Karst-Controlled Reservoir Heterogeneity ... Example from the Ellenburger... of West Texas

RI0186. Karst-Controlled Reservoir Heterogeneity and an Example from the Ellenburger (Lower Ordovician) of West Texas, by Charles Kerans. 40 p., 25 figs., 1989. ISSN: 0082335X: Print.


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ABSTRACT
This study is a product of ongoing Bureau of Economic Geology investigations, by means of geologic and petrophysical modeling, into the nature of reservoir compartmentalization or heterogeneity. The model has been designed to provide a basis for subsequent infill drilling in the reservoirs in question along trends of greatest geologic potential, rather than according to geometrically defined well spacings.

The Lower Ordovician Ellenburger Group was the site of the investigations from which more than 10,000 ft (3,050 m) of core from 63 wells was logged. Identification of six facies assemblages resulted: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod/peloid packstone. These facies assemblages record initial transgression and subsequent progradation and aggradation, indicating that paleoslope was generally southerly and easterly from the Texas Arch toward the Ouachita and Marathon orogenic belts. It was also found that all facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, have low intergranular and intercrystalline porosity.

The karst model, constructed by means of cross sections and then an isometric projection of the Ellenburger surface at Emma field
hung on the cross-section data, illustrates that porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand. This latter period occurred before transgression of Simpson Group siliciclastics, during which a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias and related fractures developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolostones (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor.

Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, and University Block 13 fields and in several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized, vertically restricted collapse structures, such as those that appear in the Shafter Lake Ellenburger reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be improved by the integration of concepts of lateral and vertical heterogeneity inferred by the karst model.

Keywords: depositional facies, Ellenburger Group, karst, Lower Ordovician, oil reservoirs, paleokarst models, reservoir heterogeneity, West Texas

CONTENTS

Abstract

Introduction

Regional Setting

Stratigraphic Framework

Structures Marginal to the Palo Duro Basin

Amarillo Uplift

Whittenburg 'Trough

Oldham-Harmon Trend

Bravo Dome

Bush Dome

Donley positive

Plaska structure

Matador Arch

Structures in Eastern New Mexico

Roosevelt positive

Surface faults in eastern New Mexico

Alamosa Creek fault

Bonita fault

San Jon high

Garcia Lake high

Structures within the Palo Duro Basin

Castro County

Arney positive

Castro Trough

Central Randall County

Lamb County

Structures of Nontectonic Origin

Subsurface Structures

Surface Structures

Summary

Acknowledgments

References

Figures

1. Tectonic elements of West Texas and adjacent states

2. Stratigraphic column and general lithologies, Palo Duro Basin and Amarillo Uplift

3. Location of seismic lines in the Palo Duro Basin

4. Structure-contour map, top of basement. southern Texas Panhandle

5. Structure-contour map. top of basement. Amarillo Uplift

6. Basement lithologies in Castro County and in the Sun Oil Company No. 1 Herring well

7. Pre-Middle Devonian subcrop map and isopach map of the Arbuckle and Ellenburger Groups

8. Isolith map of Pennsylvanian and Lower Permian arkosic

9. Structure-contour map, top of the Alibates interval, Palo Duro Basin

10. South-north cross section A-A' of western Amarillo Uplift

11. Surface geology and structure-contour map of the base of the Ogallala Formation, western Amarillo Uplift

12. Photograph of Ogallala Formation outcrop

13. Photograph of basal Ogallala Formation gravels

14. South-north cross section B-B' and structure-contour map of the base of the Ogallala Formation in Carson County

15. Isopach map of the pre-Permian Palo Duro Basin

16. Isopach map of the Leonardian Series, showing the Whittenburg Trough and western Amarillo Uplift

17. Photograph of Rita Blanca beds (Pliocene) dipping southwestward into the Whittenburg Trough from the Channing Dome

18. Structure-contour map, top of the Tubb interval, Palo Duro Basin

19. Isopach map of the middle and Upper Permian, Palo Duro Basin

20. Isopach map of the Permian System, Palo Duro Basin

21. Percent carbonate in the Upper Pennsylvanian. Palo Duro Basin

22. Structure-contour map. top of basement. Matador Arch

23. Structure-contour map, top of the San Andres Formation, east-central New Mexico

24. West-east cross section C-C' of the Alamosa Creek fault

25. West-east cross section D-D' of the Bonita fault

26. West-east cross section E-E' of the Garcia Lake high

27. Isopach map of the Mississippian System, Palo Duro Basin

28. Cross sections, Castro Trough

29. Subsurface maps, Castro Trough

30. Seismic reflection profile G-G', central Randall high

31. West-east cross section H-H' of structural low in southern Lamb County

Table

1. Structural features within or adjoining the Palo Duro Basin



Citation
Kerans, Charles, 1989, Karst-Controlled Reservoir Heterogeneity and an Example from the Ellenburger (Lower Ordovician) of West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 186, 40 p.