Delineation of Unrecovered Mobile Oil ...East Penwell San Andres Unit, University Lands

RI0194. Delineation of Unrecovered Mobile Oil in a Mature Dolomite Reservoir: East Penwell San Andres Unit, University Lands, West Texas, by R. P. Major, G. W. Vander Stoep, and M. H. Holtz. 52 p., 51 figs., 2 tables, 1990. ISSN: 0082335X: Print.

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ABSTRACT
The East Penwell San Andres Unit produces from a depth of approximately 3,400 ft (1,040 m) on the east flank of a broad, low-relief anticline on the east side of the Central Basin Platform of the Permian Basin in Ector and Crane Counties, West Texas. Since discovery in 1927 the unit has produced 43 million barrels of primary and waterflood oil of an estimated 164 million barrels of original oil in place. Approximately 30 million barrels of mobile oil remains in the main reservoir of this unit. This report presents the results of the integration of geologic and engineering studies of the reservoir to locate this remaining mobile oil.

The upper San Andres Formation [Permian, Guadalupian) at Penwell field is composed of an upward-shoaling suite of shallow-marine, carbonate-ramp facies that are now thoroughly dolomitized and that contain sulfates as nodules and cements. The lower, subtidal part of the stratigraphic section is composed primarily of pellet grainstone/packstone containing varying amounts of fusulinids and bivalves and thin zones of sponge-algal boundstone. These facies are overlain by, and/or interbedded with, thin zones of dasycladacean algae grainstone. The upper, peritidal part of the stratgraphic section is composed of nonporous mudstone and pisolite packstone, which form the reservoir seal. Pisolite packstone facies exhibit abundant subaerial-exposure features such as caliche, low-relief karst-collapse structures, and desiccation cracks.

The main reservoir facies, pellet grainstone/packstone, has two diagenetically controlled textures that give the rock a mottled appearance. The darker colored, unaltered texture was found to have permeabilities of approximately 1 md or less and oxygen-isotope compositions in the range of 3 to 5.5%0 (PDB). The lighter colored, altered texture was found to have permeabilities of approximately 10 md or more and oxygen-isotope compositions of 1 to 4%0 (PDB). Alteration was determined to have resulted from late-stage leaching of dolomite, which increased permeability by widening intercrystalline pore throats and preferentially removing early, isotopically enriched parts of dolomite rhombs. Significantly, this diagenetic mottling occurs so minutely and randomly that conventional core-analysis techniques are insufficient to detect it.

Maps of production averaged over time (to remove production/drilling-strategy artifacts such as early drilling along lease lines) and values of calculated remaimng mobile oil revealed production patterns resulting from lateral reservoir heterogeneity. The south part of the unit, where nonreservoir peritidal rocks are thick, was found to have low production and low volumes of remaining mobile oil. In contrast, the north part of the unit was found to contain higher volumes of remaining mobile oil related to inferred east-west-trending tidal channels. These tidal-channel features are targets for infill development drilling.

Keywords:  diagenesis, hydrocarbons, oil reserves, oil reservoir, PJWDM Field Complex, Penwell field, Permian Basin, petroleum engineering, San Andres, well log analysis

CONTENTS

Abstract

Introduction

   Geologic Setting

   Development History

Facies and Depositional Environments

   Pellet Grainstone/Packstone

   Algal Grainstone

   Sponge-Algal Boundstone

   Crinoid Grainstone

   Mudstone

   Pisolite Packstone

   Siltstone

   Depositional Model

Diagenesis

   Pellet Induration and Dolornitization

   Authigenic Sulfates

   Postburial Leaching

Analysis of Volumetrics

   Porosity and Permeability

   Facies Isopach Maps

   Production Patterns

   Log-Derived Porosity

   Water-Saturation Calculations

   Original and Remaining Mobile Oil

Summary

Acknowledgments

References

Appendix

   Abbreviations used in this report

Figures

1. Stratigraphic column of Paleozoic strata of the Permian Basin, West Texas and southeastern New Mexico
2. Volumetrics of major reservoirs on University Lands
3. Volumetrics of East Penwell San Andres Unit (EPSAU)
4. Location of Penwell, Jordan, Waddell, Dune, McElroy (PJWDM) Field Complex
5. Yates structure map of PJWDM Field Complex
6. Upper Paleozoic faults on Central Basin Platform
7. .Northwest-southeast stratigraphic cross section A-A' through PJWDM Field Complex
8. Structure map contoured on top of San Andres Formation in EPSAU
9. Production history of EPSAU
10. Postwaterflood production history of EPSAU
11. Type log for upper San Andres Formation in EPSAU No. 1505
12. Log/core correlation. EPSAU Nos. 207 and 1914
13. Slabbed-core photographs of pellet grainstone/packstone
14. Scanning electron micrograph of dolomitized pellet grainstone/packstone
15. Plane-light and cathodoluminescent photomicrograph pairs illustrating range of pellet present in pellet gralnstone/packstone
16. Photographs of algal grainstone
17. Photograph of sponge-algal boundstone
18. Photograph of crinoid grainstone
19. Photographs of mudstone
20. Photographs of pisolite packstone
21. Photographs of siltstone
22. West-east structural cross section EB' of EPSAU
23. Schematic reconstruction of depositional environments in EPSAU
24. Photographs of nodular anhydrite exhibiting outer edges hydrated to gypsum
25. Photomicrographs of anhydrite hydrated to gypsum
26. Photomicrograph of vug containing sulfate cement and residual hydrocarbons
27. Photograph of anhydrite nodules almost completely altered to sulfur
28. Slabbed-core photographs illustrating mottling of diagenetically unaltered and altered dolomite textures
29. Histogram of minipermeameter data collected from diagenetically unaltered and altered dolomite samples
30. Plane-light photomicrographs of diagenetically unaltered and altered dolomite crystals
31. Scanning electron micrographs of diagenetically unaltered and altered pellet grainstone/packstone
32. Carbon- and oxygen-isotope cross plot of diagenetically unaltered and altered dolomite
33. Permeability and oxygen-isotope cross plot of diagenetically unaltered and altered dolomite
34. Histograms of whole-core porosity data from reservoir facies in EPSAU Nos. 207 and 1914
35. Histograms of whole-core permeability data from reservoir facies in EPSAU Nos. 207 and 1914
36. Wireline-log and facies cross plots for EPSAU No. 2109
37. Isopach map of net peritidal facies, EPSAU
38. Isopach map of net algal grainstone facies, EPSAU
39. Map of cumulative oil production to January 1969 of wells drilled between 1930 and 1936 in EPSAU
40. Map of cumulative oil production to September 1987 of all wells in EPSAU
41. Production history of EPSAU No. 1601
42. Average annual oil production normalized to a 40-acre grid
43. Cross plots of acoustic-log transit time versus core-analysis porosity
44. Plot of the ratio of separate-vug porosity to total porosity versus cementation factor
45. Contour map of original mobile oil in place in EPSAU calculated using Rw = 0.075 ohmm
46. Contour map of original mobile oil in place in EPSAU calculated using Rw = 0.110 ohmm
47. Contour map of remaining mobile oil in EPSAU calculated using Rw = 0.075 ohmm
48. Contour map of remaining mobile oil in EPSAU calculated using Rw = 0.110 ohmm
49. Contour map of produced water Rw values in EPSAU
50. Contour map of original mobile oil in place in EPSAU calculated using Rw as variable
51. Contour map of remaining mobile oil in EPSAU calculated using Rw as variable

Tables

1. Permeability and stable carbon- and oxygen-isotope composition of unaltered dolomites from EPSAU Nos. 207 and 1914

2. Permeability and stable carbon- and oxygen-isotope composition of diagenetically altered dolomites from EPSAU Nos. 207 and 1914


Citation
Major, R. P., Vander Stoep, G. W., and Holtz, M. H.,  1990, Delineation of Unrecovered Mobile Oil in a Mature Dolomite Reservoir: East Penwell San Andres Unit, University Lands, West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 194, 52 p.