Paleokarst Related Hydrocarbon Reservoirs (SEPM Core Workshop Notes 18)

PALEOKARST RELATED HYDROCARBON RESERVOIRS

Organized and Edited by

Richard D. Fritz

James L. Wilson Donald A. Yurewicz

SEPM Core Workshop No. 18 New Orleans, April 25, 1993 ©Copyright 1993 by SEPM (Society for Sedimentary Geology)

ISBN 1-56576-004-2

Additional copies of this publication may be ordered from SEPM. Send you order to SEPM Post Office Box 4756 Tulsa, Oklahoma 74159-0756 U.S.A. ©Copyright 1993 by

SEPM (Society for Sedimentary Geology) Printed in the United States of America

PREFACE

This volume is a compilation of papers relative to paleokarst and associated reservoirs. The examples illustrate many of the rock types, and stratigraphic, structural, and paleotopographic features of carbonate strata which result chiefly from solution and collapse due to ingress of meteoric waters at and below unconformities. Examples presented here range from settings with considerable dissolution and collapse to those with significant unconformities but little evidence of meteoric alteration. Data is also presented that shows that solution and collapse can occur in deep-burial settings creating rock fabrics very similar to those produced in shallower meteoric settings. These rocks have been termed 'hydrothermal karsts' by some workers. It is estimated that 20-30% of recoverable hydrocarbons are in some way related to unconformities. Some notable examples of karst-controlled reservoirs include Precambrian in Ordovician dolomites in Renqui Field, China; Ordovician Ellenburger, Arbuckle and Knox carbonates in Texas, Oklahoma, and Alabama; Siluro-Devonian reservoirs of West Texas; the Permian Yates Formation in West Texas; Mississippian Madison Group carbonates in the Williston Basin, Wyoming and Montana; Jurassic carbonates in Casablanca Field, Gulf of Valencia; and Cretaceous El Abra carbonates in the Golden Lane fields, Mexico. Paleokarst reservoirs may also be important future reservoirs for application of horizontal drilling technology. Excellent discussions of paleokarst reservoirs can be found in publications by James and Choquette (1988), Kerans (1988, 1990), and Chandelaria and Reed (1992) We hope the papers presented in this volume will add to our understanding of paleokarst reservoirs and aid in the exploration and exploitation of hydrocarbons in these complex rocks.

We thank the editors and contributors and their respective employers, namely Amoco Production Company, Applied Geoscience, Bureau of Economic Geology, Chevron Overseas Petroleum, Colorado School of Mines, Exxon Exploration Company, Imperial Oil Canada Ltd., Indiana Geological Survey, Ohio Geological Survey, Oklahoma

State University, Marathon Oil Company, Masera Corporation, and Union Oil of California. In particular we thank Core Laboratories for their help in storing the core. We

also appreciate the SEPM for their help as well as the opportunity to provide such a forum. Finally, a special thanks to Valerie Lindsey and Sandra PaskVan of Masera Corporation for formatting and laying-out the manuscripts.

Richard D. Fritz James L. Wilson Donald A. Yurewicz

REFERENCES Candelaria, M.P., and Reed, C.L., 1992, Paleokarst, karst related diagenesis and reservoir development examples from Ordovician-Devonian age strata of west Texas and the Mid-Continent: Permian Basin Section, SEPM Publication No. 92-33

James N.P., and Choquette, P.W., 1988, Paleokarst: Springer-Verlag, New York, Berlin, 416 p.

Kerans, C., 1988, Karst-controlled heterogeneity in Ellenburger Group carbonates of West Texas: American Association of Petroleum Geologists Bull., v. 72, p. 11601183.

Kerans, C., 1990, Depositional systems and karst geology of the Ellenburger Group, (Lower Ordovician), subsurface West Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 193, 63 p.

iv

TABLE OF CONTENTS

INTRODUCTION TO KARST SYSTEMS AND PALEOKARST RESERVOIRS Mateu Esteban and James Lee Wilson (ERICO Petroleum Information and Consultant)

1

PALEOKARST FEATURES AND THERMAL OVERPRINTS OBSERVED IN SOME OF THE ARBUCICLE CORES IN OKLAHOMA Z. Al-Shaieb and M. Lynch (Oklahoma State University and UNOCAL)

11

PALEOSTRUCTURAL AND RELATED PALEOKARST CONTROLS ON RESERVOIR DEVELOPMENT IN THE LOWER ORDOVICIAN ELLENBURGER GROUP, VAL VERDE BASIN, TEXAS K.L. Canter, D.B. Stearns, R.C. Geesaman, J.L. Wilson (Consultants)

61

KARST BRECCIAS IN THE MADISON LIMESTONE (MISSISSIPPIAN), GARLAND FIELD, WYOMING A. S. Demiralin and N.F. Hurley (Colorado School of Mines and Marathon Oil Company)

101

DEEP BURIAL BRECCIATION IN THE DEVONIAN UPPER ELK POINT GROUP RAINBOW BASIN, ALBERTA, WERSTERN CANADA J. Dravis and I. Muir (Consultant and Imperial Oil Canada, Ltd.)

119

TRENTON LIMESTONETHE KARST THAT WASN'T THERE, OR WAS IT? B.D. Keith and L.H. Wickstrom (Indiana and Ohio Geological Surveys)

167

DESCRIPTION AND INTERPRETATION OF KARST-RELATED BRECCIA FABRICS, ELLENBURGER GROUP, WEST TEXAS C. Kerans (Texas Bureau of Economic Geology) (Published in Oklahoma Geologic Survey Spec. Pub. 91-3)

..... .

181

CASABLANCA FIELD, TARRAGONA BASIN, OFFSHORE SPAIN, A KARSTED CARBONATE RESERVOIR A.J. Lomando, P.M. Harris, D.E. Orlopp (Chevron Overseas Petroleum and Chevron Petroleum Technology Company)

.

201

PALEOKARST DEVELOPMENT IN DEVONIAN CHERTS IN THE ARKOMA BASIN AND BLACK WARRIOR BASIN P. Medlock and R. Fritz (Masera Corporation) ........ . ......

227

..

PALEOKARST WITHIN THE KNOX GROUP OF ALABAMA, EAST SIDE OF BLACK WARRIOR BASIN J.L. Wilson and P. Medlock (Masera Corporation) and R. Sels (Amoco Production Company) ......

vi

....

....... . . ............ . 245

INTRODUCTION TO KARST SYSTEMS AND PALEOKARST RESERVOIRS

Mateu Esteban ERICO Petroleum Information, London

James Lee Wilson Consultant

General Statement

Karst is the product of subaerial (terrestrial and coastal) exposure of carbonate rocks,

recognizable by

features

produced

by

dissolution,

precipitation,

erosion,

sedimentation and collapse in a variety of surface and subsurface landforms, and cave deposits consisting of both cements and sediments. Natural karst constitutes a drainage unit (Fig. 1) consisting of: (I) input of meteoric waters, (2) pre-existing permeability pathways enhanced or reduced by karst flow, and (3) output of resurgent waters with transported sediments and solute. There are various types of karst depending on rock types, insurgence and flow patterns, climate, and etc., corresponding to different modes of porosity creation and destruction. Lithologies can be: (a) tight (dense) with bedding plane control, (b) tight, with fracture control, and (c) porous, with intergranular porosity control. Flow patterns can be diffuse, confluent, allogenic (water collected from non-karst drainage) or authigenic (catchment surface is karst) (Fig. 2).

Most diagenetic models for subaerial exposure in carbonate rocks have been developed in Holocene Caribbean carbonates, forming a karst in very porous carbonates with diffuse recharge and flow, short exposure times, low relief and interaction with coastal exposure environments. This is but one of the many types of karst and these diagenetic models cannot be applied in many of the cases encountered by explorationist

Karst systems present zoned flow patterns, normally with many anomalies in the distribution of the hydraulic potential (hydraulic traps, confined flow) and resulting thermal and chemical zonation The level of regional groundwater saturation (water table, piezometric level) separates the infiltration or vadose zone above and the saturation or phreatic zone below (Fig 3). The water table oscillates periodically and the lower part of the infiltration zone becomes temporarily saturated, seasonally or for longer periods of time. This oscillation zone can be up to 200 m thick in alpine karst systems. Many karst systems present extremely complex flow patterns, with perched water tables and independent flow regimes in the same vertical section.

Karst systems create porosity by dissolution (corrosion), erosion (corrasion) and incasion (collapse). Most of the corrosion in karst results from carbonation of atmospheric CO2 and the formation of carbonic acid. Non-atmospheric CO2 (from organic matter,

hydrothermal sources, mylonites) can contribute under some local conditions. Other particular cases are acid corrosion (humic, sulfuric, organic acids), cooling corrosion, pressure corrosion and biocorrosion. Mixed water corrosion results from mixing of two waters in different equilibrium conditions (different partial CO2 pressures) and is extremely important in karst processes. In terms of water volumes, mixing corrosion is considerably (Continued page 4) 1

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less effective than normal corrosion. However, it is active where other forms of corrosion are inactive and is of maximum importance in the early stages of corrosion developed in deep phreatic zones. For prediction of porosity in karst, it is of critical importance to understand where, when and why the karstification process starts and what controls its evolution through time. In some special cases karst starts at the surface or in the infiltration zone, but in most situations karst is initiated in the deep phreatic zone. In these early stages mixing corrosion is very active, water velocity is low (fluid flow predominates) with very low rates of porosity creation. Once pores and interstices average 1-5 mm in diameter, turbulent flow

can lead to accelerating rates of porosity creation (youthful stage). A karst system is considered mature when or where hydrological zonation and lithological profiles are well established. The senility stage is defined by increasing rates of porosity destruction (cementation, sedimentation, collapse) and decreasing rates of porosity creation. These are 4

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5

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considered stages of development rather than timing (Fig. 4). Parts of a karst profile or even parts of the same cavity can present different stages of development.

Paleokarst Profiles A mature authigenic karst profile (Fig. 5) commonly shows the following vertical zonation: (1) Soil infiltration zone, with regoliths and penetration of the root system; (2) Percolation zone, with vertical passages showing intense sedimentation, collapse and cementation. Also includes relict features from deeper horizons (i.e., cave levels) or local saturation zones. Lost circulation and drilling-bit drops are very common but involve small volumes; (3) Oscillation (vadose - phreatic) and shallow phreatic zones, normally indistinguishable in paleokarst, characterized by predominant horizontal passages and loops with erosional features and locally well developed bedding-plane control. Lost circulation

and drops of G:_lling bits are less frequent but involve larger volumes. Cave sediments in

this zone are evidence of reducing depositional environments (gray-greenish colors), whereas in zones 1 and 2 they are pink-red. Many well logs show a characteristic kick on the gamma ray together with a decrease of the sonic velocity; and (4) Deep phreatic zone, with incipient (mixing) corrosion and/or cementation grading into the unaffected formation. Porosity enhancement (dissolution, erosion) in a karst profile occurs mainly in the oscillation and shallow phreatic zones. Porosity destruction (sedimentation, cementation, collapse) is characteristic of the infiltration and percolation zones. Juvenile stages of evolution of the karst profile favor overall porosity enhancement; advanced mature and senile stages tend to destroy karst porosity. In early stages of development a karst system will present better karst porosities towards the areas of resurgence (zone of increased potential and ascending flow). In mature stages of development of the karst system, karst porosity can be present in vertical profiles in the zone of lateral flow or even in the area of recharge. Changes in base level (sea level) can accelerate or freeze porosity evolution in the karst profile. Abandoned phreatic cave levels in the percolation zone are indicators of drops of base level. Rapid transgressions increase the chances of preservation of karst porosity by preventing long senility stages in the karst system.

Kerans Model of Ellenburger Karst Based on study of a large number of wells producing from the upper Ellenburger on and near the Central Basin Platform in West Texas, C. Kerans (1988) noted a consistent vertical succession of fracturing and breccia development a few hundred feet below the preSimpson unconformity. The sequence results from a total collapse and compaction of an early cave system beneath the unconformity. The lowest beds (cave floor) may consist of coarse breccia blocks originally fallen from the roof. There is little matrix and, if not completely spar-cemented, porosity may occur in this zone. The middle zone (cave fill) is a mixture of breccia rubble with silt or sand matrix emplaced by through--flowing water. A "tattered zone" of gamma-ray deflections marks this zone. The upper (cave roof) zone consists of crackle or mosaic breccia with some open fractures and common secondary porosity. Subsequent studies of the Lower Ordovician in the Val Verde basin and in the Arbuckle of Oklahoma and the Knox of the Black Warrior basin have recognized this same sequence although it may be obscured by tectonic movement along fault planes. See paper numbers 2 and 9 for additional references.

6

SOIL

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Caribbean karst model (James and Choquette, 1988).

7

MARINE WATER

Unconformity Types and Relationships To Reservoirs The complex controls on karstification make it difficult to predict the type of karst

-

influenced reservoirs that can be expected for different kinds of unconformities (see above). The degree example of the complexity can be seen in the three great unconformities of the North American Mid-Continent Paleozoic. The oldest is the pre-Simpson---post-Sauk break which was relatively short, (Whiterockian stage) but has abundant physiographic and lithologic evidence of its presence. It is extremely widespread, lacks evidence of oxidation, and separates the dolomitized Cambro-Ordovician from the largely non-dolomitic Middle Ordovician. The next major unconformity is post-Hunton (Siluro-Devonian) and preWoodford. This is overlain by a black shale blanket covering most of the continent which

was planed off extensively. Subcropping edge-lines of the Siluro-Devonian enable discernment of channels and sinks but there is no oxidation at the unconformity and not much breccia development below it. A rather long time is indicated at the unconformity by

biostratigraphy and most of the Middle Devonian

is

generally missing where the

unconformity is of least duration. The third unconformity at the base of the Pennsylvanian is also widespread over North America where most of the Late Mississippian and earliest Pennsylvanian strata are missing. The carbonate surface formed on the older Mississippian was extensively karstified forming several important oil accumulations. Red colors at this unconformity are common indicating a thoroughly oxidizing climate. The interplay of time, amount of sea level lowering, and climate all must be considered when comparing these important sedimentary "breaks". M. Esteban and J.L. Wilson attempted to classify karstic carbonate reservoirs based on unconformity and sedimentary-tectonic controls. These classes derive from examination of a large number of fields and are as follows: without much megakarst or Carbonate blankets over broad arches paleotopography. Commonly both dolomitization and leaching occur below the unconformity. Example: Edwards (Mid Cretaceous) over San Marcos Arch. A mosaic of paleotopographic highs at subcropping edge line of carbonate unit and channelways related to stream valleys. Examples: Hunton edgeline beneath

Woodford, east flank of Anadarko basin; Madison edgeline east flank of 3

Williston basin, north-central North Dakota. Linear trends of solution-collapse along anticlines, faults and fractures. Examples: Albion-Scipio, Michigan Basin, and Casablanca Field, Offshore Spain

Wedges and mantles of debris above unconformity. Example: Poza Rica in Mexico and Coyanosa in West Texas Layers of carbonate buildups and upward shoaling cycles. Disconformities on top of each cycle

Recognition of Unconformities in the Geologic Record Special Rock types: L Sharp contact of contrasting lithologies Glauconitic and phosphatic concentrations Blackened pebbles

8

Bored surface Oyster plasters Internal sediment filling voids beneath unconformity Caliche crusts Condensed ammonite zones Hard ground on cemented grainstone Lag of re-worked sediment, conodonts, quartz sand and silt Breccia due to solution-collapse Ferrous oxide (red-yellow) staining Dolomitization beneath unconformity Asphalt mat from degraded oil at unconformity Vuggy zones beneath breccia Preserved soil profiles Superimposed hydrothermal veins.

Special Topographic Features Sink holes, dolines Open passages marked by bit drops Unfilled fractures Fractures filled with linear rubble and neptunian dykes Remnant highs, mosaic or escarpment Traceable channel in surface of underlying strata

There remain many questions and unsolved problems relative to karst reservoirs. A greater understanding is required of karst relationships to unconformity, facies, and deep phreatic conditions. No doubt these questions and problems will be addressed and perhaps solved as exploration for paleokarst reservoirs continues.

References

Esteban, M., and Klappa, C.F., 1983, Carbonate depositional environments in subaerial exposure environment, Scholle, P.A., Bebout, D.G., and Moore, CH., eds.,: AAPG Mem. 33, 708 p.

James N.P., and Choquette, P.W., 1988, Paleokarst: Springer-Verlag, New York Berlin, 416 p.

Kerans, C., 1988, Karst-controlled hereogeneity in Ellenburger Group carbonates of West Texas: American Association of Petroleum Geologists Bull., v. 12, p. 11601183.

9

PALEOKARSTIC FEATURES AND THERMAL OVERPRINTS OBSERVED IN SOME OF THE ARBUCKLE CORES IN OKLAHOMA Zuhair Al-Shaieb Oklahoma State University, Stillwater, OK

Mark Lynch Union Oil Company of California, Oklahoma City, OK

Introduction This paper consists of two parts. Part I is a reprint of an earlier paper, "Evidence of

Paleokarstic Phenomena and Burial Diagenesis in the Ordovician Arbuckle Group of Oklahoma," published as part of Circular 92, by the Oklahoma Geological Survey in 1991. It includes detailed descriptions and analyses of paleokarstic features observed in several Arbuckle reservoirs in Oklahoma. It also provides concise evidence concerning the thermal overprints as manifested by dolomitization and other diagenetic patterns.

Part II of this report includes a detailed description of eight cores examined in this study. The petrographic, sedimentological, isotopic, and fluid-inclusion data obtained from Part II were utilized to construct various models concerning the genesis of karstification phenomenon in the Arbuckle rocks, as well as to understand various diagenetic patterns which evolved during the burial of these rocks. These analyses and models were examined in Part I of this report.

Part I Abstract

Cores of Ordovician-age Arbuckle Group carbonates from Oklahoma were examined for evidence of paleokarst. The depositional and diagenetic fabric of the rock was analyzed and compared with outcrop analogs to illustrate the nature of sedimentary, karstic, and diagenetic facies. Burial diagenesis and hydrothermal alteration have in many cases obscured the original fabric of these rocks.

Arbuckle rocks in different tectonic settings and stratigraphic intervals in the subsurface of south-central and north-central Oklahoma display surprisingly similar suites of karstic and diagenetic phenomena. Dissolution cavities, solution-enlarged fractures, collapse breccias, and vugular porosity are present in many cores and attest to the predominance of fabric-destructive processes in the development of Arbuckle paleokarst Collapse breccias

and sediment-filled solution features bear striking resemblance to outcropping analogs.

Primary speleothemic precipitates were not readily observed; either they were not precipitated or were obscured by later dolomitization. Phreatic cements were more commonly encountered than vado se cements.

A complex history of exposure, subsidence, and diagenesis is recorded in these rocks. Although the actual physical manifestations of paleokarst are not difficult to identify, interpretation of the genesis and age of these features is decidedly problematic. Arbuckle carbonates have been exposed to surficial weathering for periods of variable intensity and

duration numerous times in geologic history. Paleokarst horizons may have developed subjacent to disconformities within and between formations of the Arbuckle Group and where these rocks subcrop beneath regional unconformities. This complex hierarchy of 11

unconformities can produce numerous porous horizons whose preservation potential may depend on subsidence rates rapid enough to prevent extensive low-temperature phreatic cementation, thereby preserving the open pore network of the karst profile.

Burial diagenesis is evidenced by the multi-event dolomitization of these rocks. Ferroan and nonferroan "growth-zoned" baroque and limpid phreatic dolomite cements commonly occlude vugular and fracture porosity. Host-rock carbonates have been extensively replaced or neomorphosed. Cathodolumincscent microscopy and chemical staining indicate that "growth-zoned" baroque dolomite is commonly uniform in composition and was precipitated under mildly reducing conditions. Dolomite cementation was arrested by the migration of oil into the remaining pore space.

Introduction The limestones and dolomites of the Arbuckle Group have produced oil and gas from a wide variety of settings throughout the State of Oklahoma. While production is commonly structurally controlled, it can be demonstrated that porosity development is often due to modification of Arbuckle carbonates by (I) erosion at an unconformity surface, (2) fracturing and brecciation related to cavern collapse and/or tectonism, and (3) hydrothermal alteration of host rock in the subsurface. Recent unexpected and significant oil and gas production from Arbuckle paleokarst reservoirs (Shirley, 1988) in the Anadarko and Arkoma basins emphasizes the need for immediate, detailed inquiries into the nature of Arbuckle paleokarst.

Objectives

The primary objective of this study is to provide an inventory of paleokarstic features present in Arbuckle rocks from the subsurface in Oklahoma. The petrology, possible genesis, and subsequent diagenesis of these features are described; a complex set of dolomite types is presented and explained in relation to the burial history of these rocks.

This study is part of an on-going program at Oklahoma State University to investigate the geology of Arbuckle paleokarst in Oklahoma, both on the surface and in the subsurface .

Previous Investigations It has long been recognized that fracturing and the establishment of vugular porosity play an important role in the productivity of Arbuckle reservoirs (Bartram and others, 1950;

Walters, 1958; Gatewood, 1979) However, the study of these features in the context of karst facies is still in its infancy.

Paleokarstic sink holes and sand-filled caves have been identified in Cotter Dolomite outcrops in northeast Oklahoma by Gore (1952). Collapse breccias have been described in the Kindblade Formation in the Arbuckle Mountains by Tapp (1978) and in the Cool Creek

Formation by Ragland and Donovan (1985). Post-Permian to recent karstification of Arbuckle rocks in southern Oklahoma has been documented by Decker and Merritt (1928) and Curtis (1959) Donovan (1987) described speleotherns coeval with oil migration in a small Perrnian paleokarstic reservoir on Bally Mountain, in southwest Oklahoma

12

Disconformities within and between the various formations of the Arbuckle Group have been recognized by Ireland (1955), Chenoweth (1968), Derby (1969), and Reeder (1974), although discussion of the role these events might play in karst formation has been lacking to date.

Arbuckle paleokarst in the subsurface of Kansas has been documented by Merriam and Atkinson (1956) and Walters (1958). Paleokarst features in the Arbuckle-equivalent Ellenburger Group carbonates in Texas have been described by Ijirigho and Schreiber (1986) and Kerans (1988).

Methods and Procedures

Forty-two cores of Arbuckle Group rock housed at the Oklahoma Geological Survey Core and Sample Library in Norman, Oklahoma, were examined in a cursory manner to determine if the rock had in any way been affected by the physical or chemical processes that evidence karstification. Cores that exhibited unusual and possibly karstic features were procured for more detailed analysis. Those that did not exhibit these features were briefly described and cataloged for future reference. Seven cores from a variety of locations in Oklahoma were, on the basis of the initial macroscopic examination, selected for intensive study. Additionally, one other core was studied; it is the property of Oklahoma State University. Figure shows the locations of the cores in relation to the tectonic provinces of Oklahoma. Pertinent data for each core is presented in Table 1. Each core was slabbed and described and data were recorded on a petrolog form that was specifically designed for description of both depositional and diagenetic (karstic) features. Thin sections were prepared for petrofabric analysis utilizing both polarizing and cathodoluminescence 1

microscopes. Many thin sections were stained with alizarine red-S and potassium ferricyanide to distinguish ferroan and nonferroan varieties of dolomite and calcite (see Adams and others [1984] for procedure). A scanning electron microscope coupled with energy

dispersive

X-ray

analyzer

(SEM/EDXA)

was

used

to

investigate

the

crystallinity and chemical composition of individual crystals of each cement species. Preliminary analysis of stable carbon and oxygen isotopes was performed on 35 samples of host rock and dolomite and calcite cements by Paul Wagner, at Amoco Production Co. Research Center in Tulsa, Oklahoma. Conodont elements garnered from insoluble residues of selected intervals were identified by Scott Ritter of the Oklahoma State University School of Geology.

GENERAL GEOLOGY Arbuckle Group Stratigraphy

Figure 2 depicts the stratigraphy of the Arbuckle Group in southern and northeastern Oklahoma. Note especially the locations and relative magnitudes of unconformities within the Arbuckle Group and between the Arbuckle Group and the overlying Simpson Group.

Arbuckle rocks have suffered exposure and erosion numerous times since their deposition. The possibility exists (and must be explored) that porosity and karst may have developed at any or all of these unconformities, given sufficient time of exposure and

environmental conditions. The effects of pre-Simpson erosion on Arbuckle strata

in

northeastern Oklahoma are well documented (Walters, 1958; Reeder, 1974), but may be in question in southern Oklahoma where the Joins Formation of the Simpson Group overlies the West Spring Creek Formation with apparent conformity (Reed, 1957; Derby, 1969;

Latham, 1970). Due to the rapidly subsiding! more mobile, nature of the southern

Oklahoma paleodeep, the "pre-Simpson unconformity" may not be represented by a single

13

\-\. Northern Oklahoma \ Platform

2

i

L.,

,_.L.,

1

--i--..--; i

i I

---,

i

Ozark

I

i

r

I 3i

_ _ :I* ---' Central Oklahoma -4 Platform

i',...t..._

Anadarko Basin

Uplift

t

i

L.

L.

Amarillo

Arkoma Basin

Hunton I...Pei/Iv"; V alley ji

Uplift

Uplitt

ÇflÑìchita - Griner

r-

"

I-

--r r-

Uplif t L

5 '6 _Arbuckle r 4-4r7Ir -Ardmore

i

Oliphant 1-A Nate Oliphant 1-Lafortune Red River Uplift Getty 6-Cobb Cameron 1-Shepherd Shell 1A-3 Wesley Unit Cox 1-Wesley Unit A, TexaCo 1-Mobil Pan American 1-State "C"

Figure 1

E'ite

fi

i

¡Ouachita 4stem TJr

BasIn

20 4,0

60

BO

100

(?

miles

Locations of the cores ( ) used in this study relative to the tectonic provinces of Oklahoma (after Al-Shaieb and Shelton, 1977; Arbenz, 1956). TABLE 1. CORE LOCATION AND INTERVAL INFORMATION interval

Cored

(depth, ft)

formation

Depth below T/Arb. (ft)

Cored Core

Location

Oliphant

15-T24N-R7E

1-A Nate

SWVASWV4SE1A

Oliphant 1 -Lafortune

Getty 6-Cobb

8-T25N-R6E

Cotter

0at top

Burgen (Simpson)

Osage

-3,335 to -3,361

Cotter

0at top

Burgen (Simpson)

Creek Cushing

-2,466 tO -2,517

0at top

Bartlesville

Jefferson Dixie Area

-6,278 to -6,296

3,640

Deese

Carter Healdton

-3,584 to -3,633

Carter Healdton

-3,806 to -3,983 West SpringCr.,

Carter Wildcat

-6,300 to -6,310 -6,519 to -6,534

Cotton Wildcat

-7,475 to -7,500

SDASEV4

20-T3S-R4W

NWY4SW1/4SEV4

1-Mobil

-2,861 to -2,870

3-T17-R7E

Shell 1A-3 Wesley Unit

Texaco

Osage Naval Res.

Burbank

SE1ANW1ANEVI

I-Wesley Unit A

Strata

overlying Arbuclde

SE,ANWANEV4

Cameron 1-Shepherd

E L Cox

County/ field

3-T4S-R3W

3-T4S-R3W NEV4SWV4SW1/4

1-T5S-R1W NEV4SEV.

Pan American

36-T2S-RIOW

1-State C

N1/2NWIANE14

(poor recovery)

Kindblade

1,050

Joins

(Simpson) 870

Joins (Simpson)

420 640

Atoka

Kindblade Kindbladeb

1,775

Kindblade West SpringCr.,

Joins

(Simpson)

'Correlation based on well log signature. 'Correlation based on conodont biostratigraphy.

14

2 L1.1

Lu

to

cc

I-

>(I)

Northern Oklahoma

Southern Oklahoma

tu

cn 0-

Z

0 O

IS

CC

cc

a.

CD

cc C)

Z

, 0

OIL CREEK FM.

BURGENSS.

JOINS FM.

z a

,

0

WEST SPRING CREEK FM. ,..,..-.,..,........

5 5 cc

0 z < Lu CO

j

, 2

KINDBLADE FM.

Y COOL CREEK

co

FM.

z

m

x

.2

cc

2

U

Figure 2

z 3

COTTER

S

o

DCLO.

0

JEFFERSON CITY DOLO.

, .<

ROUBIDOUX FM.

(3)

cr

<

E

POWELL

Y

0D

cc

.:,

L TYNER FM.

,

a.

2

U..1

O

0

McKENZIE HILL

GASCONADE

FM.

DOLO.

---,_BUTTERLY SIGNAL

-2---

EMINENCE

can

DCLO.

MNTN Ls. 7R,ER FT SILL

-.'"--Can

Ls.

1

11 1.1

[1 [1

Stratigraphic column showing correlative formations of the Arbuckle Group in southern and northeastern Oklahoma (modified after Chenoweth, 1968; Ross and others, 1982).

discrete surface at the top of the Arbuckle, but may instead be represented by subtle disconformities within the West Spring Creek Formation (see Derby, 1969).

Although they are lithologically similar, carbonates of the Arbuckle Group in southern Oklahoma are predominantly limestone, while those on the northern Oklahoma platform are dolomite (Ham, 1969). It is worth noting that all of the cores examined in this study were, regardless of location, extensively dolomitized.

Arbuckle Depositional Facies Arbuckle rocks are characterized by numerous and recurring shallowing-upward, peritidal carbonate cycles. Subtidal mudstones and wackestones, intertidal bioclastic packstones and grainstones, and restricted, upper intertidal algal boundstones are commonly observed facies. Exquisite algal stromatolites and thrombolites occur in the Cool Creek Formation, and also in the Kindblade and West Spring Creek Formations more infrequently.

Supratidal, evaporite-dominated sabkha facies are not well developed; either they were never deposited or were removed by erosion subsequent to deposition. In many instances a "typical" shallowing-upward cycle culminates with an intraformational conglomerate which probably represents a period of subaerial exposure, desiccation, erosion, and redeposition. Such erosive events may be correlative to hardgrounds and karst surfaces elsewhere in the 15

Arbuckle environment (Donovan and others, 1983). Shallowing upward sequences are seldom delimited above by supratidal facies such as interlaminated evaporites and dol-algal mats. Evaporites are encountered far more frequently in the literature (Reed, 1957; Latham,

1970; Gatewood, 1978; St. John and Eby, 1978; Beales and Hardy, 1980; Ragland and Donovan, 1985) than in either the outcrop or subsurface. Nodular and remobilized fracturefilling evaporites were noted in very few cores from southern Oklahoma; interestingly, these

cores do not show signs of paleokarstification, nor do they exhibit the coarse dolomite textures found in the principle cores of this study. Evidence for evaporite presence in these

cores is scant and equivocal, such as submillimeter-size anhydrite inclusions in chert nodules, and a few silicified salt hopper crystals recovered from insoluble residues from the Cox-Wesley core.

PALEOKARST FEATURES OBSERVED IN CORES OF ARBUCKLE ROCK

Breccias and Conglomerates Breccias and conglomerates are lithotypes common to each of the Arbuckle Group cores studied. Where possible, the genesis of each conglomerate or breccia was inferred

from the fabric of the rock and its relation to surrounding structures. Breccias were described with a combination of genetic terminology and textural modifiers, as proposed by Norton (1917) and Ijirigho and Schreiber (1986). Blount and Moore (1969) outlined certain criteria for differentiating between genetic breccia types.

Crackle Breccia

The term "crackle breccia" was introduced by Norton (1917) to describe incipient brecciation in extensively fractured rocks where the fragments have not been dislodged, rotated, or otherwise moved to any appreciable degree. This is a structural term that bears no genetic connotations. Ijirigho and Schreiber (1986) modified this term to describe crackle breccias with open or slightly dilated fractures, which they termed "microdil breccias." Kelans (1988) noted that these breccia-types, which he termed "fracture breccian commonly occur in the "cave roof facies" of karstified Ellenburger Group carbonates in West Texas. Usually, this type of breccia is indistinguishable from and indeed, maybe nothing more thanintensely, and perhaps tectonically, fractured rock. It is only through its spatial relations to other karst features that this type of breccia has any use in karst-facies diagnosis.

Definitive crackle breccias were observed locally grading into mosaic or rubble collapse breccias in many of the study cores. Figure 3 shows crackle-brecciated rock subjacent to an intra-Arbuckle disconformity, in the Oliphant Nate core. Early, cracklebreccia porosity is commonly occluded by infill sediment or cement.

Collapse Breccia

Collapse breccia is a term used to imply a genetic origin for breccia texture(s) that result from structural collapse of a previously open mega-pore or cavern network. The structural integrity of the host rock maybe compromised by the dissolution of matrix carbonates and/or interbedded soluble evaporites. Foundering may be induced by gravity at

the surface if the host rock is sufficiently eroded internally, or by the application of overburden subsequent to burial. 16

f)

33

Figure 3

Crackle-brecciated rock subjacent to an intra-Arbuckle disconformity. Many fractures are enlarged and filled with micritic sediment (Oliphant-Lafortune core, -3,352 ft).

Collapse breccias are characterized by marked heterogeneity and angularity of clasts (which may be derived from numerous and lithologically dissimilar overlying stratigraphic units), poor sorting, with interstitial cement or matrix in a dominantly clast-supported fabric (Blount and Moore, 1969). Collapse breccias may exhibit the "mosaic,' "random," and "rubble" textures of Norton (1917) and Ijirigho and Schreiber (1986). Many of the breccias noted in this study exhibited fabrics suggestive of formation by

solution and collapse. A few of the characteristic breccia fabrics observed in cores are described below.

The Pan American-State core from Cotton County (Fig. 4) is comprised nearly entirely of collapse breccia. The clast populationwhich ranges from granule to boulder size and is very poorly sortedis comprised of at least six different lithologies, the most exotic of which is a silicified ooid grainstone that is not present as in-place host rock 17

7493

483 i

7466

G, 7488 I

or" 7499 I 748 a

'F1

7494 I

7497

7489

7487

(7492

tig.465

7498

1:114901

7485 PAN AMERICAN 1 STATE OF OKLA-"C" 36-28-10W NW NE Cenntv

Figure 4

Heterolithic collapse breccia exhibiting random texture (RB) and mosaic texture (MB) (Pan American-State core).

anywhere else in the core. The breccia exhibits a predominantly random fabric. Large, fractured clasts exhibit a mosaic fabric internally. The breccia matrix is a dark-brown, clayey, dolomitic mudstone.

Heterolithic, rubble collapse breccias as much as 4 ft thick were encountered in the Texaco-Mobil core (Fig. 5). These breccia immediately overlie crackle-brecciated host rock.

The Cox-Wesley core from Healdton field exhibits unique and complex "zebroid" brecciation (Beales and Hardy, 1980) and coarse dolomite cementation (Fig. 6A). This type of brecciation has been attributed to host-rock expansion due to displacive

18

Figure 5

Rubble collapse breccia (Texaco-Mobil core, -6,521 fi).

cementation subsequent to evaporite dissolution (Beales and Hardy, 1980). However, neither relict nor extant evaporites were noted in the brecciated interval. In places, the

brecciated zone exhibits a random-fragment packing suggestive of collapse (Fig. 6B). The very angular breccia fragments are cemented with a I-cm-thick rind of isopachous saddle dolomite.

Cavern-Fill Parabreccia The term "cavern-fill parabreccian was proposed by Lynch (1990) for poorly sorted, matrix-supported breccias that exhibit structures suggestive of subterranean deposition. (The precedent for this terminology is the paraconglomerate of Pettijohn [1975], which, with the exception of clast shape, this lithotype texturally resembles). The term is genetic, although structurally it is analogous to Norton's (1917) "pudding breccia," and also to the "pheno breccia" of Ijirigho and Schreiber (1986). Cavern-fill parabreccias are deposited by aqueous media flowing through an open mega-pore or cavern network within a karstified terrain. Phenoclasts may calve off the roof of the cavern in which they are deposited, or be transported from locations of collapse elsewhere in the cavern network (probably during periods of turbulent flow and rapid discharge after storm events). The fine matrix is composed of cave mud or infiltrated sediment. These breccias differ from true solution collapse breccias in that they are comprised mostly of matrix that has filled an uncollapsed cavern.

Analogous

deposits

have

been 19

observed

by

the

authors

in

lik

Figure 6

(A) Zebroid breccia. Dark laminae are laths of host rock, white laminae are subisopachous crusts of columnar and saddle dolomite (Cox-Wesley core, -3,954 11), (B) Random collapse breccia cemented with isopachous saddle dolomite (Cox-Wesley core, -3,949 11).

recent karst in the Lake of the Ozarks region of Missouri.

Figure 7A,B shows two interesting cavern-fill parabreccias in the Cox-Wesley core at depths of -3,939 and -3,972 ft. The parabreccias are lithologically similar; each is comprised of very angular phenoclasts of chert and light blue-gray limestone supported in a matrix of dark-brown, clayey, dolomitic mudstone. The parabreccia at-3,939 ft contains conspicuous limestone phenoclasts that are unlike any other lithology encountered in the cored interval. This breccia was deposited within an eviscerated chert bed, from whence the chert phenoclasts in both this deposit and the one at-3,972 ft appear to have been derived.

Thus it would appear that an open solution-tunnel network of at least 33 vertical feet (and of indeterminable lateral extent) was present within this sequence of rocks. There are no remnants of this pore network that are open currently as effective, channel porosity.

20

A

Figure 7

Cavern-fill parabreccias from the Cox-Wesley core. (A) Parabreccia within an eviscerated chert (CH) bed at -3,939 ft. Angular phenoclasts are blue-gray dolomicrite. (B) Parabreccia within a tan dolomicrite host (HR) at -3,972 ft. Chert phenoclasts (CH) derived from chert bed at -3,939 ft exhibit early, internal fractures that are healed with dolomite cement. Late, open tectonic fractures transect host rock, parabreccia matrix, and chert clasts.

Sedimentary Breccias and Conglomerates

The terms "sedimentary conglomerate" and "sedimentary breccia" are used to describe carbonate rudstones having rounded or angular clasts, respectively, that are formed by erosion of an exposed carbonate terrain adjacent to, or possibly within, the environment of deposition.

While perhaps not specifically related to karstification, these deposits do evince surficial erosion during the time of Arbuckle sedimentation. This type of breccia is analogous to the "depositional breccia" of Blount and Moore (1969).

A 1-ft-thick sedimentary conglomerate comprised of well-rounded pebbles of darkgray mudstone within a light-gray, detrital quartz-sand-rich carbonate mudstone matrix was observed in the Oliphant-Lafortune core (Fig. 8A) The noncontemporaneity of clasts and matrix is indicated by the fact that some of the clasts are internally fractured, and none 21

of them contain the quartz sand that is prevalent in the matrix. The clasts exhibit some rough imbrication which, together with the detrital quartz, suggests the possibility that this is a subaqueous channel-like deposit. The pre-burial formation of this conglomerate is indicated by the transection of both the clasts and the matrix by rather conspicuous stylolites.

The Getty-Cobb core from Cushing field exhibits a sedimentary breccia that contains phenoclasts of dolomudstone and silicified ooid grainstone(?) in a finely laminated, green shale matrix (Fig 8B). The brecciated interval is delimited above and below by transitional contacts with the host rock.

SD

OG

Figure 8

(A) Sedimentary conglomerate comprised of dark-gray dolomicrite pebbles within a sandy, dolomicrite matrix (Oliphant-Lafortune core, -3,349 ft), (B) Sedimentary breccia of clasts of tan, sucrosic dolomite (SD) and silicified ooid grainstone (OG), within a green shale matrix (Getty-Cobb core, -2,504 ft). 22

Dissolution Features The most commonly distinguishable type of dissolution observed in this study was that which was limited to the millimeter or centimeter scale, or those features that are small enough to be contained within the diameter of the core sample. Dissolution vugs, solutionenlarged fractures, and solution channels were observed in the study cores; more often than not the original pore space was occluded by infill sediment or cement.

The type of porosity created in karst terrains depends strongly upon the fabric and lithology of the host carbonate. In low-permeability carbonates such as Arbuckle rocks, vugular and solution-channel porosity (Choquette and Pray, 1970) may be created by the restriction of ground-water flow through high-permeability conduits such as fractures or bedding planes. Extensively karstified terrains may exhibit cavern porosity, which Choquette and Pray (1970) define as the smallest opening that an adult human can enter, or, if the rock is encountered in subsurface drilling, as any opening large enough to cause a noticeable drop in the drill bit.

The vugular porosity encountered in the rocks of this study typically range from 0.5 to 2 mm in diameter, and occasionally as large as 2 cm. The distinction between very large vugular porosity and channel porosity is often arbitrary, especially when determined from small-diameter core samples. Striking examples of solution-induced channel porosity and matrix-vugular porosity were observed in the Oliphant-Nate core from Osage County. The fine (<1 mm in diameter) vugular porosity is notably concentrated in halos that extend a few centimeters outward

from the eroded walls of the solution channels (Fig. 9A). Geopetal sediment, rhombic dolomite cement, and pyrobitumin (in order of emplacement) commonly fill the vugular and channel pore space.

A detailed view of a large vug in the Cox-Wesley core (Fig 9B) shows clayey sediment geopetally filling the open pore. The orientation of the infill sediment relative to

the dipping beds of the host rock suggests that the pore formedor, was at least filledafter the Pennsylvanian tectonism that induced the folding

Cements The morphology of cementation in the karst environment is controlled largely by the amount of water present. In the unsaturated (vadose) zone, cements precipitate from water

that gravitates toward the bottoms of grains or is bound by capillary pressure at grain contacts hence, these cements often display characteristic pendant

(also termed

microstalactitic, or dripstone) and meniscoid fabrics "Phreatic" cements precipitate in solution-filled cavities and tend to form isopachous coatings on pore walls and grains. Speleothemic precipitates (stalactites, stalagmites, dripstone curtains, etc.) composed of laminated calcium carbonate are typically formed in mature-stage karst close to the water table in the lower vadose zone (Esteban and Klappa, 1983)

Karst speleothems such as stalactites and stalagmites were not observed in any of the cores studied; either they were not precipitated or their presence was masked by later diagenetic events. Additionally, the tremendous improbability of actually coring one of these features cannot be discounted.

23

<

1 Inch k.

,

VP

IR

Is

Figure 9

Examples of karstic porosity: (A) Solution channels filled with clayey infill sediment (CS), idiotopic-rhombic dolomite cement (IR), and pyrobitumen (black, in pore space). Note vugular porosity halo (highlighted by black pyrobitumin mottling) surrounding the solution channels (Oliphant-Nate core, -2,863 ft), (B) Enlarged yugular porosity (VP) with infilling sediment geopetally oriented with respect to present structural dip (indicated by black line) (CoxWesley core, -3,902 fi).

Cathodoluminescent microscopy revealed the presence of vadose-type microstalactitic calcite spar cementing breccia clasts in the Texaco-Mobil core (Fig 10A,B).

The macro- and microscopically visible cements in all of the cores studied are principally subisopachous, porelining saddle and rhombic dolomite. Although these cements

are not a product of low-temperature karst cementation (see discussion below), certainly their isopachous morphology suggests precipitation under predominantly phreatic conditions

24

Infill Sediment

The phrase "infill sediment" refers to the sediments that are deposited anywhere within the karst profile, including breccia matrix and pore-filling sediments.

The lithology of the infilling sediment may reflect the origin of the sediment and, roughly, the timing of its deposition within the karst system. Arbuckle-age micrite was commonly observed filling solution-enlarged and crackle breccia fractures (see Fig. 3). Additionally, some solution tubes were filled with what appears to be Simpson Group sand, below the eroded top of the Arbuckle (Fig. 11) It is perhaps significant that the detrital feldspars, polycrystalline quartz, and metamorphic rock fragments that are nearly ubiquitous in Pennsylvanian sandstones were not encountered in any infill sediment in the study cores. The parabreccia matrix described earlier is typically comprised of an admixture of ..

terrigenous clastic clays, carbonate mud, and silty (possibly detrital?) dolomite.

DOLOMITE TYPES AND SEQUENCING The dolomite in the study cores was differentiated into eight types on the basis of crystal habit and fabric, relation to host rock, and mineralogy as determined from energy dispersive X-ray analysis, cathodoluminescence, and staining. Five types of dolomite are considered matrix replacive, or recrystallized host rock, and include: (1) host matrix dolomicrite/dolomicrospar, (2) hypidiotopic-rhombic (sucrosic) dolomite, (3) clayassociated hypidiotopicrhombic dolomite, (4) saddle-rhombic dolomite, and (5) xenotopic dolomite. Cements that were precipitated in open pore space include (1) saddle dolomite, (2) columnar dolomite, and (3) idiotopic-rhombic dolomite. The characteristics of each type of dolomite are discussed below. Many of the dolomite types described in this study are similar

to those considered by Lee and Friedman (1987) as being indicative of deep burialdiagenesis in Ellenburger Group carbonates in Texas.

Matrix-Replacive Dolomites The host rock or matrix carbonate in each of the cores studied is predominantly, if not entirely, composed of dolomite. The episodic and pervasive nature of the dolomitization has made it difficult in some instances to determine whether dolomite is syndepositional, neomorphic after a dolomitic precursor, or replaced calcitic matrix.

Host Rock Dolomicrite/Dolomicrospar The peritidal carbonate mudstones in these Arbutkle rocks are composed principally of very finely crystalline dolomicrite and/or dolomicrospar. Individual crystals range from 0.01 to 0.05 mm in diameter and are usually anhedral in shape. These matrix dolomites exhibit dull red luminescence and a dirty appearance under plane polarized light (see Fig. 12A). The fine crystal size and pervasive nature of this dolomite suggest a primary, syndepositional or perhaps eogenetic origin. Petrographic evidence indicates that this type of dolomite formed prior to fracturing, brecciation, and stylolitization. This type of dolomite was observed in all of the study cores.

25

.

. !.A

L

-,

32 mm

.. ', B

z

.

.32 mm

(A) Figure 10 Thin-section photomicrographs of microstalactitic calcite spar. Cathodoluminescence reveals the delicate spar crystals (MS) that grew downward from the bottom of a breccia clast (BC). (B) The presence of the

microstalactitic

spar (MS)

is

masked

in

this

plain

polarized

light

photomicrograph by later blocky, equant spar (BS). Faint growth zonations are visible internally (Texaco-Mobil core, -6,305 ft "Als

,

OA

3333; Figure 11 Solution channels filled with quartz-arenite sandstone (QA) of the overlying Burgen Formation (Simpson Group) (Oliphant-Lafortune core, -3,336 fi). 26

Hypldiotopic-Rhombic (Sucrosic) Dolomite

This type of dolomite is characterized by a porous fabric of loosely interlocking hypidiotopic to idiotopic rhomb shaped crystals (Fig. 12B). The highest intercrystalline porosity values (7-13%) of any of the dolomite textures described in this study were encountered in this dolomite. This friable dolomite is what is commonly referred to as "sucrosic dolomite" in the subsurface vernacular. Individual euhedra range in size from 0.1 to 0.3 mm and are commonly comprised of a dirty, inclusion-rich core and a limpid outer rim. This dolomite is slightly ferroan, and has dull red luminescence. Crystals of this dolomite are noticeably larger than those of the clay-associated hypidiotopic dolomite described below, which they otherwise closely resemble.

Sucrosic dolomite is a common constituent in the fractured and brecciated host rock

of the Getty-Cobb core. It is suggested that this dolomite is a product of eogenetic diagenesis.

Clay-Associated Hypidiotopic-Rhombic Dolomite The presence of inclusions of clay in this matrix replacive dolomite suggests that it was formed by replacement of a precursor lithology. This dolomite is limited in areal extent (on a millimeter or centimeter scale) to terrigenous clastic clay-rich seams in the carbonate host (Fig. 12C). This type of dolomite is typically fine crystalline, ranging in size from 0.05 to 0.20 mm. Individual crystals may be inclusion-free (clear throughout) or inclusion-rich (dirty throughout), or may exhibit fine growth zonation due to the inclusion of impurities imparted from the replaced host during crystal growth. Little or no mineralogical variance was observed across inclusion-rich and inclusion-free zones, and the dolomite tends to have an even, dull red luminescent quality. This type of dolomite was encountered in the CoxWesley and Shell-Wesley cores from Carter County.

Saddle-Rhombic Dolomite This interesting dolomite texture was observed only in the Shell-Wesley core. This type of dolomite is characterized by medium to coarse (0.4-1.0 mm) subhedral crystals that form a well-cemented, interlocking hypidiotopic fabric, with interstitial silty clay. The crystals are uniquely shaped, appearing rhombic in outline but with curved crystal faces reminiscent of pore-filling saddle dolomite cement (Fig. 12D). Radke and Mathis (1980) suggest that this type of dolomite is a matrix-replacive form of saddle dolomite, which is supported by the observation that the dolomite in this study contains inclusions of clay and quartz silt. Extinction is strongly undulose, often with opposing comers going extinct at the same time. This type of dolomite is characteristically growth zoned, although its luminescence is usually a non-zoned dull red.

Xenotopic Dolomite The term "xenotopic" was proposed by Friedman (1965) for diagenetically altered carbonate rocks having a fabric comprised predominantly of anhedral crystals.

Fine- to medium-crystalline xenotopic dolomites were observed in the TexacoMobil, Cox-Wesley, and Shell Wesley cores. Crystals range in size from 0.1 to 0.5 mm, and are usually arranged in a tightly interlocking fabric with little or no intercrystalline porosity. "Ghosts" of spherical and elliptical grains (perhaps ooids) were observed within 27

the dolomite crystals, attesting to its replacement origin (Fig. 12E). In addition to replacing grainstones, the uniformly dirty appearance of some of this dolomite suggests that it also replaced wackestone mudstone (Fig. 12F). Individual anhedra tend to be smaller in the mudstone-replacive variety, although both varieties tend to appear dull red under cathodoluminescence.

Anhedral crystals that grow into pore space often exhibit subhedral to euhedral terminations. Additionally, some of the anhedra exhibit faint internal rhombic growth zonation, which suggests that crystallinity diminished as the dolomite crystals grew together

in competition for the available pore space. This feature is more commonly observed in those xenotopic dolomites that replace grainstone than in the wackestone/mudstonereplacive variety. It would appear that the space available for crystal growth and the initial lithology of the replaced host carbonate exert some degree of control over the texture of the final dolomite.

Pore-Filling Dolomites Three different morphologies of pore-filling dolomite cement were observed in the study cores. These dolomites exploit many different types of porosity, including yugular, channel and fracture porosity, and breccia-intergranular porosity.

Saddle Dolomite

Spear-type saddle dolomite (Radke and Mathis,

1980)

occurs as a void-filling

cement in the Cox-Wesley, Shell-Wesley, and Texaco-Mobil cores from Carter County, and in the Cameron-Shepherd core from Jefferson County. Typically, it occurs as subisopachous

crusts of hypidiptopic to idiotopic crystals oriented normal to pore walls or grain

boundaries (Fig. 13A). Saddle-dolomite crystals, which range in size from 0.5 to 5.0 mm, commonly exhibit pronounced undulose extinction. Individual crystals may be growthzoned or, less commonly, clear or dirty throughout. Under cathodoluminescence, this type of dolomite exhibits subtle zonation comprised of dull red, bright red, yellow and nonluminescent laminae, attesting to the slight variance in chemical composition across each layer (Fig. 13B).

Radke and Mathis (1980) postulate that saddle dolomite forms at temperatures and thus would imply a deep burial originor a hydrothermal origin at shallower depthsfor this dolomite. It is interesting to note that the isotopic signature obtained for the saddle dolomites in this study (from rocks at -3,000 to 6,000 ft) is nearly identical to that observed by Lee and Friedman (1987) in saddle dolomite from Ellenburger carbonates at-15,000 to -20,000 ft. >80°C,

Columnar Dolomite

This type of dolomite is similar in appearance to the saddle dolomite described above, except that the columnar variety has a greater length-to-width ratio than the saddle variety. Columnar dolomite also occurs as normal-to-substrate isopachous crusts (Fig. 14). Strong, sweeping undulose extinction characterizes this dolomite. This type of dolomite is predominantly inclusion-rich, appearing dirty throughout. This dolomite also exhibits dull

red luminescence. Columnar dolomite was only encountered in the "zebroid" breccia interval in the Cox-Wesley core from Carter County.

28

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Figure 12 Thin section photomicrographs of host-rock-replacive dolomite types: (A) host dolomicrospar, (B) sucrosic dolomite, (C) clay-associated hypidiotopic-rhombic dolomite, (D) saddle-rhombic dolomite, (E) grainstone-replacive xenotopic dolomite, (F) mudstone-replacive xenotopic dolomite (All photos taken under plain polarized light).

Idiotopic-Rhombic Dolomite Clear, equant rhombs of dolomite occur in the vugular pore space of the OliphantNate, Cameron-Shepherd, and Pan American-State cores. Individual crystals range from 0.5

to 5 mm in diameter, have straight to slightly undulose extinction, and are typically nonluminescent. They occur as vug-lining, though not isopachous, cements (see Fig. 9A) Individual crystals are not interlocked to the extent observed in the saddle dolomites.

.

Dolomite Paragenesis

Paragenesis of the various dolomite types was inferred from the mutual crosscutting relationships exhibited by types, by their occurrence relative to the structural fabric 29

of the rock and by consideration of certain crystal textures that are known to be temperature-dependent. Although dolomite-formation temperatures have not yet been verified by fluid-inclusion analyses, preliminary investigations into the stable-isotopic signature of these dolomites suggests that most of the dolomites formed at elevated temperatures. Additionally, the mineralogy of all dolomite types, as indicated by energy dispersive X-ray analysis, is slightly ferroan.

The first dolomite to form was the fine-crystalline dolomicrite/dolomicrospar, either as sedimentary dolomite within the environment of deposition or by replacement of lime mud at shallow burial depths. The occurrence of the sucrosic hypidiotopic dolomite within oomoldic porosity and replaced matrix carbonates suggests that it is a product of eogenetic diagenesis. The clay-associated hypidiotopic-rhombic form probably began forming within clayrich seams subsequent to increasing burial; the concentration of this dolomite along clay seams may reflect incipient stylolite formation. With increasing depth of burial, Arbuckle Group rocks were exposed to elevated formation temperatures and perhaps to heated subsurface brines; at this time the xenotopic dolomite probably began to form. Gregg and Sibley (1984) suggest that xenotopic dolomite precipitates at temperatures >50°C. The idiotopic-rhombic pore-filling dolomite cement may have precipitated from fluid of similar, or slightly cooler, temperature.

The precipitation of columnar and saddle dolomite probably commenced in stages, from fluid heated to >80°C (Radke and Mathis, 1980). This would tend to suggest that this dolomite formed either at depths sufficient to encounter formation temperatures >80°C, or

that heated subsurface brines ascended through available conduits such as fractures to dolomitize the rock at shallower depths. Hagni (1976) has shown that Mississippi-Valleytype ore deposits nearly always contain sparry saddle dolomite as the most prevalent gangue

mineral, and that the single most common factor among this type of deposit is their occurrence at or near the surface. The ubiquitous presence of growth zonation in this dolomite type suggests that it accreted layer by layer, over an incalculable period of time. Petrographic evidence strongly suggests episodic migration of a fluid capable of periodically dolomitizing and corroding the carbonate host (see Fig. 15). The absolute timing of the pervasive saddle-type dolomitization is difficult to assess.

It is certainly plausible to propose that the extensive fracture systems which developed during Pennsylvanian tectonism could have acted as conduits for ascending, dolomitizing hydrothermal fluids. This premise is substantiated by Kranak (1978), who recognized Pennsylvanian-age Mississippi-Valley-type sphalerite mineralization in the Butterly Dolomite in Murray County, Oklahoma. It is likewise possible that the dolomitization could

have occurred earlier by the same general process. We suggest that the petrographic and structural data can explain the episodic ascension of dolomitizing hydrothermal fluids, subsequent to the collapse stage of karstification and Pennsylvanian tectonism. Dolomitization was probably a long-ranging process that continued throughout the burial history of the rock.

ARBUCKLE PALEOKARST MODEL AND EVOLUTION

Karst facies in the Arbuckle Group cores of this study are typified by collapse breccias and dissolution features; low temperature speleothemic cements were not observed

in appreciable amounts. The paucity of karstic cements may indicate that Arbuckle paleokarst developed in a semi-arid environment with a low water budget

30

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Figure 13 Thin-section photomicrographs of pore-filling saddle dolomite: At least two

generations of cement are present and discernible under plain light (A) and

cathodoluminescence (B) First-generation cement (SDI) is adjacent to host rock (HR), is solution pitted and has less-distinct growth zoning, and has alternating bright red and yellow luminescent zones. Second-generation cement (SD2) was precipitated as an overgrowth on the earlier cement. The second-generation dolomite exhibits more-intense growth zoning, and dull red luminescence. The establishment of karst probably began with the dissolution of massive Arbuckle

carbonates along open ground-water conduits such as fractures and bedding planes. Solution-enlarged fractures would develop into channel porosity, and perhaps into cavern porosity. Although cavernous porosity was not directly observed in this study, its former presence can be inferred from extant features such as collapse breccias. Certainly, the 20+ ft of heterolithic collapse breccia in the Pan American-State core necessitates a precursor cavity of similar vertical extent, into which the roof rock was able to collapse. The lateral extent of such features is perhaps unknowable, but for comparison purposes it is interesting to note that Curtis (1959) has cataloged present-day Arbuckle caves in Murray County that extend as much as 14,050 ft in total length. Such caves typically exhibit linear trends that may indicate a joint-controlled origin. It has been suggested that karst facies elements such as collapse breccias may attain thicknesses of 50-100 ft, and may be present hundreds of feet beneath a paleoexposure surface (Kerans, 1988).

The timing of Arbuckle paleokarstification is less obvious than the features that result from it. Data presented in this and other studies suggest that the paleokarstification of Arbuckle strata may have been an episodic phenomena that occurred (in varying degrees) when Arbuckle rocks were exposed at local or inter-regional unconformities. Numerous authors (i.e., Gore, 1952; Ireland, 1955; Merriam and Atkinson, 1956; Walters,

31

,65mm

Figure 14 Thin-section photomicrograph of pore-filling columnar dolomite This cement occurs as drusy, isopachous crusts on laths of host rock (1-IR) in the zebroid breccia interval, in the Cox-Wesley core (Cross polarized).

Figure 15 Thin-section photomicrograph of solution-pitted saddle-dolomite cement. Silty dolomite (SD) geopetally infills the solution micro-porosity (MP) (Plain polarized light).

1958; Kerans, 1988) have documented the presence of paleokarst in Arbuckle and equivalent rocks at the pre-Simpson unconformity in Oklahoma, Texas, and Kansas. Walters (1958) has shown that the porosity present in Arbuckle reservoirs along the central Kansas uplift is directly related to Pennsylvanian karstification that probably modified preSimpson paleokarst.

Data presented in this investigation suggest that the establishment of Arbuckle paleokarst may be due to multiple episodes of subaerial exposure and erosion during, and subsequent to, the time of Arbuckle deposition The presence of similar and biostratigraphically equivalent conodont species (Ritter, personal communication, 1989) in both the matrix and clasts of cavern-fill parabreccia and collapse breccia, and the complete lack of younger (Simpson, Pennsylvanian, etc.) fauna suggests that karstification occurred during Arbuckle time. An argument for karstification at the pre-Simpson unconformity can

be made for the cores from Osage County that contain Simpson-derived sandstone in solution channels and vugs.

Note that these cores also exhibit intraformational

disconformities and sedimentary breccias that are clearly of Arbuckle age. With the

exception of a thin, sandy residual zone at the top of the Getty-Cobb core, no evidence for Pennsylvanian erosion was found in the cores of this study. This in no way negates the possibility of Pennsylvanian karst occurring in other areas where Arbuckle rocks subcrop beneath the pre-Permian unconformity (for example, see Donovan, 1987). Walters (1958) speculated that the entire thickness of the Arbuckle Group in Kansas served as a vast regional aquifer during the Pennsylvanian, much as it does presently in the Tri-State region of Kansas, Missouri, and Oklahoma (Macfarlane and Hathaway, 1987) 32

Preservation of karst porosity appears fortuitous and uncommon, based on the rocks in this study. Solution channel porosity is commonly occluded by infill sediment, while the interparticle porosity in collapse breccias is typically cemented or filled with sediment. Data from the Oliphant-Nate core suggest that rnigration of hydrocarbons into solution channels

and vugs effectively arrested dolomite cementation, thereby preserving the open pore network. Much of the effective porosity currently present in these rocks appears to be intimately related to diagenetic modification of the karst profile as it was buried and then structurally modified by later tectonism.

SUMMARY AND CONCLUSIONS

Paleokarstification of upper Arbuckle Group rocks is evidenced in cores from Oklahoma by (1) breccia facies that form as a result of collapse, subterranean sedimentation, and surficial erosion; (2) vadose cements (infrequently); and (3) dissolution structures, such as enlarged vugular and channel porosity, and solution-enlarged fractures. The presence of

Arbuckle-age conodonts in breccia matrix (and the complete lack of younger fauna) suggests that much of the karstification observed may be of Arbuckle age.

A complicated burial-diagenetic history for these rocks is indicated by the variety and paragenesis of matrix and pore-filling dolomite types. Eogenetic, matrix-replacive dolomite types suggest early diagenesis, while coarse-crystalline, xenotopic matrix-replacive varieties indicate burial diagenesis at temperatures >50°C. Pore-filling saddle and columnar

dolomites likely precipitated from ascending hydrothermal fluids that may be related to hydrocarbon migration. That these fluids were alternately constructive (precipitating) and destructive (dissolving) is indicated by the presence of compositionally zoned and solutionpitted dolomite cements. Breccia-intergranular porosity (where preserved) and tectonic fractures provided the necessary conduits for fluid migration. Dolomite cementation was apparently long-ranging, but was arrested by the migration of hydrocarbons into pore space.

ACKNOWLEDGMENTS We extend our appreciation to the University Center of Energy Research (UCER) for supporting this project. This project was partially supported by a research grant awarded

to Mark Lynch by the Research Committee of the American Association of Pertroleum Geologists.

SELECTED REFERENCES Adams, A. E.; MacKenzie, W. S.; and Guilford, C., 1984, Atlas of sedimentary rocks under the microscope: John Wiley and Sons, New York, 104 p.

Al-Shaieb, Z.; and Shelton, J. W., 1977, Evaluation of uranium potential in selected Pennsylvanian and Permian units and igneous rocks in southwestern and southern Oklahoma: U.S. Department of Energy Open-File Report GIBX-3S, 248 p.

Arbenz, 1. K., 1956, Tectonic map of Oklahoma: Oklahoma Geological Survey Map GM-3, scale 1:750,000.

Bartram, 1. G.; Imbt, W. C.; and Shea, E. F., 1950, Oil and gas in Arbuckle and Ellenburger Formations, Mid-Continent region: American Association of Petroleum Geologists Bulletin, v. 34, p. 682-700. 33

Beales, F. W.; and Hardy, JI L., 1980, Criteria for the recognition of diverse dolomite types with an emphasis on studies on host rocks for Mississippi-Valley-type ore deposits, in Zenger, D. H.; Dunham, J. B.; and Ethington, R. L. (eds.), Concepts

and models of dolomitization: Society of Economic Paleontologists and Mineralogists Special Publication 28, p. 197-213.

Blount, D. N.; and Moore, C. H., 1969, Depositional and nondepositional carbonate breccias, Chiantla Quadrangle, Guatemala: Geological Society of America Bulletin, v. 80, p. 429-442.

Burgess, W. J., 1964, Stratigraphic dolomitization in Arbuckle rocks in Oldahoma, in McHugh, J. W. (ed.), Symposium on the Arbuckle: Tulsa Geological Society Digest, v. 32, p. 45 48.

Burgess, W. J., 1968, Carbonate paleoenvironments in the Arbuckle Group, West Spring Creek Formation, Lower Ordovician, in Oklahoma: Columbia University unpublished Ph.D. dissertation, 91 p,

Chenoweth, P. A., 1968, Early Paleozoic (Arbuckle) overlap, southern Mid-Continent, United States: American Association of Petroleum Geologists Bulletin, v. 52, p. 1670-1688.

Choquette, P. W.; and Pray, L. C., 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: American Association of Petroleum Geologists Bulletin, v. 54, p. 207-250.

Curtis, N. M., 1959, Caves in the Arbuckle Mountains area, Oklahoma: Oklahoma Geology Notes, v. 19, p. 20-31. Decker, C. E.; and Merritt, C. A., 1928, Physical characteristics of the Arbuckle limestone: Oklahoma Geological Survey Circular 15, 56 p.

Derby, J. R., 1969, Revision of the Lower Ordovician-Middle Ordovician boundary in western Arbuckle Mountains, Oklahoma, in Ham, W. E., Regional geology of the Arbuckle Mountains, Oklahoma: Oklahoma Geological Survey Guidebook 17, p. 35-37.

Donovan, R. N., 1987, The world's smallest oil field?: Oklahoma Geology Notes, v. 47, p. 238, 291.

Donovan, R. N.; Beauchamp, W.; Ferraro, T.; Lajek, C.; McConnell, D.; Munsil, M.; Ragland, D.; Sweet, B. and Taylor, D., 1983, Subsidence rates in Oklahoma n the Paleozoic: Shale Shaker Digest, v. 33, p. 86-88. dung Esteban, M.; and Klappa, C. F., 1983, Subaerial exposure environment, in Scholle, P. A.; Bebout, D. G.; and Moore, C. H. (eds.), Carbonate depositional environments: American Association of Petroleum Geologists Memoir 33, p. 1-54. Friedman, G. M., 1965, Terminology of crystallization textures and fabrics in sedimentary rocks: journal of Sedimentary Petrology, v. 35, p. 643-655.

Gatewood, L. E., 1978, Stratigraphic trap possibilities in the Arbuckle Group: general relationships: Shale Shaker Digest, v. 28, p. 219-227.

34

Gatewood, L. E., 1979, Some Oklahoma Arbuckle production and thoughts on fracturing: Shale Shaker Digest, v. 29, p. 4-11.

Gore, C. E., 1952, The geology of a part of the drainage basins on Spavinaw, Salina and Spring Creeks, northeastern Oklahoma: Tulsa Geological Society Digest, v. 20, p. 144-179.

Gore, C. E., 1953, Cave sandstones in Cotter Dolomite, northeastern Oklahoma [abstract]: American Association of Petroleum Geologists Bulletin, v. 37, p. 2186 2188. Gregg, J. M.; and Sibley, D. F., 1984, Epigenetic dolomitization and the origin of xenotopic dolomite texture: Journal of Sedimentary Petrology, v. 54, p. 908-931.

Hagni, R. D., 1976, Tri-State ore deposits: the character of their host rocks and their genesis, in Wolf, K H. (ed.), Handbook of strata-bound and stratiform ore deposits. II.Regional studies and specific deposits: Elsevier, New York, p. 457-494.

Ham, W. E., 1955, Origin of dolomite in the Arbuckle Group, Arbuckle Mountains, Oklahoma, in Moore, C. A. (ed.), Proceedings of the fourth symposium on subsurface geology, University of Oklahoma, p. 67-74.

Ham, W E., 1969, Regional geology of the Arbuckle Mountains, Oklahoma: Oklahoma Geological Survey Guidebook 17, 52 p.

Ham, W. E.; and Wilson, J. L., 1967, Paleozoic epeirogeny and orogeny in the central United States: American Journal of Science, v. 265, p. 332-407.

Ijirigho, B. T.; and Schreiber, J. F., Ir., 1986, Origin and classification of fractures and related breccia in the Lower Ordovician Ellenburger Group, West Texas: West Texas Geological Society Bulletin, v. 26, p. 9-15.

Ireland, H. A., 1955, Pre-Cambrian surface in northeast Oklahoma and parts of adjacent states: American Association of Petroleum Geologists Bulletin, v. 39, p. 468483.

Kerans, C., 1988, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas: American Association of Petroleum Geologists Bulletin, v. 72, p. 1160-1183.

Kranak, P. V., 1978, Petrography and geochemistry of the Butterly Dolomite, and associated sphalerite mineralization, Turner Prospect, Arbuckle Mountains, Oklahoma: Oklahoma State University unpublished M.S. thesis, 76 p.

Latham, 1. W., 1970, Petroleum geology of the Healdton Field, Carter County, Oklahoma: American Association of Petroleum Geologists Memoir 14, p. 255-276.

Lee, Y. I.; and Friedman, G. M., 1987, Deep-burial dolomitization in the Ordovician Ellenburger Group carbonates, West Texas and southeastern New Mexico: Journal of Sedimentary Petrology, v. 57, p. 544-557

Lynch, M. T., 1990, Evidence of paleokarstification and burial diagenesis in the Arbuckle Group of Oklahoma: Oklahoma State University unpublished M.S. thesis, 163 p.

35

Macfarlane, P. A.; and Hathaway, L. R., 1987, The hydrogeology and chemical quality of

ground waters from the Lower Paleozoic aquifers in the Tri-State region of Kansas, Missouri, and Oklahoma: Kansas Geological Survey Ground Water Series 9, 37 p.

Merriam, D. F.; and Atkinson, W. R., 1956, Simpson filled sinkholes in eastern Kansas: State Geological Survey of Kansas Bulletin 119, pt. 2, p. 61-80. Norton, W. H., 1917, A classification of breccias: Journal of Geology,v.25,p.160 194.

Pettijohn, F. J.,1975, Sedimentary rocks [third edition]: Harper and Row, New York, 628 p.

Radke, B. M.; and Mathis, R. L, 1980, On the formation and occurrence of saddle dolomite: Journal of Sedimentary Petrology, v. 50, p. 1 149-1 168.

Ragland, D. A., and Donovan, R. N., 1985, The Cool Creek Formation (Ordovician) at Turner Falls in the Arbuckle Mountains of southern Oklahoma: Oklahoma Geology Notes, v. 45, p. 132148. Reed, B. K, 1957, Geology of the Pre-Atokan unconformity of portions of Love and Carter Counties, Oklahoma: University of Oklahoma unpublished M.S. thesis, 59 p.

Reeder, L. R., 1974, The control of potential Arbuckle hydrocarbon traps in northeastern Oklahoma by Precambrian topography: Shale Shaker Digest, v. 24, p. 84-93. Ross, R. J.; and others, 1982, The Ordovician System in the United States, correlation chart and explanatory notes: International Union of Geological Sciences Publication 12, 73 p.

Sargent, K. A., 1969, Geology and petrology of selected tectonic dolomite areas in the Arbuckle Group, Arbuckle Mountains, south-central Oklahoma: University of Oklahoma unpublished M.S. thesis, 85 p. Shelton, J. W.; Stewart, G. F.; and Al-Shaieb, Z., 1987, Depositional environments,

diagenesis and porosity development, and petrophysical features of selected Pennsylvanian sandstones in Oklahoma: an examination of outcrop and subsurface data: American Association of Petroleum Geologists, Mid-Continent Section, Field-Trip Guidebook, 30 p.

Shirley, K., 1988, Deeper zone rejuvenates old basinOklahoma Arbuckle targeted: American Association of Petroleum Geologists Explorer, v. 9, p. 10-13.

St. John, J. W., Jr.; and Eby, D. E., 1978, Peritidal carbonates and evidence for vanished evaporites in the Lower Ordovician Cool Creek FormationArbuckle Mountains, Oklahoma: Gulf Coast Geological Society Transactions, v. 28, p. 589-599.

Tapp, J. B., 1978, Breccias and megabreccias of the Arbuckle Mountains, southern Oklahoma aulacogen, Oklahoma: University of Oklahoma unpublished M.S. thesis, 126 p.

36

Voss, R. L.; and Hagni, R. D., 1985, The application of cathodoluminescence microscopy to the study of Sparry Dolomite from the Viburnum Trend, south-east Missouri, in Hansen, D.; and Kopp, 0. C. (eds.), Mineralogy: applications to the minerals industry: American Institute of Mining Engineers, New York, p. 51-68. Walters, R. F., 1958, Differential entrapment of oil and gas in Arbuckle dolomite of central Kansas: American Association of Petroleum Geologists Bulletin, v. 42, p. 21332173

.

PART II

CORE DESCRIPTIONS, PETROLOGS, AND PHOTOGRAPHS The location of each core examined is shown in Figure 1 (Part I). Specific location and other core-interval data are presented in Table I (Part I). Each core was slabbed and

described on a petrolog form that was designed specifically for description of both

depositional and diagenetic (karstic) features. In addition, photographs of each of the cores described are also included in this part. Although Part II is focused only on detailed descriptions of the cores involved in this

study, the reader is advised to examine Part I of this report to comprehend further the descriptions and interpretations of numerous karstic features in rocks from a wide variety of settings within the state of Oklahoma.

Core: Oliphant 1-A Nate Location: Sec. 15, T24N, R7E, Osage County, OK Cored Interval: -2,861 to -2,870 ft Stratigraphic Interval: Simpson and Arbuckle (Cotter Formation) Groups Core Description: The following description is given in reference to the core photograph (Fig. 16) and to the petrolog of this core (Pl. 1). -2870 ft to -2865 This interval of interbedded, tan dolomudstones, packstones of rounded algal debris, and minor stromatolitic algal boundstone of middle to upper intertidal origin. Numerous subvertical and subhorizontal fractures transect these lithologies; some fractures are healed with ferroan dolomite or calcite cement, and some are open. Solution-enlarged fenestral porosity in the stromatolitic lithology is filled with pyrobitumen and dolomite cement. A two inch-thick zone or granule to pebble sedimentary breccia is present at -2869.5 ft.

-2865 ft to -2861 ft

This interval is comprised of cherty, dolomitic subtidal mudstones. Disconformity surfaces, dissolution channels and rubbly breccias evidence paleokarstification in these rocks. Channel porosity up to 0.75 inches in diameter has been partially or entirely occluded by infilling clay, subisopachous dolomite cement and pyrobitumen.

Arbuckle rocks are unconformably overlain by Simpson sandstone at -2861 ft. Solution-enlarged fractures below the unconformity may contain sand derived from the overlying Simpson beds.

37

Core: Oliphant 1-Lafortune Location: Section 8, T25N, R6E, Osage County, OK Cored Interval: -3335.5 to -3361.5 ft Stratigraphic Interval: Simpson and Arbuckle (Cotter Formation?) Groups Core Description: The following description is given in reference to the core photograph (Fig. 17) and to the petrolog of this core (Pl. 2). -3361.5 ft to -3350 ft This interval is comprised of dense, dolomitic mudstones, with a thin silty, greenish shale present at -3355 ft.

A faint, clast-supported brecccia of uncertain origin is present at -3359 to -3360 ft.

A thin, quartz sand-rich dolomudstone bed at -3354.9 ft shows the effects of in-place brecciation, perhaps due to foundering instigated by compaction of the underlying shale. An erosional disconformity is present at -3352 ft; mudstones below the unconformity are

crackle-brecciated, while the mudstones above appear brecciated by collapse.

The

dolomudstones of this interval may be of subtidal origin.

3350 ft to -3335.5 ft This interval comprises interbedded dolomitic mudstones, packstones, grainstones and algal laminites of probable intertidal origin. A sedimentary conglomerate containing pebbles of dolomudstone within a quartz sand-rich dolomicrite matrix occurs at -3349 to -3350 ft. Noncontemporaneity between clasts and matrix is suggested by the fact that the clast lithology is entirely devoid of quartz sand. The conglomerate is overlain by 1.5 ft of algal boundstone that has faint stromatolitic structure. The uppermost 7.5 ft of this interval (-3347.5 to -3335 ft) contains interbedded dolomudstones and intraclast packstones that

appear karstified; the rocks are randomly brecciated and are transected by dissolution channels that are filled with Arbuckle-derived bioclastic and lithoclastic debris and Simpson sand. Shaley sandstone of the Simpson of the Simpson Group (Burgen?) overlies truncated Arbuckle rock in planar unconformity at -3335.5 ft.

Core: Getty 6-Cobb Location: Sec. 3, T17N, R7E, Creek County, OK Cored Interval: -2466 to -2517 ft Stratigraphic Interval: Arbuckle Group (formation unknown) Core Description: The following description is given in reference to the core photograph (Fig. 18) and to the petrolog for this core (Pl. 3). 2517 ft to -2487 ft This interval is mostly comprised of tan to dark brown, sucrosic dolomudstones that are probably of subtidal origin. Numerous breccias are present, including collapse breccia from -2510 to -2507 ft, -2503 ft to -2498 ft, and also -2490 ft to -2487 ft. The brecciainternal sediment consists of well-rounded detrital quartz and feldspar and dolomudstone. Additionally, a sedimentary breccia is present at -2506.5 ft to -2504 ft. In this breccia, pebble- to cobble-sized clasts of dolomudstone and chert are surrounded by interlaminated green shale and dolomudstone. 2481 ft to -2466 ft This interval is comprised of collapse breccia in brown, sucrosic intertidal dolomudstones and ooid wackestones/packstones. Beccia clasts range from granule to boulder size, and considerable rotation of some clasts is evident. The sucrosic dolomite in the ooid wackestone/packstone facies has well developed oomoldic and intercrystal-line porosity. Fine, well-rounded quartz sand is present within the breccia-internal sediment; igneous and metamorphic rock fragments were not observed. 38

tir , _

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39

-2466 ft to -2465(?) ft

This thin zone comprises the residual conglomerate at the eroded top of the

Arbuckle in this core. Pebbles of reworked Arbuckle rock are present within a shaly sandstone matrix in this conglomerate. Conodonts recovered from this conglomerate (see Appendix D) indicate that it is of Pennsylvanian age.

The Arbuckle rocks in this core are overlain by the shales and sandstones of the Bartlesville Formation (not pictured in the photograph in Fig. 18).

Core: Cameron 1-Shepherd Location: Sec. 20, T3S, R4W, Jefferson County, OK Cored Interval: -6278 to -6296 ft Stratigraphic Interval: Arbuckle Group (formation uncertain) Core Description: The following description is given in reference to the petrolog for this core (Pl. 4). -6296 ft to -6296 ft The cored "interval" actually consists of a few rubbly pieces of collapse breccia; it may be that the fractured and brecciated nature of the rock prevented recovery during the coring operation.

This monomictic collapse breccia consists of nearly rectangular pebbles of dolomudstone and three lithologies of infill sediment. Geopetal dolomudstone was the first lithology to be deposited, followed by a finely laminated dolosiltite. This lithology was partially lithified, then eroded and redeposited within a matrix of shaley dolomite. Isopachous crusts of saddle dolomite coat the breccia clasts. Core: Shell 1A-3 Wesley Unit Location: Sec. 3, T4S, R3W, Carter County, OK Cored Interval: -3584 to -3633 ft Stratigraphic Interval: Arbuckle Group (Kindblade Formation) Core Description: The following description is given in reference to the core photographs (Fig. 19) and to the petrolog for this core (Pl. V).

-3633 to -3603.3 ft This interval is comprised predominantly of very finely laminated, sometimes hummocky or nodular dolomudstones that may be of subtidal origin. A thin breccia (possibly parabreccia or collapse breccia) is present at -3625 fi, and is conspicuous due to the thick rind of saddle dolomite cement present on the breccia clasts. Vugular porosity developed in medium to coarse crystalline dolomite is present from -3624 to -3620 ft. The primary depositional fabric of this zone has been obliterated by dolomitization. The rocks of this interval are extensively (tectonically) fractured. Thick stylolites in these rocks attest to considerable pressure solution. -3603.3 to -3595 ft The rocks in this interval have been extensively dolomitized; medium to coarse

crystalline xenotopic and saddle-rhombic varieties of dolomite are commonly present. Depositional fabric is obscure, but faint wavy laminations reminiscent of algal boundstones are present and suggest an upper-intertidal to lower-supratidal origin. Solution-enlarged fenestral (?) vugular porosity is present in this lithology. A small, sediment-filled solution channel is present at -3604.5 ft.

40

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This interval is comprised mostly of dense and wavey laminated, tan and grey,

clayey dolomudstones. Such lithologies are suggestive of subtidal deposition. The dolomite in this interval is typically microcrystalline or fine crystalline. These rocks are cut by numerous perpendicular and oblique-to-bedding (tectonic) fractures. A small fault (?) or fracture with considerable vertical offset is present a -3590 to -3591 ft. Swarms of variable relief, sinuous and serate stylolites are accentuated by the abundance of terrigenous clays in these rocks.

Core: E. L. Cox 1-Wesley Unit A Location: Sec. 3, T4S, R3W, Carter County, OK Cored Interval: -3806 to -3983 ft Stratigraphic Interval: West Spring Creek-Kindblade Formations Core Description: The following description refers to the core photographs (Fig. 20) and to the petrolog for this core (Pl. VI). -3983 to -3960 ft This interval is comprised of tan and grey, fine laminated and massive dolomudstones. Collapse breccias are common, notably occurring at -3979 to -3974 fi, and at -3967 to -3964 fi. These breccia commonly have light tan, dolomudstone filling the breccia-intergranular porosity. A cavern-fill parabreccia is present at -3972 to -3971 ft.

The clasts in this breccia are fractured chert, the next closest occurrence of which is at 3939 ft in the core. Solution-enlarged fractures and vugs occur frequently in these rocks. All breccias and paleokarstic dissolution features are transected by tectonic fractures.

3960 to -3937 ft This interval is almost entirely made up of "zebroid" collapse breccia--a notable exception is the cavern-fill parabreccia at -3939 ft. The "zebroid" breccia facies exhibits

both random and crackle breccia fabrics, and is almost entirely cemented by coarse columnar and saddle dolomite. Host rock in this breccia is fine to medium crystalline dolomicrite/dolomicrospar and clay-associated rhombic dolomite. Fractures in the cracklegrecciated horizones are predominantly parallel-to-bedding and cemented by dolomite; open tectonic fractures cut all lithologies and paleokarstic features. Extensive dolomitization has obliterated sedimentary structures; the depositional environment of these rocks is therefore uncertain.

-3937 to -3890 ft Tan and grey, massive, fine crystalline dolomudstones dominate this interval; locally (as at -3909 ft) faint algal laminations may be present. This interval contains a sequence of intensely fractured, dolomite-cemented rocks that resemble the "zebroid" breccia of the previous interval. However, these rocks appear to have suffered only minor movement along fractures; whereas the previously described breccia showed more obvious signs of collapse.

3890 to -3870 ft This interval is notable in that it marks the only occurrence of limestone in this core Slightly fossiliferous, hummocky-bedded, clayey blue-grey subtidal mudstones occur from --

3884 to -3876 ft; the rocks above and below the limestone beds are either dolomitic limestone or limey dolomite (the latter occurs more frequently away from the limestone beds in either direction). Unlike the dolomitic rocks above and below, this interval is very infrequently cut by open tectonic fractures. Also, fractures in the dolomitic rocks tend to be

short, open and randomly oriented (the rock typically looks shattered); whereas the fractures present in the limestone beds are quite continuous, more singular in occurrence, and tend to be cemented by ferroan calcite 43

3870 to -3806 ft This interval is comprised mostly of tan and grey dolomudstones that exhibit very regular horizontal lamination. An exception to this regularity occurs in a vugular porous zone from -3842 to -3831 ft, below what may be an erosional intra-Arbuckle disconformity at -3831 ft. Vugs up to 1.5 cm in diameter are present in this zone. Early, parallel- and

perpendicular-to-bedding fractures (crackle breccia) are cemented with coarse saddle dolomite cement. Tectonic fractures are noticeable, though they are not as prevalent here as in some dolomitic zones below.

Core: Texaco 1-Mobil Location: Sec. 1, T5S, R1W, Carter County, OK Cored Interval: -6300 to -6310 ft (core 1) -6519 to -6535 ft (core 2) Stratigraphic Interval: West Spring Creek and/or Kindblade Formation Core Description: The following description is given in reference to the core photographs (Fig. 21) and to the petrolog of this core (Pl. VII). Core 1 -6310 to -6305.7 ft This interval is comprised of fractured and crackle-brecciated blue-grey subtidal dolomudstones which may represent the host-rock floor of a paleocavern. The host rock

has slightly mottled to massive bedding which dips approximately 27 degrees from horizontal.

6305.7 to -6300 ft This interval contains a heterlithic collapse breccia adjacent to what may be the inplace host rock wall of a paleocavern. The host lithology is the same as that described above. The breccia contains at least two types of dolomudstone: one is fine crystalline and resembles the host rock, while another is coarser crystalline xenotopic dolomite that does not occur in the host rock. Also present in the breccia are clasts of calcite cement, shale rock fragments, and well-rounded quartz sand. A laminar crust of colunnar calcite coats the host-rock "wall," separating it from the juxtaposed breccia. Core 2

-6534 to -6526 ft This interval is comprised of tan and blue-grey quartz-sand-rich subtidal dolomudstones. Perpendicular-to-bedding fractures (the only effective type of porosity present) are saturated with dead oil.

6526 to -6519 ft This interval contains approximately 1.5 ft of rubbly collapse breccia (from -6522.3 to -6520.8 ft) bounded above and below by crackle breccia. The host lithology is tan, massive dolomicrospar. Where preserved, breccia-intergranular porosity is filled with dead oil. Vadose pendant calcite cements are present on the bottoms of some breccia clasts.

yugular porosity is present at -6519 to -6520 fi, and seems to be concentrated along bedding planes.

Core: Pan American 1-State C Location: Sec. 36, T2S, R1 OW, Cotton County, OK Cored Interval: -7475 to -7500 ft Stratigraphic Interval: Arbuckle Group (Kindblade Formation) Core Description: The following description is given in reference to the core photographs (Fig. 22) and to the petrolog of this core (Pl. VIII).

44

Figure 20 E. L. Cox 1-Wesley Unit A Core.

45

A.rI

APT

5141

,

17

046*

Figure 20 (Continued) 46

Figure 20 (Continued) 47

-7500 to -7475 ft This core comprises 25 ft of heterolithic collapse breccia. At least seven different lithologies (of host rockclasts and matrix) are present.

From -7500 to -7499 ft the host lithology is a darkbrown, massive dolomudstone. This lithology is extensively fractured and brecciated. Light brown and tan, fine laminated, cherty dolomudstones dominate the host rock from -7499 to -7491 ft. These beds dip

slightly (20-25 degrees from horizontal) and exhibit mosaic and random breccia fabric internally.

The dominant host lithology from -7490 to -7485.5 ft is brown, hummocky bedded, clayey dolomudstone. This lithology is more steeply dipping than the beds below (approximately 33 degrees) and occurs as cobble to boulder size breccia clasts.

Blue-grey, fine bedded (or thick laminated) dolomudstones characterize the thost rock from -7487 to -7484 ft. This lithology occurs as boulders in a breccia that also contains pebble-tocobble-sized clasts of chert, brown dolomudstone, and silicified ooid grainstone.

The breccia clast population from -7484 to -7475 ft is dominated by cobble- to boulder-sized clasts of dark brown, hummocky, burrow-mottled dolomudstone. Pebbles of chert, silicified ooid grainstone, and othe dolomite types are present in lesser amounts. The

breccia-matrix lithology throughout the core

is

a dark brown, clayey dolomicrite.

Petrographic examination of the matrix revealed the presence of dolomicrite, dolomicrospar and clay-associated rhombic dolomite types. With the notable exception of the intertidal ooid grainstone, the host-rock lithologies present in this core are most likely of subtidal origin.

48

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Figure 21 Texaco 1-Mobil core.

49

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50

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Petrolog of Oliphant 1-A Nate core, sec. 15, T.24N., R.7E., Osage County, Oklahoma. Explanation of symbols (shown on Plate 4, p. 63) applies to Plates 1-8.

51

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Petrolog of Oliphant 1-Lafortune core, sec. 8, T.25N., R.6E., Osage County, Oklahoma.

52

KARST / TECTONIC FEATURES FRAC-

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Petrolog of Getty 6-Cobb core, Oklahoma. 53

sec. 3,

MMMMM MMMMM

T.17N., R.7E., Creek County,

SED STRUCTURES/ CONSTITUENTS

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(continued on next page)

Plate 6

Petrolog of E.L. Cox 1-Wesley Unit A core, sec. 3, T.4S., R.3W., Carter County, Oklahoma.

56

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Continued.

57

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59

PALEOSTRUCTURAL AND RELATED PALEOKARST CONTROLS ON RESERVOIR DEVELOPMENT IN THE LOWER ORDOVICIAN ELLENBURGER GROUP, VAL VERDE BASIN, TEXAS K.L. Canter Applied Geoscience, Inc. Boulder, CO

D.B. Stearns Consultant Denver, CO

R.C. Geesaman Consultant Gold Hill, CO

J.L. Wilson Consultant New Braunfels, TX

Abstract In the Val Verde basin area, the Lower Ordovician Ellenburger Group represents two third-order sea-level fluctuations as determined from facies architecture, stacking patterns, and accommodation plots based on subsurface core studies. These third-order cycles are superimposed on part of a second-order sea level rise and fall of the upper Sauk

Ellenburger Group deposition ceased in response to a major sea-level fall represented by a second-order unconformity at the end of the Sauk Sequence. This upper bounding surface is characterized by an extensively developed karst profile, indicative of prolonged subaerial exposure. Sequence.

Carbonate dissolution and cave formation were most significant at and along major

block boundaries and resulted in the generation of vugs, caverns, caves, and solutionenlarged fractures and joints. The roofs of larger caves were brecciated as the caves were buried and subjected to static loading by flooding of the platform and deposition of the Simpson Group. The fracture and breccia porosity found in the cave roof portions of these karst profiles accounts for much of the regionally significant porosity developed within the Ellenburger Group.

A detailed upper Ellenburger Group isopach of the Brown-Bassett/JM fields area illustrates a linear trend of isopach thins coincident with the crest of the present structural trend indicating that, not only were the structures active during Ellenburger time, but that the entire structural trend was regionally high. A well-developed paleokarst system was described from cores taken from the Ellenburger interval in this area. This karst system has a distinctive log signature characterized by elevated gamma ray response that has been interpreted to represent more radioactive clay-rich sediment deposited as cave-fill material. Correlation and isopach mapping of the cave-fill portion of the cave zone, shows the main portion of the paleo-cave network to extend across the entire Brown-Bassett/JM trend in a west-northwesterly direction, paralleling the principal bounding faults. The caves are thickest (up to 70') and best developed adjacent to, but not necessarily coincident with the crests of the structures. It is interpreted that the maximum cave development was localized along the main basement fault zones which acted as secondary conduits for fluid flow.

61

Introduction Large reserves of natural gas are structurally trapped within the dolomite reservoirs of the Ellenburger Group in the Val Verde basin of west Texas (Fig. 1). All of the larger fields, including Puckett, Brown-Bassett, Grey Ranch, and JM, were discovered in a period from 1952 to 1965. Since these discoveries were made, explorationists have been unable to define additional Ellenburger fields of similar magnitude in the Val Verde basin. The lack of new discoveries is partly due to poor quality seismic data and the difficulty in recognizing deep structural traps. It may also be partly due to a lack of understanding of the nature and distribution of porous and permeable Ellenburger Group reservoir intervals. Recently, however, refined seismic data acquisition and processing techniques have resulted in

significantly improved data quality, allowing for delineation of multiple prospective structural trends. The focus of this study was to describe and characterize the

Pecos

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The study area within the Val Verde basin of west Texas and includes all of Val

Verde and Terrell counties and the southern portions of Pecos and Crockett counties. 62

strata of the Ellenburger Group in the Val Verde basin area in order to better understand potential reservoir development. More specifically, our goal was to define the stratigraphic

and depositional framework of the Ellenburger Group and relate these to ancient and present-day structure. The data utilized for this investigation include detailed descriptions of nearly 8500 feet of conventional core from 27 wells; quantitative and qualitative descriptions of approximately 400 thin sections; and construction of a stratigraphic database of approximately 450 wells that includes formation-level tops from the Pennsylvanian Strawn Group to Precambrian basement, and the detailed stratigraphy of six internal units within the Lower Ordovician Ellenburger Group.

The Ellenburger Group and equivalent units collectively comprise the carbonate dominated "Great American Bank". These strata are both distinctive and unique in the rock record. Lower Ordovician strata range in thickness from 1500 to more than 6000 feet across the "Bank", with the interval absent across the Transcontinental Arch (Fig. 2). This thick section of essentially pure carbonate rock was deposited in restricted shallow shelf and intertidal environments. The continuity of rocks deposited in these environments indicates the continental margin was a featureless, low-gradient ramp throughout Lower Ordovician time. Ellenburger Group carbonates consist of upward shallowing tidal-flat cycles. The

dominance of restricted shallow water carbonates implies a unique balance between depositional rates and relatively uniform subsidence along the margin of the North American craton. Rock types within this carbonate-rich interval are dominated by dolomite.

Limestone percentage increases basinward toward areas with more open marine, less restricted environments. Sandstone and chert are minor constituents, and there is a noted lack of evaporites.

Solution collapse breccias of varying magnitude occur throughout the Lower

Ordovician carbonate section. These karst-related features are the result of regional exposure associated with major unconformities. The most significant of the hiatuses affecting Ellenburger Group reservoirs in this area are the pre-Simpson and Pennsylvanian unconformities.

Additionally, repeated episodes of subaerial exposure at the top of

shallowing upward peritidal parasequences resulted in localized solution collapse brecciation, which may have partially controlled the distribution of karst systems.

Tectonic History and Structural Framework The North American craton was fragmented during an episode of rifting in Late Precambrian through Early Cambrian time (Thomas, 1991). This rifting resulted in a grid of intersecting northeasterly and northwesterly normal faults across the passive margin of the continent. These ancient faults have had a controlling effect on the later orientation and location of tectonically related structural features, and have directly influenced the nature and distribution of the overlying sediments. The Val Verde basin area was located on the southern trailing margin of the craton from Middle Cambrian through Middle Mississippian time. Structural activity was relatively subtle, as reflected in sedimentation patterns. However, local uplift of basement blocks

resulted in anomalous thinning of the upper part of the Ellenburger Group stratigraphic section (Fig. 3). The Paleozoic-aged Tobosa basin began to subside in late Lower to Middle Ordovician time, with its depocenter located to the west of the Val Verde basin area (Fig. 4).

The convergence of the Gondwanan landmass with the North American craton in Late Mississippian through Early Pennsylvanian time resulted in a compressional tectonic regime (Sacks and Secor, 1990). Ancient basement faults were reactivated; northeasterly faults became reverse faults with an element of left lateral slip. Northwesterly faults had (Continued page 66) 63

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65

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vertical and right lateral movement. Associated fracture sets trended from north to south. The Val Verde basin began to actively subside at this time. The axis of this foredeep depression shifted northward as compression continued. Synorogenic sediments were subjected to later stages of compression and structural deformation. A period of relative tectonic quiescence occurred during the Middle Pennsylvanian during a reorientation of relative plate motion. Much of the rim of the synorogenic trough, the predecessor to the Val Verde basin, became carbonate platforms. Compression with attendant thrusting resumed from Late Pennsylvanian through Wolfcampian time. The sense of motion of the Gondwanan plate relative to the North American plate changed from northerly to northwesterly. Deformation associated with this second phase of collision and

compression was intense, affecting tectonic patterns far north into the interior of the continent. The axis of compression was dominantly aligned with preexistent northwesterly faults, such as the northern bounding fault at the Brown-Bassett field. These faults were characterized by right lateral transpressional displacement during this phase of reactivation. West to northwesterly anticlines and north-northeasterly oriented fracture sets are

associated with this activity.

The Devils River uplift and other rimming highlands 66

to the south of the basin were the source of thick sections of synorogenic elastics prograding into the Val Verde and Kerr foredeeps (Fig. 5).

Since the cessation of compression at the end of Wolfcampian time, the Val Verde basin area has been in an extended period of relative tectonic quiescence. However, associated with the Triassic rifling of the proto-Atlantic, easterly and northerly tension features were reactivated, and northerly faults experienced limited right lateral movement. Laramide (Late Cretaceous-Early Tertiary) compression resulted in broad folds with northwesterly trending axes. Some left lateral wrench faulting occurred, such as at the Carta Valley fault zone on the north flank of the Devils River uplift (Fig. 5). These post-

Permian phases of deformation, though minor in scale compared to the preceding compressional events, had greater potential for trap destruction due to leakage than for reservoir enhancement.

Pre-Pennsylvanian Regional Stratigraphy

Lower and Middle Paleozoic formations correlated in this study include, in

ascending order: Wilberns Formation, Ellenburger Group, Simpson Group, Montoya Group, Sylvan Shale, SD-1 (Fusselman Formation), SD-2 (Wristen Formation), and SD-3 (Thirtyone Formation), and Woodford Shale. These strata were deposited on a shelf characterized by low topographic relief and depositional gradient. The spatial distribution of these units provides valuable information about the location and magnitude of basementinvolved structures. These structures were re-activated throughout the Paleozoic, and thus had an influence on the facies distribution and stratigraphic thicknesses through time.

The Cambro-Ordovician Wilberns Formation is a clastic-dominated interval of sandstone, granite wash, and shales that filled-in and draped the irregular surface produced during Late Precambrian through Early Cambrian rifting (Fig. 6). The Wilberns Formation and its equivalents all exhibit progressive onlap onto the southern margin of the craton, with

older strata resting on basement to the south and east. The amount of interbedded carbonate increases upward. The contact with the overlying Ellenburger Group is gradational.

The Lower Ordovician Ellenburger Group is dominantly dolomite, with lesser amounts of limestone, sandstone, and chert. The Ellenburger Group is subdivided into six time-stratigraphic subunits based on facies distribution and significant exposure events observed in core, which in turn are reflected in log signature in the subsurface. The stratigraphy of the Ellenburger Group will be summarized later in this paper.

The Middle Ordovician Simpson Group unconformably overlies the Ellenburger Group through most of the Val Verde basin. This unconformity is a supersequence boundary referred to as the Sauk-Tippecanoe unconformity (Sloss, 1963). The Simpson Group contains a thick section of complex interbedded carbonate, sandstone, and shale deposited in at least 4 third-order sequences. The upper contact of the Simpson Group is an angular unconformity.

Cherty carbonates of the Montoya Group are restricted to the western portion of the study area. From Late Ordovician through Devonian time, the Val Verde basin was a dissected shelf margin characterized by periodic carbonate deposition, and punctuated by multiple episodes of subaerial exposure. An exception to this pattern is the Late Ordovician Sylvan Shale. This distinctive unconformity-bound shale, deposited in a shallow restricted marginal marine setting, occurs in an isolated north-south trending trough in the eastern part of the study area. Silurian and Devonian carbonate rocks are characterized by (Continued page 71) 67

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Paleokarst Deposits Fracture Breccia Lacks clast rotation Single lithology

Mosaic Breccia Minor clast rotation Fitted, angular clasts

Matrix-Supported Breccia Polymictic Siliciclastic or carbonate matrix

Clast-Supported Breccia Polymictic Saddle dolomite cement

Figure 7

Schematic representation of types of karst breccias recognized in Ellenburger Group rocks (after Kerans, 1990) 70

abrupt lateral changes in thickness and facies that are controlled in part by subtle variations in paleotopographic setting related to shifting basement blocks.

The Woodford Shale, one of the most important source rocks in the Permian basin, This shale is characterized by organic-rich, pyritic shale and siltstone, and minor sandstone. Where preserved, the contact between the Woodford Shale and the Mississippian interval appears conformable. In the Val Verde basin area, however, the Mississippian section was either not deposited or was eroded prior to the deposition of the Lower Pennsylvanian Strawn Group.

occurs throughout much of the Val Verde basin area.

Ellenburger Group Microfacies Ellenburger Group deposits in this study are consist of shallow marine carbonates that fall within the platform interior facies belt of Wilson (1975). The platform interior depositional setting contains four carbonate facies belts plus a narrow siliciclastic belt. From most normal marine to most restricted and supratidal, these are: Organic buildup (SMF 7); Lagoon (SMF 8 and 9); Shoal (SMF 15, 16, and 17); and Restricted marine protected environment (SMF 19, 20, 22, 23, and 24). Clastic units are generally the most regressive deposits, and occur as thin beds that cap the carbonate-dominated parasequences.

Organic Buildups Organisms capable of constructing rigid organic buildups were rare in the Upper Cambrian and Lower Ordovician, limited to bushy algae (such as Nuia, Renalcis, Girvanella, and Epiphyton), dasycladaceans (Calathium), and sponges (Sphaerocodium and Archeoscyphia). In the Ellenburger Group rocks of the Val Verde basin, the buildups were

mainly constructed by bushy algae and accessory amounts of sponges, and are called

thrombolites (SMF 7). Mat-like buildups constructed by binding algae are also common in these rocks, and typically cap buildups constructed by the various types of bushy algae. Vuggy pores are common in these microfacies.

Lagoon Open, to more commonly restricted, lagoon deposition occurred in protected areas shoreward of the mud-rich thrombolite buildups The burrowed and bioturbated mudstones and wackestones of this low energy environment typically contain skeletal debris derived from adjacent buildups, but also contain increasing amounts of grains indicative of restricted shallow marine conditions in areas where this facies grades into intertidal flats. These muddy lithologies lack reservoir potential Shoal

Grain-supported lithologies occur throughout the Ellenburger Group, but are especially common in the middle portion. A variety of packstones and grainstones were recognized, including skeletal (SMF 12), lag (SMF 14), oolite and oolite -- intraclast (SMF 15), peloid (SMF 16), and grapestone-peloid-intraclast (SMF 17). Most of these grainstones and packstones are associated with restricted, shallow inner platform deposition under moderately to intensely agitated conditions. Oolite grainstones are particularly common at the bases of parasequences. A variety of pore types are common in these lithologies, including interparticle, intercrystalline, and moldic 71

Restricted Marine Protected Environment

This facies belt includes the intertidal and supratidal portions of the peritidal environment, and subtidal portions such as tidal channels and ponds. The belt is characterized by mud-supported lithologies containing a plethora of sedimentary features and structures, each representing a specific component of the tidal-flat system. The most common microfacies, SMF 19, is divided into six sub-microfacies (SMF 19A-E), reflecting the complexity of this depositional environment. Other microfacies include stromatolitic algal bindstone (SMF 20), oncolite mudstone (SMF 22), homogeneous dolomite mudstone (SMF 23), and flat pebble conglomerate (SMF 24). In general, these microfacies occur at or near the tops of parasequences, and are associated with local or regional disconformities. Siliciclastic-rich rocks may also cap parasequences.

Periods of subaerial exposure of varying duration have affected many of these carbonate deposits, especially those present at parasequence boundaries. These episodes of

subaerial exposure are represented by a variety of diagenetic alterations ranging from angular exposure breccias to mantle breccias. Short-term exposure features include sheet cracks, teepee structures, mudcracks, and oxidized layers. More extensive subaerial exposure is indicated by the occurrence of caliches and paleosols. Sequence boundaries may be marked by mantle breccias (or carbonate residuals) and regoliths. Karst systems may also develop in response to long-term subaerial exposure.

Paleokarst Breccia Types

The most common breccia types in the Ellenburger Group are karstic solution-

collapse breccias induced by cavern collapse, and tectonic fracturing, which play important roles in localizing solution collapse and an accessory role in reservoir development. Three

karst breccias facies have been recognized in the Ellenburger Group: upper cave roof; middle cave-fill; and lower cave or collapse rubble (Fig. 7). The cave roof karst facies consists of unbrecciated Ellenburger Group dolomite at the top grading downward into mosaic breccias ('puzzle' or 'crackle') at the base (Fig. 8A). In the angular-clast mosaic breccias slight clast rotation has occurred and some geopetal carbonate mud and veins of saddle dolomite occur (Fig. 8B). Porosity ranges from 2 to 20% and the cave roof breccias commonly form the main Ellenburger pay interval.

Figure 8

Examples of karst breccia types from the Magnolia Morrison #1 well, Will-0 field, northwestern Val Verde County. (A) Some of the most porous intervals recognized in the Ellenburger Group cores occurs within the cave roof breccia of a cave profile. Solution-enlarged fractures often occur in the intact roof portion of a cave. Bitumen lines many of the fractures in this roof breccia sample from Will-0 field (sample depth 13733'); (B) Many of the clasts and

fractures in the mosaic breccia part of the cave exhibit solution rounding. Consequently, interstitial pores within these breccias and large open fractures are common in this part of the karst profile (sample depth 13656'); (C) Most pore space is filled with either infiltrated or current-deposited, fine-grained sediment in the matrix-supported part of the cave system. The cave-fill breccias in the

Ellenburger Group typically display many generations of internal sediment,

indicating a complex, karst-dominated diagenetic history (sample depth 13722;); (D) The clast-supported portion of a cave system is characterized by solutionrounded clasts of many different lithologies. Internal sediment may be present in these karst breccias, but generally some karst breccia porosity remains (sample depth 13837'). 72

Middle cave-fill breccias grade down from the cave roof facies and consists of chaotic and polymictic clasts in a sandy-silty matrix, interspersed with graded, crudely bedded sand and shale (Fig. 8 C-D). The zone is highly radioactive compared to the other two zones and forms a distinctive field-scale traceable level, usually a few tens or hundred feet below the top of the Ellenburger Group (Fig. 9). It is interpreted to be the collapsed and in-filled passageways of ancient cavern systems.

Lower collapse or rubble zone breccias lie below the siliciclastic cave-fill facies and consists chiefly of chaotic, clast-supported dolomite breccia with some minor carbonate matrix. It may grade downward through intermittent intervals of breccia to unaltered dolomite or limestone. The unit is generally massive and includes intervals of intact unbrecciated dolomite up to 10 feet thick. Clasts average a few centimeters in size but some are as large as 1 meter. They are typically angular although some contain rounded and scalloped surfaces. Most breccias in the lower part of the paleokarst are polymictic. Since clasts of both burrow-mottled subtidal sediment and finely laminated high intertidal clasts are often juxtaposed, it is assumed that a considerable section has been lost by dissolution in the process of breccia formation. Original pore space is commonly filled with micrite, dark shale, or veinous, sparry, white and pink saddle (baroque) dolomite. Porosity varies from less than 1 to 15%. The lower collapse karst facies may form a lower reservoir

level in the Ellenburger Group, separated from the upper cave roof facies by the impermeable cave-fill facies.

Cyclicity and Stratigraphy of the Ellenburger Group Cycles within vertical sequences of carbonate rocks have long been used to interpret both the ancient depositional system responsible for the genesis of the rock body, and to place this environmental interpretation into a more regionally correlatable stratigraphic framework. Recently, there have been numerous attempts to refine the use of cyclicity and cyclostratigraphy in ancient carbonate deposits to predict ancient rises and falls in sea level (i.e. Read and others, 1986; Goldhammer, Dunn, and Hardie, 1987 and 1990; and Read and Goldhammer, 1988). Predictable cyclicity has a profound influence in ascertaining where porosity is developed and preserved within a sequence.

Most of the interpretation of the cyclicity of the Ellenburger Group sediments is based on the smallest scale cycle -- the parasequence. Once parasequences are defined in core, their stacking patterns can be analyzed by plotting cycle thickness versus time, using an average cycle period. This type of plot was originally defined by Fischer (1964) for the lofer cycles in the Triassic section of the Alps. Fischer (1964) believed that the cyclicity seen in Lofer strata was caused by relative sea-level fluctuations induced by episodic tectonic activity. Application of this method by many workers today assumes a regularly occurring eustatic mechanism superimposed on steady state linear subsidence which increases or decreases accommodation space as sea-level rises or falls.

An accommodation plot was constructed from a composite core of the Phillips Puckett "C" #1 and Phillips Glenna #1 cores from Puckett Field, Pecos County, Texas (Fig. 10). Three major stratigraphic intervals can be identified in this composite core based on lithology, facies associations, and vertical stacking patterns: a lower interval of sandstones

and sand-rich carbonates; a middle interval of muddy carbonates overlain by grainsupported carbonates; and a mud-dominated upper interval that becomes sandy in our study

area near the erosional contact that separates the Ellenburger Group from the Simpson Group. Each of these intervals can further be divided into two parts based on facies and stacking patterns: the E-1 and E-2 intervals represent the sand-rich lower part; the E-3 and E-4 represent the mud-dominated to grain-dominated middle portion; and the E-5 and E-6 intervals contain the sandy carbonates of the upper interval (Fig. 11). 74

Log Signature in Karsted Ellenburger Section Magnolia Morrision #1, Val Verde County. Intact Roof

Gamma

Cave Roof

Fracture Porosity (4-1

Cr,

Cave Fill

Collapse Zone Vuggy Porosity

Figure 9

Distribution of intact roof, cave roof, cave-fill, and collapse zones of the paleokarst deposit at the E-4/E-5 boundary in the Magnolia Morrison #1 core from Will-0 field The elevated gamma ray response at the base of this cave system (also the base of the E-5 interval) was most likely produced by infiltrated sediments within the lower part of the cave.

75

El Paso Group 30-

-30 o

6

3

Time (Millions of Years)

60

Ellenburger Group 7,4

30

.-

o

2

o

E-1

E-2

E-4

E-3

E-5

E-6

-30 o

6

3

Time (Millions of Years) Figure 10 Comparison of the accommodation plot constructed for the Ellenburger Group composite core of the Val Verde basin to a plot constructed for the El Paso Group, Franklin Mountains (by Kerans, 1988)

76

Ellenburger Cyclicity

/ C/D-v

Sloss Sequence

2nd Order Sequence

T

Fischer Plot of 3rd Order Sequences +

R

o

7=,

Facies Sequence -= -Er-

_Z:7-

=

4:::,

4.) c.

=

1)

V CD

= ct

C4

\/

-E6

v

_a -ET

_.=

.

_.= .E 4

.

..ALNAvII-

.,

.

-,,

E3

Inr..

-v-Z7--v

.7.---,:s-/-4---,----" .,-.

_An,

_ET

-LT

0 o

0

0

_.= o

El

0.0 o-.0.

.0 0-0 0 .0

Figure 11 The Ellenburger Group of the Val Verde basin lies within the upper part of the Portions of two third-order sea-level cycles are Sauk Supersequence. recognized within this interval. See text for a discussion of the cyclicity. Portions of two third-order cycles are illustrated in the accommodation plot of the composite core (Fig. 10). The E-1 and E-2 intervals contain thin, regressive, progradational, sand-rich parasequences that represent a third-order sea-level fall (Table 1). The transgressive and highstand systems tracts are illustrated by rapid increases in cumulative cycle thickness on the plot. The initial transgressive event correlates to the base of the E-3 interval. A highstand systems tract characterizes the remainder of the E-3 interval. Another

transgressive event followed by a highstand can be seen in the lower part of the E-4 interval. The E-4 interval also contains the 'turn-around' point (represented by the apex of the curve) where 'subtidal' deposition is replaced by progradational, peritidal deposition. The accommodation potential created by the third-order sea-level rise has been essentially filled-up by this time, thus peritidal, inner platform cycles replace the deeper water cycles. The two shallowing upward cycles contained within the E-5 and E-6 intervals represent a period of time when subsidence and sedimentation rates were nearly at equilibrium, manifested by the gentle oscillations in the accommodation plot curve (Fig. 10). The upper part of the E-6 interval records the onset of a third-order sea-level fall, represented by thin, sandy, mud-dominated parasequences (Fig. 11).

77

Formation

Ellenburger Group Interval (this study)

Sea-Level Change; Systems Tract

Upper Tanyard, Gorman Fm, including Chamizal Sandstone Honeycut Fm

E-1 and E-2

Sea-level fall; Late Highstand to Lowstand

E-3 & E-4

Sea-level rise; 2 Transgressive and Highstand cycles Stillstand; Highstand to Initial Lowstand Sea-level fall; Late Highstand to Lowstand

(Lower)

Honeycut Fm. (Upper) and Cindy Sandstone Post-Honeycut

Table 1

E-5 & E-6

Uppermost E-6 in southern Val Verde basin

Relationship of detailed stratigraphic intervals terminology, eustacy, and carbonate systems tracts.

to

Ellenburger

Group

E-1 Interval The lower third of the Ellenburger Group contains units designated as the E-1 and

Like the underlying Wilberns Formation, the lower intervals of the Ellenburger Group are rich in siliciclastics, but differ from the Cambrian section in that the elastic material contained within these dolomites is better sorted, texturally mature, and much finer grained. A typical shallowing upward cycle or parasequence in the E-1 interval consists of thin beds of sandstone and grain-supported dolomite packstones that are rich in peloids, ooids, and pisolites (Fig. 12). Individual parasequences range from 2-8 feet in thickness, and average

E-2 intervals which are characterized by thin, sand-rich dolomite cycles.

4.5 feet in thickness.

Parasequence boundaries are difficult to resolve in this siliciclastic-rich interval and are best defined by bounding surfaces and zones marked by intense diagenetic alteration (that is, exposure surfaces, vadose fabrics, or caliches). This lack of vertical variability in lithology and sedimentary structures may be attributed to the highstand systems tract represented by this interval. Sandstone content increases upward and sandstones predominate at the top of the interval. Collectively, these features indicate that deposition occurred within a high intertidal to supratidal or emergent environment that was deposited as a progradational sequence during a third-order fall in sea-level (Fig. 11). For the most part, the parasequences of this interval are compact or abbreviated, containing only the lower part of the depositional cycle and an upper diagenetic capping bed, or they may consist of amalgamated beds of sandy dolomite and dolomitic sandstone (Fig. 12). Both types of "cycles" indicate that accommodation potential had been greatly reduced and that the cycles had aggraded high enough on the platform so that oscillations in sea level were not great enough to rise above the lag depth or to cover the top of the platform. Because of this lack of accommodation space, many eustatic cycles were not recorded on the platform. These incomplete cycles can be classified as condensed megacycles (Goldhammer and others, 1990).

78

E-1 and E-2 Facies Cross Section SE

NW

E-2

Depth

v,

Mg ' AMO 51

-_-.

C O

+1)-

..1

U

T.

u

O

.

co (1)

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co)

=

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ne .9'.9. 4

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ty,

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rn

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cn

a

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,

od

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500

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_

c4

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OO

o

(ft) 550

g

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a

cn

O

oo

O

Phillips Wilson #1

Humble Mills Min. Trust #1

Phillips Puckett/Glenna

300

-

e-. t.419

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AN.

atia-Ar- .-

4-)

C....,,11.

-1.

. .

.

-2

a.

_L\ .=.-..

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el

'.

"O

7-,- o

---0-

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c,

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o

Irffieeatir,

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-

200 "O

-o

re,j2.q-

04

.z g

o

.,-.1

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,0.0

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1::1 II

,,o

cP!!

~sor-

CP tli .°.

o o ort --V....)

2 E

100

O En

O

..: ...!

o

Figure 12 Facies cross section of the E-1 and E-2 intervals of the Ellenburger Group. Sandy dolomite mudstones are common in the E-1 interval whereas the E-2 interval is carbonate grain-dominated and contains a karst breccia immediately below the E-2/E-3 contact. 79

E-2 Interval Sandy dolomites and algal-rich dolomite boundstones characterize the E-2 interval. Parasequences average 5 feet in thickness, with a range between 2-8 feet. Although similar in many respects to the E-1 interval, this interval contains less sand and is rich in thrombolites and other lithologies indicative of shallow water deposition. A typical parasequence or cycle is rich in grain-supported lithologies such as ooid and intraclastic packstones and grainstones. Thrombolitic boundstones occur most frequently near the base of a cycle, while sandstones, caliches, vadose fabrics, and desiccation breccias mark cycle tops. Solution collapse breccias overprint many of these grainstones, further attesting to the importance of fresh water diagenesis and the establishment of near-surface aquifer(s) during lowstand deposition or during lowstand emergence.

Although less rich in quartz sand than the underlying interval, the E-2 interval was deposited during the same third-order fall in sea-level, and represents maximum lowstand deposition (Fig. 12). Carbonate grainstones replaced siliciclastic sediments, presumably in response to a decreasing supply of clastic material Precambrian basement highs had apparently been eroded to low-relief positive features by E-2 time, and only sporadically contributed siliciclastic sediment to the peritidal-supratidal environment. Consequently, grain-rich sediments and thrombolites accumulated during this time This interval is distinguished from the E-1 interval on the accommodation plot by a slight increase in cycle thickness at the base of the E-2 interval attributed to a small-scale sea-level rise and by a dramatic decrease in quartz sand content in the cores. Like the incomplete cycles of E-1, the cycles of E-2 are also condensed megacycles.

E-3 Interval The contact between the E-2 and E-3 intervals is abrupt and unconformable, and signifies the onset of a third-order sea-level rise (Fig. 11). In core, this contact is defined by a number of features, including a change in color of the dolomites from light gray below to dark gray above, a lithologic change from grainstones and packstones in the E-2 section to bioturbated mudstones and wackestones in the E-3 interval, and by a change in average parasequence thickness. In the E-3 interval, parasequence thickness varies from 4 feet near the top of the interval to 14 feet near the base of the unit, with the average being approximately 9.5 feet. This is a dramatic contrast to an average cycle thickness of 5 feet in the E-2 interval. The effect of this increase in average cycle thickness can be seen as a significant rise on the accommodation plot, interpreted to represent a third-order sea-level rise. This deepening event is the most significant paleo-environmental change recognized in the Lower Ordovician strata of the Val Verde basin. In general, the rocks of the E-3 interval are muddy and dark colored, consisting of a thick lower unit of burrowed to bioturbated mudstone and wackestone and interbedded peloid wackestone or packstone (Fig. 13). Shell lags and intraclast packstones mark the bases of parsequences in some places, while boundaries between others are indistinct. The

depositional setting of this interval was not shallow enough to record high intertidal or supratidal deposits at the tops of parsequences. Instead, they typically terminated prior to significant accumulation of shallow intertidal sediments. What remains in the geologic record are incomplete, amalgamated deposits of subtidal on subtidal cycles. Goldhammer and others (1990) refer to this stacking pattern as amalgamated megacycles, and explain that these 'cycles' may actually consist of two or more subtidal parasequences. This would allow for the dramatic increase in parasequence thickness resulting from indistinguishable unit boundaries. These mud-dominated cycles are catch-up deposits which represent the earliest portion of the highstand systems tract when rates of sedimentation were low (Sarg, 1988). 80

E-3 and E-4 Facies Cross Section

SE

NW

Pucket Glenna

-.

E-4

lp

-

a..,.. 514...,. a

a:. él

,

600 gro,:

4:

cr, k>

C.,

--.

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,...

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.

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.

200

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cn

=

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300

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ct

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s.

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.=

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o

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= o

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a.

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.--

eu

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cu

o

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I,

o o

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....

a

,

3_

c.n

frgiaZiri

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e

(fi)

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Depth

Phillips Wilson #1

Humble Mills Min. Trust #1

Phillips

.111

1:1 .".2 41; rz dit,

45

--.

A ° e.'"1";

F.;?p (y,,D

Ich

0

Figure 13 Facies cross section of the E-3 and E-4 intervals of the Ellenburger Group. The

thicker parasequences in these intervals represent transgressive and highstand

deposition characterized by dark gray, bioturbated mudstones in the E-3 interval. Grain-rich carbonate parasequences are common in the E-4 interval. 81

E-4 Interval

A gradational contact separates the E-3 from the E-4 interval. Parasequence thicknesses increase above this contact, however, indicating that a slight deepening event occurred at this boundary. Like the E-3 interval, this interval was deposited during an overall sea level rise that began with the onset of E-3 deposition, as depicted on the accommodation plot (Fig. 11). It differs from the E-3 interval in that this part of the highstand deposit is characterized by grain-rich parasequences, representing the keep-up carbonate system where carbonate sedimentation rate was rapid (Sarg, 1988). Incomplete parasequences with thick grainstone and packstone bases and thin bioturbated mudstone caps are common in the lower part of this interval (Fig. 13). High subtidal to low intertidal bioturbated wackestone and mudstones cap the cycles in the lower part. Much of the accommodation potential created by the third-order sea-level rise was filled during E-3 and early E-4 time, so it is logical that this part of the interval would be characterized by thinner (6 to 8 feet thick) shallow subtidal to intertidal parasequences. The E-4 interval also contains the 'turn-around' point or maximum flooding surface (represented by the apex of the curve on the accommodation plot). This point marks the interval of time when aggradational 'subtidal' deposition was replaced by progradational, peritidal sedimentation. Accommodation space created by the third-order-sea-level rise was filled up by mid- E-4 interval time, thus peritidal, inner platform parasequences replace the deeper water deposits. Collectively, these more complete, progradational parsequences are referred to as rhythmic megacycles (Goldhammer and others, 1990). Parsequences are much thinner near the top of this interval, recording extensive progradation of the tidal flat system across the platform during waning highstand deposition.

E-5 Interval

The E-5 interval represents continuing deposition of rhythmic megacycles of complete peritidal parasequences across the platform area. On wireline logs, this interval is

defined by a cleaning upward (or shallowing upward) signature.

A stack of grain-

dominated parasequences at the base of this interval indicates that deposition began during a slight rise in sea-level, expressed in the rocks by a return to slightly thicker grainstone-rich units and on the accommodation plot by a slight sea-level perturbation (Fig. 11). The grain-

rich interval is typically overprinted by a karst profile where vuggy porosity, solutionenlarged fractures, and fissures are common. This regionally extensive grainstone deposit can be seen on wireline logs throughout much of Crockett and Val Verde counties.

Rhythmic, thin (< 6 feet thick) peritidal parasequences dominate the remainder of the interval. Thin burrowed or bioturbated wackestones and mudstones mark the lower beds of a typical parasequence (Fig. 14). Lithologies indicative of intertidal to supratidal conditions such as centimeter bedded mudstones, laminites, stromatolite bindstones, and rare sandy dolomites occur in the upper part of the cycles. These lithologies record the rhythmic, extensive progradation of the tidal flat across the platform as represented by the 'flat' area on the plot (Fig. 10).

E-6 Interval The uppermost interval recognized in the Ellenburger Group is similar in many respects to the lowermost interval. Both record late highstand to lowstand sedimentation marking third-order falls in sea-level. Like the E-1 interval, the E-6 interval contains

82

E-5 and E-6 Facies Cross Section SE

NW

Phillips Wilson #1

Magnolia Morrison #1

Phillips Puckett/Glenna

Depth (ft.)

500 E-6

--__= oO

cLu

col

-''' /

col

.

U

C.)

>, U

C.) .."'=-1---

U

=

7:1

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,

ct,

"a

-

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., N C

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g

r

:

'IMI Tr,,....

V

1

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s..-

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(t)

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o-

g

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Figure 14 Facies cross section of the E-5 and E-6 intervals of the Ellenburger Group. Both intervals represent progradational highstand to initial lowstand deposition characterized by rhythmic, symmetrical peritidal parasequences. 83

parasequences that become progressively thinner upwards. Parasequences average slightly less than 6 feet in thickness, ranging from 9 feet thick near the base of the interval to less than 2 feet near the top. This decreasing cycle thickness is manifested by the steadily falling curve on the accommodation plot (Fig. 11).

Rhythmic parasequences containing bioturbated or centimeter bedded mudstone beds or thrombolites at the bases and laminites, patterned dolomites, and loferites in the upper beds of the cycles are common in the lower part of this interval (Fig. 14). These cycles range in thickness from 5 to 8 feet, as compared to the slightly thinner cycles of the upper part of the underlying interval, indicating that accommodation space increased slightly, perhaps in response to a slight rise in sea level (Fig. 11). The upper part of the interval is characterized by incomplete cycles that bundle into somewhat condensed megasequences. These sandy dolomite mudstones are thinner, with an average cycle being about 5 feet thick. Many parasequences contain diagenetic caps, such as caliches or silcretes, micritized beds, and reworked exposure breccias. Well sorted and rounded, detrital quartz sand is common in the upper cycles, accounting for up to 50% of the deposit. This sand-rich zone may be equivalent to the Cindy Sandstone of the El Paso Group (Kerans and Lucia, 1989; Fig. 10).

In summary, accommodation plots representing the stacking patterns of peritidal carbonates in the Lower Ordovician provide a valuable aid when correlating cyclic carbonate sequences in areas where the biostratigraphy is poorly understood or in areas where there is poor sample recovery. Like biostratigraphic markers and zones, these orderly and correlatable rises and falls in sea level, as defined by major third-order cycles in

accommodation plots are chronostratigraphic markers that can provide a consistent framework for correlating thick, seemingly monotonous carbonate intervals such as the Ellenburger Group. Accommodation plots constructed for Lower Ordovician strata may be correlatable over great distances (Fig. 15).

Stratigraphy of the Ellenburger Group

The Ellenburger Group was deposited in a passive margin setting during the Canadian Stage of the Lower Ordovician System. It conformably overlies the siliciclastics

of the Upper Cambrian/Lower Ordovician Wilberns Formation and is unconformably overlain by the Simpson Group. Based on correlations between accommodation plots constructed in various sections from the El Paso Group of west Texas, the Ellenburger Group of the Val Verde basin is correlative to strata between the lower McKelligon Canyon Formation and to the Cindy Sandstone of west Texas, and the upper Gorman Formation to lowermost post-Honeycut Formation in the Llano uplift area.

The sequence stratigraphic analysis of the Ellenburger Group was based on detailed core descriptions and facies interpretation, combined with modeling of the small-scale cyclicity of the interval. Most of our core data came from the Phillips Puckett "C" #1, Phillips Glenna #1, Humble Mills Mineral Trust #1, Magnolia Morrison #1, and Phillips Wilson #1 wells. The Ellenburger Group was divided into 6 sub-intervals starting with the E-1 at the base to the E-6 at the top. Figure 16 illustrates the sub-intervals on the type log in the Puckett/Grey Ranch field area.

The E-1 and E-2 intervals comprise a late highstand to early lowstand systems tract. The E-1 and E-2 intervals are lithologically similar and are characterized by fine-grained sandy dolomites, grain-supported dolomite packstones, and scattered thin beds of sandstone. The Wilberns Formation/E-1 contact is relatively obvious on logs by a transitional, although significant, shift towards lower gamma ray intensities. The E-11E-2 (Continued page 87) 84

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Figure 16 Type log for the puckett area in the western Val Verde basin. The Superior Blackstone-Slaughter "B" #1 is similar to the nearby Phillips Puckett #1 and Phillips Glenna #1 wells. Composite cores from these wells formed the basis for the Ellenburger Group interval subdivisions. 86

contact is more subtle on logs and is not considered to be a highly consistent regional marker.

Deposition of the E-3 interval began with a flooding of the platform caused by a third-order sea-level rise. Siliciclastic content is much lower in this carbonate-dominated interval and the gamma-ray log character suggests a much cleaner section, particularly in the east and northeastern part of the study area. Carbonate-dominated deposition continued into the E-4 interval, with the E-3/E-4 boundary consisting of a subtle transition to slightly deeper water conditions. Within the upper part of the E-4 interval, a sea-level drop occurred. Peritidal deposits predominate in the upper part of the E-4 interval as sea-level began to drop and accommodation space diminished. Occasionally, this "turn-around" point can be identified on logs by a subtle shift of the gamma ray to a slightly higher intensity.

The somewhat more irregular nature of the gamma ray is also related to these thinner peritidal cycles.

The E-5 interval is characterized by extensive karst development. A regional cave system has been identified and correlated with a fair degree of consistency throughout the study area. The E-5 interval was deposited during a minor sea-level rise and is characterized by grain-rich cycles. The grainstone-dominated section has been overprinted by a karst profile. These grain-rich deposits likely functioned as a regional aquifer, and the resulting karst overprint contains a stratiform cave system (Fig. 9). The Magnolia Morrison #1 well in Val Verde County is an excellent example of a relict cave system (Fig. 17). As much as 65 feet of radioactive cave-fill material is present at the base of the E-5 interval. Core descriptions indicate the cave-fill deposit is overlain by solution-collapse cave-roof breccias. The regional development of the cave system is highly variable and is more intense in areas related to major basement-block fault trends. The lower contact of the E-6 interval with the E-5 interval is conformable, although abrupt. A noticeable increase in gamma ray intensity, which persists throughout most of the interval, is characteristic and helps to identify the contact on logs. The upper contact is erosional with the overlying Simpson Group and marks the upper boundary of the Sauk Sequence. This boundary is a regional (and global) erosional surface subjected to extensive subaerial exposure. Lithologies of the E-6 interval are mainly dolomitic mudstones which become sandier at the top of the interval. The E-6 interval is extensively karsted in certain areas, particularly in northern Crockett County where log correlations become very difficult due to dissolution and infiltration.

Several of the six Ellenburger Group sub-intervals were combined to facilitate isopach mapping due to genetic similarities. The E-1 and E-2 intervals were combined, as were the E-3 and E-4 intervals. The E-5 and E-6 were examined separately. The E-I and E-2 combined interval demonstrates regional thinning to the northeast towards northern Crockett County (Fig. 18). Onlap and local thinning is observed over pre-existing paleostructural features. A series of local thins are mapped along the Puckett/BrownBassett/JM/Will-O productive trend. The thinning implies that these large gas fields are

related to paleo-structural highs and basement block faulting that originated prior to Ellenburger deposition.

The E-3 and E-4 composite interval has conformable boundaries at its top and base and, therefore, thickness variations reflect only depositional processes and basin subsidence

There is a difference of approximately 100 feet in thickness between the northern and southern part of the study area reflecting a very uniform and low-gradient platform slope. Thinning occurs over a broad area associated with the Puckett/BrownBassett trend, as a result of continued movement of fault-bounded basement blocks along (Fig. 19).

this regional paleo-structural feature. 87

Magnolia Morrison #1 Core Description

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Description

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Millimeter and centimeter-bedded mudstones extensively overprinted by a fracture breccia; many of these fractures remain open, but some are partially filled with dolomite and/or calcite cements.

Mostly thin, rhythmic parasequences of millimeter laminated mudstones overprinted by an extensive mosaic (crackle) breccia caused by cave-roof collapse; open, bitumen-stained fractures are common.

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Millimeter-laminated mudstones are overprinted by mosaic breccia with significant solutionrounding of breccia clasts, interbedded with several levels of cave-fill (chaotic) breccia; abundant internal sediment, but many fractures and large vugs remain open.; excellent porosity and permeability.

Coarse-grained relict packstones and interbedded wackestones are overprinted by cave-fill breccia, multi-lithology clasts are solution-rounded, intemal sediment common; low porosity and permeability.

Figure 17 Example of a collapsed cave complex as described from the Magnolia Morrison #1 core, Will-0 field, Brown-Bassett/JM fields trend.

88

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The regional distribution of the E-5 interval records a slight shift of the basin axis from a predominantly east-west to a more northwest-southeast orientation (Fig. 3). This event is associated with accelerated subsidence of the Tobosa basin depocenter situated to the northwest of the study area. Significant thinning is observed over the Puckett/BrownBassett/JM/Will-0 trend, as well as over smaller producing areas such as Hokit, Yucca Butte NVV, and JNT fields in Pecos County. Sub-regional thickening is observed to the northeast similar to that documented in the deeper E-3/E-4 interval. The distribution of the E-6 interval was affected by both depositional processes and erosion below the sub-Tippecanoe unconformity (Fig. 20). Vertical block-fault movement following Ellenburger Group deposition, resulted in erosion of the higher blocks. These movements resulted in local variations of several hundred feet. Both Puckett and Brown-

Bassett fields exhibit highly variable thicknesses on a detailed level, due in part to the complex faulting associated with these large structures. Regional erosion below the Pennsylvanian

and

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County and at the Devils River uplift in southeastern Val Verde County, respectively. Pronounced northeast to southwest isopach trends are present, particularly in Crockett County. The intersection of these trends with the Puckett/Brown-Bassett/JM/Will-O trend is considered significant because cave development and karst-generated porosity is thought to be more pronounced at fault block boundaries. Production is often related to areas with

a thin E-6 interval.

Thicknesses of the overlying Simpson Group also reflect the

topography that existed following the fault movements that affected the karst development within the sediments of the Ellenburger Group (Fig. 4).

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Paleostructural Controls on Reservoir Development and Subsurface Delineation of a Regional Paleokarst System

A recurrent theme of this study is the repeated movement of basement blocks through geologic time, which has influenced the distribution, composition, and diagenetic history of the strata. Units tend to be relatively thin across the persistent fault-bound paleohighs. The most promising area to study the development of regional paleokarst system is along the Brown-Bassett/JM trend (Fig. 1). Most of the structural displacement along the Brown-Bassett/JM trend occurred in Late Pennsylvanian through Wolfcampian time. However, regional interval isopachs of 91

the sub-intervals of the Ellenburger Group across basement-involved structures, indicate subtle and persistent vertical displacement occurred throughout Ordovician time. A detailed Ellenburger to E-5 interval isopach of the Brown-Bassett/JM trend revealed a linear trend of thins coincident with the crest of the present structural trend indicating that, not only were the structures active during Ellenburger time, but that the entire structural trend was regionally high. Some of the anomalous thinning may be attributable to depositional effects over basement highs, however, relative uplift and erosion below the sub-Tippecanoe unconformity is the dominant factor. Truncation of the E-6 interval below the unconformity can be demonstrated. Isopach maps of the deeper Ellenburger Group subintervals (Figs. 3, 18 , and 19) exhibit a consistent relationship of thinning over pre-existing paleostructural high blocks. Production along the Brown-Bassett/JM/Will-0 trend generally coincides with anomalous thinning in the E-6 interval, although some of the most prolific producers do not relate to the isopachs on a direct basis. Later structural readjustments have caused the structural crests of these features to migrate somewhat. Recognition of similar anomalous thin trends in the E-6 interval, as well as deeper intervals, can be positive evidence delineating prospective structures in other areas.

Subsurface Mapping of the Basal E-5 Paleokarst System The base of the E-5 interval of the upper Ellenburger Group has a well-developed paleokarst system throughout much of the Val Verde basin. This paleokarst selectively formed in grain-rich strata which were charged with meteoric waters during the multiple periods of subaerial exposure of the Ellenburger Group. These grainstone beds, characterized by relatively higher original porosity and permeability, transmitted larger volumes of carbonate-undersaturated groundwater resulting in dissolution, cave development, infiltration, and ultimately collapse.

The E-5 paleokarst system has a distinctive log signature characterized by elevated gamma ray response (Fig. 9 and Fig. 21). The elevated gamma ray is responding to the more radioactive sediment deposited as cave-fill material. The cave zone cannot be correlated regionally bed-by-bed, but rather as an anomalous zone at the base of the E-5 interval. Correlation and isopach mapping of the cave-fill portion of the cave zone, shows the main portion of the paleo-cavern network to extend across the entire Brown-Bassett/JM trend in a west northwesterly direction, paralleling the principal bounding faults (Fig. 22). The caves are thickest (up to 70') and best developed adjacent to, but not necessarily coincident with the crests of the structures. It is interpreted that the maximum cave development was localized along the main basement fault zones which acted as secondary conduits for fluid flow and migration. The primary northeasterly cavern trend branches into numerous smaller northeasterly caves, the distribution of which was controlled by the intersecting set of northeasterly trending faults. This karst system does not extend southwest of the main productive trend, a consequence of proximity to the major fault zones bounding the structure on the northeast.

The magnitude of cave system development, as mapped in the E-5 interval, is not observed to have a direct relationship to well productivity. Some of the thickest cave-fill zones are developed off structure, presumably nearer to the trace of the major faults, which also may have acted as conduits and sources of infiltrating sediment. Brecciation and fracture porosity development is more extensive and extends laterally away from the sediment-filled portions of the cave system. Brecciation in the cave roof zone is evident in many areas proximal to the main cave-fill zone. Wash-outs, recognizable on the caliper log in many wells, are felt to be directly associated with the brecciated and fractured cave roof and provide an indication of the extent of cave development in areas where cave-fill is not particularly pronounced. Several good examples of extensive wash-outs in the E-5 interval above the cave-fill zone can be seen in the Will-0 field area. Recognition of the 92

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regional cavern network will provide explorationists with a useful tool for identifying the locations of significant basement block fault boundaries.

Porosity and Diagenesis

Multiple episodes of prolonged subaerial exposure played a key role in porosity development and distribution in the Ellenburger Group. At least five karst events have affected these rocks: the post-Ellenburger, Middle Ordovician (Sauk Unconformity), postMontoya, pre-Woodford, and Lower Pennsylvanian unconformities. With each event, these rocks were invaded by vadose and phreatic meteoric waters. These fresh waters modified

and overprinted existing pore networks, and created new porous zones.

Calcite-

undersaturated waters do not appear to have altered all lithologies in the same way. Some grain-supported lithologies that contained preserved porosity at the time of updip subaerial exposure likely functioned as freshwater aquifers during major exposure events. It appears that carbonate dissolution and cave formation were significant at and along major block boundaries within such aquifer systems.

Controls on Porosity Development and Diagenesis The most common pore types in these Lower Ordovician rocks are karst breccia, karst fracture, vuggy, tectonic fracture, and tectonic breccia porosity (Table 2). Other pore types such as channel, intercrystalline, moldic, and intergranular may be locally 94

Description

Pore Type Karst Fracture/Breccia

Tectonic Fracture/Breccia

Vuggy

Channel

Intercrystalline Moldic

Intergranular

Table 2

'Non-fabric selective most common 'Solution collapse breccias may be fabric-selective 'Includes pores between clasts, solutionenlarged fractures, and cave roof fracture and breccia pores 'Non-fabric selective 'Fractures are small and straight 'Associated cements - calcite, pyrite, silica, saddle dolomite 'Most common in karsted interval as dissolution-enlarged pores after interbreccia, intercrystalline, and moldic 'Also in dolomitized packstones and grainstones as dissolution pores after relict interparticle and intercrystalline pores 'Occurs exclusively in karsted or meteorically altered intervals 'Enhances pre-existing pore or fracture system 'Most common in karsted intervals May also preferentially occur in grainsupported intervals 'Most common in packstones and grainstones Low permeability 'Sandstones of the Wilberns, E-1 (Chamizal Sandstone) and E-2 intervals 'Also E-6 (Cindy Sandstone)

Pore types recognized in samples described from the Ellenburger Group.

important or significant in a particular interval Alteration of the Ellenburger Group began as soon as these rocks were deposited and has continued into the deep burial diagenetic realm. Syngenetic dolomitization of many of the mud-rich inner platform sediments was one of the most significant early diagenetic alterations affecting the Ellenburger Group on a regional scale. Sediment deposited at parasequence tops and at or below third-order sequence boundaries may exhibit evidence of local or regional disconformities denoted by a

variety of features, including lofer cycles, exposure breccias, calcretes and silcretes. Evidence for fresh-water dissolution at these boundaries includes vugs and moldic pores, and minor amounts of infiltrated sediment.

The influence of a global fall in sea-level (early Middle Ordovician Sauk Unconformity of Sloss, 1963) is manifested in these rocks by a variety of diagenetic features. Extensive carbonate dissolution resulted in the generation of vugs, caverns, caves, and solution-enlarged fractures and joints. Infiltrated sediment fills or partially occludes many of these voids, as internal sediment in caves and caverns, and as geopetal sediment in smaller voids. The roofs of larger caves were brecciated as the caves were buried and subjected to static loading with flooding of the platform and deposition of the Simpson

Group. The fracture and breccia porosity found in the cave roof portions of these karst profiles accounts for much of the regionally significant porosity within the Ellenburger Group. 95

More than one episode of karsting has affected the carbonates of the Ellenburger Group. Pre-Woodford karsting was likely responsible for some enhancement and reduction in porosity and for continued dolomitization. Infiltrated sediment drapes over dolomite breccia clasts lined with coarsely crystalline dolomite cement attest to the complexity of the paragenetic sequence caused by multiple karsting events. Another significant karsting episode took place during the Early Pennsylvanian unconformity. Alterations are most

extensive and best defined

in

In this area, the Lower

central Crockett County.

Pennsylvanian Strawn Group rests directly on Ellenburger Group strata. The most striking feature in many of these cores is the dark red coloration. Internal sediment that fills most of the intercrystalline porosity in these extensively dolomitized rocks has been oxidized. Many

of the karst deposits are actually mantle breccias representing sink holes and surficial deposits . The clasts within these breccias are altered and worn; many contain weathering Dedolomite and dissolution of well-formed, coarsely crystalline dolomite are also associated with this later, Pennsylvanian karsting event. The formation of microporous tripolitic chert was also noted in several of the cores from this area. .

rinds.

Cements precipitated prior to oil migration provide insight into the timing of oil emplacement in Ellenburger Group reservoirs. Most of the oil was emplaced after dolomitization, but immediately after dolomite dissolution in northern part of the study area.

These observations imply that oil migration took place during the Middle (to perhaps Upper) Pennsylvanian. Because oil migration and emplacement apparently took place immediately before or during the early phases of Pennsylvanian/Permian compressional deformation, this re-structuring may have formed traps in places and destroyed traps in other areas. Many of the tension gashes or shear fractures caused by compression are healed with ferroan calcite cement. Most late tectonic fractures and associated microfractures are filled by ferroan calcite, quartz, or pyrite cements.

Character of Ellenburger Group Reservoirs Cores representative of the Ellenburger Group reservoir in the Brown-Bassett/JM trend are present in the Magnolia Goode #2 well from Brown-Bassett field, the Shell Mitchell #2 and #8 wells from JM field, and the Magnolia Morrison #1 well from Morrison

Most observations and conclusions regarding the nature of Ellenburger Group reservoir are derived from the Magnolia Morrison #1 core, which contains excellent porosity development associated with karsted dolomite in the upper Ellenburger Group (lower E-5 interval; Fig. 17). Unfortunately, the quality and volume of the cores from Ellenburger strata from the prolific Brown-Bassett and JM fields is so poor it precludes a field.

direct comparison to the sub-economic reservoir at Morrison field

Volumetrically, the most significant types of porosity are the direct and/or indirect result of dissolution of carbonate by meteoric waters. Meteoric fluids dissolved unstable grains, matrix, or the sides of fractures and joints. Locally, dissolution led to the formation of caverns and caves. Where the roof of caves were weakened by overburden, they collapsed and fractured the cave roof. Later episodes of subaerial exposure, complete with flushing by meteoric waters and concomitant solution enlargement of these fractures, resulted in excellent porosity and permeability as evidenced in the upper portion of the Magnolia Morrison #1 core. Tectonic deformation due to repeated activity of the structures resulted in a high degree of brecciation, though most of the resultant hairline fractures observed were filled with calcite cement or bitumen.

Other less common types of pores in these reservoirs include minor amounts of intercrystalline, moldic, and even intergranular associated with the interbedded sandstones.

None of these pore types are volumetrically significant 96

in

cores from the Brown-

Bassett/JM trend, though they may locally influence the storage capacity and deliverability of the strata.

Exploration Implications

Porosity and permeability in the most prolific Ellenburger Group reservoirs is dominantly fracture controlled, with cavernous porosity and paleokarst features formed at intersections of dominant fracture sets. Faults and fractures established during Late Precambrian to Early Cambrian passive-margin development, exhibit a northwesterly and northeasterly orientation. These faults have been repeatedly reactivated and have enhanced Ellenburger Group reservoir quality in places. Sizable structures have been created during at least four orogenic episodes: Middle Ordovician block-faulting, Late Mississippian-Early Pennsylvanian block-faulting and folding, Late Pennsylvanian-Early Permian folding, thrusting and right-lateral transpression,

and possibly Late Cretaceous-Early Tertiary transpression. There is no shortage of structures; rather there is a shortage of coherent seals for traps formed later than the Early Permian. While each deformational event further fractured Ellenburger carbonates, each also disrupted, and in some cases, resulted in erosional events that affected sealing beds over existing traps.

With the present well control and improved resolution of seismic data in the Val Verde basin, exploration efforts can be focused on structures where Middle Ordovician, Devonian to Lower Mississippian, or possibly Middle Permian shales and/or basinal lime mudstones are believed present. In addition to being the better seals in the stratigraphic section of the region, they are kerogen-rich source rocks, something notably lacking in the thick packages of synorogenic sediments. In addition, they have been buried deeply enough to have yielded hydrocarbons, yet are generally not overcooked. By critical examination of surface geology, the more pronounced effects of Early and Late Tertiary disruption can be avoided. In areas where there is a thick plastic shale over the Ellenburger Group, effects of those later events will be minimal. In this region of frequent superposed deformation, it is not sufficient to concentrate only on large structures at the Ellenburger level. Trap integrity is also an important consideration. Acknowledgments The authors would like to thank the companies that subscribed to this regional study for permission to publish this paper and for allowing access to their core collections. We would also like to thank Pat Dickerson for her help with the interpretations of regional structural trends and James Mulholland for his assistance with Upper Ordovician, Silurian, and Devonian regional stratigraphy. The manuscript has been greatly improved by the careful editing by Donald Yurewicz and Rick Fritz.

97

References

Barnes, V.E., Cloud, PE., Jr., Dexon, L.P., Folk, R.L., Jonas, E.C., Palmer, A.R., and Tynan, EJ., 1959, Stratigraphy of the pre-Simpson Paleozoic subsurface rocks of Texas and southeast New Mexico: University of Texas, Austin, Bureau of Economic Geology Publication 5924, 294 p.

Fischer, A.G., 1964, The Lofer cyclothems of the Alpine Triassic: Kansas Geological Survey Bull., v. 169, p. 107-149.

Flower, R.H., 1964, Early Paleozoic to New Mexico and El Paso Region: El Paso Geological Society, Ordovician Symposium, p. 31-101.

Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1987, High frequency glacio-eustatic sea-level oscillations with Milankovitch characteristics recorded in Middle Triassic platform carbonates in northern Italy: American Journal Science, v. 287, p. 853-892.

Goldhammer, R.K., Dunn, P.A., and Hardie, L.A., 1990, Depositional cycles, composite sea level changes, cycle stacking patterns and hierarchy of stratigraphic forcing: Examples from Alpine Triassic platform carbonates: Geol. Society America Bull., v, 102, p. 535-562. Kerans, C., 1988, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas: Amer. Assoc. Petroleum Geologists Bull., v. 72, p. 1160-1183.

Kerans, Charles and Lucia, F.J., 1989, Recognition of second, third, and fourth/fifth order

scales of cyclicity in the El Paso Group and their relation to genesis and architecture of Ellenburger reservoirs: in B.K. Cunningham and D.W. Cromwell

(eds), The Lower Paleozoic of West Texas and New Mexico -- Modern Exploration Concepts, PBS-SEPM Publication 89-31, p. 105-110. Kerans, C., 1990, Depositional systems and karst geology of the Ellenburger Group (Lower

Ordovician), subsurface West Texas: Bur. Econ. Geol. Univ. Texas, Rept. Invest. 193, 63 p.

Lucia, F.J., 1968, Sedimentation and Paleogeography to the El Paso Group, in Delaware Basin Exploration: West Texas Geological Society, pub. 68-55, p. 61-75. Nicholas, R. L. and Rozendal, R. A., 1975, Subsurface positive elements within Ouachita

foldbelt in Texas and their relation to Paleozoic craton margin: American Association of Petroleum Geologists Bulletin, v. 59, p. 193-216.

Read, J.F., Grotzinger, J.P., Bova, J.A., and Koerschner, W.F., 1986, Models for generation of carbonate cycles: Geology, v. 14, p. 107-110.

Read, J.F., and Goldhammer, R.K., 1988, Use of Accommodation plots to define thirdorder sea-level curves in Ordovician peritidal cyclic carbonates, Appalachians: Geology, v. 16, p. 895-899.

Sarg, J.F., 1988, Carbonate sequence stratigraphy: in C.K. Wilgus and others (ed), Sealevel Changes: an Integrated Approach, SEPM Special Publication 42, p. 155181.

.

98

Sacks, P. E. and Secor, D. T., Jr., 1990, Kinematics of Late Paleozoic continental collision between Laurentia and Gondwana: Science, v. 250, p. 1702-1705.

Sloss, L.L., 1963, Sequences in the cratonic interior of North America: Geol. Society America Bull., v. 74, p. 933-114.

Thomas, W. A., 1991, The Appalachian-Ouachita rifted margin of southeastern North America: Geological Society of America Bulletin, v. 103, p. 415-431.

Wilson, J.L., 1974, Carbonate Facies in Geologic History. Springer-Verlag, 471 p.

99

KARST BRECCIAS IN THE MADISON LIMESTONE (MISSISSIPPIAN), GARLAND FIELD, WYOMING A. Serdar Demiralin* Colorado School of Mines Golden, Colorado

Neil F. Hurley Marathon Oil Company Littleton, Colorado

Thomas W. Oesleby Marathon Oil Company Cody, Wyoming

*Current Address: Ankara, Turkey

Abstract Garland field is an asymmetric anticlinal trap located in the north-central Big Horn basin, Wyoming. The field produces hydrocarbons from interlayered, fractured limestones and dolomites of the Madison Limestone (Mississippian) Significant karstification occurs in the form of field-wide intraformational breccias and locally developed cavernous porosity. Most breccias and caverns apparently formed during prolonged post-Madison exposure, prior to deposition of the overlying Darwin Sandstone.

Three types of karst breccia occur: (1) red, siltstone-matrix breccias, (2) claymatrix breccias, and (3) dolomicrite-matrix breccias. Red, siltstone-matrix breccias occur in the upper 30 ft (9 m) of the Madison, and are related to the exposure event at the top-ofMadison unconformity. Clay-matrix breccias form a regionally correlatable layer which is about 50 ft (15 m) thick in the Garland field area. These breccias, which occur roughly 200

ft (60 m) below the top of the Madison, probably formed by evaporite dissolution and subsequent collapse. Dolomicrite-matrix breccias occur at the tops of shallowing-upward sequences at several levels within the Madison, and they apparently pre-date clay-matrix breccias. Dolomicrite-matrix breccias may have formed during periodic intraformational exposure events.

Introduction Garland field is one of a number of anticlinal traps that produce hydrocarbons from the Madison Limestone (Mississippian) in the Big Horn basin, northwestern Wyoming (Figure 1). According to Peterson (1990), Garland is the fifth largest reservoir in the basin, with 160 MMBO (million barrels of oil) in place.

101

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102

The Madison Limestone, which is approximately 1000 ft (300 m) thick, is composed of interbedded crystalline dolomites, limestones, and karst breccias. For this study, which is

derived from Demiralin (1991), a total of 1840 ft (561 m) of core and 395 thin sections have been described from 10 different wells (Figure 2). This project was initiated when work in nearby Elk Basin field suggested that Madison karst breccias were important contributors to reservoir heterogeneity (McCaleb and Wayhan, 1969; McCaleb, 1988). Also, regional stratigraphic work by Sando (1972, 1974, 1977, 1988) indicated that karst breccias were laterally extensive throughout northwestern Wyoming. Such breccias had been recognized at Garland field, but their distribution and significance were not fully

understood. The purpose of this paper is to examine the occurrence and origin of intraformational breccias in Garland field.

Geologic Setting

Stratigraphy The Madison Limestone was deposited on a broad platform that covered much of North America during the Mississippian (Sando, 1988). The formation is approximately 1000 ft (300 m) thick in the study area (Figure 3). It is overlain unconformably by the Darwin Sandstone Member of the Amsden Formation. The hiatus that separates the two formations had a maximum duration of approximately 34 million years (Sando, 1988). The Madison consists of various open-shelf, restricted-marine, and peritidal carbonate rocks. Much of the section has been dolomitized in the Garland field area.

Figure 4 shows the informal subsurface zonation, based on wireline log signatures, that is used by Marathon Oil Company in Garland field. Zone and subzone boundaries occur

at major contacts between porous and non-porous rocks. Boundaries almost always correspond to limestone-dolomite contacts, or to contacts above and below breccia intervals. Subaerial exposure of the platform during the late Mississippian to early Pennsylvanian led to the formation of karst features at the top of the Madison Limestone

(Sando, 1972, 1974, 1982). This exposure event may also have been responsible for leaching of interbedded evaporites. Bedded anhydrite/gypsum is known in the upper few hundred feet of the Madison in the Hoback Basin, western Wyoming (Sando, 1988), in the Overthrust Belt in southwestern Wyoming (Harris et al., 1988; Sieverding and Harris,

1991), and in southern Montana (Andrichuk, 1955; Roberts, 1966). No such bedded evaporites are known in the Garland field area.

The Darwin Sandstone, which is the basal member of the overlying Amsden Formation, consists of cross-bedded quartz arenite deposited on the irregular unconformity surface. The Darwin, interpreted as transgressive beach and offshore-bar deposits, ranges in thickness from 0 to 50 ft (15 m). Anomalously high thicknesses of the Darwin Sandstone

and/or Amsden Formation are interpreted as sinkholes in the karsted upper Madison (Sando, 1988; McCaleb, 1988).

Structure The Big Horn basin is bordered by the Big Horn Mountains to the east, the Owl Creek Mountains to the south, the Absaroka and Beartooth Mountains to the west, and the Nye-Bowler fault zone to the north (Figure 1). The basin is characterized by elongate,

doubly plunging, asymmetric anticlines along its margins (Stone, 1967; Hoppin and Jennings, 1971; Paylor et al.,, 1989) 103

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Type log for Madison A, B, C, and D zonation, well E8, Garland field. Lithology and texture of the Madison Limestone have been compiled from all cored wells. Black layers in the log indicate producing intervals. Arrows indicate shallowing-upward sequences. The curves on the left and right are gamma-ray and density-neutron logs, respectively. In terms of depositional environment, 1 = restricted peritidal, 2 = slightly restricted subtidal, 3 = shallow open shelf, and 4

= deep open shelf For depositional texture, MS = mudstone, and GS = grainstone. Breccia zones are shown with symbols for matrix type on the left side of the column. 106

Garland field, which was discovered in 1906, is a doubly plunging anticline that trends northwest-southeast (Figure 2). The Madison producing area is approximately 4 mi (6.4 km) long and 1.5 mi (2.4 km) wide. Although the Madison is the principal reservoir, hydrocarbon production also occurs from the Tensleep Sandstone (Pennsylvanian) and Phosphoria Formation (Permian). The anticline is asymmetric with a steep limb (30 to 40°) on the northeast flank and a gently dipping limb (less than 20°) on the southwest flank (Risley, 1961; Wyoming Geological Association, 1989).

Karstification Host-Rock Lithology Because the Madison Limestone is so thick in Garland field, no single well has cored

the entire interval. Figure 5 shows the vertical distribution of core with respect to Marathon's informal A, B, C, and D zonation. All described cores are from the anticlinal crest or the gently dipping southwest flank of the fold. Zones A and B have the best core control, whereas zone D has been cored by only a few wells. No complete core exists for the C zone. Four wells were cored through the Darwin/Madison unconformity.

Figure 4 shows the main lithologies developed in zones A through D. In order of decreasing abundance, the Madison Limestone has 3 major lithofacies types: (1) restrictedcirculation, peritidal, laminated dolomudstones, (2) slightly restricted-circulation, shallowsubtidal, bioturbated packstones and wackestones, and (3) open-marine, subtidal, skeletal/ooid grainstones and packstones.

Zones A, B, and D are major shallowing-upward sequences which contain several subsequences (Figure 4). Zone C, which has limited core control, appears to form a gradual transition from zones D to B. A typical sequence has a succession, from bottom to top, of

open-shelf skeletal/ooid grainstones and packstones that grade into slightly restricted, bioturbated packstones and wackestones. These are overlain by restricted-marine, laminated mudstones. Such sequences are generally capped by an intraformational breccia. Dolomite and limestone intervals, as well as individual lithofacies types, correlate well across the field without significant thickness changes.

More than half of the Madison section has been completely dolomitized, especially the micritic facies. Reservoir-quality dolomites in zones A, B, and D have intercrystalline, moldic, and/or pinpoint vuggy porosity. Locally, skeletal/ooid grainstones and packstones

have small amounts of porosity. Crinoid overgrowths and pore-filling mosaic calcite cements, however, have destroyed most porosity in the limestones.

Intraformational Breccias A descriptive classification of breccias, modified from Morrow (1982) and Kerans (1989), is used in this study. Intensely fractured rocks with no significant displacement of clasts are called fracture breccias. Mosaic breccias have some displacement of clasts. Three major breccia types, which occupy definite stratigraphic intervals, have been observed in Garland field: (1) breccias with red siltstone matrix, (2) breccias with clay matrix, and (3) mosaic and fracture breccias with dolomicrite matrix. Breccias with red siltstone matrix occur at the top of the Madison Limestone. Clay-matrix breccias occur almost exclusively in subzone A-7, 200 ft (60 m) below the top of the Madison. Dolomicrite-matrix

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Red, siltstone-matrix breccias The top-of-Madison breccia (Figure 6) has variable fabric and thickness in the 4 cored wells in which it has been studied. The zone is 30 ft (10 m) thick in well CF12, 18 ft (5.5 m) thick in well S6, and 15 ft (4.5 m) thick in well H4. The breccia was not fully cored in well S8. The overlying Darwin Sandstone has silicified limestone clasts in wells CF12 and H4, whereas reworked clasts are apparently absent in well S8. Red iron-oxide staining is common on pore-filling calcite cements. Also, concentric red iron-oxide color bands are noted in some breccia fragments (Figure 6B). In general, the uppermost few feet (1 m) of the top-of-Madison breccias are matrix supported (Figure 6A). The matrix is typically red-colored, clay-rich siltstone and sandstone

that is unlike the gray, very fine- to fine-grained quartz arenite of the overlying Darwin Sandstone (Figures 6C, D). Matrix content gradually decreases downward, and the breccia becomes clast supported. Clasts are typically 0.1 to 10 cm or larger in size, angular to subangular in shape, and some outer surfaces are slightly scalloped. Breccia fragments include crystalline limestone and lime mudstone with rare evaporite casts. Brecciation diminishes downward into fracture breccias that apparently formed by infiltration of matrix material into fine cracks in the rock. The deepest red siltstone observed in core is about 30 ft (9 m) below the top of the Madison. Strangely enough, one fracture filled with material that appears to be Darwin Sandstone occurs roughly 350 ft (105 m) below the top of the Madison in well UW5.

Apparently, karstification at the top of the Madison did not locally enhance porosity or permeability. Although brecciation was significant, the red-siltstone matrix that fills pore space between clasts has effectively destroyed reservoir properties. Rocks at the top of the Madison are rarely oil stained, and this interval is probably part of the reservoir seal.

Clay-Matrix Breccias Clay-matrix breccias, present mainly in the 50 ft (15 m) thick subzone A-7, have greenish-gray clays in the spaces between dolomite, limestone, and anhydrite clasts (Figures

7A, B). Clast shape varies from angular to subangular at the top to subrounded with common scalloped surfaces near the base. Fragment size, which generally ranges from 0.1 to 5.0 cm, decreases downward. However, clasts larger than core diameter (10 cm) are common at the top of the breccia interval. Clasts are not oil stained, and porosities and permeabilities are always very low in this unit. Gamma-ray logs, which normally show little activity in the Madison Limestone, show significant activity in clay-matrix breccias of subzone A-7 (Figure 4). XRD analyses of clay-matrix breccias show that the rock is typically 20% clay. Clay is predominantly illite. Roberts (1966) has suggested that kaolinite is common in soil-related breccias whereas illite is common in evaporite solution-collapse breccias. His examples are from the Madison Limestone in southwestern Montana. Illite also occurs in karst breccias and insoluble residues of the Madison Limestone in Elk Basin field, Wyoming (McCaleb

and Wayhan, 1969), and in Madison breccias in northern Wyoming and south-central Montana (Vice, 1988).

109

Figure 6

Top-of-Madison breccia fabrics. (A) Breccia with red, siltstone and sandstone

matrix in the uppermost Madison Limestone. This slab shows the upward gradation from clast-supported to matrix-supported breccia. Core slab. Scale = 1 cm. Well CF12, depth 4085 ft. (B) Limestone breccia, uppermost Madison Limestone. Concentric, iron-oxide color banding within breccia fragments was observed in this well. Horizontal stylolites truncate both fragments and bands. Fragments have packed peloid grainstone texture. A possible clast of Darwin Sandstone (d) is also present in this breccia. Core slab. Scale = 1 cm. Well S6,

depth 4093 ft. (C) Photomicrograph of breccia in the uppermost Madison Limestone. This section is approximately 28 ft (8.5 m) below the top of the

Madison. Some quartz fragments (q) resemble the overlying Darwin Sandstone. However, most of the matrix material (m) is clay-rich siltstone. Note the detrital dolomite crystals (d), ferroan rims (blue), and remnants of calcite cement (red). Plane-polarized light. Stained thin section. Scale = 0.2 mm. Well CF12, depth 4108 ft. (D) Photomicrograph of Darwin Sandstone at the same scale as (C). Note the dissimilarity between the Darwin Sandstone and the siltstone matrix common in top-of-Madison breccias. Porosity is in purple. Quartz overgrowths (q) are common. The dark blue cement is poikilotopic ferroan saddle dolomite. Stained thin section, plane-polarized light. Scale = 0.2 mm. Well CF12, depth 4077.3 ft.

110

Figure 7

Karst fabrics in subzones well below the top of the Madison.

(A) Matrixsupported, clay-matrix breccia, subzone A-7. Lime mudstone breccia fragments are angular and have scalloped surfaces. Matrix is greenish-gray clay. Core slab. Scale = 1 cm. Well US30, depth 4416 ft. (B) Matrix-supported, clay-matrix breccia, subzone A-7. This slab is stratigraphically lower than (A). The average size of clasts has decreased in this interval. Core slab. Scale = 1 cm. Well UW5, depth 3910 ft. (C) Dolomicrite-matrix mosaic breccia, subzone B-1. This

breccia has excellent oil stain across the field. Fragments are laminated dolomudstone, and there are also anhydrite fragments (a). Core slab. Scale = 2 cm. Well UW5, depth 3943

fi.

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breccia is a clay-matrix breccia (c). Oil-stained breccia (b) has dolomicrite matrix. In core, the cream-colored breccia stratigraphically underlies the dolomicrite-matrix breccia. The boundary between breccias is sharp. Apparently, dolomicrite-matrix breccia predates clay-matrix breccia because there are dolomicrite-matrix breccia clasts (d) in the clay-matrix breccia. The clay-matrix breccia also has anhydrite clasts (a). Core slab. Scale = 1 cm. Well CF22, depth (E) Solution-enlarged fracture porosity in mudstone, subzone D-1. Such fractures, which occur sporadically throughout the Madison, may be responsible for lost circulation while drilling. Core slab. Scale = 2 cm. Well 4113 ft.

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Samples from the upper part of the clay-matrix breccia show the effects of plastic deformation, probably due to compaction and collapse. Fissile laminae in the clays may be inclined as much as 700 from horizontal. The boundary of clay-matrix breccia with the underlying dolomicrite-matrix breccia is very sharp. In one well (UW5), there is a thin (1 ft, 0.3 m) anhydrite interval at this contact. The top of subzone A-7 commonly corresponds to the top of the most clay-rich interval. Overlying fracture breccias, presumably related to the collapse of clay-matrix breccias, are known to extend to levels as high as subzone A-5.

Dolomicrite-Matrix Breccias Mosaic to fracture breccias (Figure 7C) with dolomicrite matrix are characteristic of subzone B-1. These breccias, which are about 30 ft (9 m) thick, are correlatable field-wide.

Clasts range in size from 0.1 to more than 10 cm. Clasts are angular to subangular fragments of restricted-marine, shallow-subtidal dolomites and supratidal, cryptalgallaminated dolomudstones. The amount of matrix is generally 5 to 10% of rock volume.

Breccias are best developed at the top of the B zone. The lack of petrographically detectable clay in the matrix, and the sharp boundary between overlying clay-matrix breccias and dolomicrite-matrix breccias suggest that the breccias formed at different times. In a different subzone (B-4), clay-matrix breccias actually contain fragments of dolomicrite-

matrix breccia, suggesting that clay-matrix breccias probably formed after dolomicritematrix breccias (Figure 7D).

Lost-Circulation Zones, Bit Drops, and Sinkholes Cavernous porosity in Garland field can be detected as zones with lost-circulation of drilling fluids, or as zones which had bit drops while drilling. Figure 8 shows that bit drops cluster in the brecciated A-7 and B-1 subzones. Lost circulation occurred rather uniformly throughout the section, and may have been caused by open natural fractures encountered while drilling (Figure 7E). Sinkholes of karst origin have been mapped by isopaching 2 different intervals: (1)

the Madison A zone, and (2) the lower two-thirds of the overlying Amsden Formation. Sinkholes were expressed as local thins in the A zone and corresponding thicks in the Amsden. Several sinkholes occur in the northwest and southeast parts of Garland field. These features are relatively small in areal extent, probably less than 500 to 1000 ft (150 to 300 m) in diameter. Their vertical relief is on the order of 10 to 20 ft (3 to 6 m). The main impact of sinkholes is a local, detrimental effect on lateral continuity of reservoir subzones within the A zone.

Discussion

Demiralin (1991) summarized the four major theories that have been proposed for the origin of Madison karst breccias. Breccias have been related to: (1) intraformational unconformities, including the top-of-Madison unconformity, (2) solution of interbedded evaporites and subsequent collapse, (3) dissolution along a flat paleo-water table, and (4) hydrothermal dissolution

Several observations made in this study are relevant to the question of breccia origin. First, all breccia intervals in the Madison Limestone in Garland field, including the top-of-Madison breccia, are at the tops of shallowing-upward sequences. This suggests

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that karstification associated with intraformational unconformities and leaching of interbedded evaporites are both viable mechanisms for brecciation. Second, the textures of Madison breccias differ. Some breccias show an overall decrease in clast size towards the

top. The top-of-Madison breccia is of this type. This overall normal grading, the inferred presence of sinkholes, and the characteristic red color strongly suggest an unconformityrelated origin.

It is interesting to note that the Darwin Sandstone differs from the red, clay-rich siltstone that is incorporated into the top-of-Madison breccia. Sando (1988) estimated a time lapse of as much as 34 million years between the end of Madison deposition and progradation of the Darwin Sandstone. Considering the long period of exposure to which the Madison was subjected, it is likely that numerous marine and/or continental units covered that surface, but were later eroded. Sando (1972) reported a red siltstone unit in the lower Darwin Sandstone in the Beartooth Mountains. It is possible that this unit correlates to the material in the top-of-Madison breccias at Garland field.

Other breccias in the Madison limestone display an overall increase in clast size and

angularity towards the top. This texture, which may be related to cave collapse, is best developed in clay-matrix breccias in subzone A-7. The origin of this widespread breccia is the subject of some controversy. Bedded evaporites do exist in the Madison Limestone to the north, west, and south of the Big Horn basin. Groundwater studies by Doremus (1986) and others suggest there are no significant evaporite deposits in the anticlines or in the undeformed parts of the Madison Limestone in the Big Horn basin. Wireline logs from numerous wells in the basin confirm this fact. An evaporite solution-collapse origin is likely for the widespread clay-matrix breccia. However, the removal of large volumes of evaporite

requires a large amount of flowing fresh water. It is possible that such amounts of water could have moved through the Madison during the tens of millions of years of exposure that occurred before Darwin deposition. Abundant calcite cements noted in the Madison may relate to this period of meteoric diagenesis. An evaporite solution-collapse origin for claymatrix breccias is supported by the presence of remaining cavernous porosity, anhydritic breccia fragments, and the abundance of illite clays. Such clays may be insoluble residue from dissolved evaporites. Despite the presence of local cavernous porosity, core analyses and wireline logs suggest that clay-matrix breccias are blanket-like permeability barriers in this field.

Dolomicrite-matrix breccias, which are best developed in subzone B-1, have a sharp contact with overlying clay-matrix breccias. Evidence suggests that clay-matrix breccias formed after dolomicrite-matrix breccias. Dolomicrite-matrix breccias, which are present at several levels within the Madison, may have formed during subaerial exposure events prior to deposition of overlying evaporites. Dolomicrite-matrix breccias commonly occur in shallow-marine to supratidal sediments. These breccias are porous and oil stained, and they commonly produce hydrocarbons in the subsurface.

Conclusions

Three major types of breccia have been recognized in the Madison Limestone at Garland field. Each breccia occurs at a distinctive stratigraphic interval, and the breccias are

laterally extensive. Breccia types are: (1) red, siltstone-matrix breccias at the top of the Madison, (2) clay-matrix breccias 200 ft (60 m) below the top of the Madison, and (3) mosaic and fracture breccias with dolomicrite matrix that underlie clay-matrix breccias. All

breccia intervals are located at the tops of shallowing-upward sequences. Moreover, the clasts are predominantly of restricted peritidal lithofacies.

116

The top-of-Madison breccia probably formed during exposure of the Madison shelf prior to deposition of the Darwin Sandstone. Clay-matrix breccias probably formed by dissolution of evaporite beds and subsequent collapse during this karstification event. Because clay-matrix breccias postdate dolomicrite-matrix breccias, dolomicrite-matrix breccias may have formed during subaerial exposure at intraformational unconformity surfaces.

Clay-matrix breccias today act as field-wide, blanket-like permeability barriers. The top-of-Madison breccia, which has little remaining porosity or permeability, appears to be part of the reservoir seal. Dolomicrite-matrix breccias are commonly oil stained, and they form an important part of the hydrocarbon-producing column.

References

Andrichuk, J. M., 1955, Mississippian Madison group stratigraphy and sedimentation in Wyoming and Southern Montana: AAPG Bulletin, v. 39, p. 2170-2210. Demiralin,

A. S., 1991, Geological characterization of the Madison Limestone (Mississippian) reservoir, Garland field, Big Horn basin, Wyoming: Unpublished M.S. Thesis, Colorado School of Mines, 127 p.

Doremus, D. M., 1986, Groundwater circulation and water quality associated with the Madison aquifer, northeastern Bighorn Basin, Wyoming: Unpublished M.S. Thesis, University of Wyoming, 81 p.

Harris, P M., Flynn, P. E., and Sieverding, J. L., 1988, Mission Canyon (Mississippian) reservoir study, Whitney Canyon-Carter Creek field, southwestern Wyoming, in Lomando, A. J., and Harris, P. M., eds., Giant oil and gas fields, a core workshop: SEPM Core Workshop No. 12, v. 2, p. 695-740. Hoppin, R A., and Jennings, T. V., 1971, Cenozoic tectonic elements, Bighorn Mountain region, Wyoming-Montana, in Renfro, A. R., Madison, L. V., Jarre, G. A., and Bradley, W. A., eds., Symposium on Wyoming tectonics and their economic Wyoming Geological Association, 23rd field Conference significance: Guidebook, p. 39-47.

Kerans, C , 1989, Karst-controlled reservoir heterogeneity and an example from the Ellenburger Group (Lower Ordovician) of west Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 186, 40 p.

McCaleb, J. A., 1988, Significance of paleokarst on petroleum recovery, Elk Basin field, Big Horn Basin, in Goolsby, S. M., Longman, M. W., Inden, R. F., Kerr, S. D., and Lindsay, R. F., eds., Occurrence and petrophysical properties of carbonate reservoirs in the Rocky Mountain region: Rocky Mountain Association of Geologists, p. 139-147. McCaleb, J. A., and Wayhan, D. A., 1969, Geologic reservoir analysis, Mississippian Madison Formation, Elk Basin field, Wyoming-Montana: AAPG Bulletin, v. 53, p.2094-2113.

Morrow, D. W., 1982, Descriptive classification of sedimentary and diagenetic breccia fabrics in carbonate rocks: Bulletin of Canadian Petroleum Geology, v. 30, p. 227-229. 117

Paylor, E. D., Muncy, H. L., Lang, H. R., Conel, J. E., and Adams, S. L., 1989, Testing some models of foreland deformation at the Thermopolis anticline, southern Bighorn Basin, Wyoming: The Mountain Geologist, v. 26, P. 1-22. Peterson, J. A., 1990, Petroleum potential outlined for northern Rockies, Great Plains: Oil and Gas Journal, v. 88, July 30, p. 103-110.

Risley, R. G., Jr., 1961, The structural geology of Byron-Garland anticlines, Park and Big Horn Counties, Wyoming: Unpublished M.S. Thesis, University of Wyoming, 62 p.

Roberts, A. E., 1966, Stratigraphy of Madison Group near Livingston, Montana, and discussion of karst and solution-breccia features: U.S. Geological Survey Professional Paper 526-B, p. BI-B23. Sando, W. J., 1972, Madison Group (Mississippian) and Amsden Formation (Mississippian and Pennsylvanian) in the Beartooth Mountains, northern Wyoming and southern Montana, in Crazy Mountains Basin: Montana Geological Society, 21st Annual Geological Conference, p. 57-63. Sando, W. J., 1974, Ancient solution phenomena in the Madison Limestone (Mississippian) of northcentral Wyoming: U.S. Geological Survey Journal of Research, v. 2, p. 133-141.

Sando, W. J., 1977, Stratigraphy of the Madison Group (Mississippian) in the northern part of the Wyoming-Idaho overthrust belt and adjacent areas, in Heisey, E. L., Lawson, D. E., Norwood, E. R., Wach, P. H., and Hale, L. A., eds., Rocky Mountain thrust belt geology and resources: Wyoming Geological Association 29th Annual Field Conference, p. 173-177.

Sando, W. J.,

1982, New members of the Madison Limestone (Devonian and

Mississippian), north-central Wyoming and southern Montana: U.S. Geological Survey Bulletin 1529-H, p. H125-H130. Sando, W. J., 1988, Madison Limestone (Mississippian) paleokarst: a geologic synthesis, in

James, N. P., and Choquette, P. W., eds., Paleokarst: New York, SpringerVerlag, p. 256-277 Sieverding, J. L., and Harris, P. M., 1991, Mixed carbonates and siliciclastics in a Mississippian paleokarst setting, southwestern Wyoming thrust belt, in Lomando, A. J., and Harris, P. M., eds., Mixed carbonate-siliciclastic sequences:

SEPM Core Workshop No.15, p. 541-568.

Stone, D. S., 1967, Theory of Paleozoic oil and gas accumulation in Big Horn Basin, Wyoming: AAPG Bulletin, v. 51, p. 2056-2114.

Vice, M. A., 1988, Depositional environments and diagenesis in an interval of the Mission Canyon Limestone (Madison Group, Mississippian), south-central Montana and northern Wyoming: Unpublished MS. Thesis, Southern Illinois University at Carbondale, 149 p.

Wyoming Geological Association, 1989, Garland field, in Wyoming Oil and Gas Fields Symposium, Big Horn and Wind River basins, p. 182-187.

118

DEEP-BURIAL BRECCIATION IN THE DEVONIAN UPPER ELK POINT GROUP, RAINBOW BASIN, ALBERTA, WESTERN CANADA Jeffrey J. Dravis Consultant Houston, Texas

Iain D. Muir Imperial Oil Canada Ltd. Calgary, Alberta, CANADA

Abstract Brecciation is a common diagenetic fabric in subsurface dolomitized sequences of While not generally associated with hydrocarbon production from these sequences, breccias were a product of the same deepburial diagenetic processes responsible for creating other secondary pores from which production occurs. Several relationships demonstrate conclusively that brecciation and other associated styles of dolomite dissolution were deep-burial in origin, having formed coincident with, or after, pressure solution in these rocks. These breccias, therefore, are an example of deep-burial "karstification."

the Upper Elk Point Group in western Canada

Upper Elk Point breccias are invariably associated with fractures and burial replacement anhydrite, both of which were related to local faulting. They are always associated with dolomites and show no preference for development along depositional cycle breaks or formation tops. The common presence of stylolitic clasts, rotated at all angles to each other and the horizon, demonstrates that solution collapse occurred after the onset of pressure solution at depth Contrary to popular models, brecciation is not unique to near-surface processes such as freshwater karstification or leaching of evaporites. For the Upper Elk Point Group, to invoke these processes as explanations for the observed brecciation is to totally ignore the stratigraphical, petrographical and geochemical attributes of these sequences. Our case study shows that given the right tectonic and diagenetic settings, impressive deep-burial dissolution can occur in buried carbonate sequences, resulting in creation of substantial secondary porosity and brecciation.

Introduction Hydrocarbons are produced from a number of pools on Comet Platform adjacent to the Rainbow Basin in northwestern Alberta (Figs. 1 and 2). Production typically is from dolomitized platform-interior cycles associated with combination traps influenced by local structural conditions (Muir and Dravis, 1990; 1991) These pools are much different from

those that occur in the classical basinal pinnacle reefs which occur in the central and peripheral parts of the Rainbow Basin (Langton and Chin, 1968; Hriskevich, 1970; Barss, et al., 1970; Schmidt, et al., 1985).

Brecciation and other styles of spectacular dissolution within dolomites were associated with many of these pools, especially those on Comet Platform. Historically, (Continued page 122) 119

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121

brecciation and secondary porosity development have been used as the major expressions of near-surface karstification, especially when formation tops or unconformities exist relatively near to these breccias (Sangster, 1988; Wilson, et al., 1992). However, the timing of brecciation must be consistent with other diagenetic and tectonic elements expressed in a sequence which has been brecciated. If it is not, perhaps an alternative explanation of the timing of brecciation is in order. Interestingly, Hill (1992) has offered one alternative explanation. Brecciation is a common diagenetic fabric in all three formations which comprise the

Upper Elk Point Group, namely, the Keg River, Muskeg and Sulphur Point Formations While it would be convenient to relate this brecciation to subaerial exposure and either dissolution of carbonates by freshwater, or leaching of precursor evaporites, these mechanisms are entirely without merit based on regional stratigraphic and facies relationships, and on the petrographic and geochemical attributes of all three formations.

The key to resolving the timing of brecciation in these carbonate sequences is utilizing a regional integrated approach which relates brecciation to depositional facies and cyclicity, and establishes that the timing of brecciation is consistent with other diagenetic elements in the same sequences. If brecciation was related to early, near-surface dissolution, then these sequences should show other evidence of this early diagenesis, most notably, precompaction cementation. If brecciation occurred at depth after the host rock had already experienced deep-burial pressure solution, then the surrounding non-brecciated rocks likewise should show evidence of this deep-burial diagenesis Many ancient breccias, including those from Comet Platform, seem to be developed in dolomites (Wilson, et al., 1992). The key to resolving the timing of brecciation in dolomitized strata is to utilize enhanced petrographic techniques such as fluorescence microscopy and diffused plane-polarized light (Dravis and Yurewicz, 1985; Dravis, 1991). These techniques can establish whether the dolomite formed early before grain interpenetration, or after, by revealing relict grains and diagenetic fabrics normally invisible with standard light microscopy. If the dolomite formed under deeper-burial conditions, and was subsequently dissolved and brecciated, then the brecciation must also have been deepburial in origin.

Our examination of the Devonian Upper Elk Point Group on Comet Platform and in some adjacent pinnacle reefs shows that secondary porosity due to deeper-burial dissolution of dolomites was responsible for reservoir quality in these pools (Muir and Dravis, 1990; Dravis and Muir, 1991a,b; Muir and Dravis, 1991; Dravis, 1992). By deeper-burial

dissolution, we mean that dissolution occurred coincident with, or after, the onset of

pressure solution. The scale of dolomite dissolution varied from pin-point microporosity to larger vuggy pores, including fist-sized or larger pores associated with zebra dolomites and some breccias. Breccias associated with all three sequences were also created by dissolution of burial dolomites under deep-burial conditions. Simply put, these breccias were the grander expression of this deep-burial dissolution.

This paper discusses evidence for the timing of brecciation in the Upper Elk Point However, an in-depth discussion of these controls is beyond the scope of this paper and is best left to another

Group and some of the major controls on its development. paper.

122

Regional Setting

The middle Devonian of northwestern Alberta contains two major basins, the

Rainbow and Zama Basins, surrounded by shallow-marine carbonate platforms, including the Comet Platform (Figs. 1 and 2). A major tectonic positive element, the Peace River Arch, occurs to the south and the Comet Platform is located less than 15 kilometers (9 mi) from the Hay River Fault, a major strike-slip fault system to the southeast (Fig. 3). The Hay River Fault is a 1300 kilometer (780 mi)-long shear zone which extends from the

foothills of northeastern British Columbia to the southern side of Great Slave Lake, recording dextral transcurrent motion of up to 700 kilometers (420 mi) (Hanmer, 1987) Wrench faults typically are characterized by two sets of vertical fractures that have a predictable geometric orientation with respect to the principal shear (Wilcox, et al., 1973). Aeromagnetic data for the Comet Platform area indicates two sets of fracture trends at different angles to this shear zone.

This structural grain, as reflected in Figure 4, played a major role in the diagenesis and porosity evolution of the Upper Elk Point Group on Comet Platform (Muir and Dravis, 1990, 1991; Dravis, 1992). Regional stratigraphic cross sections and seismic data indicate that basement faults were reactivated at least two times during the Devonian. Their influence on these sequences is expressed by the presence of vertical stylolites and healed horizontal fractures in many of the cores examined The abnormally high geothermal gradient of 40° C/Km in the Rainbow-Zama area (Dunsmore, 1971; Hitchon, 1984) is a further indication of the unique structural setting of this area.

Stratigraphy Figure 5 depicts the general stratigraphy of the Elk Point Group in the Rainbow Basin. The Lower Elk Point Group consists of basal red beds and siliciclastics and evaporites of the Ernestina Lake Formation, all generally less than 30 meters (100 fi) in thickness. These deposits grade upwards into the Cold Lake Salt, a 30-60 meter (90-180 fi)-thick halite sequence. These deposits are overlain by the Chinchaga Formation, a 60-75 meter (180-225 ft)-thick anhydrite-dolomudstone sequence reflecting sabkha and restricted shallow-marine carbonate deposition.

The Upper Elk Point Group was initiated by deposition of the lower Keg River Member. This sequence consists of porous and permeable crinoidal shoals deposited along a ramp, and provides strong aquifer support to many Keg River pools in the Rainbow-Zama areas. These deposits grade up into the Middle and Upper Keg River Members which host most of the hydrocarbons in this area. In the Rainbow Basin, these members are dominated by thick, pinnacle, coral and stromatoporoid reefs; on Comet Platform, these members

consist of more restricted platform-interior facies packaged into repetitive cycles of sedimentation. On Comet Platform, the combined thicknesses of these members are on the order of 120-180 meters (400-600 fi). Prior to Upper Keg River deposition, the Black Creek Salt Member was deposited between the pinnacles in the basin. No evidence for salt deposition on Comet Platform has been found, however.

The Keg River is overlain by the Muskeg Formation which is up to about 200 meters (650 fi) thick in the Rainbow Basin but only 30-40 meters (100-130 fi) thick on Comet Platform. The Muskeg represents more restricted shallow-marine carbonate and (Continued page 127) 123

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evaporite (anhydrite) deposition compared to the underlying Keg River. Historically, the boundary between the Keg River and Muskeg Formations has been picked on the basis of the first occurrence of anhydrite (Law, 1955).

The Sulphur Point Formation consists of a progradational limestone sequence variably dolomitized and laterally gradational with the Muskeg Formation. The Sulphur Point is overlain by the Watt Mountain Formation, an impermeable 5-20 meter (15-66 ft)thick lime mudstone-calcareous shale sequence. The regional stratigraphy along the northern margin of Rainbow Basin influenced the diagenesis and porosity evolution of all three upper Elk Point Group formations. It is important to reiterate that the carbonates in these formations are underlain by evaporites and granitic basement which are generally less than 150 to 180 meters (500 to 600 fi) below the top of the lower Keg River member. Upper Keg River, Muskeg, and even Sulphur Point carbonates are cut by basement faults which penetrated underlying evaporites. These faults not only supplied hot, basement-derived fluids but also calcium- and sulfate-rich fluids derived from the evaporites (Muir and Dravis, 1991; Dravis and Muir, 1991b, Dravis, 1992).

These fluids helped promote the impressive dissolution of dolomites which

characterizes all the Upper Elk Point pools on Comet Platform.

Data Base

Evaluation of the regional and local controls on Upper Elk Point Group pool entrapment, including resolution of the brecciation of these sequences, was accomplished through a rigorous approach which integrated geological, geophysical and engineering data. Using wireline log data from 145 wells (Fig. 6), 12 east-west stratigraphic cross sections were generated across Comet Platform, subsequently tied by three north-south sections (Fig. 7). These data were ground-truthed with nearly 3050 meters (10,000 fi) of core from 57 wells (Fig. 6) and over 750 thin sections of representative samples. Geophysical data consisted of standard and 3-D seismic; engineering data included interference tests, pressure data, drill stem test data, and non-associated gas analyses.

The petrography of Upper Elk Point Group dolomites was accomplished by integrating standard thin section observations with blue-light fluorescence microscopy (Dravis and Yurewicz, 1985) and diffused plane-polarized light (Dravis, 1991). These newer petrographic approaches revealed relict depositional and diagenetic fabrics not seen with standard thin sections, including microporosity. By using these techniques, reservoir quality was more accurately related to depositional fabrics and facies type and the relative timing of porosity evolution was clearly established. Fluorescence microscopy also quickly revealed the occurrence of diagenetic fluorites and sphalerites whose presence was a reflection of the composition of diagenetic brines. Stable oxygen and carbon isotopes, as well as sulfur isotopes, were used to help refine the timing of carbonate and anhydrite diagenesis, as well as better understand the origin of diagenetic fluids (Dravis, 1992). Standard staining techniques were used to differentiate calcites from dolomites and to reveal the presence of iron-rich carbonate phases.

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Depositional Environments

Keg River Formation The Keg River Formation on Comet Platform is dominated by typical platforminterior facies. Three major facies are represented in core: open-marine, platform-interior subtidal; restricted platform-interior subtidal; and tidal flat. These facies relationships are illustrated schematically in Figure 8. Both the open-marine and restricted platform-interior subtidal facies are dominated by burrowed, Amphipora-peloidal dolopackstones; however,

the open-marine platform-interior facies contains a more varied skeletal fauna including robust cylindrical and bulbous stromatoporoids, corals and brachiopods. Tidal flat facies are predominantly peloidal dolomudstones to dolograinstones with crinkly algal laminations and common fenestral fabrics. Near the upper part of the Keg River, replacement anhydrites are more common and may be mistaken for depositional evaporites. However, the facies succession and fluorescence microscopy establish that these anhydrites are late, burial replacement fabrics because they replace stylolitic limestones and dolomites (Dravis, 1992).

The predominance of these facies types on Comet Platform implies a barrier to circulation along the margin of this platform (Fig. 8) However, facies deposited along the platform margin could not be determined because of the lack of core control

Muskeg Formation The lower portion of the Muskeg Formation consists of restricted platform-interior subtidal and tidal-flat facies comparable to those in the Keg River. However, replacement anhydrites are more common toward the top of the Muskeg. Most of these anhydrites appeared to be syndepositional in origin based on the facies succession and increasing lack of skeletal fauna upwards in the formation. These "brining-upwards sequences" represent

more restricted environmental conditions and are characterized by bedded to massive nodular anhydrites (Fig. 9).

A few intervals within the upper Muskeg are characterized by peloidal dolopackstones with robust stromatoporoids. These deposits reflect incursions of more normal marine waters associated with minor flooding events. Sulphur Point Formation Cores from this formation are rare and were not described systematically for their depositional facies. However, thin sections of samples from this sequence indicate facies comparable to that observed in the Keg River, mainly restricted subtidal facies. Most of

these rocks were completely dolomitized and later altered by styles of burial diagenesis similar to those observed in the Keg River and Muskeg Formations.

Depositional Cyclicity A time-stratigraphic framework for the Upper Keg River and Muskeg Formations was established by delineating major and minor cycles consisting of repetitive upwardshoaling or upward-brining sequences (Figs. 8-12). The change from one cycle to another (Continued page 136) 130

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was marked by a relatively deeper-water facies abruptly overlying a shallower-water facies, cycle contacts, therefore, were used as time lines and facies within that time-stratigraphic framework were then correlated, guided by established principles of carbonate sedimentology.

Major cycles ranging in thickness from 7-12 meters (23-39 fi) were correlatable across the entire Comet Platform for the Upper Keg River and Muskeg (Muir and Dravis, 1991a; see Figs. 11 & 12). Minor cycles, averaging 1-2 meters (3-6 fi) in thickness, are locally correlatable only over hundreds of meters and only within the platform-interior setting. Major cycles are comprised of minor cycles (Fig. 10). The minor cycles within

each major cycle successively thinned upward and contained a progressively greater proportion of tidal-flat facies. The abrupt change back to a thicker minor cycle with a higher proportion of subtidal facies marks a major cycle break.

The importance of these stratigraphic relationships should be obvious. This more rigorous time-stratigraphic framework helped relate facies, associated porosity and permeability, and brecciation to their positions within depositional cycles, thus providing a better understanding of the lateral continuity of reservoir quality.

Evaluation of the "Karst Model" Brecciation is often the key fabric on which an interpretation of subaerial karstification and paleocave development is based (Loucks and Anderson, 1985; Choquette and James, 1988; Kerans, 1988, 1989; Wilson, et al , 1992, Loucks and Handford, 1992). Therefore, it is appropriate to discuss the aspects of this model because subaerial karstification is used by some to explain the brecciation of these Devonian sequences.

The "Karst Model" and associated paleocavern formation is currently a very popular model which seeks to explain reservoir development and continuity in carbonate sequences which are highly brecciated. Two sequences which have been reinterpreted according to this model are the Ordovician Ellenburger Group in west Texas (Kerans, 1988, 1989) and the Arbuckle Group in Oklahoma (Bliefnick and Belfield, 1991; Bliefnick, 1992).

Kerans (1989) and Loucks and Handford (1992) discussed in detail the diagenetic fabrics of near-surface karstification and associated cavern formation. One of the major diagenetic fabrics around which this model hinges are breccias, which are interpreted to represent cave-fill deposits associated with cavern roof collapse. Certainly, such deposits form in modern-day caverns. What is very troubling, however, is that other diagenetic fabrics normally associated with prolonged subaerial exposure and cavern formation are often missing in ancient paleocave systems. Some of these fabrics include: I. Reddish soil crusts (calcretes), often with associated soil pisolites, sinkholes and other solution pipes, at least somewhere along the top of the true exposure surface. 2. Speleothems, such as stalactites, stalagmites, and flowstones. Instead, burial cements, such as saddle dolomites, usually are encountered between breccia clasts. This is very puzzling since early carbonate dissolution in a cave system generates early cements.

136

Reddish or brownish clays. Many "karsted" sequences such as the Ellenburger and Arbuckle, however, contain green clays as breccia infills. This is enigmatic since these

green clays formed from reduced pore fluids and not oxidized ones as one would expect.

Precompaction cements in non-karsted portions of the sequence, since early freshwater dissolution results in early calcite cements which pre-date pressure solution and grain interpenetration. Cavernous porosity and evidence of primary shelter porosity (preserved or occluded). These are some of the diagenetic fabrics which one intuitively would expect to find in a subsurface sequence interpreted to have experienced near-surface freshwater diagenesis and associated karstification. Of course, in any one core, one might not see all these elements present. Limited core data, in fact, usually provides a convenient "out" for explaining why other fabrics are not present. However, if a rigorous regional study is conducted, most, if not all, of these elements should be present. One of the most critical diagenetic elements which should be present in all cases, if a sequence has experienced subaerial exposure to freshwater, is precompaction

cementation. Dissolution of carbonate leads to back-precipitation of pore-filling calcite cements (the donor-receptor relationship of Wendte and Gensamer, 1979). These cements transform a sediment into a lithified rock before burial and create a "frozen" depositional texture. Because this cementation is precompaction, individual grains resist grain interpenetration due to pressure solution because they have already been cemented in place. This style of diagenesis should be closely associated not only with paleocave systems but also with surrounding parts of the sequence exposed to the same freshwater influences.

Lastly, why do these spectacular styles of karstification frequently, if not always, occur in the dolomitized portions of sequences, like the Ellenburger or Arbuckle Groups, and not in coeval limestones? Is there a causal relationship? Wilson, et al. (1992) suggested that most , if not all, diagenesis (dolomitization, chertification, etc.) post-dated brecciation. If that were the case, why do we not see similar styles of brecciation in coeval limestones?

The timing of karstification hinges on the timing of dolomitization since it is the dolomite which is usually brecciated. Did the dolomite post-date limestone karstification? Did dolomitization occur during karstification? Or did dolomitization post-date the karstification? In all cases, one should be able to demonstrate the timing petrographically, especially if enhanced petrographic techniques are employed on enough samples. It is obvious that if the karstification and dolomitization were near-surfaces processes, then these diagenetic reactions occurred before the onset of pressure solution. Depositional grains should not be sutured by pressure solution, since early, near-surface limestone precompaction cementation, or early replacement dolomitization, would create a rigid framework which would then resist grain interpenetration by pressure solution. This is clearly seen in the Holocene and Pleistocene (Wanless and Dravis, 1989) and in ancient sequences (Ottmann, et al., 1973; Wendte and Gensamer, 1979). If dolomitization occurred under burial conditions after the onset of pressure solution, then the relict limestone fabrics subsequently replaced by dolomite (as seen with enhanced petrographic techniques) would exhibit grain interpenetration reflecting their burial origin If that is the case, then the dissolution to create breccias must also be deep-burial. This is part of the approach used to evaluate the timing of brecciation in the Upper Elk Point Group.

137

As this case study will show, brecciation by itself is insufficient to conclusively demonstrate that "karst" was developed early and related to an unconformity or subaerial exposure, as is commonly assumed. More enhanced petrography is required, especially when the karstification is associated with dolomites, to fully resolve the timing of brecciation, and other associated styles of dissolution, with respect to dolomitization. Deep-Burial Brecciation

Non-tectonic breccias in carbonate strata are the product of solution-collapse, related by most workers to early, near-surface meteoric dissolution (see Choquette and James, 1988). Even Mississippi Valley Type mineralization is thought by many to be hosted in breccias which formed by such a near-surface process (Sangster, 1988). Breccias can also be generated by dissolution of evaporite minerals and subsequent solution-collapse. Breccias are common diagenetic fabrics in the Upper Elk Point Group (Figs. 13-17). The dissolution fabrics in these or time-equivalent strata elsewhere have been attributed to the following processes: (1) either near-surface, solution-collapse following dissolution of

evaporites (Beales and Oldershaw, 1969); or (2) subaerial exposure and freshwater

dissolution (Skall, 1975; 1977; Rhodes, et al., 1984; Krebs and Macqueen, 1984; Schmidt, et al., 1985; Qing and Mountjoy, 1989). In either case, the breccias, and any associated porosity, were considered a near-surface phenomenon which occurred before the onset of burial and pressure solution. Another diagenetic fabric found in the Keg River and Muskeg sequences is grayish,

finely-crystalline dolomite commonly observed along the floors of large vuggy pores. Resembling Dunham's (1969) "vadose silt," these dolomites were interpreted as possible protodolomites and used as evidence for early subaerial exposure as well (Qing and Mountjoy, 1989). These dolomites are discussed later.

Despite the above-mentioned published reports which purport subaerial exposure for the Keg River in this region, no definitive evidence was found on Comet Platform to support regional subaerial exposure capable of promoting such karstification. As discussed below, brecciation in the Upper Elk Point Group on and near Comet Platform is a product of deep-burial dolomite dissolution, a grander-scale of dissolution which created other types of secondary porosity in these rocks.

Breccias occur in many cores from all three formations in the Upper Elk Point Group (Figs. 13-15). They consist of variably-sized dolomitized clasts, invariably rotated as

a result of solution-collapse, either supporting each other or floating in anhydrites or crystalline dolomites, including saddle dolomites (Fig. 17). Some breccias contain preserved vuggy porosity (Fig. 17B) but most are nonporous. Significantly, brecciation is always confined to dolomitized facies and is never observed in coeval limestones. Many breccias are closely associated with massive, late replacement anhydrites (Fig. 17D) which apparently were emplaced along major faults. Commonly, breccias occur at the bases and tops of these massive anhydrites which, in some cores, are nine meters (thirty fi) or more in length.

Based on extensive core observations augmented by petrography, both the breccias and the finely-crystalline gray dolomites in the Upper Elk Point Group were products of deep-burial diagenesis and have no relationship to subaerial exposure and karstification, or (Continued page 150)

138

Figure 13 Photographs of core from the Sulphur Point Formation on Comet Platform showing intense deeper-burial brecciation in the lower part of this core (Panel C). The breccias and surrounding host rock are completely dolomitized. Several dolomite clasts in this brecciated zone contain stylolites in which the stylolites are now rotated at different angles to each other and the horizontal (arrows). This relationship substantiates that the breccias were created by dissolution after the precursor limestone had been stylolitized and replaced by dolomite. Zebra dolomites with vuggy porosity and white saddle dolomites and anhydrite cement in the upper part of the core (Panel A) are also deep-burial in origin and are another reflection of burial dolomite dissolution. Fractures with white anhydrite cement are scattered in this cored interval. Well 1-8-110-7W6K Pool. Length of each core box is approximately 2.5 feet.

Figure 14 Photographs of core from the Keg River Formation on Comet Platform showing spectacular dolomite dissolution (arrows) associated with massive white anhydrite emplacement (A). Sharp vertical contacts (arrows) between the white anhydrite and darker dolomites are typical of the brecciated boundaries along the tops and bottoms of these massive anhydrites in core. Massive anhydrite emplacement at depth appears to have played a role in the dolomite dissolution which produced this brecciation. This timing relationship is confirmed by

petrographic observations in the dolomites, like those shown in Figure 18.

Cores which exhibit this style of massive anhydrite emplacement and associated brecciation on Comet Platform are invariably associated with major fault/fracture systems. Open to cemented fractures and vertical stylolites (Panel C) were a reflection of this fault influence. Well 14-29-110-7W6-N Pool. Length of each box is approximately 2.5 feet.

Figure 15 Photographs of core from the Keg River Formation from a pinnacle reef pool off Comet Platform showing extensive brecciation of dolomites along the top and

bottom contacts (arrows -- Panels A and H) with massive anhydrite (A). Secondary vuggy porosity development (V), including that found in zebra

dolomites (Z), also occurs along these contacts. The speckled appearance of anhydrite throughout this core (for example, Panels B, D, and E) is due to consumption of relict dolomite host rock now replaced with anhydrite as well. Some of this anhydrite has also been re-brecciated (see Panel G). This pool has been affected by both major faulting and fracturing. Well 4-29-110-5W6-Z Pool. Length of each core box is approximately 2.5 feet long.

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Figure 16 Typical breccia fabrics seen in Keg River and Muskeg cores from Comet Platform and Rainbow Basin. (A) Core photograph of a Keg River breccia from a pinnacle reef pool in Rainbow Basin consisting of numerous dolomite clasts

(C) of varying sizes, colors and shapes. The clasts are surrounded by a crystalline dolomite matrix. Well 12-29-108-7W6-F Pool (5675'); core is

approximately three inches wide. (B) Core photograph of a chaotic breccia in

the Muskeg Formation on Comet Platform. The clasts and "matrix" are principally dolomite; some replacement anhydrite occurs between the clasts. One large clast contains what appears to be crinkly laminated dolomite (arrows). These "laminations" are also pressure solution surfaces (wispy microstylolites).

The rotation of these pressure solution seams in this clast indicates that dissolution and brecciation occurred after pressure solution and dolomitization, namely, under deeper-burial conditions. Compare to Figure 17. Well 3-11-1107W6-M Pool (5470'); core is about three inches across for scale. (C) Core

photograph of a Keg River breccia from Comet Platform associated with massive anhydrite. Dolomite clasts are brownish in color; anhydrite ranges in

color from white to dark gray. Dolomite clasts were corroded by dissolution related to late anhydrite emplacement. Dolomite clasts become gradually totally consumed by this anhydrite. Well 3-29-111-7W6-BBB Pool (5170'); width of core is three inches. (D) Core photograph of a more massive anhydrite (white color) containing a few floating clasts of brownish dolomite. This is the typical style of brecciation observed in Upper Elk Point Group dolomites along the edges of massive anhydrites emplaced along major fault and fracture planes on Comet Platform (see Figs. 14 and 15). Well 6-22-110-7W6 (5840'); width of core is three inches.

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Figure 17 Breccias and other dissolution fabrics in Upper Elk Point carbonates. (A) Core photograph of Sulphur Point dolomite showing a spectacular brecciated fabric

This fabric was produced by deep-burial dissolution of dolomite and not subaerial karstification because the stylolites in the clasts (arrows) are at different angles to the horizontal and each other. This

created by solution collapse.

implies the rock was already stylolitized and dolomitized before the dissolution and subsequent collapse leading to clast rotation. Well 1-8-110-7W6-K Pool (5650'); core is approximately 3 inches wide. Up is to the left. (B) Core

photograph of a Muskeg breccia with preserved coarse secondary vuggy

porosity. The rotated stylolitic clasts again indicate this breccia formed under Well 6-14-110-7W6(6064'); width of core is deep-burial conditions.

approximately 3 inches. Up is to the left. (C) Thin section photomicrograph of a Muskeg brecciated dolomite previously attributed to near-surface subaerial karstification. However, wispy microstylolitic seams (arrows) at different angles to each other indicate a deep-burial origin. Well 12-29-108-7W6-F Pool (5675'); diffused plane-polarized light. Field of view is 11 mm. (D) Core photograph of a Keg River Formation breccia intimately associated with massive anhydrite. This observance was typical for most breccias and implies that calcium-rich fluids precipitating the anhydrite also caused the dolomite to dissolve and collapse. Well 14-29-110-7W6-N Pool (6029'); width of core is approximately 3 inches. Up is to the left. (E) Core photograph from S Pool showing light gray dolomite (solid arrows) associated with coarse vuggy pores. This "vadose geopetal silt" is often used as supporting evidence for subareial

However, some of the dolomite "silt" sits atop vugs (open arrows), implying it was a diagenetic reaction product and not depositional sediment derived by subaerial freshwater dissolution. Well 13-26-110-7W6 (5762'); width of core is 3 inches. Up is to the left. (F) Thin section photomicrograph of finely crystalline dolomite along the underside of coarse

karstification.

vuggy porosity, comparable to that in Figure 17E. Note that this dark-brownish material (arrows) is at an angle to the horizontal, a relationship inconsistent with "geopetal vadose silt". White material infilling the vug is anhydrite (A). Well 18-110-7W6-K Pool (5838'); Keg River Formation. Plane-polarized light; field of view is 5.5 mm.

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to leaching of syndepositional evaporites.

Several lines of evidence support the deep-burial origin of these breccias. First, the breccias were not associated with the tops of either upward-shoaling sequences (including major cycle breaks) or with major formation boundaries, stratigraphic positions which are most likely to be surfaces of subaerial exposure (Muir and Dravis, 1991). In fact, the tops of depositional cycles never showed other fabrics associated with subaerial exposure. Second, the common occurrence of these breccias in the Upper Keg River and Sulphur Point Formations, which lack faunal and other environmental indicators of syndepositional evaporites, implies that dissolution promoting the brecciation was not due to leaching of syndepositional or other evaporites. Third, the association of these breccias only within dolomitized sequences and not in limestones further implies that their formation was related not to subaerial exposure and freshwater dissolution but to dissolution of dolomites. In addition, no early freshwater precompaction cements occur in these breccia zones, including cave formations; cements are coarse dolomites, often saddle dolomites, and late, burial

Likewise, no precompaction fabrics occur in dolomites adjacent to these brecciated intervals. Dolomites, when viewed with enhanced petrographic techniques, reveal relict sutured grains (Fig. 18), substantiating their burial origin. Fourth, these anhydrites.

breccias were formed by dissolution of dolomites which petrography establishes is burial in origin (Fig. 18); thus, the brecciation must also be burial in origin In addition, petrography

shows clearly that dissolution of coeval dolomites in non-brecciated zones of these

sequences, and resultant secondary pores, are deep-burial (Figs. 19-22). Fifth, several cores contain breccias whose rotated clasts consist of highly stylolitic dolomite. The stylolites in these clasts are now at all different angles to each other and the horizontal (Figs. 17 A,B). Rotation of stylolitic clasts can only be achieved if the sequences were first deeply buried, stylolitized, dolomitized, and then dissolved to produce the rotated stylolitic clasts. This fabric could not form by near-surface subaerial karstification and dissolution. This is a key observation which can be seen in core where stylolitic clasts were preserved. Where stylolites in clasts are absent or masked by dolomitization, relict interpenetrated grains in the clasts also reflects the same timing relationship. Quite simply, breccias in the Upper Elk Point sequences reflect the high degree of burial dissolution of dolomites which occurred pervasively throughout these sequences (Figs. 19-22).

Finely-crystalline, grayish dolomites also are not products of early subaerial

exposure in the Upper Elk Point Group. These dolomites are only observed in dolomitized sequences and never in limestones on Comet Platform (Fig. 17E,F). No comparable calcitic fabrics are ever observed in these vugs. They are only found in secondary pores related to dolomite dissolution and never seen in primary pores, either in dolomites or limestones. These dolomites are not usually associated with the breccias.

Some might call these finely crystalline dolomites "cave fill deposits" because they are observed to commonly floor large vuggy pores (Fig. 17 E). However, no grains or other sediments have been observed in these dolomites and without that evidence, it is improper to imply that these dolomites were ever a sedimentary deposit. In addition, their distribution is inconsistent with gravity-induced sedimentation, implying a diagenetic origin. Petrography suggests that these fabrics are a product of dolomite recrystallization along the edges of larger pores, perhaps back-precipitated during the dissolution which created these vuggy pores (Dravis, 1992). This conclusion is supported by two observations. First, these finely-crystalline, grayish dolomites not only occur on the floors of dissolution pores in the

dolomites but they also occur along the tops and edges (Fig. 17 D).

This negates a

sedimentary infill origin. Second, the highly depleted oxygen isotopic values indicate these dolomites precipitated at elevated burial temperatures, a relationship consistent with the

150

deep-burial origin of the secondary pores in which these dolomites precipitated (Dravis, 1992).

Controls on Burial Dolomite Dissolution and Brecciation Dissolution of large quantities of dolomite at depth to create reservoir rock may be paradoxical to many. However, more recent case studies demonstrate exactly this relationship and shed light on at least some of the controls responsible for this dissolution (Packard, et al., 1990; Dravis and Muir, 1991a,b; Muir and Dravis, 1991; Dravis, 1992). Time and space do not allow for the full and proper development of these controls for the Upper Elk Point Group in Alberta; the reader is referred to Dravis (1992) for a more indepth discussion. As such, these controls are briefly summarized below.

Comet Platform pools were influenced by faults which introduced hot, calcium-rich fluids into Upper Elk Point sequences. The movement of these fluids was clearly controlled by faults and fractures, based on regional seismic and stratigraphic/structural cross sections (Muir and Dravis, 1990; 1991). At least some of these brines were derived from the underlying basement rock; others were derived from subjacent evaporitic sequences, such as the Chinchaga Formation. The following observations support this relationship: fault

planes lined with massive anhydrite; fractures which cut stylolites and are lined with anhydrite cements; abundant anhydrite cements which infill secondary porosity and overlie saddle dolomite cements; presence of fluorite and helium, indicating derivation of at least

some of these fluids from the granitic basement rocks; and presence of associated mineralization, including sphalerite, galena, pyrite and marcasite. Comet Platform is also situated down-dip from a major lead-zinc district, Pine Point (Krebs and MacQueen, 1984); dolomites and fluorites in these sequences precipitated at high temperatures, based on fluid inclusions and/or stable isotope geochemistry (Aulstead, et al., 1988; Dravis, 1992). This

relationship is consistent with the high geothermal gradient in this area, and with the presence of pyrobitumen which post-dates most of the burial dissolution of these dolomites, including brecciation (Dravis, 1992). Collectively, these observations indicate the passage of hot, calcium-rich fluids from underlying sequences into the Upper Elk Point Group in this area.

On a finer petrographic scale, breccias and other areas of massive dolomite dissolution were fed by fractures or faults (Muir and Dravis, 1991). In addition, these zones of dissolution cut across stylolites, confirming the burial origin of this dissolution but also suggesting a causal relationship In other words, the stylolites, when cross cut by fractures, served as conduits along which dolomite dissolution occurred (Dravis, 1992).

Regionally consistent observations presented above indicate that hot, upwardmoving, calcium-rich fluids were at least responsible for the observed burial dissolution of dolomites in these three sequences. This dissolution also may have been augmented by the presence of hydrogen sulfide. In fact, the Upper Elk Point Group in this part of Alberta is a

strong candidate for testing the thermochemical sulfate reduction model, since all the elements for this reaction, including hydrogen sulfide, were present (Dravis, 1992). Hydrogen sulfide is a by-product of thermochemical sulfate reduction (Orr, 1974; Eliuk, 1984; Krouse, et al., 1988) In fact, many of the pools on Comet Platform produce several percent or more of hydrogen sulfide.

Hence, regional structural gradients, combined with the upward movement of hot, calcium-rich fluids containing H2S, are the major controls on massive dolomite dissolution which promoted, in part, the spectacular brecciation in these sequences. Other styles of (Continued page 162) 151

Figure 18 Timing of dolomite replacement in Upper Elk Point carbonates.

(A) Thin

section photomicrograph of a nonporous dolomite from the Keg River Formation in which all vestiges of depositional fabric have been masked by replacement dolomitization. Given this petrographic view, nothing can be said about the timing of this dolomitization. Well 3-29-111-7W6 - Bunny Pool (5165'); plane-polarized light. Field of view is 3.0 mm. (B) Same view as in Figure 14 A but taken under blue-fluorescent light. This view reveals distinct irregularly-shaped cryptocrystalline grains (C), ovoid-shaped peloids (P), and a dolopackstone texture. The fact that these grains have been sutured by pressure solution (arrows), and that the dolomite crystals overlie these microstylolitic contacts, indicates that the replacement of these grains by dolomite occurred under deeper-burial conditions coincident with, or post-dating, pressure solution. (C) Thin section photomicrograph of Muskeg dolomite in which

depositional fabric has not been preserved by replacement dolomitization. Well 4-17-110-7W6 -- K Pool (1803.75m); plane-polarized light. Field of view is 5.5 mm. (D) Same view as in Figure 14 C but taken under enhanced blue-light fluorescence to reveal a distinct peloidal (P)-cryptocrystalline grain (C) dolopackstone fabric. Most of these relict grains are again sutured by pressure solution, implying the replacement dolomitization occurred under deeper-burial conditions. Blue-light fluorescence was extremely successful in delineating relict depositional fabric and texture in Upper Elk Point dolomites and in resolving the relative timing of dolomite formation. (E). Thin section photomicrograph of massive Sulphur Point dolomite in which a few vague grains (arrows) can be seen. However, the texture and timing of replacement dolomitization cannot be discerned with this standard thin section view. Well 6-14-110-7W6 (5978'); plane-polarized light. Field of view is 3.0 mm. (F) Same view as in Figure 14 E but taken under blue-light fluorescence, The peloidal grains are quite clear, as are the pressure solution contacts (arrows). Again, this dolomite was formed under deeper-burial conditions. Given this relationship, there is little, if any, evidence for substantial early replacement dolomitization in Upper Elk Point carbonates.

152

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Figure 19 Secondary moldic porosity in Upper Elk Point dolomites. (A). Core slab of a Muskeg Amphipora dolofloatstone in which several of these stromatoporoids have been partially to completely dissolved to create secondary moldic porosity. Because many of the dolomitized stromatoporoids are still well preserved, grain dissolution which occurred in this sample occurred after they were dolomitized and not because of the dolomitization. Well 16-18-110-7W6 - K Pool (1851m); width of core is approximately 3 inches. (B). Thin section photomicrograph of a Muskeg dolomite showing two moderately well preserved dolomitized Amphipora fragments in a dolomitized matrix. The porosity within their skeletons is primary intraparticle. Well 6-22-1] 0-7W6 (5599'); plane-polarized light. Field of view is 11 mm. (C). Thin section photomicrograph of secondary macromoldic porosity created by dissolution of the Amphipora in Figure 19 A. The general outline of the circular grain is still somewhat obvious. Well 16-18110-7W6 -- K Pool (1851m); Muskeg Formation. Plane-polarized light; field of view is 11 mm. (D). Thin section photomicrograph of Keg River dolomite showing well developed secondary micromoldic porosity within individual dolomite crystals (arrows). This type of porosity development is quite common in Muskeg and Keg River dolomites. Its presence is difficult, if not impossible, to detect with standard microscopic techniques. Well 4-7-110-7W6 -- 0 Pool (6205'); diffused plane-polarized light. Field of view is 5.5 mm. (E). Thin section photomicrograph of a Keg River dolomite with apparently only very minor amounts of preserved secondary porosity. Well 4-36-110-7W6 -- I Pool (5883'); plane-polarized light. Field of view is 5.5 mm. (F). Same view as in Figure 19 E but taken under diffused plane-polarized light. With this view, abundant secondary microporosity within the matrix is quite apparent. In addition, two vertical tectonic stylolites are also now apparent (arrows); the microporosity may have developed in association with horizontal microfractures off the vertical stylolites.

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Figure 20 Secondary vug and intercrystalline porosity in Upper Elk Point dolomites. (A) Core slab of Muskeg dolomite with well developed secondary vuggy (larger holes) and intercrystalline porosity. Darker material in porosity is bitumen. Well 6-14-110-8W6 (5983'); width of core is about 3 inches. Up is to the left. (B) Thin section photomicrograph of relatively large secondary vug porosity with some finer intercrystalline pores. Note the high degree of dolomite corrosion, including the leached interior of a rhombic dolomite crystal (arrow), suggesting that most of this porosity resulted from dissolution of dolomitized grains or matrix. Well 4-16-110-7W6-K Pool (1758.6m); Muskeg Formation. Planepolarized light; field of view is 5.5 mm. (C) Thin section photomicrograph of a Keg River dolomite with secondary fine intercrystalline porosity (blue). Such intercrystalline porosity is thought to develop from the intergrowth of dolomite crystals. Well 6-19-110-7W6 -- K Pool (6184.5'); diffused plane-polarized light. Field of view is 3.0 mm. (D) Thin section photomicrograph of another Keg River dolomite with abundant secondary intercrystalline porosity (blue), some of which is occluded by black bitumen. Note the obvious corroded exteriors of the dolomite crystals which reflect crystal dissolution. The abundant evidence for dolomite dissolution in Keg River and Muskeg dolomites raises the question as to how much of this porosity is really true intercrystalline and how much is moldic developed as a result of dolomite dissolution. Well 15-36-110-7W6 -Bunny Pool (5696'); diffused plane-polarized light. Field of view is 3.0 mm. (E) SEM micrograph of a secondary pore infilled with euhedral dolomite cements showing fine-scale dissolution of not only the pore-filling cement (solid arrow) but also the matrix dolomite crystals around these cements (open arrows). Well 4-33-110-7W6 -DD Pool (5937') Keg River Formation. Fractured chip; scale bar is 100 microns long. (F). SEM micrograph and closer view of the matrix dolomite in Figure 21 E showing the pitted and corroded surfaces of these dolomite crystals produced by dissolution. Scale bar is 30 microns long.

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Figure 21 Evidence for deep-burial dissolution in Muskeg dolomites.

(A) Thin section photomicrograph showing a horizontal stylolite terminating directly into secondary vuggy porosity (arrow). This relationship establishes that the porosity formed during or after emplacement of this pressure solution seam, under deep-burial conditions.Well 5-26-110-7W6 - S Pool (1726m); diffused plane-polarized light. Field of view is 11 mm. (B) Thin section photomicrograph showing solution enlargement along a stylolite (solid arrows), implying the porosity is deep-burial and was generated by burial fluids moved along the stylolite. Sphalerite crystals also occur along a stylolitic seam (open arrow). Well 6-14-110-8W6 (6113'); diffused plane-polarized light. Field of view is 5.5 mm. (C) Thin section photomicrograph showing a horizontal

stylolite (arrows) connecting two relatively large vuggy pores.

Such a

relationship again implies that the porosity formed during or after the stylolitic seams, under deep-burial conditions. Well 4-29-111-5W6 -- Z Pool (1507.9m); plane-polarized light. Field of view is 11 mm. (D) Thin section photomicrograph of another Muskeg dolomite showing the same relationship where a horizontal stylolite terminates into two vugs (arrows). The evidence for deep-burial porosity evolution is nearly identical to that observed in the Keg River Formation. Well 5-26-110-7W6 -- S Pool (I736m); plane-polarized light. Field of view is 11 mm. (E) Thin section photomicrograph showing extensive development of secondary vug porosity adjacent to a major horizontal stylolite. The close juxtaposition of this porosity with the pressure solution seam implies this porosity formed under deep-burial conditions. Well 4-33-110-7W6 -- DD Pool (5975'); diffused plane-polarized light. Field of view is 11 mm. (F) Thin section photomicrograph of a Muskeg dolomite showing well preserved porosity in a highly stylolitic dolomite, including well preserved porosity directly adjacent to the pressure solution seams (arrows). This relationship, also seen in the Keg River, is anomalous unless the porosity formed after emplacement of the

stylolites. Well 6-22-110-7W6 (5671.5'); diffused plane-polarized light. Field of view is 11 mm.

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159

Figure 22 Evidence for deep-burial dissolution in Keg River dolomites. (A) Thin section

photomicrograph showing a horizontal stylolite terminating directly into a relatively large secondary vuggy pore (solid arrow). Also note a tension gash off this same stylolite enlarged by dissolution (open arrow). Termination of pressure solution seams into secondary porosity implies the porosity formed during or after emplacement of the seams, under deep-burial conditions. Well 15-14-111-8W6 (1573m); plane-polarized light. Field of view is 11 mm. (B) Thin section photomicrograph showing solution enlargement along a horizontal stylolite (solid arrow) as well as a stylolite which terminates into vuggy porosity (open arrow). This relationship not only confirms the deep-burial origin of this porosity but implies that these pressure solution seams served as pathways for burial fluids promoting the dissolution. Well 3-28-110-7W6 - N Pool (5980.5'); diffused plane-polarized light. Field of view is 5.5 mm. (C) Thin section photomicrograph of a dolomite with apparent secondary vuggy porosity (V).

With this view, the origin and timing of porosity development cannot be Well 3-29-111-7W6 -- BBB Pool (5145'); plane-polarized light. Field of view is 5.5 mm. (D) Same view as in Figure 23 C but taken under enhanced blue-light fluorescence to reveal two large dolomitized grains determined.

separated by a microstylolitic contact (arrows). With this view, the porosity in both grains is clearly moldic and not vuggy. The fact that the grains have been sutured by pressure solution, and that dolomite crystals overlie the pressure solution seam, implies that the dolomitization and subsequent grain dissolution occurred under deep-burial conditions. (E) Thin section photomicrograph of a highly stylolitic dolomite showing high amounts of secondary porosity preserved between and directly adjacent to the stylolites (arrows). Normally, areas adjacent to pressure solution seams are nonporous if the porosity existed before pressure solution. In this case, the porosity must have developed during or after stylolitization by deep-burial dissolution. Well 4-7-110-7W6 -0 Pool (5898'); diffused plane-polarized light. Field of view is 5.5 mm. (F) Thin section photomicrograph showing stylolites (arrows) terminating into secondary porosity filled with anhydrite (A). The porosity must have formed after the stylolites. Well 11-13-111-8W6 (5185'); diffused plane-polarized light. Field of view is 5.5 mm.

160

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dolomite dissolution are more fabric-selective and they, in turn, were controlled by depositional facies and position within depositional cycles (Muir and Dravis, 1991)

Conclusions Brecciation in the Keg River, Muskeg and Sulphur Point Formations of the Upper Elk Point Group in northwestern Alberta is a product of pervasive deep-burial dissolution which drastically modified all three sequences and accounts for present-day reservoir quality. These breccias only developed in dolomites whose petrography shows that they formed under deep-burial conditions coincident with, or post-dating, pressure solution The presence of stylolitic dolomite clasts rotated at all angles to each other, and to the horizontal, is clear-cut evidence of the deep-burial timing of this brecciation. Other petrographic evidence, both within brecciated zones as well as in adjacent dolomites, further supports burial dissolution as the mechanism to create these breccias.

Regional controls on the development of brecciation in these sequences include faults and fractures which introduced hot, calcium-rich fluids into these dolomitized sequences at depth; it is these fluids, perhaps combined with hydrogen sulfide, which promoted dolomite brecciation and other styles of burial dissolution in these units.

The lesson from this case study is that one particular diagenetic fabric, such as brecciation, is insufficient to document subaerial exposure and associated near-surface diagenesis. This case study shows that as a result of pervasive deep-burial dissolution, spectacular brecciation can result Therefore, brecciation can be intimately associated with "deep-burial karstification."

Acknowledgements The results of this case study are part of a more extensive regional study undertaken by the authors for Imperial Oil Resources Canada Ltd., Calgary. The authors were assisted by Doug Leach, Howard Lee, Loren Snyder, Rob Klettl, and Mike Peacock. We thank Imperial Oil Resources Canada Ltd. for permission to publish this article.

References Aulstead, K.L., Spencer, R.J. and Krouse, H.R., 1988, Fluid inclusion and isotopic evidence on dolomitization, Devonian of western Canada: Geochim. et Cosmochim. Acta, p. 1027-1035.

Barss, DL., Copland, A.B. and Ritchie, W.D., 1970, Geology of middle Devonian reefs,

Rainbow area, Alberta, Canada, in Halbouty, M. (Ed.), Geology of Giant Petroleum Fields: Am. Assoc. Petrol. Geologists Memoir 14, p. 19-49.

Beales, F.W. and Oldershaw, A.E., 1969, Evaporite-solution brecciation and Devonian carbonate reservoir porosity in western Canada: Am. Assoc. Petrol. Geol. Bull., p. 503-512.

162

Bliefnick, D.M. and Belfield, W.C., 1991, The Ordovician Arbuckle Group of Oklahoma - a karsted dolomite reservoir: Dolomieu Conference on Carbonate Platforms and Dolomitization, Abstracts, p. 17, Ortisei, Italy.

Bliefnick, DM., 1992, Karst-related diagenesis and reservoir development in the Arbuckle Group, Paschall #2 core, Wilberton Field, Oklahoma: in Candelaria, M.P. and Reed, C.L. (eds.), Paleokarst, karst related diagenesis and reservoir development: examples from Ordovician-Devonian age strata of west Texas and the Mid-Continent, Permian Basin Section: SEPM Publication No. 92-33, p. 137-152.

Choquette, P.W. and James, N.P., 1988, Introduction: in James, N.P. and Choquette, P.W., (Eds.), Paleokarst: Springer-Verlag, New York, p. 1-21.

Dravis, J.J., 1991 Discussion: Update on new carbonate petrographic techniques and applications: J. Sed. Petrology, v. 61, p. 626-628. Dravis, J.J., 1992, Burial dissolution in limestones and dolomites -- criteria for recognition and discussion of controls: a case study approach. Part 1. Upper Jurassic

Haynesville Limestone, east Texas; Part 2. Devonian Upper Elk Point AAPG-CSPG Short Course "Subsurface Dolomites, western Canada: Dissolution Porosity in Carbonates, Recognition, Causes and Implications," Calgary, Canada, 176 p.

Dravis, J.J. and Yurewicz, DA., 1985, Enhanced carbonate petrography using fluorescence microscopy: J. Sed. Petrology, v. 55, p. 795-804 Dravis, J.J. and Muir, ID., 1991a, Pervasive burial dissolution of early and late dolomites in Devonian pools, Alberta, western Canada: Am. Assoc. Petrol Geologists Bull., v. 75, p. 564.

Dravis, J.J., and Muir, ID., 1991 b, Controls on widespread dissolution of burial dolomites, Middle Devonian Upper Elk Point Group, Western Canada: Dolomieu

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Dunham, R. J., 1969, Vadose silt in Townsend Mound (reef), New Mexico: SEPM Special Publ. No. 14, p. 139-181.

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Eliuk, L.S., 1984, A hypothesis for the origin of hydrogen sulfide in Devonian Crossfield Member dolomite, Wabamun Formation, Alberta, Canada: Canadian Soc. Petrol. Geologists Core Conf,, p. 245-289. Hanmer, S., 1987, Great Slave Lake Shear Zone Canadian Shield: depth sections through a crustal scale fault zone: Tectonophysics, v. 149, p. 245-264.

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Hill, C.A., 1992, Sulfuric acid oil-field karst: in Candelaria, M.P. and Reed, C.L. (eds.), Paleokarst, karst related diagenesis and reservoir development: examples

from Ordovician-Devonian age strata of west Texas and the Mid-Continent, Permian Basin Section: SEPM Publication No. 92-33, p. 192-194.

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Hriskevich, ME., 1970, Middle Devonian reef production, Rainbow area, Alberta, Canada: Am. Assoc. Petrol, Geologists Bull., v. 54, p. 2260-2281. Kerans, C., 1988, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of west Texas: Am. Assoc. Petrol. Geologists Bull., v. 72, p. 1160-1183.

Kerans, C., 1989. Karst-controlled reservoir heterogeneity and an example from the Ellenburger Group (lower Ordovician) of west Texas: Bureau of Econ. Geology Rept. Investigations No. 186, 40 p. Krebs, W. and MacQueen, R., 1984, Sequence of diagenetic and mineralization events, Pine Point lead-zinc property, Northwestern Territories: Canada, Bull. Can. Petrol. Geol., v. 32, p. 434-464.

Krouse, H.R., Viau, C.A., Eliuk, L.S., Ueda, A. and Halas, S., 1988, Chemical and isotopic evidence of thermochemical sulphate reduction by light hydrocarbon gases in deep carbonate reservoirs: Nature, v. 333, p. 415-419.

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Muir, I.D. and Dravis, J.J., 1990, Comet Platform study - controls on reservoir continuity and pool entrapment on the Upper Elk Point Group: Esso Resources Proprietary Report, 230 p.

Muir, ID., and Dravis, J.J., 1991, Controls on pool entrapment in Middle Devonian dolomitized Keg River sequences, Comet Platform, Western Canada. Dolomieu Conference on Carbonate Platforms and Dolomitization, Abstracts, p. 182-183, Ortisei, Italy.

Orr, W.L., 1974, Changes in sulfur content and isotopic ratios of sulfur during petroleum maturation - study of Big Horn Basin Paleozoic oils: Am. Assoc. Petrol. Geologists, Bull., v. 58, p. 2295-2318.

Ottmann, R.D., Keyes, P.L. and Zieglar, M.A., 1973, Jay Field - a Jurassic stratigraphic trap: Trans. Gulf Coast Geol. Soc., v. 23, p. 146-157. Packard, J.J., Pellegrin, G.J., Al-Asam, I.S., Samson, I., and Gagnon, J., 1990, Diagenesis and dolomitization associated with hydrothermal karst in Fammenian Upper Wabamun ramp sediments, northwestern Alberta: in Bloy, G. and Hadley, M. (Eds.), The Development of Porosity in Carbonate Reservoirs: C.S.P.G. Short Course, section 9, 19 p.

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165

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166

TRENTON LIMESTONE -- THE KARST THAT WASN'T THERE, OR WAS IT? Brian D. Keith Indiana Geological Survey and Department of Geological Sciences Bloomington, Indiana

Lawrence H. Wickstrom Ohio Geological Survey Columbus, Ohio

Abstract The top surface of the Trenton Limestone and equivalent carbonate units has be variously described as a subaerial exposure surface (paleokarst), a submarine erosion surface, and a submarine hardground. Detailed study of the contact between the carbonates and overlying shale in outcrop and core and regional stratigraphic analysis indicate that the surface represents a drowning unconformity on the Galena and Lexington carbonate platforms in Ohio and Indiana. This unconformity also appears within the Sebree Trough in Indiana between the platforms, but it is within the overlying shale section rather than at its base. The unconformity has not been recognized in the Point Pleasant Basin in central and southern Ohio. Paleokarst may locally exist on this surface in southern Ontario.

Introduction

The top of the Trenton Limestone in the five-state area of Indiana, Illinois, Kentucky, Michigan, and Ohio has been considered to be either a subaerial exposure surface (Rooney, 1966; DeHaas and Jones, 1988) or in part a submarine eroded valley (Schwalb, 1980). This interpretation has been based on the relief on the surface, the unusual thickness

distribution of the Trenton in Indiana, and the presence of cavernous porosity at the Albion-Scipio Field in Michigan

The consideration of whether karst developed on the top of the Trenton Limestone in the Indiana-Ohio area (the subject of this paper) hinges on the nature of the contact itself

and the age relationship between the Trenton Limestone and equivalent units and the overlying shales of Cincinnatian age (Figure 1). This subject has inspired considerable

comment in the literature, going back to almost 50 years to Du Bois (1945) The

preponderance of comments have suggested the presence of an unconformity or erosional unconformity at the contact (see Rooney, 1966 for a discussion of some of the literature). The noteworthy exceptions are Gutstadt (1958), who suggested a facies relationship in part between the Trenton and overlying shales as the Trenton thins into southern Indiana (see Figure 2), and Freeman (1953), who suggested a similar facies relationship for these units in western Kentucky. The paper by Rooney (1966) represents the first attempt to systematically present evidence of an unconformity at the contact. More recently DeHaas and Jones (1988) have interpreted drilling data in the Albion-Scipio field in Michigan, particularly reports of drill bits dropping and lost circulation zones in numerous wells, (Continued page 1 7 0)

167

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contact between the Trenton Limestone and equivalent units (dominantly carbonate) and the overlying rocks of the Maquoketa and equivalent units (dominantly shale). Position of the contact taken from various publications cited in text. 168

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as indicative of cave formation along a fracture system related to subaerial exposure and subsequent karstification that followed Trenton deposition. In contrast, Keith (1985), Fara

and Keith (1988), Wickstrom and Gray (1988), and Hurley and Budros (1990) have presented petrographic, lithologic, and regional stratigraphic evidence against subaerial exposure, but acknowledge that a break in sedimentation did occur between the Trenton and overlying Cincinnatian shales.

Nature of the Contact The actual contact between the Trenton and equivalent carbonates and the overlying

shale has been described in outcrop by Agnew (1955) and Delgado (1983) in Iowa, Hohman and Keith (in press) in Missouri, Willman and Kolata (1978) and Kolata and Graese (1985) in Illinois, and by Rooney (1966) in Indiana at the quarry of the Kentland cryptovolcanic feature. In addition, the contact has been described in the subsurface in cores from Indiana (Rooney, 1966; Keith, 1985; Fara and Keith, 1988), Ohio (Wickstrom et al., 1992), and Michigan (Hiatt and Nordeng, 1985; DeHaas and Jones, 1988). In these examples the contact between the Trenton and equivalent carbonates and overlying shale is described as being quite sharp, but the upper surface of the Trenton is irregular with several cm's of relief and characterized by pyrite mineralization with phosphatic pieces and rubble (Figure 3). There are also features described as borings into the contact surface (Fara and Keith, 1988). Greater relief on the surface, up to about five feet (1.5 m), was interpreted by Rooney (1966) to be present at the Kentland exposure. Careful study of the photograph published by Rooney (1966, Fig. 3) and field observation at the quarry by Keith show that this greater relief resulted instead from a fault that obliquely cuts the contact.

Interpretations differ as to the of origin of this surface. Agnew (1955), Rooney (1966), and DeHaas and Jones (1988) have all interpreted this surface as evidence of subaerial exposure and erosion. In contrast, Willman and Kolata (1978) referred to it as a ferruginous corrosion surface. Delgado (1983), Keith (1985), and Keith and Fara (1988) describe it as a submarine hardground.

Examination of the contact on a regional scale provides some valuable insight. Figure 4 (Indiana) and Figure 5 (Ohio) show the variation in the contact from the Galena Platform to the Sebree Trough in Indiana and the Point Pleasant Basin in Ohio (Figure 6). The contact of the Trenton Limestone with the overlying shales on the Galena Platform is sharp and characterized by abundant pyrite mineralization and phosphate debris and has a few cm's of relief This represents an extensively developed hardground surface (or multiple hardground surfaces) with no evidence of subaerial exposure (Fara and Keith, 1988). In the Sebree Trough, however, the contact between carbonate and overlying shale is simply a sharp depositional contact without the mineralization seen on the Galena Platform. About 50 ft (15 m) above the lithologic contact, however, a mineralized hardground surface is present within the shale (Figure 4). Farther south on the Lexington Platform (Figure 6), the contact of carbonate and overlying shale is once again a mineralized hardground (Figure 4),

but it is thinner with less relief than the surface on the Galena Platform. In the Point Pleasant Basin in Ohio, the contact appears as gradational between carbonate and calcareous shale (Figure 5).

Regional Stratigraphic Considerations

The thinning of the Trenton carbonates from northern Illinois to southeastern Indiana and the presence of progressively younger units in the upper part of the Trenton (Continued page 175) 170

E LO

Figure 3

Core photograph (from Keith I 985a) showing details of the Trenton-Maquoketa contact from the Peru Field, Miami County, Indiana. The major hardground surface is labeled (H) although this may actually be the amalgamation of several hardground surfaces. Present on the back side of the core (not shown) are approximately 0.5cm holes in the surface that represent borings. Pyritized

peloids (P) and other unidentified material, including phosphatic grains (Ph), infilled the surface before deposition of overlying Maquoketa shale (S).

171

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174

towards the west has been commented upon as evidence of erosion at the top of the Trenton (Rooney, 1966). In southern Indiana the Trenton thins to only a few feet or possibly zero (Keith, 1985b) and then is replaced by the Lexington Limestone which thickens rapidly into Kentucky. This area of thin carbonate has been referred to as the Sebree Trough (Mitchell and Bergstrom, 1991) replacing earlier names of Sebree Valley (Schwalb, 1980) and Kope Trough (Keith, 1985a) and its extent is shown in Figure 6. The

origin of this feature is pivotal to understanding the events that took place immediate following deposition of the Trenton Limestone. Keith (1988) used regional facies analysis to suggest that the trough formed by differential sedimentation of shale in the trough between two carbonate platforms with greater sedimentation rates. The exact depositional relationships between these features was still not clear, however. The terrigenous clastics in the trough were derived from the Taconic collisional event to the east. This picture has been expanded by Wickstrom et al. (1992) to connect the Sebree Trough with the Point Pleasant

Basin in Ohio (Figure 6). This basin was the site of deposition of calcareous shales and argillaceous limestones of the Point Pleasant Formation in a restricted environment that was contemporaneous with the carbonate platforms to either side (Wickstrom et al., 1992). A lateral facies change from the Trenton Limestone to the Point Pleasant Formation is also

suggested by Wickstrom et al (1992). The rocks of the Point Pleasant also show a gradational relationship with the shales and limestones of the overlying Cincinnatian group.

The lack of reliable data to establish the age relationships between the carbonates of the Trenton and Lexington Platforms and the shales of the Sebree Trough has long been a problem. Sequence stratigraphic analysis of the Upper Ordovician in Indiana, Kentucky, and Illinois (Hohman and Keith, in press) along with recent stratigraphic study in Ohio (Wickstrom et al., 1992) sheds some light on this problem. The basal unit of the Trenton extends to the south beneath the Lexington Limestone in Indiana and the Point Pleasant in

Ohio (Figure 7). The Trenton, thus, appears to be slightly older than the Lexington Limestone in Indiana, and the basal part of the Trenton in Ohio is older than the Point Pleasant. The contact between the Trenton Limestone and the overlying shale and between the Lexington Limestone and overlying shale is interpreted as a drowning unconformity in Indiana (Hohman and Keith, in press). Carbonate production was halted on the carbonate platforms by a rapid sea-level rise, probably related to response of the underlying crust to collision taking place along the eastern boundary of North America. As noted earlier, the hardground surface representing the drowning unconformity within the Sebree Trough is not at the contact between the carbonate and shale, but higher up in the shale indicating that the shale deposited below the unconformity was a trough facies of the carbonates. The presence of interbedding of the shale and Lexington in one Indiana core provides additional

confirmation. Deposition in the Point Pleasant Basin appears to have been continuous during this time of transition from carbonate to increasingly shale-dominated deposition, although, there may have been breaks in deposition that have not been recognized.

The stratigraphic evidence suggests that a clear break in sedimentation occurred, and an unconformity developed over the Galena and Lexington carbonate platforms, but none of the cores contain any evidence of subaerial exposure.

Dolomitization and Cave Formation One argument presented for exposure and development of karst at the top of the Trenton is presented by DeHaas and Jones (1988) for the Albion-Scipio and Stoney Point Fields in southern Michigan. They interpret the physical nature of the Trenton contract with the overlying

shale and the progressive southward thinning of the Trenton into

175

OHIO SE

NW

Cincinnatian shale and limestone

Wyandot Co.

Wood Co.

Williams Co.

Point Pleasant shale

Trenton limestone

Point Pleasant limestone and shale

Black River limestone

INDIANA SE

NW

Maquoketa shale

Jefferson Co. Marshall Co.

Miami Co.

Co. C Wayne Co.lark

Lexington limestone

Trenton limestone Maq. shale

Black River limestone Figure 7

Diagrammatic cross sections from Ohio and Indiana. Locations of sections shown on Figure 6. Approximate positions of cores of contact between underlying Trenton/Lexington/Point Pleasant rocks with overlying Cincinnatianage shales are shown. (A. Modified from Wickstrom et al., 1992; B. Modified from Hohman and Keith in press). 176

southern Indiana and central Ohio as evidence of subaerial exposure and erosion. In fact, their Figure 12, which is a core photograph of the contact in the Albion-Scipio area (exact location not identified), looks identical to the contact in northern Indiana and Ohio. The bulk of their argument, however, rests on their interpretation of the origin of solution zones and associated white sparry dolomite in the reservoir of these fields as being the result of karstification. This part of their argument is refuted by Hurley and Budros (1990) who used geochemical evidence to suggest that dissolution and sparry dolomite occurred in response to the influx of hot saline brines from depth along the fracture system in the fields sometime during the Late Silurian to Early Devonian (not Late Ordovician, as suggested by Dehaas and Jones). They also cite physical evidence related to collapse features in the fields that supports this timing. In addition, Hurley and Budros (1990) note a lack of evidence of any

cave features such as flowstone, cave sediments, etc. that have been reported from documented karst in other reservoirs. Nearly identical solution features and dolomitization

patterns are found along the Bowling Green Fault Zone in northern Ohio and are also attributed to hydrothermal fluids (Wickstrom et al., 1992).

One example of a similar reservoir, but which is attributed to paleokarst is reported by Trevail (1988) who presented systematic evidence and an extensive analysis to support local phreatic paleokarst development in southern Ontario in Trenton and Black River equivalent rocks. In a core and geophysical log study Trevail (1988) describes anomalous beds of shale and stratified internal sediment 26 meters below the top of the Trenton, zones of abnormally high porosity, zones of intense brecciation, solution-enlarged fractures, and the development of a phoscrete profile at the contact of the carbonate and overlying shale. Trevail (1988) suggests that local uplift in the area of the present day Algonquin Arch exposed this part of the carbonate platform to subaerial exposure and karst development. While a number a key features of paleokarst (James and Choquette, 1983) such as karst land forms, karren, and calcrete are not present, Trevail (pers. comm.) feels that paleokarst is still the best explanation for the features.

Implications for Reservoir Development The most productive reservoirs in the Trenton, by an order of magnitude or more are those associated with solution development and cavernous porosity, such as found at Albion-Scipio and Stoney point Fields and the Bowling Green Fault Zone. With paleokarst development as an exploration model for these reservoirs, one looks in the same area of exposure for similar features. However, using a hydrothermal model the explorationist's horizons are both expanded and limited. Exploration would not be limited to areas of significant subaerial exposure where paleokarst might have developed. Applying the hydrothermal model, any area of the Galena Platform or the Lexington platform is a

potential site for reservoir formation if the requisite geologic conditions have been met: the presence of the hydrothermal fluids, the development of a fracture or fault system to serve as a conduit for the fluids, and the proper timing of fluid movement. Additional reservoirs of this type have not been found because traditional exploration techniques do not seem to work (see Hurley and Budros, 1990). As noted by Hurley and Budros (1990), other similar reservoirs probably exist in the Indiana, Michigan, and Ohio area, and probably in other

basins with similar carbonate sequences and tectonic histories. When found, solutionenhanced fracture reservoirs can contain significant hydrocarbon reserves.

177

References

Agnew, A.F., 1955, Facies of Middle and Upper Ordovician rocks of Iowa: American Association Petroleum Geologists Bulletin, v. 39, p. 1703-1752.

Dehaas, R.J. and Jones, M.W., Cave-levels of the Trenton-Black River Formations in Central Southern Michigan, in Keith, B.D., ed., The Trenton Group (Upper Ordovician Series) of Eastern North America: American Association Petroleum Geologists Studies in Geology 29, p. 237-266.

Delgado, D.J., 1983, Deposition and diagenesis of the Galena Group in the upper Mississippi Valley, in Delgado, D.J., ed., Ordovician Galena Group of the upper Mississippi Valley--deposition, diagenesis, and paleoecology: Guidebook for 13th annual field conference, Great Lakes Section SEPM, p. A 1 -A17.

Dubois, E.P., 1945, Subsurface relations of the Maquoketa and "Trenton" formations in Illinois: Illinois State Geological Survey Report of Investigation 105, pt. 1, p. 533

Fara, D.R. and Keith, B.D., 1988, Depositional facies and diagenetic history of the Trenton Limestone in northern Indiana, in Keith, B.D., ed., The Trenton Group (Upper Ordovician Series) of Eastern North America: American Association Petroleum Geologists Studies in Geology 29, p. 277-298.

Freeman, L.B., 1953, Regional subsurface stratigraphy of the Cambrian and Ordovician in Kentucky and vicinity: Kentucky Geological Survey, Series 9, Bulletin 12, 352p. Gutstadt, A.M., 1958, Cambrian and Ordovician stratigraphy and oil and gas possibilities in Indiana: Indiana Geological Survey Bulletin 14, 103p.

Hiatt, C.R. and Nordeng, S., 1985, A petrographic and well log analysis of five wells in the Trenton-Utica transition in the northern Michigan Basin, in Cercone, C.R. and Budai, J.M., eds., Ordovician and Silurian rocks of the Michigan Basin and its margins: Michigan Basin Geological Society Special Paper no. 4, p. 33-43.

Hohman, J.C. and Keith, B.D., in press, Upper Ordovician sequence stratigraphy in the southern part of the Illinois Basin, in Witzke, B.J., Ludvigson, G.A., and Day, J.E., eds., Paleozoic sequence stratigraphy: North American perspectives: Geological Society of America Special Paper.

Hurley, N.F. and Budros, R., 1990, Albion-Scipio and Stoney Point Fields--U.S.A.,

Michigan Basin, in Beaumont, E.A. and Foster, N.H., compilers, Stratigraphic Traps I: American Association of Petroleum Geologists Treatise of Petroleum Geology Atlas of Oil and Gas Fields, p. 1-37.

James, N.P. and Choquette, P.W., 1983, Introduction, in James, N.P. and Choquette, P.W., eds., Paleokarst: Springer-Verlag, New York, p. 1-21.

Keith, B.D., 1985a, Facies, diagenesis, and the upper contact of the Trenton Limestone of

northern Indiana, in Cercone, C.R. and Budai, J.M., eds., Ordovician and Silurian rocks of the Michigan Basin and its margins: Michigan Basin Geological Society Special Paper no. 4, p. 15-32.

178

Keith, B.D., 1985b, Map of Indiana showing thickness, extent, and oil and gas fields of the Trenton and Lexington Limestone: Indiana Geological Survey Miscellaneous Map 45.

Keith, B.D. and Wickstrom, L.H., 1992, Lima-Indiana Trend--U.S.A. Cincinnati and Findlay Arches, Ohio and Indiana, in Beaumont, E.A. and Foster, N.H., compilers, Stratigraphic Traps III: American Association of Petroleum Geologists Treatise of Petroleum Geology Atlas of Oil and Gas Fields, p. 347367.

Kolata, D.R. and Graese, A.M., 1983, Lithostratigraphy and depositional environments of the Maquoketa Group (Ordovician) in northern Illinois: Illinois State Geological Survey Circular 528, 49p. Mitchell, C.E. and Bergstrom, S.M., 1991, New graptolite and lithostratigraphic evidence from the Cincinnati region, U.S.A., for the definition and correlation of the base of the Cincinnatian Series (Upper Ordovician), in Barnes, C.R. and Williams, S.H., eds., Advances in Ordovician Geology: Geological Survey of Canada Paper 90-9, p. 59-77. Rooney, L.F., 1966, Evidence of unconformity at top of Trenton Limestone in Indiana and adjacent states: American Association Petroleum Geologists Bulletin, v. 50, p. 533-546.

Trevail, R.A., 1988, The enigma of karst features in the Trenton and Black River Groups of southwestern Ontario: fact of fantasy? (abstract): Ontario Petroleum Institute Twenty-Seventh Annual Conference Technical Session Proceedings.

Wickstrom, L.H. and Gray, J.D., 1988, Geology of the Trenton Limestone in northwestern

Ohio, in Keith, B.D., ed., The Trenton Group (Upper Ordovician Series) of Eastern North America: American Association Petroleum Geologists Studies in Geology 29, p. 159-172.

Wickstrom, L.H., Gray, J.D., and Stieglitz, R.D., 1992, Stratigraphy, structure, and production history of the Trenton Limestone (Ordovician) and adjacent strata in northwestern Ohio: Ohio Geological Survey Report of Investigation 143, 78p.

Willman, H.B. and Kolata, DR., 1978, The Platteville and Galena Groups in northern Illinois: Illinois State Geological Survey Circular 502, 75p.

179

DESCRIPTION AND INTERPRETATION OF KARST-RELATED BRECCIA FABRICS, ELLENBURGER GROUP, WEST TEXAS

Charles Kerans Bureau of Economic Geology Austin, TX

Abstract The Lower Ordovician Ellenburger Group of West Texas is a prolific oil and gas producer in the Permian Basin of West Texas. Regional analysis of depositional and diagenetic fabrics within the Ellenburger show reservoir facies to be dominated by a variety of breccia fabrics. A descriptive classification of Ellenburger breccias, including fracture, mosaic, clast-supported chaotic, siliciclastic-matrix-supported chaotic, and carbonatematrix-supported chaotic types, allows simplified but genetically significant characterization of these highly varied breccia types. Although undoubtedly Ellenburger breccias are of diverse origins, vertical sequences from unbrecciated Ellenburger upward through chaotic, mosaic, and fracture breccia types in sub-Simpson Group Ellenburger reservoirs are best interpreted in terms of a karst model of cave formation, infill, and collapse. Roof and lower collapse portions of these sequences form the best reservoir intervals with siliciclastic-rich cave fill sediments commonly acting as baffles to fluid migration.

Core from the Gulf McElroy St. No. 1 well illustrates the characteristic succession of breccia fabrics used to develop the karst model. Karst facies recognized in the McElroy St. No. 1 core are lower collapse, cave-fill, and cave-roof, fracture breccias are not well developed in this core. Additional core material from producing zones in other wells is also

displayed to illustrate pore types and breccia fabrics most commonly associated with producing intervals.

Introduction The Lower Ordovician Ellenburger Group of West Texas is a prolific reservoir of oil and gas in the Permian Basin. In a regional study of Ellenburger reservoir facies, almost one-third of the 15,000 ft of core examined was classified as breccia (Kerans, 1988, 1990). Ellenburger breccias have complex origins, but their classification using a simple descriptive scheme proved highly useful for both documentation of lithologic variability and for genetic interpretation. Emphasis was placed on constructing vertical successions of breccia fabrics to aid interpretation of breccia origin and in prediction of breccia body geometry. The core material presented here is from the Gulf McElroy St. No. 1, Upton County, situated in the interior of the Lower Ordovician Ellenburger platform of West Texas (Figure 1). In this area the Ellenburger is directly overlain by the Middle Ordovician Simpson Group. This core is a representative vertical succession through the karst collapse sequence of breccias. Comparison of the breccia fabrics in core with the classification and with the karst model will aid in the understanding of these complex reservoir strata.

181

-r.

Fault

Cored well N

Line of

\I

section Franklin Mts.

\PEW MEXICO \ :ao g 0 4-

0

Llano Uplift

Marathon Basin 200 mi

100

I

1

I

Ok/GC1\ksa

QA9550

300km

o

Contour interval 500 ft Figure 1

Isopach map of the Ellenburger Group showing the location of the Gulf McElroy St. No.

1

well (modified from Texas Water Development Board,

1972).

Stratigraphic Framework Ross (1976) discussed three generalized depositional facies in Lower Ordovician carbonates of North America, including restricted shelf dolostone, open shelf limestone, and slope/basin carbonate-rich shales (Figure 2). This three-fold depositional zonation is present in Texas, with most of the Ellenburger reservoirs in the present-day Central Basin Platform and Delaware/Val Verde Basin occurring in the restricted shelf dolostone tract. Eastern Shelf Ellenburger reservoirs, Ellenburger Group outcrops in the Llano Uplift, and El Paso

Group outcrops in the Beach Mts. and Franklin Mts., are characteristic of the open shelf limestone style. The time-equivalent shale-rich turbidites of the Marathon Group, Marathon area, are of slope/basin affinity (Young, 1968).

Detailed analysis of outcrops of open-shelf strata in the Franklin and Arbuckle Mts. indicates that Ellenburger facies patterns are the result of high-frequency, cyclic deposition during the Early Ordovician (Kerans and Lucia, 1989; Goldhammer and others, 1991). Comparison of Lower Ordovician depositional facies and sequence development in Texas,

Oklahoma, and the Appalachian states using data from these open shelf depositional systems suggests many similarities between these geographically widely separated areas. Read and Goldhammer (1988) published Fischer plots (plots that compare changes in

accommodation space on the platform vs time) for the Knox Group of the Virginia

Appalachians that compare favorably with those for the El Paso Group of Texas and the Arbuckle Group of Oklahoma (Kerans and Lucia, 1989; Goldhammer and others, 1991; Wilson and others, 1991). 182

o

Figure 2

600 km

EXPLANATION 11......

Open shelf

Restricted shelf

Slope/basin

Land

Map showing regional depositional setting during the Early Ordovician (modified from Ross, 1976).

183

Attempts to recognize these patterns of high-frequency cyclicity in restricted shelf Ellenburger sequences has met with little success. Apparently, extensive dolomitization of the Ellenburger in the restricted shelf setting has obscured the high-frequency stratigraphic signature. In place of this framework, a succession of depositional systems has been defined

on the basis of lithology, sedimentary structures, and fabrics seen in cores. This depositional system framework treats only the gross lithostratigraphic elements of the Ellenburger, recognizing that intra-Ellenburger unconformities and high-frequency cyclicity

in this region are not resolved (Figure 3; table 1; Kerans, 1990b). These depositional systems and their dominant lithofacies (in parentheses) are fan delta - marginal marine (litharenite) lower tidal-flat (mixed siliciclastic/carbonate packstone/grainstone) high-energy restricted shelf (ooid and peloid grainstone) low-energy restricted shelf (mottled mudstone) upper tidal flat (laminated mudstone) open shallow-water shelf (gastropod-intraclast-peloid packstone/grainstone)

The McElroy St. No. 1 core exhibits depositional systems 3 and 4, and the portion selected for this workshop only includes facies from system 4. Further documentation of Ellenburger depositional systems in West Texas can be found in Loucks and Anderson (1980) and Kerans (1988, 1990b).

Post-Lower Ordovician Paleokarst The focus on karst-related breccias in this core display reflects the importance of these fabrics in Ellenburger reservoirs. Karst-modified sequences dominate Ellenburger reservoirs in the Andrews/Crane/Ector/Midland/Upton County area of West Texas from which greater than 80 percent of the total Ellenburger oil production has come (Kerans and others, 1988; Kerans, 1990a). Core data, which is essential for analysis of breccia types and origin, is most abundant in this area. A general classification of breccia fabrics derived from study of these cores is presented below and in figure 4. This classification is considered generic and should have application to breccias in the Arbuckle Group and other karstmodified units.

Breccia Classification The abundance and variety of breccia fabrics in the Ellenburger cores examined for

this study required development of a breccia classification scheme to be used in both descriptive and interpretive phases of the study. The classification was found useful for both karst-related and fault-related (tectonic) breccias.

Fracture and Mosaic Breccias

Fracture breccias and mosaic breccias, also known as crackle breccias, are intergradational fabrics. Fracture breccias are formed by a dense fracture network that outlines a system of tightly packed, incipient carbonate clasts. Fractures display extensional displacements, with millimeter-wide pore systems that are either open or filled by saddle dolomite cement (Figure 5A). Porosity is generally highest in fracture breccias and reaches 15 percent where dolomite cement is sparse.

184

West

EXPLANATION

Ea st

FACIES ASSEMBLAGES

00

ft o

Fzi

1500 -

E G

u_

-v-

o

o>,0. c o o

Open shallow shelf

70

0 T_

oo 20

E

Z1

Go

rir.=1

00

00 00

00 0

Low-energy restricted subtidal-intertidal High-energy restricted subtidal Lower tidal flat

-u-

Fan delta-marginal marine

"LT 1000 -

STRUCTURES

-u-

Desiccation cracks

-I-7-

000

00

o C?.0

00000

0

.

500 -

Siliciclastic sand and pebbles Burrows

000000 0 0 o

0 0 o 0 o 0o

o

Upper tidal flat

Ooids

mt3,0 eeN? \rs,s,

N\.Nsi

Cross stratification

/97070Yoro7 0 oc 900

ZO

Cryptalgal laminite

0000000

CP

Parallel larnination

000

Current ripples Rip-up clasts U-

Stromatolites

cr,

.

0-

Figure 3

o GG

Gastropods

Schematic representation of depositional systems in West Texas compared with

formalized Ellenburger stratigraphy in the Llano area. complete section is approximate (from Kerans, 1990b).

185

Total thickness of

Fan Delta Marginal Marine

Lower Ti da 1 -

Flat

High-Energy RestrictedSubtidal

Low-Energy RestrictedShelf

Upper Tidal-

R C R

R C R

R-C

A R

Flat

Open Shallow Shelf

Lithology Lithic Fragments Quartz Sand

A*

Chert Limestone

R

C

C C

C

A

Dolomite Texture > 100 gm (CC)**

C

C

A

R

R

R

10-1004m (FMC)

C

C

C

A

C

C

1-10 p.m (VFC)

R

R

R

R

C

R

C C C R

C C C R

A C R R

R C

R R

C

C

A

A

C C C C

''

C C R C

A C

R

R A

R C

R-C

C

A

C

R

R

A

C

R

R R R

R

C R

Fabrics i

Grainstone Packstone Wackestone Mudstone

Structures Cross-Stratified Current Laminated Stromatolites Flat Cryptalgal Mat Bioturbation

C

Alloc hems Gastropods Cephalopods Bivalves Sponges Ooids Peloids

R

A

R R C

C

R R

A A

R C

C C

* A = abundant, C = common, R = rare, = not available. ** CC = coarse crystalline, FMC = fine to medium crystalline, VFC = very fine crystalline.

Table 1. Characteristics of Ellenburger depositional systems.

186

R

C

FRACTURE BRECCIA dense fracture network defines clasts, Fabricno significant rotation of clasts incipient clasts composed of single host Clastsdolomite type; highly angular outline Cements- minor to pervasive saddle dolomite with rare anhydrite and calcite In terna!

Sediment- rare geopetal dolomicrite and fine siliciclastics Porosity- 1-15% MOSAIC BRECCIA Fabricfitted, clast-supported, with discrete clasts displaying minor rotation Clastsangular to slightly rounded, monomict breccia of host lithology, size usually 5-10 cm Cements- common saddle dolomite rim cement, rare anhydrite calcite and quartz Internal Sediment- geopetal dolomicrite and fine-grained siliciclastics Porosity- 2-20% CHAOTIC BRECCIA, SIUCICLASTIC-MATRIX-SUPPORTED Fabricrandomly oriented clasts in massive, upward-fining, or upward-coarsening units 10-200 cm thick Clasts -angular to rounded dolomite derived from various facies, chert, sandstone, and shale fragments; clast size-1-50 cm Matrix -mixture of shale, very fine to medium-grained silieiclastic sand and minor dolomicrite and glauconite Cements- minor dolomite cement in shale matrix In terna!

Sediment- none Porosity --

1-3%

CHAOTIC BRECCIA, CARBONATE-CLAST-SUPPORTED Fabricrandomly oriented clasts in massive units tens of centimeters to tens of meters thick Clastsangular to rounded dolomite and rare chert fragments from a v-ariety of depositional facies; clast size-5-50 cm Cements- well-developed saddle dolomite rims with rare anhydrite, calcite, quartz, pyrite, marcasite Internal Sedment- geopetal dolomite with rare fine-grained siliciclastics typically perched on clasts, but can fill intraclast space completely as a sieve-fill matrix Porosity- 1-15%

Figure 4

Schematic representation of breccia types in Ellenburger paleokarst deposits (from Kerans, 1990b).

187

Mosaic breccias resemble fracture breccias in that they consist entirely of host dolostone clasts (monomict), but they differ in the greater degree of rotation and relative movement of clasts (Figure 5B). This relative movement produces a fabric in which open pore spaces developed along fracture planes and between juxtaposed clasts. Clast size in both the fracture and mosaic breccias is similar (generally being 2 to 8 inches [5 to 20 cm]), and clasts are highly angular. Void-filling deposits in mosaic breccias include geopetal carbonate mud, saddle dolomite cement, and siliciclastic sand and shale (Figure 5B). Porosity values in the mosaic breccias generally range from 2 to 10 percent. Fracture and mosaic breccias commonly occur in close association in zones ranging

from 6 inches (10 cm) to several tens of feet thick. Upper and lower contacts of these breccias with host dolostones are gradational. Internal sediment is typically minor in mosaic breccias, but white to pink to gray saddle dolomite cement is ubiquitous, and megaquartz, coarse crystalline calcite, and anhydrite cements also occur.

Chaotic Breccias Chaotic breccias include both clast-supported and matrix-supported varieties, the matrix-supported breccias being further divided into siliciclastic-matrix-supported and carbonate-matrix-supported types. Clast-supported chaotic breccias are highly variable in terms of clast character (composition, shape, size), matrix composition, extent of cementation, and total porosity (Figure 5C). Clast-supported breccias typically occur as massive, uninterrupted intervals but locally contain unbrecciated dolostone intervals as much as 10 ft (3 m) thick.

Carbonate clast size varies from a few inches to more than 3 ft (1 m), averaging 2 to

6 inches (5 to 10 cm) (Figure 5C). Clasts are typically angular, although rounded and embayed surfaces have been observed. Laminations within clasts document their random orientation; many clasts are subvertical. Carbonate clasts are generally locally derived and belong to only one depositional system. However, clasts of burrow-mottled dolostone and cryptalgal laminite are commonly juxtaposed, indicating displacement of depositional units by at least several feet. Pore space in clast-supported chaotic breccias is infilled by a variety of materials, including carbonate mudstone, green to black calcareous shale, and coarse crystalline white to pink saddle dolomite Porosity ranges from less than 1 to 15 percent locally, depending on the degree of cementation by saddle dolomite.

Siliciclastic-matrix-supported chaotic breccias are associated with intervals of crudely bedded sandstone and shale locally displaying soft-sediment faults and folds (Figure 7C). Clasts compose 5 to 30 percent of the siliciclastic-matrix-supported breccias and are a

mixture of Ellenburger dolostone clasts and sandstone and shale fragments. These sandstone and shale fragments are identical in lithology to intervals of the overlying Simpson Group Shale clasts display intense compactional soft-sediment deformation, whereas sandstone fragments are rounded.

Intercalations of bedded sandstone and shale between and 10 ft (0.3 and 3 m) thick showing delicately graded bedding, thin planar lamination, and climbing-ripple cross1

stratification also occur (Figure 7C) Shales are both green and dark-gray, whereas sandstone clasts and quartzose sand within the breccia matrix are white to light grey and texturally and compositionally mature. Disruption of sedimentary structures by softsediment deformation is ubiquitous (Figure 7C). Visually estimated porosity of siliciclasticmatrix-supported breccias is low, generally between 1 and 3 percent.

188

A

\

Now,

E

oo

0 1- 0

'V:

oE

c\I

Lo

o o

0 o

Figure 5

Examples of typical Ellenburger paleokarst breccias (A) Fracture breccia showing pervasive network of fractures lined by saddle dolomite and locally filled by internal sediment. Low-energy restricted-shelf depositional system, Texaco Seaboard TXL D-1 well, 13,007 ft, (B) Mosaic breccia showing slightly rotated clasts and saddle-dolomite cement. Low-energy restricted-shelf depositional system, Cities Service Foster No. well, (C) Clast-supported chaotic breccia showing mixture of clast types and saddle-dolomite cement. Low-energy restricted-shelf depositional system, Gulf McElroy St. No. 1 well, 11,817 fi, (D) Siliciclastic-matrix-supported chaotic breccia with large subvertical clast. Both matrix and clasts were extensively recrystallized by pervasive late dolomitization. Low-energy restricted-shelf depositional system, Gulf McElroy St. No. 1 well, 12,035 ft. 1

189

Carbonate-matrix-supported chaotic breccias are only a minor-component breccia type. These matrix-supported breccias are gradational upward into the siliciclastic-matrixsupported breccias in many breccia sequences. Grading is absent, and bedding is only rarely apparent. Clasts are supported in a matrix of medium-crystalline dolomudstone, the same

lithology as that of the component clasts, but the matrix also contains quartz sand and angular, granule-size chert fragments. Clasts range from 2 to 16 inches (5 to 40 cm) long and are rounded to subangular and randomly oriented. The abundance of fine-grained matrix in the carbonate-matrix-supported breccias results in overall low porosity, ranging from 1 to 5 percent.

Local intense dolomitization of both siliciclastic-matrix-supported and carbonatematrix-supported chaotic breccias produces a complex fabric. Clast-matrix boundaries are obscured, original depositional fabrics of clasts are obliterated, and much of the original shale matrix of the breccia may be replaced (Figure 5D). Similar replacement of shale by dolomite in Tertiary strata was reported by Heald and Baker (1977).

Karst Model Throughout the majority of larger Ellenburger reservoirs that occur beneath the Simpson Group, a repetitive vertical succession of breccia types is observed. This sequence, from clast-supported chaotic breccias at the base through siliciclastic-matrixsupported (and lesser carbonate-matrix-supported) chaotic breccias into upper fracture and mosaic breccias, is well represented in the McElroy St. No. 1 core (Figures 6 and 7). The karst model proposed for this vertical succession of breccia types includes (1) prolonged exposure of the Ellenburger carbonate platform during a major (second-order) sea-level lowstand, (2) dissolution and cave development, (3) enlargement of cavern systems through dissolution and mechanical failure and stoping of portions of cave margins forming clastsupported chaotic breccias that accumulated on the floors of caves (lower collapse zone), (4) cessation of cave development caused by subsequent (Middle Ordovician) sea-level rise and transgression that resulted in deposition of siliciclastic-matrix-supported breccias filling cavern systems (cave fill), (5) burial of the partially to completely filled cave system, and (6)

compaction-driven fracturing of cave roof material due to increased loading over undercompacted cave-fill material causing formation of cave roof fracture/mosaic breccias (cave roof) (Figure 8).

Karst versus Fault-Related Origin of Ellenburger Breccias Many of the breccia deposits and fracture systems in the Ellenburger Group have been variously interpreted either as of karst (Loucks and Anderson, 1985) or tectonic (Ijirigho and Schreiber, 1986) origin. The term tectonic fracture is used here to describe fracturing attributed to fault-displacement unrelated to karst-collapse. The dominant tectonic event causing fault-related or tectonic fractures in the Ellenburger is the Pennsylvanian-age faulting associated with development of the Marathon-Ouachita orogen.

In this study, fracture systems and associated breccias interpreted to have a tectonic origin were seen to be localized and generally not causally linked to the widespread karst breccias. Most karst-related fractures can be distinguished from tectonic fractures using the criteria of spatial distribution, timing, association with particular breccia fabrics, fracture

geometry, and nature of fracture fill. In the case of sub-Simpson Group, pre-Middle Ordovician karst systems, karst-related fractures are generally restricted to the upper 100 to 300 ft (30 to 90 m) of the Ellenburger Group. Typically these are developed (Continued page 194) 190

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brecciated roof zone, cave-infill zone, and lower collapse-breccia zone in Gulf McElroy St. No. 1 well, Upton County. The distinctive radioactive zone that occurs in the log 40 fi (12 m) below the unconformity corresponds to the shalesandstone infill zone. Letters A through D correspond to core photographs in Figure 7, whereas numbered zones 9, 10, and 11 refer to core photographs in Figure 7, whereas numbered zones 9, 10, and 11 refer to core photographs of Figures 9, 10, and U. 191

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193

within a narrow zone immediately above a distinctive siliciclastic-rich chaotic breccia zone (interpreted in this report as a cave-fill deposit). Karst-related fractures are contemporaneous with the main phase of breccia development and do not crosscut breccias.

These fractures also predate precipitation of saddle dolomite cement and are partly to completely filled with this late dolomite. This is an important observation because this dolomite phase can be shown to postdate at least basal Simpson Group strata on the basis of crosscutting relationships (Kerans, 1990b). Besides dolomite cement, geopetal internal sediments of both siliciclastic and carbonate composition occlude karst-associated pore space.

In contrast, a suite of fractures that could be confidently related to tectonism origin had the following characteristics: (1) random occurrence within cored intervals and (2) cross-cutting of all diagenetic phases including youngest saddle dolomite cements and karst breccias. Infill deposits include rare megaquartz cement and dark internal sediment. Criteria used to differentiate tectonic from karst-related fractures are given in Table 2.

Gulf McElroy St. No. 1 Core: Depositional and Breccia Fabrics Core from the Gulf McElroy St No. 1 well in Upton County, Texas, is featured in this presentation (Figures 1, 6, 7, 9, 10, and 11). Primary depositional facies are bioturbated peloidal dolomudstones and dolowackestones of the low-energy restricted subtidal-intertidal depositional system. In terms of breccia fabrics, the core displays a thin cave roof facies from directly below the Ellenburger/Simpson unconformity (not cored) at 11,600 to 11,636 ft. This upper 36 ft contains rare fractures that are filled by dolomite cement and dolomudstone internal sediment (Figures 7A and 9). Fracture and mosaic breccias and their associated pore networks are not developed in this uppermost portion of the McElroy St No. 1 well. Interestingly, this absence of breccia-related porosity is consistent with the drill stem tests of this interval, which reported low shut-in pressures for this zone. The interval between 11,637 and 11,725 ft that gives the diagnostic high API gamma-ray log response is composed of a complex array of siliciclastic-matrix-supported breccias (Figures 7B, 7C, 9, and 10) with a variety of soft-sediment deformation and sediment gravity-flow features. Also important in this unit are dolostone fragments of probable Ellenburger affinity as well as exotic rock fragments including orthoquartzitic sandstone and shale.

Below this siliciclastic-matrix-supported breccia unit is one composed of clastsupported dolostone chaotic breccia with less common carbonate-matrix-supported chaotic

breccias (Figures 7D and 11).

In clast-supported zones breccias are cemented with

pervasive saddle dolomite cement but also retain some inter-clast porosity. This porosity, though not visually impressive, is responsible for very high drill-stem-test flow rates. Clast types in this lower breccia interval include typical Ellenburger restricted shelf bioturbated dolomudstones. Rare siliciclastic material has filtered down into this lower breccia unit, forming geopetal perched sediment in inter-clast voids. This sequence from clast-supported chaotic breccia, through siliciclastic-matrix-supported chaotic breccia, to uppermost intact dolostone with minor fracture breccia comprises the ideal vertical profile attributed to cave formation, infill, and collapse as discussed above. In terms of this model, the upper unit of the McElroy St. No. 1 core is interpreted as a cave-roof zone (11,601-11636 ft; Figure 9). The middle unit of siliciclastic-matrix-supported breccia would represent cave-fill (11,63711,725; Figures 9 and 10), and the lower carbonate clast-supported chaotic breccia interval (11726-12,050 ft; Figure 11) is interpreted as lower-collapse-zone.

194

TABLE 2.

COMPARISON OF FAULT-RELATED AND KARST BRECCIAS

Fault-Related

Karst

Breccia Fabric

Tightly fitted clasts, interclast areas filled with mechanically disaggregated host rocks and cement

Variable, fracture, mosaic (= fitted), and chaotic breccias all common, both clastsupported and matrix-supported, typically open space between ciaste filled with cement or cavity-filling sediment

Clast Morphology

Variable, highly angular to rounded, depending on degree of lateral displacement

Variable, angular to rounded, some clast embayment related to solution

Clast Composition

Monomict, derived from immediately adjacent strata

Either monomict or oligomict, depending on lithologic variability of section involved in collapse

Geometry

Tabular, restricted to area immediately adjacent to fault plane

Tabular upright bodies developed along joint trends and horizontal bodies controlled by selectively dissolved zones

Table 2. Comparison of fault-related and karst breccias. Su in

Regional analysis of core material from the Ellenburger of West Texas demonstrates that the majority of the producing reservoirs are developed in facies of the restricted shelf subdivision of Ross (1976). Extensive dolomitization of these restricted shelf carbonates prohibits high-frequency cyclostratigraphy to be applied to these strata, but a simplified grouping into depositional systems is possible These depositional systems include (1) fan delta -- marginal marine, (2) lower tidal-flat, (3) high-energy restricted shelf, (4) low-energy restricted shelf, (5) upper tidal flat, and (6) open shallow-water shelf Only system 4 will be observed in the core selected for this workshop.

The McElroy St No. 1 core provides an excellent opportunity to observe the different types of breccias commonly associated with karst-modified reservoirs of the Ellenburger Group in the area where the Ellenburger is overlain by Simpson Group strata (Andrews/Crane/Ector/Midland/Upton Counties). A simple descriptive classification can be

applied to these breccias that aids in consistent and potentially genetically significant observation. Major breccia types are fracture, mosaic, clast-supported chaotic, siliciclasticmatrix-supported chaotic, and carbonate-matrix-supported chaotic types The major karst breccia facies are lower-collapse zone (clast-supported chaotic breccias), cave-fill (siliciclastic-matrix-supported chaotic and lesser carbonate-matrix-supported chaotic breccias), and cave-roof (fracture and mosaic breccias and intact host dolostone). The consistent and complex stratification of breccia types and their timing relative to late diagenetic events are considered critical for differentiating these breccias from those of fault-related origin.

195

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References

Goldhammer, R. K., Lehmann, P. J., and Dunn, P. A., 1991, Third -order sequences and parasequence stacking patterns of Lower Ordovician platform carbonates of the El Paso Group, Franklin Mountains, West Texas (abs): American Association of Petroleum Geologists Bulletin, v. 75, p. 582. Heald, M.T., and Baker, G.F., 1977, Diagenesis of the Mt. Simon and Rose Run sandstones in western West Virginia and southern Ohio: Journal of Sedimentary Petrology, v. 47, p. 66-77.

Ijirigho, B.T., and Schreiber, J.F., Jr., 1986, Origin and classification of fractures and related breccia in the Lower Ordovician Ellenburger Group, West Texas: West Texas Geological Society Bulletin, v. 26, p. 9-15.

Kerans, C., 1988, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas: American Association of Petroleum Geologists Bulletin, v. 72, p. 1160-1183. Kerans, C., 1990, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas: REPLY: American Association of Petroleum Geologists Bulletin, v.74, no. 7, p. 1124-1125.

Kerans, C., 1990, Depositional systems and karst geology of the Ellenburger Group, (Lower Ordovician), subsurface West Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No.193, 63 p. Kerans, C., Holtz, M.H., and Tyler, Noel, 1989, Contrasting styles of reservoir heterogeneity in Ellenburger Group carbonates, West Texas (abs.), in

Cunningham, B. K., and Cromwell, D. W., eds., The lower Paleozoic of West Texas and southern New Mexico--modern exploration concepts: Permian Basin Section, Society of Economic Paleontologists and Mineralogists Publication No. 89-31, p. 131.

Kerans, C. and Lucia, F.J., 1989, Recognition of second, third, and fourth/fifth order scales of cyclicity in the El Paso Group and their relation to genesis and architecture of Ellenburger reservoirs, in Cunningham, B. K., and Cromwell, D. W., eds., The lower Paleozoic of West Texas and southern New Mexico--modern exploration

concepts: Permian Basin Section, Society of Economic Paleontologists and Mineralogists Publication No. 89-31, p. 131.

Loucks, R.G., and Anderson, J.H., 1980, Depositional facies and porosity development in Lower Ordovician Ellenburger dolostone, Puckett field, Pecos County, Texas, in Notes for Society of Economic Paleontologists and Mineralogists Core Workshop No. 1, p. 1-31. Loucks, 1985, Depositional facies, diagenetic terranes, and porosity development in Lower Ordovician Ellenburger dolomite, Puckett field, West Texas, in Roehl P.O., and Choquette, P.W., eds., Carbonate Petroleum Reservoirs: New York, SpringerVerlag, p. 21-37.

Read, J.F., and Goldhammer, R.K., 1988, Use of Fischer plots to define third-order sea level curves in peritidal cyclic carbonates, Ordovician, Appalachians: Geology, v. 16, p. 895-899. 199

Ross, R.J., 1976, Ordovician sedimentation in the western United States, in Bassett, MG.,

ed., The Ordovician System: proceedings of a Paleontological Association symposium: Birmingham, p. 73-105.

Wilson, J. L., Fritz, R. P., and Medlock, P., 1991, The Arbuckle Group, relations of core and outcrop analysis to cyclic straigraphy and correlation: Oklahoma Geological Survey Circular 92, p. 61-63.

Young, L.M., 1968, Sedimentary petrology of the Marathon Formation, (Lower Ordovician), Trans-Pecos Texas: The University of Texas at Austin, Ph.D. dissertation, 234 p.

200

CASABLANCA FIELD, TARRAGONA BASIN, OFFSHORE SPAIN: A KARSTED CARBONATE RESERVOIR Anthony J. Lomando Chevron Overseas Petroleum Inc.

Paul M. Harris Chevron Petroleum Technology Company

Donald E. Orlopp Chevron Overseas Petroleum Inc.

Abstract Casablanca Field, offshore Spain, produces oil from karsted Jurassic - Cretaceous

carbonates. Subaerial exposure that produced the paleokarst was significant and affected up to 386 meters of section. Locally, karst dissolution was extensive enough to form large, solution-enhanced fractures or small, probably horizontal, caves. Multiple phreatic zones that developed during regional uplift probably produced the various cave levels recognized in cores.

Cores contain representative and distinctive attributes of paleokarst including breccias, cave-fill sediment, and fractures. Fitted, mosaic, and rubble breccias which are

distributed throughout the cored interval formed in part during cave-roof collapse and

compaction of cave-fill sediments. The cave-fill is principally dolomitized carbonate mud or clast-supported sediment that is red in the upper portions of the cored interval and green in the lower portions. Fractures, in which a significant volume of the reservoir pore volume is contained, formed during both karst-collapse and tectonism.

Introduction Location Casablanca oil field, Spain's largest, is located in the Spanish Mediterranean Gulf of Valencia Basin approximately 45 km south-southeast of Tarragona (Fig. 1). The geology and development history of the field have been presented in detail by Orlopp (1988), Watson (1982), and others, so only a brief discussion is presented here.

Field Discovery and Development The Amposta oil field was discovered by Shell in 1970. This discovery established for the region a trap model of significant karst porosity in a Mesozoic carbonate paleohigh just below a basal Miocene unconformity. Subsequently, a regional seismic grid was recorded in the area, and the interpretation of these data indicated several deep Miocene closures overlying diffuse seismic anomalies that were presumed to be Mesozoic carbonate paleohighs. Several anomalies mapped along the Casablanca trend on seismic data were thought to be karsted carbonate hills (Figs. 2 and 3; Watson, 1982).

Ultimate oil production from Casablanca field is estimated to be 114-120 million barrels without assisted recovery. Chevron discovered and was the operator throughout the appraisal and early development phase for Casablanca. The Casablanca platform was (Continued page 205) 201

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Structure map (in meters subsea) on the top of the Mesozoic in Casablanca Field. The cored well (Casablanca-1A) and line of section for the seismic line of Figure 3 are shown.

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placed on location in early 1982. REPSOL, the Spanish State oil company, succeeded to the operatorship during the main production phase. The nature of the reservoir surface and wide range of water depths across the field imposed constraints on the development scheme (Orlopp, 1988). Peak production of 44,000 barrels of oil per day was achieved in 1983.

Drilled in late 1975, the Casablanca-1 discovery well (located on Figure 2) encountered a thick, oil saturated, karsted Jurassic carbonate sequence. The original oil column was estimated to exceed 250 meters, and the highest test rate achieved was 10,670 barrels of oil per day. To determine the reservoir's productive potential, Casablanca-1 was placed on a long term test until mechanical problems forced its abandonment and replacement by side-track well Casablanca-1A. Cumulatively, these wells produced about 8.5 million barrels of oil.

Appraisal drilling over a two year period established that high fluid transmissibility, due to fracturing and paleokarst, was pervasive over most of the field. Several dry holes were drilled on the Casablanca-Montanazo trend. This subsequently established that karstification and porosity preservation are highly variable. This aspect of the Casablanca case history demonstrates the difficulty of predicting reservoir continuity and quality in karsted carbonates.

Structural Setting Original field structure maps showed that the Casablanca paleohigh, representing one of a series of ridges and hills forming the Castellon-Montanazo trend, was narrow and Casablanca ridge is bounded by a series of substantial faults and elongate (Fig. 2). topographic scarps along its northwest and southeast flanks. The scarps probably relate initially to flank faults that offset the Casablanca ridge from adjacent depressions with throws of 400 to 600 meters. Erosion and karsting probably modified the topographic form

of the ridge so that the preserved topographic scarps no longer represent actual fault positions in the Mesozoic substrate. Faulting owes its origin to Tertiary tectonism initiated by plate motion during the Pyrenean orogeny (Eocene) and culminating during the time of Betic orogenesis (Early Miocene). The structure of the field, which measures 1 by 11 km in map view and covers 2,575

acres (1,042 hectares), portrays the karsted and eroded remnant of a paleotopographic, fault-bounded ridge (Fig. 2). The field has five significant high spots or culminations all of which represent erosional remnants, i.e. the tops of buried hills. These culminations account for most of the significant oil production from the field.

Stratigraphy and Paleokarst The carbonate beds equivalent to the Casablanca reservoir are widespread across much of the Gulf of Valencia. This thick carbonate interval, now mostly dolomitized, is

bounded below by generally tight Triassic to Paleozoic rocks and is sealed above by Tertiary fine-grained marl beds. The Mesozoic carbonates are exposed in the coastal Catalan Ranges at elevations over 1000 meters. Varied degrees of karstification are common in the outcrops (Alvarez, 1987). As a consequence, the carbonate zone acts as an excellent regional aquifer and Casablanca has a strong water drive. The karsted and eroded Mesozoic sequence is overlain unconformably by the onlapping sediments of the Middle Miocene Alcanar Group. The Alcanar is a silty marlstone to marly limestone with a variable organic, glauconite and pyrite content (Watson, 1982). Off structure, the

205

Alcanar beds served as the source rocks for Casablanca oil (Demaison and Bourgeois, 1984; Fig. 1).

The first post-unconformity sediment deposited on top of the Casablanca ridge is biostratigraphically dated as late Serravalian. In the adjacent Tarragona depocenter (Fig. 1), the first post-unconformity sediment deposited is dated as probable late Burdigalian. Therefore, the Casablanca ridge would have been subaerially exposed for at least 6 million years. Actual exposure time was probably longer depending on when the region was first upwarped and exposed during Tertiary interplate movements and major sea level lowstands. Regionally, some areas may have been exposed as long as 40 million years (del Olmo and Esteban, 1983). Locally in Casablanca Field, karst dissolution was extensive enough to form large, solution enhanced fractures or small, probably horizontal caverns. Evidence from drilling for these features are frequent bit drops on a scale of meters, common partial to total fluid circulation losses, brief but significant increases in drilling rates, expansion of the caliper

tool, and a variable, choppy aspect on the sonic log with slow zones exceeding 100 microseconds per foot in otherwise invariant carbonate. The strong deflections of the sonic log trace may represent open, solution-enlarged fractures, or locally, small caverns. Shaly zones may be points of cavern collapse or infiltrated cave-fill sediment accumulations.

Major, non-shale related drilling breaks may represent cavernous zones or possibly collapsed caverns that still retain some open porosity. These zones span intervals on the downhole logs ranging from a few meters up to 10 meters in thickness.

Other field wells were not drilled as deeply into the carbonate section as the Casablanca-1. In these other wells, similar drill break patterns and porosity variations at different levels suggest an intricate and widespread network of cavernous porosity exists in Casablanca Field. The total karsted interval of the reservoir was not penetrated by Casablanca-1A but was identified in the Casablanca-1 well. The base of the paleo-phreatic

zone is interpreted from downhole logs to be at 3025 meters in the Casablanca-1 well with the karsted zones spanning 386 meters.

Casablanca-1A Cores Core control in the Casablanca Field is limited except for the Casablanca-1A well (located on Figures 2 and 3). In that well, the upper 109 meters of reservoir column was extensively cored with generally good core recovery (Figs. 4 to 7). Figure 4 shows the downhole logs, generalized dolomite lithology for the interval, and location of cores in the Casablanca-1A well. In addition, the figure also shows our general interpretations of paleokarst that are based on examination of core and log data and application of distinctive criteria for recognizing paleokarst from Esteban and Klappa (1983), James and Choquette (1984, 1988), and other sources. The characteristics of paleokarst which are of both economic and geologic significance in Casablanca Field are represented in these cores. The main features we will emphasize are the formation and nature of breccias, the character and style of cave-fill sediments, and the distribution of fractures and porosity. Breccias Three types of breccia are found in cores from Casablanca-1A (Figs. 5, 6 and 7). Fitted breccia occurs where little to no displacement of fragments has occurred. Mosaic (Continued page 212) 206

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Drilling rate, gamma ray and sonic logs, and location of cores for the Casablanca lA well. Also shown are depth of sediment filled caves recognized in cores and inferred karst profile. 207

Figures 5, 6, and 7

Core descriptions for the Casablanca-1A well. Legend on Figure 6 refers to all. Columns indicate the following: core recovery and generalized lithology; core number corresponding with that of Figure 4 and predominant features recognized in core slabs; location of fractures, three breccia types, and vuggy porosity; comments relative to paleokarst interpretation; and location of core slab or thin section photos.

208

.

Depth LITHOLOGY

8700

I,..

itL.,.

11.1111MIIIIM

wwwwra

8710

E Mi OINIa

IWIMII

I

..

>

I

Sr

41,02 k

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:grayA

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IIIIMIIMVA NIIIIIM111

11

COMMENTS

grey rnatriv w/ reddish dolornicrite cave fil

:c3 0 ..../.-

8715 immommoor. mAmi

8720

vs

TYPES

3

FEATURES

amiummum

8705

BRECCIA

CORE

.

Cave fill color changes lo greenish

'A' A N ily

j-

.

3Z.-

S

A°

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IMF/MINIM MEINIMIWINNI

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Cave roof w/ large sediment filled yogi

t

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MMIWN 8735

11,MMIIIIIIMa

8740

8745

11`11 MINIIIMI

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we roof

N

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Cave fill

IL. N

8750 IMMIMINIII MIINVJ=1

Cave floor?

mimmtrom

.4..

111=IMIENIII INIMIIIIIIMIN IIIMININEMM

8755

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S.

a ,,,,,,,...0"'

S

Cam roof

IIIIMI

NMMIIMIMA IIIINIMININIVA

8760

1111 1111=.401

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IAIMINI

COW.

fill S

.

TS

N ---------------.1

Cave floor

8775

8780

8785

87,0

8795

209

Figure

5.

Depth UTHOLOGY

6820

E

CORE

.q

FEATURES

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8835

COMMENTS

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6845

LEGEND Scoured surfaces

Wavy lamincrtion

Fine lamination Bioturbation Geopeds

o

Open fractures Cemented fractures

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Thin section photo

TS

Slab photo

S

Red

Green Tan

Stylolites Breccia

210

Figure 6.

il p ...

'Depth (ft.)

8895

UTHOLOGY

FEATURES

pa IIII

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CORE

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I

TYPES

13.

on

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Figure 7.

No8990

211

breccia contains fragments which are displaced and disorganized. The fragments commonly display only minor rotation, the texture is still fragment-supported, and some fitted textures may still be visible. Rubble breccias occur where fragments are completely and randomly disorganized. The fragments in rubble breccias can be cement or sediment supported.

Fitted breccias are only sporadically distributed in the cores, and as is shown on Figures 5, 6 and 7 mosaic and rubble breccias are most common. Sediment composed of micrite-sized particles, in addition to coarse crystalline dolomite and calcite cements, fill interfragment areas of the breccias (Fig. 8). In fitted breccias (Figs. 8D), fine fracturing is often intense but little movement and rotation of clasts is observed. Mosaic to rubble breccia textures range from examples where several episodes of fracturing, cementation, and movement are observed (Fig. 8A) to examples of total chaotic brecciation of zones with complex cementation and sedimentation between clasts (Fig. 8C).

Breccias are most common between 2720 and 2740 meters (8925 and 8990 feet) in Casablanca-1A cores. This interval is interpreted as collapsed cave roofs (Fig. 7) and may have in part formed during compaction of underlying and overlying cave-fill sediments. Cave-Fill

Cave-fill in Casablanca-1A is dominated by dolomitized carbonate sediment that varies texturally from mud-supported to clast-supported (Figs. 9 and 10). These rocks are generally poorly sorted and can be classified as gravel to cobble carbonate conglomerates. The "mud matrix" ranges from micrite to silt- and sand-sized particles (Fig. 11A). The associated clasts are fine sand- to cobble-sized particles and are angular to well rounded (Figs. 9A and 11B). Clast types range from host rock to multigeneration reworked fragments (Fig. 9A).

The observations cited above indicate that active erosion, transport, and cementation occurred in multiple episodes during the formation and filling of the cave systems penetrated by the cores. The brecciation and cementation along fractures predated some episodes of erosion and cave sedimentation, as is evidenced by the presence of clasts composed of cemented breccia and fracture-fill cement (Fig. 11C).

Color is a distinctive characteristic of the cave-fill sediment. Red sediment (terra rosa) occurs only in the upper portion of the cored interval (Fig. 5). We infer from this distribution a lower limit for oxidizing conditions in the local karst terrain. The transition from red to green (terra verde) cave-fill can be observed within a single cave-fill (Fig. 10). Below the transition, green cave-fill sediment and tan/brown cement are dominant in the cores (Figs. 6 and 7).

A common cave-fill succession is shown in Figure 10. The cave roof is composed of poorly-laminated and bioturbated dark brown dolomite. The cave fill is dominated by red (transitional to green) mud-supported textures with abundant dark clasts of the host

carbonate ranging up to cobble size. The cave floor also serves as the roof of the underlying cave. Cave floor/roof intervals in the cores are commonly riddled with large sediment-filled vugs and fissures which make clear boundary distinctions difficult.

Sedimentary structures are rare in the cave-fill sediments. When observed, they range from distinct lamination in mud-rich portions (Fig. 12A) to faint laminae in coarser, but still mud-dominated, sediment (Fig. 9B). In cave-fill sections that are rich in (Continued page 217) 212

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CASABLANCA-1A

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conglomeratic textures, fine-scale structures are absent but crudely graded beds with locally scoured bases are observed (Fig. 9C).

Fractures and Porosity Reservoir pore volume in Casablanca Field is contained in vuggy porosity and associated fractures (Figs. 12 and 13). Fractures were formed by both tectonic and karstcollapse processes. Most fractures viewed in core are vertical to subvertical. These may trace their origin to the period of Paleogene uplift.

Open, cemented, and sediment-filled fractures are most common in host rock dolomites. They occur in both brecciated and unbrecciated sections and are rarely observed in cave-fill intervals (Figs. 5, 7 and 12). The latest generation fractures occur on natural breaks in the core and contain sparse cement. The next older generation of fractures range from being completely to partially-cemented by coarse crystalline calcite (Fig. 13C).

The earliest formed fractures, of which there are probably several generations, contain dolomicrite sediment and linings of medium to coarse crystalline euhedral dolomite (Fig. 13D).

The fractures are commonly cross cut by stylolites (Fig. 14). The relative timing of fracturing and stylolitization is ambiguous, suggesting that these processes were intermittent and may have been reactivated several times to create the complex relationships observed in cores. Cataclastic textures are also present in the form of granulated shear zones (Fig. 13B) which are in turn cross cut by younger dolomite-lined fractures. Porosity occurs locally as partially-cemented or reopened fractures (Figs. 12B, 13A

and B), irregular-shaped vugs associated with fractures (Fig. 12C), and interfragment spaces in breccias. The porosity between breccia fragments formed either during brecciation, or more likely, was reopened or enlarged prior to oil migration and entrapment (Fig. 12D). Porosity in cave-fill sediments is rare, being generally restricted to isolated pinpoint vugs in matrix (Fig. 11B) or clasts that were selectively dissolved due to textural or compositional solubility contrast with adjacent matrix and other clasts (Fig. 9B). Discussion

A thorough understanding of the paleokarst features described from the cores, and therefore the reservoir architecture, is limited by a lack of core data from other wells to place the Casablanca-1A in perspective. Even with these data limitations, however, some

interesting questions can be posed and insight gained regarding the paleokarst of Casablanca Field

Dissolution of carbonate occurs in the vadose zone but it is commonly most pronounced in the phreatic zone where a freshwater base level is established and cave formation may occur (Fig. 15). With uplift, a new base level becomes established and a new cave system may form "deeper" in the section. Continued periodic uplift is then capable of generating multiple cave levels with evolution of the karst terrain, during which the vadose zone is extended downward and an overlying cave floor is also the cave roof of the next deeper cave level.

217

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Figure 14 Whole thin-section photomicrograph from 2720 m (8924 fi) in the Casablanca1A core. Breccia with dolomite-cemented fractures. Note the open, vuggy porosity and stylolite which cross cuts and offsets the preexisting fractures. 222

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Figure 15 Schematic diagrams (modified from Alvarez, 1987) based on outcrop studies and showing the effects of changes in base level on cave-forming phreatic levels. With regional uplift and associated base level drop, the host carbonate section is imprinted with multiple levels of cave zones 223

Multiple phreatic zones and cave levels likely formed the various caves recognized in the Casablanca-1A cores. The paleo-phreatic zone was inferred to be "multilevel" from examination of downhole logs in the Casablanca-1 well and other deep wells in the field, as well as from outcrop studies in the coastal Catalan Ranges (Alvarez, 1987). While the area was being regionally uplifted during Tertiary orogenesis, karstification would have proceeded progressively downward with local base level. At Casablanca, this is interpreted to have formed an overall karsted profile up to approximately 386 meters in thickness. Diagnostic characteristics of the paleokarst recognized in cores from the Casablanca-1A well include breccias, cave-fill sediment, and porosity mostly in fractures associated with the breccias. These characteristics indicate a particular type of karst system, i.e., the free flow system of White (1969), interregional karst of Choquette and James (1988), or conduit-flow of Kerans (1989). Reservoirs that are developed in this type of karsted carbonate are extremely heterogeneous and, as discussed by Kerans (1989), are highly compartmentalized due to large-scale collapse features, tabular phreatic cave systems filled with generally low permeability cave sediments, and strong development of brecciarelated fracture porosity.

In the Casablanca-1A, there is a striking similarity between the thicknesses of cave roof/floor sections and cave-fill sections as shown on Figures 5, 6, and 7. There are two possible scenarios for this regularity: (1) dissolution processes may have been most

dominant along bedding planes in a sequence of beds of uniform thickness; or (2) dissolution occurred preferentially within specific facies in a stacked cyclic succession,

controlled by a contrast in inherent permeability related to early fracturing or residing within the matrix in different portions of each cycle. The cave roof/floor intervals in the upper cored interval (Fig. 5) are coherent, whereas similar units have collapsed and are brecciated in the lower cored portion (Fig. 7).

Cave-fill sediments show the active breakdown of host rock and reworking of preexisting cave-fill sediment. The size and sorting of material and sedimentary structures indicates an energetic flow system in a very wet environment. This same evidence also suggests common episodic or pulsed flow. Another interesting characteristic of the cave-fill sediment is the abundance of clay and silt-sized carbonate. There are several authocthonous mechanisms which could generate large amounts of mud, including the mechanical breakdown of carbonate host rock by abrasion and precipitation of fine crystals and needles in the phreatic environment. However, if precipitation is a source of carbonate mud one would logically expect to also find significant carbonate cement crusts in the form of drip and flowstones. These features are conspicuously absent in the Casablanca-1A cores.

Large volumes of carbonate-rich mud and silt are common in modern caves in areas that have been glaciated (Ford, 1988). The fine fraction in these instances formed as "glacial flour" that was easily transported into the underlying cave systems. A similar mechanism is an alternative origin for generating the fine cave-fill sediment in the Casablanca cores. A post-Cretaceous to pre-Miocene (Paleogene) Alpine-glaciation period could have locally generated the significant amount of carbonate fines observed in the Casablanca-1A cores.

A glacial scenario would also be capable of generating the high intensity, periodic flow needed to transport, round, and repeatedly rework cave-fill sediments.

224

Acknowledgements Mateu Esteban viewed the Casablanca cores with us, provided a valuable overview

of the regional geology and encouraged us to continue our work which resulted in this paper. We thank Chevron for support and permission to publish this paper.

References

Alvarez, C., 1987, Procesos karsticos: aplicacion al estudio del karst en materiales Mesozoicos de las Cordielleras Costero Catalanas: unpublished HISPANOIL report.

Choquette, P.W. and James, N.P., 1988, Introduction, in James, N.P. and Choquette, P.W. (eds), Paleokarst: Springer-Verlag, New York, p. 1-24.

Demaison, G. and Bourgeois, F.T., 1984, Environment of deposition of Middle Miocene (Alcanar) carbonate source beds, Casablanca Field, Tarragona Basin, offshore Spain, in Palacas, J.G. (ed), Petroleum geochemistry and source rock potential of carbonate rocks, Studies in Geology No. 18: American Association of Petroleum Geologists, Tul sa, p. 151-161. Esteban, M. and Klappa, C.F., 1983, Subaerial exposure, in Scholle, P. A., Bebout, D.G.,

and Moore, C.H. (eds), Carbonate depositional environments, Memoir 33: American Association of Petroleum Geologists, Tulsa, p. 1-54.

Ford, D., 1988, Characteristics of dissolutional cave systems in carbonate rocks, in James, N.P. and Choquette, P.W. (eds), Paleokarst: Springer-Verlag, New York, p. 25-57. James, N.P. and Choquette, P.W., 1984, Limestones -- the meteoric diagenetic environment: Geoscience Canada, v. 11, p. 161-194. James, N.P., 1988, Paleokarst: Springer - Verlag, New York, 416 p.

Kerans, C., 1989, Karst-controlled reservoir heterogeneity and an example from the Ellenburger Group (Lower Ordovician) of West Texas, Report of Investigations No. 186: Bureau of Economic Geology, University of Texas, Austin, 40 p.

del Olmo, W., and Esteban, M. 1983, Paleokarst development, in Scholle, P.A., Bebout, D.G., and Moore, C.H. (eds), Carbonate depositional environments, Memoir 33: American Association of Petroleum Geologists, Tulsa, p. 93-95. Orlopp, DE., 1988, Casablanca Oilfield, Spain: a karsted carbonate trap at the shelf edge: Proceedings of the Offshore Technology Conference, OTC 5734, p. 441-448. Watson, H.J., 1982, Casablanca Field, offshore Spain, a paleogeomorphic trap, in Halbouty, M.T. (ed), The deliberate search for the subtle trap, Memoir 32: American Association of Petroleum Geologists, Tulsa, p. 237-250.

White, W.B., 1969, Conceptual models for carbonate aquifers: Ground Water, v. 7, p. 1521.

225

PALEOKARST DEVELOPMENT IN DEVONIAN CHERTS IN THE ARKOMA BASIN AND BLACK WARRIOR BASIN Patrick Medlock and Richard Fritz MASERA Corporation Tulsa, OK

Introduction The Devonian Chert of the Black Warrior and Arkoma basins is part of a regional chert accumulation across the southern North American Continent. In the Arkoma Basin the Devonian is called the Penters Chert in the subsurface and the Sallisaw Formation on outcrop (Figure 1). In the Black Warrior Basin the Devonian Chen is an unnamed formation (Figure 2). The Arkansas Novaculite and the Caballos Formation are equivalents found in the Ouachita and Marathon region, respectively. Other shelfward equivalents of

the novaculites are the Thirty-one Formation of the Permian Basin, Camden Chert of Tennessee, and Clear Creek Chert of Illinois. The Devonian Chert of the Black Warrior and Arkoma basins has a greater affinity towards its shelfal equivalents to the west, east, and north than to Ouachita equivalents to the south.

Whereas there has been little published on the shelfal equivalents, a large amount of

data exists on the Arkansas and Caballos novaculites. The source of silica and the depositional environment for the novaculites and cherts in general have been debated for years. A biogenic source for the silica has been accepted by most workers. Other theories for the silica source advocate alteration of volcanic ash and volcanism promoting growth of siliceous organisms and the production of siliceous sediments; however the evidence of

volcanism in the Ouachitas is scarce (limited to scattered tuffs in the Mississippian).

General agreement on the depositional environment has not been reached for the

novaculites with McBride (1989), Thomas (1988), Sholes (1978), McBride and Folk (1977), Folk and McBride (1976) interpreting the novaculites to represent deep-water depositional environments. Conversely, a shallow-water depositional model has been proposed by Lowe (1975), McBride and Folk (1977), Folk and McBride (1976), and Lowe (1989). By analogy to the underlying succession of shallow-water marine limestones of the Arkoma and Black Warrior basins, the Devonian Chert is also interpreted to be a normalmarine deposit. A normal marine fauna is typically present in the Devonian Chert and includes sponges, pelmatozoans, brachiopods, and bryozoans. Thomas (1988) suggested that there may be an intermediate shaly facies separating shelfal Devonian Chert in the Black Warrior Basin from the more basinward chert/novaculite facies to the southwest.

Stratigraphic Distribution and Geometry of the Devonian Chert Portions of the Devonian, especially in the Black Warrior Basin, exhibits some stratification of chert and carbonate. In the northeastern portion of the Black Warrior Basin the Devonian can be subdivided into an upper unit that is composed of limestone with minor amounts of chert, and a lower unit that is composed of mostly silica (Figure 3). However, where the Devonian is thickest in the Black Warrior Basin and throughout the the Arkoma Basin, the chert and carbonate occurs at several stratigraphic intervals (Figure 4). It is common for all the carbonates that are present in the Devonian to have a varying degree of silica present from highly siliceous to scarely siliceous.

227

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In the Black Warrior Basin the subsurface geometry of the Devonian Chert resembles a bank. A thick interval of the Devonian parallels the northwest-southeast Ouachita structural trend (Figure 2). This trend begins approximaely in northern Greene County, Alabama and continues to the northwest into central Mississippi. The northwestern limit is not well constrained due to the lack of subsurface information. To the southwest in the Black Warrior Basin, the Devonian Chert thins in a basinward direction and to the northeast it thins and disappears; possibly pre-Chattanooga/Woodford erosion has removed the Devonian from this area. The Devonian Chert (Penters Formation) is thin or absent in the western Arkoma (Oklahoma portion); however, it thickens to more than 250 ft in central Arkansas and in a similar fashion as in the Black Warrior Basin, a thick parallels the Ouachita structural front (Figure 1). Subcrop limits in eastern Oklahoma and northern Arkansas are erosional, but there is the possibility of non-deposition. Southern limits are inferred in western Arkansas, but in central Arkansas there is no indication of a southern subcrop limit.

Some of the variation in thickness of the Devonian Chert is related to preChattanooga/Woodford erosion and the development of karst conditions. Typically, near the northeastern subcrop limit in the Black Warrior Basin, the percent silica maps are 100% indicating that the uppermost carbonate unit has been removed, presumably by erosion.

Correlation

The Devonian Chen has a distinctive log profile and is generally quite easy to correlate in the subsurface. Log response is directly tied to the lithologic characteristics of the Devonian Chert which are turn related to diagenesis in particular karstification and silicification. Large amounts of replacive chert and detrital infill can give a distinctive "dirty carbonate GR response which contrasts sharply with the underlying carbonates.

Petrography The Devonian is composed of carbonate and chert with some intervals that are represented by a chert breccia which consists of angular to subrounded fragments to blocks

of chert in a fine-grained matrix of clay, silt and sand-sized carbonate, and chert (monocrystalline quartz).

Commonly, fossil fragments have been completely silicified or dolomitized and are difficult to recognize. In the more limy portions of the Devonian fossil fragments are

readily recognized and appear to represent a normal marine fauna which consists of pelmatozoans, brachiopods, mollusks, and sponge spicules. Radiolarian fragments were identified but are rare. An important aspect of the matrix is that it has been extensively dolomitized. Dolomite commonly occurs as euhedral rhombs encased in chert indicating formation during early diagenesis. Intermediate and later stage dolomites are also present and generally cements the breccias. Figure 5 represent the typical sequence of diagenesis for the Devonian Chen; however variations due occur. Although chert is the major constituent of the silicified breccia, chalcedony occurs as

clast coatings and infillings, linings of intraparticle porosity and sporadic patches within chert fragments. Some fractures are open in the extensively fractured breccia, wheras others are filled with chalcedony or dolomite.

232

DI AGENETIC SEQUENCE OF THE

DEVONIAN CHERT Mineral Paragenesis

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Figure 5

Typical diagenetic sequence of the Devonian Chen in the Black Warrior and Arkoma Basins. 233

Depositional Environment Silica Source

Important to the depositional model for the Devonian Chert, and any chert formation, is the source of the silica. This controversial subject has been debated for the novaculites in the Ouachita orogen. Several sources that have been hypothesized include crystallization by siliceous oozes, alteration of volcanic ash, volcanic ash which increases silica available for siliceous organisms, silicification of a limestone precursor, and biogenic sourced silica (McBride, 1989). The probability of a volcanic source for silica is not very likely in that ash beds have not been identified in the Devonian section; although, scattered tuffs are present in the Mississippian in the Ouachita Mountains. Silicification of a limestone precursor by a mixture of marine and meteoric waters has been proposed by Knauth (1979). In the mixing zone water that is saturated with silica

and undersaturated with calcium carbonate may cause silicification of the sediment; however, it is necessary for the sediments to be siliceous. Subaerial exposure and karstification during the pre-Chattanooga unconformity would have allowed the siliceous sediments to be flushed by meteoric waters facilitating the transformation from biogenic silica to chert .

The primary source of silica in the Devonian Chert in the Black Warrior and Arkoma basins is most likely biogenic. Siliceous fossils, specifically sponge spicules and rare radiolarian, have been identified petrographically. The occurrence of radiolarians is much less common than sponge spicules. The absence of siliceous fossils from some thin sections is to be expected because biogenic silica (opal) is unstable to metastable and with increasing diagenesis this mineral converts to more stable forms of quartz. The question arises as to why the apparent proliferation of siliceous organisms in the Devonian? Greater concentrate of silica in sea water may have occurred in several different

settings including areas of upwelling, areas where surface currents diverge (equatorial regions), and along the west coasts of continents (Calvert, 1974). Lowe (1975) proposes that upwelling occurred along the Paleozoic southern margin of North America, i.e., the Ouachita Basin and shelf area to the north (Figure 6). Siliceous organisms flourished with the advent of silica-rich waters resulting in an increase in siliceous sediments.

Depositional Model

The presence of sponges, pelmatozoans, brachiopods, mollusks, bryozoans, and radiolarian indicate that the Devonian Chert was deposited under normal marine conditions. The abundance of silica and siliceous organisms indicates that the Devonian Chert may have been deposited in an area where currents supplied silica-rich waters.

Although the water depth for the deposition of Devonian sediments is difficult to estimate, the geometry and constituents of the Devonian Chert indicate a high stand to shelf margin systems tract. The fauna that is present precludes deposition in a peritidaVtidal flat depositional environment. The absence of coated grains indicates that the formation

probably was deposited below active wave base; calcareous algae and coral were not present, possibly indicating deposition below the photic zone. Fragmentation of the calcareous fossil fragments may have been accomplished by bioerosion and/or intermittent storm events. It is also unlikely that the fauna present and the subsurface

234

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Sediments

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Convergent plate junction

Shales and mudstones

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Orogenic highlands and associated delta and alluvial sediments

<

Inferred relative plate movements

Transcurrent fault Craton

km

Figure 6

Ord.-Dev.

Late Devonian-Early Mississippian paleogeography, paleotectonics and local sedimentary facies around the western Atlantic Ocean (Lowe, 1975). Key to geographical locations-North America: MC-Mexico City; A-Austin, Texas; BBirmingham, Alabama; NY-New York City. Laurasia: L-London. Gondwana: D-Dakar. Senegal; B-Bogota, Columbia.

235

geometry of the Devonian would represent an abyssal or basinal deep-water environment of deposition. It is probable that the Devonian was deposited in a shelf environment, under normal marine conditions with water depths near or below the photic zone (Figure 7). The thicker areas of the Devonian Chert which parallel the Ouachitas may indicate proximating to the shelf edge.

Karstification and Diagenesis

In many ways the Devonian Chert is a product of karstification and diagenesis. Much of the silica, dolomite, and calcite formed diagenetically. It is possible to subdivide the diagenesis into early, intermediate, and late events (Figures 5 and 7). Early diagensis began just after and possibly during the deposition with dolomitization, silicification, and calcite cementation. Late diagenesis occurred after lithification, karstification, and some burial. Subaerial conditions, as a result of the pre-Chattanooga/Woodford unconformity, allowed karstification to occur.

Pre-Chattanooga/VVoodford unconformity and Karstification The updip limit of the Devonian probably represents an erosional edge. Subaerial conditions, karstification, and brecciation has been advocated for zones within the Arkansas

Novaculite and Caballos Chert (Lowe, 1989 and Folk and McBride, 1976) the down dip equivalents to the Devonian Chert.

Breccias have been identified at Devonian outcrops in the Ozarks and also in core Some of these breccias certainly represent subaerial exposure and and samples. karstification; however, later tectonic fractures also appear to be present and appear to crosscut earlier fractures.

During the pre-Chattanooga/Woodford unconformity subaerial conditions, erosion and karstification affected the Devonian sediments (Figure 7).

Meteoric waters

undersaturated with respect to silica would have quickly mobilized the metastable biogenic silica. Silicification would have occurred once these waters became saturated with respect to quartz.

During subaerial conditions meteoric waters infilitrated the Devonian sediment. A hydrogeologic profile formed with vadose and phreatic zones (Figure 7). Dissolution of carbonates is favored in the vadose and upper phreatic sones. It is also likely that any remaining biogenic silica would be either converted to quartz or go into solution to and later be precipiated as quartz. It is likely that some of the calcium carbonate (aragonite and calcite) was also dissolved during karstification. The clasts are predominately chert indicating that solution of calcium carbonate promoted the loss of support and resulted in mechanical failure and brecciation of siliceous intervals. The matrix or cement between clasts includes diagenetic silica, sand, and carbonates. It is probable that the sand was introduced during subaerial exposure. Some of the internal sediment between clasts is laminated.

Conclusion Many mature to senile karst breccias do not make good reservoirs due to the lack of porosity and general hetergeniety; however the good quality reservoirs are present in the Devonian in the Bonanza Field in the Arkoma Basin. The Penters Chert of the Arkoma

Basin and the Devonian Chert of the Black Warrior Basin are unusual in that their (Continued page 239) 236

EARLY DEVONIAN - DEPOSITIONAL

-1

S41_

SILICA RICH SALT WATER

SPONGE SPICULES and RADIOLARIA

2

MIDDLE DEVONIAN -- JUVENILE KARST

MIXING ZONE

--I.- METEORIC WATER CHERT

Figure 7

Devonian Chert depositional sequence showing (1) deposition, (2 and 3) early karstification during the pre-ChattanoogaNVoodford unconformity and (4) deposition of the Chattanooga/Woodford Shale.

237

LATE DEVONIAN - MATURE KARST

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Continued 238

composition is predominantly chert, chalcedoney and dolomite. All of these lithologies are subject to fracture development which provides both porosity and permeability pathways. Combined with the intercrystalline and vuggy porosity of the dolomite the Devonian Chert can develop excellent reservoirs which make good candidates for oil and gas exploration. WELL:

Huber Fargo No. 1

LOCATION:

4-10N-24E, Sequoyah County, Oklahoma

CORED DEPTH:

3092-3121 ftlog, (3090-3119 ft--core)

STRATIGRAPHIC 'UNITS: Hunton F (Penters Chert), Hunton E (Frisco) Analysis and Interpretation The Penters Chert (3092-3104 ft) is a collapse-breccia complex, which is stylolitized and contains steeply inclined stratification. Major constituents are chert, dolomite, and sand grains.

Chert replaced the precursor wackestone or packstone during karstification but

after a minor dolomitization episode, as is evident by small rhombs encased in chert. Larger and occasionally zoned dolomite occurs, along with sand grains, between chert clasts. A second dolomitization episode probably occurred during and after collapse. Inclined strata formed due to collapse and infill of sediments. Two stages of fracturing are indicated by cross-cutting relationships, one occurring before collapse, the other after collapse. Second stage fractures are more likely to remain open. A late stage of silicification is seen as pore lining megaquartz and chalcedony.

1.

3092 ft

Brecciated, dolomitic chert. Karst. 239

COMPANY

SHELL OIL

WELL:

Western Coal & Mining Company No. 3

LOCATION:

31-7N-31W, Sebastian County, Arkansas

CORED DEPTHS

7748-7751 ft

STRATIGRAPHIC UNITS: Penters Chert Analysis and Interpretation

Hunton F (Penters Chert)

is

a collapse breccia with steeply dipping strata.

Constituents include chert, dolomite and sand grains in order of abundance. The chert appears to have replaced a pre-existing limestone. The majority of the dolomite and the sand grains appear to have been formed or introduced during karstification and after silicification of the precusor rock. The rock is heavily fractured with fractures tending to remain open in the chert portion of the rock. Chalcedony and megaquartz lining and filling pores are late stage events.

1.

7749 ft

Penters Chert, showing fractures and dissolution features associated with karst development 240

WELL:

Greg Drilling Gilmore Puckett No.1

DEPTH:

1586-1596, and 1625-1645 ft

STRATIGRAPHIC INTERVAL:

Tuscumbia/Ft Payne, Chattanooga (?), and Devonian

FIELD:

Wildcat

LOCATION:

18-9S-9E Itawamba County, Mississippi

CORE DESCRIPTION:

At 1594 ft thpre is an undulatory contact between the Devonian Chen and a black shale which is interpreted to be the Chattanooga Shale. The contact appears to be erosional with the black shale having small clasts of chert incorporated in it. The chert is present in two cored intervals (1594-1596 and 1625-1645 ft).

The upper interval of chert (1594-1596 ft) is gray and some what dense (nonMost of the fractures and the intergranular spaces have been filled with carbonate cement; however tripolitic). Brecciation is present in this interval and is chaotic to fracture.

minor amounts of fracture porosity has been preserved.

The lower interval of the core consists of chert to tripolitic chat. Like the upper interval, the chert here is also brecciated with most of the breccia resembling a fracture breccia. Some oil staining is present in this interval.

z

1

1593 ft

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o

z

Contact (arrow) of the Devonian Chert and black shale, Chattanooga. Several clasts (c) of the chert are present in the shale.

2.

1594 ft

Chert breccia from the Devonian.

3.

1626 ft

Minor amounts of oil staining in a tripolitic portion of the Devonian. 241

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Photomicrograph of fractured chert with minor amounts of porosity. The material in the fracture consists of calcite (c), microquartz or chert (m), quartz grains/crystals (q), and minor amounts of pyrite (opaque). 242

WELL:

Pruet and Hughes Vaughan No. 1

DEPTH:

3500-3519 ft

STRATIGRAPHIC INTERVAL:

Devonian

FIELD:

Wildcat

LOCATION:

10-13S-19W Monroe County, Mississippi

CORE DESCRIPTION:

The interval from 3500 to 3519 ft is from the Devonian based on log correlations. This interval is composed of chert to tripolitic chert with minor amounts of dolomite and calcite. Dolomite generally is present as fine euhedral rhombs with in the chert. Calcite typically is present cementing fractures.

Brecciation is common throughout this interval of the Devonian. Because of breccia the original texture is hard to discern; fossil fragments and any other primary features are not recognizable. Secondary features are much more common such as fractures, internal sediments, and geopetals. Internal sediments typically are present in some of the fractures. Where present geopetals are located in fractures and consist of internal sediment that is overlain by calcite cement.

Fractures, small clasts of chert, and internal sediments are interpreted to have formed during kartification; however, most of the fracture porosity has been destroyed by cementation and sedimentation. There is some microporosity associated with the more tripolitic portions of the Devonian. Occasionally, stylolites segregate zones of microporosity.

3503 ft

Chert breccia. Brecciation appears to crosscut stylolite (arrow). Fractures are filled with both calcite and internal sediment which consists of very fine-grained carbonate and chert debris.

3505 ft

Fracture that

is filled

internal sediment (arrow) which is very finely

laminated. Several chert clasts are present at the top of the fracture. 243

Acknowledgements

The authors would like to thank Masera for their support in preparation of this paper and permission to publish it. Special thanks to Eldon Cox of the Oklahoma Geologic Survey for use of the Huber Fargo core and to Rick Ericksen of the Mississippi Department

of Geology for use of the Gregg Gilmore Puckett and the Pruet and Hughes Vaughan. Thanks also go to Shell for the use of the Coal and Mining No.4. Special thanks to Valerie Lindsey, Masera for typing the manuscript; Sandra PaskVan, Masera for assembly of the manuscript; and Rick Elliot for drafting the diagrams.

References Calvert, S. E., 1974, Deposition and diagenesis of silica in marine sediments in Hsu, K. J. and Jenkyns, H. C. eds. Pelagic sediments: of Land and under the Sea: Spec. publs. inter. assoc. sediment. v. 1. p. 273-299.

Folk, R.L. and McBride, E. F., 1976, The Caballos Novaculite revisited part I: origin of Novaculite members, Jour. of sed. pet. v. 46. p. 659-669. Knauth, L. P., 1979, A model for the origin of chert in limestone: Geology. v. 7. p. 274-277

Lowe, D. R., 1975, Regional controls on silica sedimentation in the Ouachita system: GSA Bull. v. 86. p. 1123-1127

Lowe, D. R., 1989, Stratigraphy, sedimentology, and depositional setting of pre-orogenic rocks of the Ouachita Mountains, Arkansas and Oklahoma, in Hatcher Jr., R.D., Thomas, W.A., and Viele, G.W., eds., The Appalachian and Ouachita orogen in the United States, Boulder, Colorado: GSA, The Geol. of N.A., v. F-2. p.575590

McBride E. F. and Folk, R. L., 1977, The Caballos Novaculite revisited: part II: chert and shale members and synthesis. v. 47. p. 1261-1286. McBride, E. F., 1989, Stratigraphy and sedimentary history of Pre-Permian Paleozoic rocks of the Marathon uplift, in Hatcher Jr., R.D., Thomas, W.A., and Viele, G.W.,

eds., The Appalachian and Ouachita orogen in the United States, Boulder, Colorado: GSA, The Geol. of N.A., v. F-2. p. 603-620. Sholes, M. A., 1978, Stratigraphy and petrography of the Arkansas Novaculite of Arkansas and Oklahoma: unpublished PhD dissertation Univ. of Texas at Austin.

Thomas,W. A., 1988, The Black Warrior Basin, Chpt. 16 in Sloss, L.L., ed., The Geology of North America, DNAG v. D-2 Sedimentary Cover--North American Craton: GSA Publ., p.471-492. Wise, S. W and Weaver, F. M., 1974, Chertification of oceanic sediments in Hsu, K. J. and Jenkyns, H. C. eds. Pelagic sediments: of Land and under the Sea: Spec. pubis. inter. assoc. sediment. v. 1. p. 301-326.

244

PALEOKARST WITHIN THE KNOX GROUP OF ALABAMA, EAST SIDE OF THE BLACK WARRIOR BASIN James Lee Wilson Consultant New Braunfels, TX

Patrick Medlock MASERA Corporation Austin, TX Roger SeIs Amoco Production Company Houston, TX

Introduction Rocks of the Cambro-Ordovician systems are exposed in the Appalachian fold belt in the states of Tennessee, Virginia, Georgia, and Alabama, and extend westward from the southern Appalachians under the Black Warrior Basin of Alabama-Mississippi. These strata are practically all carbonate and were deposited on the eastern and southern passive margins of the North American craton. The great thickness of strata (up to 7000 ft. for the Lower Ordovician to Middle Cambrian) constitutes what R.N. Ginsburg has termed the Great American Bank, a vast Early Paleozoic carbonate platform extending from Newfoundland down the Appalachians, completely across the southern part of North America, and part way up the western part of North America, thus encircling more than half of the craton. The typical bank facies extends far up on the eastern craton, its characteristic strata being present as far north as the Canadian border. Throughout this extensive area the CambroOrdovician facies are very similar (Figs. I and 2). The Middle and Upper Cambrian develop sandy "pinch out" edges along the Transcontinental Arch and are onlapped by the Lower Ordovician. Practically all of the Lower Ordovician carbonate facies consist of shallow, restricted marine, dolomitized, upward shoaling, subtidal to tidal flat cycles. These occur over a broad area of more than 1000 miles (e.g., from the Oneota-Shackopee dolomites of Wisconsin to the Knox Group of the Warrior Basin in Alabama (Fig. 3 and 4).

The Guy Smith core of Morgan County (sec. 19-T6S-R3W) of northern Alabama which is displayed for examination is a part of a series of long cores (3000 to 5000 ft. each)

taken by Amoco Production Company, Atlantic Richfield (Arco), and Humble Oil Company. Four of these stratigraphic tests form a line along, and west of, the Appalachian fold-thrust belt front. These are from south to north: cores #3 (Collins) and #4 (Collins) close to Cambro-Ordovician outcrops in Bibb County, Alabama; #10 (Snead) in Etowah County, Alabama some 80 miles to the north; and the Amoco Guy Smith well in Morgan

County, Alabama, about 50 miles west of the Snead test and far in front of the Late Paleozoic Appalachian fold-thrust belt (Figure 5a and 5b).

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Paleogeography of Early Middle Ordovician (Kay, 1951) at time of post-Sauk unconformity (Whiterockian Stage) in North America.

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Post-Sauk Unconformity

In this core the top of the Knox is at a depth of 1531 ft and is marked by an important unconformity present over most of North America. In the Guy Smith core it is marked at depth 1531, (1) by a sharp contact of differing rock types and a lithoclastic conglomerate, (2) by green shale and limestone partly filling a vug with overlying spar, forming a geopetal structure, and (3) by extensive dolomitization below the "break". Lower Ordovician conodonts of about Middle Kindblade age or younger occur 137 ft. lower in the Knox indicating some erosion or non-deposition of the upper Knox in the Guy Smith core. About 50 ft. above the 1531 ft. "break" Middle Ordovician conodonts are present.

Regionally the post-Sauk unconformity is present over all neutral and positive elements across the North American craton. It is absent only within the center of the Michigan Basin, the Southern Oklahoma Aulacogen, within the Reelfoot Rift, and within the southern part of the Black Warrior Basin. Usually the Whiterockian Stage (Middle Ordovician), or part of it, is missing at the unconformity. It thus represents at most a few million years in time; strong physical evidence commonly exists to generally separate the dolomitized Cambro-Ordovician from the much less dolomitized Middle Ordovician. Mussman and Read (1986) list many well-known paleokarst criteria useful in recognizing the unconformity in field studies in Virginia and Tennessee. Among these are: (1) eroded topographic highs on the underlying Knox surface, (2) sinkhole depressions filled with carbonate detritus and boulders, or fossiliferous marine carbonate, (3) discordant bodies of dolomite below the unconformity which may represent cross sections of filled caverns, (4) laminated argillaceous water-laid sediment, now dolomitized, representing meteoric cave fills, (5) breccia beds indicating compacted and collapsed caverns beneath the unconformity. A predictable sequence of breccia types can be discerned in some of these collapse zones (Kearns, 1988) and is discussed below.

Depositional Cyclicity and Use of Fischer Plots in Correlation Upward shoaling tidal flat cycles from the Cambro-Ordovician of the central and southern Appalachians have been exhaustively described by J.F. Read and students over several years of study (Read, 1989; Bova and Read, 1986; Koerschner and Read, 1988; Montanez and Read, 1992). The same types of cycles occur in outcrop exposures and core of the Lower Ordovician (Fig. 6). An ingenious way of graphing thickness variations in a column of cyclic sediments

was devised by A.G. Fischer (1964) who plotted cumulative cycle thicknesses on the horizontal axis which represents time subdivided into equal periods. (Read and Goldhammer, 1988; Montanez and Read 1992; Sadler, Osleger, and Montanez, 1992) (preprint). When a graphical representation is made of stacking patterns of tidal flat cycles taking into consideration variation from mean subsidence, these parasequences may be naturally grouped into sequences of thinning and thickening cycles. Some groups are believed to represent regressive periods, thinner cycles indicating less accommodation space for sediments due to falling sea level. Transgressive periods are marked by thicker cycles

owing to sea level rise plus regular subsidence which resulted in more accommodation space.

Despite some questionable assumptions such as conceiving eustatic variations at a regular time period and assuming only regular linear subsidence, the Fischer plot device appears to be useful in regional correlation at the level of "third order" cycles Figures 7a and 7b indicate similarity in Fischer plots; the larger perturbations are interpreted to (Continued page 256) 252

Desiccated Facies with Laminated Anhydrite and Anhydrite Nodules

Desiccated Facies Sandstone Facies Laminated Facies (mm-scale)

Stromatolitic Facies (LLH Type)

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Laminated Facies (cm-scale)

Digitate \Stromatolitic4

;

Facies

Flat Pebble Congi2merate Faci Pelmicrite Laminated Facies -I

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,

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Shni Thrombolitic Facies

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1

Figure 6

Typical shallowing upward cycle of the Knox Group.

253

Fischer Plot of Cycles On Interstate 35, Oklahoma Arbuckle Mountains

Mid K

Low K

K = Kindblade Fm. WSC = West Spring Creek Fm. S = sandstone HC JC = Honeycut-Jeff City equiv. (biostratigraphic marker) C = cycle number

C 88

C 57 WSC

est top HC JC

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13 m below top WSC

29 m to base of K Top HC JC

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Cindy top

Vertical scale for Arbuckle Mt. cycles

Ranger Peak

Fischer plot of El Paso Limestone o Franklin Mountains, El Paso.

80 m to top of El Paso Fm.

Figure 7a Comparison of Fischer (accomodation) plots of El Paso and Arbuckle Mountain outcrops of upper half of Lower Ordovician. Note that thickness of total Lower

Ordovician at El Paso is about 1/3 that of equivalent strata in the Arbuckle exposures of the Southern Oklahoma aulacogen 254

-4.-- KINGSPORT FM.

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Figure 7b Fischer (accomodation) plots at same vertical scale as Figure 7a, of total Lower Ordovician of Appalachian outcrop sections in Tennessee and Virginia (from Montanez and Read 1992) X-Y-Z peaks are marked to enable comparison with Fischer plots of Figure 7a. 255

represent major eustatic variations in sea level which are seen from the El Paso and Arbuckle Mountains sections all the way to Virginia and Tennessee (Montanez and Read, 1992). Fischer plots of the higher-middle part of the Lower Ordovician (McKelligon

Canyon Fm., El Paso; Honeycut of Llano Uplift, Ellenburger; Kindblade Fm. of the Arbuckle Group; Jefferson City of the Ozark sections; and Newala of the Alabama Knox Group) persistently show an ascending pattern owing to stacking of thicker, more subtidally dominated cycles (y). This pattern possesses subsidiary "bumps" which can also be widely correlated (x). The decline of slope marked by numerous sandy-shaly thin cycles followed by a minor rise (z) is typical for highest beds of the Lower Ordovician. In the Guy Smith well the Kindblade rise is present, but the uppermost (West Spring Creek) decline and rise are not observed. Perhaps this indicates some erosional truncation at the post-Sauk unconformity (Top of Knox Dolomite) in this area of Morgan County, Alabama. Biostratigra ph ic Control

Since the studies of Butts (1926), Bridge (1930), Cloud and Barnes (1948), and Cullison (1944), the restricted marine Lower Ordovician east of the Transcontinental Arch has been divided into about a half dozen general biostratigraphic zones based chiefly on gastropods, sponges, and a few brachiopods and trilobites. Derby et al. (1991) has the most up-to-date review. In recent years conodont studies by Ethington and Repetski (1984),

Ethington and Clark (1981), Repetski (1982), and Harris et

al.

(1979) have aided

substantially in the biostratigraphy of Lower Ordovician strata, particularly because larger invertebrate remains are almost impossible to extract from core material. Conodont zonation has to date been defined in at least two standard sections of reference: El Paso Group and Arbuckle Group, both in predominantly limestone facies. To date, however, the

biostratigraphic conodont subdivisions of the Lower Ordovician have about the same precision as those of the gastropod-sponge faunas. The most detailed subdivision of the Lower Ordovician shelf faunas is to be found in the Cordilleran sections studied by Ross (1976) and Hinze (1952, 1973) and based principally on brachiopods and trilobites. The ascending Fischer plot pattern of the middle Lower Ordovician lies above the Lecanospira zone and accords with the Honeycut-Jefferson City fauna (plus some younger beds). This

fauna is characterized by abundance and rapid evolution of the gastropod operculum Ceratopea. The brachiopods Xenelasma and Finkelnburgia, the trilobite Jeffersonia, and the coiled nautiloid Tarphyceras also characterized this fauna. The sponge Archaeoscyphia and the supposed dasycladacean Calathium are also common. It is approximately the algal-

sponge unit described by Alberstadt and Repetski (1989) who noted that grains of the supposed algal form Nuia were also common in these strata.

Specific Information About the Amoco Guy Smith Core The Guy Smith well is located in Sec. 19-T6S-R3W in Morgan County in northern Alabama. It was cored continuously from top to total depth of 5218 ft and the core was

slabbed and excellently prepared by Amoco Production Company. It is from 1 to 3 inches in diameter. This well is located about 30 miles west of the Sequatchie anticline, the frontal fold of the Appalachian belt, and penetrated structurally undisturbed strata. Extensive

brecciation occurs in the Knox strata but no faulted or structurally dipping beds were

identified. The section begins in the Mississippian carbonates of the Ft. Payne Formation

and reaches down to well within the Middle Cambrian and thus includes a complete Middle and Lower Ordovician section. Wells logged from ditch cuttings from 15 to 50 miles away enable good correlation of stratigraphic units. These correlations accord with those derived from petrophysical logs as well.

256

Tectonically, the well is located on the northeastern shelf of the Late Paleozoic Black Warrior Basin. It permits observation of typical platform interior facies of the Cambro-Ordovician Knox Group as well as 666 ft. of overlying Middle Ordovician. Like all the Cambro-Ordovician of the Great American Bank, the total Knox and Middle Cambrian section is composed entirely of upward-shoaling tidal flat to shallow subtidal sedimentary cycles. (See Figure 6). An ideal such cycle has a pebbly lag at the base, grades upward to a foot or so of peloidal grainstone, and is overlain by a few more feet of cm-banded dolostone which is dark at the base and lighter brown above. In some cycles a mass of thrombolite (mottled algal micrite) will occur in this position. The cycle is topped by several feet of light grey fenestral laminite with a sharp upper contact with the next overlying subtidal unit. Even in sections dominated by limestone the laminite is almost entirely very fine rhombed dolomite. This is the most persistent and easily traceable unit of the cycle. The percentage of tidal flat rock types in cycles of three wells is almost the same-about 40% (Collins #4 = 45%, Snead #10 = 38%, and Smith #1 = 44%). In the whole Cambro-Ordovician (Knox-Bibb-Ketona-Brierfield) section from 1531 Average cycle thickness is therefore 16.5 ft. but with a wide variation. Hooks (1985) determined about 40

to 5218 ft.) there are 224 such cycles through the 3687 foot section.

cycles through 1280 ft. of Lower Ordovician (aver. 31 ft/cycle) and 82 cycles in the underlying 1830 ft. of Upper and Middle Cambrian (aver. 22 ft./cycle in the Snead #10 well of Etowah County, Alabama. The cyclicity is made irregular by a few thicker units (35-70 ft.) of thrombolite or breccias. Assuming 62 million years for the Lower Ordovician-Late Cambrian-Middle Cambrian each cycle might be about 280,000 years but the error inherent

in absolute age dating in the Early Paleozoic, as well as the assumption of continuous sedimentation during cycle formation, makes this an equivocal figure.

The following are some notes on special rock types seen in the cyclic Upper Knox portion of the section.

Fossils are extremely rare in the Knox strata of this core. Usually gastropod steinkerns are visible in Lower Ordovician rocks, but only a few of these were observed here in the upper Knox despite the large amount of fine-grained limestone. Even trilobite tests are rare. The grainstones are not generally bioclastic but consists of peloids.

Peloids in the grainstones or peloidal mudstones are of uniform size, from .5 to 1 mm in diameter; they are generally well-sorted. Oolite is present but rare. In the lower part of a graded sequence lithoclasts commonly occur, composed of lime mudstone; in places they are flat and rounded and are termed pebble conglomerates. In some dark coarsely dolomitized rock it is difficult to distinguish pelleted micritic sediment from grainstone.

Lag deposits at the base of some cycles have coarser grains including lithoclasts

with blackened rinds and rounded shapes, and a variety of grain sizes (poor sorting); matrix is peloidal and some units are graded, fining upward.

lags may be difficult to distinguish from normal pebble conglomerate. Lags are usually only a few cm thick. Basal contacts of lags are

Lithoclastic

always sharp.

Tops of cycles, particularly in the lower portion of the core, may consist of dark chocolate-colored homogeneous dolomudstone, but more commonly the top of each cycle is marked by distinctive gray-green millimeter-planar

257

laminated dolomudstone; this may be associated with a mudstone with

"patterned or crumpled-contorted" fabric which may be fenestral. The graygreen color is caused by disseminated pyrite which also occurs in tiny blebs and indicates a reducing environment. This rock type is considered to represent tidal flat deposition along natural levees and other relatively high places on the tidal

flats where lime mud sediment

is

glued and trapped by cyanobacteria.

Occasionally algal stromatolites occur, both digitate and laterally linked hemispheroidal forms, but planar laminite is most common. 5.

Thrombolite algal buildups are mottled micrite (looking almost bioturbated) in

which dark splotches contain the microscopic algal structure Girvanella,

Renalcis and Epiphyton. Thrombolites may be capped by stromatolites. They were originally somewhat porous because (a) they are more coarsely dolomitized, (b) their void spaces are commonly filled with kaolinitic-tripolitic internal sediment, (c) they are commonly replaced by chert, and (d) vuggy porosity is almost always confined to thrombolitic units. A matrix of peloidal

grainstone occurs generally between the heads of thrombolites or digitate stromatolites and in some places a foot or so of peloidal grainstone or oolite underlies these algal heads.

Comparison of core description with petrophysical logs on Guy Smith #1 (Amoco) well in Morgan County, Alabama Some cycles may be recognized on gamma ray logs when they are used with sonic, neutron, and density logs (Wilson et al., 1992, Figures 8 and 9). The lower, or subtidal portion of the cycle shows a more massive cleaner response on the gamma ray, whereas the upper, tidal flat portion is often more serrated and irregular because of its slightly increased terrigenous component. Many minor sharp "blips" equate with laminites and help to identify the upper parts of cycles. Sharp gamma ray deflections, unfortunately, are not confined to the intertidal part of the cycle but in places occur in the subtidal part apparently induced by thin seams of argillaceous material within pure carbonate. Other significant petrophysical log responses include:

About 30-40 ft. below the unconformity at 1531 ft. there is a green shale which accords with a strong gamma-ray/neutron deflection.

Throughout the core from 1480 to 3150 ft. there are low density "blips" caused by microporous tripolite, internal sediment, or chalky dolomite. Chen beds and large nodules also cause low density responses. Limestones can be noted by movement of the density curve on or slightly inside the general neutron line. E.g., from 1720-1870 ft.

Breccias are not always indicated by gamma ray and neutron density logs. E.g., the first main breccia from 2415-2510 ft. does not show on petrophysical logs because it is cemented tight and does not have a relatively high gamma ray response in internal sediment (Fig. 10). E.g., breccia can be readily discerned from 2330-2360 and 2700-2800 ft. along with internal sediment from deflections on gamma ray and neutron logs (Fig. 10). Base of Chepultepec = top of Copper Ridge and second main breccia is at 27602800 ft. on petrophysical log. (Continued page 262) 258

Subtidally Dominated Cycles Laminite

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Figure 10 Two collapse breccias in Guy Smith No Gamma-ray and porosity logs in collapse Zone A exhibit minimal response to breccia. The lower portion of I

collapse Zone B has marked increase in radioactivity owing to internal sediment and cave fill.

261

Breccia down in the Copper Ridge at 3 100-3 150 ft. is marked by strong density and neutron deflections, but not much gamma ray deflection.

Reservoir quality appears poor base on petrophysical log calculations and whole core analysis. Although over 100 ft of porosity exceeding 4% exists in the

Knox, porous zones are thin and scattered. The most effective porosity is a combination of vuggy and fracture.

TOPS IN THE GUY SMITH WELL FEET Top of Middle Ordovician 865 (Conodonts at 1482 ft , about 50 ft. above base of Middle Ordovician Stones River Group are Chazyan (Tulip Creek-McLish) upper Whiterockian in age) Unconformable top of Knox (correlates with unconformable top of Knox at 30 ft. below top in Snead #10 well, Etowah County) Newala-Odenville conodonts (not earlier than Middle Kindblade)

1531

.

1658

Top of first major breccia sequence (correlates with 640 ft. in Snead #10 well; thickness of 659 ft. of Knox above upper major breccia in Guy Smith well, and 610 fi. in Snead #10 well)

2190

Top of Copper Ridge and top of 2nd series of major breccia (correlates with 1170 ft. in #10 Snead well, also with the top of Copper Ridge picked by Hooks, 1985). Although traditionally selected as the boundary between Upper Cambrian and Lower Ordovician, this is not a precise contact, but very gradational. Usually increase of chert and sandy carbonate and thorough dolomitization are used to mark the boundary. (Thickness of Copper Ridge is 1230 ft. in Guy Smith well and 1170 ft. in Snead #10).

2720

3950 Base of important breccias Top of abundant grainstone units at cycle tops and supposed top of Bibb-Ketona-Brierfield Middle Cambrian unit. (correlates with 2380 ft. in Snead #10 well at total depth in both cores strata were still in cyclic platform carbonates. BKB is 1268+ ft. thick in Guy Smith and 620+ ft. thick in Snead #10; both are incomplete sections). T.D.

5218

(T.D. in Snead #10 at 3000 ft.)

Paleokarst-Collapse Breccias in Guy Smith and Adjacent Wells The several types of breccias and models for their genesis have been discussed by Blount and Moore (1969), Morrow (1982), Kerans (1988 and 1990), and Wilson et al., 262

To interpret breccia genesis several factors need consideration: (1) location of breccias above or below regional unconformities, i.e., how deep within the formation do breccias occur; (2) relationship of breccia to depositional facies; (3) systematic vertical sequence of breccia types; (4) lithology of clasts: rock types, shape and size of clasts, gross textural relations, bedding styles; (5) internal sediment, its color and mineralogy, chert and detrital sand content; (6) susceptibility to later diagenesis such as dolomitization and silicification; (7) relationship to ease of tectonic fracturing; and (8) presence or absence of porosity in breccias, including occurrence of vuggy or retained primary interclast pore (1992).

space.

Kerans in 1988 noticed that a few hundred feet below the unconformable top of the Ellenburger in West Texas a tripartite predictable sequence of breccia types commonly occurs resulting from cavern collapse. From bottom to top these are: a lower collapse zone composed of rubble fallen from the cavern roof, a middle zone composed of cave-fill rubble plus matrix of sandy or argillaceous or fine carbonate debris washed through the collapsing

cavern, and an upper zone of fractured but not badly displaced cave roof carbonate. The sequence is widely visible in all areas of West Texas Ellenburger production and has been seen also in the Knox cores in the area under consideration here. It thus forms a valuable model for interpretation of cavern collapse history and for porosity prediction in local field areas.

Petrographic Description of Breccias Cambro-Ordovician breccias have been classified by various authors (Hooks, 1985, Ijirigho and Schreiber, 1988, and Kerans, 1990) Much of the terminology developed by petroleum geologists has a foundation in earlier studies by ore geologists (Ohle, 1959 and 1985, Kyle, 1976, and Sangster, 1988). Knox breccias can be classified using similar nomenclature. Three basic breccia types seem to exist: (1) fracture or crackle breccia (2) mosaic, and (3) chaotic or megabreccias. Further subdivision of the chaotic breccias can be based on matrix- versus clast-support and the presence and type of matrix.

Fractured or Crackle Breccias Fractured or crackle breccias have more than one mode of origin; these breccias may form from either tectonic stress or during cavern collapse where the roof or walls lose support and where mechanical failure results. Generally, there is no displacement in fracture breccia and the clasts are angular and of homogeneous texture (Plates 1.1, 2.4, and 2.5). This breccia type has a network of closely spaced fractures that may not be open. In open fracture systems significant porosity can occur. However, most fracture breccias tend to be closed, cemented by sparry calcite or dolomite; less commonly, internal sediment fills the fractures. Mosaic or Fitted Breccias Mosaic or fitted breccias constitute actually a transitional type between chaotic and fractured breccias Whereas fractured breccias have no displacement between clasts, mosaic breccias have some displacement between angular rock fragments But, unlike chaotic breccias, the original position of clasts in mosaic types can be determined and can be

visually fitted back into place, there is only minor displacement of the angular clasts

263

(Plates 2.4 and 3.3). Mosaic breccias may be porous but also, like fracture breccias, sparry cements or internal sediment may close pore spaces

Chaotic Breccias There is much variation in the textures of chaotic breccias. Clasts can be angular, subrounded and even-rounded, and may cut across both bedding and rock types in the section. It is evident that the sediment was well-lithified prior to brecciation. Clast size is

quite variable with diameters from 1 cm to several centimenters being common; large boulder-sized clasts are occasionally present. The lithology and texture of chaotic breccias may be homogeneous (Plates 1.6 and 2.1) or polymictic (Plates 1.4, 2.3, and 2.6). The

most obvious feature of chaotic breccias is that there has been displacement of clasts presumably due to void space collapse. Subdivision of chaotic breccias can be based on matrix-supported (Plates 1.3, 1.5, and 2.3) and clast-supported textures (Plates 1.4, 3.2, 3.3, 3.4, 3.5) with the latter being the

most common. Further subdivision can be done based on type of matrix: cement versus internal sediments (Plates 1.5 and 2.4). Several different cements are present in breccias including dolomite, calcite, silica, and in rare places metallic sulfide minerals. Matrix supported textures rarely have any porosity preserved, whereas clast supported textures may or may not have preserved pore space.

Internal Sediments Within collapse breccias internal sediments are very common. They do not consist merely of insoluble material; rather there exists a wide variety of sediments, some of which include precipitates of dolomite, silica, clay, and mixtures of these components. The most common type is a buff-colored, fine-rhombed dolomite with minor to abundant siliciclastic material. Internal sediments are commonly laminated. They can be a late stage event occurring after thermal dolomite, megaquartz, and late calcite, but generation and deposition of internal sediment prior to mineral precipitation also occurs. Tripolitic chert is a common internal sediment in silicified collapse breccias.

Timing of Brecciation But when do the breccia sequences form? Are cavern formation and collapse almost

contemporaneous? Do the breccias represent a single major event under an important unconformity with very deep water tables or do they form pencontemporaneously at multiple surfaces at cycle tops at the scale of parasequences ("4th and 5th order" cycles). Answers to these and other questions may be derived from detailed study of breccias and their total geological setting.

As noted above, the Guy Smith well is one of several extensively cored stratigraphic

tests by Humble Oil Company, Arco, and Amoco along the Appalachian front of northeastern Alabama. Breccia beds from 10 to more than 40 ft thick occur in the Knox in all these tests whose Lower Ordovician strata have been described by Sternbach (1984) and Hooks (1985). The Knox in all of these cores contains solution-collapse breccias, some more than 2000 ft. below the post Sauk unconformity. Petrographic study of the breccias reveals that most of them are polymictic, derived from lithified rock and are angular to subrounded, Where cemented, contacts rotated in the matrix and a few are uncemented. 264

between cement and clasts are sharp. In some breccias there is much coarse, veinous hydrothermal-type dolospar with baroque crystals and vugs. Hooks (1985) noted that some breccias have a matrix of comminuted clasts and rubble and that others contain identifiable internal sediment which is either peloidal micrite or greenish "clay" composed of finegrained carbonate and quartz with very few clay minerals. Rarely, reddish internal sediment is noted. Some internal sediment appears by its laminated or graded structures to be gravity

or traction-laid, but other types, such as the "greenish clay" may have been precipitated from solution. Hooks concludes that his breccia types are cavern collapse in origin. The present writers agree. They correspond to Keran's (1990) chaotic breccia in the cave fill sequence.

Rarely, finely fractured tectonic breccia with fitted clast contacts resembles the "crackle breccia" of some authors, or the cave-roof fracture breccia of Keran's (1988 and 1990) but in the Knox some of this may be a later feature related to stress induced by nearby Late Paleozoic Appalachian folding.

Within the five deep stratigraphic tests studied, 30 to 40 breccias appear in each core. In the Amoco Guy Smith core the upper 900 ft. of the Knox contains only minor

breccias, but in the interval 2428-4920 ti. (through about 2500 ft. of section) at least 14 breccias occur, a few of them about 40 ft. or more thick. It is believed that two major breccias, one in the upper Knox and one at top of Copper Ridge can be correlated between Guy Smith and Snead wells, a distance of 50 miles (Figure 5b). Most of the breccias, including characteristic cave fill sequences (Kerans, 1988 and 1990) occur in the Copper Ridge Formation below 2720 ft., some 1200 ft. down in the Knox.

Beds above each of 92 breccias from various cores were recorded to ascertain what preferred rock types are brecciated. Peloidal sediment, bioturbated peloidal sediment, and dolomudstone each are associated with about 20% of the breccias. Attention was also given to clast lithology. Very few breccia beds had clasts of thrombolite, stromatolite, or grainstone. Obviously, the breccias are preferably associated with normal marine, subtidal, thick-bedded dolomite strata and not directly with high intertidal or supratidal environments where laminite and stromatolite formed.

As in the Ellenburger and Arbuckle groups, these solution-collapse breccias may occur hundreds of feet below the top of the Knox. The lowest major breccias in the long cores examined occur as much as 2000 ft (600 m) below the top of the Knox and minor breccias occur even deeper in the Guy Smith well.

Hooks (1985) divided the outcropping Knox sections of central Alabama into 5 major units of regression (perhaps of "third order" scale). Possibly unusual drops in sea level during some of these major regressive phases could have been associated with multiple water tables and karst levels as the great pile of Cambro-Ordovician sediment was building up. But no rock types exist to show that meteoric water tables were regularly associated with intertidal phases of the smaller scale cycles. Little evidence of karst exists at tops of minor cycles within Knox strata and few oxidized regoliths are seen. In summary, most Knox breccias were formed by collapse of well-lithified rock into caverns formed by solution by meteoric water. Substantial erosion of carbonates occurred to provide infilling internal sediment. The ideal cavern collapse profile of Kerans (1988) is

seen in some Knox breccias, but not all.

Some of the major breccias correlate over

distances of scores of miles meaning that certain times of great regional sea level lowering might be responsible. So many breccias were formed so far beneath the Middle Ordovicianpost Sauk unconformity (down to 2000+ ft.) that it seems likely that periodic

265

sea level drops during Knox deposition could have been responsible for cave development at various levels, particularly at regressions related to "3rd order" cycles.

It is possible that water tables existed more than 2000 ft. below the post-Sauk unconformity and that solution below this one surface was solely responsible for cavern formation. No major disconformities are indicated within the Knox Group by presently understood biostratigraphy. However, if the single post-Sauk unconformity was responsible for the deeper water tables, a very considerable amount of sea level lowering must have taken place, more than regional stratigraphy indicates.

Acknowledgements The authors would like to thank Amoco and Masera for their support in preparation of this paper and permission to publish it. Special thank to Vernon Buchanan, Amoco for preparing the core; Ed Manning, Amoco for core photography; Dell Wilson and Valerie Lindsay, Masera for typing the manuscript; Sandra PaskVan, Masera for assembly of the manuscript; and Rick Elliot and John Nichols for drafting the diagrams.

266

References

Alberstadt, L. and Repetski, J.E., 1989, A Lower Ordovician sponge/algal facies in the southern United States and its counterparts elsewhere in North America: Research Reports, Palaios, v. 4, p. 225-242. Blount, D.N., and Moore, C., 1969, Depositional and non depositional carbonate breccias, Chiantla quadrangle, Guatemala. Geological Society of America Bulletin, v. 80, p. 429-442. Bova, J.A., and Read, J.F., 1986, Incipiently drowned facies within a cyclic peritidal ramp sequence, Early Ordovician Chepultepec interval, Virginia Appalachians: Geological Society of America Bulletin, v. 98, p. 714-727.

Bridge, J., 1930, Geology of the Eminence and Cardareva Quadrangles: Missouri Bureau Geology and Mines, 2d Series, v. 24.

Butts, C., 1926, The Paleozoic rock of Alabama, in Adams, G.T., Butts, C., Stephenson, L.W., and Cooke, W., eds. Geology of Alabama: Geology Survey of Alabama, Special Report No. 14, P. 40-230. Cloud, P.E. and Barnes, VE., 1948, The Ellenburger Group of Central Texas: University Texas Pub., 4621, 473 p. Cullison, J.S., 1944, The Stratigraphy of some Lower Ordovician formations of the Ozark uplift: University Missouri, School of Mines and Metallurgy, Bull. Tech. Ser., v. 15, no. 2.

Derby, J.R. et al., 1991, Biostratigraphy of the Timbered Hills, Arbuckle, and Simpson groups, Cambrian and Ordovician, Oklahoma: Oklahoma Geological Survey Cir. 92, p. 15-41.

Ethington, R.L., and Clark, D.L. 1981, Lower and Middle Ordovician conodonts from the Ibex area, western Millard County, Utah: Brigham Young University Geology Studies, v. 18, part 2, 155 p.

Ethington, R.L., and Repetski, J.E., 1984, Paleobiogeographic distribution of Early Ordovician conodonts in central and western United States: in Clark, DL., ed., Conodont Biofacies and Provincialism. Paper 196, p. 89-101.

Geological Society of America Special

Fischer, A.G., 1964, The Lofer cyclothems of the Alpine Triassic: in Merriam, D.F. ed., Symposium on Cyclic Sedimentation, Bull. 169, v. 1, p. 107-170.

Harris, AG., Bergstrom, S.M., Ethington, EL., and Ross, R.J., Jr., 1979, Aspects of middle and upper Ordovician conodont biostratigraphy of carbonate facies in Nevada and southeast California and comparison with some Appalachian successions: Brigham Young University, Geology Studies, v. 26, pt. 3, p. 7-33, 5 pis.

Hintze, L.F., 1952, Lower Ordovician trilobites from western Utah and eastern Nevada: Utah Geological and Mineralogical Survey, Bull. 48, 249p., 28 pls.

267

Hintze, L.F., 1973, Lower and Middle Ordovician stratigraphic sections in the Ibex area, Millard County, Utah: Brigham Young University, Geology Studies, v. 20, pt. 4, p. 3-36.

Hooks, J.D., 1985, Stratigraphy, deposition environments, and silica diagenesis of the Cambro-Ordovician carbonate sequence, Appalachian Valley and Ridge, Alabama (M.S. thesis): Birmingham, Alabama, University of Alabama, 197 p.

Kay, M., 1951, North American Geosynclines: Geological Society of America, Memoir 48, 143 p.

Kerans, C., 1988, Karst-controlled reservoir heterogeneity in Ellenburger Group carbonates of West Texas: American Association Petroleum Geologists Bull., v. 72, p. 11601183

.

Koerschner, W.F., and Read, J.F., 1989, Field and modeling studies of Cambrian carbonate cycles, Virginia Appalachians; Journal Sedimentary Petrology, v. 59, P. 654-687.

Montanez, I.P., and Read, J.F., 1992, Eustatic control on early dolomitization of cyclic peritidal carbonates, evidence from the Early Ordovician Upper Knox Group, Appalachians: Geological Society of America Bull., v. 104, p. 872-886.

Morrow, D.W., 1982, Descriptive field classification of sedimentary and diagenetic breccia fabrics in carbonate rocks, Bull. Canadian Petroleum Geology, v. 30, P. 227-229.

Mussman, W.J. and Read, J.F., 1986, Sedimentology and development of a passive to convergent-margin unconformity-Middle Ordovician Knox unconformity, Virginia Appalachians: Geological Society of America Bull., v. 97, p. 282-295.

Read, J.F., 1989, Controls on evolution of Cambrian-Ordovician passive margin, U.S. Appalachians: in Crevello, P., Wilson, J.L., Sarg, J.F., and Read, J.F. eds., Controls on Carbonate Platform and Basin Development, S.E.P.M. Spec. Pub. 44, P. 147166.

Read, J.F., and Goldhammer, R.K., 1988, Use of Fischer plots to define 3rd-order sea-level curves in peritidal cyclic carbonates, Early Ordovician, Appalachians: Geology, v. 16, p. 895-899.

Repetski, J.E., 1982, Conodonts of the El Paso Group (Lower Ordovician) of westernmost

Texas and southern New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 40, 121 p.

Ross, R.J., Jr., 1976, Ordovician sedimentation in the western United States: in Bassett, M.G., ed., The Ordovician system: proceedings of Paleontological Association symposium, Birmingham, September 1974: The University of Wales Press and National Museum of Wales, Cardiff, p. 73-106.

Sadler, P.M., Osleger, DA., and Montanez, I.P., 1993, On labeling, length, and objective basis of Fischer plots: Journal of Sedimentary Petrology, v. 63, p. 360-363.

268

Sternback, L.R., 1984, Carbonate Facies and Diagenesis of the Cambrian-Ordovician shelf and slope-margin (The Knox Group and Conosauga Formation) Appalachian fold belt, Alabama: M.S. Thesis, Rensselaer Polytechnic Institute, Troy, N.Y., 165 p. Wilson, J.L., 1975, Carbonate Facies in Geologic History: New York, Springer-Verlag, 471 P.

Wilson, J.L., Fritz, R.D., and Medlock, P.L., 1991 The Arbuckle Group--relationship of core and outcrop analyses to cyclic stratigraphy and correlation: Oklahoma Geological Survey Spec. Pub. 91-3, p. 133-144.

Wilson, J.L., Medlock, P.L., Fritz, R.D., Canter, K.L., and Geesaman, R.G., 1992, A review of Cambro-Ordovician breccias in North America: in Candelaria, M.P., and Reed, C.L. eds., Paleokarst, Karst Related Diagenesis and Reservoir Development: Permian Basin Section - SEPM, Pub. No. 92-33, p. 19-29.

269

Plate 1 1,

2284 ft

Mosaic/fitted to crackle breccia. Only minor displacement of clasts some internal sediment (is) and sparry cement is present.

2285 ft

Fracture/crackle breccia with vuggy porosity. Many of the fractures are closed; however some are open and augment the porosity network.

2340 ft

Matrix-supported, chaotic collapse breccia; gray dolomite clasts are supported by internal sediment that consists of quartz sand, clay, and dolomite. Small clasts of chert (c) are also present.

2436 ft

Clast-supported collapse breccia. The clasts are composed of limestone and dolomite; note how textures differ between adjacent clasts.

2441 ft

Geopetal formed between internal sediment (is) and calcite in a matrixsupported breccia. The lower clast is oriented in a near vertical position.

2443 ft

Sparry calcite cementing dolomite clasts in a collapse breccia that appears to be cement supported

270

r

N

PLATE I

I

Plate 2 2463 ft

Chaotic collapse breccia. The matrix is composed of sand, clay and dolomite and the majority of the clasts appear to be burrow-mottled.

2480 ft

Collapse feature. Left side of the sample is essentially horizontal; this may

represent the wall of the cavern or it may represent a large clast that is oriented horizontal to the original depositional bedding. 2500 ft

Polymictic clasts in a matrix-supported, chaotic collapse breccia. Rock types that are present are laminated shale, dolomite, chat, and limestone.

2512 ft

Crackle breccia to mosaic breccia; there has been very little movement of clasts in this portion of the karst system. Fractures are filled with a coarse internal sediment that consists of dolomite and chert clasts. A geopetal (arrow) is present between calcite and internal sediment in one fracture.

2538 ft

Crackle breccia, there is no displacement of the clasts and most of the fractures are closed or filled with carbonate cement.

6.

2577 ft

Chaotic collapse breccia which is both matrix and clast supported. Most of the clasts are dolomite with some chert clasts present; the internal sediment is composed of tripolite.

272

273

PLATE 2

Plate 3 1

2618 ft

In situ interval between collapse breccias; fracture and vuggy porosity is present.

2736 ft

Chaotic breccia with polymictic clasts supported by clasts and internal sediment composed of tripolite.

2739 ft

Collapse breccia to fitted breccia; there has been minor displacement as is indicated by a similar texture exhibited by the clasts.

2757 ft

Clast-supported, chaotic breccia.

Matrix consists of tripolitic internal

sediment, sand grains, calcite, and fluorite.

2764 ft

Chert and dolomite clasts in chaotic breccia.

2779 ft

Collapse feature with tripolite filling the fractures and interparticle spaces; dedolomitization is present in this interval.

274

In order to facilitate the most timely publication, SEPM Short Course Notes are not subjected to the level of review required of an SEPM Special Publication.

ISBN 1-56576-004-2