Stratigraphy, microfacies, and petroleum potential of the Mauddud Formation (Albian-Cenomanian) in the Persian Gulf basin

Stratigraphy, microfacies, and petroleum potential of the Mauddud Formation (Albian– Cenomanian) in the Arabian Gulf basin F. N. Sadooni and A. S. Alsharhan

ABSTRACT The Albian –Cenomanian Mauddud Formation extends over most parts of the Arabian basin including north Iraq. The formation consists mainly of Orbitolina-bearing limestone with local basin margin rudist buildups in the offshore North field of Qatar and northeast Iraq. Extensive dolomitization, with wide variations in both extent and texture, has been reported from both outcrops and wells. The Jurassic – Cretaceous pelagic strata are probably the possible source for the Mauddud Formation oil in northern Iraq, whereas indigenous sources in the Mauddud strata and Nahr Umr shales, as well as the Upper Jurassic rocks, are probably the source rocks in the southern parts in the basin. Porosity of 10 –35% and permeability of 10 –110 md have been reported from different fields of the basin. This porosity is attributed to a combination of dolomitization, fracturing, and dissolution. There are two main oil provinces where the Mauddud Formation is a major oil-producing reservoir. The northern province includes Iraq’s oil fields such as Ain Zalah, Bai Hassan, and Jambur. The southern province includes the Ratawi field in southern Iraq, Raudhatain, Sabriya, and Bahra fields in Kuwait, Bahrain (Awali) field in Bahrain, and Fahud and Natih fields in Oman. The formation has high oil potential in the southern and southeastern fields of Iraq and the offshore areas of Qatar and Saudi Arabia.

INTRODUCTION The Mauddud Formation is a shallow-water carbonate with widespread distribution in the subsurface of the Arabian Gulf. Originally defined in Qatar, it has subsequently been recognized in the

Copyright #2003. The American Association of Petroleum Geologists. All rights reserved. Manuscript received February 7, 2001; provisional acceptance October 15, 2001; revised manuscript received May 8, 2002; final acceptance April 22, 2003.

AAPG Bulletin, v. 87, no. 10 (October 2003), pp. 1653 – 1680

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AUTHORS F. N. Sadooni  Department of Geology, University of United Arab Emirates, Al-Ain, United Arab Emirates; [email protected] F. N. Sadooni has been an associate professor and chairman of the Department of Geology, United Arab Emirates University since September 2001. He received a Ph.D. in petroleum geology from the University of Bristol, United Kingdom, in 1978. After working with Iraq National Oil as a senior exploration geologist for 13 years, Fadhil joined Yarmouk University, Jordan in 1991 and then worked as a consultant petroleum geologist in Auckland, New Zealand. In 1998, he joined the University of Qatar as assistant professor before moving to the United Arab Emirates University. His research interests include carbonate reservoir characterization and evaporites. He is a member of the AAPG. A. S. Alsharhan  Department of Geology, University of United Arab Emirates, Al-Ain, United Arab Emirates; [email protected] A. S. Alsharhan is professor of geology and dean of the Faculty of Science at the United Arab Emirates University. He received a Ph.D. in petroleum geology from the University of South Carolina in 1985. He has authored and published over 80 scientific papers. He coauthored Sedimentary Basins and Petroleum Geology of the Middle East (1997) with A. E. Nairn and Hydrogoelogy of an Arid Region: Arabian Gulf and Adjacent Areas (2001) with Z. Rizk, A. E. Nairn, D. Bakhit, and S. Al-Hajari. He coedited Quaternary Deserts and Climate Change (1998) with K. W. Glennie, G. Whittle, and C. Kendall and Middle East Models of Jurassic/Cretaceous Carbonate Systems (2000) with R. W. Scott. His research interests include Holocene coastal sabkhas of the Arabian Gulf region and the geology and hydrocarbon habitats of the Middle East and north Africa. He is a member of the AAPG, SEPM, the International Association of Sedimentologists, and the Geological Society of London.

ACKNOWLEDGEMENTS The authors are indebted to the Department of Geology at the University of Qatar and the University of the United Arab Emirates for providing the required technical assistance and to Christopher Kendall and A. E. M. Nairn, who reviewed the original manuscript and made many useful amendments. The paper benefited significantly from an extensive review by James Lee Wilson.

northern Gulf (Iraq and Kuwait), eastern Arabia (Saudi Arabia, Bahrain), and the southern Gulf in United Arab Emirates and Oman (Figure 1). Vertical and lateral facies variations in the formation have resulted in a variety of names that have been compounded further by varying terminology developed by the operating oil companies. The Mauddud Formation is an important rock unit in terms of regional distribution and thickness all over the Arabian basin. Furthermore, the formation has produced large quantities of oil in northwest Iraq and Bahrain since the 1940s, and it may contain more oil in other potential areas. The objective of this paper is to evaluate the rocks of the Mauddud Formation in the Arabian Gulf basin in terms of their historical nomenclature and then describe their facies and lithofacies variation and depositional setting and pinpoint new areas in which the formation may contain considerable reserves of hydrocarbons (Table 1).

REGIONAL SETTING The Mauddud Formation was described for the first time by Henson in an unpublished report in 1940 (cited by Sugden and Standring, 1975) from the Dukhan-1 well in Qatar (Figure 1). The formation is named after the area of Ain Mauddud near Dukhan. Prior to its formal definition, the rocks of the Mauddud Formation were designated as the Orbitolina concava limestone by Barber (1948) or the Second Pay in Bahrain. The original description was amended by Sugden in an unpublished report in 1958. According to this report, which was transcribed by Bellen et al. (1959, p. 178), the formation consists of around 55 m of ‘‘limestone, light gray, earthy, mostly of fairly high porosity except for the bottom few feet, which are rather marly. Much of the limestone appears to be silty, reflecting the presence of fine calcareous detritus.’’ The first formal description of the type section of the formation in Qatar was published by Sugden and Standring (1975) as consisting of foraminiferal limestone ranging from lime mudstone at the bottom through wackestone to pelletal and skeletal packstone and wackestone at the top. In some horizons, the formation consists predominantly of tests of Orbitolina and Trocholina. In the northern offshore areas of Qatar, the Mauddud Formation consists of rudist grainstone and packstone with peloidal, Orbitolina-bearing limestone (Focke et al., 1986). In its type locality at Qatar, the Mauddud Formation rests conformably on the Nahr Umr Formation and is defined by the change from the clastic rocks of the Nahr Umr. In the subsurface north area, the lower part of the Mauddud Formation is called the lower marly member and represents a transitional zone between the Nahr Umr clastics and the limestones proper of the so-called ‘‘Upper Mauddud’’ (Figure 2) (Focke et al., 1986). The overlying formation in the type locality is the Ahmadi Formation, and its contact is also conformable (Sugden and Standring, 1975) and marked by a sharp 1654

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 1. Location map of the Arabian Gulf basin showing the main and adjacent areas, localities, fields, and wells mentioned in the text.

change from the Orbitolina-bearing limestone of the Mauddud Formation to the clastics of the Ahmadi Formation. In the subsurface north area, the overlying formation is the lower Khatiyah. In Kuwait, the reference section of the Mauddud Formation is in the Burgan oil field. It consists of a single limestone unit that ranges in thickness from less

than 2 m in the Umm Gudair field to more than 90 m in the Raudhatain field in northern Kuwait (Adasani, 1967; Al Shamlan, 1975) (Figure 3). The Mauddud Formation consists of dolomite and dolomitic limestone with Orbitolina and Trocholina foraminifera and dasycladacean algae. It is overlain by the Wara Formation and underlain by the Burgan Formation. The Sadooni and Alsharhan

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Kuwait

Orbitolina sp., Trocholina sp., Cyclamina sp., Oligostegina sp., Globigerina sp., Cuneolina sp., and Guembelina sp.

35 – 150 formation Albian – Cenomanian

Thickness (m) Status Stage

2 – 110 formation upper Albian

Iraqia simplex Henson, Trocholina altispira Henson, Trocholina lenticularis cf. concava (Lamarck), Rabanitina basraensis Smout, and milliolid

pseudo-oolitic orbitolinid packstone/wackestone contains interbedded locally recrystallized finely crystalline limestone, few shales dolomitic limestone and marly limestone; beds and soft porous the limestones are limestone with some spotted with pyrite; thin sands and dark glauconite has gray marl near the base been noted toward the base

South Iraq

Fossil content

Lithology

Components

Qatar light gray, limestone of fairly high porosity, except for the bottom few feet, which is rather marly; much of the limestone (compact lime mudstone) appears to be silty, because of the presence of fine calcareous detritus, and the upper part dominated by wackestone/pelletal skeletal packstones, and wackestones contain beds rich in foraminifera and skeleton debris Orbitolina cf. concava (Lamarck), O. concava (Lamarck) var. qatarica Henson, Trocholina arabica Henson, Trocholina lenticularis Henson, Trocholina altispira Henson, and Cyclammina whitei Henson 55 formation Albian – Cenomanian

Saudi Arabia dolomitic, pyritic, glauconitic, aphanitic limestone, with some shaley or silty sandstone interbeds; eastward and northward in the direction of the central part of the Arabian Gulf, the limestone becomes cleaner, the shale decreases, and the sand disappears

37 – 105 formation upper Albian

5 – 55 member Albian

Orbitolina concava Orbitolina concava (Lamarck) var. qatarica (Lamarck), Cyclammina Henson and whitei Henson, Trocholina sp., milliolids, Hemicylammina whitei Henson abundant echinoid, and algal debris

cream to tan limestone, which appears somewhat sandy because of the presence of abundant biogenic debris

Saudi Arabia –Kuwait – Neutral Zone

Country

Table 1. Carbonate Sediments and Their Characteristics of the Mauddud Formation, Arabian Gulf

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producing reservoir in the Greater Burgan field

oil bearing, heavy oil with 22j API at Ratawi field

Lithology

Components

gray, bioclastic limestone and peloidal-bioclastic wackestone and packstone

Bahrain skeletal and peloidal packstone and wackestone, with locally rudist fragments; alternations of gray-green shale and limestone occur toward the base

United Arab Emirates

leaching allochems leaving void spaces and channel solutions, intercrystalline porosity in the highly dolomitized limestone, and interparticle and intraparticle porosity in the microfacies; porosity ranges 12 – 30%

disconformably overlain by the Wara Formation and conformably underlain by the Burgan Formation shallow marine with scattered shell banks

conformably overlies Nahr Umr Formation and is conformably overlain by the Wara Formation shallow-shelf carbonate

Petrophysical leaching and moldic characterization porosity; average porosity is 18%

Hydrocarbon occurrence

Depositional setting

Contacts

Subsurface alternation of dolomite and dolomitic of foraminifera (mainly orbitoids), with dasycladacean algae, peloids, and miliolids changing into anhydritic facies at the western side of Kirkuk field (Jawan facies)

Outcrop undifferentiated from Shuaiba Formation; alternations of coarsely crystalline, granular, rhombic, and mosaic dolomite with organic, detrital dolomitic limestone; sparse disseminated quartz silt

Offshore Iran thick-bedded to massive gray to brown Orbitolina-rich limestone

microporous, dolomitized bioclastic and peloidal wackestone and packstone, with local development of radiolitid and rudist biostrome

Northern Iraq

vuggy, interparticle, and leaching matrix porosity; porosity ranges from 11 to 32% and permeability from 1 to 80 md

shallow-shelf carbonates, with somewhat deeper water conditions toward offshore no production, oil shows only

the overlying and underlying contacts are conformable

Oman

Country

conformable contacts with the overlying and underlying members of the Wasia Formation very shallow-water intermittently to reducing environment moderately agitated of a restricted-shelf, water, reflecting a lagoonal and gradual shallowing of tidal-flat complex the basin oil-bearing in Darra field oil-bearing at Safaniya and produced sweet and oil stained in Khafji gas at Dammam and Hout fields, but not field productive vuggy and leaching leaching and matrix matrix porosity porosity contacts with Wara and Burgan formations are conformable

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Orbitolina concava (Lamarck)

30 – 37 member

Albian conformable with the underlying Nahr Umr silts and shales and the overlying Wara shale

shallow agitated marine waters

oil producer in Awali (Bahrain) field

Thickness (m) Status

Stage Contacts

Depositional setting

Hydrocarbon occurrence

Bahrain

Fossil content

Components

Table 1. Continued

Oman

no production, shows in some offshore wells

shallow-shelf carbonates

hydrocarbon bearing in some central Oman fields

shallow-shelf carbonates

Orbitolina concava Trocholina altispira (Lamarck) Henson, Trocholina sp., Orbitolina concava (Lamarck), Hemicyclammina whitei Henson, Pseudothalmmaninella subticinesis, Ticinella sp., and Trocholina lenticularis Henson 38 – 75 200 formation equivalent to members E, F, and G of the Natih Formation late Albian latest Albian conformable with conformable with the overlying and the overlying and underlying formations underlying formations

United Arab Emirates

Outcrop

Northern Iraq Subsurface

not applicable no production, but shows in some offshore wells

shallow-shelf carbonates

intra-Hauterivian – Albian conformably overlies the Sarmord Formation and is overlain by the Kometan Formation, the contact is erosional unconformity but without angular discordance shallow-shelf carbonates

799 formation, equivalent to Balambo

Albian conformably overlies the Kazhdumi Formation and conformably overlain by the Ahmadi Member

55 – 122 member of the Sarvak Formation

shelf carbonates with Orbitolina shoals and possible rudist buildups on the shelf margin oil in Jambur and Khabbaz, oil shows in Kirkuk field

Albian conformably overlain by the Dokan Formation and underlain conformably by the Nahr Umr shale

380 formation, equivalent to Balambo and Jawan

occurs in association with glauconitic horizons Orbitolina concava, Cuneolina sp., Orbitolina concava (Lammack) Favusella (Lamarck), Trocholina sp., Pseudochrysalidina sp., washitensis Muneira baconica(?), Trocholina cf. lenticularis Salingoporella muhlbergii(?), Henson, Iraqia simplex Permocalculus inopinatus, Henson, Dictyoconus sp., Orbitolina cf. discoidea, miliolid, textularid with Trocholina cf. lenticularis algal remains Henson miliolids

Offshore Iran

Country

leached intercrystalline porosity in the dolomitic suites with some interparticle porosity in the Orbitolina packstone/grainstone facies secondary porosity leaching and because of subaerial moldic porosity leaching, porosity has been enhanced through the preferential solution of rudists, with porosity of 15 – 25% and permeability of 5 – 300 md matrix and leaching Petrophysical the Mauddud has porosity characterization higher proportion of leaching porosity than intercrystalline porosity; it also has some original interparticle porosity; porosity can have as much as 35% and permeability from 1 to about 1000 md

formation has been divided into seven high-frequency depositional sequences. The lower transgressive sequence changes from limestone in northern Kuwait to siliciclastic deposits of the Burgan Formation in the south. The upper highstand sequence is carbonate dominated and commonly thins southward because of depositional thinning and postdepositional erosion (Strohmenger et al., 2002). In southern Iraq, the reference section of the formation was described from the Zubair-3 well by Owen and Nasr (1958) (Figure 1). Here, the Mauddud Formation consists of organic, detrital, occasionally pseudooolitic limestone with streaks of green or bluish shale. In central Iraq, the formation was also described from the Awasil-5, Falluja-1, and Makhul-1 and Makhul-2 wells and more recently from East Baghdad and Ahdab fields. In all these areas, the Mauddud Formation is composed of organic detrital limestone with a marly matrix and an Orbitolina –Trocholina fauna (Bellen et al., 1959). In northern Iraq, a different stratigraphic nomenclature had been adapted for the Mauddud-equivalent strata. This is because the stratigraphic nomenclature there was based on the fieldwork in the mountainous region of northern Iraq that preceded the exploration drilling in that area. In that particular region, the outcrops of the Hauterivian –Albian strata include the exposed, vertical, massive, black to dark-brown dolomites and dolomitic limestones that form the cores and carapaces of most of the main anticlines of northern Iraq. These strata were assigned to the Qamchuqa Formation by Wetzel in an unpublished report in 1950. A transcription of some parts of that report appeared in Bellen et al. (1959). Wetzel divided the Qamchuqa Formation into three limestone units that are separated by three dolomite units (Figure 4). It was then recognized that these subdivisions are impractical because they depend on the extent of dolomitization, which varies from place to place. An alternative scheme was proposed by Hudson (1954), who described a section of the formation from the Bekhme Gorge area of northern Iraq. His subdivisions were based on a mixture of lithology and age. In 1953, the Qamchuqa Formation was encountered in the subsurface at the deep test well Kirkuk109 (Figure 1). The formation was divided in that well into an upper dolomite unit and a lower Orbitolina unit. The same rocks were described from Kirkuk-116 and Kirkuk-117 wells and were divided into the Upper and the Lower Qamchuqa formations separated by a shale unit. This shale unit is designated as the upper Sarmord Formation. This subdivision scheme was then adapted for northern Iraq by Bellen et al. (1959) (Figure 5). Sadooni and Alsharhan

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 2. General stratigraphy of the Cretaceous units in the northern offshore area of Qatar showing the stratigraphic status of the Mauddud Formation (modified from Focke et al., 1986).

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Figure 3. Lithologic subdivisions and microfauna content of the Mauddud Formation in the Raudhatian-1 well in Kuwait (modified from Al Shamlan, 1975).

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 4. Lithologic components of the Qamchuqa Formation in its type locality, Qamchuqa village, northern Iraq.

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Figure 5. Major sequences of the Cretaceous strata from a typical well in Kirkuk field, northern Iraq (modified from Al Shdidi et al., 1995).

Instead of geologic considerations, this double nomenclature for south and north Iraq was the result of concession boundaries between Iraq Petroleum that operated in the northern areas and Basra Petroleum, which operated in southern Iraq. Working on the regional correlation of the stratigraphic units in Iraq, Chaton and Hart (1961, in an unpublished report cited by Sadooni and Aqrawi, 2000) recognized this double terminology and suggested replacing the term Upper Qamchuqa with the name Mauddud and the Lower Qamchuqa with the name Shu’aiba to unify the stratigraphic nomenclature between south and north Iraq.

Sadooni (1978, cited by Sadooni and Aqrawi, 2000) suggested using the term Qamchuqa Group to include the whole Hautervian–Albian interval in all the subsurface sections of Iraq. In northwestern Iraq, a slightly different regime prevailed in response to the influence of the Mosul Block on sedimentation during most of the Cretaceous. The Mauddud Formation at the Ain Zalah field consists of around 160 m of dolomite, and it is overlain unconformably by the Gir Bir (equivalent of Mishrif Formation) and rests conformably on the Nahr Umr Formation (Figure 6).

Figure 6. The Cretaceous strata of the Ain Zalah-16 well, northwest Iraq (modified from Chatton and Hart, 1961, cited by Sadooni and Aqrawi, 2000).

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

In Saudi Arabia, the Mauddud was described as a member by Powers et al. (1966) from the Safaniya well 17 with a thickness of 5.2 m. The unit, however, thickens toward the southeast and reaches a maximum thickness of around 60 m in the central Rub’ Al-Khali. In that area, the Mauddud Formation consists of marly dolomite with Orbitolina, but changes along the western margin to a coarse dolomite or pyritic, glauconitic lime mudstone with some interbedded shale and silty sandstone. To the southeast of Qatar and in the United Arab Emirates, the dominant carbonate lithology continues to be a fossiliferous wackestone/packstone, which is more argillaceous toward its base, reaching about 61 m in thickness. Still farther east in Oman, the Mauddud Formation (equivalent to Members E, F, and G of the Natih Formation) reaches about 200 m in thickness and consists of skeletal and peloidal wackestones and lime mudstones, with the upper part of the succession formed of packstones with a local development of rudist rubble (Alsharhan and Nairn, 1988). Henson (1948) proposed a Cenomanian age to the Mauddud Formation in Qatar based on the association of O. concava with Praealveolina cretacea. Bellen et al. (1959) suggested that the P. cretacea is a caving contamination from the overlying beds and proposed an Albian age to the formation. Ibrahim et al. (2000) suggested a late Albian to early Cenomanian age to the formation in the Dukhan field, Qatar. This is based on the coexistence of the pollens Florentinia radiculata, Florentinia resex, and Florentinia berran. There are many arguments in favor of this diachronous age of the formation. These include the following: 1. The carbonate sediments of the Mauddud Formation were deposited on the retreating clastic front of the Nahr Umr Formation, hence, the deposition of the formation in the areas far from the source of the clastics along the present-day Iraq–Iran border and gradually transgressed toward the southern and southwestern parts of the Arabian platform as the clastic source was shut off. Examples of this are numerous, among them the presence of Orbitolina-bearing limestone intertonguing with the clastic facies in the Halfaiya field near the Iraq – Iran border and the presence of Orbitolina in the outcrops of the Nahr Umr Formation in Oman. 2. In Saudi Arabia, the upper contact of the Mauddud Formation in Saudi Arabia is also diachronous because of the inclusion of the clastic components in the formation. The stratigraphy indicates that thickening takes place by addition of beds at the top. In

the Dammam field, for instance, Hemicyclammina whitei and O. concava appear at the top of the Mauddud Formation. In cases where the carbonate component alternates with the sandier beds, H. whitei replaces O. concava in the sandier interval. The two forms have parallel ranges and can be used interchangeably (Powers, 1968). Therefore, the presence of H. whitei in clastic strata above the Mauddud limestone units in wells south of Safaniya indicates partial equivalence with the limestone units of the upper part of the Mauddud Formation at the northern end of that field. Similar variation in the position of the carbonate beds that marks the top of the Mauddud Formation was reported from other fields, including the Khursaniyah, Qatif, and Dammam. Powers (1968) concluded that although a faunal zone associated with the top of the Mauddud Formation can be recognized throughout the fields located at the coastal areas of Saudi Arabia, the actual lithologic top is highly variable. 3. The Mauddud Formation is capped by a regional conformity that eroded variable thicknesses of the formation depending on the configuration of the old paleotopographic surface.

MAIN LITHOLOGIES AND FACIES INTERPRETATION The Mauddud Formation was originally designated to describe the Orbitolina-bearing limestone of the Arabian basin. Other facies, however, were described from the formation, and the following are the major lithologies. Orbitolina-Bearing Limestone Orbitolina-bearing limestones form a large bulk of the Mauddud Formation all over the Arabian Gulf basin. This facies has been described from the region by Henson (1948), from Qatar by Focke et al. (1986), from Kuwait by Al Shamlan (1975), from several oil wells in southern Iraq by Mohammed and Al-Sayyab (1993), from northern Iraq by Sadooni (1978, cited by Sadooni and Aqrawi, 2000) and Al Shdidi et al. (1995), and from other parts of Arabia by Alsharhan and Nairn (1988, 1997). Several species had been described, including O. concava (Lamarck), O. concava var. qatarica Henson, O. concava var. iraqi Henson, O. concava var. sefini Henson, Orbitolina discoidea Gras, and Orbitolina aff. curdica Henson (Henson, 1948). Sadooni and Alsharhan

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Figure 7. Orbitolina rudstone from the Mauddud Formation, Kirkuk field, northern Iraq.

Orbitolina occurs in several facies combinations varying in its abundance and the nature of the other components. The Orbitolina rudstone (Figure 7) is mainly composed of Orbitolina tests (80–90% of the whole rock) with very little or no matrix. This facies was described from Kuwait by Al Shamlan (1975) and from the Kirkuk field, north Iraq by Sadooni (1978, cited by Sadooni and Aqrawi, 2000). The second most common association is the Orbitolina-rudist limestone. In this facies, Orbitolina is mixed with large shell fragments of rudists along with other mollusks and echinoderms. This facies was described by Focke et al. (1986) from the North field of Qatar and by Sadooni (1978, cited by Sadooni and Aqrawi, 2000) from the Jambur field of northeast Iraq. The third facies is an Orbitolinamiliolids limestone, in which Orbitolina is found associated with dasycladacean algae, pellets, and small benthonic foraminifera including miliolids. The Orbitolina in this facies are extensively micritized and blackened by organic pyrite. This facies has been described from the Jawan Formation, the lagoonal equivalent of the Mauddud Formation in northern Iraq fields, and by Al Shamlan (1975) from Kuwait. The Orbitolina flourished in tropical to subtropical water along shallow coastlines where the temperature would have been between 15 and 35jC (Douglass, 1960a, b). The water depths tied to Orbitolina seem to be variable, and it is probably best inferred on the basis of the associated organisms. Orbitolina co-occurrence with miliolids and green algae suggests shallow water, 1666

whereas its association with rudists may indicate deeper, more open water. Rudist Rudstone/Grainstone Rudists are common components in the Cretaceous carbonates of the Arabian basin. Rudist fragments were described in the early reports of the Iraq Petroleum wellsite geologists in northern Iraq as ‘‘horned limestone’’ or ‘‘horned shell fragments.’’ Wilson (1975) commented on the possible occurrence of basin margin rudist buildups in the Albian sediments of northern Iraq. Sadooni (1978, cited by Sadooni and Aqrawi, 2000) described rudist fragments from the Jambur field in northeastern Iraq, where they are mixed with corals, gastropods, or echinoderms. Mohammed and Al-Sayyab (1993) described as much as 40 m of rudist biostrome from the Mauddud Formation in the well Abu Ghirab-5 on the Iraq –Iran border in southern Iraq. In that well, the strata are formed of bioclastic rudist limestone with occasional rudist boundstone. Focke et al. (1986) described limestone rich in large fragments of caprinidcaprotinid rudists in the upper part of the Mauddud Formation in the northern areas of Qatar (Figure 8). Radiolitid rudists also occur farther to the south of the Qatari offshore fields, and some rudist debris was described from the Mauddud member of the Natih Formation in Oman (Harris and Frost, 1984; Alsharhan and Nairn, 1997). In all these cases, it is difficult, because of the limitation of samples, to suggest the

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 8. Radiolitid rudist fragment in a dolomitic groundmass from the Mauddud Formation in the well of Abu Ghirab-5 near the Iraqi –Iranian borders.

presence of rudist buildup in these areas. The high degree of fragmentation of the rudist debris in both northern Iraq and Qatar indicates the possible presence of local basin margin buildups that were eroded and distributed over larger areas.

shelf-environment inhabitants. The algal-bearing limestone represents, then, the lagoonal or semilagoonal settings in the Mauddud basin. These algae are commonly associated with miliolids, pellets, and clotted limestone. Pelletal Packstone/Wackestone

Algal Packstone/Grainstone The green dasycladcean algae were also a major rockforming fossil in the Mauddud Formation and its equivalent formations. The most common species is the Salingoporella dinarica, which characterized the Lower Cretaceous strata of the Tethys (Elliot, 1958). Furthermore, the species Salingoporella muhlbergii had been described from the type locality of the Qamchuqa Formation in northern Iraq (Bellen et al. 1959). The second group of algae that was recorded from the Mauddud Formation belongs to red algae of the Gymnocidiaceae. Permocalculus debris is the most common algal debris from this family. They have characteristically small, numerous pores and occur as a Permoclaculus debris facies, similar to the facies described by Johnson and Kaska (1965) from the Lower Cretaceous of Guatemala. Dasycladacean algae flourish in tropical to subtropical areas, below low tides, and commonly in water less than 5 m deep in low-energy, sheltered areas below intense wave agitation (Wray, 1977). Elliot (1958) found that the maximum abundance of these algae is from the low-tide level to a depth of 5 – 6 m and extends down to 10 m while diminishing in abundance. The gymnocodiacean algae are also considered by Wray (1977) as

This facies consists of pellets, peloids, green algae, micritized miliolids, and Orbitolina in a slightly argillaceous micritic groundmass. The facies was described by Sadooni (1978, cited by Sadooni and Aqrawi, 2000) from the western parts of Kirkuk field, where the Mauddud Formation grades to its lagoonal equivalent, the Jawan Formation. Two similar facies from the Mauddud Formation in the northern fields of Qatar were reported by Focke et al. (1986), namely, the peloidal Orbitolina/echinoid packstone and the pelletoidal grainstone. Many of the peloids in this facies are possibly micritized benthic foraminifera. The facies underwent partial leaching and is cemented by blocky calcite cement. Whether the present peloids are fecal pellets or micritized small benthic foraminifera, this facies probably formed in a relatively sheltered area and may represent deposition in a restricted shallow-marine shelf or a lagoon. Chalky Limestone Chalky limestone is used here as a collective term to include deeper water carbonates in the Mauddud Formation. Tintinnid-bearing limestone from the Mauddud Sadooni and Alsharhan

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equivalent sediments in northern Iraq were described by Bellen et al. (1959) and Chatton and Hart (1961, cited by Sadooni and Aqrawi, 2000). They suggested that this facies represents a transition between the Mauddud Formation and the pelagic Balambo Formation. In the Jambur-13 well, which is slightly to the east, the Mauddud Formation has wedged out and is replaced by a thick sequence of the Balambo Formation. Chalky limestone with calpionella was described from the Mauddud Formation in Badra field on the Iraq – Iran borders. The facies is important as a separating marker between the shallow-water carbonates of the Mauddud Formation and the pelagic facies of the Balambo Formation to the east. Anhydritic Limestone This is a limited facies and was described from the western parts of the Kirkuk field (mainly from the Kirkuk-116 and Kirkuk-117 wells) and the Jawan structure of northern Iraq as a part of the sabkha suite of the Jawan Formation. The facies consists of pelletal limestone, dismicrite, and stromatolitic lime mudstone that is partially to extensively replaced by nodular anhydrite. From the available data, it seems that the sabkha to the west of the Mauddud basin was restricted to only a small area to the west of Kirkuk field, but this may be the result of limited data, and the sabkha may extend in a longitudinal zone to the south. Dolomitic Limestone Dolomitic limestones and dolomites form the bulk of the Mauddud rocks in some parts of the Arabian Gulf basin. They have been described from northern Iraq by Sadooni (1978, cited by Sadooni and Aqrawi, 2000) and from Kuwait by Al Shamlan (1975). Dolomite forms a significant lithologic component of the Mauddud Formation in the subsurface sections in Iraq, Kuwait, and Saudi Arabia, as well as in the outcrops of northern Iraq, but it is of less importance in the southern parts of the Gulf region such as Bahrain, Qatar, United Arab Emirates, and Oman. Dolomite lithologies are varied and complex in terms of texture and size. Table 2 and Figure 9 list their most distinctive petrographic characters and distribution. Some of these types (e.g., saddle dolomites) may be associated with the development of the Cretaceous ophiolite suite in northeastern Iraq (Buday, 1980). Such dolomite has large rhombs (lengths as much as 4 cm in the Mauddud Formation section in the well 1668

Kirkuk-109). These are also associated with bitumen in the Jambur oil field. Other textures, such as those associated with fractures or stylolites, may be the result of only local conditions. Al Shamlan (1975) suggested a mixed saline to freshwater regime to explain the origin of dolomite in the Mauddud Formation in Kuwait. The origin of dolomite in the Mauddud Formation is beyond the scope of this paper.

DEPOSITIONAL SETTING The Mauddud Formation rests on the Nahr Umr Formation in most parts of the basin. The contact between the two formations may result from a stratigraphic discontinuity developed during flooding of the clastic-dominated shelf, resulting in the deposition of shallow-water carbonates. The mechanism that shut off the supply of fine clastics from the source area to the west may be related to climatic change (Murris, 1980), or it could be attributed to a period of relative tectonic stability. Accordingly, the lower part of the Mauddud Formation represents a transgressive system that culminated and stabilized as a highstand systems tract with the development of the Orbitolina-bearing limestone. The vertical and horizontal extension of this facies all over the Arabian basin indicates longterm stability of the basin. In areas of relatively higher energy, localized rudist banks were established on the basin margin that gradually graded to the deep-water carbonates of the Sarmord Formation or the pelagic facies of the Balambo Formation to the east. Local lagoons were also formed wherever the topographic or hydrodynamic settings allowed. These were characterized by algal-pelletal facies. A gradual change from the Orbitolina-bearing limestone to pelletal and anhydritic limestone was noticed in the Kirkuk-116 and Kirkuk117 wells, which are situated on the western part of Kirkuk field in northern Iraq. In the areas to the south and southwest, clastics sediments were still the dominant component (Figures 10, 11). The upper contact of the formation with the strata of the Ahmadi Formation (and its equivalents) suggests that clastics dominated the shelf again. Dunnington (1967) indicated the presence of a regional unconformity at the top of the Mauddud Formation in both Kuwait and southern Iraq. Sugden and Standring (1975) mentioned that there is no physical evidence of such unconformity in Qatar. However, the upper part of the Mauddud Formation in the northern fields of Qatar is fractured and penetrated by the overlying soft marls of

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Table 2. Petrographic Classification of Dolomite in the Mauddud Formation Selective dolomitization

postfracture

poststylolite

local source

water-controlled

Extensive dolomitization

aphanitic

limpid

saddle

Dolomite rhombs follow fractures that may provide ions and water necessary for their formation. In some cases, the fractures are minute and can be recognized only under the microscope (Figure 9A). In other cases, the growth of dolomites along fractures may be associated with the same stress that created the fractures. Dolomite rhombs grow in such cases with a preferred orientation (Figure 9B). Stylolite seams act as channels instead of permeability barriers. This dolomite appears as irregular, meandering zones contrasting with the nearly straight character of that following fractures (Figure 9C). The dolomite may have formed by Mg2 + released during pressure solution (Alsharhan and Sadd, 2000). The term ‘‘styloreactates’’ has been introduced by Logan and Semeniuk (1976) to describe minerals that form by reaction in or along the stylolite seams. High magnesian calcite grains, such as red algae, might act as nuclei from which dolomitization starts and expands (Sibley, 1982). In some cases, the dolomite crystals are restricted to the skeletal parts or extend to the neighboring groundmass (Figure 9D). Such type of dolomitization is normally associated with high-dissolution porosity. Murray (1964) suggested the term ‘‘solution cannibalization’’ to describe the process by which dolomite grows from a limestone groundmass. Dolomite is controlled by other factors that are not related directly to the rock properties. As Figure 9E shows, the dolomitization front has a sharp contact. This may be the result of high moisture content induced by the level of ground water at that time. This dolomite is mainly present in the lagoonal miliolids-bearing limestones of the Mauddud Formation. It may be associated with the evaporites of the equivalent Jawan Formation to the west. In some cases, dolomite crystals may agglutinate together during neomorphism or cementation, forming pseudopellets made up of fine-grained dolomite crystals (Figure 9F). This dolomite forms the main reservoir unit of the Mauddud Formation. It consists of well-developed, euhedral to subhedral dolomite rhombs of medium size and is associated with intercrystalline and dissolution porosity (average 15%). Some of the dolomite rhombs are zoned and have clayey nuclei. This dolomite apparently mainly replaced the skeletal sand of the Mauddud Formation, as it is the case in the reefal and shelf carbonates of the formation in the Kirkuk-109 and Jambur oil field wells. The dolomite has produced large quantities of oil. Scanning electron microscopy (SEM) photomicrograph of this type (Figure 9G) shows the presence of a relatively thin crust of dolomite covering a core of clayey material. This dolomite consists of interlocked anhedral, xenotopic, or nondescript dolomites that lack any recognizable crystal features (Friedman, 1965). These have been found mainly in the Kirkuk-109 well, Jambur wells, and in the outcrops of northern Iraq. In some cases, it is associated with orthoclase crystals, which may be pyroclastic, and also with bitumen and heavy oils in Jambur oil field. In some cases, the dolomite crystals may have grown for several centimeters. Examination of this type of dolomite by SEM indicates that it is formed of interlocked crystals with no porosity (Figure 9H). Sadooni and Alsharhan

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Figure 9. The different types of dolomite found in the Mauddud Formation in northern Iraq. See Table 2 for details. Selective dolomitization: (A –B) postfracture dolomite; (C) poststylolite dolomite; (D) local source dolomite; (E) water-controlled dolomite. Extensive dolomitization: (F) aphanitic dolomite; (G) limpid dolomite; (H) saddle dolomite. the lower Khatiyah (Ahmadi), suggesting the presence of such a depositional break. In some parts of central Iraq (e.g., Awasil-5 well) and in northwestern Iraq (e.g., Ain Zalah-16 well), the Mauddud Formation is overlain directly by the Cenomanian Mishrif Formation or its equivalents (the Gir Bir or the Mahliban). This means that the Ahmadi Formation in these areas was either removed or never deposited. In northern Iraq, the Mauddud Formation is overlain by the Calpionellabearing marly limestone of the Dokan Formation, and the contact between the two formations may represent a eustatic sea level change that signaled the start of a new transgressive sequence. The widespread distribution of the Mauddud rocks marks the end of the clastic phase that dominated most parts of the basin. This late Albian transgression re1670

sulted in a rapid shutdown of the clastic materials. The presence of clastics (as it is the case in the western desert of Iraq (Jassim et al., 1984, cited by Sadooni and Aqrawi, 2000) and in Saudi Arabia (Powers et al., 1966) in the limestones suggests a nearby but restricted clastic source.

PETROLEUM SYSTEM Source Rocks In northern Iraq, the Mauddud-equivalent Upper Qamchuqa Formation is a well-established reservoir that has produced oil from fields like Jambur and Bai Hassan for decades. The combination of rich and thick source rocks

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 9. Continued. (the pelagic Chia Gara-Balambo formations) with clean shelf and basin margin carbonates of the Upper Qamchuqa (Mauddud) created an efficient oil system that may work for other fields in that region. The geochemical properties of the major potential source rocks in northern Iraq have been investigated by Al-Habba and Abdulla (1989) and Odisho and Othman (1992). These formations include the Cretaceous Balambo and Sarmord and the Jurassic Chia Gara, Naokelekan, and Sargelu. Al-Habba and Abdulla (1989) found that the average total organic carbon (TOC) value of the Balambo Formation is about 1.18 wt.% and might be classified as good source rock. They suggest that oil started to be generated in the lower part of the Balambo Formation at 15 Ma. Intertonguing of the Mauddud Formation with the Balambo Formation occurs beneath the Jambur field of northeastern Iraq. This suggests that at least some of the Mauddud Formation oil in the northern parts of the Arabian platform may have been generated in the Balambo Formation. Odisho and Othman (1992) investigated the potential of the Cretaceous – Jurassic strata of northern Iraq as source

rocks. Using samples from three outcrops and the Kand-1 and Taq Taq-1 wells, they found that the Jurassic Chia Gara Formation ranges from very rich in the Shiranish and Geli Mezerka outcrops (11.83 and 11.99% TOC, respectively) to rich in the Taq Taq-1 well (2.15% TOC) to fair in the Bekhme outcrops (0.73% TOC). The Sargelu Formation is very rich in the Kand-1 well (3.10% TOC), but poor in the Taq Taq-1 well (0.49% TOC); the Naokelekan Formation is very rich in the Taq Taq-1 well (4.66% TOC), and the Cretaceous Sarmord Formation is poor in the Taq Taq-1 well (0.32% TOC). Some of their data are shown in Table 3 and Figure 12. They concluded that the Chia Gara Formation possesses the highest potential as a source rock. They also proposed that all these formations might act as a single source layer because of their stratigraphic continuity. In fact, Bellen et al. (1959) had already noticed that the Cretaceous Balambo and the Jurassic Chia Gara are of the same facies, but had been divided according to their ages. The extension of the Chia Gara-Balambo facies to the south (e.g., Kuwait) could solve the source dilemma in these areas. Sadooni and Alsharhan

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Figure 10. Paleogeographic map of the Albian –upper Cenomanian strata of the Arabian Gulf basin (modified from Murris, 1980 and Alsharhan and Nairn, 1997).

The notion of vertical migration of Jurassic oil into Cretaceous reservoirs in northern Iraq was proposed originally by Dunnington (1958), disputed by Weeks (1958), and later supported by Al-Shaharistani and Al-Atyia (1972), who used vanadium and nickel con1672

centration to correlate between source rocks and the generated oil. In Kuwait, the TOC in the Mauddud rocks ranges between 0.64 and 3.4% and is classified as fair to good in the upper part and good to very good in the lower

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Sadooni and Alsharhan

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Figure 11. Cross section in the Mauddud strata showing the regional facies changes.

Table 3. Main Pyrolysis Parameters Used in Source Rock Evaluation from Northern Iraq* Formation

Location

Depth (m)

TOC (wt.%)

S1 (mg HC/g sample)

S2 (mg HC/g sample)

HI (S2/TOC)

PI (S1/S1 +S2)

PP (S1 +S2)

T max Cj

Chia Gara Chia Gara Chia Gara Sargelu Sarmord Chia Gara Naokelekan Sargelu

Shiranish Valley Geli Mezerka Bekhme Kand-1 Taq Taq-1 Taq Taq-1 Taq Taq-1 Taq Taq-1

outcrop outcrop outcrop 2271 2898 3130 3236 3308

11.83 11.99 0.73 3.10 0.32 2.15 4.66 0.49

1.75 0.74 1.01 0.94 0.05 0.52 0.47 0.06

53.94 59.51 3.84 3.23 0.28 2.14 1.04 0.50

455 496 526 426 87 99 22 102

0.03 0.01 0.21 0.07 0.16 0.21 0.31 0.11

55.69 60.25 4.85 14.17 0.33 2.66 1.51 0.56

437 436 449 441 439 423 568 515

*After Odisho and Othman, 1992.

part. Kerogen elemental analysis and pyrolysis indicate the presence of type II–III kerogen, and the formation is mature (Abdullah and Kinghorn, 1996). Toward the southern parts of the Arabian basin, the possible source rocks for the oil in the Mauddud Formation is one of the challenging problems facing exploration of this reservoir. The shaly, middle section of Nahr Umr Formation is the most probable source. The Nahr Umr clastic front changes to clastic marine shelf near the central part of the present Arabian Gulf (Murris, 1980). There are few published studies on the source rock potential of the Nahr Umr and its equivalent formations in the Gulf region as a source rock or its thermal maturation (see Newell and Hennington, 1983; Alsharhan, 1991, 1994). Another possible source for the Mauddud oil is that it is indigenous. As recorded by Alsharhan and Nairn (1994, 1997), the Mauddud Formation in the eastern and southeastern parts of Qatar consists of interbedded globigerinid marl and calcareous shale with organic-rich intercalations. The TOC content of these intercalations ranges between 3 and 8.3%; it is mainly sapropelic, with lesser amounts of liptodetrinites and algae. The organic matter is kerogenous to mainly kerogenous, and its source potential oil is excellent. Ibrahim et al. (2000) investigated the thermal maturation of the Mauddud Formation in Dukhan field in Qatar and concluded that the abundance of dinoflagellate cysts with amorphous organic matter and opaques indicates oil-prone type II kerogen. They also suggested that the rocks of the Mauddud Formation in Dukhan field are slightly thermally mature, as reflected by the orange color of the spores. Therefore, the possibility of a local source for the Mauddud Formation oil in the southern parts of the Gulf remains a possibility. A Jurassic source for the Mauddud Formation oil in the southern parts of the Arabian basin should not be 1674

excluded. Geochemical analyses of Mauddud and Arab formation oils conducted by Cities Services (1986, cited by Chaube and Samahiji, 1995) have shown that both oils have identical pristine/phytane ratios, indicating a common origin. Furthermore, even-odd carbon number preference in the n-paraffins, naphthenicaromatic ratio, the medium gravities, and gas chromatograms of the saturate and aromatic C 15 + fractions of both oils are also very similar.

Figure 12. Types and evolutionary stages of the kerogen from the Upper Jurassic to Cretaceous source rocks in northern Iraq (modified from Odisho and Othman, 1992).

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Figure 13. Karstification of the Qamchuqa Formation at an outcrop from Sefin Dag Mountain (near Shaqlawa), northern Iraq.

Reservoir Characterization Two factors strongly affected the porosity evolution and destruction in the Mauddud Formation. First, the upper part of the formation is marked by the presence of an unconformity and thus may be subjected to widespread meteoric diagenesis. Second, after its deposition, some parts of the Mauddud basin witnessed strong igneous activities, including ophiolite intrusion in Oman and northeastern Iraq (the Qulqula and Mawat intrusions) (Bolton, 1958, cited by Buday, 1980; Aswad et al. 1993). The first factor created and/or probably blocked regional porosity in the Mauddud rocks; the second factor may be responsible for the development of extensive interlocked and, at least, partially thermally induced dolomitization. Dissolution by meteoric water is a characteristic feature of the Mauddud rocks in surface and subsurface section. Large caves and karsts have been described from many of the northern Iraq dolomite outcrops of the formation in the mountains of Sefin Dagh (near Shaqlawa town) (Figure 13). Significant vuggy porosity was also found in the subsurface sections of Kirkuk, Bai Hassan, and Jambur fields in northern Iraq (Sadooni, 1978, cited by Sadooni and Aqrawi, 2000). Similarly, Focke et al. (1986) noticed that practically all the major rock types of the Mauddud Formation in the northern fields of Qatar were leached and associated with the development of dissolution porosity in the

rudist facies. Dissolution also enhanced the matrix porosity of the formation. In the producing fields of northern Iraq, dolomite is the main reservoir unit in the Mauddud Formation. A crossplot of the porosity versus dolomite percentage (Figure 14) indicates the presence of two distinctive

Figure 14. Relation between porosity and dolomite percentage in the Mauddud Formation from the northern fields of Iraq. Sadooni and Alsharhan

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groups: dolomitic limestone (lower part) and dolomite. The lower part of the graph suggests that dolomitization may increase porosity during the early stages, but as the dolomitization proceeds, there is no clear relation between the two at the late stages. This probably is related to the growth of the dolomite rhombs blocking the intervening pores. For the purpose of petrophysical analysis, the dolomite is divided into two main forms: planar and nonplanar (Sibley and Gregg, 1987; Woody et al., 1996). The planar forms are further subdivided into planar-e, which is formed mainly of euhedral crystals (euhedral dolomite of Friedman, 1965) and planar-s, in which most of the dolomite crystals are subhedral to anhedral. The nonplanar dolomite consists of closely packed anhedral crystals (the xenotopic or interlocked dolomite of Friedman, 1965). Mercury porosity for the Mauddud dolomite in north Iraq oil fields (e.g., Khabbaz) ranges from 0.20 to 21%. With few exceptions, most of the high porosity values are associated with the planar-e dolomite. The average effective porosity of planar dolomite is about 11%, whereas it is only 6% in nonplanar dolomite. Permeability values have a reverse order. Planar dolomite has a lower permeability (average 13 md) when compared to the nonplanar type (average 214 md). These high values, however, are attributed to a few samples instead of a general trend and may be attributed to fracturing instead of the dolomitization itself (Salih and Sadooni, 1987). In general, there is a positive relationship between porosity and permeability associated with the two major types of dolomite. In the planar dolomites, permeability values increase sharply when the porosity is higher than 15%. This may represent a significant threshold at which pores may start to connect with each other, enhancing the permeability. In the nonplanar dolomite, higher permeability concentrates about 10– 15% porosity and starts to decline again. It seems that higher permeability values are associated with medium porosity values. Although it is understandable that low permeability is associated with low-porosity levels, it is more difficult to explain the reason for permeability drop with high porosity values. The decline of permeability in such cases can probably be attributed to the destruction of pore throats by extensive xenotopic dolomite. The high porosity values may be the result of dissolution of large isolated vugs. High porosity (10–22%) with low permeability between 10 and 15 md was reported by Alsharhan and Nairn (1997) from some Kuwaiti oil fields, including Bahra, Raudhatain, and Sabriya. It seems also that most 1676

of these porous zones are restricted to the fossiliferous packstone/grainstone suites of the formation. In Bahrain, the Mauddud reservoir consists of bioclastic packstone/ grainstone, with an average thickness of about 28 m. Here, porosity ranges between 20 and 35% and is mainly vuggy, with permeability values between 10 and 110 md. In Qatar, there are two main reservoir rock types in the Mauddud rocks: the leached rudist packstone/ grainstone and the Orbitolina-echinoderm packstone. Porosity is 14–40%, and permeability is 0.20–70 md in the rudist grainstone/packstone, and porosity is 12.0– 30%, and permeability is 0.14–13 md in the Orbitolinaechinoid packstone in the North field area of Qatar (Focke et al., 1986). Most of these latter values are attributed to the presence of large vugs (as much as 3 cm in size) and the leaching of the unstable components. Porosity-permeability crossplots from the North field of Qatar (Figure 15) imply overlapping between the two major facies, indicating that dissolution is probably the major controlling factor in the development of porosity and permeability in the two facies.

Figure 15. Porosity versus permeability from the Mauddud Formation in the offshore North field of Qatar showing the overlapping of values for the two main facies (data from Focke et al., 1986).

Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Oil Potential Judging from the distribution of hydrocarbon occurrence in the different parts of the Arabian basin (Table 4), it seems that there are at least two oil provinces in which the formation is a major producer. These are a northern province that includes the fields of northern Iraq and a southern province that includes Ratawi field in southern Iraq and Raudhatain, Sabriya, and Bahra fields in Kuwait and Bahrain (Awali) in Bahrain. Geological conditions in these provinces are, however, heterogeneous, and reservoir characters are different. In the northwestern fields of northern Iraq such as Ain Zalah, where oil was discovered in 1957, the Mauddud reservoir is the result of a combination of dolomitization and fracturing (El Zarka, 1993). In the central northern fields such as Bai Hassan, the main producing zones are dolomite and Orbitolina-bearing limestone, whereas toward the northeast, in Khabbaz field, for example, dolomite is the main producing rock. In Jambur, dolomite also forms the major producing beds but with bioclastic units that contain some rudist debris. Jambur field is located near a basin margin buildup of rudists and algae similar to the depositional setting of the Mauddud Formation in the Abu Ghirab-5 well in southeastern Iraq and the offshore North field of Qatar. In the Jambur-13 well, there is an intertonguing of the Mauddud Formation with the pelagic Balambo Formation. In Jambur, there is also a considerable amount of bituminous matter associated with the dolomite. The Mauddud Formation has been mentioned as containing gas in the Ghouna field in Syria (Alsharhan and Nairn, 1997). The scarcity of data and the slightly different stratigraphic nomenclature applied in Syria make it difficult to evaluate this record, but given the huge oil reserves in the Mauddud Formation in northern Iraq oil fields, the potential of this formation in Syria may be worth further investigation. In the southern province, Badra field on the Iraqi– Iranian border produced on test from a combination of dolomite and microporosity in a chalky limestone. It probably represents slightly deeper water depositional conditions. Farther to the south, the Mauddud Formation consists of rudist bioclasts in the Abu Ghirab field (Mohammed and Al-Sayyab, 1993), but there are no data on the petroleum potential of the formation in that field or the other neighboring fields such as Majnoon. The presence of rudist basin-margin buildup in the Abu Ghirab-5 well may represent a continuation of the rudist basin margin already producing oil in both Bahrain and Qatar in the south and in Jambur field in

the north. Heavy oil was found, however, in the Mauddud Formation in the Ratawi field of southern Iraq. There are no obvious reasons why the Mauddud Formation should not be a reservoir in the other major oil fields of southern Iraq. In our opinion, its insignificance as a reservoir is mainly because of the exploration policy that targeted the main reservoirs, such as the Mishrif and the Zubair formations. The heavy oil in Ratawi is probably caused by the absence of a good seal because of the truncation or wedge-out of the Ahmadi Formation and its equivalents. In Bahrain, oil was produced from the formation in Bahrain (Awali) field in 1933, but was stopped by 1936 because of the absence of a water drive to maintain production. Gas injection was started as an alternative drive to maintain production. Continued gas injection led to the formation of a secondary gas cap that divided the reservoir into two zones (Ba and Bb) (Alsharhan and Nairn, 1997). The main oil-producing reservoir unit is the leached bioclastic unit (Bb), and the gas-producing reservoir is the peloidal-bioclastic wackestone/packstone. In Kuwait, the relatively thin section (5–10 m) reduces the importance of this formation as a major reservoir rock. The formation, however, has produced oil in the Raudhatain, Bahra, and Sabriya fields. The major reservoir units are dolomite and Orbitolina-bearing limestones. Heavy oil was reported from Bahra field with API gravity of 17 –28j. The geological settings of these fields may be similar to that of the Ratawi in southern Iraq. In the northern offshore areas of Qatar, the Mauddud Formation consists of relatively well-developed rudist buildups, with Orbitolina- and echinodermbearing limestones, but with no significant dolomitization. Porosity and permeability are mainly the result of extensive dissolution of the rocks similar to that in the Bahrain (Awali) field. The disappearance of dolomite in Qatar and Bahrain is an interesting point that requires further investigation beyond the scope of this paper. Most of the Saudi fields that contain significant oil potential in the Mauddud Formation are located in the offshore areas such as Marjan and Hasba. There is little available information on these fields, but it seems that the Mauddud Formation is similar in the Saudi offshore and Qatar northern areas. In Oman, the Mauddud-equivalent strata encompass three zones in the Natih Formation. All these members are oil producing. The formation produces from a combination of dissolution and dolomitization Sadooni and Alsharhan

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation

Ain Zala

Iraq

North

Safaniya Marjan Hasba Sharja-A-1 Fahud and Natih

Bahrain

Qatar

Saudi Arabia

UAE Oman

10 –22 22 24

Sabriya Bahra Dorra Mutriba Medina Bahrain

30

12– 39.5

20 –35

16 –22

Raudhatain

18

20

Kuwait

Ratawi

Khabbaz Badra

Bai Hassan Jambur

Ghouna

Field

Syria

Country

Porosity (%)

15 –500

50 –80

10 –110

13

22

Permeability (md)

40 32

27

28.5 17 – 28 27 – 28 23 23 31

29

22

29

16

Gravity (j API)

218

240

1944

27

2.9

2.1

556

608

Gas-oil ratio

3.4

3.28

5.1

2.5

Sulfur (%)

Table 4. Hydrocarbon Potential of the Mauddud Formation in the Arabian Gulf basin

28

30

60

55

19

165

800 undifferentiated

Thickness (m)

chalky limestone, fractured

bioclastic packstone and grainstone local rudist buildups and Orbitolina-bearing limestone

dolomitic and marly limestones fractured limestone and dolomite as above dolomite mainly with local rudist buildups dolomite mainly dolomite and chalky limestone Orbitolina- and algae-bearing limestones and dolomites Orbitolina-bearing and dolomitic limestones as above as above

Lithology

heavy oil oil (no production yet) oil (no production yet) condensate oil production, members E, F, G of the Natih formation

oil on test

oil production

oil production oil production oil and gas

oil production

heavy oil

oil on test oil on test

oil production oil and gas production

oil production

gas

Hydrocarbons

porosity. These are believed to be associated with subaerial exposure (Harris and Frost, 1984). Future Potential Several areas in the Arabian basin may contain significant hydrocarbon accumulation in the Mauddud Formation. The most important of these is the northern offshore area of Qatar. The leached bioclastic packstone and grainstone, which are the main reservoir units in Bahrain, are present in the North field (KNA-1 and KNA-3 wells) and Umm Al Irshan field (UISE-1 well). The same zone might extend northward into the Kuwaiti offshore areas and into southeastern Iraq in fields like Sinbad and Siba and Abu Ghirab (southeastern Iraq) and southward in the Saudi offshore area in fields like Marjan and Hasba, which have already produced oil on test from the Mauddud Formation. The southern fields of Iraq, particularly those neighboring Ratawi field, may also contain significant reserves in this formation if more efforts are directed toward exploring it. The well-developed, porous dolomites of Badra field could be found on both sides of the Iraq–Iran border.

CONCLUSIONS 1. The Mauddud Formation includes the Albian – Cenomanian Orbitolina-bearing limestone and dolomites that cover most of the Arabian basin, including the extreme extensions of the Arabian platform. 2. Regional correlation over the area has been enhanced by new data, which indicate that the stratigraphic nomenclature of the southern Arabian platform can be applied to its northern extension. 3. The Mauddud Formation represents a very shallow to shallow carbonate shelf with local basin margin rudist buildups that was established following the shutdown of the clastic front of the Nahr Umr Formation. 4. The Upper Jurassic and Cretaceous pelagic strata are probably the source rocks for the Mauddud Formation oil in the producing fields of northern Iraq. The Mauddud Formation might contain mature source rocks in the southern parts of the basin as is the case in Qatar, but the shales of the Nahr Umr and the Upper Jurassic rocks might also be the source rocks there.

5. Extensive dolomitization, fractures, and dissolution of the grainstone/packstone facies of the formation are the main controlling factors that shaped its petrophysical characters. 6. Porosity ranges between 10 and 35%, and permeability ranges between 10 and 110 md. These are found in the leached bioclastic limestone, dolomite, and Orbitolina-bearing limestones. 7. The Mauddud Formation is a major oil-producing reservoir in two provinces. The northern province encompasses the fields of northern Iraq, such as Ain Zalah, Bai Hassan, and Jambur. The southern province includes the fields of Ratawi in southern Iraq, Raudhatain, Sabriya, and Bahra in Kuwait, and Bahrain (Awali) in Bahrain. 8. The Mauddud Formation may represent a reservoir rock in the northern offshore area of Qatar and Saudi Arabia, as well as the southern and southeastern fields of Iraq.

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Stratigraphy, Microfacies, and Petroleum Potential of the Mauddud Formation