Depositional Systems and Karst Geology of the Ellenburger Group (Lower Ordovician), Subsurface West Texas

RI0193. Depositional Systems and Karst Geology of the Ellenburger Group (Lower Ordovician), Subsurface West Texas, by Charles Kerans. 63 p., 37 figs., 2 tables, 1 appendix, 6 pls., 1990. ISSN: 0082335X:Print.

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ABSTRACT
The Ellenburger Group (Lower Ordovician) of Texas is a laterally extensive peritidal carbonate shelf sequence. It forms a major deep oil reservoir, having estimated reserves of 1.15 billion barrels of oil, and it also contains an estimated 2.2 billion barrels of oil equivalent. Despite its economic and geologic significance, comparatively little is known about the subsurface Ellenburger in West Texas; thus, this study presents a regional model of Ellenburger deposition and diagenesis.

Six depositional systems, based on associations of lithologies and sedimentary structures observed in core, were recognized in the Ellenburger: (1) fan delta-marginal marine (litharenite); (2) lower tidal-flat. (mixed silliciclastic-carbonate packstone-grainstone); (3) high-energy restricted-shelf (ooid-peloid grainstone); (4) low-energy restricted-shelf (mottled mudstone); (5) upper tidal-flat (laminated mudstone); and (6) open shallow-water shelf (packstone-grainstone). The first two depositional systems record retrogradational sedimentation during initial transgression. The high-energy restricted-shelf system forms a laterally extensive sheet throughout most of Central and West Texas and represents relatively rapid and widespread flooding of the shelf. The latter three depositional systems record gradual progradational or aggradational sedimentation, or both. The open shallow-water shelf depositional system occurs as a broad fringe along the cratonward margin of the Ouachita foldbelt, and it represents the most open marine conditions on the shelf during middle to late Ellenburger sedimentation.

Evidence of subaerial exposure and karst development is ubiquitous in Ellenburger carbonates. The most prolific karst event occurred prior to deposition of the Middle Ordovician Simpson Group associated with a global eustatic sea-level lowstand. This karst system is represented by fracture, mosaic, and chaotic breccias, and siliciclastic and carbonate infill sediments. Karst collapse breccias locally extend more than 600 ft (200 m) below the Ellenburger-Simpson contact, and laterally extensive phreatic cave systems developed between 30 and 300 ft (10 and 300 m)below this unconformity. Additional local karst development occurred in the Silurian-Devonian, Mississippian, and Pennsylvanian Periods.

Diagenesis of the Ellenburger Group was dominated by three major styles of dolomitization. Very fine crystalline dolomite (5-20um) is restricted to tidal-flat facies and is interpreted to be a penecontemporaneous replacement fabric. Fine to medium crystalline dolomite (20-100 um), which is widespread in all facies, probably resulted from regionally extensive reflux processes operative during Ellenburger sedimentation. Coarse crystalline replacement mosaic dolomite and saddle dolomite cement formed in a burial setting after pre-Simpson karst formation and before Pennsylvanian faulting, uplift, and erosion. Other diagenetic events were karst-related dissolution episodes associated with repeated uplift and exposure of the Ellenburger platform and subsequent dedolomitization.

The most common porosity type in Ellenburger reservoirs occurs in fractures and brecciated dolostones within paleokarst collapse zones. These porosity zones may be continuous from the upper Ellenburger erosion surface downward, or they may be represented by impermeable cave-infill sediments, resulting in vertical reservoir compartmentalization. Other porosity types are late, tectonically generated fracture porosity and vuggy and intercrystalline porosity produced during burial dolomitization, particularly in the high-energy restricted-shelf depositional system.

Keywords: carbonate facies, depositional systems, diagenesis, Ellenburger Group, karst, Lower Ordovician reservoirs, West Texas

CONTENTS

Abstract

Introduction

Regional Geologic Setting

Previous Work

Depositional Systems and Facies Analysis

Sedimentary Facies and Depositional Systems

Fan Delta - Marginal Marine Depositional System

Lower Tidal-Flat Depositional System

High-Energy Restricted-Shelf Depositional System

Low-Energy Restricted-Shelf Depositional System

Upper Tidal-Flat Depositional System

Open Shallow-Water Shelf Depositional System

Depositional History

Regional Depositional Setting

Paleokarst Features

Middle 0rdovician Unconformity

Age Relationships at the Unconformity

Paleotopography

Paleokarst Deposits

Fracture and Mosaic Breccias

Chaotic Breccias

Vertical Distribution of Paleokarst Deposits

Lateral Distribution of Paleokarst Deposits

Regional Distribution of Paleokarst Deposits

Model of Middle Ordovician Paleokarst

Silurian-Devonian Paleokarst Features

Carboniferous Paleokarst Features.

Nature of Carboniferous Paleokarst.

Diagenetic History

Marine Components

Early Diagenetic Phases

Dolomitization

Very Fine Crystalline Dolomite

Fine to Medium Crystalline Dolomite

Coarse Crystalline Dolomite

Other Late-Stage Diagenetic Phases

Summary of Diagenetic History

Implications for Petroleum Exploration and Production Conclusions

Acknowledgments

References

Appendix: Core Examined in This Study (in inside back pocket of book)

Figures

1. Location map showing cores and lines of cross section studied. tectonic features, and Ellenburger and El Paso Group outcrops
2. Map showing regional depositional setting during
3. Isopach map of the Ellenburger Group
4. Simplified regional cross section of the Permian Basin
5. Schematic representation of depositional systems in West Texas compared with formalized Ellenburger stratigraphy in the Llano area
6. Graphic log of fan delta -marginal marine depositional system, Phillips Puckett No. 1- C well, Pecos County
7. Core photographs of fan delta -marginal marine depositional system
8. Graphic log of lower tidal-flat depositional system, Gulf Keystone 108-E well, Winkler County
9. Core photographs of lower tidal-flat depositional system
10. Graphic log of high-energy restricted-shelf depositional system, Gulf McElroy St. No. 1 well, Upton County
11. Core photographs and photomicrograph of high-energy restricted-shelf depositional system
12. Graphic log of low-energy restricted-shelf depositional system, Phillips Glenna No. 1 well, Pecos County
13. Core photographs of low-energy restricted-shelf depositional system
14. Graphic log of upper tidal-flat depositional system, Phillips Puckett No. 1-6 well, Pecos County
15. Core photographs of upper tidal-flat depositional system, Magnolia Below No. 1 well, Kendall County
16. Graphic log of open shallow-water shelf depositional system, Humble Alma Cox No. 1-D well, Crockett County
17. Core photograph and photomicrograph of open shalllow-water shelf depositional system
18. Generalized cross section showing distribution of depositional systems in a section oriented oblique to depositional strike
19. Schematic illustration of principal depositional and erosional phases exhibited by the Ellenburger Group
20. Subcrop map of Ellenburger depositional systems showing progressively deeper erosion toward the northwest
21. Schematic reconstruction of early Middle Ordovician paleogeography of North America
22. Schematic representation of breccia types in Ellenburger paleokarst deposits
23. Typical examples of Ellenburger paleokarst breccias
24. Typical infill deposits of the Ellenburger paleokarst system
25. Core photographs, Gulf McElroy St. No. 1 well, Upton County
26. Gamma-ray, neutron, and graphic logs showing distribution of fracture-brecciated roof zone, cave-infill zone, and lower collapse breccia zone in Gulf McElroy St. No. 1well, Upton County
27. Gamma-ray, spontaneous potential, resistivity, and graphic logs showing distribution of paleokarst deposits, Gulf TXL 000-1 well, Emma field, Andrews County
28. Core photographs of paleokarst features, Gulf TW, 000-1 well, Andrews County
29. Gamma-ray, spontaneous potential, resistivity, and graphic logs showing character and distribution of Ellenburger paleokarst breccias, Phillips Wilson No. 1 well, Val Verde County
30. Spontaneous potential and resistivity log cross section of the northern Andector Ellenburger reservoir showing the lateral continuity of the infill zone
31. Schematic model of development of Ellenburger paleokarst deposits
32. Core photographs of paleokarst features associated with the pre-Pennsylvanian unconformity in the upper 40 ft (12 m) of the Houston Oil and Minerals Co. Brown B-8-1 well
33. Photomicrograph of marine components and eogenetic diagenetic fabrics
34. Photomicrographs of very fine crystalline and fine to medium crystalline dolomite generations
35. Photomicrographs of coarse crystalline dolomite
36. Photomicrographs of late diagenetic textures postdating or intergrown with coarse crystalline dolomite generation
37. Sequence of diagenetic events recorded in Ellenburger carbonates

Tables

1. Characteristics of Ellenburger depositional systems
2. Comparison of fault-related and karst breccias

Plates (in inside back pocket of book)

1. West-east cross section, Lipscomb to Montague Counties

2. West-east cross section, Andrews County

3. West-east cross section, Winkler to Brown Counties

4. West-east cross section, Winkler to Reagan Counties

5. West-east cross section, Presidio to Schleicher Counties

6. West-east cross section, Val Verde to Kendall Counties


Citation
Kerans, Charles, 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.