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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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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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.

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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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breccias occur mainly in subzone B-1, and directly underlie clay-matrix breccias.

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.

(D)

Subzone B-4 breccia interval. Light-colored

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.

UVV5, depth 4293 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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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.

146

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147

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.

148

FIGURE 17 re

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t

-

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;

149

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

FIGURE 18 Of:

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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.

154

FIGURE 19

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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.

156

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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.

158

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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.

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in

Roehl, P.O. and Choquette, P.W. (Eds.), Carbonate

Petroleum Reservoirs: Springer-Verlag, New York, p. 141-160. Skall, H., 1975, The paleoenvironment of the Pine Point lead-zinc district: Econ. Geology, v. 870, p. 22-47.

Wanless, H.R. and Dravis, Ji., 1989, Carbonate environments and sequences of Caicos Platform: Int. Geol. Congress Field Trip Guidebook T 374, 75 p. Wendte, J.C. and Gensamer, A.R., 1979, Pore systems in Jurassic carbonate reservoirs, United States Gulf Coast: Am. Assoc. Petrol. Geologists Bull., v. 63, p. 551. Wilcox, RE., Harding, T.P. and Seely, DR., 1973, Basic wrench tectonics: Am. Assoc. Petrol. Geologists Bull., v. 57, p. 74-96.

165

Wilson, J.L., Medlock, PL., 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: examples from Ordovician-Devonian age strata of west Texas and the Mid-Continent, Permian Basin Section: SEPM Publication No. 92-33, p. 19-28.

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)

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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

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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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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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DRILL CURVE :

'

80

1=

goc il

P2=1111,1111= rwmi

D

u

r

.

0 v, 0 P z 51 47

*.

.....

on

iur E u. -1

Z

19

0

T.D. 2759m(M.D.) (-2693mT.V.D.) ORIGINAL. 0/ W = 40m BELOW :T.

FEE T/HR

'

.,

,

Figure 4

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

,, . /

IIMMIAMIII

:grayA

45 , ,ft...,

.,

Immow.

i

4.

:4,4

, ,C..6.. ,,.

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°

C2 --,....,-..,._-,,...-,..._,...-

IMF/MINIM MEINIMIWINNI

MIEFII=1 INIMIIIMINIMI

--v-

IMIIMEMIM

IMI/1 ow IMINWIIIli

b.

8725 INIIIIIMIIIIMIla IMINIIIMININAM

--,

IPMIONIMMIla

IMI.MMIil rMIIMI IIIMPWEI

8730 mommrom

--,- k

. ........,,..._ ...--

Cave roof w/ large sediment filled yogi

t

.---

MMIWN 8735

11,MMIIIIIIMa

8740

8745

11`11 MINIIIMI

-----

wwarawra IIMMINIIMMIII 1111=1=1= IIIMIIIIININIME IIIIIIMMOIIIIII

we roof

N

I.

Cave fill

IL. N

8750 IMMIMINIII MIINVJ=1

Cave floor?

mimmtrom

.4..

111=IMIENIII INIMIIIIIIMIN IIIMININEMM

8755

.m.m.....

ta -_-

S

S.

a ,,,,,,,...0"'

S

Cam roof

IIIIMI

NMMIIMIMA IIIINIMININIVA

8760

1111 1111=.401

.

INIIIMMEMEM

8765

inommr c, OVINIIIIIMIDA

IIMIP eMININIIl 1IIIMIIIMI 8770

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

--a-

IMINIMI111

'--- 1,,, --, -..,

AMINuM 8825 IIMMOVIIIIIIII

rAM MIErANINI

NIMMENII=1 IMIEWINffira

,'

----------__,--

1

8830

Cl

rr

111111 WINNIIIIa

Faintly lamincrted

\

\

1MIIMr vrIMIMIMIMA CI IMNIP

i

---,----__-, '

iramb INIIIMIAMVA 8840

i-

In

krbi -----, \ =wara tall11 sa \\ wan/UM. waismr.m.

8835

COMMENTS

7

-------

IMIMirarani

"e)

>.

,_ i

--------.J-

ININIIIMIIMINM

TYPES

1

E_ogiltli

*4 IMMIMMIII

BRECCIA

y

7-7.. Highly fractured

TS S

----,:i----,ti-

6845

LEGEND Scoured surfaces

Wavy lamincrtion

Fine lamination Bioturbation Geopeds

o

Open fractures Cemented fractures

/7


O mi E C.)

(TI

"Ci O I

15 Lt /2

cri

c.)

,(,)

.

.

...O -.'

m--m.--, ,) c -,.., >, 4-, ..-1745-1

c_ ,._ d., 7,r., c, cn....,,_r---o-

..c

z: ..c

00

cd

cci

-..._

a.)

,,, v)

C...) 5 _a - ,..._. -.,9 ..a-

;-...

(1.)

(1)

7.,

(,) E --.m

m ,I) '''' -. E