Controls on Porosity and Permeability of Hydrocarbon Reservoirs in Lower Tertiary Sandstones along the Texas Gulf Coast

RI0149. Controls on Porosity and Permeability of Hydrocarbon Reservoirs in Lower Tertiary Sandstones along the Texas Gulf Coast, by R. G. Loucks, M. M. Dodge, and W. E. Galloway. 78 p., 71 figs., 3 tables, 5 plates, 1 appendix, 1986. ISSN: 0082335X: Print.

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ABSTRACT
Examination of porosity and permeability (reservoir quality) data, as determined by whole core, acoustic log, and petrographic analyses of lower Tertiary sandstones along the Texas Gulf Coast, made it possible to delineate areas most favorable for development of hydrocarbon reservoirs. Deep (about 3,350 m [11,000 ft] or greater) Wilcox sandstones exhibit no systematic regional reservoir-quality trends. Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth. Yegua sandstone porosity is intermediate between that of the Vicksburg and Wilcox sandstones. Vicksburg sandstones have the poorest reservoir quality of all the deep sandstones. Frio sandstones improve systematically in reservoir quality from the lower to the upper Texas Gulf Coast owing to grain composition and geothermal gradient.

Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses that become richer in quartz from the upper to the lower Texas Gulf Coast. Most rock fragments are metamorphic or volcanic. Yegua sandstones are moderately sorted, fine-grained, lithic arkoses to quartzose lithic arkoses. Volcanic and carbonate rock fragments are common along the lower Texas Gulf Coast, whereas volcanic and metamorphic rock fragments are common along the middle and the upper Texas Gulf Coast. Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses. Rock fragments are mainly volcanic clasts containing lesser amounts of carbonate and minor amounts of metamorphic clasts. Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithic arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast. Volcanic and carbonate rock fragments are common along the lower Texas Gulf Coast.

Although they vary in composition, lower Tertiary sandstones exhibit similar diagenetic sequences that may be idealized as follows:
Surface to shallow subsurface diagenesis (0 to 1,200 m+/- [0 to 4,000 ft+/-]) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite. Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite were locally precipitated. Porosity was commonly reduced by compaction and cementation from an estimated original 40 percent to less than 30 percent.
 

Intermediate subsurface diagenesis (1,200 to 3,400 m+/- [4,000 to 11,000 ft+/-], involved dissolution of early-formed carbonate cements and subsequent cementation first by quartz overgrowths and later by carbonate cement. Cementation may have reduced porosity to 10 percent or less, but this trend could have been reversed by later dissolution of feldspar grains, rock fragments, and possibly carbonate cements. Porosity was restored in some sandstones to more than 30 percent, but some porosity was later reduced by cementation by kaolinite, Fe-rich dolomite, and ankerite.

Deep subsurface diagenesis (>3,350 m +/- [>11,000 ft+/-]) was a continuation of late-stage Fe-rich and Fe-poor carbonate cement precipitation. Plagioclase was albitized during this stage. Differences in intensity of diagenetic features that were related to changes in rock composition and geothermal gradient distinguish areas of high reservoir quality in the deep subsurface along the Texas Gulf Coast. Vicksburg and Frio reservoirs along the lower Texas Gulf Coast have extensive late-formed carbonate cements, whereas along the upper Texas Gulf Coast late-formed carbonate cements are minor. Wilcox reservoirs show no simple regional trend; quartz and carbonate are the dominant porosity-reducing cements, and their precipitation was governed by local chemical and physical conditions.

Keywords: diagenesis, geopressure, Gulf Coastal Plain, permeability, porosity, reservoir properties, reservoir rocks, sandstone, Tertiary, Texas


CONTENTS

ABSTRACT

INTRODUCTION

General statement

Objectives and scope of study

Methodology

REGIONAL GEOLOGY

RESERVOIR QUALITY

Sources of data

Porosity and permeability trends

Porosity by formation and area

REGIONAL CONTROLS ON RESERVOIR QUALITY

General statement

Effect of time on porosity

Effect of pressure on porosity

Effect of temperature on porosity

Mineralogy and diagenesis: controls on reservoir quality

General statement

General diagenetic sequence

Mineralogy and diagenesis: stratigraphic and geographic distribution of reservoir quality

Sandstone classification

Wilcox Group

Texture and mineralogy

Diagenesis

Reservoir quality

Yegua Formation

Texture and mineralogy

Diagenesis

Reservoir quality

Vicksburg Formation

Texture and mineralogy

Diagenesis Reservoir quality

Frio Formation

Texture and mineralogy

Diagenesis

Reservoir quality

Summary of reservoir quality in lower Tertiary Gulf Coast sandstones

PETROPHYSICAL PARAMETERS AND DIAGENESlS

Rock consolidation gradient from interval-transit-time plots

Regional variation in rock consolidation gradient

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX: WELL DATA

 

FIGURES

1. Stratigraphic chart of the Cenozoic Era, Texas Gulf Coast

2. Area of investigation

3. Distribution of whole core by area, unit, and depth

4. Location of wells with porosity and permeability data

5. Location of wells with acoustic logs

6. Depositional style of Cenozoic strata along the Texas Gulf Coast

7. Salt domes and major faults of the Gulf of Mexico region

8. Mean porosity versus depth from both whole core and sidewall core

9. Mean sandstone porosity versus depth from whole core analyses

10. Sandstone porosity versus depth from whole core analyses

11. Permeability versus depth from whole core analyses

12. Mean porosity versus depth for sandstones with and without clay matrix

13. Relation of porosity to permeability for Wilcox Group and Vicksburg and Frio Formations

14. Mean sandstone porosity versus depth by formation

15. Mean Wilcox sandstone porosity by area

16. Mean Frio sandstone porosity by area

17. Relation of porosity to depth of burial in late Cenozoic sands and sandstones and Cenozoic shales in the hydropressured zone and in the geopressured zone in the Louisiana Gulf Coast basin

18. Porosity loss relative to pressure gradient

19. Pressure at 3,050 m (10,000 ft) in the geopressured zone

20. Pressure at 3,660 m (12,500 ft) in the geopressured zone

21. Porosity versus corrected bottom-hole temperature for Wilcox sandstones

22. Porosity versus corrected bottom-hole temperature for Frio sandstones

23. Corrected temperature at 2,290 m (7,500 ft) in the hydropressured zone

24. Corrected temperature at 3,050 m (10,000 ft) (a) in the hydropressured zone and (b) in the geopressured zone

25. General pathways of porosity and interval transit time with depth

26. Secondary porosity as a percent of total porosity versus depth

27. General diagenetic stages with increasing depth of burial

28. Sandstone classification

29. Wilcox sandstone composition

30. Basic data for Wilcox sandstone composition by area

31. Distribution of sorting in Wilcox sandstones by area

32. Distribution of grain size in Wilcox sandstones by area

33. Wilcox sandstone rock fragment composition

34. Diagenetic events versus depth for Wilcox sandstones

35. Permeability versus depth by area for Wilcox Group

36. Secondary porosity as a percent of total porosity versus depth for Wilcox sandstones

37. Yegua sandstone composition

38. Basic data for Yegua sandstone composition by area

39. Distribution of sorting in Yegua sandstones by area

40. Distribution of grain size in Yegua sandstones by area

41. Yegua sandstone rock fragment composition

42. Diagenetic events versus depth for Yegua sandstones.

43. Secondary porosity as a percent of total porosity versus depth for Yegua sandstones

44. Vicksburg sandstone composition.

45. Basic data for Vicksburg sandstone composition by area

46. Distribution of sorting in Vicksburg sandstones by area

47. Distribution of grain size in Vicksburg sandstones by area

48. Source areas for sediments of the Vicksburg and Frio Formations

49. Vicksburg sandstone rock fragment composition

50. Diagenetic events versus depth for Vicksburg and Frio sandstones

51. Permeability versus depth by area for Vicksburg Formation

52. Secondary porosity as a percent of total porosity versus depth for Vicksburg sandstones

53. Frio sandstone composition

54. Basic data for Frio sandstone composition by area

55. Distribution of sorting in Frio sandstones by area

56. Distribution of grain size in Frio sandstones by area

57. Relation of percent quartz to average grain size for Frio sandstones along the Texas Gulf Coast

58. Frio sandstone rock fragment composition

59. Permeability versus depth by area for Frio Formation

60. Secondary porosity as a percent of total porosity versus depth for Frio sandstones

61. Packing proximity versus depth for Vicksburg sandstones from area 1 and Frio sandstones from areas 5 and 6

62. Interval transit time as a function of depth showing consolidation effect for Louisiana Tertiary sandstones

63. Sandstone interval transit time versus depth for the Coastal States and Greenbriar Ltd. 72 No. 1 Echols in Starr County

64. Sandstone interval transit time versus depth for the George Mitchell No. 1 Peschel in Austin County

65. Sandstone interval transit time versus depth for three wells in area 1 of the Vicksburg/Frio trend showing effect of formation changes

66. Sandstone interval transit time versus depth by area for the Vicksburg/ Frio trend

67. Basic data for plot of interval transit time versus depth for the Vicksburg/ Frio trend sandstone

68. Sandstone interval transit time versus depth by area for the Wilcox trend

69. Basic data for plot of interval transit time versus depth for the Wilcox trend sandstone

70. Potential for high-quality, deep reservoirs in lower Tertiary strata along the Texas Gulf Coast

71. Schematic porosity versus depth/ temperature curve for lower Tertiary sandstones showing relative significance of primary versus secondary porosity

 

TABLES

1. Porosity loss relative to pressure gradient, calculated using Atwater and Miller's data from south Louisiana

2. Porosity loss relative to pressure gradient for Wilcox and Frio sandstones along the onshore Texas Gulf Coast

3. Average geothermal gradients by area for the updip Wilcox and downdip Vicksburg/ Frio trends

 

PLATES

1. Criteria for recognition of secondary porosity

(a) Patchy remnants of Fe-rich carbonate cement

(b) Partial dissolution of a plagioclase grain

 

2. Criteria for recognition of secondary porosity

(a) Partial dissolution of a volcanic rock fragment forming secondary porosity

(b) Dissolution of feldspar grain forming honeycombed structure

 

3. Criteria for recognition of secondary porosity

(a) Almost complete dissolution of feldspar grain with a minor honeycombed structure and clay coat remaining

(b) Dissolved feldspars outlined by clay coat and partially filled with kaolinite cement

 

4. Criteria for recognition of secondary porosity.

(a) Sandstone showing oversized pore spaces resulting from dissolution of feldspars

(b) Embayment in quartz overgrowth

5. Generalized rock consolidation sequence with increasing depth for Tertiary formations of the Texas Gulf Coast


Citation
Loucks, R. G., Dodge, M. M., and Galloway, W. E., 1986, Controls on Porosity and Permeability of Hydrocarbon Reservoirs in Lower Tertiary Sandstones along the Texas Gulf Coast: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 149, 78 p.