Canyon Sandstones--A Geologically Complex Natural Gas Play in... Val Verde Basin, Southwest Texas

RI0232. Canyon Sandstones--A Geologically Complex Natural Gas Play in Slope and Basin Facies, Val Verde Basin, Southwest Texas, by H. S. Hamlin, S. J. Clift, S. P. Dutton, T. F. Hentz, and S. E. Laubach. 74 p., 68 figs., 7 tables, 3 appendices, 1995.  Print.

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ABSTRACT
Canyon sandstones form a prolific low-permeability gas play in the Val Verde Basin of southwest Texas. Exploration and development activity is at a high level, but little published information on Canyon geology is available. Although several geographically and stratigraphically distinct "Canyon" intervals (Upper Pennsylvanian-Lower Permian) are productive, our study focused on characterizing the stratigraphy, diagenesis, and natural fractures of Sonora Canyon sandstones in Sutton County and Ozona Canyon sandstones in Crockett County. These Canyon intervals are composed of hundreds to thousands of feet of thinly interbedded sandstone and mudstone, which formed in slope and basin depositional systems.

Because outcrops are not present in the Sonora and Ozona intervals and seismic data were not available, well logs and cores formed the data base for this study. Sandstone distribution was mapped regionally and locally in Sutton County, where dense well control and core allowed field-scale facies characterization. Sonora sandstones lie in a wedge-shaped interval along the southwest margin of the Eastern Shelf, whereas the more tabular Ozona interval occupies a basin-floor position adjacent to the south margins of the Ozona Arch and the Central Basin Platform. Other Canyon or "Wolfcamp" sandstone intervals lie adjacent to the Ouachita orogenic belt in the south and west parts of the Val Verde Basin.

Sonora and Ozona sandstones were deposited in deep-water, turbidite systems. Several depositional facies were identified in core: conglomeratic sandstone, thick- and thin-bedded turbidites, chaotic facies, and hemipelagic mudstone. These facies recur in characteristic associations to form the elements of submarine fans: slope channel, proximal channelized fan lobe, distal lobe, and lobe fringe. Individual fan lobes are a few hundred feet thick and a few miles wide, and fan channels are less than 100 ft (< 30 m) thick and less than 1 mi (< 1.6 km) wide. Channel and lobe depositional elements are complexly interbedded and laterally coalesced. Regionally, Canyon fans form multiple-sourced, strike-elongate submarine ramps and slope aprons.

Most Sonora and Ozona sandstones are fine- to medium-grained litharenites. Chertand sedimentary and low-rank-metamorphic rock fragments are the predominant lithic grains. Original porosity and permeability were largely destroyed by compaction and by cementation by quartz and carbonate minerals. Pervasive diagenetic modification masked the expected relationship between depositional facies and reservoir quality. In Sonora sandstones, however, early siderite cementation preserved some intergranular porosity by inhibiting mechanical compaction and precipitation of quartz cement. Although siderite-cemented layers developed preferentially in Bouma T, turbidite divisions, predicting siderite-enhanced porosity will require more comprehensive investigations.

Natural fractures in Canyon core were mapped and described to determine their attributes and orientations. Ozona and Sonora fractures are typically subvertical extension fractures that terminate at the boundaries of beds or cementation zones. Clay-filled fractures in siderite-cemented zones form the most common fracture class in Sonora sandstones but may be flow barriers. Although quartz- and carbonate-cemented fractures are less common in core, fracture porosity is preserved locally along their traces. Spacing between larger, more permeable fractures could not be observed directly but is probably comparable to the thickness of the quartz-cemented intervals that contain the fractures-several feet to tens of feet. A wide range in fracture strike was observed in oriented Sonora core, but subsurface fractures trending generally northeastward are most prone to be open because of in situ stress conditions.

Complexly interacting geologic variables determine the intrinsic attributes of Canyon gas reservoirs. Although depositional environment controls reservoir shapes, dimensions, and internal compartmentalization, our findings suggest that diagenesis and natural fractures exert equal or greater influence on reservoir quality and gas productivity.

Keywords: Canyon sandstone, diagenesis, fractures, natural gas, reservoir properties, stratigraphy, submarine fan, Texas

CONTENTS

Abstract

Introduction

            Methods and Data

            Reservoir Attributes

            Val Verde Basin

Stratigraphic Framework

            Sonora Canyon

                        Regional Sandstone Distribution

            Ozona Canyon

                        Regional Sandstone Distribution

            Depositional Facies

            Depositional Elements

                        Miers Area

                        Sonora Area

Sandstone Composition and Diagenesis

            Framework Grains

            Cements and Replacive Minerals

            Porosity

            Siderite in Sonora Sandstones

            Controls on Siderite Distribution

Natural Fractures

            Fracture Description

            Fracture Classes

            Timing of Fracturing

            Fracture Networks

            Fracture Prediction

Summary: Geologic Controls on Reservoir Attributes

Acknowledgments

References

Appendix A: Sources of Canyon Sandstone Core Data

Appendix B. Sonora Maps and Cross Sections

Appendix C. Ozona Maps and Cross Sections

Figures

1. Principal Late Pennsylvanian-Early Permian tectonic elements, Central and West Texas
2. Relative proportions of tight-gas production, new completions, and reserves in existing wells for formations in non-Appalachian, continental United States basins
3. Sonora and Ozona Canyon gas fields
4. Typical well logs from the Sonora trend in Sutton County and the Ozona trend in Crockett County
5. Southwest-northeast schematic cross section showing Late Pennsylvanian-Early Permian depositional topography, northeastern Val Verde Basin

6. Upper Pennsylvanian-Lower Permian stratigraphy, Val Verde Basin and Eastern Shelf

7. Annual gas production and net new completions for Canyon reservoirs, Val Verde Basin

8. Index map of subsurface data

9. Southwest-northeast schematic cross section of the Val Verde Basin showing contrasting tectonic settings and depositional styles

10. Map of the Val Verde Basin showing Canyon sandstone intervals

11. Structure map, contoured on top of pre-Canyon carbonates, northern Val Verde Basin

12. Structure map, contoured on top of Ozona Canyon interval

13. Southwest-northeast regional cross section extending from southwest margin of Eastern Shelf into the deep Val Verde Basin and showing structural and stratigraphic configurations of major Canyon sandstone intervals

14. lsopach map, interval between Strawn limestone and top of Sonora Canyon

15. West-east Sonora stratigraphic cross section B-B' oriented approximately parallel to depositional dip and showing gamma-ray logs

16. Net sandstone thickness map, lower Sonora Canyon map unit

17. Sonora Canyon sandstone depocenters and inferred shelf-edge positions

18. lsopach map, Ozona Canyon interval

19. Southwest-northeast Ozona stratigraphic cross section F-F' oriented along the axis of the Ozona trend and showing gamma-ray logs

20. West-east schematic cross section showing stratigraphic relationships along the north margin of the Val Verde Basin

21. Ozona Canyon sandstone depocenters

22. Simplified submarine-fan model showing depositional elements and typical gamma-ray log responses

23. Photograph of conglomeratic sandstone facies, Sonora Canyon

24. Typical facies characteristics and vertical associations in Sonora core

25. Photographs of thick-bedded and thin-bedded turbidites, Sonora Canyon

26. Photograph of conglomeratic mudstone facies, Sonora Canyon

27. Structure map, contoured on top of Sonora Canyon

28. Net sandstone thickness map, middle Sonora Canyon map unit, Miers field area, south-central Sutton County

29. West-east stratigraphic cross section K-K', Miers field area, showing gamma-ray logs

30. lsopach map, lower Sonora Canyon map unit, Sonora area

31. South-north stratigraphic cross section L-L' connecting the GRI cooperative wells

32. lsopach map, combined middle and upper Sonora Canyon map units

33. Net sandstone map, lower Sonora Canyon map unit, Sonora area

34. Maximum sandstone map, lower Sonora Canyon map unit, Sonora area

35. QFR ternary diagram illustrating detrital components of Sonora and Ozona sandstone samples

36. SEM photograph of siderite and chlorite cement rimming detrital quartz grains in Sonora Sandstone

37. Photomicrograph showing quartz cementation in Sonora Sandstone in the absence of early siderite cementation

38. Photomicrograph of Sonora sandstone showing thick, continuous siderite rims around quartz grains and intergranular porosity

39. Photomicrograph of Ozona sandstone showing plagioclase grains partially replaced by ankerite cement

40. SEM photograph of a cluster of siderite rhombs in Sonora sandstone

41. Typical distribution of fractures in Sonora sandstone core

42. Fracture width versus depth of Sonora sandstones

43. Fracture height versus depth of Sonora sandstones

44. Photographs of fractures in Sonora core
45. Map of initial potentials from wells completed in Sonora interval in the Sonora area
46. Porosity and permeability relationship of Sonora Canyon facies

Tables

1. Canyon sandstone production data, reservoir properties, and engineering parameters

2. Characteristics of Canyon depositional facies

3. Petrographic analyses of Sonora and Ozona Canyon sandstones

4. Comparison of siderite-poor and siderite-rich Sonora Canyon sandstones

5. Summary of fracture-class attributes

Appendix Figures

B1. Net sandstone thickness map, middle Sonora Canyon map unit

B2. Net sandstone thickness map, upper Sonora Canyon map unit

B3. West-east stratigraphic cross section A-A' oriented approximately parallel to depositional dip
B4. West-east stratigraphic cross section C-C' oriented approximately parallel to depositional dip
B5. Northwest-southeast stratigraphic cross section D-D' oriented approximately parallel to depositional strike
B6. Northwest-southeast stratigraphic cross section E-E' oriented approximately parallel to depositional strike

C1. Net sandstone thickness map, Ozona zone 1

C2. Net sandstone thickness map, Ozona zone 2

C3. Net sandstone thickness map, Ozona zone 3

C4. Net sandstone thickness map, Ozona zone 4

C5. Maximum sandstone map, Ozona zone 1

C6. Maximum sandstone map, Ozona zone 2

C7. Maximum sandstone map, Ozona zone 3

C8. Maximum sandstone map, Ozona zone 4

C9. Log facies map, Ozona zone 1

C10. Log facies map, Ozona zone 2

C11. Log facies map, Ozona zone 3

C12. Log facies map, Ozona zone 4

C13. Northwest-southeast stratigraphic cross section G-G' oriented perpendicular to the main Ozona trend

C14. Northwest-southeast stratigraphic cross section H-H' oriented perpendicular to the main Ozona trend

C15. Northwest-southeast stratigraphic cross section I-I' oriented perpendicular to the main Ozona trend

C16. Northwest-southeast stratigraphic cross section J-J' oriented perpendicular to the main Ozona trend

Appendix Tables

A1. Gas Research Institute cooperative wells

A2. Canyon core intervals from older wells


Citation
Hamlin, H. S., Clift, S. J., Sutton, S. P. Hentz, T. F., and Laubach, S. E., 1995, Canyon Sandstones--A Geologically Complex Natural Gas Play in Slope and Basin Facies, Val Verde Basin, Southwest Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 232, 74 p.