environment.an indexbasedon that classis particularlyvaluahle.This is thecasetorthe lighthydrocarbons C3-Cr,whicharepracticallyabsenrin Recentsediments. The distributionof long-chain n-alkanes is alsosimilarin manyRecentsediments, but thereare exceptions. c) Light Hydrocarbons Amount and compositionof the light hydrocarbonfracrion,comprisingcom_ poundswith one to about eightcarbonatoms.havebeenfound to be suitable maturityindicatorsof sourcerocks.The basicconceptof the applicationof light hydrocarbons asa maturityindicatorrestsuponthe factthat li;ing organisms"do not synthesizea complexspectrumof Cr to C8 hydrocarbons, especiallvnot aromaticsandnaphthenes. Furthermore.light hydrocarbons aregenerated from kerogenat a muchhigherratewith increasing maturationascompiredto theC1,* hydrocarbon fraction,whichneedslessactivationenergies for their formationas comparedto light hydrocarbons.A great variety of techniqueshas been developedto extractand subsequently analyzelight hydrocarbinsfrom rock samples.Basically.all of them usegaschromatography andthe methodscanbe classified accordingto the molecularrangeof compounds analyzed: - gasanalysis(C,-C,,)of eitherheadspace gasor cuttingsgas, - gasoline-range analysis(Ca-C')of coresandcuttings. Essentialto all of these techniquesis to collect and store rne samples immediatelyafter recoveryin gas-tightcontainers(e.g., tin cans).The simplest methodis the analysis of the headspace gasaccumulated at ambienrremperature in,thesamplecontainerbearingthewater-covered rocksample.A knownvolume of the headspace is takenby a syringeandinjecteddirectlyinto a gaschromatograph.Heatingof the containerto just below100.Candsimultaneous highspeed grindingof the rocksampleincreases the yieldof the hydrocarbons accumuliting in the headspace (Hunt, 1975).Otherextractionprocedures mobilizelow boilin! hydrocarbonscontinuouslyfrom the rock and collect them in a cold trapl Mobilizationcan be carriedout by combinedheatingand grindingeitherunder
528
of SourceRocks Identification
reducedpressureor with the aid of an auxiliarygas(Durand and Espitalie,1972; Le Tran, 1975;Philippi,1975) Durandand Espitalid(1975)alsoappliedCFCII extractionto rock samples,however,becauseof the needto removethe solvent by evaporation,this method has applicationsonly in a higher molecular range (> Cui. I-e Tran (1971) isolated occludedlight hydrocarbonsfrom carbonate iocks'6y acid digestionof the mineral matrix. Jonathanet al (1975)usedthermal vaporizationof the C6-Crshydrocarbonspresentin coresor cuttings.The hydrocirbons are trapped and subsequentlyanalyzedby gaschromatography' Thus, a completefingerprint of the fraction < Crsis obtained. Using a comparable technique, Schaeferet al. (1978a, b) developedthe hydrogin stripping method in order to lecover hydrocarbonsfrom CsCe' This method combinesbothmobilizationof C2-Q hydrocarbonsfrom the rock and a subsequentcapillary gas chromatographyinto a single step. The stripping gas (hydrogen)acti alsoasthe carriergasin the capillarycolumn,andsplitlesssample iniroduition ensureshigh sensitivity.Samplerequirementsarevery low (between samplesfrom of hand-picked 0.1 and 1.0g) and,therefore,facilitatethe analysis drill cuttings. Bailey eI al. (1974)analyzedthe cuttingsgas(Cr{a) and the hydrocarbonsof the gasolinerange (Ca-C7)in Western Canada.The variation of composition observedcan be summarizedas follows: L diageneticstage:the gasis mainly composedof methane;the gasolinefraction absent, is lean,with mostcomponents gas abundant C2 to Ca hydrocarbons;the contains the stage: 2. catagenetic gasolinefraction is rich, with all componentspresent, 3. hetageneticstage: the gas is mainly composedof methanei the gasoline fraction is lean.
of wet Fig.V.1.22. Percentage gas (C2-Ca hydrocarbons) in the total Cr-C,rcuttingsgasfor 14wellslocatedin the SverdruP Basin,North Canada.The Plot versus burial depth delineates the main zoneof oil andwet gas generation.-Ar Main stageof HC generation (oil and wet gas).(After Snowdonand RoY, 191s)
1.3Maturationof the OrganicMatter
52g
oflight hydrocarbons duringthestage -Anothergoodexampleofthe generation of catagenesis is presentedin FigureV.1.22basedon datafrom 14 wills in the Sverdrupbasin,North Canada(Snowdonand Roy, 1975).A plot of ,,wetgas',, defined as the proportion of C2-Cahydrocarbonsin the total -,-Cn gasmiiture versus depth clearly delineates the stage of catagenesis(oil and wet gas generation)between4,000and 14.000ft. Leythaeuser et al. (1979a, b) studiedthe generationof light hydrocarbons in sourcerocks in relation to their maturity and type of organicmatter for selected explorationwells usingthe "hydrogenstripping"technique.They found, for instance,that over a shalysource-bedintervii oflurassic aseof about 1000m in WesternEuropelight hydrocarbons are progressively geneiated. An overalltenfold increaseof the total C2-C7hydrocarboncontentnormalized with respect to organic carbon is observedover the interval studied. Fieure V.1.23showsthe trendof concentration vs. depthfor selectedn-alkanes.lican be observedat an early maturationstagethat the hydrocarbonconcentratrons increasewith differentratesfor differenthydrocarbonspeciesby the following approximateorder of magnitude:ld5 for ethane. 101for propane,10r for n-butane,elc.This meansthat the activationenergies of the foimaiionreactions increasewith decreasing chainlengthof rheproducts. The maturity-relatedlight hydrocarboncontentsin sourcerocks have been investigatedby Schaeferet al. (1982).As shownin Figure V.l.24 the total saturatedlight hydrocarboncontents,asdeterminedby J combinedhydrogen_ stripping/thermovaporization technique,of selectedpetroleum source beds increaseby morethan two ordersof magnitudeif maturityincreases from 0.4to
yleld t 9/9Cdql+
Fig. V.1.23. Depth trendsfor carbon-normalized yieldsof ethaneand propane(upper)andpentaneto heptane (loh)er)for ^ I\rassic shaleunit of FastnetBasin, Ireland. Hexane valuesfelf within the stippled arca. (After Leythaeuser et al., 1979)
Identificationof SourceRocks
530 ' gSl 9 ' o r q ] ONF a 2 _ C g 5 A T U n A T EHDY D R O C A R B o[N CINCENTFATIC
,i'
lO,
o s D PL e g? r s t e 5 r r J u f o s s r - A p t o ng l @ k s h o l e s [RFo.05"4
6l 2
4
6
5 2.
lighthydrocar' Fig.V.1.24. Saturated h o n c u n c e n t r a t i o n : 1n igng C . . r )i r t h e h l a c k. h a l e s o f S i l e5 l l o t t h ( D e e f S e a D r i l l i n gP r o j e c t( D S D P ) comparedto those of Potentialoil source rocks of different maturit) .ldges.Thr\ compari'onindicatesadd i t i . r n r l l vt h e l o w m a l u r i t ) o f t h e ' ' b l a c k' h a l e s t r o m S i l e 5 l l ( A f t e r et al.. 1983) Schaefer
12, ,Cl 8. 6.
F-.J66 c-r%l
2 ) J .
lufo.sc
e 6l
1PDf.lp.e B.sn fte o.d I
t8 ot.";l [R..0 -
2. c' r!.
105
, ; Ii l , ro"
ro5
Jurassic about0.9 R,,.More than l0r ng/gC,,,,are reachedin the hydrogen-rich V l 2 4 ) s o u r c eb c r li r l t h e m a l u r i l yi n l e r v a0l . 7 8 0 . q 4 q R ' { F i g Severalauthorsproposedusingfor maturationstudiesthe distributionof the Philippi(1975)usesthe C7hydrocarbons ol C6or ir hydrocarbons. mainclasses and plots in a tiiangulai diagramthe abundanceof paraffins(normal plus maturaandaromaticsWith increasing (cycloalkanes)' naphthenes isoalkanes). it is diagram triangrl.lar the On tion the proportionof paraffinsusuallyincreases. emphasized be pole lt should. paraffin lhe markedLy u .ou. in t'h. directionof that similarchangesalso occur in the higher rangeof molecularweight For example.a similir trend on the sametriangularplot (paraffins'naphthenes' fractionC,.* of sourcerockswith maybe observedfor the extractable aromatics) maturity. increasing ratio of specificcompounds'e g ' Jonathinet al. i1975)useda concentration index The basisforinterpretaa maturation as ' n-hexaneto methylcyclopentane plots, i e, an increasein the C? or global C6 the tions is still the samea; in The authorshavecalibratedtheir molecules. f-portion of linearversuscyclii reflectance' vitrinite as such parame:ers' indicesagainstmore classical employeda large (1979) wh-o by Thompson u;ed was A similar approach indices'e g ' paraffinicity so-called to establish data numberoflighiirydrocarbon maturatlon' with increaslng whichincreases the "heptane-value"
L3 Maturation of theOrganic Matter
511
d) CarbonPreference Indcx (Cpl) The carbonpreference indexhasprobabll,receivedthegreatestattcntlonamong
"^ll*j|r. cre,l91lcomqo;ition of bitumen. riwasoriginauy p,upo,.i' ll:tX:-u.: o y n r a y . a n d - t r \ a n s( 1 9 6 1 ) .a n d
i s b a s e do n t h e p r o g r e s s i vcet a n g . o f t n . disrributionof long-chain rl_alkanes duringmaturatio;.I; Re;e;t ;ediments,rhe malor sourceof theser-alkancsis wax from higherplants. Therelore.it was rhc fingerprintof the long-chain alkinesr;flectsthecontributionof ,rs\umed.rhJt n l g n e rp t a n t sr n n t o s tm a r i n eo r t e r r e s t r i aeln v i r o n m c n t s . U n d e rt n e s ec r r c u m _ stJnces.ther!-is a_strongpredominance of moleculesrvith an ocidnumberof r l r r x ) l l i L ) n 1l\n R c ' c ( . nSt r L J i n t e nT t sh.e r m a ld e g r a d l t i O o n f k e r o g e nC u r r n g cataqencsis -fhus subscqucntly generatcs ncwalkaneswitiout predominance. the prelercncefor odd-numberedmoletulcsprtrgressrvelr ji."pp."rr. This pheno_ i n u r t , i r r crrt t . r r r i n l C h r p r e rf t . : , n O i t i w n , t . J r n F i g u r c iul .. i-li'.,n, :.r. r, dt s: ,g:c1n. "e(rt l \ e o n s l J e r r t, h J a tt h e r ci s l i t t l eo r n o o d d p r e f c r e n cber : t h c t r n l et h e p c a ko l o i l g e n e r a t i oins r e a c h e d . S ( \ . s r ael \ p r e \ s i ' , nosf t h c r r t i dp r t , f e r c n chcr v e h c e np r ( ) p o \ e d : L t h e( r n r t i n rrl l e i i n i t i o nh r B r a va n dE v a n s( 1 9 6 1 u) s e d t h e C . ] _ C 1i .n, t e r v a l : CPI
l r cc.. . .+ c , r + . . . + c , l
2r ltcc. .r ,+ c , 6 +, , . ,. \( t- :
\
C,.+C.r+...+Ct1 j
'
.
\
'
.
'
t
s
.
.
'
'
t
)
l
? . P h i l i p p i( 1 9 6 5 )u s e dt h e predominance of the C', l-alkane interYal: D
l
in the C,5-Cj11
-
C.N + Cirl
3. ScalanandSmith(1970)usedan expression whichis applicablcto ary desired flve-carbon-number interval: C, + 6c,.:+ ( -
oEP=[
l
1 C'*t + 1 C*,.
]'" - { . T i s s o tc r r l . . ( 1 9 7 7 )p r o p o s e d a m a t h e m a t i c tarle a t m e not f t h e d i s t r i b u t i o n nilseoon t he fresttrtof I parrbola.anda numerical expression of thc deviation lorm thitt curvc. Although.thecarbonpreferenceindexhasbeenwidely applied.underany of ., these formulationsthere are severallimitationsto its uie.'ftre ipl valucsare irtfluenced rhg type.,rf organicmatter, and Uy ttrc .teg.eeoi maturitv. In .h1 P . l r t l c u l alr1. h i t sh e e no h s c r r e dl h a l s e d i m e n tosf t h e , a r n f3 g s .h r o u g h t , , t h e sametemperature dueto burial.mayresultin differcntCpl vaiues.Forixample. E r d m a n( 1 9 7 5 )r c p o r t e dt h a t T e r t i l r v s h a l e sr e a c h i n g a ,.,r[,,,,ur" of 110.C showan odd preference of 2 to 2.5in the Gulf of papualanOt -6in'itreNorth Sea. respectively. In addition,in theNorth Aquitainebisin, Lowercreraccous shales tt.9lC:f odd preferencethan the Upper ju.orr,. t"a, ot u llwals lalc,o il -5lr. Furthermore.thereaie situationswherehigh ::T|:.:::.1:,*^qj.irrahtc rno ro\\ L t t vatuesare llternarivelvobservedin successive bedsfrom the same w c l l( F i e .I l . - 5 . 1 6 ) .
532
Identificationof SourceRocks
is thatthe alkanedistributiondependson the The reasonfor suchobservations original distributionof the n-alkanesand on the amountof the new alkanes generated.First,thereare environments wherethe hypothesis on the universal shapeof the distributionof long-chainn-alkanesin immaturesedimentsis not valid. Huc (1976)hasshownthat in coresof the Lower Toarcianshalesof the Parisbasin.whichhaveneverbeenburiedto more than 500m, the CPI ranges from 1.0to 2.2.The highervaluesarelocatedcloseto theborderof the basin,and Furthermore,in the samecorehole,the the lowervaluesare locatedbasinward. CPI changes from 2.2 downto 1.1over a verticaldistanceof 15m (Fig.V.1.25). proportionofthe long-chain Wherethe terrestrialinputis negligible, a significant suchasplanktonicalgaeandbacteria. alkanesmaybe derivedfrom othersources, CoreDeplh ^^. hole lm) rrr A A A
n-Alkone dislribulion
0
50xm
G2 156 21
G6
7.5 2.2
G6
22
G10 324 1o
15 20 25 l0
zZi:Z/ .r' -.-BZ
Apprcrnole etlenslonol lhecanlinenlba/deinglhe lawel JurosstcSea Presenloulcropsoflo*et fo1tct1nsh0/e5 otld o/de o.ks Presenloulcropsaf P1leozotc shoto* carc-hole
Fig. V.1.25. Changesof the Carbon PreferenceIndex in immatureshalesof Lower arefrom shallowcoreholes,andtheirmaximumdepth Toarcian.ParisBasin.All samples of burial has never been more than 500 m. The changesof CP1 are relatedto the importance of the terrestrialcontributionfrom the Continentlocatednorth of the I-ower (Adaptedfrom Huc, 1976) Basin. Jurassic
1.3Maturationof the OrganicMatter
533
Theseorganisms do not usuallygeneratea predominance of the odd-numbered long-chainmolecules.Thus, the related CPI is near 1.0, even in immature sediments. At depth,thissituationmay explainthe observedalternationof high and low CPI observedin the obviously immature sedimentsof the Pannonian b a s i n( F i g .I I . 5 . 1 6 ) . The changeof the alkanedistributionwith increasing depthis directlyrelated to the amountof new alkanesgenerated.We havelearnedin ChapterIL5 that differenttypesof kerogengeneratesmalleror largeramountsof hydrocarbons duringcatagenesis. In the North Aquitainebasin,the Lower Cretaceous beds, havinga lower sourcerock potentialthan the Upper Jurassic,yield a smaller amountof hydrocarbons, and keep a higherCPI than the Upper Jurassicbeds (Table IL5.3). Furthermore,the n-alkane content in the total petroleum generated variesto a largeextentaccording to the typeof kerogen(Fig.II.5.15). Thus, the samevaluesof CPI may correspondto the formationof different quantitiesof petroleum,dependingon the kerogentype. The aboveobservations representlimitationsto the useof absoluteCPI values for definingmaturity.Despitethesevariouslimitations,the carbon-preference indexcanoffervaluableinformationon the maturationof sourcerocks.orovided it is consideredas a qualitativeindicatorand usedin association with another independentindex. High CPI values(above 1.5) alwaysrefer to relatively immaturesamples.Low CPI values,however,do not necessarily meanhigher maturity,theycanalsomeana lackof highern-alkanes stemmingfrom terrestrial input. Finally,it shouldbe mentionedthat an evenpredominance (CPI valuesof 0.8 or less)may be observedin carbonate-evaporite sediments. This hasto be interpretedalsoasa signof immaturity.
e) IsoprenoidDistribution Isoprenoidabundancealsovariesduring catagenesis (Sect.IL5.4.4) and may providean indicationof maturation.The pristane:phytane ratio hasbeenproved to be changedby thermalevolutionin coal(Brooks,1970),andthisobservation may be extendedto kerogensof comparable composition. However,no comparablevariationhasbeenobservedin association with typicalI or II kerogens. The ratio of isoprenoidsto n-alkanesdecreases with depth in all typesof organic matter, but the rate of decreaseis not directly related to the amount of hydrocarbons or alkanesgenerated. Thisis dueto thefactthatisoprenoidchains. linkedto kerogenby weak bonds,may be releasedfrom kerogen,and thusthe phenomenonis more complexthan a simpledilution of isoprenoidsby other alkanes.Nevertheless, the pristane:n-C17 or phytane:n-C13alkaneratio may providesomeinformationon thermalevolution.
f) Ring-NumberDistributionof Naphthenes Naphtheneshave also been consideredwith respectto thermal evolutionby Philippi(1965).He defineda naphtheneindex(NI) asthe percentage of 1-ring
of SourceRocks Identification
534
plus 2-ring naphthenesin the 420-470"Cisoparaffin-naphthenelraction.The aim dilutionof biogenic4- and5-ringnaphthenes bt Nt ls to quintliy the progressive by more simplemono- or dicyclicnaphthenesgeneratedfrom kerogen This ciiterion can be applied to a given soutce rock to compare various stagesof decrease maturation.For example,FigureIL5.17 clearlyshowsthe progressive However, basin. the Paris Toarcian of in the Lower of polycyclicnaphthenes difierent source rocks cannot be directly compared with each other, as the This content may vary considerably. originalcontentof polycyclicnaphthenes -II with low in association and or kerogens with type-l .un b. highin association type-lll kerogen. Index (MPI) g) Methylphenanthrene Index (MPI) has beendevelopedrecentlyby Radke Thc Methylphenanthrene andthreeor four of et al. ( 1932i). Il is basedon the distributionof phenanthrene changeduringmaturation its methylhomologswhichshowsa progressive organicmatteraregenetically from petroleumandsedimentary Phenanthrenes relatedto steroidsand triterpenoidsfound in biologicalsourcematerials(Mair, 19641Greiner et al., 1976).However,partial dealkylationof thesepotential only and' precursormoleculeswould result in 1- and 2-methylphenanthrene of 1-and9-methylphenanthrene' iherefore,maynot accountfor a predominance has Thus,methylationof phenanthrene asfrequentlyseenfor immaturesamples. the 1of depth trends as an alternativemechanism.Coinciding beendiscussed in the observed have been ratiosthal and 9-methylphenanthrene/phenanthrene WesternCanida Basin (Radke et al., 1982a)are likely to reflectthe similar rcactivitiesof the 1- and the 9-positionof the phenanthrenemoleculein methylationreactions.The relativeabundanceincreaseof 2- and 3-methylduringmaturationcanbe cxplainedon the basisof rearrangement phenanthrene more favor thesethermodynamically reactions.which. at highertemperatures, stableisomers. IndexMPI I the Methylphenanthrene considerations, Basedon mechanistical been introduced: has "t''
-
+ 3-methylphenanthrene) 1.5(2-methylphenanthrene pf"n"ttf,tt"*
r l - m c r h y l p h e n a n l h r e n-e' l - m e l h y l p h e n a n t h r e n c
IndexMPI 2, maybe usedasa the Methylphenanthrene A similarexpression. controlmeansor as a substitutefor the MPI l: M P I2 :
3(2-melh)lphenanlhrene) + 9-methylphenanthrene phenanthrene + 1-m€thylphenanthrene
higherthanthatof the The numericalvalueof the MPI 2 is generallysomewhat of 2- over3-methylphenanMPI 1; the differencereflectsa slightpredominance distribution threnewhichis commonto the methylphenanthrene A strongcorrelationof the MPI 1 and vitrinite reflectancedata within the 0.Gl.3Vc R, intervalhasbeenobservedin a WesternCanadaBasinwell (Fig' influencedby changesin the V.1.26).Obviously,the MPI wasnot significantly
I . . l M a t u r a r i o no f r h e O r g a n i cM a t t e r
53s
i/ETHYLpffNAMTlnEt€ ttO€X(Mpt1) oF_0t7 _ qej1 . 19- 1.s , 17
Fig. V.1.26. Correlationbetweenthe Methylp h e n a n t h r e nI ned e r a n d t h e m e a nv r t r i n i l er e tlectancefor samplcsfrom Elmworth 103571 W6M well. (After Radkeet al., 1982a)
o9 2
(.)
5 E
z
E
',.', I 'rl
=
I 4
i
15r 17-
1sf
Fig. V.1.27. Relationship between Methylphenanthrene Index(MPI 1) and mean vitrinite reflectance(7. R",) asbasedon datafrom Type-lII keroeen-bearing rock and bituminouscoalsanrples. (After Radkeand Welte. 1982)
organicfacies.e. g.. interbedded coallayers.Thisis understandablc, because the shalesin betwecnthe coalsconrainmainlytype-lll kerogen.The validityof the MPI as a maturityparameterhasbeenfurtherdemonstiatedin the Ruhr area. WestGermany.wherea good correlationbetweenthe Mpl I and R. hasbeen shownto exist for a seriesof Upper Carboniferousbituminouscoals(Radke et al.. 1982b).Furthermore,evidencehasbeenobtainedfor a generalrelations h i pb e t w e e tnh eM P I I a n dR " ,( F i g .V . 1 . 2 7 . B ) .a s e d o n r hi sr e l r r i o n s h i p v j.t r i n i t e reflectance values(R.) havebeencalculated from the Mpl 1 (RadkeandWelrc, 1982): R. R. -
0.60MPI 1 + 0.40for 0.65S R. < 1.35 0.60MPI 1 + 2.30for 1.35< R, < 2.00.
The ideais thatthe MPI 1valuedetermined for an extractindicates thevitrinite reflectance value(R") of the corresponding sourcerock unlessit hasreceivedan i np u t o f n o n i n d i g c n o u a sr o m a t i c s .
Identificationof SourceRocks
536 I'€AN REFLECTAICE Rln %)
LITI]O- ffiI'tATION IlGY
AGE
E I
I I
data(R') fromMPI1(R.)andfrommicroscopic calculated Fio.V.l.28.Meanreflectance is presence hydrocarbons c1(of migrated Alps. No,rtr.rn ,.,la""ir,r- " *!iiiitt"J in tn. a n dw e l t e l q 8 2 ) i n d i c a l ebdv\ o l i ds t g n a t u rt eA f t e rR a d k e for numerouswellswhere The validityof thisconcepthasbeendemonstrated of a rock impregnation An wasnegligible freavyhydiocarbons *airitit"i.".f R' value the i e value; ' R. 'levated oii generallvresttltsin an ;;l;;;;tg;,;d the Thus' ofthatsample value R. determined is hisherthanthe mlcroscoplcally presumed its to origin of unknown ,iru,in! petroleum MPiil; ;;J;;.i'*tt.n source ""il; rocK. organicmatterin Mti is particularlyusefulto measurethe maturityof asshownfor a rare, frequently particles are ,ou.i.1"Jrirn.iones wherevitrinite t h e o r e t i c avl i t r i n i t e T h e V 1 2 8 ) a t p s t n i g well drilledin the Northern from the tentatrve ;"fi;.i;;." gradientbasedon the MPI 1 differssignificanlly values vitrinitereflectance data Thus,calculated g."JJ", U"i"a "" microscopic data' or questionactualmicroscopic [R.; ura , possiblemeansto support Hydrocarbons h) lsomerizationand Aromatizationof BiologicalMarker environfossils(biologicalmarkers;in geological of geochemical The occurrence charactenstlc ofthese rn"",. tt". U"." a"lussedin ChapterIt 3 The importance and geologicalhistory was n-'oi".ut". as indicatorsof depositionalenvironment is to€xaminewhether paragraph pt"t"rrt.J i" Cn"pterIV.3. Ti.repurposeof.this areuseful quantitatively measured whichcanbe i..iuin ,utio, ot titesemolecules the usage was approach an such of "r'*ri"rrii." *oices. The first consideration naphthenethe (d) or paragraph in index(CPI) described ;;;;;;;J;t""ce indexdesciibedin paragraph(f) of thissection'
1.3Maturationof the OrganicMatter
53:
The assessment of the extent of thermal maturationof orsanicmatter in sedimentary rocksby biologicalmarkerparameters hasrecentlybeenreviewed by Mackenzie(1984).The best and most reliableexamplesof this approach involvethe monitoringofthe extentof isomerization andaromatizatron reacnons by measuringthe relativeconcentrations of the reactants (A) and products(B): k,
of u
Ideally,the concentration of B shouldbe zero at the start of the maturation process.and the extentof the reactionis measuredaspercentase of B A + B usuallyby combinedgas chromatograph/mass spectrometry(GC/MS). When the reactionrateconstantk, is very muchlargerthan k3.the reactionideallywill
--f- t
"".<
Cryn€rcr Ts
-_
--{-I
"o-k
CIdur--r PR'OUCYS
Fig. V.1.29. Isomerization and aromatizationreactions used to assessthe extent of thermal maturationof sedimentaryorganicmatter (Mackenzie.198'l)
538
Identificationof SourceRocks
maturity.lf thereis a noticeableback proceedfrom 0 to 100%with increasing rcaction.and k1, for example,equalsk2, then a reactionequilibriumwill be at a value of 50%. When a reactionhas reachedcompletionor established the extentof thermalmaturation. equilibriumit canno longerbe usedto assess Figure V.1.29 givcs four exampleswhere the initial concentrationof the occursat isomerization productsnearlyalwavsis zero. In acyclichydrocarbons The biogenic chiral centres.like C-6 and C-10 in pristane(A in Fig. V.1.29). (6R, 10R two enantiomers (6R, l0S configuration)is convertedto mesoisomer complete reaction is apparently and65. 10S)undergeobgicalconditions,but this with an equilibrium value of 50% at a relativelyearlv stage of maturity ( M a c k e n z icet a l . . l 9 i l 0 ) .S i m i l a r l yi.s o m c r i z a t i oant C - 2 4i n t h e s i d ec h a i n o f v e r ye a r l y( M a x w e lel t a l . , 1 9 8 0 ) . s t e r a n ecso m e st o a n e q u i l i b r i u m e si n F i g .V . 1. 2 9 )c a nb e T h ei s o m e r i z a t i oant C - 2 2i n l 7 c ( H ) . 2 1 B ( H ) - h o p a n( B followcdin the Crr to Cr5extendedhopanesby the m/z 191massfragmentograms (22Rand 22S)for eachhomologexceptin the most showingtwo diastereomcrs of all carbonnumbersvalucs(C11C,5)or immaturesedimcnts. Eitheran average the value for the Cql cxtendedhopanesis used to measutethc extent of isomerizationwhich vuries from 0 to about 60% with increasingmaturity ( E n s m i n g eer t a l . . 1 9 7 . 1S: e i f c r ta n d M o l d o w a n ,1 9 8 0 ) a . l t h o u g ht h e a c t u a l equilibriumvalucscemsto varyslightlybetwcencarbonnumbers.The isomerizat i o n a t C - 2 2o f t 7 a ( H ) . 2 1 B ( H ) - h o p a nr e sq u i r e sa h i g h e rl e v e lo l m a t u r i t yt o reachcompletionthanat C-6/C-l0 in pristane.but it is normallycompletebefore t h c o n s c to f i n t e n s eh v d r o c a r b ognc n e r a t i o(nM a c k e n z iaen dM a x w e l l .1 9 8 1 ) . Despitethe fact that isomerizationreactionsof steranesunder geological at conditionsare very complex(Seifertand Moldowan.1979),the isomerization (C in Fig. V. 1 29)appearsto be mostuseful in assessing the C-20of C],r-steranes thermalmaturityof both sedimentaryrocksand crudeoils (Mackenzieet al., 1980:Seifertand Moldowan. 1981),sinceit can extendwell into the zoneof hydrocarbongeneration(Mackenzieand Maxwell. 1981).The extent of this usingthe peak steranes isomerization is measured for 5n(H),14a(H),174(H)-C2e areas(or hights)of the20Sand20Risomersin the m/z217massfragmentograms. The valuesrisefrom 0 to 50-607c,but it is difficultto determinethe equilibrium of the complexityof the steranedistributions. exactll'because lsomerizationreactionsat chiral centerswhich are part of ring systemsare more complex, but they are occasionallyused as supplementarymaturity at C-17andC-21in hopanes(van parameters. Amongthesearethe isomerization Dorssclacr.1975:SeifertandMoldowan,1980)andat C-14andC-17in steranes (Serfcra t n d M o l d o w a n .1 9 7 9 1M a c k e n z i ce t a l . , 1 9 8 0 ;c f . a l s o M a c k e n z i e , 198,1). (D in Fig. V.1.29) is Aromatizationof C-ringaromaticsteroidhydrocarbons transformationwhere quantitationhas been atthe only aromatization-type temptedto usethisreactionasa maturationparameter(Mackenzieet al., 1981). The main reactionpathwayis the conversionof C-ring monoaromaticsteroid with two nuclearmethylgroupsto ABC-ring triaromaticsteroid hydrocarbons hydrocarbonswith one nuclear methyl substituentprobably via diaromatic The chiral centreat C-5 is lost during aromatization,and the intermediates. reactants reactionextentcanbe measuredon the basisof two C',rmonoaromatic
1.3Maturation of theOrganic Matter
539
(m/z 253 fragmcntogram)and a C., triaromaticproduct(m/z 231 fragmentogram).Thc cxtent of aromatizationincreases from 0 to 100% with increasing maturitvover the rangeof late diagenesis to the peakof oil generation,similar to that covcredbv thc isomerizationat C-20 in the steranes(reactionC in F i g .V . 1 . 2 9 ) . Within eachreactiontypea sequence of changes is alwaysobserved. andFigure V. L30 attemptsa summaryof the rangescoveredby thosereactionsdiscussed abovein relationto other reactions.like porphyrinconversion. and to vitrinite retlectanceand oil gencration.This interrelationis basedon the analvsisof Mcsozoicsamplesuitesfrom the ParisBasinand NW Germanv(Mrckcniie antl Maxwell.1981).I t is only a roughguidesincethc relativeratesof the reacrrons are governedbv the kineticconstantsof the reactionlates and varv with thermal h i s t o r vT. h o s er e a c t i o nwsh o s er a n g ees r t e n di n t ot h eo i l q e n e r l t i t i zno n ch a v ea n applicationto the measurcment of crudeoil maturityas well as of sourcerock maturit\'. Howevcr. it is to bc expectedthat the rclationshiDbet\\,eenthese d i f f e r c npt a r a m e t e rwso u l db e d i f f e i e n u t n d c lr l r i o u sg e o l o g i c a l t t i n g sa.st h e se rcitctronratesare stronglvinflucncedby hcatingrates.For instanccMackenzie c t a l . ( 1 9 8 2 s) u g g e s t ctdh a t i n s c q u e n c eosf s e d i m e n tws h o s et e m p e r a t u rw cas I n c r e a s eadt h i g ha v e r a g rea t c s( e .g . . P a n n o n i aBna s i nc. a . 1 5 " C M . a r ) t h er a t c of aromatization wasaccelerated to a greaterextentthanthe isomenzauoD rare. w h e nc o m p a r e dw i t h l o w e ra v c r a g eh e a t i n gr a t e s( e . g . . E a s tS h e t l a n dB a s i n . N o r t hS c a .l ' C . M a r ) ( F i g .V . 1 . 3 1 )T . h i sw a sp r o p o s e tdo b e p a r r l )d, u c t o t h e
t/to(Ecu_aFMTtos {nE€c)
{%)
5-Ilo
PORPHYBTNS AFOitATC(tn) STERAiES (%) IICKEL IANADYL
Arx rcs (%) 5()
ji o
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F i g . V . l . 3 0 . I l a n e eos f i n d i ri d u a n l t o l c clua rm e a s u r e m e n o tf st h er m a lm a t u r a t i opnl o t t e d againstthc hvclrocarbon gcnerationcurveantlvitriniteretlectance values.The datawere compilecl from rcsultsfor Toarcianshalesof thePilrisBasinandfor Pliensbachian shalcsof N. W. Gcrnrany(ad/ptedwith changes from Mackcnzieand Maxwell.l9E a
540
Iz
Identificationof SourceRocks Fig. V.1.31. ComParisonof the variationof the extent ot of C-ringmonoaromatization aromaticsteroidhYdrocarbons relative to the extent of isomerization at C-20 of 5a(H).14a(H).17c(H)-C1 steranes between Jurassic shalesof the EastShetlandBasin. North Sea. and the Plioceneshalesof the PannonianBasin.SE t{ungarY (Mackenzie . 198'l)
aromatization reaction having a higher activation energy than the isomerizatron reaction. These two reactions. however' may not be independent since it is conceivablethat saturatedsteranesare converted to aromatic sterords.
of PotentialSourceRocks on Characterization 1.4 Conclusions of the source AII the methodsreviewedmay contributeto the characterization excellent low to ' andthere from varies of efficiency degree their rocks.However. in Table summarized are aspects These use. their limitations-to mav be some sophisticaof degree on the depending considerably, varies also cost V. i.5. Their tion. obtainedthrough of organicmatteris quicklyandconveniently Theabund.ance the measureof total oiganiccarbon.It can be carriedout routinelyon a large number of samples Qzalit1' and maturationof the organic matter can be evaluatedseparitely.Ho*.u... due to the strongrelationshipbetweentheir influence.it is preferableto evaluatequalityand maturationsimulresDective is probably the bestroutine tool t'or determitittg t)-peand taniously. P-r'rol,vsis maturai;n of organicmatterar the sametime. Furthermoreit allowsa semiratio Pyrolysis quantitative;val;ation of the Senelicpotentialandtransforma.tion Optical routinely be used and can are alsoquick and inexpensive, t'echniques light transmitted in examination are alsooi greatvalue,particularly techniques and matter, of organic type the for determining with fluorescence associaied somewhat are techniques These maturation to characterize vitrinitereflectance time' than pyrolysis;theyalsorequiremoresample-preparation moreexpensive to a applied often are they Thus, quickly. and are thereforenoi'uuailubleas the Finally' results pyrolysis of basis the limitednumberof samplesselectedon requtres which also of kerogen' analysis sameconclusionappliesto elemental
1.4 Conclusionson Characterizationof potential SourceRocks
541
Table V.1.5. Principalmethodsusedfor sourcerock characterization, and theirdegreeof efficiency
t -
Class
Typ"
v
E 9
;
of the analysis .o 9n
< 6
Chemical (on rock)
Organic carbon Transmitted light (Palynof acies, alteration)
Optical microscopy
Reflected light
Rock-Eval
Composition of HC in pyrolysate _ Elementalanalysis Infraredspectroscopy Thermalanalysis (TGA) Electronmicrodiff raction
Physicochemical (on kerogen)
o
E S R .N M R
a
a
Isotopes(C. H. S etc.)
a
n-Alkanes
a
a
Isoprenoids
a
a
Steroids,terpenes
a
o
Aromatlcs
a
Porphyrins, metals
I
I
a (
o o
o o o o a
Isotopes(C. H. S etc.)
cood or excellent
o
o oo o a o
Light HC
Physical (on bitumen,oil or gas)
oo o oo oo o oo oo a
Amount of HC
Chemical (on bitumen or crude oil)
o
> b
a
Fluorescentlight
Pyrolysis(on rock)
t 9 0
tui.
o low or limitationsof use
542
of SourceRocks Idenrification
time andcare.andthe techniquecouldbe restrictedto calibratingthe maintypes by pyrolysis. distinguished As a generalremark, optical andcherzicaltechniquesshouldnot be considered as competitivealternatives,but as complementarymethods.In fact, optical techniquesare well designedto discriminatebetween autochthonousand reworkedor alteredorganicmaterial,whereaschemicaltechniquesconsider organic matter as a whole and measureaverages.In counterpart,chemical the whole organicmatter, whereasoptical techniques techniquesencompass oftenrelyon a certainfractionof organicmatterdueto the laboratoryprocedures (CorreiaandPeniguel,1975)or observation: thisfractionis not of concentration part of kerogen.Furthermoreopticaltechthe main oil-generative necessarily niquesalone do not alwaysoffer a sufficientdiscriminationof the quality of organicmatter.Thus,the combinationof two differentmethods,one chemical and one optical,is certainlythe mostrewarding. of the results.It wouldbe Thereis obviouslya needfor a commonexpression matterto be expressed maturation stage of the organic desirablefor the typeand in comparabletermsby all laboratories,althoughthey may be determinedby differentmethods.The questionis relativelysimplewith respectto the typeof organicmatter.In this book, we preferredto usenumerals(type I, II and III) rather than geneticterms (sapropelic,humic, etc. ...). The reasonis that "sapropelic",may in some denominations basedon opticalexamination,e.g., However,thereis somehopethat casesincludedifferentchemicalcompositions. furtherwork may allowthe definitionof a generalnomenclature. The situationis ratherdifferentwith respectto the maturationstageof source rocks.The variousscalesare basedon differentchemicalor physical(including optical) propertiesof the organicmatter. In turn, the propertiesdependon different aspectsof the structuralevolutionof the organicmatter:e. g., elimination of carboxylicgroups,abundanceof free radicals,sizeof the polyaromatic generatingthesephenomenaobey nuclei, etc. The chemicaltransformations to temperadifferentkineticlaws.with differentactivationenergiesin response of thisfact. First,thereis no way to ture increase.Thereare two consequences sinceboth time andtemperaexpressmaturationin termsof paleotemperature, ture affectthe maturationindicators.Eachscaleis in facta temperature-history scale.integratingthe effectsof temperatureand time. Furthermorc.the timeThus, relationship is differentfor thevariouspropertiesconsidered. temperature variations affected by strong as one may be the scalescannotbe superimposed. For example,fluorescence while the other variesonly slightlv.and vice-versa. and the beginningof catagenesis, and O/C ratio vary stronglvduringdiagenesis smaller extent. ln turn. reflectance H/C are affected to a whilstreflectance and during late catagenesis. and H/C ratio changestronglv For this reason.we haveto considerthe useof one scaleof maturationasa reference.providingan approximatecalibrationwith the other indicatorsof reflectance scale thermalevolution.Due to its wide use.the huminite-vitrinite has often been consideredas a reference.Hood et al. (1975)proposedan designedto independentscale.calledLOM (levelof organicmetamorphism) overthewhole havea linearvariationasa functionof depth.andto be continuous rangeof generationand destructionof petroleum.More recently.Dow (1977)
1.4Conclusions on Characterization of PotentialSourceRocks
543
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'9
i 6
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t Lr.
Identificationof SourceRocks
544
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1.4Conclusions on Characterization of PotentialSourceRocks
545
I
t ..111r,"..'., E € '6
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546
Identificationof SourceRocks
(log R") for reflectance suggested usinga logarithmicscaleof huminite-vitrinite purpose. approximately linear with depth over the same This scaleseemsto be and catagenesis stages, but not necessarily during most of the diagenesis metagenesis.The temperatureindex of pyrolysis Z-,, has the advantageover vitrinitereflectance of beingbasedon a propertyof the bulk of organicmatter, whereasvitrinite may be subordinateor even absentin type-I or -II kerogen. Thereis, however,an influenceof the type of kerogenat low evolutionstages, whichmay be accounted for by usinga plot of I.", versushydrogenindex. In terms of vitrinite reflectance,the_boundarybetween diagenesisand catagenesis corresponds approximately te R, = 0.5% andthe boundarybetween catagenesisand metagenesisto about Ro = 2Va. Metamorphism (greenschist valuesof LOM are shown facies)appearsat aboutR, = 4alo.The corresponding in FigureV.1.32and V.1l.33.Whateverscaleis used,it shouldbe remembered that. due to the occurrenceof different types of organic matter with different kineticsof transformation, thereare no exactlimitsfor the stagesof oil andgas generation. differslightly The exactboundaries ofthe petroleumgenerationzone with the typeof kerogen.A maturationscale,therefore.shouldbe basedon one oil generationfiaystartat a vitrinite of the mainkerogentypes.If not specified, refleclance R" from 0.5 to 0.7Vaandmay reacha peakat R, rangingfrom 0.8to 1.1% accordingto the type of kerogen. in for comparison The mainindicatorsof sourcerock evolutionare presented FigureV.1.32 (indicatorsbasedon kerogen),and in FigureV.1.33 (indicators basedon composition of bitumen).The readeris referredto FigureV.1.19for the pyrolysisindices.The courseof changeof a maturationparameter.e.g., its linearityversusmaturationscale,definesthe preferredintewalsof application: (b) ESR and H/C ratio: catagenesis; (a) fluorescence and O/C ratio: diagenesis; (d) pyrolysisandalteration (c) vitrinitereflectance: catagenesis andmetagenesis; o I p a l y n o m o r p hfsr:o md i a g e n e stios m e t a g e n e s i s . The introductionof fast but powerful and reliabletechniquesof pyrolysis analysis on a largenumberof samples. makesit now possibleto run geochemical For instance.cuttingsfrom a bore hole can be analyzedroutinelyevery 10or 20 m. Furthermore,pyrolysismay evenbe performedon a well site.The results canbe plottedagainstdepthin the form of a log wherethe typeof organicmatter and the geneticpotentialare shown,togetherwith the maturationstageand samthetransformation ratio (Fig.V.1.34).Afier evaluationof thislog,selective plingcanbe donein orderto run a smallernumberof samplesfor moredetailed analvsis,
SummaryandConclusion There are three $ucia\ tactols tor charactezation of a petroleumsourcerock: the amountof organicmatteror organicichness,thetypeor qualityof organicmatterand its stateof maturitY.
Summaryand Conclusion The amount of organic matter can easily be estimatedfrom an organic carbon analysis.Quality and maturation of organic matter can be evaluatedby different chemicaland optical techniques.It is advisableto evaluatequality and maturation simultaneously,becauseof tbe stronginterrelationof thesetwo parameten. Pyrolysisis probably the best routine tool for determiningtype and maturationof organic matter at the same time. Optical techniquesare also of great value in this respect.Examinationof organicmatter in transmittedlight is valuablefor a determination of the type of organic matter. Vit.inite reflectancestudiesare excellentfor a characterizationofthe stateof maturity. The methylphenanthrene indexis a verygood meansto measurethe matudty of extractsand crude oils over the catagenesis stage. Optical and chemicaltechniquesshouldnot be consideredascompetitivealternatives, but as complementarymethods. Quality or type of organicmatter is here divided with the help of chemicalmethods (pyrolysis,elementalanalysis.etc.) into type-I.type-II and type-III kerogens.Divisionsbasedon opticalexaminationsdo not per sedistinguishbetweenkerogenson the basisof different chemicalcomposition. Maturation stagescan be determined using various physicochemicalor optical techniques.The intenelation of the resultingmaturation scalesare currently being established.Each scalerepresentsa temperature-historyscale,integratingthe effects of temperature and time. The most widely applied maturation scale is based on huminite-vitrinitereflectancedata. Whateverscaleis used.it shouldbe remembered that, due to the occurrenceofdifferent typesoforganicmatterwith differingkineticsof transformation,there are no sharpboundariesfor the stagesof oil and gasgeneration.
Chapter2 Oil andSourceRockCorrelation
The objectivesin crudeoil correlationvaryconsiderably, dependingon exploration andproductionproblems.In principleit is desirable to correlatedifferentoils with eachother, oils with their sourcerocks,gaseswith gasesand, if possible, gaseswith oils and sourcerocks(Fig.V.2.1). Questionsfrequentlyposedto the exploraliongeologistare the following: - is it possibleto identifyvarioustypesor familiesof oils, condensates or gases occurringin a sedimentary basin?Doeseachfamily havea singleor multiple origin? is it possibleto identifythe sourcerock of a givenoil, condensate or gas?Is it possibleto detectintra-basinal facieschanges in a givensourcerockthatwould resultin the releaseof differentand identifiablecrudeoils acrossthe basin? The solution of such questionshelps to define the kind and number of explorationtargets.and to developan explorationconcept.In oil production, especiallyin oil-field developmentof stronglytectonicallydissectedproductive
G€ocha.dcal l6sas
Ratb6 ol .pecilic
retios ol so.citic l€ qc/nc, o/H. !s/3,s
a.d non-l€.
lrc (c,-cD) and non-Hc pop€dirs
Ratro6 ol spocifb HC (C?-Cs) rtd non-Hc,'t/'C. o/H
Fig. V.2.1. Schematicrepresentation of desirablestudiesin oil, gas and sourcerock correlation.SomeusefulcorrelationDarametersfor certaintvDesof correlationare listed
2, 1 CorrelationParameters
areas, or in multipay fields, oil-oil correlation can quickly provide valuable information on possiblecommunicationbetween different fault blocks or different productivehorizons. Oil or gas entrapped in a reservoir has been generated from kerogen disseminatedthroughout a sourcerock. During primary migrationonly a certain proportion of the petroleum compounds generated from kerogen can be released,a certainpart is alwaysretainedin the sourcerock. Basedon this concept, the three principle objects of study for correlation are the insoluble kerogenoI a source rock, the bitumen extractablefrom this source rock and pooled hydrocarbons(oil, cond.ensate, or gas),possiblyderivedfrom this source rock. Geochemical correlationbetweenpetroleumand sourcebedsis basedupon recognitionof compositionalsimilaritiesor dissimilarities. Such featuresare usually establishedon relative distributionpatternsof certain compounds, althoughabsoluteconcentration valuesmayoccasionally provideusefulinformation for correlation.Generally,compositional parameters suitableforcorrelation should: - not be too seriouslyaffectedby any processes actingupon sourcerocksand petroleum during and after their physicalseparation(e.g., preferential separationby migrationprocesses, thermaland bacterialalteration); - havesufficiently characteristic compounddistributions in bothrocksandoilsto permit differentiationbetweenindividualsourcerocksand oils. The basicidea is to recognizeidentical,similar, or different,,fingerprint', patternsexhibitedby correlationparameters for kerogens,bitumensandpooled hydrocarbons. The degreeof similarityor dissimilarity thenproves,suggests, or disprovesa relationshipbetweenpetroleumsand sourceblds. It is advisable neverto relyon onecorrelationparameteronly,but to applyseveralindependent correlationcriteria.Any correlationbasedon minorconstituents, e. g., biological markerdistributions, shouldbe reasonably consistent with bulk parameters like aliphatic/aromatic hydrocarbonratiosor carbonisotopevalues.bne of the first successful correlationstudieson oils waspresentedby Jonesand Smith(1965), who comparedgrosschemicaldifferencesof distillation fractions of crude oils from the PermianBasin of West Texasand New Mexico. Correlationproblems havebeenreviewedby Koonset al. (1974),Williams(1974),Welteet al. (1975), Deroo (1976),SeifertandMoldowan(1978),Seifertet al. (1980)andSeiferrand M o l d o w a n( 1 9 8 1 ) .
2.1 CorrelationParameters The parameterssuitablelor oil-oil coruelationcanbe chosenfrom variousclasses of hydrocarbons andnonhydrocarbons normallypresentin crudeoils.Oneof the best correlation tools is provided by geochemicalfossilsor biological markers. Geochemicalfossilshave been discussed in detail in ChaDtersII.3 and IV.3. Compoundsof the steroidand terpenoidclassesare of spicial interestin this
550
Oil and SourceRock Correlation
molecules with a respect.Furthermore.the distributionpatternsof hydrocarbon ofnaphthenes lessspecificorigin,suchasn- or iso-alkanes, or the variousclasses and aromaticscanbe successfully utilized.In addition,porphyrinratiosor even somenonbiogenicheterocompounds suchasthiopheneor carbazolederivatives In oil-oil correlation,it is possibleto mayprovideusefulcorrelationparameters. in sucha way thata wide molecularweightrangeis coveredtbr selectparameters anygivencorrelationparameter.Suchinformationcanbe provided,for instance, (GC) recordof by the n-alkanedistributioncurveor by the gaschromatography the saturatedhydrocarbondfractionon a capillarycolumn.Usefulinformation may alsobe obtainedby suchgrossparameters as isotoperatios(lrc/r:C. D/H. !S/rrS)or aliphatic/aromatic hydroctrrbon ratios.Isotoperatiosmayalsobc used in a more specificform by determiningthe isotopiccompositionof certain or evenindividualcompounds. compoundclasses The correlationbetweenoils andcondensates, andespecially betweenoils,or condensates, and gasesis a morc delicatcproblem.becausegasesand to some extent condensates are related by preferentialgenerationand accumulation which may mechanisms be differentfrom thosefor oils.This fact mustbe taken molecular weightrangeof thesevariouspetrolcumproductsis into account.The so different,that the success of a correlationanalysisdependslargelyon the depreeof overlapbetweenthe molecularweightrangesof the differentproducts (e.g., light and gasolinerangehydrocarbons for oil-condensate correlation). and light oils may be Usefulinformationfor the correlationof condensates analysisof the gasoline-range obtainedby a detailed gas chromatographic hydrocarbonfraction.In the caseof gases,thereare severelimitationsbecause compounds. theircomposition is too simple,andtheyconsistof ratherunspecific However, carbon isotope ratios have been shown to be useful correlation parameters for groupinggases(Stahlet al..1917:'Schoell,1980). The correlation between pooled hydrocarbonsand source rocks is more haveaccumulated complicated thanthe oil-oil correlation.Pooledhydrocarbons migration.Thereare composithroughthe processes of primaryand secondary tional differencesbetweenthe bitumenleft behindin a sourcerock and that in a reservoir.Due to migralion fractionof bitumenthat finally accumulated phenomena.pooledcrudeoils are stronglyenrichedin saturatedhydrocarbons, moderatelyenrichedin aromatichydrocarbonsand depletedin polar NSO compoundswhen comparedto sourcerock bitumen(seealsoChap. IIL3; Fig. are thereloremore suitablefor III.3.1). Saturatedand aromatichydrocarbons Any group of compounds oil-sourcerock correlationthan heterocompounds. selectedfor correlationshould be easy to isolateand characterize,but the individualcomponentsshouldbe similarin their physicalpropertics,suchas polarity.solubilityand molecularweight.On thesecriteriaagain.geochemical fossilsof the steroidand triterpenoidhydrocarbongroupserveas mostsuitable parameters may be correlationparameters. Likewise,other hydrocarbon-based previously mentioned for oil oil correlation. appliedas Re-distributionof bitumen in sourcebeds, especiallyimpregnationwith foreignbitumenfrom an outsidesource,canobliterateoil-sourcerock correlation basedonly on sourcerock bitumenwhile neglectingthe kerogen.An oilsourcerock correlation,therefore,ideallyshouldalsoincludea studyofkerogen
2.2Oil-OilCorrelation Examples
551
demonstrating that the bitumenof a sourcerock is indigenous, andcanbelinked to the kerogenfrom which it was supposedlygeneiated.Alternatively,the indigenousnatureof the bitumenshouldbe proven indirectlyby relatingthe amountof_extractable organicmatter, the percentage of hydiocarbonsin the extractor the transformation ratio (productionindexjfrom Iiock-Evalpyrolysis $ee Chap.V.l ) to the maturitylevelof the kerogen.The choiceofparameters for correlatingbitumendirectlywith kerogenis li;ited. The mostfiequentlyused techniqueis a carbonisotopeanalysis.Bitumenderivedfrom a ceriainkirogen shouldbe isotopicallylighter than the kerogenitself. The differencein dFC_ values,however,shouldnot be more than abou 2-3%".An oil connectedto a certainsourcerock bitumenshouldbe isotopicallyidenticalor, again,slightly lighterthan the relatedbitumen.Grossisotopedata, however.strouldnot le overemphasized when a correlationis indicated.In contrast.noncorrelation basedon isotopevalues,i. e.. whenan oil is isotopically heavierthana kerogen.is a clearindicationfor a misfit. Furthermore.carbonisotoperatiosof pooledgases.i. e., of methane,ethane andpropane,havebeenappliedto obtaininformationabouttheirDossible source rocks (Stahland Carey. 1975).This applicationis hasedon the conceprrnar carbonisotoperatios of gasesare controlledmainly by the maturit),of their sourcematerial.A refinementof this methodis achievedwhen carbonrsotoDe valuesare appliedtogetherwith hydrogenisotopevaluesandare plo ed rgainst eachother. ln this wav, Schoell(1980)cameto a verv detailedciassification of gasesot variousorigins.It shouldbe mentioned,however,that the conceptthat carbonisotoperatioscan be usedto determinethe maturityof the sourcerock f r o mu h i c ha g a sh a sh e e nd e r i r e d .i s n o t u n d i s p u t e d . Anothermethodto correlatebitumenwith kerogen,employingpyrolysisofthe kerogen,hasbeensuggested by Welre (1972\.Tie basis'foithis methodis the idea that exhaustively extractedkerogenstill exhibitsa potentialto generate hydrocarbons (seealsoChap. IL5), and, on pyrolysis,wili yield new hydrocar_ bonswith a patternsimilarto thosewhich havebien previouslygeneratedrn a naturalway. RecentlySeifert(1978)reportedthe succeisful appiicitionof sucha technique.Pre-extracted kerogenwaspyrolyzed,andthehydiocarbonpatternof the -pyrolyzatecomparedto the previouslyextractedbitumen usingGC_MSanalyses. On the basisof a matchbetweensteraneandtriterpanepatleins,he was able.to provethe indigenousnatureof the bitumen.The sameprinciplecan be used.tocorrelatedegradedwith non-degraded oils by pyrolyzingthe asphaltene fractionof the degradedoil (Rubinsteinet al., 1979;A;efjev et-at.,flS^O;.
2.2 Oil-Oil CorrelationExamoles Oil-oil correlationis sometimes straightforward. In suchcases,a simplecomparison of the GC recordsof the saturatedhydrocarbons may clearlydemonstrate identicalpatternsfor severalgroupsof compounds(n-alkanes,branchedand cyclicalkanes).Suchan exampleis givenin FigureV.2.2.
552
Oil and SourceRock Correlation
oil Audignon Albion
Sourcorock Ak€ -sur-Adour (upper Juroslic)
c
3
0
2
5
Fig. V.2.2a c. Comparisonof the gas chromatographic tracingsof the saturated hydrocarbons as a meansfor oil-oil and oil-sourcerock correlation.SouthAquitaine Basin.France.(After Deroo. 1976)
Koons et al. (1974)studiedpetroleumcharacteristics of crudeoils in lower Tuscaloosa Cretaceous reservoirs in the centralGulf Coastarea,USA, utilizinga multiple-parameter approach.The parameters chosenfor oil-oil correlationwerc the carbonisotopevaluesof the saturates, the paraffincontentof the saturated hydrocarbon fraction.the amountof steranes, ratios, andsomelighthydrocarbon suchasthoseof the cyclopentanes to n -paraffins.Utilizingthesefour parameters, crudeoilsreservoired in the lowerTuscaloosa formationcouldbe separated into two families.One family appearsto be indigenousto the lower Tuscaloosa (brackishinterval,whichconsists ofsediments deposited in anintermedrate{ype marine),nearshore environmentreceivingvaryingamountsofterrestrialorganic matter.The carbonisotopevaluesof the indigenous lowerTuscaloosa oils(d''C : -27 .9 + 0.6%"PDB)arelighterby about2%,thanthoseof theotheroil family. Theyalsocontainmoreparaffins(27vs. 137o),twicetheamountof steranes anda higherratio of cyclopentanes to n-paraffins.The relativelylight carbonisotope values and the high paraffin content are in agreement with a terrestrially influencedsource,as representedby the lower Tuscaloosa.The other family of
2.2 Oil-Oil CorrelationExamples
553 Fig. V.2.3. Relation betweencarbonisotoperatios andopticalrotationin crude oils of the WillistonBasin. USA. Two familiesof crude oils can be recognized.lncreasrngmaturationof oils causesa shift to the loh'er /e/t. (Modified after Williams. 1974). Biodegradatron would have causeda shift to the lower right corre,"of the diagram
3l .9
r0
-.€ 5 4
2t
E.t
.2-
n
oils is probablyderivedfrom moremarineorganicmatterand is apparentlynot indigenous to the lowerTuscaloosa formation. Three differenttypesof oil havebeenrecognizedby Williams(1974)in the WillistonBasinin the northwestern UnitedStateson the basisof carbonisotoDe ratios,hydrocarbon composition andopticalrotationmeasurements. A semi-log plot of carbonisotoperatio versusopticalrotationwashelpfulin characterizing oil types (Fig. V.2.3). This method is of specialinterest with respectto recognitionof maturationand biodegradation effectsupon crudeoils (iee also Chap. IV.5). Thermal maturationprocesses decreaseopticalrotation (Louis, 1968)and causea shift toward heaviercarbon isotopevaluesin the residue (Silverman,1967).Contraryto this. biodegradarion increases opticalrotation (Philippi,1977)due to a selectiveremovalof opticalenantiomer;from racemic mrxturespresentin the originalcrudeoil and/oradditionof newopticallvactive biogeniccompounds.However,biodegradation, like thermalmaiuration,also causesa shift toward heaviercarbon isotopevaluesdue to the preferential cleavageof the r:C-rrc bond, and hencelosi of the isotopicallyligirtercarbon atoms.In FigureV.2.3. analyticaldata,representing 125oil samples,showtwo distinctelongatedclustersorientedin the graphfrom upperrighi to lower left. Thiselongationtowardloweropticalrotationandheaviercarbonisotopevalues is the resultof thermalmaturationon theseoils(Williams,1974).Theseparation into two differentoil familieswith differentisotopevaluesis due to their origin from different source beds. In such a plot as presentedin Figure V.2.3. biodegradation would resultin a shift from upperleft to lowerright. Recentlythe identificationof crude oil familiesand also the correlationof crude oils with their sourcerocks basedon sulfur isotooe ratios has been performedby Thode(1981)in the WilistonBasin(USAlf2n|6u.;.Sulfurisotope ratiosare expressed as6raSvalueswhichare definedasfollows: ( " S r 2 S s) a m p l e- ( ' o S . ' 2 S s t) a n d a r d o 5 { ' ; ' ) = # < l a t 0 0 . -5) |
5/
Standard
The oils of the WillistonBasinweredistinguished by the dsaSvaluesof their sulfur-containing compounds in the bulk oil. Threemajortypesof crudeoil were
Oil and SourceRock Correlation
554
s f t h et h r e ef a m i l i ew s e r e5 . 8+ 1 . 2 7 , , . 2 .+8 0 . 8 %ac n d 4 f o u n d .T h ed r u Sv a l u e o secondarymigrationover some 150 km t 0.7%rrespectivelv.Long-distance_ resultcdin little or no changein the d"S value.Hower,er.elcvatedtemperatures or advancedoil maturationand crude oil alteration,such as water washing mav changesulfurisotoperatiosof thc bulk oil. and biodegradation. Considerableeffortshave been undertakenin recentyearsto developthe fossil)parameters. in combination applicationof biologicalmarker(geochemical (e. g., with others.for oil-oil correlation Seifertand Moldowan.1978.1981;Shi et al., 1982;Welte et al., 1982).They all usesteroidandterpenoidhydrocarbon spectrometryas distributionsderivedfrom capillarygaschromatography/mass an analyticaltechniquethat makes correlationparamelers. maior source-related beingeluted useof a massspectrometer asa veryspecificdetectorfor molecules T E R P A N E S- G C M S
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GCTIMEOIRECTION (mlz 191)offour crudeoilsfrom differentoil Fig.V.2..1.TerpancmassfragmentoSrams characteristics. fieldsin Sumatraindicatingthree oil farniliesbasedon compositional (After Seifertand Moldowan.l98l )
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ill,: l r o i l r . :ll,l r t ) t J:"i'.:1,I l l c . l t r ( ' rhdr d r o c a r h o nm\ i t \ h e r e p r c s e n t ebd\ t l t e m a s .[ r . r u m e n t u _ gramol a singlemassspcctrometric k.1-t,"g.J"ifrr.,,if-l i.rnp.ru,r.r,yp". a. on example.FigureV.2.;l showsthe m/z 191terpanemas,frag.cnt,,grams of tour cruileoilstiom ditTerent oil fieldsin Sumatra.ihe originot ti" iUi ,nrs trugrn"nt is marnlr lrom rrrcvcliclnd pcntacvclicterpanes.?roii,.r"- Jut, tnr". o,t I a m l l l c \( ; r n h e d t l l c r e n t i a t chda s e dc r nt h e i rc h a r a c l e r i \ t i c tritcrpancomlo\i_ tions.It wasrealizedearlv.however.rhatbiologicalmarke, ..riririU'uilon:i oi.ruO. oils are stronglvinfluencedbv their temperaiurehistorv (i.e.. maturitv) and
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556
Oil andSource RockCorrelation
possiblyalso by migrationeffects(Seifertand Moldowan,1978).Implications in more detailin contextwith oil-sourcerock relatedto this will be discussed correlation. for crudeoil correlationwasillustratedby An applicationof nonhvdrocarbons thecrudeoilsfrom to characterize Deroo(1976).He usedthiopheniccompounds the Alberta Basin of WesternCanada.The thiopheniccompoundsusedfor and naphdibenzothiophenes correlationcomprisemainly benzothiophenes, (Fig. V.2.5). For crude oil characterization,a gas thobenzothiophenes record of the total aromaticfraction is used employinga chromatographic a portionof the flame ionizationdetector(FID). Simultaneously, conventional columnandrelayedto a effluentis splitoff afterleavingthe gaschromatographic dctcctor(flame photometricdetector.FPD), which is only sensitiveto sulturcontainingcompounds.In this way, two gas chromatographicrecordsare andanother obtnined.onc of the total aromaticfractioninclusiveof thiophenes, An example aromaticcompounds(thiophenes). showingonly sulfur-containing is givenin FigureV.2.5.Threedifferenttypesof oil canbe of thistvpeof analysis -fhe recordsallow a aromaticand thiophenicgaschromatographic recognized. in the (a) the unconformity of the groupsof oils located above ditTerentiation main younger), (b) at or near the (Colorado group and Upper Cretaccous (c) in the the unconformity Basin. and below in the Alberta unconformitv in total sulfur content of oil also differ The three different types Dcvonian. Crude oils becomeincreasinglvsimilarwith maturation.In particular,the polycyclicnaphlesscharacteristic: saturatedhvdrocarbonsare progressively andtendto thenes(geochemical fossils)are greatlydiminishedin concentration, distributionpatternslook moreand be lessdistinctive:n-alkaneand iso-alkane morealike.Therefore.a correlationbetweenolderandmorematurecrudeoils, is ratherdifficultand might yield ambiguous basedon saturatedhydrocarbons, and aromatic hydrocarbonsand results. In such casesnaphthenoaromatic appliedto oil-oil correlations thiopheniccompoundshave been successfully (Tissotet al.. 19741Deroo. 1976:RullkotterandWelte, t980). FiguresV.2.6 and V.2.7 give examplesof crudeoil correlationsbasedonly oils from the Illizi of the Paleozoic upon the aromaticsand naphthenoaromatics of thesecrudeoils,andgaschromatoBasin(Algeria).A globaloverallanalysis doesnot allow a graphyand massspectrometry of the saturatedhydrocarbons in Ordovician,Lower andUpper Devonian distinctionbetweenoils reservoired with respectto analysis andCarboniferous strata.However,a massspectrometric naphthenoarotype of ring structureand carbonnumberof the aromaticsand betweenthegroupsofoils.The providesa fair differentiation matichydrocarbons differentiationis mainly basedon tetracyclicmoleculesvery probablyderived molecules arehardlydetectable from stcroids.Althoughthe saturatedtetracyclic any more in theseoils, thereare sufficienttetracyclicmoleculespresentin the suchasthe monoaromatic aromaticfraction.Aromatizedtetracyclicmolecules, rings,are morestable three naphthenic containingone aromaticand structures. total aromaticsand the oils. The and henceare preservedin more mature in the Illizi Basin for oil oil correlations molecules used monoaromatic tetracyclic rock correlation oil-source parameters for a successful servedalsoascorrelation (Tissotet al.. 1974).
2 . 2O r l - O rC l o r r e l a t r oEnx a m p l e s
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AL AROMA
1 9
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Fig. V.2.6. Oil oil correlation basedon aromaticsand naphthenoaromatics. Comparison betweencrudeoilsfront LowerDevonianand(Jrcfurician reservoirs in the Tinfou\'€-Tabankorroil f i e l d . I l l i z i B a s i n .E a s t c r nS a hara. Alseria. (After Tissot er a l . .1 9 7 . 1 )
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Irig. V.2.7. Oil-oil and oil \ourccrock correlationbasedon aromatics and naphthenoaromatics.Thc distributionof the naphthenoaromatics (monoarom a t r c sC. , , l l . ,r : ) p e r m i t sa d i s criminationbetweenthrec diffcrenttvpesof oil whichalsocan bc relatedto their sourcerocks. Carboniferous andDevonianoils are from the oil field Tigouentourine.thc Ordovicanoils from Quan'faredcrtin the IIlizi tsasin. EasternSahara.Algeria. (Atter Tissoter al.. 197.1)
Oil and SourceRock Correlation
558
hampcrsor makeslmposIn additionto maturationeffects.biodegradation y r m o d c r a t e lbyi o d e g r a d ecdr u d co i l s 'c h a r a c s i b l eo i l o i l c o r r c l a t i o nl n. s l i g h t l o teristic petroleum componentshave to be selectedwhich arc not readil)' Positivecorrelationwasindicatedb.v eliminaticlor alteredby microorganisms. and a moderately Molclowanand Scifert (1980)betweena non-biodegradcd biodegradedcrudc oil from Minas and Duri oil fields.Sumatra'basedon the and a preseiceof a rare isoprenoidbiologicalmarker.namelybotrvococcane' V lt) ieriesof commonisoprenoidalkanesin both oils (Fig 2 in Steranesand tritcrpanesare often found unaffectedby biodegradation cases are in thcse thus moderatelydegradedcrudeoils (Welteet al.. 1982)and seemto be still suitablefor correlation.Evcntually.however'microorganisms casc steranes ln this (see IV 5). also Chap andtriterpanes ablcto attacksteranes w. h e r e a s 1 9 7 9 ) M o l d o w a n . a n d ( R e e d . 1 9 7 7 S : e i f e r t a r ep r e f e r c n t i a ldl ve s t r o y c d and to demethylation leading altercd. be specificallv triterpanestructuresmav thc capillarl" V 2 9 shows 19132). Figure (Rullkijttcr Wendisch. and ring opening fractionsof a non-biodegradcd gaichio-"iogra.s of the saturatedh1'drocarbon ( u ' e l l B N ' l L - l )f r o m M o r o n d a r rI tl a s i n ' a s p h a l t h i o t l c g r a d c d a iucll VIID 2tancl of n- and isoprcnoiil removal complete the $'hich dcmonstratc Madagascar. h o p a n c t 1 pt rei t e r P a n casso n l v t h e c a v i n g b e h i n d B M L I a s p h a l t i n t h e alkancs clcarlyshow that the maior components.The adjacentmassfragmentograms Mino5 Field 2500 FT
uurl ileto
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gn r i t i r c F i g . \ ' . 2 . t . C r p i l l i l r \ g i l s c h r o n a t o { r r l m so l t w o S u n ' l l t r ac r u d c o i l s i n d i c r r t i n p oils The hoth in of botrlococcane presence thcbl de\pile bi(;egradation corrclation bioclegradeclciucle oil lithe botton shows a predominance of branched isoprenoid a l k a n e so v c r l i - i r l k t n c s .( A f t e r N l o l c l o u ' aann d S c i f e r t .1 9 8 0 )
2.2 Oil-Oil CorrelationExamples
559 N I
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Oil and SourceRock Correlation
560
by an equivalentseries regularC1 (h, j) to Cj, (w) hopaneseriesis accompanied asphalt(c, d correspondhopanes(m/z 177)in the biodegraded of demethylated i n g t o h . j . r e s p e c t i v e leyt.c . ) . appearto be evenless andthiopheniccompounds Polyaromatic hydrocarbons (steranes. terpanes). than are polycyclicnaphthenes affectedby biodegradation An example of correlatingoils with various degreesof biodegradationis presented by Claretet al. (1977).Oilsfrom the offshoreoil field Emeraudein the Congo,West Africa, show differentgrosschemicalproperties,dependingon reservoirdepthand stratigraphichorizon.The oil bearinghorizonsof Cretaceous agein the Emeraudefield aresituatedat relativelyshallowdepth(200to 600m). The oils are obviously affected by the products of biodegradation.and the questionariseswhetheror not theseoilsmighthavea commonorigin.A detailed analysis revealedthetypicalsignsof gaschromatographic andmassspectrometric Furthermore,the (e. g., normal and isoprenoid alkanes). loss of biodegradation oils show good correlationon the basis of their aromatic and thiophene components(Fig. V.2.10).This pointsto a commonorigin. In additionto GC analysis ofthe a moredetailedmassspectrometric recordsof the total aromatics, distributionof mono-, di-, and polycyclicaromaticsprovidedcorroborative evidencefor a commonorigin. Similarly,Welte et al. (1982)providedevidenceby low eV gaschromatograand fractionsthat biodegraded phy/mass of aromatichydrocarbon spcctrometrv crudeoilsfrom the ViennaBasinbelongto the sameoil family non-biodegraded The Thesedataweresupportedby steraneandtriterpanemassfragmentograms. oils investigatedwere from reservoirsbetween5500 and 800 m depth The l-ow degradatioi
High degradation
Soluroted HC ( gos chromotoqrophy)
Aromoiic HC ( gos chromotogrophy )
crudeoils in the Fig. V.2.10aand b. Comparisonbetrveendegradedand nondegraded hydrocartracingsof the saturated Emeraudeoil fjeld.Congo.The gaschromatographic still showan bons(a) are differentdue to degradation.Ilowever. polycylicmolecules (FID) and tracingsof total-aromatics excellentcorrelation.(b): gaschromatographic thiophenederivatives i/FPDl.(After Claretet al.. 1977)
2.3Oil-Source RockC'orrelation Examples
561
compositional similaritiesof most of the oils in the ViennaBasin.apart from biodegradation effects.point to a commonongtn. Finallv.vcrv heavill'degradedoils can rlso he correlaredbv pyrolyzing the asphaltene fraction.ln thisway. the compositional patternof the originalciude oil can be rcconstructed: thisfeaturehasbeendesciibedin ChapterIV. 1.g.
2.3 Oil-SourceRock CorrelationExamples ln principlefor oil-sourcerock correlationthe samemethodsare utilizedas mentionedbetbrefor oil-oil correlation.For instance,FigureV.2.2 shows that the GC record of the saturatedfraction of two crude"oilsfrom the South AquitaineBasinin Francecaneasilybe comparedto theequivalentGC recordof the sourcerock. The generalapproach.however.is somewhatdifferent, in that m o r e . d e t a i l e da n a l y s e a s re retluiredT . h i s i s s o b e c a u s ea p o o l e do i l h a s p r o c e s s eo \l m i g r a t i o n w h i c hm a vh a r ep a r r l yc h a n g e idr r c o m p o s i r: ,o1npl.r1o' "mnt.h: 9 a l( ) tt h ec o r r e s p o n r l i h n iul u m e no I t h es o u r c er o c k .O i l _ s o u r creo c k c{)rrelationparameters.thercfore.shouldincludecletailecl informatronon hy_ drocarbondistributionpatternsol specificmolecules.suchas steroidor triter_ pcnoid hvdrocarbons.which exhibit comparablephysicochemical behavior. Furthermore.crxrelationparameters shouldbe chosenfrom amongat leasttwo ditferentmolecuia r weightranges.e. g.. C1;-C25 saturates andaromatics andfrom L r ' + - c y c l ih( r d r o c a r b o n cs r. o s sc h e m i c ai n l f o r m a t i o an r o n er. i k et h a tb a s e do n the amountof saturatednormal or aromatichydrocarbons. sulfur conrenr.or c!::l.carl9n rsotoperatios.is generallvnot sufficient.but oftcn givesvaluable additionalinformation.In oil-sourcerock correlationthe indigenousnature of the bitumenin a givensourcerock shouldalsobe verifietl.esp'eciall1, if thereis susprcion of .il impregnation of a rockfrom an outsidesource.I ndicatrons tor this from roo high hl,drocarbon: roral organic carbon rarios (above :11 !:.9..i":,1 l5(I-200mg hydrocarbors:g C,,,*)and from too high bitumen:roral organrc carbon.ratios (above200-300mg bitumen:g C",,,).B,vfar the mostconvenient methodis a succession of Rock-Evalrneasurem.l,ts presentingfrec oil and gas contentsandtransformation ratios(productionindex): FigureV. 1.22.Anv ,o-n" of migrationalhycirocarbon enrichmentwilr exceedthe citabrished,r."J. ri.un a l s ob e . a c h i e v eb d v c o m p a r i n gt h e m a t u r i t ) o, l t h e b i t u m c nb y a c h e m i c a l maturationparamctersuchas the Methyl phenanthrene lndex values(Radke c t a l . . l 9 l l 2 )w i r h t h e m a t u r i r vo f t h c i m m o b i l ek e r o g e n( c . g . . v i t r i n i t er c f l e c _ tance). Earlv examplesfor oil sourccrock correlationsbascdon severalcorrelation p a r a m e t e rw s e r e p r e s e n t e dt r v W e l t e e t a l . ( 1 9 7 - 5 )A. m o n g t h c c o r r e l a t i o n p a r a m e t e russ e dw e r et h e d i s t r i b u t i opna t t e r n o s f s t e r a n ea. n d t r i t e r p a n e S s .i x molecularion intensitiesof homologous seriesweredeterminedbv GC/MS and r e l a t i v ed i s t r i b u t i o ness t a b l i s h ei nds o u r c cr o c kb i r u m e nasn do i t s i n i g .V . 2 . t t . y . Other parameters usedwereratiosof two or morehydrocarbon., ihJ C,.,to C.,,, n-alkaneseries.the isoprenoidhydrocarbons and cirbon isotoperatios.When makingconclusions abouta geneticrelationship for an oil_sourcerock pair. the
Oil and SourceRockCorrelation
562
0 i s t r i b u t ipoant t e r nosf s i x p o l y c y c lni Ia p h t h e n B s ( g e o c h e mfiocssa il l s)
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piltternof sterncandtriterpane molecular ionintcnsitics used Fig.\'.1.IL Distribution e ro i l s o u r creo c kc o r r e l a t i o(nA.f t c rw c l t cc t a 1 .1. 9 7 5 ) a sc o r r e l l t i opnu r i r n r e t o
to the C..r-cvclics.FigureV.2.1I showsthree highestdecisivevalucwasassigned differentoil-sourcerock situationsfrom top to bottom: D-YID-l: an oil-sourcerock pair of Oligocencagefrom thc Upper RhineG r a b c ni n S W G e r m a n y( W i l dC a tw e l l a t H r . i t t c n h e i m ) . E-YIE-1.an oil sourccrock pair of Eoccneagefrom thc Uinta Basinin Utah ( B l u e b e lol i l f i e i d) . from the Uppcr Rhine-Grabcnin K-Y/K l. K-2. K-3: u sourcerock sequencc i n t h e F o r s to i l f i e l df r o r nu r c s c r v o i r K Y w a s c o l l e c t c d S W G e r m a n l .T h c o j l of Eoceneagc.The Jurassicrock samplesK- I . K-2. K-3. all exhibitingsource rock propcrties.are from thc sameoil field and representthc Doggeralpha ( K - 1 ) .t h e L i a se p s i l o n( K - 2 )a n dt h e L i a sb c t a( K - 3 ) . betwecnoilsandsourccrocksfor the thc correlatiOn Basedon thc Ct,*-c-vclics. stlurce to bc sood. In caseof the multiple-choice pairsD and E wasconsidered bctweenthe Dog€ler canbe established rock situationK. the closcstrelationship a l p h as h a l e( K - 1 )a n dt h eT e r t i a r yo i l ( K - Y ) . In thissamestudyWeltect al. ( 1975)alsoshowedthat of the morethan 10oil no one showedcompletcagreementbetweenall sourcerock pairsinvestigated. correlationparametersapplied.This indicatesthe necessitvto aPpl) several independentcorrelationparameters.In a follow-uppapcr. Levthaeuseret al. (1977)expandcdon this approachto oil-sourcerock correlation.and replaced
1.3Oil-Source RockCorrelation hxamples
563
the visualcomparisonbetweendifterentdistributionpatterns(correlation parameters) b)'a statistical analvsis. Thcvalsoshowedwittrthehelpof theC.-,_cvclics distributionpatternthat the homogeneitv of somesour." ,,.,.t unit, .ll i iun.i,on of samplinqlocationwasexcellent. An exampleof how to approachsourcerockicrudeoil correlation in a complex geological by a more sophisticated biologicalmrrke, tecnn,que*as .s1'stem presentedbv Seifertet al. (1980)for rhe area oithe prudhoe Bav oil fierd. N o r t h e r nA l a s k a .T h e m o l e c u l a5r t r u c t u r eus: e d t o r f i n g e r p r i n t i n g rheshale extractsandthecrudeoilsincludedsteranes. terpancsan(l;t)n()aromaticsteroid h;-drocarbons. The diagrammatic east westcross_section ofthe prudhoeBa),oil ficld area in Figure V.2.12 showsthe generalized structuraiani stratigraphic situarionand rhe approximateoriginof ihe sampiesanailzed t,i CC.ll,tS.gasea o n r h e i rJ a t a . . S c i t e rt ra l .( l q x o ) c o n c l u dlehdr l r h e S h ; b l i kt . i r r a s r r cKl .i n u r r l r J u r l r \ \ r ci ]l n L lI h e d e e pp o s t _ N c o c o m isahnl l e ru e r e t h e m a j o rs ( | u r c s r . l l h . principaloil accumulations. Otherpossiblesourcerockson a geologrcal andbulk geochemical reasoning.like the dtep Kavak (Mississippi"" frfrrf"".i-fr. ,f,Jf"r, post-Neocomian. the Neocomianan.i the Uppe, Cretaceous Jates were clis_ cardcdafter biologicalmarkeranalvsisas the irigin of the accumulated petro_ leum. This is shownfor theselour shalesand th; Sadlerochit oil (202in Fig. V.l ll I hi rh('\rr'rdnc mrsstftrgmentograms( mlz 217)in FigureV.2.l3 a. Black \niidL-LI pr.ilk\rndicirtefor eachshalcsamplewheretheycliffer significantly from thc oil so that they can be ruredout as iis srurce.A,in''iru, tr,.i..nt.o.r.totinn
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Oil and SourceRock Correlation
56,1
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(Seifertel al . Ba!'.Alaska.sourcerock oil correlation Fig.\'.2.13aandb. Prudhoe (m/z217).(b)Key mass fragmentograms (a) Proofof lackof correlation bl sterane 1980). uil hv terpancmasrfragmentograms for the Ssdlerochit sourcerock-oilcorrelation ( n t , zl 9 l )
oil wasevidentfrom the terpanemass betweenthesamerocksandthe Sadlerochit (mlz 191).lncontrastto this.the terpanemassfragmentograms fragmentograms oil in FigureV.2.l3 b providethe key for positivecorrelationof the Sadlerochit to all otheroilsfrom the PrudhoeBay oil whichis similarin sourcecharacteristics field areaexceptthe Kingakoil (219).All biologicalmarkerdistributionr.gross between relationship parameters anCcarbonisotopevaluesconfirma one-to-one oil (202) the Kingakoil (219)andthe Kingakshaleasits source.The Sadlerochit however,showshybrid valueswhich can be explainedby a combinationof namelythe Shublikshale,the Kingak lrom threedifferentsources, hydrocarbons exhibit, major shale,which amongthemselves shaleand the post-Neocomian (Fig. differences, e. g., in their triterpanemassfragmentograms compositional
2.3Oil-Source RockCorrelation Examples
565
V.2.t3). but alsoin their steraneand monoaromatic steroidhydrocarbonmass fragmentograms. The clueto the multiplesourceconceptfor the Sadlerochit oil (and thosewith similarcomposition)was finally derivedfrom a quantitative assessment of the sourcespecificbiologicalmarkerparameters(TableV.2.1). This quantitationprovidesthreeaspects: a) the consistcncy of data within a given sourcerock formationand a clear distinctionbetweendifferentformations: b) the one-to-onecorrelationof the Kingakoil and the Kingakshale; c) a multiplesourceconceptasthe bestexplanationfor the compositionof the Sadlerochit oil. Sourcerock/crudcoil correlationby biologicalmarkertechniques is a powerful tool aslongasit canbe reasonably wellassured thattheoriginalbiologicalmarker pattern of the sourcehas not been alteredby subsequent processes such as migration.maturationor biodegradation ofa crudeoil. The effectof biodegradation on biologicalmarkershas alreadl'been mentioned(seeChaps.IV.5 and V.2.2).Usualll morcserious.however.is the influenceof thermalmaturationon the biokrgicalmarkerdistribution:it caninfluenceboth the patternandthc total a b u n d a n c e( s e e C h a p . V . 1 . 3 ) . S t e r o i dh y d r o c a r b o nfso r i n s t a n c eu n d e r g o significantisomcrizationand aromatizationreactionsin the early phaseof petroleumgeneration(Mackenzieet al.. 1982a).andthusfingerprintcorrelation mayfail if the sourcerock.althoughthe right one.is not availableat the suitablc maturationlevel.In thesecasescarbonnumberdistributit)ns of regularsteranes (C,-C,r). often displayedin triangulardiagrams.may still be usedfor correlation. Carbonnumberdistributions of steranes. in contrastto isomerdistributions (seeChap. V.1.3). appearto be largelyunaffectedby thermalmaturationbut parameters TableV.2.1. Source-specific for threemajorsource rocksandtwodifferent oilsin Prudhoe Ba,v.Alaska.(AfterSeifert et al.. 1980)
GCMS No.
Formation
Age
I ricvLlic I7c(H)- . 5d___.1_11' Terpanes.Hopanes. '13 51, c/c' %' Steranes
S h a l e2 1 0 K i n g a k Jurassic 2.3 S h a l e2 1 3 K i n g a k Jurassic 2.7 Shale207 Shublik Triassic ll.5 Shale208 Shublik Triassic 11.7 Shale209 Post-Neocomian Lower Cretaceous 4.0 Shale223 Post-Neocomian Lower Cretaceous 3.1
b4 b4
Oil 202 Oil 219
55 63
Sadlerochit Kingak
Permo-Triassic Jurassic
8.9 2.2
52 65 67
3.9 2.8 6.0 fr.I 1.1
3.9 ,+.1
'' Cls5/i. 1,1n.17d(20R)+ C1 5a. 1,{J,11t(20R+ 20S)+ Cr85/i. 1qJ. l7P(20R+ 20S) n Cr, 5a. 1,1a.17,1(20R) ' Area n of peaks ,,/: 191massfragmentogram on
566
Oil and SourceRock Correlation
definedby the envtronratherrepresentthe originalorganicmattercomposition (HuangandMeinschein.1979;Shiet al., mentalconditionsduringsedimentation 1982:Mackenzieet al., 1982b).Anotherlimitingfactorfor the useofbiological are is relatedto thefactthatthesecompounds markersfor correlationparameters often relativelyminor componentswithin the hydrocarbonmixturegenerated from a sourcerock andthatthcy arethermallylessstablethanmostof theothers. in sourcerockssuggcst Recentquantitativebiologicalmarker determinations that theirconcentration maydrop bv a factorof morethanonehundredfrom the stage.Thiseffectmavbecausedboth bv the latcdiagenesis to peakoil generation overwhelminggenerationof othcr hydrocarbonsand by the early thermal maturecrude oils may breakdownof biologicalmarkers.As a consequence. Any later additionto sucha containvery little biologicalmarkercomponents. lessimportant.but maturecrudeoil from a secondsourcewhichis quantitatively lessmatureand thcreforericherin biologicalmarkers,ma1-givcan imprinton a reservoiredcrude oil which is not consistentwith the bulk. more mature. the neccssitv of usinsmultipleparamehydrocarbons. This againdemonstrates tersin sourcerock/crudeoil and alsoin oil lil correlation. An interesting examplefor an oil sourcerockcorrelation.basedon thiophcnic thatthe wasgivenby Deroo(1976).He demonstrated compounds andaromatics. from the Arzacq Basin in SW France,reservoiredin the Roussecondensate hada doubleorigin.from sourcerocksof AlbianandOxfordian UpperJurassic. age(Fig. V.2.14).With thc help of the distributionpatternsof the diaromatics (type C"H1"'1) and dibenzothiophenes. eventhe relativecontributionof each sourcecouldbe estimated.This is possibleasboth sourcerocksare at aboutthe s a m em a t u r a t i o lnc v c l . featuresof compositional Oil-sourcerock correlationbasedon characteristic the light hydrocarbonfractionwas performedby Schaeferet al. (1978b).The datafor the lighthydrocarbons availabilityof sufficientlyaccurateconcentration "hydrogenstripping" in a large number of rock samples.obtainedby the technique.permittedthe applicationof a statisticalapproachto this problem. ratios(correlaFirst.a numberof characteristic Iighrhydrocarbon concentration -butane. -pentane. methylbutane/r tion parameterssuch as methylpropane/n etc. cyclohexane/methylcyclopcntane. 2-methylpentane/3-methylpentane. ) were parameters coefficient tbr sample. Based on these a correlation calculated each matrix is producedb! a computerprogram.which. upon request.may plot a d e n d r o g r a nAr .s s h o w ni n F i g u r eV . 2 . 1 5 t. h ec o m p o s i t i o nsailm i l a r i t i ebsc t r v c c n in a largenumberof rock samplesfrom an one crudeoil and thc hydrocarbons by explorationwell (PorcupineBasin.offshorclreland)may be demonstratcd plottingthc correlationcoeflicients betweenoil androcksagainstdepth.On the two intervalsshow basisof a similaritylimit of 0.80for thecorrelationcoefficient, significantsimilaritieswith the oil. thc shallowerintervalexhibitingthc grcater similarity.[:romothergeochcmical informationit wasdeducedthat thisinterval by migratedpetroleum.The is not a sourcerock.but a zonewhichis impregnated wereconsidercd asthe likely lowerintervalor the appropriatelateralequivalents sourcerock for the oil. Correlationof crudeoilswith their oil sourceformation,usinghighresolution wasperformedby Philippi( 1981). gaschromatography of CinC1hvdrocarbons.
1.3Oil-SourceRock CorrelationExamples
567
Fig. \'.2.11. C)il sourcerock correlationbascdon clistributionof thiophcniclnd polraromaticconrpounds. 1 he double originfrom sourcerocksof Albian and Oxfordianageof the Roussecondensalc in the ArzacqBasin.SW France.canbc (Alter Deroo. 1976) demonstrated.
300
t"
j
ROUSSE u-1./urcSsrc
I souf'dE-F6ai-! --------6F5-
0istribution of CnH2nJ2dioromclicso n d LOO dibenzoihiophenes
TSorrJ-uRc-E-ffin GER
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10o
s 9
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to C - A T O M SP E R M O I E C U L E \
15
20
25
to
!5
C-ATOMSPER MOLECULE\ -2
568
Oil and SourceRock Correlation Fig.V.2.l5. Compositionalsimilaritiesbel$een ,ril and rtrcl 'lmple. (Porcupine Basin.offshorelreland).Clusteranalysis of onecrudeoil and 51 rock samplesfrom wellbasedon eightdifferent ancxploration parameters(concentrationratios) calculated from light hydrocarbondata. The similarityvalues(correlationcoefficients) betweentherocksampleandtheoilsample areplottedagainstdepth.A similaritylimit of 0.8Uis representedby a broken line (Schaefer et al.. 1978b)
The conceptis similarto the previousexample.Ideally.a givencomposition parametershouldhavethe samevaluefor a crudeoil and the sourcerockwhich generated andexpelledthatcrudeoil. A "similaritycoefficient"hasbeendevised to definethe degreeof similarity.Philippi'smethodwastestedon ten selected presumedoil sourceformationpairsand it wasconcludedthat it is functioning properly. It has to be realized.however.that oil sourcerock correlationbasedon characteristiccompositionalfeaturesof light hydrocarbonsis only possible becausethermodynamicequilibriumof isomer distributionsis normally not reachedduringnaturalhydrocarbon generation (seealsoTable1V.1.3).Furthermore.thiskind of correlationcanonly be appliedwhencrudeoil andsourcerock a r c a t a b o u tt h es a m el e v e lo f m a t u r a t i o n . A specialapplicationof a kind of oil sourcerock correlationis the prediction, and if possible.recognition.ol a sourcerock basedon certaincharacteristics of reservoired petroleums.Certainpetroleumshavebeenfoundto exhibitan even predominance of the 11-alkanes in the rangefrom C,6to C12.whichis frequently combinedwith a predominance of phytaneover pristane.Degreeand extent (with respectto C number)of thesecharacteristics vary wrth ti'pe of oil and maturity. I t hasbcenshownthatanevenpredominance is typical of thez -alkanes for highlysaline.carbonateand/orevaporiticenvironments (WelteandWaples, 1973:Dembickiet al.. 1976)andthatthisfeatureis veryfrequentlvaccompanied (Wapleset al., 1974;Kalbassiand bv a phytaneover p stanepredominance Welte 1975).Thusa crudeoil with thesecharacteristics is mostlikelyderivedfrom a carbonateor evaporitetypesourcerock (seealsoChap.II.3 andFig. IV.3.4). This conclusion wasalsoverifiedby Tissotet al. (1977)by statisticalanalysis of n-alkanepatternsof numerousrock extractsandcrudeoils. Crudeoilsexhibitingan odd predominance of the highern-alkanes, especially in the rangeC;3-C.-,, andwhichsimultaneously aboveC20 containmoren-alkanes
1.3 C)il,Source Rock CorrelationExamplcs
569
C ; c n e r a l i o na n d m i g r a l r o n o l l j g h t h \ d r o c a r b o n \
..-@ @@ 'R
/
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/
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t/////,,*.!.:. :\ Z////)8|.;4 .-<.*J 1
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\)
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l lrmrue
I COAL
Fig. \'.1.lt,. Hr cirocurbont\ pe composilion ( .i ) of C,r and C- contpourrcigroups lor manne lhalc (t\pe-ll kero{en). nonmarine shale l predonrnirnrl\ t!pe-lll Kerogen) and coals (mean r alues) from (ipper Carbonilirous (Westphalian) striltl ot Corehole V. Ruhr area. Federal Republic of Germanr'. This information shouldbe contparedto r jmil a r f i n d i n g sa m o n gC r ; _ h v orocarhons as presented in F i g u r cI L 5 . l 5
cycLo-aLxanes @ ARoiralcs
thanbclow.aremostcertainlvderivedfrom a sourcerockwith a largeproportton o f t e r r e s r r i aolr g a n i cm a t t e r i r fh i g h e rp l a n to r i g i n( s e e a l s o Fi!. fV.:.:;. Sr.t crudeoils have.frcquentlv a slightprisianeovei phytanepredtiirnance.How_ amounrof isoprenoids is ratherlow. Oils of this kind usually 11,.._l ll^. o r r g_l n a t,o.].roll eIdr o m n o n m a l i n e s o r l:c r ( rocKs'or lrom stratarcprescnting nearshore' brackish-marine environments. Leythaeuseret al. (1979b)tbund that significantdifferences in the light , h1'drocarbon compositionare associ.rted with ihangesin the t1:peot ttreorganic mattcrthe),originatefrom. This dependcnce on th; faciestype'isexcmplifiedin FigureV.2.16wherethe hydrocarb,rn-type composirion of Cnond C, .o-pornd groupsare shownfbr marine shale.nonmarine shaleand coalsof advanced maturatronlcvel (abour 1.27. R,,) from Upper Carboniferous(Westphalian) strataot a core hole in the Ruhr district.The moststriking effectconcernsthe concentration of aromatics(benzeneandtoluene)with resp-ect to the othernonaromatichydrocarbons. ln thc caseof the coalsamples. boih aromatichvdrocar_ bons representa signiticantlvhigher proportionin the Co "nd a, g;;p.-;, c o m p a r e tdo t h c m a r i n es h a l ecso n t a i n i ntgv p e _ l kl e r o g e nw. i t h a ni n t e r m e d i a t e p o s i t i o nl b r t h e n o n m a r i n cs h a l e s( p r e d o m i n a n t tl yy i e _ l l l k e r o g c n )F . or the c t c l o a l k a n et sh e r ci s a t e n d e n c fvo r h i g h e rr e l a t i u ep r o p o r t i o n s ' i n themarine shalcascomparedto coal for ttLeC, hv-drocarbonr. Ii,t,,inrrnr,1,.ttreCoand C. hltlrocarbonmixturesgeneratedfrom coalvorganicmutte, areielativelyrich in aromaticand poor in naphthenic compounds. in comparison with thosclound in marineshalcsat equivalentmaturationlevel.The preclominance ol oenzeneand tolueneappearsto reflectthe influenceof highei plants. Simrlarooservanons $ere Inadefor two adjacentsource-.be.l typeshile intcrvals.bearingtvpe_lIIand ttpe-ll kcrogen.rcspectively,in rhc main phaseot petroleumg'eneratron. of Jurasic agein.the PorcupineBasin(Schaefer and Leythaeuser. tlSO).Cornpu_ rableobservations on C,r* mcdiumto heavyhvdrocaibons havebeenpresented i n F i g u r eI I . - 5 . 1 - 5 .
57t)
Oil and SourceRock Correlation
in crudeoil or bitumenhasnot yet Recognition of tvpicalfaciescharacteristics been extensivelydeveloped.However, more researchin this directionwill certainlyhelp to characterizc crudeoils in more detail.whichin turn will allow the predictionof the type of sourcerock from which it probablyderived.In ChaptersL4 and IV.3, somedata are presentedand thoughtsdevelopedwhich and type of biomass environments are pertinentto recognitionof depositional deposited. Finally, it should be remarkedthat suchfeaturesas the existenceof irregularitiesin the n-alkane distributionpattern in oils, or an even or odd predominance etc.. is alwaysa signof relativelylow maturity.More maturecrude of theoriginofan oilsshowsmoothn -alkanedistributioncurves.Suchknowledge oil with respectto faciesand level of mat[ration may be of greathelp when developingan explorationconcept.
Summaryand Conclusion Most important objectivesi. oil and sourcerock correlation are the recognitionof variousfamiliesof pooledhydrocarbonsin a basin,and the identificationof the source rock of a given petroleum. Such knowledge helps to define exploration targets. Geochemicalcorrelation is based upon recognition of compositionalsimilarities betweenpetroleumsand sourcebeds.Relativedistibution patternsof hydrocarbons especially those are typicallyusedas correlationparameters, and nonhydrocarbons with very specificchemicalstructures,suchascompoundsof the steroidand terpenoid classes.LIc/llc-isotope ratios are also used. especiallyto relate oil and bitumen to a certainkerogen.It is advisable to usealwaysmorethanone correlationparameter. Oil-oil and oil-source rock correlationis hamperedby both strong maturation or andaromatichydrocarbons biodegradationof pooledpetroleums.Naphthenoaromatic and thiophenic compoundshave been successfullyapplied for oil-oil-correlation of matured oils. Aromatic hyd(ocarbonsand thiopheniccompoundshave been used to correlate biodegradedoils in addition to biological markers of the sterane and triterpanetype which are not alwaysaffectedby biodegradation.
Chapter-l LocatingPetroleumProspects: Applicationof Principle of Petroleum Generation andMigration- Geological Modeline
I n t h e e a r l v d a v s o f p e t r o l e u me x p l o r a t i o n .w e l l s w c r e d r i l l e d w h e r c o i l o r g a s seepagcsindicated the prcsenceof petroleum. Later. with incrcasingsophistication of reological knowlerlge. and cspeciallv with the advent of exploration qeophvsics.the ilecisionto drill a well was additionallv takcn on the basisof thc r e c o q n i t i o no f s u i t a b l es t r u c t u r e sr. u c h a s a n t i c l i n c s .t u u l t t r r p s . u n c o n f o r n r i t v traps anclrccfs. Frequentlv. the selectionof a structureto be drillcd w as hasetlon i n t u i t i o n a n d t e n e r a l e x p c r i e n c er a t h e r t h a n o n p e r t i n e n ti n f o r m a t r o n .b e c a u s e verv Iittlc information u'as available whethcr or not a trap would contain hvclrocarbons. A s v s t e m r t i cs t u d v . u t i l i z i n gt h c n e w u n d e r s t a n d i n og f p e t r o l e u mg ( ' n c r i r t i ( ) n a n d m i g r t i o n . c a n h c l p t o t l e c r e a s ct h c u n c e r t a i n t vi n p r e d i c t i n ga p e t r o l c u m f i l l c d s t r u c t u r c .a n d h c n c et h e f i n a n c i a rl i s k w h e n d r i l l i n ga w e l l . T h i s i s o f s p e c i a l importancc in offshore areas and remote. hostilc exploration rc{ions uhere d r i l l i n ei s c x t r c n l c l vc o s t l v .T h c c o n s e q u e nat p p l i c a t i o no f g e o c h e m i s t rcya n a l s o cielinenel exploration targetsin rclirtivclv well-known basinswhere the ratc of d i s c o v e r i ch s a sd c c l i n e d . The purposeof this chapter is to show how petroleum exploration can benefit. firstlv bl collecting sourcc rock and maturational informatiot't (acquired at relutircll lou' cost) from applicationof organicgeochemicalstudics.and secitndlv b v r c l a t i n s t h i s k n o w l c d s ei n t e l l i g e n t l vt o t h e s e o l o c i c a fl r a m e r v u r ko f a s i v e n exploralrola l r e a . T h c t r a s i ci d e a o f t h i s c o n c e p ti s t o i d c n t i f v t h c s o u r c er o c k s p r e s c n t .t h c h l d r o c a r b o np o t e n t i a lo f e a c h s o u r c er o c k i n t i m c a n c ls p a c e .a n d r c l i r t (tJh i s i n t i ) r n ] a l i o nt o t h e g e o l o g i c acl v o l u t i o no f t h c b a s i n .A t t h e e n d o f s u c h i r s t u d v . m o s t f a v o r a b l ez o n e s f o r p c t r o l e u m a c c u m u l a t i o na r e l o c a t e di n t h c b a s i n .T h i s i s d o n e b r ,r c l a t i n t t h e h v t l r o c a r b o n c e n e r a t i o no f a s o u r c cr o c k i t t a n v q i r c n t i m e d u r i n gb a s i ne v o l u t i o nt o t h e m o s tl i k e l vp a t h so f p e t r o l e u nm r iqration. anil to thc tbrnation and l{e of a trap. T h c d e t e r m i n a t i o no f t h e m o s tf a \ o r a b l cz o n e so f p e t r o l c u ma c c u m u l a t i o ri nr i t basin consistsof a scqucnce of steps to prcpare and combine thc nccessarv qeoloqicalanclgeochenricalintbrmation. antl produce mapsand sectionsshowing a n d r a t i n e t h c p o t c n t i a lt a r { e t s .T h i s c a n b e t l o n e b v h a n d ' i n a c o n v e n t i o n a l n l a n n c r .o r \ \ ' i t ht h c h c l p o f c o m p u t e r sl.n F i g u r eV . 3 . l a s c h c m ci s p r e s e n t e d t h a t s h o w s t h c s c q u e n c eo f s t e p s a n d . i n a s e n e r a l w a v . t h e k i n d o f g e o l o g i c a l infonnation rcquired in ortler to applv nrganic geochemicalstudiesto petroleunl cxploration. This schcmccan easilvbe tbllou,edin practicc. It is no nore dilficult than the introciuctionof faciesanalvses.isopachmaps. t)r depth contour mapshas p r e v i o u s l vb c e n . l n t h e p a s t . a l l t h c s ct e c h n i q u e sh a v c a l s ob c c n a s s i m i l a t e b t lv
Geochemistrv Application of C)rganic Prospects: Locating Petroleum
572
A d e c i s i veer l l o r a t i 0pnh i l 0 s 0 0ihnyc 0 r p 0 r a tgi neg0 c h e m idc a tl a : 0 e t e r m i 0 a l0i 0f nm 0 s lt a v 0 u r a bIl0en e s0 l p e t r 0 l e uemc c u m u l a t i 0 n
organicget|chemical input
Geological Input
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IDENTIFICATIONOF SOURCE ROCX
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M I G R A T I O NA V E N U € S IN THE EASIN
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Fig.\'.3. ). Schcmrtic summar!of t hc \ ariousstepsthatleid to the detenninationof most in ir basin favomblezonesof petroleumaccunrulation
petroleumgcologists. geochemical ciatashoulclbe incorporltedin andnowadays t h e p h i l o s o p h0vf e x p l o r a t i o r . modelingwith the help of high speedcornputr.rs is Geological-geochemical possiblythe mosteffective.suitable.andcertainlythe mostadvanced methodfor It integrates determinationof the favorablezonesof petroleumaccumulation. knowledgeon petroleumformation andquantifiesthe ncwlvgainedgeochemical frameworkof basinevolution.Thiscomputerand migrationinto the geological processes has the aided numericalsimulationof geologicaland geochemical geologiderived enormousadvantage thatthroughthe combinationof regionall,v
3.1Acquisition of theGeochemical Information
5:/3
cal datawith informationaboutthe principalprocesses of petroleumgeneration andmigration.tailor-mademodelsofa specificexplorationarea are availablefor the€xplorationist. ln thiswaya rankingof explorationtargetsbasedon a network of logicallyconsistentdata can be established.This ;iminishes greatly the cxplorationrisk. The followingchapteroutlinesthe acquisition of the geochemical information .^ (Sect.3.1) and presentsa first conceptualmodelof pJtroleumgeneration in a basin(Sect._ 3.2)- Then geologicalmodelingis presented(Secti3.3),whereas geochemical modelingwill be introducedin the next ChaDterV.4
3.1 Acquisitionof the Geochemical Information When a new explorationprogramis designedin a more or lessunknownarea, only a limited amountof gcneralgeologiialinformationis normallyavailable. This. of course.appliesnowadavsonly to certainremote areasand offshore rcgrons.Samplematerialof the mostimportantstratigraphic andlithologicunits reprcsented in the basinmustbecollected.outcropsat tharim of thebasi;beinga readilyaccessible source.A carefulstudyof thi availableinformationon the sedimentary filling of the basin(a faciesanalysis)might resultin a preliminary identificationof one or morepotentiqlsoura"ioak, or"aourcarock sequences. A sourcerock-identification program using organicgeochemicaltechniquesto analyzeavailablesamplematerialshouldhelp to definepotentialsourcerocks moreclearly.At thesametime,neighboring nonsource ro;k stratashouldalsobe checkedfor porosity and permeability.and their role in offering possible migrationavenues.If little or no samplemarerialis available.a limiteJsimpling programmay be startedto obtainsurfacesamplesandcomplementary unweafh_ eredcore samplesfrom a few shallowdrill holes(up to 100'mdepth)at strategic locations.In offshorebasins.informationfrom adjacent,comparablestrata on land,andsometime-s samplestakenfrom the seabottomby dredgingor shallow corrngmay be helpful. A diffcrentcaseis presentedwherebasinshavebeenabandoned somevears ago. and where fresh interestarisesfor new exploration.The adventoi the geochemicaltechniquesand neir knowledgeof the principlesof petroleum generation andmigrationfrequentlyjustifiesa freshbok at old prospeCts. ln such cases. old samplematerialfrom earlywellscanstillbe usedto evaluatethesource rock potential.However,in all cases,the first stepin a basinstudyis to identify the numberand kind of potentialsourcerocksin ihe basin.The samplemateriil hasto be analvzedfor amountand type of kerogen.andits stageof maturity. In fact.the conceptofsourcerockidentification anddeterminitionofthe main zonesof oil andgasformationcanbe introducedinto an explorationcampaignat alrv stage.In particular.beforea well is drilled, it is criiical to ensurethat a maximumof informationcan be gainedwith respectto both sourceand carrier rocks.To obtainthis informationfrom the well. an effectivesamplingprogram must.tte prepared.Major lithologiesand regular depth intervalsshould be sampled.Cuttingsshouldbe collectedat intervalsof3 to 30 m. tJepending on the
5'74
Geochemistr) Locating Petroleum Prospects: Application of Organic
changesin lithology.Preferablv.samplesshouldbc cannedin small air-tight containers(about 300 500 cc). with ample water to keep the original moist saturation.particularl!if light hydrocarbonstudiesare intended.In addition. somecoreor sidewall coresamplesshouldbe obtained.in orderto supportand vcrjfvinformationgaincdfrom cuttings.C--ore samplingis alreadydoncroutinely for large-scale testingof physicalpropcrties.Thc samplematerialshouldbe in sucha waythat basicanalvticalinformation subjectedto geochemical analyses onsource r o c kp r o p e r t i cest c .i s a v a i l a b l ei d. e a l l yb y c o m p l e t i oonf l h e w e l lo r , a t 'l'herefore, least.with a minimumtime-lag. rapld rnethods hate to be appliedfor techniques. sourcercrckanallscs.SuchmethodsJhouldbeconsitlered asscreening ratherthan asa mcansto obtaindetailcdanalvticalresults.Methodssuitablefor s c r e e n i nagr et h ep v r o l ) s i tse c h n i q uacn dt h c q u a n t i t a t i vdce t e r m i n a t i oonf l i g h t h v d r o c a r b o nBs o . t h h a v eb e e nd e s c r i b eidn C h a p t c V r .1. 'I'hese geoclremical trvotechniques will provideindependcnt logs.rvhichcanbe r c a d i l vi n t c r p r c t c icni t e r m so f h y d r o c a r b opno t e n t i aal n dl e v e l o lm a t u r i t v( m a i n z o n c so f o i l a n dg a sf o r m a t i o n ) . Evenintirrmationwith respectto hvdrocarbon migrationcanbe derivedfrom methodscanbe theset\\,oIoqs.Analyticalinstrumcnts for both kindsof screening considerinstalledanduseddirectll at the well site.Undcr thesecircumstances. can be simplified(no canning. able time is savcdbccauscsampleprocessing packing w h i l ed r i l l i n gi ss ti l l i n a n dt r a n s p o r t a t i oe nt c .) . a n dr e s u l tcsa nb eu t i l i z e d progress.The advantages of 'real-time"gcochemical loggingare obvious.and ccrtainlycanhclp to cut costsandgainvaluableinformation.For instancc.wellloggingby pvrolvsiscan indicatewhen a overmaturczonc is headgcochcmical reachcd.and no moreoil is to be expected.Useless continuationof drilling.and total amountof hencethe wasteof money.can thusbc prevented.Conversely. light hydrocarbons and certainhvdrocarbonratiosmay givc an indicationfor nearby hydrocarbonaccumulation.and thus stimulatefurther drilling and will alsostimulate improvcthc success rate.Knowledgeof a nearbyaccumulation necessar-rsafetymeasures for furtherdrilling. In addition to the screeninganalvses.further samplc material has to be gcochemical laboratorv,to verifvandcomplctcthe analvzedin a wcll-cr'1uippcd informationgainedfrom screening. Thc numbcrof samplesto be analyzedin moredetailfor sourcerock identification,typeof kerogen.andmaturitycanbc reducedconsiderablyb1' utilizing the screeningresults to select zoncs ot homogeneous organicfacies.in particularthoseof mostpromisinghvdrocarbon potential.Wheneverpossiblc.a fcw core samples(sidervalJcoresor othcrs) shouldbe includedto supportresultsobtainedfrom cuttingsand/orserveasfixed informationon geochemical reference pointsin a well.Suchdetailedgeochcmical physicochemical and selectedrock samples.basedon variouscomplementary (Chap.V. I ). serveto refineresultsfromscreening opticalmicroscopic techniqucs techniques and allorvthe improvementand updatingof the preliminaryconceppossiblycollectedfrom tual model.In addition.analyses of pooledhydrocarbons (FT provide informationon the sourcerock-oil tbrmationtests samples).can relationship:identificationof the sourceby directcorrelationand the stagcof maturityat the time of hydrocarbon expulsioncanbe attemptcd(Chap.V.2). ln the caseof severethermalalterationoccurringin thc rcservoir.suchinformation
3.2 FirstConceptuitl Modelof pctroleurn Ceneration in a tsasin
575
cannotbe obtained.In somecases.wheresourcerockshave not yct tteenfound. the_analyses of pooledhydrocarbons ntavallowpredictionof thi typc of source rock to be expccted(Chap.V.2).
3.2 First ConceptualModel of petroleumGenerationin a Basin Thc scicntificapproachfor locatingpetrolcumprospects. described below.canbe ", an\ \rag-cof exptoration.In fict. it is dcsirabteto updatethis :I::ld":.g d e t ( ' r m t n i i t t or rntm o s tf u r o r a h l cz o n e so f p e t r o l e u ma c c u m u l a t i o n continuouslv J u r r n rt :h er . r t t j reer p l r r r r t i rcr n irmpaiun. W i t h t h eh c l po f t h c g e o c h e m i cdailt r . i r p r e l i m i n u rc\ o n c c p r u a l modeo i fthe . basinrvithrespectto sourcerocksanclgeneration of peiroleumis dcveloped.For cachsourcerockidentified.a mapis producedshowingthe regionalextcnt ofthe sourcebcd. its thickness.and dcpth levcl. It is alio advisable ro construct preliminarvdiagramsol the subsidence historyfor eachsourcclaverat stratcgrc p o i n t si n t h c b a s i n( F i g .I I . 5 .l l ) . T h i sp r o b a b rcyo u r db c c l o n ea t a i e r a t i v e leva r r v stageof Ihe.explorationcampaignin basinj situatedat a passire .u"fn"niii margin.In the passivcmargin setting.the depth timc relationship is usualiv rather-simplc.as normallvthere has bcen minrmalerosion. The siratigraphii idcntificationof seismicmarker horizonsis the basisfor thg constructrcnof diagramsof subsidencebefore thc drilling of the first exploration wells. Bv interpretationof the maturitv data of ihe rock sample'si"""r,,gur"J. ,h; 'apfroximate subsidence curvesof sourcerjck units help to preOi.t levelsof matuntyin difterentpartsof the basin.In thisway. it is poisibl. to obtaina first ideaaboutthe depthlevelofthemainzoneof oil formationthroughout thc basin. For a determinationof the mostfavorablezonesof petroleuil accumulation the followingseriesof questionshasto be answcred: 1. Which bedshavea sourcerock potential?What is their regional extentand their relationship to palcography.l 2. Duringwhichgeological timc interval.at whatdcpth.anclin whichareaofthe basinwcre the above-mentioned sourcerocksmatureenoughto generatc petroieum? 3. Duringwhichgcologicaltime interval.and llong whichprclerrcd routeswas l c t r ( , l c u mm i g r r it , , np o s s i b l e . ' '+. At whatgeological time did the formationof trapsoccur.ascomparedto the timingof petroleumgcnerationandmigrationi)*here arethesetraps Iocated, with respectto possiblemigratlonroutes? Thesequestionscan be answeredon the basisof a set of mapsand sections p r e p a r e d" b t h a n d ' . H o w e v e rt.h e q u a n t i t a t i vaep p r o a c h u s i n gg c o l o g i c aaln d g e o c h e m i em a lr r L l c il s. a m o r ec f f e c t i r em e r h o dt , , i a n r u e r i n g tie-.lue.ti.,n.,.
576
Locating PetroleumProspects:Application of OrganicGeochemistry
Basin 3.3 NumericalSimulationof theEvolutionof a Sedimentary - GeologicalModeling handlingof the The startingpoint for a quantitative.mathematical-numerical of a petroleum of the development is the simulation formationand migrationof are represented basin.The relevantfactorsof suchan undertaking sedimentary steps.whichfolin FigureV.3.2. The ideais to retracegeological schematically at time beginning from an initial condition low eachother as a causalsequence (t: T,,), over intermediateconditions(t - T'), and to the basinconfiguration observedtoday(t = T*). A particulardifficultyis that onlv the end resultat etc.- andnot the initialcon(t : T*) today'sstratigraphic section.porosities. dition at (t = T,,) is known for the observedsedimentarybasinas empirical basin data availablefor comparison.The evolutionof a particularsedimentary must be completclycomputedout. usingcstimatedempiricallydcrivedrnitral data, and then the resultmust be comparedwith the real basinto seewhether the presentconditionat time (t = T,) hasbeenreached. The conceptualmodel is first set up for a real system,that is. for a real sedimentary basin.whichnaturallyis lesswellknownat the startof anexploration modelis the basisfor a mathematical campaignthan at the end. The conceptual initialdata.The first throughwith estimated model.whichthenwill be calculated with the valuesthat iterationwill generallynot achievegood correspondence basin.Now the initialinput the present-day conditionofthe sedimentary describe modifiedaslongasnecessary datamustbe changedand/orthe conceptualmodel known end result.A flow diagram to give the geologically for the calculations (Fig. V.3.3) illustratesthis procedureschematically.
To
EN at \ 3l
\
| |
th€rmal conductivity
basinfrom an initialconditionat time (t : T,') to Fig.V.3.2. Evolutionof a sedimentary p\rrlr\it\. aspressure. observedtodav(t: T*) Suchparameters the basinconfiguration unit in each sedimentary continuouslv are changing andthcrmalconductivity temperature with increasing deDthof burial.(After welte. 1982)
.l --lNumericalSimulation of the Evoturronot a Sedimentary Basin
I1t J;iii,
for three_dimensional quantitarive basinmodet.(Afterwelte ii.il,r'agram
The computedvaluesare made.to resemblethoseobservedin nature within toleranceby meansof successive itera,i";;.' i".-,#;;"t'#liiiru,ut,on o,oo"l workslts way from a lowerlevel,ot ,o un.rttar-f,lgnarleveluntil -correspondenc" the whotesimutarion is ended.rle tactrtrat ttrem;,;;.;;;,;,;."meters ro be aremutually dependent
:i'::p].,1 an Inherent logic is followed thr
i. of iurourn.niur'dilan#l
lt means that ,5l paramerers roitr,","oJ.i.,ujf:hx::?,,j:"il:-l;$*;rumi:l* temperatureand maturity. The core for the three_dimensional rieterministic
;;;;;;;;."i,ilii"q"u,ion,,r :#:JT":?l,.T:i,'j.: ;"i:'J,:l,l*,".;; car piocesses. rh.;;;k;;;;;::'ffJi"'"HT:;i:::fifi.Jil:il::'ifiiiil ( l a RI ) A r c o k r g i c al . a h r d r o d y n a m r ca. l h e r m o d v n a m i c . a n da g e o c h e m i c a l
part hc Jirringuished irmung rheinpurdarutFig.V.J.a).The to bc investisa
s:1,1T. e c t m eLr,,Crlr n t a r vh r s..an in
"l:'of km' and a depthof severat Tu"v.tho"usancl km t, oi"ia;atll up lll^t']1s,1l rnto a three-dimensional
grid. rather' into elementarvp;;i ;;;;.'"uu The essential parameters. suchaspressure.temperature, porostty,degreeof thekerosen.
erc..areihen.ur.utut.i-*iti'ii,Ii".,iu.,uunn, o.,".
g:1r,:11i.:.1 e o t o g t c at li m e s p a n sf o r t h e n u m r
:j,^,h:.j1,:;;:;tii;",,"""i'riili'To.J'"rJ::lr"J;,:,Tiiil
calculatrons at alrgrid pointsseveralti... in u r"qu"n.i Jiii"i.utrJn,*u, urr.uoy porntedout. The computedvalues. *trictr are mutirat,r;.;;;;:;r. "oapredto eachother.and rheyare brouphrinlo-agreement witit tfrir.In.r.i.,f 'i, in nature hy rhis
iterarion process (Fisi.v.3.3ind t.j.+i.'ii
i"ffi thatonlyrhe tastesland most capable of large computerscan be used for such calculations.
57lJ
LocatingPetroleumProspects: Applicationof OreanicGeochemistry
Fig. V.3..1.Development of thrcc-climcnsional deterministic dynamicmodel.Groupsof inprrtparameters and resultsafter solutionare schematicall) shown.(Aftcr Weltc and Y n k l e r l. 9 8 l )
tffi;.dR.;
-:_,,
-
-----: T!
ar
througha realbasinas basedon quantitatilebasin Fig. V.3.5. Computedcross-section modeling.(irid pointsin the horizontalplainareshownwith trumbers1-15(N S). (After Welte. 191i2)
l.-l Numerical Sinulation of the L\ ()llltion of a Scdirrrcntar I lJasin
-i79
The first importantstagein thesimulationof a basin.s historvfrom a getlloercal standpointis the calculation0t crosssections(Fig. V.3.5) rif the sedrmentarv h a s i ,tnh r t i . u n d e ri n \ c s t i r a l i , , nf.5 * . n ' n n r r ' . , , nu l th. *^O"ili;J;;.;i;;i t n r c k n c so\ n t h c c r o , i s - r c c t i o un i.t h l h i c k r r e sl sc l u a l l ) m c i s u r c d rn r*ell,,,r seismicprofilesis one of the first controlpossibilities tbr bringingconceprand reality into agreemcntthrough purposefuliterations.How iinelv the cross_ sectionis dividedinto discretcunitsin the horizontaland vcrtical .fi.."ri"^ i hoth.the complerityof the geotogyand bv rhe lcvet of geologicai 11::::: .b..'I hc InteBrati(rn rnTo.rmtll()n. over timc of the individual'corrciponOing p.ocerscs uf th. sl,item leadsto the computationof sedimentary thickness in the geologic a lt l m es D a n s . imp(,rtanrgoalsof integratedbasinstudiesfor pctroleum ]ll"_,*.sr c^-._?1. x l l L l r :f a tIrn ( r \ I h ec ,' m p u a l li ' ' n o f I h e h rd r o c lr h o np o l c n t i Iao f a p a t I i c ; l i rr \ o ur c c rock.ateverl'givenpointintimedurinethehisto^ofasedimeut!r'oasrn. t.nisis " i , h " ,r c J a r i v c h t ri g hl e t c lt r t r r r n l i d e n cuei t h r h eh c l po f r h . . " " n , " r i . o f : . iuht1. r9t l Lo,ln sl :rl m t h c h x s i ns d e \e l o p m e n tT. . h cm a t u r i t vm a p p i n igr f a s o u r c e r o c ki n spaceandtime is an cssential part,whe-n detcrminingthe h'vclricarbrn generarrng z o n c .I n . l . f i r \ 1 . \ t c pi.t i s i r c c c l t a b l feo r a n e x p l o r i t i o nc a r l p a i t n t o e s t a b l i sah vrld rlinktngol e\plorntiontrir-qets in_agivenbasinratherthant., p.cr.nt u truly quantrtatrvc prcdictionof amountsol hvdrocarbons. Whilsta truly quantitativ; evaluationnccessitatcs thc use of an elaborateklnetlc moJet of kerogen degradation (prescnted in the ncxtchap. v.'t). it is sufficientfor a iirstevaruation olexplorationtargetsto calculatcin a semi_empirical manncryitrinitcreflcctance valucs(i. e.. maturitv).A relativel), simplemet-hod to arriveat calfutatcdi,itrinite rcUectance valuesusingtime temperaiurerelationships wnr. f,,, inrtan... pru_ p o s e db 1 -L o p a t i n( 1 9 7 1 a ) n c tl a t c i m o d i f i e db y W a p f c s V S i f ,T1 h. i sm e t h o d lf is rh ( r e \ r c h l p rer . F o l tr,ui n g e, , i rh , , u g rhn p r e , e nr _ . 1 . , r lm : l al :ll u : ]n: .l^\ :m. Ui r l h i rl n\ c, d o n r i l l i ' , l t a r e l l c c t . r n rrhr iv. itri n luesi:presenlcrlinfigurcV.r.r,t,,r a sourcerock ln an arcaof activeexplorationof about165.(XX) kmt. l.hesevalues can be convertedinto amountsof hvclrocarbon generatcdpcr rock unit. if thc q u a l i t va n dt h i c k n c sosf t h c s o u r c er o c k _ iksn o w n . - l n t h i se x a m p l et .h ec o m p u t e d basinmodeltvpicallvcontainsabout3-50datapointsfor one siice of sourcebecl exrcndrnsover ar areaof l6-5.00(, kmr. Mapsof this kinclcan bc drarvnbv thc c r ) m p u l c r : irll \ t i r r i r e r F l e o h r g i ctl il m c l r r rc \ \ . r \ p r i r m c l efrh : r ti s c | , n l a i r r c J in l n C : ( l o l L l l l l c r Ln t t i t lc q u i l l t o n sr r l l h c s i n t u l l r t i o n Dtrrurum.
' l ' . T , ] , 1 1 1 1 , ' c rpr l, ' ,1, c1n11 1, ,,t,l1s o u r c cn , i k h , , .h . c n e , ) m l u l c d \ l i r ( c rn t ( .t.hr \ r nt o r m t t ) n i l-nTu.Ilr:m i , c n nb ec o m b i n e idn a p p r o p r i a tf co r mw i t hr n l o r n t a t r o n about the prescnceof porouscarrierand reservoir.oatt; unA their geological posrtlonIn the dcvelopingbasin.Structuralmapsof such reservoirrocksor of anv canalsobe calledup from the computerfor anvgivenp.;;i ,; ,;; ::11i".C..^"._\ ( F i s .V . 3 . 7 1 ihe numericalsimulationof the geotogicaldevelopment of a sedimentarv . basin makcs possiblean uncommondensityof gejogical inior_uti.,n n,rj geologic:rl dataot r complerelv newcluality.Thegreaiadva-ntage ofthe numerical \ r m u l a l r od n \ . s e r i h ehde r el i e sn o r , r n l \ I n t h e q u a n t i l ) . i n, s, f g l o l o g i c rp l rramc_ ters.whichnormallvareonlv availablethroughwells.hut-nror!imp.rrtantty rnthe fact that a continuousnet of geological infor-mation coveringthe iahoreareaano
580
-=*lt
-1.0- lso maturation lines. vilriniie reflectance (Rmi)
-\" I \;, V
i.r4
/ L?"/12 \:,-' ----13-
? : ,
--l;" ,.," ' -
,i.-,,
\
,..: '
'l/,:t 1.6
.t|:g,:,jL" " ,:::,:-{
valuesfor a sourcerockof a realbasinfor the Fig.V.3.6. Computedvitrinitereflcctance presentdal asbasedon quantitativcbasinmodeling.(After Welte.19E2)
Fig. V.l.7. Three-dimensional subsurfacccontour map of the top of a reservoirformationin a real basjnas basedon quantitative basin modelinS. (After weltc. 1982)
Summary andConclusion
5gl
volumeis madeavailablein dependence with time. The computedmapshavea considerably higher "reality content"than conventionalgeologicalsubsurface mapsthroughthe mutualinfluenceof individualparameters on eachother and throughthe cross-checks with realgeological conirolpoints. It is quite obvious that the applicationof computer-aidedmodelingfor geological andgeochemical processes isonlyin itsinitialstages. Furthermore,itis realizedthat a truly quantitativepredictionof hydrocarbonpotentialsand ultimateoil and gasreservees of sedimentary basinsand not oniy a rankingof explorationtargetsis onlypossibleif the mostimportantgeological andgeochemical processes responsible for the formationof hydrocarbonaccumulations are understood in greaterdetailthanat present.Next to the geological modelingthe geochemical modeling,especiallyof the kineticsof kerogendegradationand hydrocarbongeneration,is of greatimportancein this connection.This will be discussed in the nextchapter.
Summaryand Conclusion A systematicutilizationof the new understandingofpetroleum generation,migration, and accumulationcan help to decreasethe uncertaintyin predictingpetroleum-filled traps. This is done by identifyingthe sourcerocks, the hydrocarbonpotential ofeach sourcerock in time and space,and by relating this information with the geological evolution of the basin. This determinationof most favorable zones of petroleum accumulationin a basincan be done "by hand" in a conventionalmanneror with the help of computers. In the initial stageof an explorationprogram, even with (lirnited) information on potential souraerocks, a first conceptualmodel of petroleumgenerationin a basinis developed.In this model the relativepositionof potentialsourcerocks,their presumed stateof matu ty and their extensionthroughoutthe basinare depictedin maps and profiles. Sucha conceptualmodel may provide valuableinformation for the selectionofthe siteof exploratorywells.Rapidgeochemicalscreeningtechniquescanbe appliedat the well siteto providereal-timegeochemicallogswhichcanbe readilyinterpretedin terms ofhydrocarbonpotentialof sourcerocksandlevelofmatu ty. Furthersamplematerial hasto be analyzed in a laboratory,to verifyandcompleterheinformationgainedfrom screening. A numericalsimulation of geologicaland geochemicalprocessesthat lead to the formation of hydrocarbon accumulationsis the prerequisiteto make guantitative predictionsabout oil and gasreservesin a basin.Simulationof geochemicalprocesses necessitates a kinetic model of hydrocarbonformation, and alsoa model of migration and accumulationmechanisms whichare not yet known in sufficientdetail. However,a ranking of exploration targets can be achieved by simulating the evolution of a sedimentarybasin,ie., by geologicalmodelingand a semi-empiricalincorporationof relevantgeochemicalprocesses. This diminishesgreatly the explorationrisk. The backbone of geologicalmodelilg is a set of nonlinear partial differential equationsofenergy and massbalancethat descdbethe courseofrespectivegeological
582
LocatingPetroleumProspects: Applicationof OrganicGeochemistry
processes. Main resultsof the modelcalculations are. for instance,computermaps showingporositvdata, temperatures. maturitiesor sedimentarythicknesses after compactionfor any givensedimentary unit duringevolutionof the basin.A typical three-dimensional gridsvstemof anexploration areaisin therangeof 10,000 to 100.000 grid points.It is thus obviousthat geologicalmodelingmakespossiblea formerly uncommondensityaf geological informationandgeological dataof a completelynew ouahtv.
Chapter4 Geochemical Modeling:A euantitativeApproachto the Evaluationof Oil and Gasprospects
4.1 Necessity of a QuantitativeApproachto petroleumpotential of Sedimentary Basins The purposeof thc evaluationof a sedimentarybasinin termsof petroleum cxplorationis Io knorvthe amounrof oil and gai that hasbeengcnerated and accumulated and then to locateir. In thisrespect,the followinginformationis desirable: a) Amount.of.oiland gasgenerated(per kmr) in eachsourcerock and in every p a r to f t h e b a s i n . b) Time of hvdrocarbonformationto compareit with the ageof deposition of impcrmeable sealsandwith the ageof foldingandfaulting,i. e., with respect to tbrmationof traps. c) Amount of oil andgaswhichhasbeenexpelledout of the sourcerock into the porous reservotrs.amount and locationof hydrocarbonsaccumulated in IraDs. d) Ev;luationof the ultimateoil and gasreserves of a sedimentary basin.This parameterwill be requiredincreasinglywhen explorationreaches an advancedstage_ Then the problemariseswhelherto;ontinue or stopexplora_ tron,dependingon the possiblereserves remainingto be discovered. A quantitativeapproachallowinga computationof the amountof oil and gas generated andmigratedin anyplaceof the basinasa functionof timeir necessiry to answerthc abovequestions. As petroleumandnaturalgasresultfrom kerogen degradation, variousindiceshavebeenproposedto charicterizethe qualityind evolutronstageof the organicmatter.Thcsehavebcenreviewedin Chaptei V.l. They canbe usedasa first approach.in a semi-empirical manner,for rankingof explorationtargets(seeabove,Chap.V.3). However.theseinclices do not allow a trulyquantitative evaluationof hydrocarbon generation asa functionof time.In particular.mostof themdo not accountfor therespective kineticsof degradation of the varioustypesof organicmatterand alsofoi complexburialhistories. Mathematical modelsbasedon kineticsof kerogendegradation andpolyphasic fluid flowsand usingcomputersimulationalloJa trul! quantitativeapproach. The problem.seemsto be adequatelyexpressedin iespectto formation of petroleumandgasasa functionof time,but additionalreseirchis stillrequiredon migrationof hydrocarbons to achievethe evaluationof the ultimatereserves.
of Prospects Approach to theEvaluation Modeling: A Quantitative 581 Geochemical wasfirstintroducedthroughtheconsideration Kineticsof kerogendegradation for eachother. that temperatureand time might, to someextent,compensate Thisviewwasknownfor a longtime,asMaierandZimmerley(1924)investigated the kineticsof bitumengenerationfrom the GreenRivershales.AIong that line. various experimentally McNab et al. (1952)and Abelson (1963)investigated an that by using cracking)and showed degradationschemes(decarboxylation, in the low temperature Arrheniusplot. suchreactionsmayoccurat the relatively periodsof time.Anotheraspectis the relation sedimentary basinsovergeological betweenageor heatingtime of sourcerocksand temperaturethresholdof the main zoneof oil generation.For a giventype of organicmatter,the threshold generallydecreases with increasing ageor heatingtime (Connan,I974,1976)ol relationship. the sourcerock. accordingto an Arrhenius-type also consideredthe kineticsof the On the other hand. coal researchers andthe progressive eithcrnaturalor artiticialduringcokefaction. carbonization. related eliminationof volatile matter and increaseof carbon content and reflectance. Kineticswereintroducedby usingnomograms,or an approximate computation(Huck and Karweil. 1955;van Krevelen,1961;Lopatin. 1971: Lopatin and Bostick.1973).In respectto pyrolysisof oil shales,experimental wereobtainedby HubbardandRobinson dataon kineticsof kerogendegradation proposedby Allred (1966), (1950)and mathematical modelsweresubsequently Braun and Rothman(1975). Fausett(197,1). Lopatin(1971),followedby Waples(1980),madean attemptto calculatein a on malurationof the simplemannerthe combinedeffectof timeandtemperature organic matter, without using the actual kinetic equations.which require a numericalsimulationand the useof computers.It is assumedthat the ratesof chemicalreactionsapproximatelydouble for every 10'C rise in temperature aninterval burialtheycalculate (Lopatin,19711 Waples.1980).Upon progressive of interval residence / T in a temperature maturityby multiplyingthe time of The total every 10"C. to 2n+r,2n*2, etc. 10"Cby a factor2", whichitselfchanges index)whichis the sumof the by TTI (time-temperature maturityis expressed intervalmaturities: -rrr -
I
2"(/r,).
and.through Thena tableis providedto calibrateTTI versusvitrinitereflectancc it, to interpretthis indexin termsof stagesof oil and gasgeneration. it doesnot require Althoughthis calculationprocedureis attractive.because that the rateof reactionsdoublesfor the useof computers.the basicassumption with an activationenergyin therangeof everyl0"C riseis onlyvalidfor reactions from 20oto 160'C whichincludethe E = 10 to 25 kcal mol ' (at temperatures at the stagesof oil and gasgeneration).This order of magnitudeis acceptable onset of oil generation,when relativelyweak chemicalbondsare broken in kerogen(Tissot,1969;Connan,1974). (WeitkampandGutberlet, from experiments However,it hasbeenrecognized in sedimentary basins(TissotandEspitali€.1975)thatthe 1968)andobservations with progressively increase, activationenergiesinvolvedin kerogendegradation at broken.For instance, increasing maturation,asstrongerbondsaresuccessively
.1.2Mathematical Modelof Kerogen Degradation andHydrocarbon Gencration
5g5
of oit.generationrhe aycragcadivation energv is E = .10to 60 kcal lfe,q;ak mol and in thc gaszonc it reachcsE = 50 to z0 kcaim.r J. It shoutdbe re_ .'. membcredthatan activationenerg),,E i corr"riona, = S0l<^calmol oppror:,r.,ot"ty to rates multiplied by 5 every 10.C and f = OS'tiat"mor ro rates lgaglion multipliedbv 10. in thi temp'erature rangescorresponding to tl,e peak of oil gcnerationand the gaszone,respectively.lnthat intervalo?maturation.over a Icmperaturcrncrease of 30"C. thcTTl. will accrrunt for I ratemuItipliedby 2r : g. whereasthe actualratewill be muhipliedby 125 to 1000.if the ave'rage E = -50to 65 kcal mol r. More informationa'bouttt e .ote anOsigniii.-* "f activation encrgicsmal be lirund in SectionsV.-1.1and V i..s i" f,rrtii.",ar. rherc are or degradation jlI::t:::",]. dueto differences in chemrcalcomposi_ lincrics non or rherc\pccrrvc kcrogcnlypes.Thusit is difficultto accountwith a single parametcrfor hvdrocarbongenerationfrom the various typei oi sourcerocrs.
4.2 MathematicalModel of KerogenDegradationand HvdrocarbonGeneration A mathematical model of petroleum generation. accounting expticitlv for gcological time. was firsr introduced by Tissot (1969) and i., ijitv 0,..u.."a in
E-spitalii (1975). T'hemodet is tor",iontii.ii.. ot tlrog"no.gru,tu_ Il::1,-Td tron and usesthe gcnerar
schemeof evolution,describedin part Ii ot thisbook. Kcrogenis a macromolecule composedof polycondensed nu.i"i t"unng oltyl charnsandfunctionalgroups,the linksbetwecn nucleibeingheieroatomic bonds or carbonchains.As the burial depthand temperature in&.u.., tn. bondsare broken, roughly in the order ot in.r"urtn!',rftu." energy. t.he :T.,:::l]:ly pr()ducts generated arcfirst heavyheteroatomic compoun-ds. carbondioxide.and w.rrer;rnen progrc\srvel\smallermolecules: and finally hydrocarbons. At the residualkerogenoecomesprogressrvely morc aromaticand :i.i,: liT".the evolvestowardsa carbonresidue.All thesemech"anir*, l.,uu.U..n clescribecl in Part II and are summarizcdin FigureV.4.1. The;u;;;.. o'titiJ.ut ".uti"or modclis to represent the kineticsof theparallelandsuci".r.iu.-r"o.tlon, ,frn*n in this figure.
Corbon residue
Corbon residue
Fig. V.,1.1.Generalschemeof kerogendegradation (Tissotand Espitalid.1975)
586 Geochemical Modeling: A Quantitative Approach to theEvaluation of Prospects Reactionsare consideredhere as not reversible,In fact. when sourcerocks havebeenburiedto a certaindepth.thenbroughtagainto surfacebv subsequent foldinganderosion,theorganiccontentkeepsthe composition andphvsicochemical propertiescorresponding to the maximumburialdepth.Furthcrmore.some of the bv-products of kerogenevolution,suchaswaterand carbondioxide.are highlymobilein sub-surface conditionsandcouldnot be availablefor recombination. shouldit be the case. For simulatingthe systemof reactions, shownin FigureV.4.f. it is advisable to consideronlv a limitednumberof stepsandparticularly: A . B r . B " r . * h i c h w a st h c f i r s tv e r s i o np r o p o s e b d y T i s s o (t 1 9 6 9 ) . A. B".r. 8,,.whichappearsfrom experience to be sufficientto accountfor thc successivc oil and gasformation(Tissotand Espitalid.1975).anduill bc uscd here.
." Ker'cen <.t"::.<.ii:.-" Thc lastforrnulationcorrcsponds to:
(l)
The following svmbols will be used. .,1is kerogen. comprising rr1bonds of a given type i, at the time Ii r, is the amount of organic matter reacting in thc i'h r e a c t i o n( b r c a k i n go l r - t v p eb o n d ) .B ) t t o B j , , , a r c t h c p r o c l u c t st oh fe f i r s t s t c p o f re actions (formation of oil), their respectiveamount at time I being _yr to ).. B,r to B.,r are the products of the secondstep of reactions(cracking). their respective amount at time t being ul to l.{,,.
A (n,x)
B'' (-yr)
Brt (ut)
B'r (y,)
Br, (rr)
(2)
B1
Br, (y) B,^ (y^)
(y)
Br, (ui)
B. (u,)
The first sct of transformations represents a certainnumberof parallcland/or successive rcactions.It is assumed thattheprobabilityof breakinga bondof tvpci is indepcndent of the abundancc of bondsof the othertypesandalsoindcpcndent of the time elapsed.Then the breakingof l-typebond obevsthe Poissonlaw: -
-
dn, n;
= ktidt,
,1.2Mathematical Modelof Kerogen Degradation andHydrocarbon Generation 587 ft11 beinga constantat a giventemperature. If the structureis homogenous, l e.. the densityof bondsis statistically the samein the kerogenvolume,we have: where4 is a constant,and, with regardto parallelreactions: J , o= r o 4 . { beingthe frequencyof i-typebondat the originr : 0, andxo : )x;e beingthe - organicmatter.Then, it becomes: total amountof labile- or pyrolyzable - T dx, = at
ktix,.
Therefore.we obtainfor eachof the i reactionsa kineticequationsimilarto that of the first-orderreactions.It has beenshownthat suchformulationis able to accountfor kerogendegradation in geological conditions(Tissot,1969). If we makethe additionalhypothesis thar the 8 r, consriruents of oil havethe samegeneralbehaviorin respectto cracking,we canconsiderfor the secondstep of reactionsBr = ) 8,,. The respective amounts-ri,y; andu;canbe obtainedfrom t h e f o l l o u i n gs y s t e m o f d i f f e r e n r i aelq u a l i o nIsE q . t 3 r ] : _
dt,
_ kri xi
dt dui
:
dt
kzif
(3)
u:l :l + t + tup l +l -,! lln lr, 7Y,' 7u, The first two equationsexpressthe kineticsof the system.whereasthe lasttwo expressthe massbalance.The variationof k,, and k;, with temperaturemay be describedbv usingthe Arrheniusformula: _ -\, h , : A t , e RT which is basicallyvalid for fast laboratoryor industrialreactions.but may be extendedto slow reactionsoccurringundergeologicalsituations(Tissot,1969; C o n n a n1. 9 7 4 ) . f r , i s t h e a c t i v a t i o n e n e r g y o f t h e l ' h r e a c t i o n , A r i i s a c o n s t a n t a n d Tis the absolutetemperature(Kelvin).In geological situations, temperature is a functionof time throughsubsidence andrelatedburial.As burialis reconstructed by geologists on the basisof successive time intervals(e.g., Lower Cretaceous, UpperCretaceous, Paleocene, etc.) depthis considered to be a linearfunctionof timeduringeachinterval.Thustemperature alsois approximated asa succession
588 Geochemical Modelingr A Quantitative Approach to theEvaluation of Prospects of linear functionsof time during the successive time intervals.Under these conditionsthe system[Eq. (3)] cannotbe easilyintegrated,as ftri becomesa complexfunction of the time r. The easiestway of solvingthe systemis by numericalintegration,with successive,j/ increments. This is doneby usingthe computer,andtheprogramincludesthe adjustment of thevariousparameters"riO, E11, A 1iandy0,whichis in factthecalibrationof themodelto theparticulartypeof organlcmalter. For this calibration,only the first set of reactions(formation of oil) is considered. At the beginning,valuesmeasured recentsediments on comparable (e.g., BlackSeasediments for type-Il kerogen)areusedfor y0,andapproximate valuesfor A,, (Tissot,1969).In respectto the Er,, it is necessary to considerall activationenergiesfrom a few kcalmol I, corresponding to the ruptureofweak bonds,as in adsorption,to about 80 kcal mol-', corresponding to breaking carbon-+arbon bonds.The objectiveof the adjustmentshouldbe to determine the frequency,.f, of each/-type of bonds.or the amountx,uof labileorganic materialinvolved in the i'" reaction.The calibrationis basedon valuesof extractableorganic matter (hydrocarbons+ resins+ asphaltenes) naturally presentin corestaken at differentdepthsin thc sedimcntarybasinand/oron comparable figuresresultingfrom laboratoryexperiments at highertemperature. The adjustmentis based on the method of the least squares.The results concerning the threemaintypesof kerogenI, II andIII, areshownin TableV.4.1 and FigureV.4.2. Thesefiguresare basedon the data obtainedfrom the sourcerocksof the Uinta, Parisand Douala basinswhich were originallyusedto definethe three maintypesof kerogen.They offer a first approachto calculatethe amountof oil TableV..1.1.Distribution of activation potential energies andgenetic of the principal kerogentypes Kerogentyp€s
Activation energies
Typ. I
(kcal mol r)
E,, Erz Elr E', Er: E,o
l0 30 50 60 70 80
0.02.1 0.064 0.136 0.152 lJ.341 o.t72
Geneticpotential of kerogen ro : xro
.1.7510s 3.04 t0'6 2.28 10' 5 3.98 10r') 4.1'7 ](l3) 1.1010"
0.895
,4 is expressedas 10"yr ' value of yo
Type I : Type Il : Tlpe III:
0.051 0.03s 0.018
Typc II
0.022 0.03.1 0.25r 0.152 0.1l6 0.120
0.695
1.27 105 7.,17l0'. 1.,18l0:7 5.52 l0?'r 2.04 1015 3.80 l03s
Tlpe ltl
0.023 0.053 0.072 0.091 0.0,1e 0.027
0.3r3
5.20 10r ,f.20 10'' ,1.33l0r5 1.97l0r: 1.2010rr 7.56 l0r'
I
1.3 GeneticPotentialof SourceRocks.Transformation Ratio
589
Fig. V.4.2. Distributionof rhe achvauon energiesinvolvedin the degradation of the lhree main rypes ol kerogen(Tissot and E s p i r a l r e1.q 7 5 )
.9
I
10 -E
30
50 60 70 ao ( K c o t . / m o t e ) -_ _ _ _ |
andgasgenerated from othersourcerocksof comparable type.However,oueto thevariabilityof kerogencomposi.tion, a specificaijustment'baJon laboratory assays, whichprovidea directcalibrationof the moiel for eachparticular source rock, is generallypreferable. Calibrationof the secondsetof reactions _ formationof gas_ hasbeen made from laboratoryexperiments on cracking,includingthe reiultsof McNab et al. ( 1952),andJohnsandShimoyama Consideritionof a singlereactronwith 11972.1. an activationenergyof 50 kcal moj r seemsconvenientro i.count fo. ga, generationin the deeppartsof the sedimentary basins.
4.3 GeneticPotentialof SourceRocks.Transformation Ratio The total amountof hydrocarbons whichcanbe producedby a certainkerogen, providedit is heatedto a sufficienttemperatureover a su"fficient tlme, is the qu a n t i t v
o=I that appearsin Tablg Vaa f This quantity is equivalentto the gen eticpotentialof , , 0 :O" : o _ r , " d - !e. f i n e di n C h a p r elrl . 1 2 1 6 . t u l u . O e p e n d r o n r l i e r y p e o f k e r o g e n . 1 . e . .o n t l s o r r g r n acl h e m i c acl o m p o s i t i o nA. s o u r c er o c kc o u l d b e d e f i n e da sa
590 Geochemical Modeling: A Quantitative Approach of Prospects to theEvaluation rockwhosegeneticpotentialis abovea thresholdvalue,e. g., 0.25or 0.30related to the unit weightof kerogen,or (if we consideran averagevalueof 1% organic carbonin rock) 0.25to 0.30Vcrelatedto unit weightof rock. At any time, the stageof evolutionis measuredby usingthe ftansformation ratio r, tlelinedin ChapterIL7.2. which is the ratio of the kerogenalready transformedto the geneticpotential: ).r4 - Lr, -to - )li _
-I9
The transformation ratio is zeroat shallowdepthsandprogressively increases to 1, whichis reachedwhenall labileorganicmaterialhasbeenexpelledleavinga carbonaceous residue.
4.4 Validityof the Model In varioussedimentarybasins,comparisonbetweenthe figurescomputedby usingthe modelandthe corresponding amountsof petroleumgenerated through burialshowsexcellentagreement, with a quadraticdeviationlowerthan10 ', and a correlationcoefficientbetter than 0.9. The modelhasbeenusedto simulate experimental heating- eitherisothermalor with a regularrate of temperature increase during varioustimes from an hour to one year. againwith satisfactoryagreement.Furthermore.the sameset of constants,4,and 81,shown in TableV.4.1. is sufficientto accountfor all conditionsof kerogendegradation (TissotandEspitalid,1975)including(a) naturalevolutionin sedimentary basins at relativelylow temperature(50-150'C)overa periodof 10to 400millionyears; (b) artificialevolutionthrough laboratoryexperiments(180-250'C)(c) hightemperature(400-500'C)retortingof oil shales.The factthata singlemodelwith the same set of constantsis able to simulatesuch different situationsis a confirmationof the validit)'of the hypothesis madeon kinetics(statistics of bonds and activationenergies,first-orderreactions,etc.). The timingof oil generationprovidedby the modelmay alsoin somecasesbe checkedagainstgeological data.An examplefrom northernSahara(Algeria)has beenshownin Chapter1I.7.6.
4.5 Significance of the ActivationEnergiesin Relationto the Type of OrganicMatter for thesakeof The parameters E1iusedin the modelarecalledactivationenergies simplification. They havea role similarto that of activationenergy,but theyare not strictlyactivationenergies,asthe latterare normallydefinedin respectto a particularandsinglereaction.Thisis not thecasein the model,whereweconsider reacthe "formationof petroleum"from numerousparalleland/orsuccessive tions.
,1.5Significance of theActivation Energies
591
Many t)'pesof bondsare originallypresentin kerogen.with disrincrrupture energies.and particularly: - weak bonds correspondingto physicalor chemical adsorption(hydrogen b o n d s e. t c . ) . - carbonvland carboxylbonds, etherii, I sulfurbonds. carbon-ci.bon bonds. Furthermore,the ruptureenergyof mosttypesof bondingmayvaryaccording to neighboringfunctionalgroupsor substituents, leneth of chains.etc. Conre_ quently,consideration of a distributionof rhe acrivationenergies611from 0 to 80 kcalmol '. aswe did in Section2.2,is probablycloserto thJeffectivemecha_ nismsthan a hypotheticalmeasurement of eachindividualtype of bond.There_ forc. the bestrepresentation of kerogenc()mpo\itionmay'bethe histogramof activationenergies.derivedfrom Table V.,1.1,presentedin Fisure V.,1.2for t y p e ' 1 -. l l . a n d - l l l k e r o g e n sW . i t h i n c r e a s i nhgu r i a la n d r e m p e r i t u r e ( a n dd e _ I creasing-) the variousbondscorrcsponding to the successive actiratirtncner_ gies E, arc progressively broken, roughly in order of increasing8,. This is suggested by the tcmperaturedependcnce of the reactionparameters k, in Figure V..1.3. The geneticpotential-r,,= >,r,ir.andthe distributionof the activationenergies changeaccordingto the tvpe of kerogen(Tissotand Espitalid,1975): a) TypeJ kerogencontainsa largeproportionof labileorganicmaterialrr),and thusits geneticpotentialis high. The distributionof the activationencrsies includefew with low valuescorresponding to weak bonds.Most valueslre
Fig. V.4.3. Temperaturedependence ofthe reactionparameters kiinvolvedln kerogendegradation. The figureshows thatthe variousbondscorresponding to the successive activationenergiesare p r o q r e \ r i v e lh\ r o k e n .i n o r d ( r o l i n creasingactivationenergy(Tissotand Espitali6.1975)
Modeling:A Quantitative Approachto the Evaluationof Prospects 592 Geochemical bonds. groupedaround70 kcalmol-r, andmaycorrespond to carbon-carbon generatfor particular higher temperatures this kerogen requires Therefore, ing oil than the othertypes. TypeJI kerogencontainsslightlylessof thelabileorganicmaterialresultingin b) a geneticpotentialslightlylowerthan in typeI. However,the distributionof activationenergiesis wide andincludeslowervaluesthan typeI with a mode at 50 kcal mol-r. Thus,the mainformationof petroleumstartsat somewhat Iowertemperaturethan in the previoustype. c) Type-III kerogencontainsstill lessof the labile organicmaterialthan the others,and its geneticpotentialis low. As a result,the total amountof oil is generated is comparatively small.The distributionof the activationenergies s m o o t hw i l h a m a x i m u mf r e q u e n cayr 6 0 k c a lm o l - r . A highconcentration of organicmatterrelatedto typeI or II resultsin a richoil shalewith a high oil yield. The Green River shalesbelongto type I and their valuesof .xuis 0.8 to 0.9, i.e., 80 to 907oof the organicmatter is able to be convertedto oil. The Toarcianshalesof WesternEuropebelongto type II, and valueof re is 0.6, i.e., 60% of the organicmatter can be the corresponding convertedto oil. Type-III organicmatter, on the other hand, may be conshales,but with a low oil yield centratedto form certaincoalsor carbonaceous (.r, = 0.25)and no commercialoil shales. The relationshipbetweenthe threemaintypesof kerogen,asdefinedby their distributionof activationenergies,ma1 chemicalproperties,and the respective be usedfor a quick calibrationof the model.If one knowsthe specifickerogen theappropriate or elementalanalysis, type(1,U or III) from opticalexamination valuesof A, and E, from TableV.4.1 can be usedin the model. A reviewof the apparentactivationenergiesproposedin the literaturefor petroleumgeneration, carbonization of coal,or oil-shaleretortingshowsthatthe valuesrangefrom 8 to 65 kcal mol-t. Theseapparentactivationenergieswere computedon the basisof a singlereaction,althoughit dealsin factwith a setof successive and/orparallelreactions.As the true activationenergies81,correspondingto the breakingof variousbonds, are affectedroughly in order of ascending valueswith increasing temperature,the apparentactivationenergyis closeto the lowestE/ at low temperatureand closeto the highestEi at hiSh temperature.This behavioris confirmedby experimentaldata obtainedfrom laboratorypyrolysisof oil shalesby Weitkamp and Gutberlet (1968):they observeda progressiveincreaseof the apparentactivationenergyfrom 20 to 60 from 0 to 80%. This kcal mol ', whenthe conversionrateof kerogenincreased considerationis sufficientto explainthe conclusronthat apparentactivatlon energiesrelatedto the beginningof oil formationare on the average10-15kcal related mol r(Tissot,1969;Connan,1974)whereasapparentactivationenergies ' to oil shalepyrolysisor carbonizationof coal are about 50 to 65 kcal mol (Abelson,1963;Hanbabaand Jiintgen,1969,etc.).
4.6 Applicationof the Mathematical Model to petroleumExploration
593
4.6 Applicationof the Mathematical Model to PetroleumExploration The mathematical modelof kerogendegradation providesquantitativevalueof oil andgasgenerated asa functionof time.Therefore,it is diiectlyapplicable for evaluationof the oil andgaspotentialof a sedimentary basin,andfoi determination of the timing of petroleumformation for comparisonwith the age of structuralor stratigraphic traps. The datarequestedfor useof the modelare presentedin FigureV.4.4: (a) a direct calibrationof the d distributionon laboratoryassavi,which in fact J:pre.Tnts.the chemicalcompositionof rhe organicrnrrr.r, o, alternatively identification by opticalor chemicalmethodsof the kerogentypeandsubsequeni useof the corresponding distributionshownin TableV.a.1; (0 the burialcurve, i. e., the depthversustime relationship. from the time of sourcerockcleposition to the presenttime. and (c) the geothermalgradient.for computationof the temperaturcversustime relationship.The gradientmay vary with geological periods accordingto geotectonicconditions,and its determinationwill be discussed in the followingparagraphs. Providedsuchilata are available,the amountof oil and gasper ton of rock generatcdin anv placeof the basincan he compuredasa fuiction of time (Fig. V.,1.4).In order to coverthe wholesedimentarl' basin.it is convenrent to divide up thevolumeof the basininto a three-dimensional grid (asshownin theprevious chapter,Fig. V.3.4)andto makethe computationfor eachunit. The elementary unitsmay be definedin differentways:in platformbasinswith gentlefolding.; grid basedon IatitudeandIongitudeis generallyadequate;inmo-bile areaswhere foldingis fairly strong,elementaryunitsdelineatedby isobathcurvesare more nearly homogeneous in respectto depth and temperaturehistory.The most t
^t'i:.
I = E I 0rsTRtSuI0t{ 0F ACTIVATION Ti/ERGI€S
BUBIAT HISTORY
o'nt#lfftl
Ef
.,
--
;-]
otA
il
Jl -?o-ToilC--60 ETIME
I i
8P( Y)
ca Purar/o0f a/LI GAS 'ftytRArtD -T|lllE+ G"C/km
Fig. V.,1.4. Principleof use of the kineticmodel of oil and gasgeneration.The data providedarethe typeof organicmatrer(or therelateddistribution oi actirationenergies), the.burialhistoryand the georhermal gradient.The modelcalculates the quantitiesof oil andgasgenerated. asa functionof time (Tissotet al.. 19g0)
594
Modeling:A QuantitativeApproachto the Evaluationof Prospects Geochemical
)
dt' oui +50
" a
)
tr
'i23 c, -
!Oii"a
-5oa o,gao
, ne! r O S n qe p r o d L r c w
.O
show
oDryhore
1.6Application ofthe Mathematical Modelto Petrolcum Exploration
595
effectiveway'to use this geochemical modelingis obviouslvto introducethis simulationinto the basingeological modelingpresented in the previouschapter. The applicationof the mathematical modelto oil generation in Jurassrc source rocksof the ParisBasinis shownin FigureV.4.5, It can be expressed by the fractionof kerogen,whichhasbecnconvertedto petroleum,i. e.. rru in grams petroleumper kilogramof organicmatter.Alternatively.if the distributionof organiccarbonacrossthe basinis alsoknown,it canbe relatedto the wholerock andexpressed asgramspetroleumper ton of rock.From thefigure.it is clearthat all knownoil fieldsfall insidethe area,wheremorethan110g petroleumhasbeen generated per kilogramof organiccarbon,andinsidethe area,wheremorethan 1500g petroleumhasbeengenerated per ton of rock. Regardingthe timingof oil generation. mostpetroleumhasbeenformedin thisbasinduringCretaceous and Tertiary. The exampleof oil and gasgenerationin Paleozoicsourcerocksof thc Illizi basin.Algeria,is shownin FigureV.4.6 anddealswith a moreadvanced stageof thermal evolution. Geologicalhistorv of the basin includestwo periods of sedimentationand subsidenccseparatedbv Hercvnianfolding and erosion. During Paleozoicsubsidence, the Silurian and Devoniansourcerocks were buriedto a greatdepth(about3000m) in the southwestern part of the basinand the kerogenwasdeeplvdegraded(transformation ratio r = 1) to generategas. This gtrssubsequentlv migratedinto anticlinesformed by Hercynianfolding. However.a deeppost-Hercynian erosionreachedthe lowerPaleozoic reservoirs and prccludedconservation of gas.In the other partsof the basin,Paleozoic subsidence wasmoderateandonly initiatedoil generation.Foldinganderosion weretbllowedby an importantMesozoicsubsidence. particularlvin the northeastcrnpart of the basin.Thc new burialexceeded the maximumpre-Hercynian depthsin all placesof thebasin,exceptthe southwestern part,wherekerogenwas definitell unableto evolveanv more. In all other areas.kerogendegradation continuedgcneratingabundantoil andalsogasin the northernandeasternparts of the basinwhere Cretaceoussedimentation was very thick. The amountof petroleumformed has been calculated.with considcrationof the respccti!e timingol generation. Thus,oil andgaspotentialmaybe described as:(a) low for b o t ho i l a n dq a si n t h es o u t h \ \ ' e sdtu. ct o c r o s i o na f t e rg e n e r a t i o n ( b: ) g o o cflo ro i l i n t h c c e n t r aul n ( le a s t e r np a r t so f t h e b a s i nw i t h s o m cg i i sa s s o c i a t ei nd t h e c a s t : ( c) c o o ctlo rq a si n I h cn o r t h c a sw t .i t hs o n eo i la s s o c i a t ci ndt h cl c s sb u r i c do r c o l d e r areits.
The exampleof thc HassiMessaoud areain northernSahara,Algeria.hasbeen presentedin ChapterIL7.6. There, purely gcologicalconsiderations allow an independent confirmationofthe timingofpetroleumgeneration (Figs.IL7.5 and 1 1 . 7 . 6 )T.h c m a t h e m a t i c m a l o d e ls h o w st h a t t h c m a i np h a s eo f o i l g e n e r a t i o n lrom Siluriansourcerocks is not reachcduntil Cretaceous time. and extends
Fig. V.1.5. Applicationof the mathematical modelto the generation of oil from Lower Jurassic sourcerocksofthe ParisBasin.Iop. theoil generated is expressed in gpetroJeum per kg organiccarbon.Bottom:the oil generated is expressed in g petroleumper t rock ( D e r o oe t a l . . 1 9 6 9T; i s s o at n dP e l e t .1 9 7 1 )
596
Geochemical Modeling:A QuantitativeApproachto the Evaluationof Prospects
Gosgeneroled duringPoleozoic Tropsore eroded
0ilgeneroied
270-
\ mosllyduring
Mesozoic
Oilgeneroled mostlyduring Poleozoic
0
1001m
I Devonion oulcropsondsubcrops underneoih lhe Mesozoic unconformily Middle a UpperDevonion E LowerDevoniqn ffi 2 Oilondgosfieldsproducingtrom Corbonifurous ond Devonion Oil s Gos poientio Pelroleum I Nil
lIIITll Oilpolenlrol fffl tn H i g h
lll I co.po,"n,ior Hiqh I
Fig. V.,1.6. Hvdrocarbonpotentialof the Lower Devonianbeds in the lllizi Basin (Algeria).asdeterminedby useof the mathematical model(Tissotand Espitali6.1975) mostly over Cretaceousand Tertiary. In this area all traps are equallv prospective, whatever age thev are. from Paleozoicto Cretaceous.
4.7 Reconstruction of the AncientGeothermalGradienr 1.7.1 HeatFlow and Geothermal Grudient The temperatureincrease with increasing depthresultsfrom a heatflow from the insideto the outsideof the earth.The temperarure I is a functionof depthz and ,tT the geothermal gradientis G |. At a givenplaceof the earth'scrustthe heat flow @ and the geothermalgradientG satisfythe relarion:
G= E . a .
4.7 Reconstruction of the AncientGeothermalGradient
where1/{ is the thermalconductivity.G is expressed asthe temperature increase p e ru n i to f d e p t h e , .g . . d e g r e eCs e l s i uksm ' . I t m a yr a n g ef r o m1 0t o 8 0 " Ck m r , w i t h a n a v e r a gv ea l u e o af b o u t 3 0 " C k mr . @ i s e x p r e s s iend4 c a lc m : s r 1 1 0o caloriespe-rsquarecentimeterper second).It may rangefrom lessthan I to 8 pcal cm-' s-'. with an averagevalueapproximatelyl-2 pcal cm-2s I. Thermalconductivityvarieswith the compositionof rocks.For instance,the valuesare quite high in salt (Fig. III.2.3). The variationscausea variabilityof geothermal gradient,whereasvaluesof heatflow are lessscattered. TableV.4.2 showsthe averagevaluesof heatflow in continentalandoceanicbasinsaccording to geotectonic conditions. Heat flow is low in Precambrianshields (0.9 4cal cm-: s r) and in areaswhere orogenyand/ormagmatismare of Paleozoicage or older (1.2 to 1l.3 's-'). It is somewhathigherin areaswhereorogenyand/ormag4cal cm matismisofMesozoicorTertiaryage(1.9pcalcm-2sr);alsoinTertiaryvolcanic areas(2.2 4cal cm-'.s-'); and moreoverin thermalareaswherethe flow may exceed3 4calcm ' s '. Thushighvaluesof heatflow andgeothermal gradientare reportedfrom synorogenic or postorogenic basinsof the Alpine belt (Pannonian basinextendingover Hungary,Yugoslaviaand Romania)and of the circumPacificbelt: Okhotskand Japanseas,Sakhalin,North Sumatra,Fiji, etc. Oceanicareasalso show a large rangeof heat-flowvariation,from oceanic ridgesto oceanicbasinsand troughs.Furthermore,the heat flow is quite high alongthe axial zoneof the ridges,i.e., alongthe crestand rift valleys.where valuesashigh as 8 pcal cm 's-' are measured.It decreases progressively away from the crest.Within 150km on both sidesof the crestof the EastPacificridee. halfofthe recordedvaluesarestill in therangeof3 to 8,acalcm 2s rl beyondtiat distance. the heatflow is alwayslowerthan3 pcal cm 2s I . A similarobservation hasbeenmadealongthe mid-Atlanticridgein regardto a 50-kmzoneon both sidesof the cresl. The presentoceanicexpansionbeing approximately3 to 6 cm yr ' in the EastPacificand I to 2 cm yr I in the Atlantic, the belt with a highheatflow corresponds approximately to an oceanicbasement youngerthan 5 millionyears. More preciselv.Le Pichonand Langseth(1969)havecomputedthe average valuesof heatflow as a functionof distanceto the axisof oceanicridges(Table V.4.3). To measurethe distanceD they used. insteadof kilometers,a unit corresponding to magneticanomalyNo. 5. whichis definitelydatedasslightlyless than 10millionvears.Thus,distances arein factcomputedin termsof ageof the oceaniccrust. and oceanswith differentratesof expansioncan be compared. From TableV.4.3.it is clearthat highheatflow (2 or 3 timesthe averagevaluein oceanicbasins)are relatedto a belt with an agelowerthan5 millionyears.Then the heat flow dccreases with increasingagc and is no longerdistinctfrom the averageoceanicbasinwherethe ageof the basementreaches15 to 30 million vears,dependingon the area. Thisviewis confirmedby heat-flowvaluesrecordedin narrowelongated basins corresponding to UpperTertiaryand/orRecentexpansion (in the Aden Gulf, the R e dS e a a , n dt h eC a l i f o r n i G a u l f ; 3 . 9 , 3 . 4a n d3 . 4 p c acl m 2 s I r e s p e c t i v e layn) d alsoin continentalgrabensof similarorigin (Baikal Lake and Rhine-Graben), whereflowsashighas2.5and4pcalcm-r s-1,respectively, havebeenmeasured.
596
Geochemical Modeling:A QuantitativeApproachto the Evaluationof prospects Gosgeneroted l\4esozoic
Gosgeneroied duringPoleozoic Tropsoreeroded Oilgeneroted - \mostly during Mesozoic
Oilgeneroted mostlyduring Poleozoic 1q0im
I
1 Devonion oulcropsondsubcrops underneoth theMesozoic unconformitv Middte I UpperDevonion E Lowe,Devonion E 2 Oilondgosfieldsproducingtron Corbonifcrous ond Devonion -
Oil
I
Gos
Pslroleumpoientiol
nfin lJl I mnnlll H i q hI
o',0o,"n,,0,
Nil F:;:-l
f-'-r
Gospotenliol High
Fig. V.4.6. Hydrocarbon potentialof the LowerDevonianbedsin the lllizi Basin (Algeria). asdetermined by useof themathematical model(Tjssot andEspitalid. 1975) mostlyover Cretaceous and Tertiary.In this areaall trapsare equallvprospective. whateveragethey are, from Paleozoic to Cretaceous.
4.7 Reconstruction of the AncientGeothermalGradient 1.7.1 HeatFlow and Geothermql Gradient The temperature increase with increasing depthresultsfrom a heatflow from the insideto the outsideofthe earth.The temperatureIis a functionofdepth z and .
the geothermalgradientis G =
dT
. At a givenplaceof the earth'scrustthe heat .1: flow @ and the geothermalgradientG satisfythe relation: G- €.@,
.1.7Reconstruction of theAncientGeothermal Gradient
5g,l
where1/6is the thermalconductivity.G is expressed asthe temperature increase p e ru n i to f d e p t h e . .g . . d e g r e eCs e l s i uksm ' . l t m a yr a n g ef r o m 1 0t o 8 0 . Ck m r , w i t h a n a v c r a g ve a l u eo f a b o u t3 0 ' C k m ' . @i s e x p r e s s ei nd p c a lc m r s | ( 1 0 - 6 caloriesper squarecentimeterper second).It may rangefrom lessthan 1 to 81rcalcm 's '. with an averagevalueapproximatelyl-2 pcalcm-2s t. Thermalconductivityvarieswith the compositionof rocks.For instance,the valuesare quite high in salt (Fig. III.2.3). The variationscausea variabilityof geothermalgradient,whereasvaluesof heatflow are Iessscattered. TableV.4.2 showsthe averagevaluesof heatflow in continental andoceanicbasinsaccordins 1 og e oer c l o ni c c o n d iilo n s . Heat llow is low in Precambrianshields (0.9 4cal cm-2 s r) and in areaswhereorogenyand/ormagmatismare of Paleozoicageor older (1.2 to 1.34cal cm t s '1. It is somewhathigherin areaswhereorogenyand/ormagmatismisofMesotoicorTertiaryagell.g4calcm-2sr):alsoinTerriaryvolcanic areas(2.2 4cal cm ' s '): and moreoverin thermalareaswherethe flow mav exceed3pcalcm 2s-r. Thushighvaluesof heatflow andgeothermal gradientarl reportedfrom svnorogenic or postorogenic basinsof the Alpine belt (pannonian basinextendingover Hungary.Yugoslaviaand Romania)and of the circumPacificbelt: OkhotskandJapanseas.Sakhalin,North Sumatra,Fiji, etc. Oceanicareasalsoshow a largerangeof heat-flowvariation,from oceanic ridgesto oceanicbasinsand troughs.Furthermore,the heat flow is quite high alongthe axial zoneof the ridges,i.e., alongthe crestand rift valleys.wheie valuesashigh as 8 4cal cm ' s-r are measured.It decreases progressively away from the crest.Within 150km on both sidesof the crestof the Eastpacificritiee. h a l f o f t h er e c o r d evda l u e sa r es t i l li n t h er a n g eo f 3 t o 8 p c a l c m 2 s , ; b e y o n d t h a t distance, the heatflow is alwayslowerthan31rcalcm 2s-1.A similarooservarron hasbeenmadealongthe mid-Atlanticridgein regardto a 50-kmzoneon both sidesof the crest. The presentoceanicexpansionbeing approximately3 to 6 c m \ r ' i n r h c E a s rP u c i f i ca n d 1 t o 2 c m y r I i n t h e A t l a n t i c .t h e b e l tw i t h a high heatflow corresponds approximately to an oceanicbasement youngerthan 5 millionyears. More precisely,Le Pichonand Langseth(1969)havecomputedthe average valuesof heat flow as a functionof distanceto the axisof oceanicridses(Table V.4.3). To measurethe distanceD they used, insleadof kilometers,a unit corresponding to magneticanomalyNo. 5, whichis definitelydatedasslightlyless than 10million vears.Thus,distances are in factcomputedin termsof ageof the oceaniccrust. and oceanswith differentratesof expansioncan be compared. FromTableV.4.3,it is clearthat highheatflow (2 or 3 timesthe averagevaluein oceanicbasins)are relatcdto a belt with an agelowerthan5 mi ion years.Then the hcat flo\\' decreases with increasingage and is no longcrdistinctfrom the averageoceanicbasinwherethe ageof the basementreachesl5 to 30 million vears,dependingon the area. Thisviewis confirmedbv hea!flowvaluesrecordedin narrowelongated basins corresponding to UpperTertiaryand/orRecentexpansion (in the Aden Gulf, the RedSea,andthe CalifbrniaGulf; 3.9,3.4and3.4pcalcm-2s rrespectively) and alsoin continentalgrabensof similarorigin (Baikal Lake and Rhine-Graben). whereflowsashighas2.-5and4 4calcm : s r. respectively. havebeenmeasured.
59ti
Geochemical Modeling:A QuantitativeApproachto the Evaluationof Prospects
Table V..1.2. Averagc valucs of Seothermal flux. (After Lee and Uyed.L,
1e6s )
Geotectonic Provinccs
Averagegeothermal Number of flux mcasurements ( 1 0 6 c a lc m 2 s r )
Precambrianshiclds
o.92
26 l6 2l
AreasstablesincePrecarnbrian' l 32 Areaswith Paleozoic orogeniesl 23 Areas with Mesozoic or Cenozoic orogcnies Areas of Cenozoic volcanism Oceanic basins Oceanic ridges Oceanic troughs
|.92 2.t6 I.28 l.u2 0.99
19 ll 213 338
zl
a Excluding Australia (Cenozoic volcanism).
Table V.4.3. Geothermal flux on oceanicridges as a function of normalizeddistanccto thc axcs of thc ridsc (Lc Pichonand Langseth,1969) A. East Pacific ridge
Flux Normalized distance
(D)
Average value (!cal cm '\ ')
Standard devlallrrn
< D<0.5.1 0 0 . 5 .<1D < L l t l tt
.3.3I 2.00 1.17
1.9.1 I.:9 1.03
B. Atlantic Indian ridgc Flux Normalized distance (D)
< D< 0.:16 0 0.:16< D < 1..1 1 . . 1< D < 3 . 1
Average value (FCarcm - \ ')
2..'t2 1.45 l.l0
Standard uevra!()n
2.33 0.94 L l.l
4.7Reconstruction of theAncientGeothermal Gradient
599
4.7.2 GeneraLRulesfor Reconstitutionof Ancient GeothermalGradient From the obseryeddistributionof geothermaldata,somegeneralrulesmay be deducedfor the reconstitution of paleogeothermy, accordingto the geotectonic 5erting o f t h e s e d i m e n t a rbya s i n s . 4.7.2.1 StablePlatforms Presentand ancientgeothermalgradientscanbe aboutthe samein sedimentarv basinswhereno tectonicor magmaticactivityhasbeenknownsincedepositionoi the potentialsourcerocks.This normallyoccursin areaswhich, after possible foldingand magmaticintrusions,are incorporated into a stableplatform.Then, subsequent sedimentation of potentialsourceandreservoirrocksresultsin a new sedimentarybasin.now generallyflat-lyingor gentlyfolded. In this case,the prescntgeothermalgradientis generallyapplicableto sourcerock evolution. Thissituationoccursin undisturbed Paleozoic basinsrestingon foldedor even flat-lyingpre-Cambrian basemenr. suchasseveralPaleozoic basinsof Ausfralia, and in the northern and easternSaharain Algeria; in Mesozoicundisturbed basinsrestingon Paleozoicor older folded basement,like most of the West Canadianbasin,the ParisBasin.etc. In suchcasesthe valuesof geothermal gradientare often 25 to 35'C km r. ,1.7.2.2OrogenicBasins The similarityof pastandpresentgeothermal gradients is not acceptable in basins wherethe lastorogenyand/ormagmaticintrusionis contemporaneous with, or youngerthan, solrce rock deposition.This situationoccursin synorogenic or postorogenicintiainontanebasinssuch as the Pannonianbasin in Central Europe,associated with the Carpathes alpinefolding,or in permo-Carboniferous basinsassociated with the Hercynianorogenyin WesternEuropeand Eastern Australia. In thesebasins,geothermalgradientand flux may be high and quitevariable with respectto locationduringthe periodsof orogenicacrivityor immediately followingit.Forinstance,valuesashighas50"Ckmrarenowobservedinseveral partsof the PannonianTertiarybasin,wherePliocenesourcerockshavebeen broughtto sufficientlyhigh temperaturefor generatingoil, despitethe short periodof time.
4.7.2.3StableContinentalMargins The basinsof depositionlocatedalongthe stablecontinentalmarginsresulting from oceanicopeningandexpansion areworth a specialconsideration. From the datareportedin Section4.7.1andTableV.4.3,it canbe assumed thatdurinsthe first 5 million yearsaftercreationof a newoceanicbasement, the first sedimints
600 Geochemical Modeling:A Ouantitative Approachto the Evaluationof Prospects depositedon the new basementhavebeensubjectedto a heatflow two or three timeshigherthantheaverage. Thentheflow progressively decreases, to reachthe average valueafter15or 20 millionyears.Undersuchcircumstances, moderately buriedsourcerocksmay reachthe principalzoneof oil formationwithin 5 or 10 million yearsafter deposition. This situationoccursmainly at the beginningof ocean spreading,when continentalmassesare still closeto the hot axial zone. and orovideabundant materialfor a thick sedimentation overthiszonefollowedby quickburial.In later stagesof oceanicevolution.whenthe oceanicspreadinghasalreadymovedthe continentsfar awayfrom the axialzone.this hot axialzonegenerallyreceives a reducedand slow sedimentation. Examplesof thick sedimentation associated with a highheatflow mayoccurin Cretaceous coastalbasinsof WestAfrica and SouthAmericaasa resultofthe openingof the SouthAtlanticOcean;in Jurassic and Cretaceous basinsresultingfrom the openingof the North Atlantic Ocean; and basinsof Westernand NorthwesternAustralia:anclin Mio-pliocenesedim c n t so f t h e R e d S e a .A d c n G u l f. e t c . Theseconsiderations. deducedfrom the presentdistributionof heatflows.are in agreement with observations madeon the kerogenof ancientsediments of the Aquitainebasin,whoseopeningis linkedto the expansion of the AtlanticOcean. Robert (1971)obsenesthe reflectanceof the organicmatter as a functionof depth in bore holes. and finds an abrupt increasecorrelatedwith a lower Cretaceous stage.Sucha shiftis explainedby the influenceofa considerable heat flow over the periodof time associated with the openingof the Aquitainebasin.
4.7.2.4Evolutionof Heat Flow The heatflow throughundisturbed sedimentary basinsis usuallyeitherconstant or decreasing as a functionof geologicaltime. This is alsotrue with respectto orogenicand magmaticactivity:heat flowsare high whenthis activityis young anddecrease whenit becomesolder.After a sufficienttime.it remainsconstant, aspresentvaluesin Precambrian The or Paleozoic foldedbasinsarc comparable. samegeneralscheme- high values,then decrease to reacha ratherconstant state- is, for otherreasons. validfor stablemarginsrelatedto oceanicopening, asdiscussed in Section4.7.2.3. gradientsmaybe. However.the reversemavhappen,andpresentgeothermal in certaincases,higherthan the ancientones.In parlicular,continentalgrabens are associated with rathercomplexmechanisms, includingpossiblerifting,rising gradients of thermalwaters,etc.In the Rhine-Graben. heatflowsandgeothermal aregenerallyhighandvariablewith the location.Theymayreachpeakvalueslike 80'C km t in the Landauborehole.However,the stageof kerogendegradation, with sucha high value(Doebl as well as vitrinite reflectance, is not consistent gradientwasmodet al., 1974).Mathematical simulationshowsthat geothermal erate until late Tertiary, and then increasedsubstantiallyup 1o the present value,probablyasa resultof faultingand risingof thermalwaters.
,1.7Reconstruction of theAncientGeothermal Gradient
601
1.7.3 Evaluationof the presentGeorhermalGradient b^t^:f stableplatforms,the_presenr valueof geothermal gradientis a l:,T.1ll satlstactory evaluationof thepaleo_gradient (Sect.,1.7.2. i). Determinationof the presen t.gradientrequiresa sufficientnumberof temperutur" rn"oru.arn"n,a,n boreholes.Electric.acousric or nuclearloggingd.ui""i or" u.uuity"qulppedwith a maximumreadingthermometer.Howc*vei.tne time Ueiweendrilling and krggingis usuallyrestrainedto a minimum,so that ttrermaiequih Orrumwith the formation is nor estabtished, and the foimation a";;;r,;;;, are generally u n d e r e : t i m a l erde. . u l t i n gi n l o o l o $ g r a d i e n\ta l u e \ . tJetterresultslre ohtaincd if extendedtestingis made on reservoirbeds containingoil or gas.lbllowedby iongpressure Uuiia-rp.if,.n tt " equitibriumis reached.resultingin accurateformat ion_temperarure measuremetrs anocompu_ t a t i o no l g r a d i e n t .
1.7.1 Evuluationof the Ancient CieothermalGradienr M c K e n z i e( l 9 7 l J .l 9 8 l ) s t u d i e d . t h et i m e _ tm e p e r a t u r eh r yi n s e d i m e n t a r y 'iti s t o'rtr.t.t,ing basinsformed bv exrension.followed by ,r6.i0"".". J' rnnO"t calculates the subsidence and the temperature gradient.within a subsiding basin with compaction.The principal,.rult, -uv il* rppiy i"'."rne other basins producedb)' certainthermaldisturban..,oflithorphar". Reconstitutiorof ancientgeothermaldatamay ;lso . be attemptedby measur_ rngphysicalor chemicalDroDerties of kerogenandsimulating its ;volutionby use ofa mathematical modefuniilsatisfa.,".yia;"rin.'.", i, iruot *iir, ,n" n,'"u.u..0 Thir techniclu.e requiresthe knowledgeoi rt " trurJu.rrus trmecurveJ Illl". basedon geologicalreconstitution_. It is best achievedby integratlngthis .rlff!::l^Tl: geologica t.modetingpresenred in the previouschapter. llewhrch r ne parameters mav be usedincludevitrinitereflictance.eleitronspin resonance of kerogen(ESR),hyorocarbon compositions, etc.someauthorshave tried the directuseof suchmeaiurements as,,maximumreadingthermometers,,. Tie variation-s of rhesepropertiesall resultfrom degraJotl- oi?.rog.n, *f,i.f,i, governedby kineticlaws.and thusis a functionoi both time anJ re-pe.uture. Thus,it is not possibleto relateany of thesepropertier.f [;;;;; ro maximum j:Tl:.11,-r-*.""ty. as rhev resulrfrom the whole-thermal histoi. Furrhermore, me {rrrerent pJrarnerers may he linked to differentaspectsof ihe evolution of Kerogenstructute.and thus trme and temperature miy influencerne vanous indicesin differentwavs. o t n . l g t h ep h ; s i c api r o p e r r i eost k e r o g e nw. h i c h c a nh e u r e dl o r r e c o n s t r r u -ron.or tnermarhrstory.one of them,vitrinitereflectance, is discussed here. The qualitativeaspects of the increase of vitriniterefleciancewiihOurialdepth rn Chapter V.t. A model of vitrinite degradation has been il: ll*":r".1 oeveroped on the basisof a sequenceof Logbabashalesin the Douala basin t L a m e J o o n t .l h e r e , t h e o r g a n i cm a t t e rc o n s i s t sm a i n l yo f v i t r i n i t e and of amorphous organiccementwith the samephysicalproperties(elemenrar compo_ sition.IR specrra,reflectance erc.).Tt usli tris be.n p,isriul"-ti.uiil.ut" u _oo.r
Geochemical Modeling:A QuantitativeApproachto the Evaluationof Prospects
602
_averoqe
r e f l e c t o n c(e% i n o i l ) _ _
(>1)
/ltinite
>.---
ton sfol notton rcl 1b
rc/leclonce
,--->
Fig. V.,1.7.Changesof the transformation ratioof vitrinite and of its reflectance in the Logbabawells,Douala Basin(Tissotand Espitali€, l97s)
s
_Tronsformorion
rorioof v t.inite=_--)
-<
d
E E
E €
2
:
o -
Fig. V.1.8. Relationshipbctween the transformirlionratio of vitrio.5 1 nitc and its reflectance(Tissotand Tronsformorion rotio of viiriniie____________)E s p i t a l i 61. 9 7 5 )
J.7Reconstruction of theAncientGeothermal Gradient
603
(adjustedalongthe methodsoutlinedin Sect.,1.2)to simulate the degradation of the organicmatter of vitrinite type. Furthermore,it is possibleto establisha relationship betweenreflectance andtransformation ratioLf vitrinire.Dymeasur_ ing bothparameters on the samesamples. FigureV.,1.7showsboth paramerers as a,functionof depthin the sedimentary sequence of the Doualabasin.andFigure V.,18 show-s the relationshipbetweenrefiectance ancldegradation of vitrinite. Thus..itbecomespossibleto make a quantitativeinteipretarion,rn rermsor . thermalhistory,ol a well locatedin any basin.providedthe rockscontarn some vitrinite.The interpretationis basedon measurements madeat differentdepths in a bore hole. and reconstiturion of burial for eachsample,madeon purely eeologicalinformation.Each reflectancemeasurement is convertedinto the corresponding transfornationratio, accordingto FigureV.4.g. Independently, the modelof vitriniteevolutionis usedto calcr]late airansformationratio, based on the burial history. as a function of geothermalgradient G. Then, an a d l u s t m e ni ts m a d c o n v a r i a b l e( i , u n t i l m i n i m i z i n gt i e q u a d r a t i cd e v i a t i o n betweenthe computedvaluesof the transformation raiio andthe valuesdeduced from rr[r5gry211nns throughFigureV.4.g. This procedurecan be usedeitherto ]uJustii-slmp-lc lawexpressing C asa functionof time,or to calculatean average valueof (i whichis the bestevaluationof the pastgeothermalgradient over the periodof maximumburial. Thismethodhasbecnusedwith success in severaltypesof basins,wrthancient gradrents equal.higheror lowerthanthepresentone(iable V.4.4).In the Rhine Grabcn.the calculatedvaluesare in agreementwith the observaiions madeby Doebletal. (1974)thatthe presentgeothermal gradientcouldnot hau"fu.,,f for Table V.4.21.Presentand ancient geothermal gradients Geothermal gradients
Basin
('c tt--'1 Well
Period of time presently
computed
Paris Basin (France)
Essises
33
-r1.0
From middle Jurassic
AquitaineBasin (France)
LA 10.1 PTSI RSE I Nat I
31 33 31 26
33.0 I5..1 32.(l 25..1
From middle Cretaceous
Illizi Basin (Algeria)
lrarraren Oudoume
l3
32 . 9 26.1
Paleozoic Paleozoic to Cretaceous
Rhine-Graben (W. Germany)
Landau2 11 Sandhausen 1 .11
-58 30
Tertiary Tertiary
Douala Basin (Cameroon)
J-i)ghaba
t7
CretaceousPaleocene
2-s
32
60.1 Geochemical Modeling:A QuantitativeApproachto the Evaluationof Prospects
DOUALA BASIN
Fig. V..1.9. Comparisonof the calcud l a t e da m o u not f o i l a n dg a sg e n e r a t ei n theDoualaBasin.by usingtwo different valuesof the geothermalgradientG. with the presentgradientG Calculation = 32'C km rdiffer markedlyfrom the observedvalues. whereascalculation w i t h l h e c o m n u l e dp a s tg r r d r e n t6 = : 1 7 ' Ck m ' s h o w s a g o o d a g r e e m e n t with actualvaluesmeasuredon cores
E
200
t
300
(mq/qorq HYoRoCARBoNS C) GENERATED
a verv long time. The high valuespresentlyobservedare probablya resultof risingof thermalwatersthrougha fault system.This interpretationwould also explainthe variabilityof the observedgeothermalgradientover relativelyshort horizontaldistances. In the Douala Basin. the ancient geothermalgradientwas rather high. probablydueto thc vicinityof the midoceanic ridgein theearlystageof thebasin. During the first -5millionyearsheatflow andgeothermal gradientwereprobably well over average.progressively decreasing to reacha standardvalueafterca.20 million vears. Figure V.,1.9showsthe amount of oil and gas generatedat Logbaba.computedby the model of hydrocarbongenerationusingthe burial vcrsusdepth curveand two differentvaluesof the geothermalgradientG; the prescntG = 32'C km I andthecomputedpastvalueG - 47'C km- I. It isobvious that the observedquantitiesof oil generated(measured on corematerial)are in agreementwith the simulationusing the calculatedvalue of averagepast geothermalgradient(G - ,17'Ckm r) and differ markedlvfrom rhe quantities basedon the prescntgradient(G = 32'C km ').
4.8 MigrationModeling The othermajor aspectof geochemical modelingis migrationfrom sourcerocks to reservoirs andtraps,andpossiblyto the surface.Althoughthe mechanisms of migrationare lesscompletelyunderstoodthan thoseof generationof hydrocarbons.a first attemptto simulateshalecompaction andwaterexpulsionby usinga
I
4.ll Mieration Modeling
605
numericalmodelwasmadebv Smith( 1971).More recently,Ungereret al. ( l9g3) and Durandet al. (1983)presentedan integratedmodeiof oil generatronand migration,wherethe generationpart is the liineticmodeldescribedabove.The mlgrationpart is basedon the conclusion, expressed in part III, that theprincipal modeof fluid migrationin sedimentary basinsis a polyphasic modewithieparite hydrocarbonand waterphases. The model presentedby Ungereret al. (19g3)and Durand et al. (19g3)is diphasic:, fluid.movements arc.governed by the Darcy'slaw appliedto polyphasic tlo\.\'s with the helpof the relativepermeabilityconcept.The maindrivingfbrceis compactiondue ro the sedimentary load,whichis supportedpartly by ihe solid mineral phaseand partly by the pore fluids, watei and hydrocarbons.thus lncreaslng lnternalpressure. Anothercauseof pressure risein sourcerocksis the conversion of solidkerogeninto fluid hydrocarbons with a relatedincrease of the averagespecificvolume of the organicphases.Capillarypressures,and the ciangeof oil viscositywith temperatureare alsou".ount.d for. As presented by theseauthors,the model is two-dimensional, i. e., it works alonga geologicil section.This sectionis represented by a grid of finite elements.eachelement havinga specificlithologyand organiccontent.The model calculates tbr each elementand eachintervalof time: porosity,hydrocarbonsaturation.pore fluid pressure!and velocityof fluid flows. DYrlnd et al. (1983)appliedthis schemeto part of the Viking Grabenin the ., "Indonesia. North Sea.and to the Mahakamdelta in eastirn Kalimantan, The resultsfrom the Viking Grabenare presented in FiguresV.4.10andV.4.11.The sectionrepresented is borderedby two majorfaults,on both easternandwestern sides,and comprisesa tilted block of Jurassicbeds underneaththe major unconformity.The sourcerocks belong to KimmeridgeClay. Heather and D u n l i nf o r m a t i o n sa.n da l s ot o B r e n tc o a l s a, l l o f J u r a s s ]acg c .S a n dr e s e r v o i r s belongto Brent and Statfjordformations.The two major fiults borderingthc scctionare considered asimpermeable barriersto fluid flow. The otl saturations a r e p r e s e n t e di n F i g u r e V . , 1 . 1 0a t 6 5 m . y . b . p . ( M a e s t r i c h t i a nw ) ,h e n a n accumulationis beginningto form in the Upper Brent sands.and also at the presenttime, when accumulations are locatcd in Jurassicsandsrones on the structuralhigh.In sourcerocks.highsaturations areobserved in the syncline.It is notedthat the onsetof oil expulsionfrom sourcerocksoccursbeyond20o/roil saturation.Thc model also calculatesthe excesspr"raur" u"rau, hvdrostatic pressure, shownin FigureV.,l.l l for thesameintervilsof time.lmportintexces, pressure is predictedbeyond3000m depth.andthishasbeeneffectivelv observed in wells.At a givendepth.thisexcess pressure is moreimporlanton the structural high. but the maximum valuesare computedwhere burial of the Heather formationis the deepest. The samemodelusedin a very differentsituation.i. e., the Mahakamdeltain Indonesia.hasbeenableto calculatethe locationof the oil pools,the hydrocar_ bon reserves in placeandthe distributionof abnormalpressures, all valuesbeing in agreementwith observeddata.The fact that the modelis ableto accountfor verydifferentgeological histories,suchasthe North Seaandthe Mahakamdelta. suggests that the hypotheses and approximations usedin the moderare reason_ ably satisfactory.
606
Gcochemical Modeling:A QuantitativeApproachto the Evaluationof Prospects
OILMIGRATION
= F F
z
la ats% " . t , F a , f ) =
--
ra tcatqtlunIt L 'tt! I.P 0f tlt 0/t lftltt'y
ar i4t,lttt
t a alnl
" 2- -
I r:ra >7. z
l -.4 5lrr1lf a: - tt )'r sttt
Fig. V.'1.10. Hvdrocarbons generationand migrationmodelingin part of the Viking Grabcn.North Sea.The sectionis borderedbv two majorfaultson the castern1/,"!glt) and westernr//di)sidcs.and comprises a tilted blockof Jurassic bedsunderneaththe major unconformitv.The modcl calculates oil saturationin all betisfor eachintervalof time: which is the beginningofoil expulsion: f/op./situationat 65 m.y.b.p. (Maestrichtian) (Durand fborlom)situationat the prescnttime. showingaccumulation in Jurassic sancis et al..19E3)
Poll'phasic modelsof oil migrationcertainlvrepresent a significant advance.as m o s tt u n d a n e n t aslt u d i c sl l a v cn o wc o n c l u d etdh a tt h i si s t h ep r i n c i p am l o d eo f hvdrocarbon migration.Thereare,howevcr.specificdifficultiesinherentto this tvpe of model. Darcy's law is widelv used in reservoirengineering,but the conditionsin sourcerocksare very differentwith respectto hetcrogeneitv. and alsoextensionof the relativepermeabilitvconcept.Thus.the resultspresented hereshouldbe considered asa first approach.Thev haveshownthat. in certain geologicalsituations,the model is able to generatedata in agreementwith observedfacts:locationof petroleumaccumulations, distributionof pressure includingoverpressured zones.A presentrestrictionis the two-dimensional character whichis acceptable in grabens, suchastheVikingGraben,orelongated basins.Thisshouldsoonbe overcomeby the increasing availabilityof highspeed computers,whichwill makethe useof three-dimensional modelspossible.
4.qConclusion
607
FLUID PRESSURE DISTRIBUTION
rf*
r*
:l
l'*
f.* ; ooc S
=
000 I'| t: :a 5 c,Fc 'c ?a 4tb a:) \fr>iakto
- aattl L':11 ttttl l - t4tr 5,4 ttr1/t t: a st[r4,r, a:= sttr slta
Fic. V.4.11. Excesspressurein part of the Viking Graben.The sectionis the sameas .hown in Figurc V.,1.10.The model calculates the excesspressureversushydrostatic pressurc for the sameintervalsof time (65 m. y. b. p. and0 m. y.) (Durandet al., 19g3)
-1.9Conclusion It is nowpossibleto simulatethermaldegradation ofthe maintypesofkerogenby usinga mathematical model.The modelis calibratedagainsteachtypeofoiganii matterby usinga certaindistributionof activationenergies. A sinslemodelisable to simulatethe whole thermalhistory from low tem-pe ratures'in sedimentary basinsto high temperatures in the oil shaleindustry. Althoughmigrationmechanisms are lesscompletelvunderstood thankinetics of kerogendegradation, it is alreadypossibleto iimulite migrationandcalculate hvdrocarbon saturationandfluid pressure in eachindividualbed,asa functionof time and locationin the basin. It is realizedthat modelingof geological processes is still in an initialphaseand * ill rapidlvdevelop.especially with respectto truly three-climensional simulation procedures. However.somcpresentandfutureapplications of modelingcanbe iistedasfollou's.
Approachto the Evaluationof Prospects Geochemical Modeling:A Quantitative basins; areasfor oil and gasin sedimentary of prospective a ) determination with the ageof andmigration,for comparison D l timingof oil andgasgeneration seals; formationof trapsand impermeable basin; of a sedimentary c ) evaluationof the ultimatereserves historyof waterflow in the basin; d ) distributionof abnormalpressures; reconstitution of ancient geothermals gradients and thermal history of sediments;this applicationis not limited to petroleumexplorationbut concernsgeologyin general; f) simulationof retortingconditionsof oil shales. modelsusedin petroleumexploration The numberof geological/geochemical is certainlygrowingfast.Chenetet al. (1983)andUngereret al. (1983)discussed and compaction, the influenceof the varioussubsidiarymodels(subsidence of thermodynamics thermalhistory,hydrocarbongeneration,fluid movements, modelof basinevolution. phaseseparationand exchange)in a comprehensive the quantitiesof oil andgasprovided Bishopet al. (1983)estimatedsuccessively by the sourcerock within the drainagearea,the quantitiesof gaslostby diffusion andthe trapvolume.Then,by comparingthesevalues,theymade anddissolution volumeandnature(onlyoil. oil a probabilistic estimationof trappedhydrocarbon and gas, or only gas). A different type of model, mainly on a statistical. probabilisticbasiswas presentedby Nederlof(1980)and Sluijk and Nederlof thata giventrapcontainsmorethan (1983).It is designed to calculatethechances a certainvolume of oil and/or gas.It drawsfrom a world-widecollectionof basisand calibration geologicaland geochemical datawhichform the statistical setfor a prospectappraisal.
Summaryand Conclusion Geochemical models of petroleum generation offer a quantitative approach to explorationproblems.They providethe amountof oil and gasgenerated(askg perton of rock) irom the different sourcerocksin the vaious partsof the basin.Furthermore, these results are expressedas a function of tirne, and the timing of petroleum generationcan be comparedwith the ageof traps. The data required asinput to the model are the type of kerogen,the burial history (depth ve$us time curve) and the geothermalgradient. On stable platforms, the ancient gradient can frequently be approximatedby the presentgradient. In other basins,the ancientgeothermalgradientcan be calculatedby usinga model basedon vitrinite reflectance. Migration modelsprovide information on hydrocarbonand water movement.They define the areasand sedimentarybedsof hl'drocarbonaccumulation,and the occurrenceof abnormalpressures.They alsooffer a clueto the problemof ultimatereseryes basin. of a sedim€ntarv
I
Chapter5 Habitatof Petroleum
Four selcctedcasehistoriesof geologically differentpetroleumregionswill be p r e s e n t chde r e :t h c A r a b i a nC a r b o n a t P c l a t f b r my. o u n gd c l t aa r e a s i.. c . . t h e N i g e ra n dt h e M a h a k a mD el r a .t h e L i n ri B a s i no t f h i n r r l n dr h c D e e pB a s i no f WesternCanada.Thesecasehistoriesshowtheapplication of thenewknowledge aboutthe principlesof petroleumgeneration, migrationandaccumulation. Thev demonstratethe understanding of sourcerock maturationand hldrocarbon generationaspart of the geological evolutionol a sedimentary basin.Following the considerations about the origin of hydrocarbonsin each of the basini dcscribed,thc most likely pathwaysand modesof migrationinto the rcservoir rocksand trapsare discussed. Finallythe qualityof the hydrocarbons found in traps is briefly outlined, with the implicationswith respectto the overall g e o l o g i c as e l tring. The four casehistoriesselectcdencompass the mostimportantprototypesof petroleumhabitats: the Arabian CarbonatePlarform. one of the most important petroleum provincesin the world. with Mesozoicmarinecarbonatesourcerocks and reservoirs,with a simpletectonichistoryand consequently simplestructural teatureswith an enormoushorizontalscale: - youngdeltaareas.i. e., the Nigerandthe MahakamDelta.Tertiarypetroleum provincescomposedof detrital rock sequences and heavilyinfluencedby terrestriallyderivedsediments with a ratherfastsubsidence history; the Linyi Basinof China,an assymetric grabensystemof Tertiaryagcwrrna sedimentary fillingof detritalrocks,representing limnic,fluviatileto lacustrine environments: the Decp Basinof WesternCanada,a gas-proneareaconsistingof a thick Mesozoicsectionof interlayeredsandsand shaleswith numerouscoal seams representing a marineto brackishnear-shore environment. The gasoccurrence in theDeepBasinis of specialimportancehecause rhesituationii the reverse of a convcntionalgas field. i.e.. the gas-bearing zonescorrespondto the less porousandlesspermeablerockssituatedin a downdipposition.whereasmore porousand permeablerocksare updip andabove.
610
Ilabitat 0f Petroleum
5.1 Habitatof Petroleumin the ArabianCarbonatePlatform The ArabianCarbonatePlattbrmwith the ArabianBasinin a centralpositionis oneofthe mostimportantpetroleumprovinccsin theworld. lt canbe considered largelyto be a foreland{ypebasinlike the WesternCanadaBasinand Eastern Venezuela(Demaison.1977).Thesethreebasinshavein commonthat thev are geometryshowinglittle tectonic largebasinsrvitha relativelysimpleasymmetric complexitvand onc long, gentlydippingflank. They containby far the largest petroleumaccumulations on Ourplanet.Thc trulv giganticheavyoil andtar sand accumulations at the easternrim of thc WcsternCanadabasinare found at the updip easternedgeof the homoclinewhere the sedimcntswedgeout on thc shield.ln EasternVcnczuelathe heav\ oil bclt North of the Orinoctr Canaciian and-Iertiarv Rivcr occursin a comparablegcologicsettingwhereCretaccaous wcdgeout towardthc southof the Brasilianshiclcl. sediments goingfrom Iran in the Eastwith the foldedbeltandovcrthrustA cross-section Gulf ing of the ZagrosMountainstowardthe Westthroughthe Persian-Arabian areaandinto SaudiArabiatowardthe Arabianshieldin the Westagainshowsan ovcrallsimilarity.However,onc js temptedto saythatthc geologicsettingon the in so far as there\\'asno ArabianC'artronate Platformwas|]toreadvantagcous jor waters itftcr theoil hadaccumulated. of mcteoric ma crosionandior ingression Platformwere Arabian Carbonate of the Thereti)re.thc Detroleum accumulations reportson tar although there are not largelyconvertedinto heavyoils and tars. matsin existingfields.This areanow holdssomeof thc largestoil fieldsof the world: Ghawa (Saudi Arabia). estimatedultimate recoveryof 1.1 < I(lu mr (83 x 10'barrels)lBurgan(Kuwait),estimated ultimaterecoveryof 11.5 x l0''m' (72 x 10"barrcls);Safaniya-Khafji (SaudiArabia),estimatedultimaterecoverv of 5 x l0emr (30 x 10ebarrels).Thesetieldsandseveralothersin thisgeneralarea belongto the categoryof so-called"giantoil fields".Any fieldwith an estimated to be such ultimaterecoveryof 0.5 x 10ebarrels(80 x 106mr) is oftenconsidered a g i a n to i l f i e l d ( s e eC h a p .V . 6 . 5 ) .l n t h i s c o n n e c t i o nh.o w c v e r i. t s h o u l db e rememberedthat the amountof heavyoil in placein thc Athabascadepositsin WesternCanadaalone is far higherthan in evcn the Ghawarfield in Saudi Arabia. here.the Persian-Arabian Gulf rcgionandSaudi The areaunderconsideration Arabiais schematically shownin FigureV.5.1. For manyyearslittle information wasavailableaboutsourcerocks.hydrocarbongenerationand migrationin this area.Only recentlyinformationwasmadeavailablefor thepublic.The following discussion on the habitatof oil in the ArabianCarbonatePlatformis largelybased o n t w o p a p e r sb y M u r r i s( 1 9 8 0 a) n dA y r c se t a l . ( 1 9 8 2 ) . The physicalboundaryof the ArabianCarbonatePlatformto the southwcstis crvstallinerocksin the soutlt[esternhalf of the thc outcropof Precambrianr Arabian Peninsula.The Iimits to the northeastare lesswell-defined.but are usualllconsidcrcdto bc the foldedbelt of the ZagrosMountains.This bor.rndarr in the would llso roughl) separatethe Cretrccousand Jurassicshclfsecliments the northeast. The Alpine flysch to and southwcstfrom open marinesediments rvere uplifted rocks that of basin is formed by crystalline southeastern cnd the wasnot a basinin thc parallelto transformfaults.The arcaunderconsideration
5.1 Habitatof Petroleumin rhe ArabianCarbonatePlatform
6ll
Fig. V.5.i. The Arabian CarbonatePlatfornt and rts principaltectonicfeaturcs.(After Ayreset al., l9ri2)
geometricscnseduringtheJurassic andCretaceous periodswhilethethickestand most importantsedinentswere being deposited.Insteadit was a broad, flat carbonate shelfdeveloping out into the Tethysfrom theAfro-Arabianconrrnent. Two principaltvpesof depositional systemsalternatedin time (Mirrris. l9tt0): 1 ramp-tvpc mixed carbonate-clastic units; during regressive periodsclastics wcre broughtinto the basin; 2. differentiatedcarbonateshelves:transgressive periodswere dominatedbv carbonatecvcles.Starvedeuxinic basinswere separatedfrom carbonatec v : r p o r i tpcl u t l o r m sh v h i g h - e n e r gmva r g i n s . The structuraltcctonicdevelopmentof the Arabian Platformcan be subdividedinto threestages(Murris. 1980).The first stageendedwith the Turonian and was characterized by stableplatformconditions.During the secondstage from Turonianto Maestrichtian therewasorogcnicactivitvandtheformationofa foredeepalongthe marginof the Tethysocean.Duringthe third stagefrom late Clretaceous to Miocenethe platformrcgainedits stabilitv. Broad and elongatedanticlinal structure developcdwith a north south directionasdrapefoldsoverrejuvenated basement upliftsthatresultedoriginally from an casl-westdirccted earlv (Prccamhrian?)compressional stressfield ( A y r e se t a l . . 1 9 8 2 )-.f h e s es t r u c t u r easn d a l s on e a r l vc i r c u l a sr t r u c t u r ei sn t h e easternpart of the area.stemmingfrom the movementof lnfracambrian Hofmuz salt.containthe largeoil ficld of the ArabianPlatform.
612
Habitat of Petroleum
Thereis now ampleevidence(Murris, 1980;Ayres et al., 1982;Welte,1982) thatthemainsourcerocksin the ArabianCarbonatePlatformareMiddleJurassic to Upper Jurassicorganic-richcarbonates.Regionally,where sufficient Upper Cretaceous and Tertiary overburdenhas been accumulated, Lower to Middle Cretaceousorganicrich carbonatesand marls alsoreachedsufficientmaturity to generateappreciableamountsof petroleum.The acceptedsourcerocks of Jurassicand Cretaceous agecontainmainlytype-Il kerogen.The sourcerocks periodswhereon the differentiatedcarbonateshelves belongto the transgressive euxinicbasinscould havebeendeveloped.Namesfor the stratigraphic equivalentsof sourcerock units vary regionally.Well-knownrepresentatives are the Upper Jurassic Hanifasourcerock andthe Aptian Shuaibasourcerock (Murris, 1980). A ratherdetaileddescriptionof a Jurassicsourcerock section(Callovianto Oxfordian)in NorthernSaudiArabiahasbeenpresented by Ayreset al. (1982). The unit wasinitiallyidentifiedfrom geochemical analyses of well cuttingsand core sampfes.The total organiccarboncontentaveragesaround37t,to 5Voby weight. Visual analysesshowedthe kerogento be dominantlyamorphous. Elementalanalysis of kerogen(Ayreset al., 1982)andpyrolysisindicesshowthat the organicmatterbelongsto typeII. An interestingtechniquewasappliedto map the regionaloccurrence of these sourcerocks throughoutNorthern SaudiArabia (Ayres et al., 1982).Source rocks, being very rich in kerogen and extractablebitumen. have distinct geophysical wire line characteristics, showinghigh electricalresistivities com-
*Eg*-\
Ca) otn n 3OOlt - 91m
Fig. V.5.2. Distribution and isopachs of a Jurassic sourcerock section(Callovian to Oxfordian) in Northern Saudi Arabia with more than 17, total organic carbon. (After Ayreset al., 1982)
5.1 Habitatof petroieumin the ArabianCarbonateplatform
613
yi,l low sonicvelociries(Chap.IIL2.4 5). Neulronlogsrespondsimilarly 91":d ton:: respectto rhe.organic tuci.s. Cumrna1rui"."rponr. lo l9g: .with *u, generally highbut variable.Thisvariationin gamma-ray response ofsourcerocks ls more pronouncedin carbonate_source rocks than in shalysourcerockswhich are moresteadyas,for instance.documented in the caseof ihe Kimmeridgianof the North Sea..Logresponses in theseuniformsourcelitfroiogls;f SauAlaraUia werecalibratedto geochemicalana-lyticdata andusedto "riiriui" unOrnuprou... rockrichness. FigureV.5.2showsthedistriUution andisop..ir ofiir. .ou."..o.t with a total organiccarboncontentwhich is lo/o or greaiei bf " ' wergntin the A r a b i a nB a s i na n dr h ea d j a c e nG r o r n i aB a s i ni n t t r en J i r f r . ' - ' I ne rrmtngot maturationandhydrocarbon generationis relatedto burialand temperatureexposureof the sourcerocks. In carbonataayp" ,ou.a" rocks the vitrinile reflectancetechniqueto determine sourcerock mituration frequently cannotbe usedbecausevitrinite due to the lack of terrestrialJetrrtalinput is scarceor absentin most carbor sequences' The sameproblemhas been encountered with Arabian"uroo,tut"
marurarion orthe k".;s.; il;;#'."jffi'.Ttr::::T:iilitii:"f ffi:j H/C ratios and from calculatedmaturitiesUur"j on f_oputin;.'ririJ_,".p".u,u."
index method (Lopatin, 1971;Waples, 1930). i;;;.';;'Hii ratios for a determinationof maturitiesis only possible;hen the iyp.-oi tJ.og.n o ,.rr knownanduniformoverrheareau;d;..onria..ution.'i ,i,"uo Ji,i.'Hlc .urio,ot kerogen within Callovian and Oxfordian source rock fr";;;;;*" rn Figure V.5.3.The H/C rariosbetween1.3and0.8of thi, k;;;g*;;; ieln internrete,t
o 40 ao O
oir riaro
Fig. V.5.3. Map of H/C ratiosof a Jurassic soulce rock section(Callovianto Oxfordian) in Northern Saudi Arabia. (After Avreset al.. 1982)
6l,l
Habitarof Petroleum
to correspondmore or lessto the oil windowequivalentto vitrinitereflectance v a l u e so f 0 . 6 % t o a b o u t1 . 3 7 o . In generalthere seemsto be agreement(Murris. 1980;Avres et al.. 1982; Welte.1982)thatthe Middleto UpperJurassic sourcerocksarematureovermost of the areaof the ArabianCarbonatePlatform.The maturationhistory.i. e., the evolutionof the onsetand main phaseof hydrocarbongenerationin different partsof thatareacanbe inferredfrom measurements andcomputations basedon geothermaldataand burialhistories.In termsof absoluteagevaluesdepending on localburial historyand temperatures. Middle to UpperJurassicsourcerocks reachedthe stageof maximumoil generation,l5to 70 million yearsago.while Lowerto MiddleCretaceous sourcerocksareevenat presentimmaturein certain reglons. Petroleumgenerated from thesesourcerocksis foundtodayin reservoirrocks whichwereformedby regressive sandsandby the high-energv ooidalgrainstones terminatingthe carbonatecyclesor in rudisl"reefs"andalgalboundstones along the shelfmargins(Murris, 1980).Leachingandcementation processes playedan importantrole in determiningthe final reservoirqualityof the carbonaterocks. The principalregionalsealingformationsarethe UpperJurassicHith Anh_vdrite andthe Cretaceous Nahr Umr Shale.They controllargelythe verticalmigration of petroleum. Regionallyvertical fracturesand faults must have provided migrationavenuesfrom sourcerocks throughseveralhundredsof metersof relativelytightcarbonate rocksbeforepetroleumwasemplaced in a reservoirand trappedby anetTective seal.The GhawarFieldcanbecitedasancxamplefor such extensivevcrtical migration (Steinekeet al.. 1958). However, it must be emphasizedat this point that in spite of this obviousverticalmigration.the outstandingfeature of migration on the Arabian CarbonatePlatfornris a substantial lateralmigration(ARAMCO-staff,1959:Murris, 1980). Accordingto Ayreset al. (19t12). the oils in Mesozoicreservoirs within Saudi Arabia are similarin composition. The well-knowndifferencebetweenArabian Iightandheavv(Safaniya) orlsmaybe dueto maturationdifferences. Thisoverall uniformitvof oilslrom numerousfieldsandproductionzonessuggests a common or similarsource.This is supportedby correlationanalysesthat link oils and sourcerocks. The qualityof mostoils is suchthat they fall into the aromatic-intermediate class(see also Fig. IV.2.l). Some oils have to be classifiedas paraffinicnaphthenic(Abu Dhabi,Qatar).A fewoilsapparentlv havesufferedbiodegradation andlostan importantpart of thesaturatedfraction.The sulfurcontcntof the oils is generallyhigh (1-4olc)and may reachvaluesup to 8% by weight.Stable carbonisotoperatiosvarv by lessthan 2.\c/oandthe contentin vanadiumand nickelporphl'rinsis low. Murris (1980)pointsout what makesthe Middle Eastsuchan extraordinary rich petroleumhabitat.It is not exceptionally rich or thick sourcerocks;it is not extraordinary reservoirs or seals.It is, however.the horizontalscaleof the basin that providesthe three essentialingredientsof a petroleumprovince.source rocks,reservoirrocksandsealswith an uncommonlywide extent.The Mesozoic platformwas2000to 3000km wide and at leasttwiceas long.The depositional degreeof differentiation,facieschanges,and variationsin the sedimentation-
5.2Habitatof Petroleum in youngDeltaAreas
615
related morphologyof the platform were minor. Therefore,sourcerocks. reservoirsand sealsall have very wide lateral extent and rather uniform consistency. Likewisethe structuresare alsovery wide and havegentleslopes. This hasa twofold effectin that no fracturingin generaldamagesthe seal,and large trap volumesare attachedto even relitively smallvertiial closure.The horizontalextentof a closureof 1000km2or evenmoreis no exception. Finally,because of thesewide horizontalfeaturesmigrationeffiiiencyis very high: large areasof maturesourcerockscould be drainedover long geologicalperiodsandnot muchoil waslostalongthe pathsof migrationandto the sur_ face. The major differenceto WesternCanadaand EasternVenezuelais the fact thatthe flanksof the ArabianBasinwerenot cut openby erosion.Leakageto the surfacewas thereforemuch lessimportant and likewisebiodegradationand c o n v e r s i otno l a r i s r e l a l i v e l ryi l r e .
5.2 Habitatof Petroleumin youns Delta Areas Largeyoungdeltaareasrepresenta specialsettingfor the origin,migrationand accumulation of petroleum.The essential featuresand proceir., foi the occur_ renceof petroleumareamongthe characterisrics of a delia. providedthatthereis an organicrich sourceformation.The geologicsettingis auchthat the marine shalesof the delta fronl and the prodeltaaria, receivingmainlv land_derived organicmatter.canact assourcerocks.The inrerlayered mixedsandsandshales of the paralicsequenceof the delta plan provide reservoiropportunitiesand sealingcap rocksin the direct neighborhoodof the delta front. Furthermore. youngdeltasduringtheir build-upandsedimentaccumulation undergocomparativelvstrongsubsidence withoutuplifting.Thisis in turn essential for sourcerock maturation,generationof hvdrocarbons, andmigration.Thusthe maingeologic featuresandprocesses requiredfor petroleumocJurrences aresuitablvcombined in spaceandtime in deltaareas:i. e., theyoffer a compactscenariooi perroleum generation,migrationand accumulation For the specialimportanceof voung deltasthree reasonscan be cited: the generalfall of the sealevelthroughoutthe world oceansfrom about +350 m in UpperCretaceous time to +60 m in MiddleMiocene;the development andrapid spreading of an angiosperm flora that conqueredthe continents itartingwith ihe Upper Cretaceous;finally the Alpine folding, mainly occurringduring late Cretaceous andTertiary.The fall of the sealevelcreatedlargedrainagepatterns and funneledthe water run-off from the continentsto th-eoceans.The new angiosperm flora providedgreatamountsof terrestrialorganicmatterof higher plant origin to be transportedbv hugeriver systems.Th; Alpine foldingfinal_ ly gave rise to a strongrelief in certainareas.providingabundantclasticrock oeDns. ln deltaareasthe commongeological rulethatyoungersediments areon ropor , older sedimentsis modifiedsomewhat,encompassing alsoa lateralcomponent with respectto the ageof sediments. Generallyspeaking,due to the prograding
616
Habitatof Petroleum
delta, older sedimentsare on the landwardside, and going in the opposite direction toward the sea,sedimentsbecomeprogressivelyyounger.The progradation of the delta in time causesa shift of faciespatternsasthe delta movesout the faciesofcontinental, into the sea.This resultsin a well-knowndiachronism: fluviatile sediments,the mixed sedimentsof the deltaic plain and those of the units.Due to their nature, marineprodeltashalescut acrosstime stratigraphic deltasare situatedat the edgeof a land massand very often at the junction between the continentaland oceaniccrust (Audley-Charleset al., 1977). Therefore, the depositionaland structural evolution of the progradingdelta is more stronglyinfluencedby basementtectonicsthan is normallythe casewith other basins. The TertiaryNigerDeltaon the westernedgeof the Africancontinentandthe TertiaryMahakamDeltaon the easterncoastof Kalimantan(Borneo),Indonesia are next described.Both deltasare equatorialand provideabundantorganic materialfrom the tropicalforestsand swampsof the backlandand alsoautochthonousorganicmaterialfrom the shallowwatersof the delta front and thus ensurethe depositionof potentialsourcerocksmainlyin front of the delta.
5.2.1 The TertiaryNiger Deha deltaareas asoneof the best-known The TertiaryNigerDeltacanbe considered habitatof the NigerDeltahavebeendescribed The geologyandthe hydrocarbon in a seriesof papers(Weberand Daukoru,1976;Evamyet al., 1978;Ekweozor and Okoye. 1980;Ejedawe, 1981)in considerabledetail. The Niger Delta representsa regressiveclasticsequencewhich reachesa maximum thicknessof 9,000to 12,000m of sediment.It coversan areaof about75,fi10km' andmeasures acrossthe Tertiary delta well over 300km. The principal geologicalfeaturesare shownin a SSWto NNE crosssection(Fig. V.5.4). The formationof an initial basinat the site of the presentdelta is very probablyrelatedto Cretaceousrifting when South America startedto be separatedfrom Aftica. Basementtectonicsat the junction of oceanicand continentalcrust determinedthe original sitesof the of the delta. andthe development mainriver andthuscontrolledthe depocenter This developmentof the delta has been dependenton the balancebetweenthe This balanceand the resulting and the rate of subsidence. rateof sedimentation sedimentarypatternsseemto be influencedby basementtectonicsof individual in theNigerDelta faultblocks(Evamyet al., 1978).The thickwedgeof sediments can be subdividedinto three units: mainly marine shalesat the bottom (Akata Formation), an overlying paralic sequenceconsistingof interbeddedsandsand shales(Agbada Formation), and a topmost continentalunit composedof fluviatile gravelsand sands(Benin Formation). Thesethree formationsare also shown in Figure V.5.4. They are stronglydiachronousand cut acrosstime stratigraphic units. As mentioned before, this latter effect is causedby the advancementof the delta toward the sea,mainly during Middle and Upper Tertiary, and the accompanyingfaciesshift. The marine shales of the Akata Formation are generally over-pressured (under-compacted) to be the mainsourceofthe hydrocarbons andareconsidered
I
5.2 Habitatof Petroleurn in YoungDeltaAreas Coast Line
bl /
AGBADA BENIN Kokori
rc tc.ooo p.ooo to.oo0
a
Contin.ntal {= BENTN)
EI
P.ralic ( =AGBADA)
E
l,larine (Shalcs ) (=AKATA) Marine sh.le diapirs with sandy-silty sedirhents in interspac€d depressions
a
n B
Oceahic crust Continental crust Shale tlowage
Fig-V.5..1.Cross-section throughtheNigerDeltaandits principal geological features. ( A f t e rE ! a l n ve t a l . .1 9 7 8 ) foundthroughoutthe delta.Thc paralicsequence of the AgbadaFormationwirh thealternatingsandandshalelayerscontainsnearlyall petroleumaccumulations. It plays.however,a minorroleasa sourceformation.The greatmajorityof these petroleumaccumulations are found in trapsassociated with so-called,,growth faults".Thesegrowthfaults(seealsoFig. III.4.9) are synsedimenrary g-ravitationalfaultsthat remainactiveover extendedperiodsof time. Therefbremore and fastersedimentation is possiblein the downthrownblock relativeto the upthrownblock (WeberandDaukoru,l926).Growthfaultsarealsowell known from the Gulf Coastareain the US. Ejedawe(1981)hasstudiedthe distriburionofoil reserves in the NigerDelta and its relation to the geologicalevolutionof the progradingTertiary delta, especially with respectto the tectonicandsedimentologiCcontrol. He fo;nd that a prolificbelt (Fig. V.5.5), represenring an areaof prelerentialconcentration of giantandmedium-sized oil fieldsis closeto thecontinental edse.Il wasconcluded that the prolificbelt eithercoincideswith the transitionzone.wherethe oceanic andcontinentalcrustsate thoughtto meet,or is flooredby oceaniccrust.In any casethe prolificbelt is the zoneof maximumsedimentary thickness andof a higit rateof subsidence. It containsa properblendof sandsandshalesencomDasslng sourcerocks, reservoirrocks and seals.The enormoussedimentarvthicknesi allowssourcerocksat greaterdepthto be in the oil- or gas-generati;g window. Landwardthere are fewer shalesto providepotentialiourie rocksand seals, seawardthereare too few reservoirsands.There is agreementthat the marine Akata shalesconstitutethe majorsourcerocksfor the hydrocarbons of the Niger Delta.Thedeeplyburiedlowerpartsofthe paralicsequence (AgbadaFormation) cannotbe discountedas sourcesfor oil and gas (Weber and Daukoru, 1975; Ekweozorand Okoye, 1980),howeverthey seemto be of lesserimportanceas comparedto the Akata Formation.A wide varietyof shalesamplesfrom the
618
Habitatof Petroleum
BExrirctTY -/ z'6
,"'a/
Fig.V.5.5.Mapshowins relations of prolificbelt. position of continentaledge.andmajor oil fieldsof NigerDelta.(AfterEjedawe1981) Akata andAgbadaFormationshowedorganiccarboncontentsof aboutl7c with vervlew valuesin excess of27oC,,.r.The qualityofkerogenis mainlytypelllwith varying admixturesof type IL Occasionallyliptinite enrichmentscan occur. Folbwing the terminologyusedby Shell(ShellStandardLegend,1976)it would correspond to humicand mixedtypesof organicmatter. The thresholdof intensehydrocarbon generationasdeducedfrom a few wells basedon analyses of extractable organicmatterwasfoundin the offshoreareaat approximately2900m and in the onshorearea at considerably greaterdepth around 3300 m (Ekweozor and Okoye, 1980).A correctionfor migrated hydrocarbons obviouslywas not made,so that the main zoneof hydrocarbon generationin reality could be somewhatdeeper offshore and onshore.In addition,it hasto be keptin mindthatthetemperature distributionin theTeftiary deltais variable.resultingin differentdepthlevelsof the top of the "oil kitchen" (Evamy et al., 1978)throughoutthe delta. A detailedtemperaturesurvey throughoutthe deltaencompassing all threesedimentary unitshasshownthatthe present-davtemperaturegradientvariesconsistently with the sandstone/shale ratio in a given well (Fig. V.5.6). The temperaturegradientincreaseswith diminishingsand percentagefrom less than 1.0'F/100ft (1.8,1"C/100 m) in continentalsands.to aboutI .5'F/100ft (2.73'Cl100 m) in the paralicsection.1oa maximumof about3.0"F/100ft (5.a7'ClI00m) in the continuousmarineshales. This temperaturepatternis relatedto freelymovinggroundwatersespecially in the shallowersandstones of the continentalsandsand the existenceof a high pressure zoneat the bottom.Freelymovinggroundwatersact ascoolingagents and high pressurezonesshowa ratherrapid temperatureincrease. (BeninFormation)andthe thickestparalic The thickestcontinental sandstones (AgbadaFormation)with alternatingsandsand shalesis found in the sequence
5.2 Habitatof Petroleumin YoungDeltaAreas
619
f i 3 0
Et*
i
qlY E:_ frr
ro !:E
.i . C. .': .'.'. . . r. r.. . .. J-...,.*fr.rl " "ir--t€t
lrl
i l o SAI{OP€RCEXTAGE Fig-V.5.6 Re'lation het\\eengcotemperature gradientandsandpercentage in the Niser D c l t a .( A f r e rE v a m !e t a l . . l 9 7 l l )
\n".(2{tg)},,,*.
ttt*t*,n
"o*o-" ,ouu." nop oil knch.nl
O l----r-
25 -
50 r----J
75km
Fig.V.5.7. Depthcontours of theI l5'C isoremperature plainin theNigerDelta.(After h v a m ve t a l . .1 9 7 8 )
middlepart ofthe delta(seeFigs.V.5.rlandV.5.5).Followingtheaooveconcepr of temperaturedistributions. it is not surprisingto find on isotcmperature plots t he deepestsubsurface contoursfor an isotemperature plain(e.g.. 115.C) in the middlepart of lhe delta(Fig.V.5.7).The temperature of I l5.C hasbeenrefered to asthc top of the oil kitchenin the Tertiarvdeltabv Evamyet al. (1978).It is possibleto usea certaintemperature valuehereratherthana maturityindicator for definingthe maturityof sourcerocksasmaturationandoil generarion areverv voungin the deltaand relatedto a rapid.equallvvounssubsidence in that area:
620
Habitatof petroleum
Therewasno additionaluplift anderosion.It is interesting to notethat Evamyet al. (1978)and Ekweozorand Okoye (1980)reportthe samedepthdifferenceof approximately 400m with respectto the top of the oil kitchenor onsetof intense hydrocarbongenerationbetweengiven offshoreand onshoreareas,although theydo not agreeon absolutedepthof the principalzoneof oil generation. In the onshoreareamaturationis achievedat greaterdepth. The mature sourcerocks have apparentlygeneratedlight and paraffinic oils. The initialparaffinwaxcontentmaydifferwith thefaciesof the sourcerock,i. e., more input of terrestrialhigher plants or, more precisely,a higher liptinite contentin kerogenwould causethe wax contentto rise.Among Nigeriancrude oils in principlethree variationsare found: paraffiniclight oils, intermediate paraffinicoils with a certainwax content(both belongto the paraffinicclassChap.IV.2) and biodegraded heavieroils.The degreeof biodegradation varies. The generalcoincidenceof the boundarybetweenbacteriallydegradedand unalteredparaffinicoils,within a rathernarrowtemperature range(65.to 80"C), accordingto Evamy et al. (1978),implies that little or no subsidence with simultaneousincreasein geotemperatureof the oil-bearingreservoirshas occurred after emplacementand degradationof the oils. This distinct oil distributionpatternmaybe explainedasfollows:differences amongthe original paraffinicoils are either due to sourcerock heterogeneityand/or possibly migrationalchanges andthereis no long-distance lateralmigrationof oil, whereas there may be extensiveverticalmigration.Thus the original oil distribulion patternin the deltahasbeenlargelypreservedexceptfor the alterationof light primary oils by bacterialdegradationin zonesthat were invadedby meteoric watersand had formationtemperatures not higherthan 65"to 80.C. Therethe primarylight paraffinicoils aretransformed into medium-gravity low pour-point crudes.Philippi(1977)hasreporreda similarfinding(seealsop. ,164). WeberandDaukoru(1976)andEvamyet al. (1978)haveemphasized therole otgrowthfaultsasmigrationavenues for hydrocarbons from theundercompacted Akata shalesinto Agbadareservoirs.The faults,at greaterdepth wherethey becomemore horizontalin the plasticmarine shales,are not consideredto provideeffectivemigrationpaths.However,that part of the faultswhichis more verticalmayvervwell providean effectivemigrationpath (seealsoFig. III.4.9). The growth faults thus help migrationin a doublesense,they can bring the overpressured mature sourcerock into juxtapositionwith the downthrown reservoirsequence (Agbada),dependingon the throw, and throughthe permeable verticalpart of the fault they can providemigrationavenues.Where the hydrocarbonkitchenlies well below the top of the continuousshales,the oil apparentlyhas only a remotechanceof migratingthroughthe shalesinto the overlyingreservoirs of the AgbadaFormation.
5.2.2 The TertiaryMahakam Delta The TertiaryMahakamDelta and its hydrocarbonoccurrences haveonly been investigatedover the last few years.The hydrocarbonhabitat, mainly with respect to source rock and migrational phenomena,has been describedby
5.2 Habitatof Petroleumin young DeltaAreas
621
al. (1970).combaz and De Matharel(1978),Durano and oudin Y:gli.:,., (rvdu1.vandenbroucke and Durand (1993)and by Schoellet al. (1983).These publicationsservedasthe main sourceof iniormati,cnfor the following consider_ ationson the MahakamDelta. The TertiarvMahakamDeltais locatedin Eastern Kalimantan,at the eastern edgeof the Kutei Basin. The deltaicsystemgoing landwarJ trorn r},. A"ttu i.ont from the offshoreareainto the hinterlind meisur-esappro*ir*i"iyiO t rn unOtt " "lengrh" is nearly100km followingthecoastline(Fig.V.5.S).Deliaic seAimenta_ tion is known to have occurredat least sinceif,J VlAOf" Uiocene,,.e., tor I5rnillionyears.TheeastwardprogradingsedimentarysequenceisaUoutOttOOto 6ruu m rnrck.aboul40{t0 m of w hichhavebeendrilled.The exactposition of the basement and the ageof the first deltaicdepositsare not known.ih. Muhukurn Delta is considerably smalrerin sizeandhaiaccumulatea r"i. s"alrn.nt.tr,antt. Niger Delta. Sincethe Middle Miocenethree major deltaic l-mptexes have accumulated. separated by two marinetransgressions. Underneaththeinterbed_ deo clavsand sandsof rhe deltaicplainrhereare slrongly marine_influenced or marineargillaceous sediments of the prodelta.TheselariJ. ,.0irn.nt., "sp..iuffy massive -siltyprodeltashales,arefrequenrlyoverp.a.ru."a.in" utt"rnatlngclays andsandsofthe deltaicplainin the.M-ahakim DeitaarecornpaiuUie to tt,.pa.uti. sequence (AgbadaFormation)in the NigerDertaandthe massive prodeltashales
=
Oil ticlds
Ca3 ti.tds = O 5 lokm
Fig. V.5.8. The Tertiary Mahakam Delta in Eastern Kalimantan (lndonesra) wrth major anticlinalaxis and its oil and gas fields. (After Vandenbrouckeet al.. 1983)
622
Habitatof Petroleun')
SANGA-SANGA HANDL
o
N
w
t
,,' t
2 6
a
r.-x" Seisdc Markef Horizons
E n E f:l ---
De[a Plain DeltaFront Plattorm ProgradatronTalus lso T*,4iro'Cftop ot Oil Generatbn Zone) Topof HaghPressurezone
Fig. V.5.9. Cross-sectionthrough the Mahakam Delta (lndonesia) and its principal geologicalfeatures. (After Durand et al.. 1983)
to the marineshales(Akata Formation)in Nigeriarespectively. As in the Niger Delta.thesourcerocksarefoundin thefrequentlyovcrpressured deeperprodelta shalesandthereservoirrocksamongthealternating of theplainof sandandshales the MahakamDelta. In the NigerDeltathe dominatingstructuralfeaturesaregrowthfaults.Thisis not so in thc MahakamDelta. There.the mostimportantstructuresare major anticlinalaxeswhich roughly parallelthe coastline (Fig. V.5.tl) and have a decreasing amplitudefrom wcst to east(Fig. V.-5.9).The oil and gasfieldsare alignedalongtheseanticlinalaxes(Fig. V.5.8).The sourcerocksfor oil andgas and alsocoals(Combazand are reportedto be mainlyshalyprodeltasediments and Durand. 1983). Matharel,197tJ: Durandand Oudin. 1980;Vandenbroucke The averageorganiccarboncontentis usuallyabove2% C,,,r.The argillaceous sediments in front of the deltacontainca. 2 3VcC.,r.Coalylayersmayreachup in to 80e/rin C,,.r.Practically all the kerogenis typeIII. andthereis no difference chemicalcompositionbctweenthe organicmatter of shalesand that of coaly layers(Vandenbroucke and Durand. 1983). . . 5 . 8a n dV . 5 . 9 )t h em a i nz o n eo f o i l f o r m a t i o n I n t h e H a n d i lO i l F i e l d( F i g s V wasdeterminedby DurandandOudin(1980)asoccurringat T.., valuesbetween to a vitrinitc 440' and 455"C as basedon Rock-Evalanalyses. This corresponds reflectancevalue between0.7c/oand 1.1% of type-lll kerogen.The zone of hydrocarbonformation in the Handil field was found to be far below the hydrocarbon-bearing reservoirs.between500and 1000m deeper.The zoneof hydrocarbon formationfollowedthecontoursof the structure.i. e., it wasdeeper at the flanksandhighestat the crestof the structure(Fig. V.5.9).The top ofthe oil windowat Handilis around2500m. Many excellentpotentialsourcerocksin the MahakamDelta are still immature. maturit)'for SimilarJyasin the NigerDelta, the sourcerocksreachsufTicient shalesor at leastin the so-called hydrocarbongenerationin the overpressured of thistransitionzone The thickness transitionzonewith intermediate oressures.
5.2flabitatoi Petroleum in youngDeltaAreas
623
is about300m in Handilfield andprobablyabout700min NilamandBadakfield. It isarguedconvincingly thatgaseous andverylighthydrocarbons canmrgrateout of the overpressured zone more easilythan heavierhydrocarbonsand non_ hydrocarbons. Thenthe tighthydrocarbons on their way;pward exrracrneavrer compounds andcarrythemalong.tn the '.hot',overpres,suied shalesthereseems to bea singlesupercritical hydrocarbonphasethat,whenmovingupwardto lower pressures and temperatures, progressively losesthe heavieren-dof it, hvdro.r._ bons by retrogradecondensation. In this mannerthe inversespecifii gravity relationshipof hydrocarbonoccurrences in Handil can be explained.l. e., ligtri productsat the top and heavierat the bottom. In the MahakamDeltafractureplanesseemto be of onlyminorimporrance ror secondarv migrationascomparedto the Niger Delta. Wherefracturesor faults areabsentin the MahakamDeltathe sealingefficiencyandthe distance between the top of the maturezone in the sourcesequence(top oil kitchen)and the reservoirzonecontrolthe amountof hydrocarbons whichcanbe expelled.Ifthis barrieris thick,fewhydrocarbons, exceptthemorevolatilecompounds. canpass through. Thereforeit wascctncluded thatin the MahakamDeltathenatureof theDooled hvdrocarbons, i.e.. oil or gas,and the amountthat accumulaledin differenr fields,wasdependenton the relativepositionof the hydrocarbonkitchenwith respectto the reservoir.A thicker shaleinterval betweenthe hydrocarbongeneratingzone and the reservoirsand would allow fewer hydrbcarbonsto accumulate thanonly a thin barrierbetweenthe maturesourceandthe reservoir. The relativepositionof differentreservoirbedsvs.the underlyingoverpressured sourcezone would, due to retrogradecondensation increasingly, allow lighter hydrocarbon mixturesto accumulate in reservoirs, the farthertf,evareabovethe vturce. Thus. unlike in the Niger Delta, thc differencesamong the pooled h1'drocarbons are not seenasconsequences of sourcerock qualitvbut asrelated to migration. Oil-source rock correlationstudiesas baied on steroid and triterpenoidhvdrocarbons and stableisotopes(Schoellet al., l9g3) haveshown thattheoilsaresourcedfrom humickerogens (typeIII ). The qualityofthe pooled hvdrocarbons varies.Due to retrogradecondensation there are gaspoolsand condensates at shallowerdepthand alsolight paraffinicoils. A slightbiodegra_ dationof primary light paraffinicoils may occur but doesnot seemto be fre(ruent. Summarizing the hydrocarbon habitatin the NigerandMahakamDeltamanv similarities betweenthe two Tertiarvdeltaareascanbe observed.The sedimen_ tologicalsuccession with sourcerocks,reservoirrocksandsealsis nearlyidentical in both cases.The sourcerocksare the moremarineshalesin front of the delta and at the bottom of the sedimentarysequence.The reservoirrocks are the altcrnatingsandsand shalesof the paralicsequence in the Niger Delta and the comparablealternatingsequence of the deltak plain in rhe Mihakam Delta.In both deltasthe maturesourcerocksare generallydeepand the top of the oil windowis in or closeto the overpressured zone.The thermalgradientsin both areasarenon-linearandvaryregionally.The rypeof kerogenis dominatedby the terrestrialinflux of organicmatter, i.e., sourcerockscontainmainlvtvD;_III kerogen.Migrationfrom sourcerocksto reservoirrocksis mainlvvertiialin both
624
Habitatof Petroleum
examples.The primaryoils in both deltaareasare light paraffinicoils.Generation, migrationandaccumulation in bothinstances is veryyoung of hydrocarbons and apparentlystill in progress.Although the two deltasdiffer in size,their approximatereservesper area are comparablewith eachoJ about 105mr oil equivalentper km'. The NigerDeltacontainsabout6 x 10'm'oil equivalentand thel\4ahakam D e l r aa b o u t0 . 5 . I 0 " m ' o i l e q u i v a l e n t . Finally.it shouldbe mentionedthat are al!o somedissimilarities betweenthe petroleumhabitatof the two deltaareas.Whereasin the MahakamDelta coals play a major role asa sourcefor hydrocarbons, thereis no indicationfor thisin Nigeria.The typicaltrapsin Nigeriaare growthfaultswhich are, however,not found in the Mahakam Delta. Consequentlyfaults are of importancefor migrationin Nigeriabut not in the MahakamDeltaarea.Conversely agas-phase migrationand retrogradecondensation are mechanisms which prevail in the MahakamDelta, but seemto be of no or minor importancein Nigena.
5.3 The Linyi Basinin the People'sRepublicof China The followinginformationon the Linyi Basinis basedon thefirstresultsprovided project(beingcontinuedat present)betweenthe Geological by the joint research ResearchInstitute of the Shengli Oil Field (ShandongProvince,People's at Republicof China)andthe Instituteof Petroleumand OrganicGeochemistry the Nuclear ResearchCenter in Jiilich (KFA-Ji.ilich).West-Germany'r.The geological dataof the ShengliOil.Fieldand informationis basedon unpublished geochemical resultsobtainedduringthe cooperation of bothsides andgeological at the KFA Jnlich. The casehistory of the Linyi Basin makesuse of data, of sourcerock maturation,obtainedthrough especiallyduring the discussion geological-geochemical basinmodelingby computersimulation. The Linyi Basinis locatedon theNorth Chinaplain,southof the Gulf of Bohai nearthe mouth of the Yellow River (Fig. V.5.10)and extendsover an areaof grabensystemcontrolledby normalfaults 3969km:. It represents a asymmetric that cameinto existenceat the beginningof Oligocene.The strikeof the basin, especially of the centralgrabenandtheaccompanying faultzonesis SW-NE.The but also basinis mainly filled with detrital rocks,shales,silt- and sandstones a The basinfilling in generalrepresents containsminor amountsof carbonates. limnic,fluviatileto lacustrine environment. The grabensystemof the Linyi Basin is an extensionalstructure.The fault-controlledsubsidence during the basin evolutionin Oligocenewas interruptedduringmot of Mioceneby a period of The total uplift or nondepositionand then followed again by subsidence. in the deepestpart of the grabenamountsto about thicknessof thesesediments 5000m or more. 13 This cooperation was started in 1979 with the authorization of the Ministry of PetroleumIndustry of the People'sRepublicof China and the Ministry of Research and Technologyof the FederalRepublicof Germany
5.3 The Linvi Basinin the Peoples Republicof China 116.E
118"E
'120"E
122"E
625 F i g . V . 5 . 1 0 . L o c a t i o nm a p o f the Linvi Basin (North China) and ma!nstructuralfeatures
{w investigaled
Fig. V.5.ll. Typicalcross-section throughthe Linvi Basin.This is a computer_plotted cross-section as taken from basinmodeling.Fault zonesborderingthe main graben featuresare not depicted
Volcanismis restrictedto theperiodof maintectonicactivitvduringOligocene . time. A typicalN-S cross-section throughrhe basinis shownin fig"urei.5.tt. Thiscross-section represents a computerplottedprofileand is basedon an input data file employinga three-dimensional grid systemcoveringthe basin.There_ fore,the fault zonesborderingthe individualpartsof the basin.especially on the flanksof thedeepcentralpart of thegraben,arenot shown.Thiscross-sectron rsa typicalresultof geologicalmodeling(Chap.V.3). Among the lower Oligoceneshaly sedimentsthere are units exhibiting excellentsourcerockpropertieswith organiccarbonvaluesin excess of 2 ci . The! representa fluvio-lacustrine depositional environmentwith regionaloccurrences of typicallake facies.At a maturity level of about 0.5ct vi;inire reflectance. sourcerock samples from thesesedimentary unitshavehydrogenindexvaluesof 2llllto 51rumghydrocarbons/g C,,.,asbasedon Rock-Evalanalyses. The kerogens are classified as tvpe ll or tvpe Il-IIl, whichis in goodcorrespondance with the
Habitatof petroleum
626
typeof kerogenfound in the Dongyingdepression farthereast(Zhou Guangjia, 19U0).A maceralanalysisunder the microscoperevealedvery high liptinite contents:frequentlymore than 70yaof total maceralsidentifiedwereliptinites. Thismicroscopic observation is in goodagreement with geochemical analvses. for instancewith high hydrogenindex values.Especiallysourcerock samplesof lower Oligoceneageare characterized by mixed algae(mainlyBotryococcus), and degradedspores.Their kerogenis mainlytype II and is consideredto be a goodsourceof oil. The factthattvpc II andtvpeIII arethepredominant tvpesof kerogen,andnot tvpe I as in the Uinta Basin(anothertype of intracontinental lacustrinebasin), seemsto bc relatedto the water circulationpatternand the influx of detrital organicmaterialfrom the surrounding land.The predominance of type-lkerogen is probablvtied to quietundisturbed lakessuchasthe EocencGreenRiverinihe westernUnited States. Thc terrigenous influenceon the organicmatterin the Linvi Basinsediments is obvious from gas chromatographic investigationof the extractablesaturated h1'drocarbon fractions.All chromatograms arc dominatedb1,long-chain n-alkanesderived from higher plant waxes.Partial microbial reworking is indicatedin somesamplesby a slightshift of the r-alkane maximumto lower carbon numbers.The algal contributionrecognizedby kerogenmrcroscopy cannotbe seenin mostof the saturatedhydrocarbon distributions. Basinevolutionandsubsidence largelydeterminethe temperature historyand thereforematurationof sourcerocks.The maturationof sourcerocksand the hydrocarbongenerationin the Linyi Basinhasbeenreconstructed with the help of geological-geochemical modelingbasedon a computersimulationfollowing the conceptof Welte and Yiikler (1980).For the reconstruction of the following sourcerock maturationhistorya changingheatflow duringbasinevolutionwith 1.85HFU (heatflow unit = l0 6 cal cm I s r) in the beginningand 1.40HFU toward the end (Quarternary)was assumed.The changesin heat flow during
norihern faull system
\
Fig. V.5.12. Computer-plorted maturitymapof an Oligocenesourcerock in the central part of the grabenof the Linyi Basin25 millionyearsbeforepresent.Matunt) 15gi!en rn isoreflectance linesof vitrinite(calculatcd bv comoutcr)
5.3The Linvi Basinin the Peoples Republicof China
621
basinevolutionfollow the conceptof McKenzie(197g).Threesuccessive stages of the maturitydevelopment of an Oligocenesourcerock in rhe Linyi Basinire shownin Figure V.5.12 through V.5.14. The isoreflectance lines of vitrinite shownin thesemapsare eachconstructed from well over 100controlpointsfrom thc datamatrixprovidedby the computermodel.The time sliceswith 25 and 16 millionyearsbeforepresentare chosenarbitrarily.Dependingon the timesteps usedin the model simulationany othertime slicescan be depictedasa maturitv map fbr a givensourcerock.The computer-derived maturitydataarecalibratei by a comparison betweenmeasured andcalculated vitrinitereflectance valuestor the presenttime in areasof the basinwherethereis well control.From Figure V . 5 . l 2 t h r o u g hF i g u r eV . 5 . 1 ,i1t c a nb e s e e nt h a tt h eo n s e o t f o i l g e n e r a t t owni t h
northefn tault system
\
l--.t
ir t
scxJthefn fault syslem
Maturity l6rny B.P
Fig.V-5.13. Computer-plotted maturitymap of an C)ligocene sourcerock in the central part of the grabenof the Linyi Basinl6 mittionyearsbeforepresent.Maturity given is in isoreflectance linesof vitrinites(calculated bv computer)
|€r+
Matu.ity Present
Fig. V5.I'1. Computer-plotted maturitvmapof an Oligocenesourcerock tn the central partof the grabenof the Linyi Basinat present.Maturityis givenin isorcflectance lindsof !itrinite (calculated by computer)
628
Habitatof petroleum
respectto the sourcerock shownwasonly about25 millionyearsagoin thevery centerof the graben.Thenthe matureareawith steadilyincreasing oil generation expandedslowlyover the next 10millionyearsonly reachingpeakoil generation in the centerof the basin over the most recentmillion years (Fig. V.5.14). Knowledgederivedfrom suchsourcerock maturitymapscanbe combinedwith informationabout the spatialdistributionof carrier and reservoirrocks and structuralfeaturesthroughoutthe basin.In this way main migrationavenues, changes in drainagepatternsthroughtime, andthe availabilityof potentialtraps can be determined.The alternatingsand-shale sequence in the Linyi Basinand the structuralsettingof the grabensystemoffer numerouspossibilities for oil accumulations. The fault systemat the edge of the graben providesgood migrationavenuesfor oil derivedfrom deeplyburiedsourcerocksin the center part of the graben. The compositionof the crudeoils found in Linyi Basinreservoirsis in good agreement with an originfrom limnicorganicmatter.Biodegradation of varying degreesis not uncommonamongthe crudeoilsof thisbasin.Accordingto liquid chromatography the non-biodegraded oils are of the paraffinictype with saturatedhydrocarbons in the strippedcrudeoils(C,.*-cut)from 37to 69% (average about 557a) whereasaromaticsand N, S, O-compoundsare lessabundant (II-31%). The asphaltene contentsare relativelyhigh and rangeftom 2 to 8Ea. Many oils showa slightbut distinctoff-evenpredominance of highern-alkanes above20 C-atoms.The steranedistributionpatternstronglyfavoringC2eandC27 over C,r compoundsis consistent with a significantcontributionfrom terrestrial organlcmatter. The Linyi Basin is a good examplefor the formalion and occurrenceof petroleum in a limnic, fluviatile to lacustrineenvironmentwith terrestrial 'l'he influences. computersimulationof the basinevolutionfollowingthe concept of Welte and Yiikler ( 1980)permits,contraryto conceptsbasedon geothermal gradients,a regionalreconstruction of the paleotemperature history for each sedimentaryunit throughoutthe basinand thus detailedtiming of sourcerock malulatlon.
5.4 Habitatof Gasin the DeeDBasinof WesternCanada The Deep Basinin WesternCanadais locatedeastof the teclonicallydisturbed belt of the Rocky Mountainsin the provincesof British Columbiaand Alberta (Fig. V.5.15). It represents the deepesrpart of the huge asymmetricWestern CanadaBasin.The DeepBasinis approximately 650km longandreaches a width of about 130 km. A schematicSSW-NNE cross-section showsthe orincioal geologicalfeaturesof the basin (Fig. V.5.16). Paleozoicrocks, mainly carbonates,restunconformably on the Precambrian basement whichdipsgentlyto the SW. They are overlainby a thick Mesozoicsequencelargelyconsisting of Cretaceous dark shaleswith interlayeredsandstones and conglomerates. Coal seamsarefrequent,particularlyin the deeperpart of the LowerCretaceous. The thicknessof the Mesozoicincreases from aDprox.300m in EasternAlberta to
5.,1Habitatof Gasin the DeepBasinof WesternCanada
629
Fig. V.5.15. Locationof WesternCanadaDeepBasin
F ""; ,'.d :r
^:. Llrrr,[tBl4l
t<^ \ ri\ KEG RIVER
-
- t!J
OEEP BASIN, ALBERTA SCHEMATIC CROSSSECTION E zo{Es oF GAs sATURATto ELMWORTH
Fi-s.V.5.16.Schcmatic cross-section through DeepBasinshowing onesof gassaturation more than 4000m near the Foothiltsand the overthrustbelt which forms the borderof the basinto the west.The stratain gcneraldip southwestward into the Deep Basin,whereboth porosityand permeabilityof the sandstones decrease significantlydue to greatercompaction,higherclay content.and more intense diagenesis. Gas trccursonly in the deepestpart of the basin.over an areaof approx.67,00Ukm', wherealmostthe entireMesozoicsectionat a depthlevel exceeding1000m belowthe surfaceis gas-saturated as tlerivedfrom resistivitv
I l a h i t a to f P e t r o l e u m
logs (Masters,1979)and geochemical analysesof severalwells (Wclte et al., 1982).Thesegas-bearing zonescorrespond to the lessporousandlesspermeable rockssituatedin a downdipposition.The samestratain an updippositioneastofa transitionzone. exhibitinghigherporositiesand permeabilities, are saturated with water.Thus.the situationis the reversefrom a conventional gasfield,where abovethe gasan impermeablesealwould be expected,insteadtherc are warersaturatedreservoir-type \t rata. Throughout the Mesozoic,in the Triassic,the Jurassicand primarilv in Cretaceousrocks some 12 pay zones have been encounteredwith aveiage porosities of l0 cZandpermeabilities of about0.5md (Masters,1979).The better partsof thesepayzonesareoftenconglomeratic andexhibitpermeabilities which rangefrom 50 md to severaldarcys.To producethe gas the wells generally have to be stimulatedby hydraulicfracturingtechniques.Recoverablegasin the DeepBasinmay verywell be around50 Tcf (50 x l0r: cubicfeetequivalent to 1.416x 1u''mr, STP)or evenmore(Masters,1983personalcommunication). In the followingsomeresultsofcombinedgeological andgeochemical research are presentedthat showthat the gasoccurrences in the Deep Basincan bestbe explainedas a dynamicsituationberweengenerationof gas from coalsand carbonaceous shaleson one hand,andlossesto the shallowerupperlayersof the rocksectionandgoingupdipon the otherhand.The Mesozoicrocksectionin the areaof the ElmworthGasField.dueto its richness in organiccarbonandtvpeof EL-irlwORIH6 -28 - 68-t3WbM oRGAXTCCAR8oi{ @ TE}fT (%)
Fig. V.5.17. Organic carbon content (notelosarithmicscale)plottedvs. depth for cuttingssamplesof an Elmworthwell
5..1Habitatof Gasin the DeepBasin of WesternCanada
6l I
tull.r: represents probably oneof thefinestsource sections llll.ll. for gas.A protrlemonitoringtheorganiccarbon content asderivedt ornauiilng,unuryr", ot a typicalEtmuorrh weltlsshounin Figuret.!.it. ;;;;;+obo, a"ptr, tr," .irb.?.:j"ntent averages arounJ1%. Th.n " "..y'.i"i 1."" tolowswith :liill. values upto 807. whichreflects. in essence, thepresence of lool,auIrr,,ntf,i,pu.t (2400_3200
:l,l;o:?j""
m).From 3200';;J3a5j';ii;ff"
c,,,* rever is
,tl. of rheorganicmatterwasderermined by Rock-Evalpyrolysis.The _,,I1. slowing the hydrogenindexplotecl.versur'",,yg." rna._.1r.ig. l11-CJam V.5.l8) corresponds to the basicdiagramrepresented ln flgure'i. t. t Z. ihe oataot this well plot fairly closeto the tvDe_llicurve indicaiE," fr_"j."g""-pr.r kerogen whichis.maintyderiveclfrom i.,igt*. t"rr",ririot pr;;;;.;;;;;..";;: ,nvestigarion revealedlow amountsof liptiniie macerals. U"ifrigi.t.""r."*li if ui,.ini," lup ro known to.'be a sood sas senerator.and inerrinire..t.hus.the 1]1:]":^!:l,ll mrcroscoprc dataagreewith theresultsfrom Rock_Evalpyrniyr,.. in.r"tnr". it i, concludedthat this kerogenis nol oroir.
good source orso,ir i; i,"i;;; *ii',1j;"r:i"J:[];1,r""",'""",, FigureV.5.19
burisa
shows vitrinitereieoan;;;;;rAii;j;"r thesame Elmworth well. The m-aturity of the organrcmatteris approx.0.7Vcmean vltrinite uppJ..ort part of the'welland showsonly a siight :l-.^"^,^-:: !?" l.l li.lh" rncrease down to 1500m whereit riaches0.g%. At tfri. i"i"t thereis a ELMWORTH6_28-68_ 13W6M ROCK- EVAL PYROLYSIS
150Io
Fig. V.5.18. Van Krevelen-tvpe dia. lrom Rock-Evalpr rolv_ SramLlerived s i . d a t ar h r d r o g e ni n d c xl , r a n d o x \ _ gen index Io) for selectedrock samplesof an Elmworthwell
632
Habitat of Petroleum
ELMWORTH6-28-68-13W6M 0_6
MEAiI VlTnNrE FEFLECTA|€E t%t1.2 1.4 16 o.a 10 1t 2,0
Fig. V.5.19. Mean vit nite reflectance plotted againstdepth for rock samplesof an Elmworth well
a COI{CENTRATIONlrng/g Colgl 40
;
F uJ
a TOT L SATr,tAreO 8YOROCARAONS(CB.) . rOtAL n-ALXANES (C6-C$)
ELMI/IOFTH 6-2a-6A-FW6M
1
2 3 4 . COIICENTRATION tnEig Co'el
Fig. V.5.20. Total saturatedhydrocarbon(Cr5*)andtotal r-alkane (Crs-Cj5) concentrationsplotted against depth for rock samplesof Elmworthwell
5..1Habitatof Gasin theDeepBasinof Western Canada
633
progressive changein the reflectance gradientand maturityincreases quicklyto 2.17aar total depth(T. D.). lt is knownthat rype-III kerogennormallystarti to produceliquid hydrocarbons at a maturitylevelaround0.7% R.. Oil generation reaches a maximumaround0.9% R. andthendecreases with fuithermaturatron until the bottom of the "oil window" (at approx.1.3 to 1.4oloRm).Significant amountsof gas are thought to be generatedat maturitiesexceeding0.9 to 1.17c R.. The bottomof the ,,drygaswindow,'is not yet reachedin thiJwell Ir T.D. Concentrationsof Crs*-saturated hydrocarbonsand C,aC35 n_alkanesas . determincdfrom cuttingsamples.both normalizedto organiccarbonare de_ p i c t e Li n l F i e u r eV . 5 . 2 1 1l t. e x h i b i r rsh e r \ p i c a ls h a p eo l a g e n e r a r i , rcnu r r c r s i l would be predictedbascdon theoreticalconsiderations. ihis indicatesthat. in general.the C15-hvdrocarbons remainedat the placewhere they have been generatcd.There was no major redistribution,i.e., migration. in vertical direction.In this connection.it is importantto remembertliat rhe (liquid)C,.. hydrocarbons whichhavebeengenerated duringmaturationofthe sourcerockat shallowerdepth are being crackedwith furiher advancinqmaturationand convertedto smallermolecules (e.g.. gas)at greaterdepth.Thirefore,hydrocar_ bonconcentrations of the C1s.fractionpassthrougha maximum.asshownin this figurc.Similarcurvesexistfor both the total extractablearomaticsfractionand individualaromaticcompounds.As eventhe low boilingaromaticswhich are much more solublcin water than the corresponding sarurates, also show this VITHI{TE REFLECTANCE (%} o5
1,0
1.5
2.O
GEOLOC{C r AGE
UPF€F CAEIACEOUS
_ 15)OO E
o 20@
tow€F CREIACEOUS
JUnASS|C TFtASS|C ET WOFTH 6-2A-6A-
'3W6M
Fig. V.5.21. Calculated and measuredvitrinitereflectanceplotted against depth for selectedrock samplesof an Elmworth well. The calculatedR.value is derivedfrom the methylphenanthreneindex (MPl) derived from the bitumenof theserock samples
63,1
Habitatof Petroleum
typicaldistributioncurve,it is suggested that no long-rangeverticalwaterflow hasoccurredsincethe time of intensehydrocarbon generation.Suchwaterflow would alsobe unlikelybecause of the very low porosityand permeabilitl'of the rocks. A further independentargumentagainsta large verticalmigrationof the heavierhvdrocarbons is providedby the Methylphenanthrene Index(MPI) (Fig. V . 5 . 2 1 )T . h i sc h e m i c apl a r a m e t e w r , h i c hi s p r e s e n t eidn C h a p t e V r . 1 . 3p e r m i t s the definitionof the maturity of an oil or rock extractin termsof calculatcd vitrinite reflectance(R"). In FigureV.5.21 the hvdrocarboninternalmaturitv (R..)is comparedwith the meanvitrinite retlectanceof the kerogen(7c R,,,). Thereis obviouslvan excellentagreement betweenthe two curves.Any invasion of a more matureoil or condensate from greaterdepthswould havecauseda positivedcviationof the R.-valuesfrom the vitrinitereflectance curve. With decreasing carbonnumberthe concentration profilesof hydrocarbons in Elmworthwellsbecomebroaderandbroadcrandeventually loseanysimilarityto a generationcurvc. Contrary to a previousfigure (Fig. V.-5.20),depictinga tvpicalgeneration curvein thedepthprofileof C1.*h1'drocarbons in theshale.the propaneconccntration profileis ratherbroadand remarkablyconstantbetween 1000m and2,1(X) m in depth.belowwhichit decreases (Fig.V.5.22).Thc stepwise propanedistributionpatternas presentedin this figureshouldshow a definite trcndto$ ardslowerandIowerconcentrations goingfrom2000m dcpthupwardif ing,g of rock HYDROCARBON CONCENTRATION 1d lo, ro3 ro4 rcs 106 rc7 TIME
liRoPANEl
ELMWOFTH
Fig. V.5.22. Propttneconccntration (absolutevaluesobtainedbv combinecl thenrrovaporitation,h\drogcn stripping)plotted against depthfor selected rock silmplesof an Elmworthwell. Also shownin thisfigureisthecrlculatedmethanc concentrations as deriveclfront a pr!'ssure corebarrelgasanahsis
5.,1Habitatof Gasin theDeepBasinof Western Canada
635
it.wouldbe influencedby generationrates.Maturitiesand temperatures above 2000m are too low to warrantgasgeneranon. The absoluteconcentration of total light alkanes(Cr_CJ reachesa maximum of 1.2 x 105ng g I, or 120g per metiic ton ot roci 1nig.V.S.Z3),wtrichls consideredto be a very high value. It becomes.u"n rnoi" spectacular when considering the factthat methaneis not includedhere.Hence,thesegeochemical analyses verifythe conclusions basedon resistivitylog interpretationiby Masters (1979)concerningthe gassaturation.As the bulk o1 ttre iight hvdrocarbons is thoughtto be generated at maturitylevelsexceeding 0.97cR,,,,coircsponorng to depthrangesbelow20(X) m. it hasto be assumed thal a considerable part ofthise hydrocarbons (e.g.. C,-Cj) hnsmigratedover an appreciable distanceinto the overlvingstrata,contrarvto the heavy C1.* hydroiarbonswhich apparently remarnmore or lessat the siteof their origin. T h ec o n c e n t r a t i odniss p l a y cidn F i g s V . . 5 . 2 2a n dV . - i . 2 jw e r co h t a i n e u d s i n ga combined thermovaporizationh1'drogenstripping method developed bv Schaeferet al. (1978a).Thesedata ari in go,xt ogi..rn"nt with gasinalyse.s includingmethane(sceFig. V.5.22)of a pressurized core_barrel simple taken from anotherElmworthwell between250dm ancl3000m depth.The measured amountof methaneand higherhydrocarbons from the pressurized core_barrei samplesmakesit possiblek) cxtrapolatethe absolutemethaneyield (which cannotbe.measured from cuttingsquantitatively) from the propaneconcentra_ tlon_ata givenmaturity.This calculated valueis 6.7 x 10r'ngg I or about 11mr methaneper metricton of coal.whichis only the residualamountof gaspresent
rooo
F 2000 u,l o
ELM\NOFTH 6-28-68-t3W6M
Fig.V.5.23. C. C. hvdro, carbonconcentration (absolute valucs obtained bl combincd thermovaponzlllon-hvdrogeD strip, prng)plottedrgainstdepth lor sclectedrock samples o f a n E l m w o r t hw e l l
636
Habitatof Petroleum
today. Yet. a coal is thoughtto haveproduced8 to 10 timesas muchmethane upon reachingthis maturationstage(approx. I.3Vo R^). This total volume generated cannotbe storedby the coalitselfandthereforemigratesinto adjacent strata. Thus.the absoluteconcentration of methaneandothergaseous hydrocarbons like ethane,propaneandbutaneper rockvolumeandavailableporespaceofthe differentsedimentary unitsbecomesan importantparameterto understand the problemof gasgenerationandmigration.Furthermore,knowingthat this gasis generatedfrom coalsand type-lll kerogen,the methaneconcentrations normalizedto contentof C.,, of theserocksinformus aboutthe relativeimportance of the variousgasgenerators, i. e., the sourcesof the gas.Followingthis line of thoughtethaneconcentrations weredeterminedin detailin coal-containing rock sections.FigureV.5.24may serveas an examplewhereorganiccarboncontent and ethaneconcentrations have been analyzedfor a coal-containing 100 m interval. The figureshowsa sectionof the Lower Cretaceous in the deeperpart of the well wherea NotikewinCoalSeamis overlainby HarmonShales gradingupward into shaly sandswhich are followed by the Cadotte Sandstone.The C,,,!normalizedethanecontenlremainsfairlyconstantoverthewholedeprhinterval. Therefore,a certainvolumeof coalcontainsmuch more ethanethan the same volumeof shalethan was to be expected.A similarcurve hasbeen found for propane.lt canbe assumed that methaneconcentrations follow the sametrend. i. e.. that the absolutemethaneconcentration is highestin the coal.The factthat thc C,,,*-normalized valuesfor ethaneare in the sameorder of magnitudefor differentrocksections with varyinglithologyandalsofor coalsis interpretedasan HYDROCARBON YIELDIN SAND/SHALE,/COAL SEOUENCE @
ORGANTCCAFAO CO||TENT frel FOBMATION
E F
Fig. V.5.2,1. Organiccarbon and ethaneyield (obtained by hydrogen stripping) plottedagainstdepth for a sand/shale/coal sequenceof an Elmworthwell
5.4Habitatof Gasin theDeepBasinof Western Canada
63j
indicationthat the gasgenerationprocessis still activeand that concentration gradientsfor gasfrom the coalstowardneighboringrocksare maintained.The concentration gradientswouldhavebeeneliminatedby diffusionif the coalshad ceasedto generatean appreciableamountof methane.Theseconcentration gradientsmustbe the drivingforcefor an activegasdiffusiongoingon today. Hence,theconclusion isthatin theElmworthGasFieldthereisevenat presenta zone of activegas generationmainly in the deeperpart of the rock column exceeding2000 m. ln this zone betweenabout 2000 m and 3500 m deoth temperatures rangefrom about80'C to 120'Cand maturitiesfrom 0.9% R, to 2 . 0 7 cR ^ . Basedon dctailedpressurestudiesof the Lower Cretaceous CadominSandstonerescrvoirin the ElmworthGasField.Gies(1982)arrivedindependently at the sameconclusion that the gasaccumulation is not in staticequilibriumbut is in a dynamicstatewith ongoingupdip gas migralion.The presiuregradientsfor regionalwater and the actualgasas determinedby Gies (1982)are shownin FigureV.5.25. It can be seenthat the actualdowndipgaspressures are greater than thosepredictedby a constructedhypotheticalstaticgaspressuregradient basedon a continuous gassaturation overa depthrangeof 800m. The conclusion derivedfrom thesepressuredatain the CadominSandstone reservoiris thatgas mustbe migratingupwardin responseto the pressuredrop in an updip direction. In summarvthe followingobservations havebeenmadein the ElmworthGas Field. In the tight. low porositvcentralpart of the DeepBasinnearlythe entire Mesozoicrock sectionis gas-saturated. The gassaturationdecreases rapidlyin thc shallowcrpart of the rock columnand updip towardthc eastwheremore PRESSTJRE GRADIENTS fi
4mo
ACT(AL GAS 0.06-0'65+ PSi FT
2oo
22s 30@ PAESSURE
25o bar
Fig. V.5.25. Pressuregradients for regionalwater. -static gas" and actualgasfor the Elmworth area.(After Gies.l9il2)
63U
Habitatof perroleum
porousandpermeablerocksarefound.Detailedgeochemical analyses showthat thereis no major redistribution, i. e., migration,of heavierhydrocarbons andno flow of water in the tight part of the rock section.However,there is evidencefor massiveredistribution,i. e., migrationof gaseoushydrocarbons, and it can be shownthat the maintransportation mechanism mustbe diffusioninsidethe tight rock section.First balancecalculations haveshownthat present-day gassaturation profilesin the centerpart of the ElmworthGasFieldcanbe simulatedwhen assumingdiffusioncoefficients for methanein the rangeof 1.0 x 10-oto 2.0 x l0-'cm's 'whcreby increasingly high diffusioncoefficients havebeenadopted with increasingdepth and temperature.Obviouslythe diffusionallossesand otherlossesby conventional buoyancy-driven gastransportmainlytowardmore porousupdip-situated strataare compensated by a continuinggasgenerauon from coalsandorganic-rich shalysourcerocksin the rocksection.Temperatures between80'C and 120'C seem to be high enoughto guaranteean ongoing coalificationprocessand hencegasgeneration.Balancecalculations showthe coalmeasures to be importantsources for the gas.Cumulativecoalthicknesses of Jurassic to LowerCretaceous coalsin the Elmworthregionmayrangeup to 70m and more (Masters.1983,pers.comm.). All in all, the Elmworth gas field is a dynamic situationwhere gas is continuously beinggenerated in thecenterpartof theDeepBasinandlosttoward thesurfaceandthe moreporousedge.In the innercoreof thegas-generating rock columndiffusionprocesses seemto be the predominating modesof transportatlon.
Summaryand Conclusion Four selectedcasehistoriesof geologicallydifferent petroleum regionsare discussed with respectto the principlesof petroleumgeneration,migrationand accumulationl the ArabianCarbonatePlarform.youngdeltaareas,i. e.. the Nigerandthe Mahakam Delta.the Linyi Basinof Chinaand the Deep Basinof WesternCanada. TheArabianCarbonate Platformcontains giantoilfields.The mainsource numerous rocksare marineMiddleJurassic to UpperJurassic orginic richcarbonates containing type-ll kerogens.Thesesourcerocks becamemature45 to 70 million yeals ago. Petroleumgeneratedfrom thesesourcerocksis found today mainly in regressivesands and ooidalgrainstones terminatingthe carbonarecyclesor in rudisrriefs and algal boundstones along the shelf margins.The principalregionalsealsare the Upper Jurassic tlith AnhvdriteandtheCretaceous NahrUrnrShale.The overa)luniformityof mostoilssuggests a similarsourceenvironment. The oilsbelongmainlvto thearomaticintermediate class.Someoilshaveto be classified asparaffinic-naohrhenic. Thereason platformisthe tor the extraordinarilv richpetroleumhabitatin the ArabianCarbonate enormoushorizontal scaleof the basin, providing sourcerocks, reservoirrocks and sealswith an uncommonly wide extent. Becauseof these wide horizontal features migrationefficiencywasvery high: largeareasof maturesourcerockscould be drained over long geologicalperiods.Regionallyve.tical fracturesand faults providedmigra-
Summarvand Conclusion
6-19
tion avenuesftom sourcerocks through severalhundredsof meteISof relativelytight carbonaterocks before petroleum was emplacedin a reservoiror reacheda carrier rock. Young delta areas.like the TertiaryNiger and MahakamDelta offer a compact scenarioof petroleum generation, migration and accumulation.Source rocks are providedby the marineshalesof the delta front and the prodeltaarea.receivingmainly land derivedorganicmatter. In both deltasthe maturesourcerocksare generallvdeep and the top of the oil window is in or closeto the overpressured zone.The geothermal gradientsare non-linearand vary regionally.Sourcerockscontainmainlv tvpe-III kerogen.In the MahakamDeltacoalsseemto be a maiorsourcerock. The reservotr rocksand sealsare found in both deltasadjacentto the sourcerocksin the alternating sandsand shalesof the deltaicplain. The primary oils found in thesereservoirsof both deltasare light paraffinicoils.Certainoils in the Niser Deltaare biodeeraded. ln the MahakamDeltabiodegradation is onlvof minorimpoirrance. A n rnver.e$ecificgravitv reiationship is observedamongoils of the MahakamDelta. lt canbe explainedby a migration of a supercriticalhydrocarbonphaseout of the overpressuredshalesthat. when moving upward to lower pressuresand temperatureslosesprogressivelythe heavier end of its hydrocarbonsby retrogradecondensation.In general, migration from sourcerocksto reservoirrocksin both deltaareasis mainlyvertical.Thelypical trapsin Nigeriaare growth faults.In the MahakamDelta oil and gasare found in anticlinalaxes*hich roughl;parallelthe coaslline. The Linyi Basin of China is an assymetricgraben systemof Tertiary age with a sedimentary fillingof mainlydetritalrocks,representing limnic.fluviatileto lacustrine environments.Among the lower Oligoceneshalysedimentsthere are units with excellentsourcerock properties.The kerogensof thesesourcerocks are classifiedas type II or type II-III. The terrigenousinfluenceon the organicmatterin the Linyi Basin sediments is obvious.The temperature historyandmaturationare largelvdetermined by basinevolutionand subsidence. Basedon a computersimulationof the basin evolution,a regionalreconstruction of thepaleoremperiture historyof eachindividual sourcerock and consequentlyof its maturation history was performed. Computerderivedsourcerock maturity mapscanbe combinedwith informationaboutthe spatial dist bution of carrier and reservoirrocksand structuralfeaturesthrouehoutthe basin. The alternating sandshale sequence in the Linyi Basinandthe structu;l settingof the grabensystemoffer numerouspossibilitiesfor oil accumulations.The fault systemat the edgeof the grabenprovidesgood migration avenuesfor oil derived from deeply buriedsourcerocksin the centralpart ofthe graben.The crudeoilsfound are paraffinic with a relativelyhighasphaltene content.Aromaticsand N. S. O-comDounds areless abundant. TheDeepBasinof WesternCanadais a gas-prone areaconsisting ofa thickMesozoic sectionof interlayeredsandsand shalesrepresentinga marine to brackishnear-shore environment.The clasticMesozoicrock sectioncontainsnumerousshalvzoneswhich arefich in organicmatterandalsoa suiteof coalsrrata.Thissection,coniaining mainly type-IIIkerogen.is the idealgasgenerator. Maturityrangesfrom about0.54l.vitdnite reflectance to about2.0% in the deeperpart of the section.Apparentlythe mature sectionis still in an activephaseof hydrocarbongeneration.Due to the tightnessof the rocks, hvdrocarbontransportmechanismseemsto be dominatedby diffusionprocesses.The light hydrocarbondistributionpattemsobservedthroughoutthe wellssuggest a dynamictrapping mechanism.Light hydrocarbonsare lost at the top of the mature hydrocarbongeneratingzone and are replenishedin the middle part of the section where rich sourc€rocks are found.
640
Habitat of Petroleum
The gasoccurrencein the Deep Basinis of specialimportancebecausethe situationis the revene from a conventionalgasfield, i. e., the gas-bearing zonescorrespondto the lessporous and lessperrneablerocks situatedin a downdip position, whereasmore porousandpermeable rocksareupdipandabove.The Elmwo hGasFieldintheDeep Basin is a dynamic situationwhere gasis continuouslybeing generatedin the center part of the basinand lost toward the surfaceand the more porousedge.
Chapter6 The Distributionof World Oil andGasReserves and Geological-Geochemical Implications
6.1 Introduction The observeddistributionof known reserves of oil and gashasbeenstudiedby explorationgeologistsand also bv petroleumeconomists.The former have consideredthc distributionin relationto geologicalhistoryof the earth, and glltl!"q.ry basintypesand globaltectonics:Brod (1965).Uspenskaya (1966. 1 9 7 2 )H , a l b o u t y( 1 9 7 0 )O . l e n i n e( 1 9 7 7 )M . e y e r h o f (f 1 9 7 9 )B , o i se r a l . ( 1 9 8 0 . 1982),Klemme (198t))and Sokolow(1980).perroleumeconomisrs were more interestedin predictingultimatereserves world or national.basedon potential future discoveries and/or increaseof engineeringand economicfeasibilityof recovel,/:Desprairies(1977).Nehring (1978).Halboutl,and Moody (1979). D e s p r a i r i easn dT i s s o t( t 9 l t 0 )a n dB o y d e l a T o u r e r a l . ( 1 9 8 1 ) . In this respect.thereis an obviousneedfor a systematic classification and a fixed nomenclature to designatethe varioussolicl,liquid or gaseous petroleum productsand their reserves. In this chapterwe shallrefer to namral gasfor the fractionof petroleumwhichis in gaseous phaseunderreservoirconditions. or is in solution in crude oil and becomesa separategaseousphaseunder surface conditions.Naturirlgasmav includenon-hydrocarbon conitituents. Oonventional oil refersto all petroleumproductsnaturallyoccurringin a liquid . phase.whichcanbe exploredandproducedby conventional methodsiprimaryor secondary recovery).includingthe additionalquantitieswhichmay rcsultliom enhancedrecoveryprocedurcs. N
oll
Thc Distributionof World Oil and CiasResenes
A specialremarkconcernsthe ageof the oil andgasreserves. The ageof the reservoirrock is generallywell known and is usedthroughoutSection6.2. The actualageof the sourcerock is not alwaysavailable.In most cases.crudeoil reservoirand sourcerock belongto the samegeologicalperiod,e.g., Jurassic, Cretaceous, etc. There are. however,situationswhereoil hasmigratedfrom a sourcerock into a reservoirrock from a differentperiod.In this case,the age assignmentof the sourcerock used in Sections6.3 and 6.4 has been made according treatmentis not possiblefor theage to Boiset al. ( 1982).A comparable of gassourcerock,dueto the greatermobilityof gasandthe lackof an adequate compilation.
6.2 GeologicalSettingof Oil and GasReserves In this descriptionwe shallfollow the worldwidecompilationrecentlvprepared by Bois et al. (1980. 1982).The oil and gas accumulations are groupedby petroleum?or?es,a conceptdefinedby Bois (197-5).The petroleumzone is composedof accumulationswhich are locatedwithin the same productive sequence,with the sametypesof traps,and containhydrocarbons of similar chemicalcomposition.They are likely to derivefrom the samegroupof source beds. The mostrclcvantaspects of petroleumzonesare the folklwing: - the environmentof deposition,whichis mainlyresponsible for the natureand abundance of organicmatter: - the natureof sedimcntation, particularlythe clasticterrigenous input. or the carbonateand/or evaporiteautochthonousscdimentationresponsibletbr depositionof reservoirs and seals; - the rate of subsidence and the geothermalgradientwhich are responsiblc for conversionof kerogeninto oil and gasi in this respectthe major break is betweenthe platformsedimentation (in cratonicor pericratonicareas)with a ratcof subsidcnce in the orderof l0 or 20 m m. y. r. andthe sedimentation of rapidlv subsidingbasinsin mobile belts (foredeep,intradeep.backdeep. marginalbasinsand associated in theorder troughs)with a ratc of subsidence o r 5 0 o r 1 0 0m m . y . ' . o r c v e nm o r e i - tectoniceventswhich providemostof the trapsbut may also.in association with subsequent erosion.resultin destructionof hydrocarbonaccumulations by openingthe trapsand breakingthe seals.
6.2.1 Paleozoic Petroleumfoundin Paloezoic reservoirs oil amountsto l4% of the conventional and 29c/rof the gasreservespresentlydiscovered.Most petroleumzonesare Iocatedin North America(567a of the oil. 35e/.of the gas)and Europe(32c/.of t h eo i l a n d3 3 %o f t h eg a s ) w , h e r e aN s o r t hA f r i c aa c c o u n tfso r 1 1 %o f t h eo i l a n d thc Middle Eastfor 28% of the gas.
6.2Geoloeici,rl Settinc ol Oil andGasResenes
6.13
The geologicalsettingduring Paleozoictime was quite differentfrom the presentsiluation;furthermoremanychangeshaveoccuiredover a periodof ca. 350millionyears.Mostoil sourccrocksandreservoirs, however,weredeposited in platformenvironments, formedby shallowseason the edgeof precambrian cratons.Carbonatereservoirs predominate in North America.with reefdevelopments,oversandstones. Carbonateandsandstone reservoirs arefoundin Eastern Europe (USSR),whereassuccessive sandstoneblankelsare the reservoirsof North Africa (Algeria).Well knownsourcerocksare Simpson(MiddleOrdovician).Woodford(Devonianto Mirsissippian ) and phosphoria(permian)formationsin thc UnitedStates.the Domanikformation(LateDevonian)in the USSR. and the Silurianshalesof Sahara.Algeria. Somepctroleumzonesare associated with the late Paleozoicmobilebeltsof North America and WesternEurope: in the latter area. Carboniferouscoal measures arethesourcerockfor thegasfoundin lateCarboniferous andpermian reservoirs. Finally.whensourcerocksare associated with oldermobilebelts.no significantreservcs of oil or gashavebcenfound. Thc timing of petroleumgenerationand migrationfrom severalpaleozoic sourcerockshasbeendiscussed in ChapterII.7.6.It hasbeenshownthatin manv places.suchasHassiMessaoud (northernSahara,Atgeria)and LeduclAlberra. Canada).subsidencercmainedmoderateoyer Paleozoicplatformsand the principalstageof oil formationwasonlv reachedin Mesozoic.or evenTertiary
2 Iniracot|rineat ritt basin H Heavyoils Principall.troleun rones 0[. (r06m3) a > t000 . 1 0 0r o 1 0 0 0 ' t0 to 100 c A Sl l 0 e m 3| o > | 000 o 1 0 0r o | 0 0 0 " l0 to 100
Fig. V.6.1. World distributionof petroleumzones(or groupsof petroleumzones)in Jurassicreservoirs.Sizeof the circlesis relatedto the importanceof total recoverable reserves. Adaptedwith changesfrom Bois et al. (1982)isituationof the conrinenrs at 170m. y. modificdfrom Smith(1973).H heavyoilsand tar sands
6.r.1
The Distributionof World Oil and GasRcserves
time. This situationprevailedin Alberta and other basinsof WesternNorth America, North Africa and the Middle East. In WesternEurope the coal measures, althoughburiedat depthat the endofPaleozoic, generated mostofthe presentgasduringlateMesozoicor Tertiary.In EasternEurope,however,andin other placesof North America and North Africa (Ahnet-Mouydir basin) hydrocarbons werealreadygenerated by the end of Paleozoic. A specialremarkconcernsthe widespread occurrence of Permianor PermoTriassicevaporites,which seal off the Permianreservoirsand allowedhuge quantitiesof gasto be preserved in the southernNorth Sea,North Germanyand Netherlands. in Texasand in Southwestern Iran and other Dlacesin the Middle East.Indeed,gasis so muchsubjcctto escapeto$ard the iurface.includingby diffusion(Leythaeuser ct al.. i982). that preservation of suchlargeaccumulationsrequiresan outstanding seal 6 . 2 . 2 M e s o z o i (cF i g s .V . 6 . 1a n c lV . 6 . 2 ) Petroleumfoundin Mesozoicreservoirs amountsto 547. of thc conventional oil and4,1%of the gasreseryes presentlvdiscovered (the figurefor oil wouldbe still higher.shouldheavvoil and tar sandsbe considered).The major petrolcum
70"i
r0's Fig. V.6.2. World distributionof petroleumzones(or groupsof petroleumzones)in Ctetaceousreservoirs.Sizeof the circlesis relatedto the importanceof total recoverable reserves.Adaptedwith changesfrom Bois et al. (1982):situationof the continentsat 100m. y. modifiedfrom Smith(1913).H: heavyoilsand tar sands;"/; oil or gaspartlyor whollvderivedfrom Jurassic sourcerocks
6.JGeologicai Sctting of Oil andGasReserves
6.15
zonesare locatedin the Jurassicand/orCretaceous of the Middle East.Western Siberiaand the Gulf of Mexico for conventionaloil, and WesternCanadafor heavyoils.The Triassiccontributionis minor,asthe mainDetroleumaccumulationsconcernedwith Triassicreservoirs are in fact derivedfrom sourcerocksof differentage(Algeria,Alaska). The geologicalsettingduringJurassicand Cretaceous haschangedmarkedlv sincethe Paleozoic andis shownin FiguresV.6.1 andV.6.2.The mijor featureis the occurrence of an east-westocean,comprisingthe MesogeanOceanand the early Central Atlantic Ocean, from South East Asia to the Caribbean.It separates a northernhemisphere(North America,Eurasia)from the massive Gondwanaland.whichwasprogessively breakingup. In the meantimc.Jurassicand moreoverCretaceous periodsare markedby severalmalor transgressions over continentalmarginsand intracontinental depressions. These areasinclude the marginsof the MesogeanOcean.the depressions locatedaboveformer Paleozoicmobile belts.respectivelv. These t r a n s g r e s s i ocnr cs r l e t lm a n l s h a l l o wf.r e q u e n r llya n d l o c k eedp i c o n t i n e ; r ,ael a 5 . In suchsituation.abundanceof nutrientsfavorsalgalblooms.and insuftjcient renewalof watcr enhances preservation ol the organicmatter. Examplesof marincsourcerocksassociated with thesemajor transgressrons are numerous.Along the marginsof the Mesogeanweredepositedthc Oxfordian-Kimmeridgian carbonatesourcerocksof the Arabianpiatform.the middle Cretaceouscarbonatesourcerocks of the Arabian Gulf. and the Cretaceous sourccbedsof the Svrtebasin.Libya. Altogetherthe Mesogeanrealmconralns ca. 80% of the oil and half the gasfrom the Mesozoic. The transgression over someformer Paleozoicmobile belts has causedthe depositionof theJurassic(includingKimmeridgian) sourcebedsandoil shalesof WesternEurope. the Jurassic-Cretaceous of Mexico, and the Cretaceousof Venezucla.From the Arctic Seatransgressions coveredlargeareasin Western Siberiaduring the Upper Jurassic.resultingin thick shalysourcebcds.These shalesgenerated largeamountsof oil andthermalgas.Thewell-knownBazhenov formationwith its hugeamounts(between300and 600billion tons)of disseminatedbitumen(Bois and Monicard.1981)is containedin this area,whichalso represents a former Paleozoicmobilebelt. Transgressions overthe otherpassive marginswereresponsible for Cretaceous sourcerocksin WestAfrica, alongthe easterncoastof SouthAmerica.andfor Jurassicand Cretaceous sourcebedsin WesternAustralia. Transgressions over the foredeepof Mesozoicmobilebeltscausedthe deposition of the Lower Cretaceous sourcerocksof the North Slopc,Alaska,and the Lower to Middle Cretaceousshales,which are amongthe sourcebeclsof the A l b e r t ap r o v i n c eW , . C a n a d aA . c o m p a r a b lsei r u a t i o n p r o b u h l yp r e v a i l e di n Columbiaand Ecuadorwith the Cretaceous NaDoformation. Furthermorea worldwideriseof the sealeveiduringJurassicand Cretaceous reachedan elevationca. 350m ahovepresenrin the Lite Crelaceous (85 m. y.): Pitman(1978)and Vail (1978).This situationresultedin an imporrantdecrease of the clasticinput from the continent.and a large extensionof carbonate sedimentation.Thus carbonatesource rocks (usuallymarls or argillaceous limestone)andcarbonatereservoirs aremorefrequentin Jurassic andCretaceous
The Distributionof World Oil and GasReserves
b..16
thanin otherperiods.Underfavorableconditions,suchsourcerocksmaybe very rich (lessdilutionby mineralparticles)andcontaina particularlyfavorabletype of organicmalter(type-ll kerogen,derivedfrom marineplanktonanddeposited in a reducingenvironment),ableto generatelargeamountsof oil. In manymobilc beltsor margins,therewasrapidsubsidence andsedimentation duringCretaceous andhencefastburialandratherearlygeneration of oil in late Cretaceousto early Tertiary time. A particularaspectof the Mesozoic petroleumreservesis the frequentoccurenceof rich sourcebedsover some limitedintervalsof time duringthe Upper Jurassicperiod (roughlyKimmeridgian) and duringthe Early to Middle Cretaceous period (approximately AlboAptian and Cenomanianto Coiriacian).A possibleexplanationfor these worldwidcfcaturesis prcsentedin Section6.4. 6.2.3 Tertiar,-(Fig. V.6.31 Petroleumlound in Tertiarvreservoirs amountsto 32%,of the conventional oil antl214/c of the gasreserves in the world.The majorreserves are associated with Tertiary mobile belts. However. large deltas,such as Gulf Coast. Nigeria. Mackenzie.Mahakam,etc. alsocontaina largeproportionof Tertiarvgas(ca. 357o) and oil (ca.20c/c). The geokrgical progressively settingchanges towardthe presentconfiguration. The east-west-trending oceanis no longerthe major feature.as the openingof the AtlanticOceanhascreatedpassages for a globaloceaniccirculation,with the coldoxygenated polarwatersreachingat depththe low latitudesandpreventing the existence of largeconfinedseas.
LATE TERTIARY S
irair dll 9.s h.ai!
d.ft.
Fig. V.6.3. Distributionof petroleumzones(or groupsof petroleumzones)in Late Tertiar)reservoirs. Adaptcdwith changes from Boiser al. (1982);presentsituarionofthe continents.H: heavyoils and tar sands;Cr oil or gaspartly or wholly derivedfrom Cretaceous sourcerocks;E. from EarlvTertiarvsourcerocks
6.2Gcolocicul Scttinsof Oil andGasReserres
61j
The Mesogeanrealm (sensulalo from Caribbeanto SouthEast Asia) again containsa largeproportionof the Tertiaryreserves (83% ofoil and867oof gas). However,thesefigurescannot be readily comparedwith the figuresfor the Mesozoic,as they cover very different situations.Among the three major petroleumprovinces(Midle East,Caribbean,and Gulf Coast)two resultfrom geologicalconditionsnot strictlydependenton the evolutionof the Mesogean realmduringTertiary.In the MiddleEast(207aof theoil, 137oof the gas)theoil originatesfrom Cretaceous sourcerocksand wassubsequently transferredby a secondmigrationin largeelongatedanticlinaltrapsformedduringlateTertiary and sealedby evaporitedeposits.In the Gulf Coast,a largedeltaicsystemis responsible for ca. l0% of the oil and 30% of the gasin the Tertiary. In the other areasof the Mesogeanrealm,the occurrence of the petroleum zonesis largelycontrolledby the development of the Alpine orogenicbelt,from the Caribbeanto lndonesiaandNewZealand:a thickclasticsedimentation anda -r - sometimes highrateof subsidence over 100m m. y. - arecommonfeatures sometimesassociated with overpressured shalesand mud volcanoes.Foredeep basinsoffereda favorablesituationin the areasof continental collisionsuchasthe Caribbean,wherethe Orinocobasinincludesthe largestreserues of heavyoils andtar sandsin the world.Otherforedeepbasinsarein Europeor WesternAsia, suchasthe Molassebasin,Romaniaand the NorthernCaucasus. Furthermore. intramontanebasinswere formed in Europeand WesternAsia: e. g., Vienna, Pannonianand Caspianbasins,and representanotherfavorablesituation,The Tertiarvpetroleumzonesof Burma and lndonesiaare locatedin backdeepsor marginalbasins. The WesternAmericanmobilebeltswereveryactiveduringTertiaryandthus they providedrelativelydcepand restrictedbasinsof depositionwheremarine organicmatterwaspreserved:California,Alaska.Peru.Comparablesituations m a r s t i l le x i s lt h e r et o d a ) . Thercaretwo mainreasons for thedevelopment of largedeltaicsystems during Tertiarvtime. The mountain-building resultingfrom the varioustectonicphases of the Alpinc cycleprovidedabundantopportunitvfor erosionandthusa source for clasticmaterial.The generalfall of the sealevelfrom a maximumof *350 m above presentin Late Cretaceous(85 m. y.) to *60 m in Middle Miocene (l-5 m.y.: since that time glacialfluctuationsma1'have dominatedsea-level c h a n g e sT) .h u s ,h u g ed e l t a i -s y s t e m ss,u c ha sG u l f i l o a s r .N i g e ra n dM a c k e n z i e deltas.are a new featureof the petroleumgeologyin the Tertiaryera.Although the associatedorganic matter is from terrestrialorigin (kerogentype III), i. e.. it has a comparativelylower potentialfor oil than for gas,the enormous quantitvwhichaccumulatcs in thissituationis responsible for ca. 357cof thegas, ztndca.20c/cof the oil known in thc Tertiary. Along the passivemargins.other typesof petroleumzonesare found in the Gulf of Suez, in the Cambay basin (India) and the Gippslandbasin (SE Australia).In China.oil is in internalrift basinswhichwerefilled by nonmarine, o r p a r a l i c( B o h a ib a s i n s) e d i m e n t a t i o n .
6-ltl
l-heDistribution of WorldOil andGasReserves
6.2.4 Conclusionson GeologicalSettingof PetroleumReserves From the previousdiscussion, it is clearthat the distributionof Paleozoic oil and gasreserves with a low rate is mostlycontrolledby platformsediments deposited of subsidence in cratonicor pericratonicareas.Furthermore,when suchsediwith late mentsbecamelater incorporated in mobilebelts,only thoseassociated younger petroleum Paleozoic or beltsoffer some reserves. During Mesozoicand Tertiary, a generaltrend is observedmarked by a decrease ofthe roleofplatformdepositsandan increase ofthe influenceoffillingup basinswith a highrateof clasticsedimentation in convergence zone(foredeep, backdeepand marginalbasins,and associated troughs). More generally,the basinsassociated with mobilebeltsincludeca.40VooI the oil and gas reservesin the world, but their importanceincreasesfrom the Paleozoic,wherethey controlonly 20c/aof the oil and 40% of the gas,to the Tertiary.wherethey control70c/aol the oil and 557c of the gas. The significanceof these observationswill be discussedin Section6.4. particularlythe increasingimportanceof mobile belts and the decreasing importanceof piatformsediments with increasing for petroleumoccurrence, age o f t h eE a r t h .
6.3 Age Distributionof PetroleumReserves Thisquestionhasbeendiscussed by Tissot(1979)andBoiset al. (1982).The age distributionof the initialin-placecrudeoil is shownin FigureV.6.,1,i. e., thetotal plusthe nonamountof the oil alreadyproduced,plusthe recoverable reserves. recoverable fractionof oil in all discovered fields.The surfaceof the blocksis proportionalto the total initial in-placeoil: the ageconsidered is the ageof the sourcerock, not that of the reservoirrock.To convertrecoverable oil to in-place oil and viceversa.a world recoverylactor of 0.25is assumed. i.e., 25% of the initialoil in placeis recovered.Obviouslythisfiguremayvaryfrom lessthan5?Z with veryviscousoilsandpoor reservoirs to morethan507. underveryfavorable conditions. to Considcrationof thc in-place.not the recoverable, crudeoil is necessary includeheavvand extra-heavyoils (tar sands):for someof them the present recovervfact()r may be approximatelyzero, unlessmining techniquesare oilsaccumulations belongto the considered. Someof the heavyandextra-heavy largestin the world. The most striking remarksconcernthe concentrationof oil reservesover certainperiodsof time and the existenceof two major cyclesof sourcebeds. duringJurassic periods Prolificpetroleumsourcerocksdeposited andCretaceous are responsible for ca.70c/cof the world'sconventional crudeoil, althoughthis time interyalamountsto only 22Vcof the time elapsedsincethe Precambrian. Furthermore.a detailedexamination of the ageof the sourcebedsshowsthatthe to between180and85m. y., i. e., only 17o/cof the time spanis mainlycondensed geological (Tissot,1979). timc sincethe Precambrian
6 . 3A s c D i s t r i b u t i oonf P c t r o l e u m Reserves
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Fig.V.6.,1.Agedistribution oftheinitialconventional oil in-place andheavyoil derived hom thevarious geological periods. Theageconsidered is thatof thesource rock:it is compared u'iththemajorsea-level (E. absolute;R: changes general relative;8,: trendin Paleozoic andMcsozoic timeI seeSefi.6.,1. I ) andrheestrmared ahundance of phytoplankton. Someof the majorprolificsourcerocksarealsoindicate
The Distribution of Worlcl Oil anci(iiLs Rcserves
65()
I
Gt0t0Gtclt Ptn|00s
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c0ar G^S c0A1 ntsEnvts ItSEBVtS PnrrcPAr lt09n3/flY) l10ei/[Yl
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5TA I.EVILCIIAilGES
SESoURCES
t
.-
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Fig.V.6.5. Agedistribution of theworldreserves of gasandcoal,compared withthe majorsea-level changes. Theageconsidered for gasisthatof thereseryoir rock.Someof themajorcoalprovinces areindicated. AdaptedfromTissot(1979). withdatafromtsois et al.(1982) on gasandcoalreseryes. Vail( 1978) andPitman(1978) onsea-level changes (curves E, R andE''arethesameasin FigureV.6.,1)
biogenicgas and thermal gas are generatedunder vcry different geokrgical situationbut thev are not distinguishedin the evaluationof rescrves.The Cretaceous peak.however,may be interpretedalongthe samelinesasthe peak observed in the distributionof oil (Sect.6.4.4,below).whereasthe Permianpeak mavbe a resultof a widespread saltcoverensuringan exceptionally effectiveseal of giantgasaccumulations.
6.4 Significance of theAge andGeotectonic Distributionof Petroleumand Coal The dataon geological settingand agedistributionof world petroleumreserves (reportedin Sects.6.2 and 6.3) can be interprctedin terms of generation. maturationand destructionof crudc oil accumulations. Five aspectswill be discussed: l. Existence of two majorcyclesof sourcebedswhichcanbe correlatedwith the major cvclesof sea-levelchanges.and thus with the main eventsof global t e c t o n i c(sS e c t 6. . 4 .1 ) . 2. The conditionsfor generation andpreservation, or destruction, of oil accumulationsexplainhow the youngercycle(Mesozoic-Tertiary) is about l0 times more productive(per million year) than the older cycle (Paleozoic)if the calculationis basedon the presentlvexistingreserues (Sect.6.4.2).
of theAgeandGeotectonic 6.'lSignificance Distribution of Perroleum andCoal
651
3. The samefactorsalsoexplain the progressivechangein the geologicalsetting of crudeoil, from platformsediments in Paleozoic to mobilebeltsin Tertiary (Sect.6.4.2). 4. A more detailed study of major sourcerock occurrencesshowsa frequent relationship with major transgression phases(Sect.6.4.3). 5. Prolificpetroleumsourcebedsdepositedover short periodsof Jurassicand Cretaceouscorrespondto an associationof severalfavorableconditionsover largeareasof sedimentation (oceanicanoxicevents,Sect.6.4.4).
6.4.1 Significanceof the Two Major Cycles The two major cycles(Paleozoicand Mesozoic-Tertiary) of the crude oil distributionaccordingto the age of the sourcerocks have been presentedin Section6.3 and FigureV.6.4. Recent developmentsin marine geologyand geophysics made possiblea comparisonof theseeventswith globalseaJevelchanges(Tissot,1979).Pitman (1978)hasshownthat absoluteriseor fall of sealevelare causedby the volume changes ofthe mid-oceanic ridgesystem.In turn, thesevariationsaremainlydue to changein spreading rate andto creationor destructionof a ridgesyslem.The modelusedby Pitmantakesinto accountthe positionof theridgesandtherelated spreading ratesfor calculating the absolutesea-level changes shownby curve(E) in FiguresV.6.4andV.6.5:thesealevelwasca. +350m abovepresentin theLate (85m. y.) andfell to +60 m in theMiddleMiocene(15m. y.). Therate Cretaceous of fall was in the order of 0.4 to 0.7 cm per 1000years.Sincethat time, the glaciationshave been the major causefor sealevel changes,with successive variationsreachingI m per 1000years. Vail (1978)studiedthe seismicrecordson many continentalshelvesand margins,especially the onlapof coastaldeposits.From thesedata,he evaluated the relativechangesof sea level, i.e., its rise or fall with respectto the land surface.They are shownby curve(R) in FiguresV.6.4 and V.6.5. Vail defined two majorcyclesof sea-level riseandfall, the first onefrom Cambrianto Triassic (58t)-200 m. y.), the secondone from Jurassic to the presenttime (20G{lm. y.). Both curves(E) and(R) showa goodagreement with respectto generaltrendand average values.Thusit is possibleto extendthecurve(E) ol theabsolute sea-level changes backinto earlyMesozoicandPaleozoic time by curve(E') averaging the generaltrendof (R). Furthermore,the estimatedabundanceof phytoplankton(redrawnin Fig. V.6.4 from Fig. L2.1) also showstwo distinctmaximacorresponding to the highestsea level, i.e., the largestextensionof epicontinental seas,which are more productivethan deepoceans. A comparison of the differentcurvesin FigureV.6.4 showsthatthe two major cyclesfeaturedby the agedistributionof oil canbe correlatedwith the two major cyclesof sea-levelrise and fall (curveE-E'), and alsowith two maximaof the estimatedabundance of phytoplankton,whichis the mainprimaryproducerof marineorganicmatter.
6-i2
Tlte Di\tributionof Worl.l Orl JnJ C:r: Rc.ctrrr
6.4.2 RelativeProductivityofthe PaleozoicandtheMesozoic-TertiaryCycles; Causesfor Different GeotectonicSettingof Their Reserves There is a differenceof one order of magnitudewith respectto oil resenes betweenthe older and the youngercycle:the averageamountof crudeoil per million yearsis lower by about 1:10in the older cycle.We do not considerthis mechanisms differenceas resultingfrom differentgeologicalor geochemical beingactiveduringPaleozoic time.The distributionof coal(Fig.V.6.5)suggests andaccumulation of the organic that the conditionsfor production,preservation mattcr were equivalent.at leastduring Upper Paleozoic.On the contrary.we considerthe presentdistributionof crudeoil to resultfrom a dynamicsituation wherePaleozoicoil fieldsmay havebeensubsequently alteredor destroyedby thermal.tectonicand otherinfluences. Firsta greaterthermalmaturation,dueto burialat greatdepthfor longperiods of time. haschangedpart of the Paleozoic oil into gas.Moreover.the preservation of oil and gasaccumulations or their destructionmight be the controlling erosion.are factor.Tectonicevents,suchas faulting,folding and subsequent responsible for breakingthe sealsand openingthe traps(Fig. V.6.6; caseA). Finally.physicalleakage.includingdiffusion.is anotherwayof destruction of old for the accumulations. All thesefactorsin one or the otherway are responsible differenceof oneorderof magnitudeobservedbetweenthe reserves of the older and voungercycles. Furthermore.the chancesfor conversionto gas, or for the destructionof Paleozoicoil fieldswere particularlyimportantin mobilebelts:crackingto gas
u.PArEozotc i ltEs0z0tcIfR4s
Fig. V.6.6. lnfluenceofthe rate of generasourcerocks. tion in threePaleozoic -,l. Silurian.SouthwestIllizi Basin, Algeria: the fast temperatureincreaseduring Paleozoicled to cracking of the kerogenand the oil previouslvgeoerated:Iater Hercynian foldingandtaultingwasfollowedby a deeperosionof the trapslocated here. updipfrom the areapresented is reNo comnrercialaccumulation ported.B; Silurian,HassiMessaoud area,Algeriardueto the low rateof quantities subsidence. no significant of hydrocarbonswere generated duringPaleozoic. The bulk ofoilwas generatedin responseto the faster subsidencewhich occurredduring Cr DevoJurassicand Cretaceous. area,Westnian.Leduc-Woodbend ern Canada:the situationis comparableto B. with a still fasterrate of subsidence duringCretaceous
on hydrocarbon --lOM.Y.{BP)subsidence
6
E I
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Trapseroded Folding
MAII{HYDNOCAASOflS GEXERIITO -N0 aPPiactABtE ct tnAflotr
6.-lSigniticancc of theAgeanclGeotectonic Distribution of Pelroleum andCoal
653
because subsidence wasimportant:destructionbecause foldingandsubsequent erosionwereparticularlyactivein that geotectonic setting.Thus,evenif mobile beltsoriginallycontaineda largepart of Paleozoicoil fields,as they do in the youngerMesozoicor Tertiaryseries,thisfractionwasextensively destroyed.In particular,no oil accumulationhas actuallysurvivedwhere sourcerocks are associatcd with mobilebeltsolder than late Paleozoic or Mesozoic. On the contrary.sourcebedsdepositedon platforms(cratonicor pericratonic basins)wereburiedat a low rateandeitherreachedthe stageof oil formationby the end of Paleozoic(Devonianof the Volga-Ural)or evenremainedimmature until thevwereburiedthrougha voungercycleof sedimentation (Fig.V.6.6B and C): The Siluriansourcerocksof HassiMessaoud(Algeria)generatedoil when the Jurassicitnd Cretaceousbeds were depositcdover the Northern Sahara p l a t f o r m( s c ca l s oC h a p .I L 7 . 6 ) ;r h e D e v o n i i ns o u r c er o c k so t L e d u c( A l b e r t a ) generatedoil when the area recciveda thick filling-upsedimentation of Late Cretaceous age.as part of thc foredeepbasinof the RockyMountainsmobile belt.[nbothcasesthepreservarion.overalongperiodoftime(2-50to300m.y.). of the geneticpotentialof the sourcerockshasbeenthe essential factorfor the presentoccurrenceof theseoils. Theseconsiderations explainwhv platform sedimentation with a low rateof subsidence controlsthe distributionof oilsfrom Paleozoic sourcerocks(ca.80% of the oil). In the )'oung Tertiarv beds. the situation in almost reversed:platform s e d i m e n tdse p o s i t eadt a l o w r a t eo f s e d i m e n t a t i o n . e . g . , 1 0 o r 2 0 m m . y1.,a r e not sufficientlvburied to have reachedthe principalzone of oil formation. TableV.6.1. Approximate timingofthebeginning (takenat 20E(oilgenerated) andthe end(takenat 50% oil cracked into gas)of oil generation. The tablerefersto type-ll kerogen. witha constant rateof subsidence gradient Sanda geothermal G = 35'Ckm t. (Forfurtherdetails. seeTissotet al., 1980) Rateof subsidence S ( r nm . y . r )
Timingof oil generationDuration (m. y. afterdeposition)of oil-window B e g i n sa t ( ' n .y . ) Endsat
Source-rock Basin
t0
190
300
ll0
Paleozoic E. Illizi
20
100
170
70
Jurassic ParisBasin
50 100
25
1o 31
25 12
Cretaceous W. Canada
200 .100
12 7
liJ
l0
Geologicalexamples Average subsidence S ( m m .y . ' )
30 to 80
LateTertiary 200 Los AngelesB. lo Pannonian B. 300
TheDistribution of WorldOil andGasResen.es
6.
Greater burial depth and faster subsidenceis neededfor oil formation. This is found in basinswith rapid filling-up sedimentation,alongthe Alpine or Western Americanmobilebelts,or in internaltroughs,suchasseveralTertiarybasinsin China.Theseconsiderations explainwhy the basinswith a fastrateofsubsidence associatedwith mobile belts contain 70Voof the oil from Tertiarv sourcebeds ( T a b l eV . 6 . 1a n dF i g .V . 6 . 3 ) .
o.
654
6.4.3 Major SourceBedsund TrarcgressionPhases Worldwidetransgressions overcontinental platformshaveoccurredseveraltimes duringgeological history.Transgressions andregressions dependon: (l) the rate of absolutesealevelriseor fall and (2) the rate of subsidence minussedimentation rate, the resultantof which controlsthe movementof the continental platform.Thusworldwidetransgressions maybe causedby changes ofthe rateof sealevel rise or fall (Pitman,1978).They appearassecond-order cyclesof the curveR (Fig. V.6.4) drawnby Vail (1978). Shallowepicontinentalseastransgressing over platformsand continental depressions often representfavorableconditionsfor sourcerock deposition. There,primaryproductivityof phytoplanktonis high. Furthermore,transgressionsover continentaldepressions may form landlockedseas,surroundedby emergentlands,providingmineralnutrientsand favoringalgalblooms.In turn, bloomsresult in an abundantbacterialconsumptionof oxygen,which is not sufficientlyrenewed,thus providinganoxicconditions.Under thesecircumstances,organic-richsourcebedscontainingthe oil-pronetype-lI kerogenare deposited. In fact, the comparisonof many well-knownprolific sourcebedswith the second-order cyclesof curveR confirmsthat theyweremainlydepositedduring periodsof globalmarinetransgression. In the Sahara(Algeria)the rich Silurian sourcerockswere depositedby a largetransgression over the glacialtopography inheritedfrom the Ordoviciancontinent.In WesternEuroDe.the successive Jurassictransgressions over the Triassiccontinentdepositedt-hesourcerocksof the North Seaandthe ToarcianandKimmeridgianoil shales(France,Germany, UnitedKingdom).Along the marginsofthe Mesogean Ocean,the transgression over the Arabian platform depositedthe prolific carbonatesourcerocks of Oxfordianand Kimmeridgianage.In WesternSiberia,a largetransgression of the Arctic Seaover a former Paleozoicmobilebelt depositedthe Late Jurassic sourcebedsincludingthe very rich Bazhenovformation.In WesternCanada,a marine transgression from the Arctic Sea over the foredeepof the Rocky mountainsmobilebelt depositedthe lowerto Middle Cretaceous sourcebedsof the WesternCanadaBasin.Many other examplescouldbe cited (Fig. V.6.a), including some of the major petroleum resenes, to demonstratea direct relationship betweenthe majorsourcerockoccurrences andthe majortransgressionphases(Tissot,1979).
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6.4 Significance of the Age and GeotectonicDistributionof petroleumandCoal
6-5.5
6.4.4 OceanicAnoxic Events
Prolific petroleumsourcerocks, responsiblefor a largefraction of the petroleum reserves,were depositedover relativelyshort periodsof time, e.g., parts of Juras.sic andCretaceous. Schlanger andJenkyns( 1976)andArthur anrlSihlanger (1979)reporteda doubleworldwideoccurrence of suchsediments in lowerand middle Cretaceous and namedthem OceanicAnoxic Events.Sucheventsare characterizedby the occurrenceof rich to very rich organic beds (up lo 40Ea organrccarbon)containingmarine planktonicorganicmatter preservedin a reducingenvironment.Their lithologiccomposition variesfrom shalesto carbo_ nates. Duringthe first major Phanerozoic cycle(570to 200m. y. b. p.) someofthese oceanicanoxiceventsaresuspected, e.g.. duringlateDevonian,but insufficient data and uncertainpaleogeographic reconstruction make their assessment dif_ ficult. During the secondmajor cycle(200to 0 m.y.b.p.) three organic_rich eventsseemto havea wide, perhapsworldwide,significance: - A lateJurassic eventis presentaroundthe Northernseas:Alaska,Norwesian Sea,North Sea,northernend of AtlanticOcean,WesternSiberia;in the 6ulf of Mexico,on the Arabianplatform,etc. - A LowerCretaceous cvent,coveringmainlyAptian andlowerAlbian (ca.115 to 105m. y. b. p.), is presentin the SouthandNortheastAtlantic deeDbasins (Tissotet al.. 1979,1980),and also on the Manihiki plareauin the South CentralPacific.on the ShatskyRisein the WesternNorth pacific.andon the continentalmarginwestof Australia(Schlanger andJenkyns,t 976).Alongthe Tethysmargins,comparable organic-rich intervalsareknownin Italv. Switzerl a n da n dr h e M i d d l eE a s r . - A middleCretaceous event,often of Cenomanian-Turonian age,sometimes extendingfrom Late Albian to Coniacian(ca. 100_g5m y b.p is possibly ), the mostwidespreadevent.It is known in mostof the Ailanric deeobasins. exceplrhe cape Basin(Tissotet al., 1979,1g80;de Graciansky et al.. l9g2;on the Shatskvand HessRisesin the northernpacificand on the continental marginwestof Australia(Schlanger andJenkyns.1976).Organicrichbedsare alsoprese nt on someTethysmargins(Spain.Italy.NorthAfrica,MiddleEast), in thc Caribbeanand South America (Venezuela,Trinidad, Colombia. Ecluator).in thc Gulf of Mexico.etc. The principalcauses for the occurrence of theseorganic-rich bedsseemro oea highfertilityof the sea.highlyfavorableconditionsfor organicmatterpreservation, and a low rate of dilutionby mineralconstituents (B;is et at., 19g2). All threeeventscorrespond to majorphasesof transgiession, andthe lastone approximately corresponds to the higheststandof the ibsolutesealevel.due to the.intense activityandfastspreading of the oceanicridgesca. 115to g5 m. y. b. p. andthe relatedincrease in thevolumeof themidoceaniC ridges(pitman,l91g).in turn. this situationdetermineda worldwidetransgression over the continental platform:the globalextensionof shallowseasappioximarely doubledover that period of time. The favorableinfluenceof such a situationwas discussed in Section 6 . 4 . 3a b o v e .
656
TheDistribution of WorldOil andGasReserves
Furthermore,the high standof the sealevel,coveringplainsand lowlands, whichis the majorfactorfor decreased the importanceof detritalsedimentation, dilutionoforganicmatterby mineralconstituents. Thissituationresultedinmany limestone)beingdeposited in shallow carbonatesourcerocks(marl, argillaceous werelaiddown seas(Venezuela, MiddleEast),whereas highlyorganicrichshales in deepoceanicbasins(CentralAtlantic;Angolabasin). The lack of global oceaniccirculation, which usually provides a renewal of dissolvedoxygenin sea waters,was an importantfactor for organicmatter preservation.The main oceanicbelt wasorientedeast-west,from SoutheastAsia to the Caribbean through Tethys and Central Atlantic. Thus there was no opportunity (except in the Pacific ocean)for a generalnorth-south circulation which presentlybrings at low latitudesthe deep, oxygenatedpolar waters. Locally,the existence of physiographic barrierssuchasthe FalklandPlateauand the Rio Grande-Walvis Ridgepreventedoceanicwatersfrom circulatinginto the youngbasinsresultingfrom the beginningof SouthAtlanticopening,suchasthe Capeand AngolaBasins. temperatures up to Furthermore,a warmequableclimate,with highsea-water high latitudes,favoreda high organicfertility and a decrease of the contentof dissolved oxygenminimum oxygen.All thesefactorscontributedto a widespread layercoveringa muchwiderdepthrangethanusual;thissituationmayexplainthe occurrenceof organic-richbeds on plateau and rise of the Pacific ocean (Schlanger and Jenkyns,1976).Furthermore,the major tectoniceventsdetermined many large-sized restrictedor barredbasins,alongthe Tethys(Middle East)and in the Centraland SouthAtlantic.
6.5 Richness of Sedimentary Basins.Roleof GiantFieldsandGiant Provinces The role of giant fields and giant provincesin the world distributionof by Halboutyet al. (1970),Perrodon(1978. hydrocarbons has been discussed basins ofthe varioussedimentary 1980)andNehring(1978).The relativerichness fieldsareusuallydefined is comparedby Perrodon(1978).Giantandsupergiant asfollows: - giantfield: ultimaterecovery> 80 million m1or 0,5 billion bbl of oil; - supergiant field: ultimaterecovery> 800million m' or 5 billion bbl of oil. It should be pointed out that the definitionis dependenton the physical characteristics of the reservoir(porosity,permeability,surfaceproperties)and the fluids (composition,viscosity.etc.) which togethercontrol the ratio of recoverv,and alsoon the stateof art in production.
of Sedimentarv 6.5 Richness Basins
657
6.5.I Giant Provinces
The conceptof a petroleum province, composedof one or severalsedimentary basinshaving common geologicalfeaturesand a comparablehistory, is usedby Perrodon(1980).It generallyincludesseveralpetroleumzones.For instancethe WesternCanadaprovince coversthe Alberta basinand its extensionsin British ColumbiaandSaskatchewan; it includesthe Paleozoic crudeoils,theCretaceous oils and alsothe heavyoils and tar sandsof Athabascaandotherlocations. Nehring(1978)andPerrodon(1978)pointedout thal about20 giantprovinces (TableV.6.2:eachcontainingmorethan 1.6billionmr or 10billion bbl) contain over 85Voof the total production plus recoverablereservesof conventionaloil, althoughthey cover only 20Voof the total surfaceof sedimentarybasins.Among them,the A_rabian-Iranian province(hereaftercalledMiddleEast),containsca. 80 billion m' (ca.500billion barrels)andthusaccounts for abouthalf of thepast productionplusknownreserves of conventional oil. If heavyoilsandtar sandsare also considered,Eastern Venezuelaand WesternCanadaprovincesfall in the same order of magnitude and the three provincestogether accountfor three quartersof theworldreserves. The giantprovinces containall 33supergiant fields and227of the 272knowngiantoil fields(Nehring,1978:TableV.6.2).
6.5.2 Oil and Gas Richness Perrodon(1978)defined the averagerichnessof sedimentarybasinsas the amountof oil or gas(pastproductionplus recoverable reserves) per discovered squarekilometer.There may be somediscussion whetherthe figureshouldbe relaledto the surfaceor volumeof sediments. However,it shouldberemembered that the totalthickness of the sedimentary columndoesnot influencedirectlythe inteNalof that columnwhichbelongsto the oil generation zone.The situationis the samefor wet gas,but more questionable for dry gasftom the metagenesis zone,whichmayextendto greaterdepthsif thesedimentary columnis verythick. However,evaluationof the sedimentary volumeis difficultand we shallusethe richnessin mr km-2 for both oil and gas.FollowingPerrodon(1980,p. 340)we shallconsiderthreeclasses of provinces:very rich with more than 20 x 10rml -below km r; rich with 2 to 20 x 103m3km-2: poor 2 x 103m3km 2. TableV.6.2 showsthat all 18 major oil provincesare rich or very rich.Their averagerichnessis 13 x 103mr km 2 for oil and 6.106m3km-2 for gas.These figurescan be readilycomparedwith the averageof disseminated oil in good marinesourcerocks.whichmayrangefrom 105mr km 2fora mature10-m-thick sourcerockcontaining27oorganiccarbonto 106mr km : for a 20-m-thicksource rock containing1070organiccarbon.The ratio of accumulated to disseminated oil isin theorderof1to l0 and1to 100respectively, whichareacceptable figuresfor the efficiencyof migration,at basinscale. There are, however,large differencesin oil richness,even betweengiant provinces:Maracaibo,Reforma Campeche.Californja,Middle East,Sirteand North Caucasus provincesare veryrich andhavemorethan20 x 10rm3km-2. If heavyoils are considered,EasternVenezuelaand WesternCanadafall into the
658
The Distribution of World Oil and Gas Reserves
TableV.6.2. Giantoil provinces of the world. (Adaptedfrom Nehring,1978;Perrodon, 1978:and Boiset al.. l9ll0. with changes) Province
Number of Total oil Gas richness richness richness giant oil fields" 106mr l0r mr l0r ml
Surface Production+ (l0r km:) provedreserves
km -
Conven- Gas rionaloil (l0rr m') ( l t ) 'm r ) Arabian Irani!n (MiddleEast) 'l ampic(F ReformaCampcchc
42
'79
8
52
,{"
3
5
8
ti
6.5
2100
77
20
33
150
ll
2
.l.l
WcstSibcria
120(l
8.'l
l\lidcontincnl+ Wcst Texirs
10011
7.ll
Gulf Coast+ Mi\sissippi+ E. Texas
600
7.3
Maracaibo Volga Ural
60 500
5.7
Sirte.Libya
200
Norlh Caucasus + Kopct Dag North Sea NigerDelta California
7
6.5
1b
17 ,7
l2
t'7
29
L
Iu6 1l
t1
2.8
123 t7
5.1
0.9
27
5
150 500
3.3 3.0
| 2.5
2 2 6
1 5
150 6t)
3.0 3
1.5 0.9
20 50
2.5
0.2
2.3
2.6
2
I
1300
l0 15
32 2
9 30 65
6 12
2
0.3
t7
l9
Sumatra
210
2
0.5
Itl
12
Sabara.Algeria
6(Xl
3.5
3
9
NorrhSlope. Alaska
180
0.3
I
l3
l3
l9
Total \orld
18Giantoil province! Total world
ca. 175 105 (ca.15.1x 1(l') metrict)
{r7.5 %
10%
(Aftcr Nehring.l97tl) Excludinghcaw oil accumulations. Prcscntfigurcprobablyhigh€r
l5
9 11
120
11380+ l-51.2
ti
l1
OrinocoTrinidad
Toral lt giant Provinccs
l,r
11.5
Iu
N. China(Bohai. SungLiao) WestCanada
t
o eq. kml
221 212
6.5Richness of Sedimentarl Basins
659
samegroup-.All other giant provincesshow a moderaterichness.from 2 to 20 x 10' m' km-' includingWesternSiberia,Gulf Coast.Ural-Volga. Niger delta.and the North Sea.Non-giantprovincesusuallyshowa moderate(2 x 10r to 20 x 10rmr km-r) or low (< 2.103m3km 2)richness. If the total hydrocarbon(oil plus gas)richnessis now considered,largedeltaic provincessuch as Gulf Coast and Niger Delta join the group of the very rich provinces(TableV.6.2).Thisfactis possiblydueto an importantcontributionof terrigenous organicmatter(type-IIIkerogen)whichhasa limitedpotentialfor oil generation,but is ableto producelargeamountsof gas.
6.5.3 ConcentratedVersus Scattered Distibution of Oil Reserves. Roleof Giant Fields Nehring(1978)pointedout thatknownrecoverable crudeoil reserves areheavily concentrated in giant fields.This situationis clearlyreflectedin FigureV.6.7, wherethe numberof fieldsdecreases abruptlyasa functionof their size,whereas the fraction of world known reservesincreasesdramaticallywith the sizeof the fields.In particular,thereare between20,000and 30,000fieldssmallerthan 16 million m3 (100million bbl) and they only accountfor ca. I\Vo of known oil reserves; on thecontrary908fieldslargerthan16millionmr amountto ca.907oof oil reserves. Furthermore,2T2 giantfieldscontain threequartersofthe knownoil resewes,and only 33 supergiantfieldsconcentrate half of the reserves. Thereare, however,somedifferences in the degreeof arealconcentration of oil reservesand alsoin the role of giantfieldsfrom one provinceto another,as observedby Perrodon(1980).A concentrated distributionof oil reservesis
-
0il reservesperfield
Fig. V.6.7aandb. Sizedistribution of oil fields.(a) Numberof oil fields per classof size (the definitionof the classes is based per field). (b) on the oil reserves Distributionof oil reservesas a function of the sizeof the fields. For instance40% of the world reserves are concentrateo ln fieldslargerthan l0 billionbbl.
The Distributionof World Oil and GasReserves
defined by the occurrenceof the major fraction of reservesin a small numberof fields: there are large differencesin size between fields. The extreme casels presentlythe North Slopeof Alaska,wherePrudhoeBay encompasses the bulk of theknownreserves in thatarea.Obviously,thissituationmaychange,asaresult of further exploration. Somemore intenselydrilled petroleum provinces,however, alsoshow a concentrateddistributionof oil reserves,e. g., the Ural-Volga province where the Romashkino field contains more than one third of the reserves;the WestSiberianprovincewhereSamotlorfield contains4070of theoil reservesand Urengoy field one third of gas reserves;and the Algerian Sahara province, where Hassi Messaoudfield containstwo thirds of oil resewesand HassiR'Mel field more than one third of gasreserves.A concentrateddistribution of reservesis usually met in geologicalsituationswhere a structural or stratigraphicfeature hasfavored large drainageareassuchasa regionaluplift, a regionalunconformitywith weathered bedsor with a basalsandstone actingasan avenuefor migrationetc. An extremecaseof concentrateddistributionis offered by the huge heavyoil accumulationsof WesternCanadaand EasternVenezuela with the possibleconjunctionof a regionalunconformityfor migrationandan insitu degradation responsible for sealingoff andtrapping. A scattereddistribution is defined by a wide distribution of reservesamonga largenumberof fields;the largestfield may containonly 5 or 10% of the total reserves.This situation is met in deltaic basinsor stable intracratonic Dlatforms:Nigerdeltaand Gulf Coastprovinces,wherethe 10 major fieldsamount to only onethird of the oil reserves; Parisbasinwhere11milliontonsare spread over 18 fields; Michigan basin where more than 600 fields total slightly over 1 0 0m i l l i o nt o n s( P e r r o d o n1. 9 8 0 ) .
6.6 Ultimate World Oil and Gas Resources As of 1.1.1982,the total pastproductionof oil wasca. 62 billion tons,whereas provedreservesof conventionalcrudeoil wereevaluatedto be ca. 92 billion tons, making a total of 154 billion tons (ca. 175 billion mr.1.Beyond this value, additional reservesof conventionalcrude oil may be expectedfor two reasons (Fig.V.6.8):eithertherecanbe additionalproduction from knownfieldsthrough enhancedoil recovery(EOR) or new fields can be discovered.On Figure V.6.8, the ratio of oil recoveryis supposedto changeprogressivelyfrom 25% (present worldwideaverage)to 40%. Thisfigure,however,is morea targetfor improving existingtechnologiesor introducing new ones than an actual prediction of the level which will be reachedwithin a certainperiodof time. Along the other axisof FigureV.6.8 thereare new discoveries. It shouldbe pointedout, however,that any evaluationof future oil discoveriesis entirely speculative. Different procedureshave been applied for evaluationof not-yet-discovered reservesof conventionaloil, i.e., excludingheavy oils and tar sands,offshore areaswith great water depth (beyond400m) and polar areas: a) Consultation of numerousexperts, followed by a statisticaltreatment was (1977),updatedby Desprairies andTissot(1980).It was usedby Desprairies
6.6 Ultimateworld Oil and GasResources
i PlovenReserves Fulule Discoveries Pmduction 90 I 60 recovery Enhanced 1 Y
oil on-recovered
1
661 Fig. V.6.8. Discoveryof additionalnew fieldsand enhanced oil recovery (EOR) are two possiblewaysfor increasingthe worldreserves ofcrudeoil. The may result in two possibilities additional reseryesof comparable order of magnitude. (Adapted,with changes,from Boy de la Tour and Le Leuch, l 9 8 l)
360
concludedthat ultimate resources(includingpast productionplus known werein the 250-300billion tonsbracket reserves) roleof giantoil fieldsandmadea thepredominant emphasized D , | Nehring( 1978) that that observation.He suggests based on mainly worldwideevaluation billion 17-37 to amounting discovered, might be giant fields about 130-185 tons, plus an equivalentamountfrom non-giantfields.The additionalgain from EOR is eitimatedto be 5G97 billion tons,makinga total of ultimate recoverablecrude oil resourcesof 230-310billion tons. Half of these resources are supposedto be locatedin the Middle East. Halboutyand Moody (1979)by comparisonof unknownwith known provinces,reviewgdthe differentbasinsof the world and quotedan expected resources,with a most ranee of 40 to 345 billion tons for undiscovered resourceof ca 300 total ultimate making a tons, 141 billion pro-bable value of -billion ton.. In thisevaluation,the shareof the Middle Eastis smaller,asthe in thisareais 100billiontons,i e, mostprobablevaluefor ultimateresources one third of the world resources. d ) B o i s( i n D e s p r a i r i easn dT i s s o t1, 9 8 0 ) a n d B o i s e t a l . ( 1 9 8 0 ) u s e d t h e r e s u l t s o f explorationin United Statesasa model for an advancedstageof exploration drilling. Giant fieldsamountlo 36% of reservesin United States,whereas they represent757o of them in the rest of the world. Thus, if the rate of in the restof the worldto that in the United exploraiionbecomescomparable outsidetheUSA couldbeanticipated, Stites,a doublingof theworldreserves anynewgiantfield.The figurewouldbe 270to 300billion withoutdiscovering of giantfields(17to 37 tons,to whichsho;ld be addedanyfurtherdiscoveries a b o v e ) . b i l l i o nt o n s :a c c o r d i ntgo N e h r i n gs. e e "Americanmodel" by and Tissot,1980)usedagainthe e)- Bois (in Desprairies sedimentsin the (post-Precambrian) comparingthe volumeof Phanerozoic the and estimated 610:44 the ratio He found world with that in UnitedStates. billion tons at 320 world resources
662
TheDistribution of WorldOil andGasRescrves
f) The FederalGeologicalSurveyin Hannover,W. Germany.prepareda survey for the 11thWorld EnergyCongress(BGR, 1980)and of energyresources evaluatedthe ultimateoil resources at ca. 354billion tons. Basedon thesedifferentapproaches, there seemsto be someconsensus to placethe world ultimateresources of conventional oil in the rangeof 250to 400 billion tons.However,it shouldbe kept in mind that sucha coniensusis by no meansan argumentfor the validityof theseresourceestimates. The totalamounl-ofnaturalgasproductionplusprovedreserves is estimated to be ca. 105x l0'' m'. WesternSiberia,North Americaand Middle Eastcontain about2:3 of provedreserves.The ultimateresources were estimatedbv MeverhotT(1979)to ca. 200 x 10r:mr and by BGR (1980)to ca. 290 / l-0rrmi. Furthermore,it canbe anticipated thatnewhydrocarbon discoveries will include a larger proportion of gas accumulations. First, drilling at greater depths, especiallyin onshore basinswhere targets at moderatedepths have been extensivel-tprospected, will mostly investigatethe late caragenesisor metagenesis zones.Then, offshoreexplorationwill be tbcusedon continental marginswith its importantterrestrialrunoff,includingorganicmatter(typeIII) whichhasa moderatepotentialfor oil, but cangeneratelargeamountsof gas.
6.7 Paleogeography asa Clueto FutureOil and GasProvinces Among theresources ofconventional oil, thefractionrelatedto offshorebasinsis graduallyincreasing dueto improvements in offshoretechnologyandintensified explorationefforts: about I2Vo of the cumulativeproductionof oil has been obtainedfrom the sea,whereasmore lhan20a/cof provedreserves andpossibly 60% o'i the future discoveries are locatedin the offshorebasins.Furthermore. one of the major contributions to unconventional crudeoil is supposed to come from deepmarinebasins(morethan 400m of waterdepth). The costof explorationdrillingat sea,andthat of productionincrease at a fast ratewith increasing waterdepth.The only acceptable targetsin deepwaterswill be providedby rich provincescontaininggiantor supergiantfields.SuchIarge accumulations requireprolificandwidespread sourcerocksassociated with good reservoirsand seals.Paleogeography of the organicand mineralsedimentation providesa way of understanding the distributionof sourcerocksand reservoirs, and thus is a clue to the explorationof continentalmarginsand deepoceanic basins. For instance,interpretationof the geochemical dataobtainedfrom DeepSea Drilling (DSDP)core materialallowedTissotet al. (1979,1980)andRullkcttrer et al. (1982)to identifythe principalorganicfaciespresentin the Cretaceous blackshalesof the Atlantic basins,to reconstruct their paleogeographic selting and to evaluatetheir respectivepetroleumpotential.The three major organic taciesobservedin the Atlantic basinsare the following: a) Planktonicorganicmatter(type-IIkerogen),generallyoccursin areasofhigh productivity(see above, Sect. 6.4.4) where preservationwas ensuredby
6.7 Paleogeography asa Clueto FutureOil andGasprovinces
663
anoxrcor near-anoxic conditionsof deposition.The relatedpotentialfor oil a n dg a si s h i g h . b) Terrestrialorganicmattermainlyderivedfrom higherplants(type-III kerogen) and lessdegradableis preservedevenin placeswherean oxic warer_ columnpreventsthe depositionof planktonicorganicmatter.I r resultsfrom a rapid influx and sedimentation of terrigenous materialincludingslumpsand massflows.The potentialfor oil is only moderate,whereasthe potentialfor gasis good. c) Residualorganicmatter, either oxidizedin subaerialenvironments and/or recycledfrom oldersediments, providesan inertfractionof kerogen.lt isvery wrdespread andmaypredominate in someenvironments, suchashemipelaeii sediments. It hasno potentialfor oil or gas. Geochemicallogs were preparedfor each of the DSDp wells. usins in particularthe Rock-Evalpyrolysisto identifythedifferenttypesof organicmatter and evaluatetheir hydrocarbonsourcepotential.Figure V.6.9 shows,as an example.the datarelatedro site361(CapeBasin)andsite364(AngolaBasin).A -Albian first occurrence of organic-richblackshales,of Aptian to Lower ag;, is markedin both Capeand Angola basinsby a largeinput of marineplanktonic
CAPE BASIN-LEG 40-SITE 36] ORG.C 9
-I!111 l 0t 0l
-:
E
6
!14!t
E
llcsouRcE POTENIIAL (kqHC/t ofrock) 0t I l0 100
ANGOLA BASIN.LEG 40.SITE 364 b 9
E
HCSOURCE POTENTIAL
6
400 500 600
..-i .l-
700
E I
3
i -
800
900 000
in|
-
€H
|00 lzaa
::i:rMAR NEI TypEOF MATIER NL\I ORG -:MA [RsESTj
V 6.: Examplesof geochemicalwell logs preparedfrom cores of the Deep Sea Iiq: Drilling (DSDP) in the Cape and Angoti Basins.The hydrocarbonsource .Program potentialis obtainedfrom Rock Eval pyrolysis;r. met c ton. A first occurrenceof black shales,with marineorganicmatter,occursin Aptianto iowerAlbianbedsof bothsite361 (beyond1000m depth)andsite364(beyond930m). A secondoccurrence ofblackshales at site364comprisesa succession ofdiscreteorganicrich layersof Albian to Coniacianage with marineorganicmatter(ca.600-700m). (Tissotet al., 19g0)
664
TheDistribution of WorldOil andGasReserves
organic matter (type II). However, an important contribution of terrestrial organicmatter (type III) in the Cape Basin dilutesthe planktonicinput and reducesthe hydrocarbonsourcepotential,ascomparedto the AngolaBasin.A secondoccurrenceof black shalesis recordedin the Angola Basin during late Albian to Coniaciantime. There, a successionof discretelayerscontainsa rich planktonicorganicmatter (type II), while no comparabledepositionis known in the Cape Basin. Thesefacts are interpretedas a witnessof anoxicconditionsin the deepoceanicCapeand Angola basinsduring Aptian and Lower Albian time, when they were barredand separatedfrom the openocean.Later the CapeBasin becameopen to oceaniccirculation: the conditionswere no longer appropriate for preservation of planktonic material. On the contrary, the Angola basin remainedprotected, at leasttemporarily, by the Walvis-Rio Grande Ridge and intermittent anoxicconditionswere establishedduring late Albian to Coniacian tlme. Comparableobservationsweremadein the North andCentralAtlantic Ocean, and the distribution of organicmatter typesis reportedfor two different periods of time in FigureV.6.10. From thesedata, Tissotet al. (1980)observedthat statisticalregularitiesof distributionoccuron a regionalbasis,althoughindividual bedsmay not be correlated.Suchregularitiesof distributionsuggestthat they reflect paleogeographicconditions.Basedon the respectiveconditionsfor preservation of marineplanktonic(type II), terrestrial(type III), and residual organicmatter, they sketchedthe distributionof depositionalenvironments time in the Atlantic basinsas follows(Fig. V 6.11);these throughCretaceous et al. precisedandenlargeduponby De Graciansky weresubsequently concepts
(1e82). a) During Aptian and part of Albian, the Cape and Angola basin appear as anoxicenvironments,protectedby ridgesfrom the open ocean.Another Atlantic in the north-central at leastperiodically, anoxicsubbasin is obserued, alongthe African margin.This situationwas possiblyrelatedto upwelling currentsasis presentlythecasein placesalongtheWesterncoastof Africaand to circulationof oxicwaters.In otherareas, SouthAmerica.or to restrictions such as the northernand northwesternAtlantic, oxic conditionsprevailed exceptfor short periodsof time, and the continentwas the only sourceof organicmaterial,with an impofiantproportionof residualorganicmatter' along b) DvingCenomanlan,anoxicconditionscontinued,at leastperiodically, parts of the over large the northwestAfrican margin;a temporaryextension ln nofth and northwestAtlantic is observedaroundLate Cenomanian the SouthAtlantic, the AngolaBasinalsoremainedanoxic,but the CapeBasin becameopen to circulationof oxicwaters. c) During Turonian and moreoverConiaciun,the anoxicenvironmentwas ridgeand restrictedto the AngolaBasin,protectedby the Rio Grande-Walvis possibly with upwelling in connection the latter to the Cape Verde area, conditions(Arthur and Natland,1979). d) During the later part of Cretaceous,all Atlantic basinswere open to circulationof oxic watersandno further favorableenvironmentforplanktonic materialDreservation is found in the Atlantic.
6.7 Paleogeography asa Clueto FutureOil and GasProvinces
E-Pr4t4!qf4U I
665
5 NM Y C E N O M A N I A N - T U R O N9I A
:y'zr ^1q
u{) \ r05
U
398o/
11387
1tr386 Ztgt l37ZoB8 411/BZ
"i3:
MAN IYPESOFORCAN C MAIIIR ! I n Oa
(1ypeI) Plonklonrc (lypel[) Te(esliomoderoley deqrcded Resduol Snghhormns
Fig. V.6.10a and b. Distributionof the main types of organicmatter observedin Cretaceoussedimentsof the Central and North Atlantic. (a) Late Aptian-Albian time (positionof the continentsis only approximateat 110 m.y. (b) Late Cenomanianisonlyapproximate Turoniantime(positionofthecontinents at95m. y.) (Tissotet al.,1979. with chanses)
APTIAN.ALBIAN CENOMANIAN TURONIAN LATE CRTTACEOUS 110 tu1.Y Y 75t/Y 9 5[ / Y 90 lv1
TYPEOFOCEAN ICBASINS
FM5 ANOXIC
r-r oxlc
Fig. V.6.11. Dist bution of depositionalenvironmentsthrough Cretaceoustime in Atlantic basins. In some places,or at ce ain times, anoxic conditions were probably intermittent,particularlyin Coniaciantime (Tissotet al., 1980)
The Distribution of World Oil and Gas Reserves
Paleogeography,interpretedin the framework of plate tectonics,will afford a clue to petroleumexplorationof continentalmargins.It providesus with the rationalefor the distributionandcharacteristics ofthe variousorganicfacies,and also of the reservoirs.Combinedwith the use of basinmodeline.it offersa comprehensive approachto oil and gasexploration.
Summaryand Conclusion The age and geotectonicdistribution of world petroleum reservesare interpreted in terms of sedimentarycycles.sea-levelchanges,anoxic eventsand environmentsof deposition, and subsequenttectonic history. The role of giant fields and giant provinces,and the richnessof sedimentarybasinsare alsodiscussed. Paleogeographyand its consequeaces on sedimentologyof the source rocks and reservoirsappearto be an important contributionto the explorationof future offshore petroleumprovinces.Combinedwith the useof basinmodeling,it offersacomprehensiveapproachto oil and gasexploration.
References to PartV
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Relerencesto Part V
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Tissot.B., Durand.B.. Espitali€,J.. Combaz.A.: lnfluenceof natureand diagenesis of organicmatterin formationofpetroleum.Am. Assoc.Petr.Geol.BuI1.58,3.199-506 (1e71) Tissot.B.. Espitali€.J.. Deroo.G.. Tempere.C.. Jonathan.D.: Origineet migrationdes hydrocarbures dansle Saharaoriental(Algerie).Advancesin OrganicGeochemistrv 1 9 7 3T. i s s o tB . . . B i e n n e rF. . ( e d s . )P . a r i sT: e c h n i p1. 9 7 , 1p,p . 3 1 5 3 3 , 1 Tissot,8.. Pelet.R.. Roucachd. J.. Combaz,A.: Utilisationdesalcanes commesfossiles gdochin]iques indicateurs desenvironnements giologiques.In: Advancesin Organic G e o c h e m i s t r1y9. 7 5C . a m p o sR. . . G o n i .J . ( e d s . )M . a d r i d .1 9 7 7p. p . l l 7 t 5 , l Tissot.B.. Deroo. G.. Ilood. A.: Geochemical studyof the Uinta Basin:formationof petroleumfrom the GreenRiverformation.Geochim.Cosmochim. Acta42. 1;1691,185
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T i s s o tB. . . D e m a i s o nC.; . .N l a s s o nP.. .D e l t e i lJ. . R . . C o m b a zA. . : P a l e o e n v i r o n maenndt petroleumpotentialof MiddleCrctaceous blackshalesin AtlanticBasins.Am. Assoc. P e t .G e o l .B u l l .6 4 ,2 0 5 t - 2 0 6 3( 1 9 8 0 ) T i s s o tB . . P . . D e r o o .G . . H e r b i n .J . P . : O r g a n i cm a t t e ri n C r e t a c e o us se d i m e n tosf t h e North Atlantic:contributionto sedimentolog! and paleogeography. ln: Deeptlrilling resultsin the Atlantic Ocean:continentalmarginsand palcoenvironment. Maurice E$ing Series3. AmericanGeophvsical Union. Washingron. pp. 362-,101 (1979) Tissot. B. P.. Bard. J. F.. Espitalid.J.: Principalfactorscontrollingthe riming of petroleumgeneration.ln: Factsand principlesof world petroleumoccurrencc. Miall. A . D . ( e d .) . C a n .S o c .P e t .G e o l .M e m .6 , 1 1 3 1 5 20 9 U 0 ) U n g e r e rP . . . B e s s i sF. . . C h e n e t .P . Y . . N g o k w e yJ. . N 4 . .N o r a r e t .E . . p c r r i nJ . F . : Geological deterministic modelsandoil expl,-,rati.'n, prineiples andpracticalexamples. Am. Assoc.Pet. ceol. Bull. 67, 185(abstractonly) (1983) Uspenskaya. N. Y.. Tabasaranski. Z. A.: Perroleumprovinces and resionsofthe USSR t i n R u . s r r n .M ; . t r s c o uN . c d r a1 l a o b 1 Uspenskaya. N. Y.. Tauson.N. M.: Petroleumprovincesand regionsof the foreign countries(in Russian).Moscow.Nedra(1972) Vail. P. R., Mitchum.R. M. Jr.. ThompsonS.:Seismic stratigraphy andglobalchanges of sealevel.part ,l: Globalcyclesof relativechanges of sealevel:Am. Assoc.pet. Geol. Mem. 26, 83-97 ( 1978) Vandenbroucke. M.. Durand,B.: Detectingmigrationphenomena in a geologicalseries by meansof C1-C1.hvdrocarbon amountsanddistributions. In: Advancesin Organic Geochemistry 1981.Bjoroy. M. et al. (eds.).J. Wilev, Chichester. 1983.pp. 1,17155 Vassoevich. N- 8.. Korchagina.Yu. I.. Lopatin.N. V.. Chernvshev, V. V.: principal phaseofoil formation.Moscov.Univ. Vestnik6, 3-27. 1969(in Russian): Engl.transl. Int. Geol. Rev. 12. 1276-1296 (19j0) Waple-s. D. W.: Tine and temperature in petroleumformation:applicationof Loparin.s topetroleum e x p l o r a t i o nA.m . A s s o c p. e t .G e o l .B u l l .6 4 ,9 1 6 9 2 6 ( l O S 0 ) ...method Waples.D. W.. Haug. P.. Welte. D. H.r Occurrenceof a regularCr., isoprenoirl h],drocarbon in Tertiarl sediments representing a lagoonal-ty.pe. salinecnvironment. Geochim.Cosmochim. Acra 38, 381 387(1971) Weber.K. J., Daukoru.E.: PetroleumGeologvof the NigerDelta.gth World pet.Cong. Proc.2,209-222(1976) Weitkamp.A. W.. Gutberlet.L. C.: Applicationof a micro-retortto problemsin shalc pyrolysis.Am. Chem.Soc.Div. pet. Chem.preprints13.2. F71_F85(196g) W:lt:. H:: Pctroleumexplorationandorganicgeochemistry. J. Geochem.Explor.l, ? l r 7 - r 3 6( 1 9 7 2 ) Welte. D. H.: Informationon sourcerocks of the Arabian-persianGulf Rceion. Unpublished information.KFA-Jiilich( 1982)
67li
Rcfcrcnces to Part \,
Erddl u. Kohle. Erdgas. Welte.D. Il.: NeueWegein der Kohlenwasserstoffexploration. ( l9ll2) Pctrochem..15.503-501t W e l t e . D . I L . Y i i k l e r . A . : E v o l u t i o no f s e d i m e n t a rbva s i n sf r o m t h e s l a n d p o i not f petrolcumoriqin and ccunr lation - an approachfor a quantitntivebasinstudv. 2, l-ii (l9lt0) C)rganic Cieochernistrv in basinevolutron a Wclte. D. tl.. YLikler.A.: Petroleumorigin and ilccumulation quantitativenrodel.Am. Assoc.Pet.Geol. Bull. 65, l3lJ7 1396(1981) t Alkane in Welte. D. tl.. Waples.D W.: Uber die Berorzugunggeradzahligcr SeciimentgestciN n eant u. r w i s \ e n s c h a f6t 0e .n5 1 6 - 5 1 7( 1 9 7 3 ) w c l t e . D . I I . . l l a s c n r a n nH. . w . . H o l l e r b a c hA.. . L e v t h a e u s eDr . . S t a h l W . .:Correlation betweenpetroleum andsourcerock.9th WorldPet.Cong.Proc.2, 179 191(1975) of organic W e l t c .D . H . . Y i i k l e r .M . A . . R a d k c .M . . L e y t h a e u s eD r .. : A p p l i c a t i o n geochemistrt to petroleumexploration.ln: Originand rnd qurntitalivebasinanalysis Press.l9lll . Chemistryoi Pctroleun.Atkinson.G.. Zuckerman.J. J. {eds.). Pergamon pp. 67 t(8 w e l t e. D . H . . K r a t o c h v i lI.L . R u l l k i j t t c rJ. . . L a d w e i n H . . . S c h a c f e rR. . G . : O r g a n i c of their origin. geocheDristrv of crudeoils fronr the ViennaBasinand an assessnent C h e m .G c o l .1 5 .l l 6 8 ( 1 9 8 2 ) ol Oil Typesin WillistonBasin.Am. Assoc.Pet. Cieol. Williams.J. A.: Charactcrizatioi B u l l .5 8 , l l l l l l 5 l ( 1 9 7 . 1 ) Zhou Guangiia:Propert,r. maturationandevolutionof organicmatterin the sourcerock of continentalorigin.Int. Mectingon PetroleumGeology.Peking(1980)
Subjectlndex
a b i e t i ca c i d 3 7 . I 1 6 . I l 7 red 6.47 abnormalpressure 301.302 asa sourceof h!drocarbons .17 49. A b u D h a b ic r u d eo i l 6 l ' 1 1 0 5 .1 2 2 1 . 26 431 accumulationof perroleum 293.29,1.3,11. a s as o u r c e o f k e r o g e n1 5 1 1 5 3 . 1 5 7 . - 1 , 1 5 . 3 { 9 . 3 5- 1 5 63. 5 l t 3 6 5 256.'l'lt . ,1,12 aclds u n i c e l l u l a rl 1 i n c r u t l eo i l l 2 l . 1 0 1 . . 1 0 3 . . 1:3f 31,.1 . , 1 3 7alcalmat 112 fattv. seefattv acids Algeria.sceAfrica.North.andalsoSahara in orsanisms 3.1 36.15 ,19 alginicacid 33 i ns e d i m e n t s1 0 7 -l l 0 . 1 2 1 . 1 2 5 . , 1 3 1 . a l e i n i t e 1 5 3 .1 5 7 .1 5 9 . 5 0 , 1 .113 aliphaticchainsin kerogen 1,17, 148.15l. u n s a t u r a t e d1 5 . . 1 7 . . 1 9 r 5 3 .1 5 . 1 . 2 5 0 . 2 5 2 . 2 5 8 a c r r t a r c h1 . 1 1 6 . 2 3 .1 3 6 alkanes a c t i v a t i o n e n e r g2y2 1 . 5 ' 1 0 . 5 8 4 . 5 1 1 5 . 5 E 7branched i 1t) 116.182.183.38-5389, 592 ,16,1.,166 adamantane 39{J.392 cycllc.seecvcloalkanes A d e nG u l f 5 9 7 . 6 0 0 normal.seen-alkanes adrenosterone124 (paraffins) in crudeoil 315.316,382rdsorption307.351 38.1,.1tI, 416 ,119 capacitvof a sourcerock 497 alkenes(olefins) 48, 261.385,3i19.390 of organicmatter 58-60 alkylbenzene. alkylnaphthalene. alkyla e r o b i co.x i c 7 6 7 8 .9 2 . 2 0 1 . 4 6 4 . 6 6 3 p h e n a n t h r e n e1 9 3 3 . 9 3 - 3 9 54. 1 2 . 665 53,1-536 A f r i c aN . o r t h 1 5 2 1. 5 3 . 1 6 1 . 1 6126. 6 . allochthonous organicmatter 55. 230 1 6 8 .1 7 0 .1 8 t )2. 2 1 - 2 2 7 alterationof petroleum 122.123,45y Agbadaformation.Nigeria 617,618.620 169.1'74.478 agc 223 227,451-457 Amadeusbasin.Australia 227 Ahnet-Mouydirbasin, Algeria 221,221, A m a z o nR i v e r .d e i t a f. a n 1 3 , 5 7 U , :l.441 227. 614 aminoacids A k a t as h a l e sN. i g e r i a 6 1 6 .6 1 8 .6 2 0 rn organisms 31, 32 alaninc 32 in sediments 58. 59, 76. 80,83. 91 A l a s k a 5 6 3 5 6 5 . 6 4 5 . 6 4 7 . 6 5 5 . 6 5 8 . 6 6 0ammonia 27. 86. 87 Albert shales.Canada 258 amorphous organicmatter.kerogen 158, Alberta.seeCanada ,19i1.503-505 Aleksinacshales,Jugoslavia 142.263 Ampostacrudeoil. Spain 106,107.,133 a l g a e 1 6 .1 7 . 3 2 . 3 3 . 3 6 . 4 7 - 5130. 5 1. 2 2 . a m v r i n 1 2 0 .1 2 1 .1 2 3 151.230.237,256.426,428.,130, 442. anaerobic. anoxic 76-80.92, 123,201, 502.504.505 142.113.465.478.654_656,663_665 b l u cg r e e n 4 , 6 . 1 , 11. 5 .1 8 . 2 1 . 5 0 analyticalmethods brown 33. ,17 for chemical analysis ofextractable bitug r e e n 6 . 1 4 . 1 5 .1 1 1 , 4 7 men 378.379.526 528
680
SubjectIndex
in kerogen 142 145,1,tli,166.172 analysis ofkerogen 140 for chemical , 25,186.187, s t e r o i d s 1 1 9 ,1 2 0 1, 2 3 1 1,{7,508,509.523 525 396.397.435.436.537-540,556,557 for crude oil characterization 375 379 t r i t e r p e n o i d s1 1 8 1 2 0 , 1 2 31 2 5 , 1 8 6 , lor humic material 82 84 r87, 396,397 for light hydrocarbons 527 530 aromatic-asphalticcrudeoils'118.'119, of kerofor opticalexamination 5 121 423. 446. 474. 4'7 g e n 1 3 3 - 1 3 9 , , 1 9580 7 , 5 1 5 - 5 2 0 oils 418. aromatic-intermediatecrude for organiccarbon 495 497 4 1 9 .1 2 1 4, 2 2 ,4 4 1 , , 1 4 6 ,1 4 by pyrofor sourcerock characterization 418,419. aromatic-naphtheniccrudeoils l y s i s 5 0 9 - 5 1 3 , 5 2 05 2 3 121.123.174.175 forlitrinite reflectance 505-507.516 hydrocarbon aromatic/saturated 519 ratio 187 72 anchimetamorphism aromaticity 116,23'7-239 androsterone l2,l aromatics angiospermsl8-20. 13. 104 parameter 555 557 ascorrelation anhydrite.seeevaporite 560.566.567 a n i m a l s 4 1 .1 2 1 asmaturationparameter,135.'136. of vitrinite 166.2.12.245 anisotropj/ 534-536,538-539 anoxic,seeanaerobic 237.537-539 a n t e i s o a l k a n e s ( 3 - m e t h y l a l k a n e s ) , 1 7 aromatization . Arrheniusrelation.formula.plot 5E+, 1 1 0 .3 8 6 ,3 8 8 ,3 8 9 ,4 2 8 , , 1 3 0 . , 1 3 1 587 anthracene 393 asparticacid 32 anthracite 71, 72,111,145.229,235.231. asphalt ,171.seealsotar sands 238,241 n]at,seetar mat anticlines 352.360 anticlinalstructures, plug or seal ,182 362,611.621.647 AsphaltRidge,Utah 475 API gravity 462,171,175 479 asphaltenes projects6 and48 376,377 API research in alterationofcrudeoil 407-408,423, Appalachianbasin 227, 467 ,161-,{63, 465,478,479 pre\\ure.\ee \\atere\Pan\ion dquathermal 1 nq l u c logi l 3 8 0 , . 1 8 1 , 4 0 1 - 4 0 8 . 4 1 AquitaineBasin,France 103,104,106, ,111,413.416,418-423 205,206.210-212, 117,180 182.18.1, elementalcomposition150.176,404, 447.531, 533, 318,396,420-422.441, ,108 s 5 2 , 5 6 6s, 6 7 . 6 0 3 generation 175.176.189.196.217,'108 95, arborinol,arborinone,isoarborinol in heavyoils 104,406,40'7. 421- 123. , 21,431 9 9 ,1 1 7 1 472,473.415.176,118.4'19 a r c h a e b a c t e r i1a1 4 1 1 6 . 1 2 91,3 0 . 4 0 8 . in migration 150,306 309.332.333. 431 4 0 8 .4 1 3 Arctic Islands.of Canada 158,423 in sourcerockbitumen 175.176.189, 301 argillaceoussediments ,104 argonin naturalgas 199.207 metalsin 410 aromatic molecularweight 405,406 hydrocarbons p y r o l y s i s4 0 7 . 5 5 1 in crudeoil 379 381,383.384,393 of 378,379,404 separation 3 9 74 , 0 6 - 4 0 8 . 411, 4 1 24, 1 6 - 4 1 0 . asa sourceoffossilmolecules 130.'107, 141-413,164.465.412 ,108,551 g e n e r a t i oonf 1 8 3 . 1 8 61,8 7 1. 9 3 , structure of 150.40,1 '107 215,441-443 natural 50,51 associations, in sediments 126.127 Athabasca 317.354,422,423,465.4'71' sneets ,178.480-482 in asphaltenes.104,405.408
Subjectlndex
681
Bell Creekoil field, Montana 363,364 Bellshill,Lake oil field.Alberta 478,479 B e m o l a n ghae a v lo i l . M a l a g a s v4 2 1 Beninformation,Nigeria 618 benthos 53, 75,92 benzanthracene 393 h e n z e n e. 1 0 a1.1 2 , . 1 1341. 5 . . 1 137r.) 3 395,465 benzofluorene393 benzopyrene 126, 121,315 benzoquinoline401,402 dibenzothiophene395benzothiophene, 397,399,400,421.446,449,472-414, b a c t e r i a , 1 ,1 , 11, 6 -1 8 . 2 3 , 3 1 , 4 0 , 4 5 , 4 6 , ,178,555,556,566,567 5 0 - 5 3 , 7 4 8 0 . 8 2 . 9 09 2 ,1 0 81. 1 0 , 1 1 4 Bering Sea 80, 81, 85. 87 2 0 4 . 1 2 9 1 . 3 0 2 , 0 1 , 2 0 3 , 1 1 6 .1 1 8 1, 2 4 , Bibi-Eibatcrudeoil. USSR 403 229. 233, 234, 464 46'1,418 bile acids 40. 99,120,121,125 aerobic 21. 76, 77, 90, ,164 bimacerite 499 a n a e r o b i c7 6 .7 7 8 0 . 4 6 5 . 4 7 E ofcrudeoil 45q.461 biodegradation autotrophic 1l 61,1,620 461.413. 471,,178-480, of oil by 463-467.478,620 degradation of 407,551, oils,correlation g e n e r a t i oonf 8 a sb y 7 7 . 7 9 . 8 0 . 8 9 . 9 0 . biodegraded 560,561 201-204 biogenicgas,biogenicmethane 79,80' green 6 89.90, 199-204,209-211 heterotrophic 4,1 fossils biologicalmarkers,seegeochemical of 114 116.118.130.403 membranes of 31 53 biomass.composition methanogenic71.79.80,201 b i o p o l y m e r s7 5 , 9 0 , 9 1 red 6 evolutionof 14-19 478 biosphere, in reservoirs 463-,166. asa sourceof fossilmolecules 108.110. bi-phytane 116 14, bitumen.extractableorganicmatter . 3 0 . 4 2 5 . , 1 2483 0 . 1 r 4 1 1 61 . 1 8 1. 2 9 1 1 9 3 -196. 1 4 0 1 , 7 6 1 8 0 . 1 3 1 , 1 3 2 , 4 l l . ! ? . 412 . 71, , 3 0 3 3 24 , 1 83 2 4 8 , 2 5 1 , 3 03 81 , 73 s u l f a t e - r e d u c i n g ' 7119 , 2 0 1 , 2 0 4 , 4 4 6 . , 1 9 55. 1 3 .5 1 4 ,5 2 5 ,5 2 6 . 5 ' 1 95 5 1 .164.465,4711 coal 2,18-251 from tetrahydroxybactetetrol. bacteriohopane evolution 193 196 in experimental riohopane 125.,103 generation of 176 180 Baikallake.USSR 597 i n d i g e n o u s5 5 1 . 5 6 1 Bakkenshale.in Willistonbasin 321. 339 ratio 177-180.219.223,526 BalticSea 102 bituminouscoal 71,229.234,235,23'1' bark of plants 45,,19 238 barrier.effectof. duringmigration 352. B l a c k s e al 0 1 2 . 3 1 . 5 1 , 5 3 . 5 6 . 5 7 . 7 8 , 361 87, 88. 92. 102.118,447.507 base.of a crudeoil 416 black shales 258.264,265. 663. 664 basrn b o g h e a d c o a l s1 3 41. 3 5 ,1 4 1 1. 5 1 , 2 3 - , of deposition 56. 57,69 256-258 modeling 572. 5'76-581,625-62'7 BohaiGulfbasin,P. R. ofChina 624.647 study 571-573,575 oonos basins,hydrodynamicsof 3,18 169. breakinginkerogendegradation Baxtervillecrudeoil. gulf Coast 410 1 7 4 .1 7 5 ,1 8 9 - 1 9 15, 8 4 - 5 8 65, 9 1 - 5 9 2 Bazhenovformation.w. Siberia 645.65'1 158, type.distributioninkerogen 151,153, Beaufort-Mackenziebasin.delta 154.591 616. 647
. 23. A t l a n t i cO c e a n 2 5 , 2 6 , 5 81, 0 2 1, 1 8 1 155-157,443,445,600.646,655. 1,16. 656,662-665 atmosphere 4 7 autochthonousorganicmatter 55, 230 Australia.western 600,645,655 France 135,l5l. 256, Autun boghead. 25'7. 261. 262 azaarenes 402 Azov-Kubanbasin,USSR 178.219
6u2 BoscanCrudeoil. Venezuela 129.410, 474,475 Botryo(occus 48.13,1. 135.151.157,256. 506 Bouxwillershales. France 108.I 18.l2l .
r22
Bowen-Surat. basin.Australia 389 branchedalkanes,seealkanes,branched Brazil 441,445 BritishColumbia 27. 80, 81. 84. 85 browncoal.fignire 70,'l1, 141,229.231, 233-235 . 241, 243. 250 bubblepoint 354 bubblesof hydrocarbons310 buoyancy 3,11347,351 Bureauof Mines.analysisofcrude oil 377.378 Burganoil field, Kuwait 46,1.610 b u r i a l b. u r i a l d e p t h 1 6 21, 6 3 .1 6 5 -1 6 8 . -21' 7. 111-176.118-187. \91,21,2.215 5 7 6 .5 8 5 .5 8 7 .5 9 3 ,5 9 5 .6 1 3 , 6 5 26 5 , 1 Burma 4.15.647 b u t a n e 7 9 . 1 9 9 ,3 1 , 13, 1 9 . 5 2 9 calibrationof numericalmodels 588,593 California 103.179,180,184,222,223. 335.409.421.452, 453 . 464.4' /1 .647, 6,19.657.658 G u l f o f 6 1 .5 9 7 Southern, sea,shelf,sediments 61.76. 7 1 , 9 0 . 9 5 , 9 69. 8 , 1 0 1 ,1 0 8 2 . 24 calorificvalue 23,1 campesterol,10.41 Canada.WesternCanadaBasin.Alber. 5 5 . 1 6 01.1 J 0 1 .8 1 . 2 0 3 . 2 0 4 . t a 1 5 21 : t . 12. : 2 .t 2 1 .2 2 5 3 . 2 0 . r 4 - ., 1 5 13.i 5 . 36.1. 365.391. 396.,109,120122.110, ,!11.,1,19. ,160-462. ,165. ,166. 452.,157. .171 481. 53.1. 556.628.629. .,1?3-,178. 6,13.6.15.649.652 65,1.657.658 cannelcoals 230 c a p i l l a rpyr e s s u r e3 1 t | . 3 3 7 . 3 4 3 1 1 7 .3 5 1 cap-rock.seeseal c r r r b a z t rel ca.rb r z o l ed c rr rl t r r e s l l - . ,101.402.550 70.7,1.76. carbohydrates30-33.,15.,16. 83. 94. 233 refersto carbon:if not statedotherwise organrccarbon carbon annualproduction 3, 9. 25.29
SubjectIndex budgetand cycle 7,9 12 contentin ancientsediments 97 contentin coal 234-236 contentin Recentsediments 95,96 content(Corg)in sourcerocks 97,495497,612.618,622,625.631.655,657, 663 dioxide g e n e r a t i o n o f1 6 3 . 1 7 3 - 1 7 5 , 1 9 9 , 202 204,207,209. 214,216, 216,247 from kerogenpyrolysis 219.220. 5 1 0- 5 1 2 oxygenindex 510-512 in primarymigration 316 lsotopes,seersotopes preference predoindex.CPI.odd-even m i n a n c e 1 0 11 , 0 31 , 8 2 1. 8 4 1 , 85,2492 5 1 . 4 3 3 . 4 3 , r . 5 3513 3 ratio CR/Cr 513 carbonaceousshales 630 carbonate oil shale 255 sequence,133.,155,149 CarbonateTriangle.Alberta 471,480 c a r b o n a t e s7 9 .1 0 6 1 . 93,301.305,307. 3t2 as resenoirs 353,358,359,365.614 assource rocks 305.338,339,4.15.,146, 197,609.612,613.65,1,656 carbonization of coal.seecoalification of palynomorphs 515 carbonyl.carboxylgroupsin kerogen 1 4 3 .1 , 1 81 5 1 .1 5 3 1 5 4 ,1 6 7 .1 7 0 .1 7 4 carboxylicacids 311,401.403 perhydro-0-caro0-carotane, 0-carotene. . 2 6 .1 9 3 4. 2 8 .4 3 0 t e n e , 1 11 carotenoids 11. 47. 126.126 c a r r i e r b e d . c a r r i e r r o c2k9 3 . 2 9 , 1 , 3 , 1 1 3,1,1.3.16.35t) carveol.carvonc -16 casehistories 609-6;[0 C a s p i aS n e a .b a s i n 5 1 . 5 3 . 6 , 1 7 c a t a g e n e s i 1s 1 .7 0 . 7 1 1. 4 1 1. 6 1 1 , 6 31 . 66 1 7 0 1. 7 5 1 , 8 11 . 8 91 . 9 62, 0 0 2 . 0 1 2. 0 4 . 2 0 5 . 2 0 92 1 2 . 2 1 52 1 7 . , 1 5 0 . 5 , 1 3 . 5 4 4 . 546 end of 72 catalysts. catalyticeffect 107.192.193. r97 , 329 Caucasus. USSR 207,6,17.657
Subjectlndex cellulose 33. 14-46. 232,233 cementation 214,360 CerroNegroheavvoil. Venezuela 475 Challenger Knoll 326 characterization of potentialsource rocks 540-546 Chattanooga Shales 157 c h e m o s Y n t h e s1i s1 .l 2 China,People's Republikof 259.262. 445.621 628,647.654. 658 chiralcenters 538 chitin 32.33 c h l o r o p h y l l, 1 . 3 7 . 4 21.1 2 - 1 1 41. 2 7 1. 2 8 . 192.410 chloroplasts4 c h o l a n iac c i d 1 1 7 . , 1 0 1 cholesterol. cholestane. cholestene 10-42.17.99, 117.390.,126.562 cholicacid 40 chromatographic adsorption duringmrgra tion 339 chromalography gas 378.379.527.528 liquid 3713.379 thin layer 378.379 chrysene 126.127.393 citronellal.citronellol36 classification of coals 234,235 -,123 of crudeoils ,115 of kerogens l-51 158 ,148 clasticsequence ,145.4.16. clay dchvdration 328.329 m i n e r a l s 7 2 .1 0 7 1 . 9 21. 9 3 . 1 9 61.9 7 . 307.308.320.328329 climate 231,443,656 Closttidia 79 closureof trap 357 cloudpoint 261,384 clustersof aromaticsheetsin kerogen 112,r44. 166.172 coal association with crudeoil 252..145.622 classification of . seeclassification elementalcomposilion 141.235-237 formation 229-241 humic 229,230.241 liquids 404.107,471.172 macerals 229,230.234.231.238.241 244
683 petrography 2,11-245 ranks 23,1 239. 211. 243-215 sapropelic 229.230 asa sourceofgas 206-2112\2.221. 2,152,18.630.631 asa sourceof liquidhvdrocarbons 248 252,622 structure.chemical 237.238 coal-bearingstra2 t a3 1 coalification. carbonization of coal 23,12,11.245.2.16.592 c o a l vk e r o g e n , 1 9 1 1 . 4 9 9 coastalenvironment231 coccolithophorids15.16.5l ColdLakeheavvoil.Alberta 472.478,180.482 collinite 244.50.1 c o l l o i d asl o l u t i o n 3 l l . 3 1 2 .3 2 5 c o l o ro . f s p o r easn dp o l l e n 1 6 7 . 5 1 5 , 5 1 6 , 543 combustion. in situ '182.,183 c o m p a c t i o n2 9 9 . 3 0 13 0 5 . 3 2 7 . 3 2 9 , 3 3 5 , 336 delayetl 302 watcrs -125.335.336 computcrsimulation in basinstudies 572. 5 7 7 5 8 1 . 5 1 1 3 . 5 8558 8 . 6 0 56 0 7 condensate 199.217,338.,163,550 early-mature 338 condensation polycondensation81 84,175,23,1, 231. 215 retrograde ,1611.623 conifers 18. 19.23,1 . r ' n i l c r r l u l . , r h , ' l. 1 ? . 4 as: e ea l s ol i g n i n connatewater 353 conodonts l6 Conroecrudeoil. Texas 392 contlncntal environment, of deposition 445 m a r g i nr.i s e s. h e l fs. l o p e 2 , 1 , 2 5 , 3 1 . 5 9 . ,r42.645.646.654,662 61. 151..1,11. active 4,17 p a s s i v es.t a b l e , 1 4 35. 9 9 .6 , 1 56, 4 7 , , rg i r n i m c J t t e r\.e et e r r e \ t r i aolr g a n i c mattcr plants.seeterrestrialplants conversion factor.organiccarbonto organicmatter 496 Cooper.Basin.Australia 389 c o o r o n g i t e8 8 . 1 , 1.11 5 1 , 2 5 6 . 2 5 7
684 c o p e p o d s2 2 , 2 3 . 4 5 , 4 8 . 4 9 c o r es a m p f e s5 7 4 correlatlon index.CI 377,378 oil-oil.oil-source rock 330.333.339. 4 0 7 . 1 2 4 , 4 5 9 . 5 4587 0 . 6 1 4 , 6 2 3 coumarylalcohol 44 CPI, seecarbonpreference index CR/Cr 513,522.523 cracking 175.180,181,190,193.200,201. 205,206,217,218.215,388,389,.160. 586 cratonic.pericratonic 642.653.660 criticalheight.ofoiiorgascolumn343. 314,3.19. 350 crudeoil, seealsooil, seealsopetroleum classification415 423 composition 330,332.335.375114. 172 correlation.seecorelation c u t i c l e s4 3 , 5 0 . 2 2 9 , 2 3 7 cuticularwaxes 102 c u t i n . c u t i n i t e4 2 . 4 3 . 4 9 , 1 3 9 . 5 0 4 . 5 2 0 cuttings 573.574 gas 527.528 C1-anophyceae, blue-greenalgae 21. ,10 c y c l i cs e d i m e n t a t i o n 231 c y c l o a l k a n ensa,p h t h e n e s C17.cyclicmolecules 561-563 i n b i t u m e n 1 8 7 .5 3 3 , 5 3 4 incrudeoil 382-384.390-393.416122.464-466 formationof l1l3 cycloalkvlaromatics. seenaphthenoaromatics cyclobutane 392 cycloheptane392 cyclohexane. methylcyclohexane 95.98. 390.392 cycloparaffins.seecycloalkanes cyclopentane.methylcyclopentane 95. 9 8 .3 9 0 , 3 9 25. 3 0
SubjectIndex
D e e pS e aD r i l l i n gP r o . i e cDt ,S D P 9 5 , 9 6 , 1 1 6 1. 2 3 1 . 5 5 , 1 5 61.9 7, 2 0 2 , 5 3 0 . 6 6 2 , 663 decpseasediments. hydrocarbons in 96. 326.333 d e g r a d a t i o n o f c u r d e o3l l9 1 , 4 2 2 , 4 2 3 . 159.163-461,4'/0,478-480, 614,620 degraded oils 391.418-422.170,474 dcgradedoils.correlationof, see biodegraded oils dehydration.ofclayminerals328,329 dch,vdroabietic acid 117 deltaarcas 327.355.362,605,615 624. 6,16.6.17.660 densir)' ofcrudeoil 342-344,37'7,411.113, seealsoAPI gravityand specificgravity changewith depthand age 451 457 ofheavyoil andtar 471.475,176,478. 179 ofshaleoil 261 263.101,4'71.,472 of water 303,304 depth 85.178-180.215,222,223.327, 450 ,154..160. seealsoburial of oil and gasfields 450-452 Desulfovibrio 77,78, 446 d i a s e n e s i s1 1 , 6 9 . 7 0 . 7 1 . 7 5 , 9 0 , 9 4 , 1 4 1 , 1 7 41 . 81.200.201.203.215.216,305, 360.,150.543.5,14.5,16 carlv 70 e n do f 7 1 of kerogen 162,170.1'14 diamerers of molecules 206.306,309 of pores.seeporediameter diastcrane. diasterene 121 diatoms 15.16,21-23.27.32.31.15-4'1 , 50. 5l dibenzofuran 401.403 dibenzothiophene.seeb€nzothiophene cliffusion l o s s ebsv 3 3 9 .3 4 0 . 4 6 7 , 4 6 8 i n m i g r a t i o n 3 0 9 , 3 1 0 . 3 13 82 0 , 3 5 1 . Darcy'slaw 301.605.606 637.63u Dariuscrudeoil. Iran dihvdrophvtol I 13. I 14 deasphafting459.461-463.'167.seealso DinoJlagellares15.16.21 23,27,45,47, asphaltene separation 5l decalin 390 dinosterol 122.125 decarboxylation106.107,114.192,193. diploptene 39 204.245,426.433 dismigration 293 DeepBasinof W. Canada 628 639 disproportionation,160,461
Sub.jectIndex
685
of migration 3,17.351.355.,179.cukaryotes 15. 114.116 distances 180.614.620 euphoticzone 10,25, 26. 59 Europeanbadistillate 376 European.North-west sin 206.212.251 d i s t i l l a t i o n1 9 1 , 3 7 6 . 3 7 7 , 4 1 5 diterpanes. diterpenoids 37. euxinic 6l l. 612 diterpenes. 116.117.250.'126 elaporationof hydrocarbons465 d o l o m i t i z a t i o n3 0 5 e!aporite 106.305.433.,1,17,611,614. Domanik formation. USSR 6.13 611.61'7 ofn-alkanes 106 DoualaBasin.Cameroon 88. l,ll.l48. evenpredominance. 1 . 1 91.5 2 , 1 5 41.6 1 , 1 6 81.7 8 - l r J 01.8 2 . 1 0 8 . 1 3 2' 1. 3 3 . 5 3 3 . 5 7 0 1 8 3 , 2 1 9 . 2 2 0 . 3 4 8 . 4 5 1 . 5 1 1 . 5 1 2 . 5 1e 7v .o l u t i o n of the biosphere 14-20 525.601-604 path d r a i n a g e a r e a3 5 3 . 3 5 4 . 4 8 1 . 6 1 5 d r y g a s f o r m a t i o n1 6 7 . 1 7 5 . 2 0 0 . 2 0 5 . 2 1 7 . o f a s p h a l t e n e s , l 0 8 o f k c r o g e n 1 5 1 -1 5 8 . 2 1 82 2 1 . 5 1 1 , 221 512 stateof plants 230 EastShetlandbasin,North Sea 539.5.10 evolutionary EastTexas 362.464 exinite 153157.237.seealsoliptinite effusion 351 experimental cvolution Eh,oxidation-reduction.redoxpoteno f k e r o g e n 1 6 9 1 7 4 .1 9 3 - 1 9 6 , 5 9 0 tial 59.70.'17.232,233 of pure compounds 192.193 electron 509 of sedimcnts 193 diffraction 139.1,12-1,15. gencrationof hydrocarbons192-196 ESR. 508,523 525 spinresonance. exploratron elementalcomposition philosophy 572 of biomass 5l targets 571 of coal 235-237 well 573 o f k e r o g e n l . l i ) .1 , 1 1 . 1 5 1 , 1 6 5 . 1l 7 l0. . cxpulsionof oil 252.336 508.5l l. 523,52,1 of resinsand asphaltenes,10,1 exsudatinite 503 extract.secbitumen ellagicacid ,15 oils.seeheavyoils Elmworthgasfield,W. Canada 629-638 extra-heavy Emba.South.USSR 447 560 facies Emeraudeoil field, Congo .121.,17,1. characteristics. ofcrudeoil andbiof deposition 11,12.21 -30. environment tumen 568-570 1 0 6 ,1 1 4 ,1 5 3 . 4 2 1 . 1 2 61 3 3 . 1 1 0 - 4 1 9 organic.in source-rocks572.662,see studies ,137 environmental alsokerogentype andpalynofacies enzymes 31. 32. 76 sea 443.651.65,1 FalklandPlateau 656 epicontinental farnesol 37. ll4 epimetamorphism 72 Faskcncrude oil. Texas 3[]4 Equisetum 101 ergosterol ,10,41,46.47.95. Fastnetbasin.Ireland 529 ergostane, fats 3,1.35 562 E r m e l oo i l s h a l e sS.o u t h A f r i c a 2 5 7 . 2 6 3 . f a t t v a c i d s3 4 . 3 5 . 4 64 9 , 7 9 - 8 1 . 1 0 0 . 1 0 6 1 0 8 .l l 0 . 1 9 2 ,1 9 3 . , 1 0 3 . 4 3 3 264 iso-and anteiso- 46,47 oils 36.,19.110.111 essential unsaturated ,19 ester.esterbonds 84. 14E.15,1.172 faults 355.360-363.617,620,623-621 estradiol,estrone 124 e t h a n e 7 9 . 1 9 9 . 2 0 2 , 3 1 1 , 3 1 9 . 5 2 9 , 6 3 6 f e r m e n t a t i o n7 9 ferns 124,524 etherbonds 1.18,154.404 fichtelite 116.117,390 eubacteria 11,1
686
SubjectIndex
fingerprintpattern 549 dissolved in formationwaters 333 335 Fischerassay 260 distributionin sedimentary basins 213. fish 53 214 FloridaBay 108 formation 180.199 221,245 248, fluid 328.628-638 flow 301,304,309,3111 inorganicorigin 207,208 modelsof 348,578,605 non-hydrocarbon203,204.206-208 pressure! seepressure / o i l r a t i o ,G O R 3 5 3 . 3 5 , 1 , 4 6 2 . , { 6 3 polvphasic 310.605 fluids.monophasic, p h a s e m i g r a t i o n3 3 4 , 3 3 7 fluoranthene 126-127,393 reserves 6.19.650,658.662 fluorene 393.394 sourcerocks 2I9 -221. 251- 252 fluorenone ,101..103 Gato Ridgecrudeoil, California 402 fluorescence13,1, 136.137.139.2.13.:198.g e n e t ipco t e n t i a l 1 5 9 , 2 1 82.1 9 . 5 I1- 5 1 3 . ,199.502 50,1,506.507.519.520.5,13 589 59i foraminifera 16. 17,50 geochemical fossils.biologicalmarkers lormatronwater.seewater 93- 130..121137.527.5.18550.55.1. fossilization of organicmatter I I 13.2l 555.55E.562-566 fractionsofcrudeoil. keyfractions 377. a c y c l i ics o p r e n o i d s1 1 1 1 1 6 378.,115.,116 b r a n c h eadl k a n c s 1 1 0 .l 1 l franboids 136.l3lJ d i t e r p e n o i d s .I 1 6 . l l 7 l r e r r a d i c a l : .l 6 : . I h l . l q l . 5 2 . l i 2 5 from asphaltenes130.,107, 551 friedelin 120.121.123 f r o m k e r o g e n 1 2 9 .1 3 0 . 5 5 1 Friggfield. North Sea 210.212 indicators of despositional environfringelite 129 ment :126 ,132 fucosterol 4l, 47 of earlydiagenesis432.,133 fulvicacids 82-88. 91. 9,1 of thermalmaturation 433-436 functional in oil-oilcorrelation 5.18-550. 551. analysis of kerogen 142 5 5 5 .5 5 8 groups.in kerogenandcoal 1,18. 170. in oil-sourcerock correlation 562-566 112.173.238,239 n-alkanes 100-l l0 fungi 16, 17. 14.233 porphyrins 128.129 fungisterol 40 significance,12,1,425 F u s h u no i l s h a l eP. . R . o f C h i n a 2 5 9 . 2 6 2 stcroidsand tritcrpenoids I l7- 126 fusinite 139,2,15.504 g e o p o l y n e r s7 0 . 7 5 . 9 1 g c o \ t a l lpc r c 5 \ u r el i.l h o s t a l i cf -e.l r o . l J l i c Gabon 108.427.430.4,15.51.1 299.300 gallicacid 45 geothermal Gambacrudeoil. Gabon 445 flux, seeheatflow gammacerane39.390 history 599.6(X) gas gas 2(17 b i o g e n i c 7 9 . 8 0 . 8 9 , 9 0 . 2 0 0 - 2 0 3 . 2 1 5 , gradient 215.216.223.224,296298. l 0 r . 1 r , 4 s 0 .J 5 l . - 1 b 0 . 5 7 . 2 . 55q9.6r ._ 216 cap 353.354 60.1 carbonisotopecomposition of, see ancient 60| - 60,1 isotope geraniol.geranicacid 36, 111 chromatography. seechromatography Ghawaroil field. SaudiArabia 610.61,1 massspectrometry. GC-MS436.437 giantfield. giantprovince 656 661 composition 199 g i b b e r e l l i n s3 7 . 3 8 in cuttings.seecuttinggas Gippslandbasin,Australia 389,647 -floor 213. depthlimit.-destruction. globulesof hydrocarbons310 2 1 , 12. 1 8 Glossopteris flora 231)
SubjectIndex
b8i
kerogen -198.503.505 herbaceous (N. S. O.) 399-108 heterocompounds g e n e r a t i o n1 8 9 . 2 1 7 i n m i g r a t i o n 3 1 2 .3 1 3 , 3 3 3' .1 0 8 heterocycles in asphaltenes ,104,405 in humicacids 82 in kerogen 1,17,1tl8 530 hexane.hexaneisomers 385 3119. hexene 31i5 abnorof fluid.overpressure. highpressure 30(1.302. nralpressure.excesspressure . 1 6 6. 2 1 - 6 2 36. . 1 7 3 l 1 .3 2 1 3 . 2 i r6. 0 5 6 0 7 6 crudcoil. seesulfur high-sulfur high-waxcrudeoil. seewax higherplants.seeterrestrialhigherplants of vitrinitereflectance2.!1. histogram 215.516.519 llodonin crudeoil. Czechoslovakia392 h , ' f ' a n eJ U . J h . 5 rl rl 7. . l l x . l 2 l . l : : . se 1 2 11 . 25..126.,129.,1311.,1 3e 'l'136. alsobactcriohopane . 90 h o p c n e 1 1 7 .1 2 4 3 hormons 125 nu m l c acids u2-88. 91.9,1.158.240.442.503 coal.seecoal kerogen.organicmatter 134-136.158, 233.240.2.13.498.499.503-505 humification 229,234, 231 h u m i n 7 4 .1 J 2 . 8 5 , 8 6 , 8 8 . 9 1 ,19341. 230. 234,23'7.210,243 huminite 620 623 Handiloil field. Indonesia hydrobarbon IlarmattanEastcrudeoil. Alberta 396 changewithdepth 180 composition, Algeria 225.226.316. llassiMessaoud. 2 0 0 . 2 0 5 ,2 1 5 .2 5 0 ,2 5 1 . 4 5 1 188. 3 2 7 3 2 93 . 3 9 . 3 8 5 . 5 9654. 3 . 6 5 2 . 6 5 3 . formation 93.94,176-188,201206, 660 2 1 5 2 1 8 . 2 , 1 82 5 2 HassiR'Mel gasfield. Algeria 660 gas.seegas.seealsolight hydrocarbons H/C. O/Cdiagram.seeVan Krevelendia in crudeoil 376 379 ol identification composition gram.seealsoelemental carbonratio 96.97,seealso i'organic kerogen ratio hydrocarbon heatflow 296.297.596 598 ,183.6'11. phasemigration 320-323.334.33rt heavyoils ,119423.,165.,170 3t9 6,16.6,18.649, 657.660 643.6,1,1. ratio 177 180.223.526 API gravityof 171.415 179 ofsaturated 186.187,391. ringanalysis heightof oil or gascolumn 343 345.637 392 heliumin naturalgas 199.207.20il t l , p e d i s t r i b u t i o n1 8 3 .1 8 8 . 3 u 2 - 3 8 4 h e m i n 1 2 7 ,1 2 8 hvdrocarbons t l e m p e l d i s t i l l a t i o n4 1 5 inancicntsedimen9 t s1 . 9 4 . 9 5 ' 9 7 . 9 8 . heptane 384.386-31i9 seealso geochemicalfossils isomers 386.3i18 in coal 24il-251 value 530
g l u c o s e3 . 3 3 glutamicacid 32 glycerides 3,1 glvcerol 3.1.35. 114.I l5 e t h e r s 1 1 4 .I 1 6 gl),cine 32 GoldenLane.Mexiko 360 Gondwana 230,232..264. 615 graben 289.297.597.600.603 graphite 72. 201.235.2,11 g r a p t o l i t e s1 6 .1 3 6 gravit)'.seeAPI and specificSravitt segregation462.163 GreatPlainsof Canadaand United states 202.204 '133.4'17 Greecc l(16.,121. shales 118.122. Riverfbrmation. C;reen 1 4 0 1 , 1 2r .5 2 .1 5 3 .1 6 1 1. 7 9 . 2 5 52 5 9 . 5 .1 2 .5 l l 1 2 6 1 .2 6 3 .2 6 5 . 1 2 0,.1 2 8 . 4 3 0 g a s f i e l d L . 2 12.328. 2 0 6 . 2 1 Groningen 339.360 Grosnvicrudeoil. USSR .103 growthfaults 327.355.362.617.620.6l.l . 0 2 . 3 2 73.3 5 . G u l f C o a s t 8 4 . 2 9 13i .0 1 3 3 5 53 . 6 23 . 6 3 . ' 1 2 1 . . 144511. . 1 5 36.. 1 6 . 6.17.658660 G u l fo f M c x i c o 5 7 . 5 9 . 6 0 . 9 6 . 9180. 1 . 6,15,655 118.203.326.327,341J, gymnosperms19.20 gypsum.seeevaponte
688 in crudeoil 382-397 in cuttings 527-530 in deepoceanicsediments 95,96,202, 326. 333 in heavyoils 472,475 isotopicratios 190-192,200-203,205, 209-211 in naturalgas 199,20I-205,209 211 in organisms 36-39,41,46-49,1101 1 2 ,1 1 4 -1 1 5 ,11 8 ,, { 2 5 in Recentsediments80,81.94-96,98; seealsogeochemicalfossils in sourcerocks 97, t76-188,527-540 fluidfl ow, hydrodynamic hydrodynamics, gradient 341,3,14349 hydrodynamic interhydrogenbondsin asphaltene-resin action 406 hydrogen content in asphaltenesand resins 404 in coal 234,236.2,10 , 4 1 , 1 5 1 - 1 5135. 7 . i nk e r o g e n 1 4 01 158,162.163,166 index,of kerogen 510 512 oigin in natural gas 206,207 453, sulfide 78,79.199,204-207,448, 455 transfer 461 hydrolysis 32,33 in sediorganiccompounds hydrolyzable ments 80. 85, 88, 233 hydrophilic,hydrophobic 311 hydrostatic 299.300.341.342.345.346 hydrotroilite 79 h y d r o x ya c i d s , 1 2 ,4 3 .4 5 , , 1 9 hydrouspyrolysis 196 igneousintrusion 197 , 28,329 i l l i t e 1 9 63 Illizi Basin,Algeria 221,224,348,349. 556.557.595.596.603.652.653. immature crudeoil 441,450,477 stageof evolution 163,215,216.495. 517.518,521,533,545 indane 396 indole 402 389,409,419 Indonesia 108,109,22,1. 421, 440,441, 444, 445,64'/ 244, ine inite 139,155.237,240,242 .{98-500,503.504
Subjectlndex i.fraredspectroscopy143.146.156.167. 1 7 1 1. 7 3 , 2 2 0 , 5 0 8 injectionpressure 342 seecombustion in situ combustion. insolubilization85-89, 91 interfacialtension 310. 3.{2-344 internalsurface 306,307 intrusion 146,197 Irati shale,Brazri 257, 258,262,264 iron I34,136, iron sulfides 6-8,70,79,12'7, 138,447 oxides 72 47,109. isoalkanes(2-methylalkanes) 110,385,388,389,428,430 seearbo nol isoarborinol. isomerization 434-436,537-5'10 isomersin crudeoil andsediments 3E8. ,134,537-540 isooctane 385 i s o p r e n e 3 4 . 3 6 ,1 1 1 isoprenoidacids 403 i s o p r e n o i d s, 1 ,1 0 7 . 1 1 11 1 6 ,1 2 9 1, 3 0 . 182,185,186,250.332,389,425,126. .166,533 in biodegradation 466 isotope andmefractionationduringcatagenesls tagenesis 189-192 ratio ofbitumenandcrudeoil 189,350, 3 5 1 .5 , 1 85, 5 0 .5 5 1 ,5 5 3 .5 5 4 . 90 192. o f g a so. f m e t h a n e 8 9 . 9 0 1 199 203,205.209 212 . 89.201.508.548. ofkerogen 89.901 551 of organicmatterin youngsediments 89. 90 isotopes o f c a r b o n 5 1 , 6 1 . 1 i 9 . 9108, 9 1 9 2 .1 9 9 203.205.202 912,350.351.463,508, 548.55t|.551 of h)'drogen 89.90.190.191.200,548. 550 of nitrogen 206 of sulfur 548.550,553,554 Italy, North. seePo Valley Joboheavyoil, Venezuela 475 JohnCreekcrudeoil, Kansas 386
Subjectlndex kaoiinite 329 thermalanalysis 147,16g,169 kerogen type 151-159,181-lB3,219,220,257 . aliphaticchainsin 147.148,151,153. 426.441_443.445,497_514. 517_519, 154.156 589.591,592 amorphous 136.158.498,499,503vanKrevelendiagramof 151.152.155, 505 158,161, 257, 429, 5I2 aromaticsheetsin 142-144,14i,t48 weightlossof 1i0. 17I, t't3 catagenesis163,165,170,l'15,216,21j Kerosene shales, Australia 257,25g,260. classification151-156,498 262,264 color 167 key fraction 416 concentrates132,135-137 kinetics 539,542,546,584-587 correlationto crudeoil 548,551 Kirkuk oil field. Irak 360 definition 131.132 Kivu. lake 79 diagenesis162.165,170.1i4, 216 Kukersite.ussR t41. 257,258.261,263 electrondiffraction of 142-I1S Kutei basin,Indonesia 621 eleme n t a lc o m p o \ i o l no f I 4 0 .l 4 l Kuwaitcrudeoil 409,421.464 1 5 1 ,1 6 5 ,1 6 6 ,1 7 0 .1 7 1 evolutionpaths 15l 155.216 Lacq field. France 210,212 experimental evolutionof 169,174 lacustrine basin,lake 151,231.259,259. formationof 59, 88. 90. 91 443,145,624_626,628 functionalanalysis of 142,15:l lagoon 6l , 78, ,147 groupsin 1,12,147 149,\54, 1j2 La Luna formationandcrudeoil, Veneheterocycles in 1.17,148 zuela 339,410 hydrocarbonyield of different tvDes lamination. in organicrichsediments 92, 181-183,219.220.440,442 259 infraredspectroscopyof143.1,16. 155. lanosterol 39.40 156,167,17]l 173 leaves 45, 49, 234 internalsurfacearea 307 Leducoiifield.Canada 224.22i,461. isolation 132,133 6.13.652.653 matnx as a trap for biologicalmole_ leveloforganicmetamorphism. LOM 71, c u l e s 1 1 6 .1 2 9 .1 3 0 .1 8 6 .1 9 5 , 5 5 1 515.542-544 maturationof, seekerogendiagenesis. Libya 365.416,645,657,658 calagenests, metagenests light hydrocarbons450,527-530,57,1 metagenesis165,166,172,1j6,216lignin 3l , 32.37, 42, 44- 46,50.220,233 , 218 142. 443 microscopic constituents of 133 lj9. lignite.seebrowncoal ,198 505 lrmestone assourcerock 496.seealso nitrogenin 141.1,19 carbonates n u c l e i 1 4 7 .1 4 8 .1 5 1 .1 5 3 ,1 5 4 lrmonene I I I rn oil shales 255 25i.262. 263 linoleicacid 35 oxidationof 140.142 Linvi basin.P. R. of China 624 628 oxygenin 1.11.1.18,149 l i p i d s 3 1 ,3 3 .3 5 .4 5 4 8 ,6 0 ,7 6 ,9 1 ,9 4 .9 8 pyrofysisof 142.170-Ij3, 193_196 liptinite 139,237,242,243,246,252. 1gB. 551.seealsopyrolysisand RockEval ,199.502_504.516,518,519 in relationto coal 157.15g.241.252 lithification 30,1.305,334 iD relationto humin 74, g9.89. 131 Lloydminster heavyoil. Alberta 476. sourceoffossilmolecules 129.1g6, 478, 479 1 9 5 .5 5 1 Logbabawells329.seealsoDoualabasrn \ t r u c l u r e o f I 4 7 - t 5 0 .l 7 q t ? 6 , 2 3 R _ LOM. ree levelq[ 619.r1.r.,amorphism 239 L o p a t i nm e t h o d 5 8 4 . 5 8 5 sulfurin 141,150 L o u i s i a n a9 6 ,1 0 1 .1 6 0 , 4 - 5 1 , 4 6 4
690 LosAngelesbasin,California 103.lTll 1 8 0 ,1 8 4 . 2 2 2 , 2 2 3 . 3 3 5 . 3 6 1 . 6 5 3 l u p a n e 3 8 . 3 9 ,l l 9 - 1 2 1 . 1 2 3 l y c o p a n e 1 1 2 .l 1 6 . 1 2 6 . 3 8 9 l y c o p e n e4 1 . 1 1 2 L1-copodium 203 lysine 32
SubjectIndex
mercaptans399 M e s o g e a6 , 1 7 . 21. M e s s e l s h a lGee, r m a n y 1 0 8 1, 1 8 1 122.125.141-194,195,251.258.262. 431 '12, meta-anthracite 7 | . 211.213 metagenesis7|,12. 141, 166.172,175, l 8 l . 2 0 02 . 0 12 . 0 52 . 1 0 2. 1l . 2 1 5 2 1 8 , 212 maceral 133.229.230.234.231-239 543.544.546 groups 242.,199 nretals Mackenzie delta.seeBeaufort-Mackenzie in asphaltenes405 basin in crudeoils 408-410 MagellanBasin,SouthAmerica l()'1.105. metamorphism72 .141.4.1.1. .1.15. 419.,129. 431.4,10. 514 meteoricwater 348.463-465 m a g m a l iac c t i \ i t \ .i n t r u : i , , n\ .c e i n t r u \ i o n n r c t h a n e Mahakamdelta.Indonesia 173.252.327. biogenic.seebiogenicgas in coal 245 248 338.605.620 621.646.611 M a i l l a r dr e a c t i o n 8 3 floor 213.214.218 generation of 201 205.207,209-212Malossafield. Italy 209 Mannvilleshales. WestcrnCanada 152. 216 248 isotopiccomposition of. seeisotoperatio 155.421.122 of methane.seealsogas Maracaibopetroleumprovince.Venezuela 657.658 in youngsediments 79.80.90.201m a r i n eo r g a n im c atter 7.51.153.157. 203.216 159.219.385..126.'129.431.,1.10-,M 1 4e3t h a n o b a c t e r i q ' 7 9 . 1 2 8 bacteria 79, 112,115 marshgas 79 methanogenic (MAB) exmass methanol-acetone-benzene balance 587 tract 175.189 fragmentogram538.539.55.1.555. methylcyclohexane 95,387.390 558.559.564 methylcyclopentane 95. 392.530 s p e c t r o m e t r y ( M S ) 3 7 6 . 3 7 8 . 3 7 9 . 5 5 1m . e t h y l p h e n a n t h r e n3 e9 3 index 534-536 560 genemodelof hydrocarbon Mexico 360.363.409.441.411.645 mathematical ration 585 589 micellarsolution 311.312,334.335 micelle 306.311,312,317 m a t u r a t i o(nt h e r m aal l t e r a t i oonr e v o l u tion) ofcrudeoils.seethermalalteration micrinite 243,246.504 maturation(thermalevolution)of microbial kerogen,of organicmatter,of source activityinyoungsediments 70.75 80, r o c k 2 1 5 2 1 8 . 4 9 5 . 5 1 5 . 5 4 1 - 5 4 6 , 5 7 2 . 8 9 .9 0 .2 0 1 alteration,seebiodegradation 575 generageneration ofgas.seebacteria, of sourcerocks.chemicalindicators tion of gasby 523-540 pyrolysismethods 520 523 organicmatter 108,109.114 116.153. 428-430.443 opticalindicators 515-520 populations in peat 233 McMurray,Alberta 422 microfissures321-323, microfractures. MedicineHat gasfield.W. Canada 80. 337.338 335 sea 57. 106.,133.447 microreservoirs522 Mediterranean microscopic methods 133-139,,198-507 Melekessheavyoil, USSR480.481 . 25, M i d d l eE a s t 3 3 9 , 3 5 4 . 3 6 5 , 3 9 8 , , 1 0 9 . , { 2 0 , m e m b r a n e s4 2 . 4 6 , 1 1 41 1 6 ,1 1 8 1 421. 423. 439- 441,443, 447.451,452, 130,431,432
SubjectIndex 455.481.610-615.612.644.645.611. 6,19.657.658.661.662 GreatBritain Midlandcoalmeasures. 252 MidwaySunsetcrudeoil. California 431 migration.seealsoprimarymigrationand secondary migration in composition during 330 changes 333 distanceof. seedistance o f p r t r o l < u md. < f r n r t r o n .2 a l 2 a i vertical 355 basin 207 Mihai field, Pannonian mineralmatrix.retentioneffectof 196 Mississippi delta 98. 108 mobilebelts 6,17.648 modeling geochemical basinmodeling 583-60i1 geologicalbasinmodeling 572.576581 moisturecontentof coal 23,1 MolasseBasin.Germany 209.2ll. 6,17 molecuiar diameters of petroleumcompounds. see diameter sieves 376 s o l u t i o n 3 1 2 - 3 1 83, 2 5 .3 3 4 .3 3 5 weightolresinsandasphaltenes,103. ,105,.106 monosaccharides 32 monoterpenes36 montmoillonite,seesmectite Morichalheavyoil. Venezuela 475 Morondavabasin.Malagasy 558,559 M o s c o w l i g n i t e s1 6 0 . 2 2 3 mosses 124.,125 Mowry shale,Wyoming 177 mud volcanoes 647 m y r c e n e 3 6 .1 1 1
691 longchain.from waxes l0ll. 109 odd numberedCr(-Cr7 105.106.426 c r . - c 1 3 1 o o 1 0 5 .1 9 4 , 3 8 5 . 4 2 6 . , 1 2 8 in oil shalesor coal 104.249-251 in Recentsediments 100-103,108 n a n n o p l a n k t o n1 5 . 2 1 naphthalene393.394 index 533 naphthene naphthenes. seecycloalkanes naphthenic a c i d s 3 1 1 .4 0 3 c r u d eo i l s ' 1 1 6 - , 1 1 9 . , 1 2 1 in naphthenoaromatic hydrocarbons, crudeoil 381-397 Napoformation.Colombiaand Ecuador 645 naturalgas.see8as 474 nickel.in crudeoils ,108-410. Nigerdelta.Nigeria 60,84.103.327.348. 355.362.363.4,11. 616-620.622-624. 6.16.647.658-660 nitrate 23.27.28 nitrogencompoundsin crude oil 402 nitrogen ]n coal 237.24'1,248 in crudeoil ,102.403,,104 i n k e r o g e n 1 , 1 11. 4 9 .1 7 6 . 2 0 6 in naturalgas 199.201,206.20'7.209, 21r,213 in youngsediments 86.87 non-hydrocarbons. seeN.S.O.compounds.asphaltenes and resins gas.seegas.non-hydrocarbons organlcmatter.seeterlestllal non-maflne organicmattef North Sea 210-2\3.291. 348.363,420. 421,440.441,64,1,654,655.658.659 North Smyercrudeoil. Texas 386 Norton Sound,Alaska 317 NorwegianSea 102.108.655 N.S.O.-cornpounds1U9.403408,415, 416,419,420.465.467,4' 75 (NMR) nuclearmagneticresonance spectroscopy146.163 nucleiin kerogen 148.151,153,154 o u t r i e n t s2 5 . 2 7 , 2 8
Nagoakaplain,Japan 326,333 n-alkanes in ancientsediment.andcrudeoils 1 0 0 1 1 01 , 8 2 .1 8 43. 0 6 . 3 0 9 . 3 1 3 1 9 . 384-387, 425-432,471.531 533 biodegradationof 464 466 evennumbered 101,106.107.385.432, ,133 O/C.oxygento carbonratio,seeelemental generation of 182-184 ofkerogenandVanKrevecomposition fossils 100-110 asgeochemical len diagram in livingorganisms 47-50
692 oceanspreading,oceanicridges 597.600, 651.655 water,chemicalcomposition of 27 oceanicbasins geothermal flux in 597.598,600 petroleumpotential of 662 666 (OEP),seecarpredominance odd-even bon preferenceindex Officinabasin.Venezuela 252.445 oil, seealsocrudeoil andpetroleum column,heightof, lengthof 343 346, 349 family 548.552.553 fields,distributionof depth 451,452 formationor generation, mainstage,zoneor principalphase of 163,175.180, 181,277,227,327,336,517 gravity, seeAPI and specificgravity phasemigration.seehydrocarbon phase migratlon reserves distributionof 641-660 resources 657-662 richness 657-659 saturation 353.354 oil shale 254-265,592,645,654 composition of organicmaner 256258,262.263 compositionof shaleo1l 261,-263 density 259 of deposition 258.259 environments oil yield 260 263 organicrichness 255 pyrolysis,retorting 259.260.590,608 ofPaleozoic sediOklahoma,compaction ments 302 o l e a n a n e3 8 , 3 9 .1 2 4 olefins,seealkenes oleicacid 35 onocerin 39 OntarioLake. Canada 87 o p r i c aal c t r v i l.)o p t i c arlo l a t i o n l q I . l q 3 . 464,553 organiccarbon,seecarbon organrcmatter of 3.9. accumulation andpreservation 10. 12, 13,55. 57, 59.60 composition of l3l, 132 dissolved 13,55,57-61 of oceansediments 96, 530,662 665 particulate 13,55. 57-61
SubjectIndex productionof 3,25,seealsocarbon production quantityin sediments 96,9'/ in relation to sedimentgrain size 56 in sedimentation and earlydiagenesis 14-92 sourceof 12.45-53 type of, seekerogentype organisms autotrophic 3, 6, 16 of 45-50 composition heterotrophic 5,6. 16 OrienteBasin.Colombia,Ecuadorand Peru 409,655 Orinocodelta,Venezu€la 98,325 Orinocoheavyoil belt. EasternVenezuela 423.472 181,647 Orinoco-Trinidadpetroleumprovince 658 ostracods 16 overprelsure. seehighpressure oxic.seeaerobic oxidationof methanein nearsurfaceconditions 90 oxidation or kerooforganicmatter,orparticles, gen 139,155-157,500,663 of pooledoil 465,467.478 oxidizingconditions 232-234 oxygen in atmosphere and hydrosphere 7 compounds in crudeoil 401,403 in crudeoil ,103,404 diffusionof 55 indexofkerogen 510-513 in kerogen 141-143.146.148 150, 1 5 4 .1 6 6 .1 7 2 minimum.- verticaldistributionin ocean 26, 28,57.59 in reseryoirwaters 463 in vounssediments 76,77.86 PacifiO c cean 58,76,80,81,326 paleogeography ofsourcerocks 572,575, 655.662 666 paleohydrodynamics 347 paleotemperatures524,525,601 604 Paleozoic sourcerocks 225-227,643, 644,652.653 palmiticacid 35, 47, 49 palynofacies136
Subjectlndex
693
palynomorphs, carbonization of 515 petrostaticpressure!seegeostaticpressure Pannonianbasin,CentralEurope 184. pH 7O,232 185,207,224,225, 419, ,140, .141 phenanthrene393.394,534 , 451, ,152,539,540,597,599,647 phenols 49 Panuco-Ebano,Mexico207 phenylalanine193 paraffin,seealkane phosphate 23,26-28 paraffiniccrude oils 116-419,123,441, phospholipids31,42, 16 48 445.620.628 Phosphoriaformation, Wyoming 6rl3 -naphthenic crudeoil 416 120,123, photosynthesis3. 12 .1.11. 61,1 physiography4,13 paraffinicity. 351 phytadiene 11,1 paralic 231.,145 phvtane 111-I 14.25U.389..166 P a r i s b a s iFn r. a n c e 1 0 9 1. 2 3 .1 2 5 1. 2 7 , predominance overpristane 106.107. ,132,433.568 1 3 3 r. 3 , 11. 3 6 1 . 3 7 1. ' 1 01. , 1 1 , 1 4 8 - 1 5 - . 1 5 2 1 5 . 1r .6 2 , l 7 l . 1 7 6 -1 8 31. 8 6 _ 1 8 8 . phytanicacid I I,l r93 196.219.220.222,223,256 258, phvtobenthos 22 261.262.265.330.3.18, 420.440.441. p h y t o l . 1 . 3 7 , 9 81. 1 2 -1 1 , 11.2 8 1 4 ?5. I I . 5 1 25. 1 7 -5 l e .5 J 25. . l S5. J phvtoplankton. plankton l,t, 15,20.23. 594.595.603.653.660 3 1 . . 1 54.7 .5 1 .9 8 ,1 2 2 1 , 5 3 . , 1 2464. 3 5 . 03. :122..172. PeaceRiverheavvoils.Alberta 505.6,19.651.654.655.662 665 ,179.,180,.182 Piceance Basin,Colorado 259 peat 70. 71. 229-235,211 p i g m e n t s3 4 , 4 1 . 4 2 pectin 33 plankton.seephvtoplankton andzooPeedeeBelemnite,PDB 89 plankton Pembinacrudeoil, Alberta 396 plants,higher,terrestrial. seeterrestrial pentacyclictriterpanes,seet terpanes high€rplants pentane 388,529 platform.stableplatform 599,609-615. peptidebond 32,83, 86, 149 643,645.648.653-655 Peridineans. seeDinoflagellates Po Valley.basin,ltaly 203,209.211, permeability 294.297,301,305,358,359 335 P e r m r aB n a s i nW . e s tT e x a s 2 0 5 , 2 U 7 polarcompounds in sediments andpetro347.398.409,,121,,155,5,19 l e u m 1 2 9 , 1 3 03,1 1 3, 1 2 3 . 3 0 3. 3 3 3 , 50, perylene 126.127 437 petroleum.seealsocrudeoil and alsooil poflen 17.49.50.57, 136,137. 167,231. accumulation351-354.360 365 5 0 3 - 5 0 55, 1 5 r a v o r a ozl eo n eo l : / 1 , l / 1 . : / : pollution 434,437 alteration 459 ,168 pollaromatichydrocarhons. ptrlyclclic composition, changewithageand (PAH) 126. aromatichydrocarbons depth 451-,157 127.394.560,567 distribution. ageandgeotectonic 6.12polycondensation8l-85, 234,237,245, 656 216 formation polyhvdroxyquinone129 d e p t ha n dt i m e 9 3 , 1 8 0 , 2 1 5 . 2 2 2 polymerization 81, 90, 93, 191.234,237 227 polysaccharides3. 32, 80. 91 generalschemeof 215 218 polyterpenes 126 habitat 610,640 PoncaCity,crudeoil,Oklahoma 11,. p o t e n t i a l 2 1 8 . 5 1 1 - 5 1 3 , 5 8 3 , 5 8 8 . 5 8 9 , 376, 377, 386.389, 392 s93-596 PorcupineBasin,lreland 566,568,569 p r o r p e c t l.o. c a t i nogf 5 7 1 5 7 1 . 5 " 5 . pore 593 596.608 diameter,radius 306-308,310,3.12reserves. resources 657-662 346.358
694
SubjectIndex
watel Qatarcrudeoil 31i5,421 tiee and bound 307.308.3l I Quassmethod 132 salinity 300 quaternarvcarbonatonl 388 porosity 294.297.301,30 32 0 .4 . 3 0 6 . 3 0 7 . quinoline 127.,101.402..104 358 360 Q u i r i q u i r e o i l f i e l d , V e n e z u e3l2a8 . 3 3 9 . primaryand secondarv 305.359.360 35{t.351 porphin '1.,12.127 porphyrins .12.128.129.-l50.351.,l'ft'. ,110.435.436.539 racemicmixturesof hydrocarbons464 pour point 413 radioactivit! 207 PowderRiverbasin.Wvoming 363.36'l Radiolorii 16.27 p r e s s u r e1 9 6 . 2 2 3 . 2 9390 3 . 3 0 6 . 3 0 1 J - Rainbowareii.W. Canada 365 .11 l . . 1 . l 7 . e e a l s o a h n , r rJm rank of coaliiication 231-211.213.245 r rr(l\ r u l c pressure. geostatic pressure. capillaD/ rate of chemicalrcactions 58,1.585.592 hvdrostatic ratios.seebilumenratio.hvdrocarbon Probistipoil shale.Jusoslavia 502.506 ratio. transformation ratio primar) migration 293.29,1.296I'lt) recrvstallization305.360 . h r r n g tcr f, . o m p t r s i t idounr i n g . 1 . r 0l l l Red Sea 22,1.597.6(X) i n c o l l o i d a l omr i c e l l asro l u t i o n 3 1 1 . Red Washcrudeoil. Utah 396 312 reducingconditions.environments 106. diffusioncontrolled 318 320 I l l . l 5 l . 1 5 7l.5 q .1 3 2 . 2 1 4 . 4 3 . 1 .s6e5e5 . gasphase 337.33ll alsoanaerobic hydrocarbonphase320-323 reductionon sulfate 11.17 79- 146 449 in molecularsolution 312-318 reef 364.365.,161,462 time and depth 325 329.335-337 reflectance principalphaseofoil formation.seeoil ofhuminiteandvitrinite 70-72. 141, formation 161- 163.166-168.209.212.234 231, pristane ,19.98. I l l 114,250.389.466 211 215.249 252.499.505.506.5lb isomers ,134.538. 539 520.522.53t).539.543.54,1 to phytaneratio ,129,434 of exinite/liptinite andinertinite 162. productivity. primary 21.23 25.28,29. 1 6 3 .l ' 71 . 2 4 2 .2 4 3 .4 9 9 .5 1 6 .5 1 7 .5 1 9 56.57 petroleumprovince. Reforma-Campeche p r o k a r y o t e s1 5 . 4 11. l l l .1 2 2 1 . 2 5 1. 3 0 . Mexico 657 660.662 '125.130 refractiveindex 377 proline 32 r e g r e s s i o n5 6 . 6 1 p r ( ' p r n c ' q . l q q .2 0 2 . 1 1 1 . 3 1 q . 5. 2 , reserves. resources 5i13.60i1.657662 63,1 rcscnoir prospects, seepetroleumprospects bitumen 463 proteins 31.45. 46. 79. 233 m i g r a l r o inn . r e r s e c n n d a rmv i g r a t i o n PrudhoeBay. Alaska 563-565.660 rock 357-360 Psilopsida 18. 13 rcsiduc,solid.in reservoirs '161 Ptttidoph:-ta 230 resinite.resin-bearingtissues 23,1. 252. pycnocline 60 50.1 pyrene 126.393 resins p,vridine 127.,101..102 c o n s t i t u e n t o f c r u d e o1i l8 9 . 3 7 8 - 3 8 1 , p y r i t e 7 9 , 1 3 4 .1 3 6 .l 3 U , 5 0 i J 4 0 3 4 0 8 . 4 1 14 1 3 . , 1 1 6 , 4 I i 1 . 4 2 0 - , 1 2 3 , pyrobitumen ,160.463 172 176.418.,179. 504.505 pyrolysis 130.142.190.255.,107.509 organicparticies fronrplants 37.49. 513.520-523.55r.57,r 23'7 pvrolysis-fluorescence 513 resistivitv of formations 321.612 pyrrole ,12.127.,102 r e t e n e I 1 6 .1 1 7
SubjectIndex retentioneffcctof mineralmatrixin pvro, l)sis 196 retortingprocess of oil shales 260 r e t r o g r a dceo n d e n s a t i o n , 1 66g2. 3 reworkedmaterial 156.237,243, 499,516 RhineGraben.RhineVallev 106.t07. 1t8 . 121, 122. 296. 297, 329. 354,362. '131.4-13.,135. 441.562.597.600.603 Rhonedelta 8,1 rhvthmicsedimentation231 rift ,115.597 ring analysis. of saturatedhvdrocarbons 391.392' Rio GrandeRidge 656.66.1 Rock Eval.methodof pyrolysis 509.522 rock fracturing 31l. 321 323.337 R o c k yM o u n t a i n s2 0 7 .2 1 3 . , 1 5 5 6 .5 j Romania 363.647 Romashkino oil fietd.USSR 660 R , ' t l i e g e ntdo r m r l r o n\.o r l h S e a C . etm a n va n dN e t h c r l a n d s2 l l . 2 1 2 .3 6 0 Roussecondensate field.France 566.567 Ruhr area.Germanv 535.569
695
g e n e r a t i oonf 1 8 1 - l i l 7 .1 9 3 1 9 5 . 2 1 5 . 141.412 saturates/aromatics ratio.changewith depth 187 saturahonpressure 354 SaudiArabia 421, 447- 610-6t5 sclerotinite 50,1 Scotiansheif 8,1 screening techniques for sourcerocks 574 sealevelchanges 651.65,1 seawater 12,25-28 scal.cap-rock 294.3-51. 352.35,1. 357, 362.363,365 secondary migration 293,291,299.3tl 355.572 drstanceof 354.355.,179. 480.61,1. 620 sedimentary basins.distributionof gasin 213.21;l processes55 seotntenlatron rate 12 c v c l e 2 2 5 . 2 2 66. 5 3 sedrments hvdrocarbons in. organicmatterin. see hydrocarbons and organicmatter Saararea.Germany 2,19.252.29ll methaneln young- 90 S a f a n i voai l f i e l d .S a u dA i r a b i a 6 1 0 . 6 1 . 1 seeds ,19 S a h a r aN. o r r hA f r i c a l l 7 . 1 2 0 t. 2 - 5 . 1 5 2 . sceps 1 5 3 t. 6 i . 1 6 2 1. 6 5 _1 7 0 1. 8 6 . 2 2 5 _ 2 2 7 . I n C ' ) i l lm t n C S 2 l l 330.-.13.s.3,17. -178. -5rl .512.557.6,13. of gas 317.326 653.651.651t.660 of oil 326.465 salinitv senrifusinite2.15.504 of porewarer 300 separation-migration -355.,1611 of seavater 25.27 sesquiterpenes37. I 12. I l,l satt 2t2..226.221.297.299.362 36,1. 650 s e s t e r t e r p e n el ls2 . l l , 1 domes 362.363..165 shaleoil 260-263.401.1'71.1i2 Samotloroil field. USSR 660 shales sampltng program.for sourcerockidentifi_ compaction and porositv 301 30,1 ctt|on I /.t d i f T u s i oi n 3 t 9 . 3 2 0 sand pore drameters in 306.307 sandstone reservoirs 3-58_360 assourcerocks 3311. 339.4.16. 447.616 s h r l ( \ e q u e n C s( .e cc l a s t i sCe q u c n C ( 620.622.623.62.5.626 S a nP e d r oo i l f i e l d .A r g e n t i n a i 2 x sharkliver oil 3ll sapropelic ShatskyRise 326.655 coal 229.230 s h c l f 2 4 .2 5 .3 1 .6 1 1 .6 1 2 kerogen,organicmatter 135.136.13g. Shcnglioil field. P. R. of China 624 158.503.505 Siberia.W. Siberia.USSR 80,202-20,1. Sargasso Sea 29 6,15.654,655.658,659 saturatedhvdrocarbons siciiy. Italy 259.421.147 in crudeoils 379 382.417 sidewall coresamples 57,1 ennchmeDt in migration 332.333 silicoflagellatcs l -5.22
696
SubjectIndex
Simpsonformation 6,13 sporinite 5()4 sinapvlalcohol ,14 sporopollenin 34,36 Sirtebasin,Libya 645,657,658 Springhillformation,MagellanBasin 431 s i t o s t e r osl .i t o s t a n e4 0 , 4 1 , 5 0 .1 2 5 , 4 2 6 s q u a l a n e1 1 2 .1 1 6 .1 2 6 . 3 8 9 s m e c t i t em. o n r m o r i l l o n i t el 9 3 ,I q b . 1 2 8 , s q u a l e n e. 1 83. q , I l 2 329 Sianleyville basin,Zaire 258 soils,gasanalysis in 90 stanols 118,121 solubilityof petroleum StateLine crudeoil. Wvoming 386 in gas 337,338 steaminiection 482 in water 312-315.317 stearicacid 37. 47. 49 solubilization of organicmatter 11, 12 steranes 118 125.186.195,250, 393, sofutionmigration 312-318,335,338 426-428,430,433436,4,11.537-540, sourcerock 558.560.562.564.565628 carbonates as.seecarbonates aromatic,seesteroids,aromatic casehistories ofgassourcerocks 209 /hopaneratio 122.426,429 212.631-637 isomerization.134-436. 537-540 casehistorie sof oiI sourcerock 478. sterenes 119.122,186.389 479,612-614,616-620,622-623,625 stereochemistry, stereoisomersll3, 124, 628 4 2 5 . 4 3 44 3 7 , 5 3 75, 3 8 characterization495,540-546 steroidacid 117,120 122,125,121.131 bvC1.--hydrocarbons 513,51,1, 525steroids 3,1,38.40,41,46,50,117-126, 5 2 7 , 5 3 15 4 0 1 8 6 . 1 8 71,9 5 , 3 9 3 . 3 9 6 , 3, 49 2' 75 - 1 2 8 , by kerogenanalysis 508.509,523 430.,133-436 525 a r o m a t i c 1 1 71 . 1 91 , 2 01 . 23,125,186. b y l i g h t h y d r o c a r b o n5s2 7 - 5 3 0 187.396.397,.125.433.435.546,538. by opticalmethods ,198507,515539 520 i n s e d i m e n t s1 1 7 - 1 2 6 b y p y r o l y s i s5 0 9 - 5 1 3 . 5 2 05 2 3 s t e r o l s 4 0 .' 1 6 . , 1 7 . 5101,8 .1 2 2 1 . 23,433. coalas 251.252 442 ,10,41.46.50. c o n d i t i o nosf d e p o s i t i o n1 1 , 1 3 .1 4 . 6 1 s t i g m a s t e rsotli.g m a s t a n e /crudeoil correlation.seecorrelation 121.426 f o r m a t i o n o f p e t r o l e u m i1n7 6 - 1 8 9 . s t r a t i g r a p h i c t r a p3s5 7 , 3 6 0 . 3 6 4 . 3 6 5 215 217,222.-221 structuraltraps 294,357.360-363 gasversusoil 219-221 structured water.adsorbed on claysurfageneticpotentialof 159.218,5U9.590 ces.seewater.adsorbed identification in basins 572 575 suberin 42.43, ,19 inmigration293.294,308.330-333.33 6 b s i d e n c e1 6 0 , 2 2 3 . 2 2 4 . 2. 51'71 2 . 6 1 6 , su organiccontentof 55,97.495-497 624,626.647,652654 specific rate of 616.642,617.652,653 gravity Suez,Goif of 224.647 of crudeoils 294,4\3,446,449.453, sugars 32. 33, 81 462,463.467 sulfate 7 9. 214.218,465 ofheavyoils 471,472,175,477 reductionby bacteria 77 79,201,201, volumeof water 304 446.147.165,478 spillingplane.spillpointofaccumulasulfides 8.70,79,148,150.399,400,447. tion 355,360,362 465 SpitsbergenIsland, Norway 331 sulfur spores 17,18.34,49.50,136,229,230, aromaticderivatives in crudeoil 395234,237,243,503-505.507 397,399,400,411,172.473 andpollencarbonization, in asphaltenes405,408 color 167, 515,516,543 in biodegradation465-467
SubjectIndex
697
in coal 238,239 104,108,122,128.154,157,159,219compoundsincrudeoil 398-400 221.229,23O,233.234,385,426,431. content 440 443 i n c r u d eo i l 3 5 1 , 3 9 8 , 4 1 1 , 4 1 3 , 4 1 7 . h i g h epr l a n t ss, e et e r r e s t r i oa rl g a n i c 418.420 422,446,448.449.454.,156 matter in heavvoils 473,474 tertiary correlationwith density 413,,149 carbonatom 388 high-sulfurcrude oil 445 ,150 migration 341 incorporation intosediment 77 79, Tertiarydeltas 615 624,646,647,658, 442.447 659 i s o t o p e s ,e ei s o t o p e s Tethys 4,17.611.655,656 in kerogen 148,150,447 tetraethers of glycerol 115,116 origin in crude otl 411.413, 442.447 tetrahydronaphthalene, tetralin 394,396 S u m a t r a , I n d o n e s i4a4 4 . 5 5 4 . 5 5 8 . 6 5 8 t e t r a h y m a n o 3l 9 Sunnyside, Utah 475 tetraterpenes, Can-terpenes41,112,126 surlacearea,internal,specific 307,see thermal alsointemalsurface alteration.evolution 422,423,459 ,163,467,556 suspended matter 56, 57, 59 Sverdrupbasin,N. Canada 528,529 indexTAI 516,543 SwanHills formation,W. Canada 339 analysis of kerogen 147 cracking 190,193,205,seealso TAI, seethermalalterationindex cracking tannin 42,44,45 conductivity of rock 296,297,299 tar 421,422,465.411 historv 596-600 mats 464,610 maturationof kerogen,seematuration sand, 422,423,465,470483 waters 600 tasmanite 134.141,151,256.251.262- thermocline 60 265 thermodynamicequilibrium,isomerization Tasmanites137,162,256 equilibrium 193,388,460 tectonic thiabutane 399 eventsanddestructionofaccumulathiacyclohexane,thiacyclopentane 399, tions 595,652.653 400 orogenicactivityandheatflux 597-599 thin laverchromatography,seechromatography telinite 244,504 Temblador oil field, Venezuela ,179 thiols 399,400 temperature 162,114,222,223,296 thiophenede vativesincrude 299. thiophene. 302-304,306,450,460.596,601,see oil 395-397,399,4O0,41,1,412,421, alsothermal 146,449.472474,478,550,555,556, measurements601 560,566,567 profile 297,298 threshold(depth,temperature)to oil orgas versustime,seetime-temperaturerela- generation 1'19,181,223 tion time terpanes in basinmodeling 586-588,593 generationfrom kerogen 195 of hydrocarbondiffusion (to sudace) in oil correlation 554,555,564,565 339,340 terpenes.monoterpenes34,36, 110,111 ofmigration 320,325 328,461,605, terpenoids 34, 36-40, 46, 50, 251 608 terpinene 36 of oil andgasformatior. 222-22'7,584, terrestrial 590,595-596,608 -temperature or continental organicmatter 7, 17,18, relationship in petroleum 2 0 , 3 1 , 3 6 , 3 8 , 4 0 , 4 1 , 4 5 , 4 6 , 4 9 - 5 2 , 1 0 2 g e n e r a t i o n2 2 2 . 2 2 3 , 4 6 0
698
SubjectIndex
timingof oil and gasformation,seetime vanadium.vanadyl 128,129.402,408Toarcianshales.seeParisbasin 110,471 of WesternEurope 2513, 261.654 vanKrevelendiagram 151,152.157.159. toluene 95, 314.315.393,465 16l.216,219.220.239 241,257,429. t o r b a n i t e1 3 , 1 , 1 5 1 , 2 5 6 - 2 5 8 . 2 6 2 511.512.523 t r a c ee l e m e n t si n. c r u d eo i l 8 , 1 . , 1 0 U - 4 1 0 V e n e z u e l a3 0 2 . 3 2 5 . 3 2 8 , 3 3 9 . 3 5 0 . 3 5 1 . transformation ratio 177.218.219.521. 410.,121,123.,151 39t1..1011 453.472526.590 481.610,615.645.655 658 t r a n s g r e s s i o5n6 . 6 1 , 6 1 2 . 6 2 1 . 6 4 5 , 6 5 1 . V e n t u r ab a s i nC . alifornia179.181.223 654.655 verticalmigration.seemigration.vertical trap 227,294,351.357.360-365.,159. Victoria.Australia 231 572.596 ViennaBasin.Austria,151.452.560.561. formationof 227,575,583.608 647 tricJ'clodecane. seeadamantane Viking Graben.North Sea 291.605,607 triglyceride 35 Viking shales.W. Canada 56. 157 trilobites 16 r'iscositv 301.,107. 41i. ,113. 421.449.,150 trimacerals 139 of heavyoils 4'70,4'71.415-118,482. trimacerite 499.500 483 triphenylene 126.127 vitrinire 139,153.157.230,231.231 244. triterpanes 118 122,124,186.195.25U. 2,16.,198 501,50.1 393.428.,134.558,562 reflectance. seereflectance triterpenes 38.39,42,46.120-125.250. volatilematter 234-236,245-248 393 volcanicgas 207 triterpenoid alcohol.acid 121,124.125. volcanoes. mud. seemud volcanoes 431 V o l g a - U r a l b a s i n4 0 9 , 4 5 5 . : 1 8 0 . , 1 8 1 . 6 5 3 , triterpenoids 38 ,40.117 127.130.186. 65il-660 426 Vokzia 106 a r o m a t i c 1 1 7 - 1 2 01. 2 3 ,1 2 5 .1 8 6 3 . 96 diagenesis of 121-126 Wabasca heavyoils.W.Canada 422,4'72. .179.480.482 troilite 79 Tunisia 106.107.409.433 WalvisBay 118 Tuscaloosa reservoir. Gulfcoast 552. WalvisRidge 656,664 553 Wassoncrudeoil. Texas 376.,100 typeof kerogen.of organicmatter.see water kerogentype adsorbedon claysurfaces.structured water 307.308 boundor fixed,in claydehydration 328.329 U i n t ab a s i nU , t a h 1 0 4 , 1 0 5 . 1 0180, 9 1. 5 2 . 178-180,182,183,219.265,386.389, connate 353 3 9 1 . 3 9 6 , 4 2 0 , 4 2 8 . 4 3 0 , 4 3 1 , 4 . 1 1 . 4 4 4 . e x p a n s i o n o f3 0 3 , 3 0 4 , 3 4 1 flow of 297.331J. 445.474,480. 341-346.352 526.562.626.seealso generation GreenRiverformation of. from kerogen 163.174, unconformity 225-227,347,355,478, 216.219,220 481.482 irreducible or residualamount of 353. unsaturated hydrocarbons, 360 seealkenes meteoric.seemeteoricwater upwelling 25, 26, 29, 59 role in pyrolysis 196 ureaadduction 379 Urengoygasfield, USSR 660 saturation 353 washing 465.,167 ursane 38,39. 124 wax 34,237,384,385,425,442 445,620. U.S.BureauofMines.seeBureauof 626 Mines
S u h j e c tI n d e r
esters 48-50 high-waxcrudeoil 252.384.385.,119. .110.441,459.471 weathering. of organicmatter 156 weightlossof kerogen 168-173 well cuttingsandcores 573.57,1 well-headsamples of crudeoil 376 WestAfrica.westcoastof Africa 86.104. 1 0 5 I, 1 7 .1 7 7 . 3 6 3 . 3 9 1 . 4 0 9 . 4 1 9 . 4 2 1 . 4 2 2 , 1 4 0 . 1 4 1 . 4 5415 3 . 4 6 . 1 , , 1 7 4 , 6 0 0 . 6'15 WesternCanadabasin.seeCanada WesternSiberia.seeSiberia w€t gasgeneration 163.175.200.217 qettahilin.oil-*etoruater-w . l el |l| . 320.336 Whiterocks.Canyonasphalt.Utah,l02. 474 W i l l i s t o n b a s i n . U S A3 2 1 . 3 2 9 . 3 3 5 . 3 3 9 . 553
699 Wilmingtonoil field. California 361.402 wood 32 Woodbine s a n d s t o nT ee . xas 362 Woodfordshale 643 woddvkerogen 498.505 Wiistrowfield. Germany 212.213 x a n t h o p h y l l s, 1 1 xviene 95.314,393,,165
Yallournlignite,Australia 194,195
Zagrosmountains 610 Zechstein formation,GermanyandNorth Sea 106.212 z o o p l a n k t o n1 6 . 2 0 . 3 1 . 4 5 . 5513 , 9 8 , 1 5 3 .1 5 7 . 4 2 6
Tissot/Welte,PetroleumFormationand Occurrence Wriftenby world-renowned expertsin the field of petroleum research,this is the first comprehensive textbookon origin, migration, and accumulation of petroleum. The authorshave not only updatedtheir previoustext, but have added completelynewchapterson gas,heavyoilsand tar sands,distribution of worldpetroleumresenes,casehistorieson habitatof petroleum and geologicaland geochemical modeling. Other chaptersor sectionshave been enlargedand rearranged, suchas thoseon geochemical fossils(biologicalmarkers)primary migration,asphaltenesand resins,coal as a possible sourcerock,newtechniquesof sourcerock characterization, oil-sourcerock correlations.About 270new referenceshave been addedand the index has been considerablyimproved andenlarged. "Few technicalbooks have
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tsBN 3-540-13281-3 rsBN0-387-13281-3
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