Heterogeneous Deep-Sea Fan Reservoirs, Shackelford and Preston Waterflood Units, Spraberry Trend, West Texas

RI0171. Heterogeneous Deep-Sea Fan Reservoirs, Shackelford and Preston Waterflood Units, Spraberry Trend, West Texas, by Noel Tyler and J. C. Gholston. 38 p., 29 figs., 3 tables, 1988. ISSN: 0082335X: Print.

To purchase this publication as a downloadable PDF, please order RI0171D.

ABSTRACT
The Permian Spraberry Trend, once regarded as the world's largest uneconomic oil field, is a prime candidate for reserve growth through extended conventional recovery. More than 9.4 billion barrels (Bbbl) of oil was discovered in the trend, which is part of a giant oil play (10.5 Bbbl of in-place oil) that produces from submarine fan reservoirs of the Spraberry and Dean Formations in the Midland Basin, West Texas. Historically, development of the naturally fractured, very fine grained sandstone and siltstone reservoirs was based on the assumption that reservoirs were laterally continuous and that fractures would foster efficient drainage at 160-acre well spacings. Yet ultimate recovery of oil is estimated to average 6 percent of the original oil in place. Almost 10 Bbbl of oil will remain in Spraberry-Dean reservoirs at abandonment. Of this volume, 4 Bbbl is nonresidual mobile oil that awaits extended conventional recovery in untapped or poorly drained reservoir compartments. This unproduced mobile oil forms a major component of the target for Spraberry reserve growth.

Geological characterization of the Shackelford and Preston waterflood units (SPWU) in the central Spraberry Trend established that three separate submarine fans (two in the upper and one in the lower Spraberry), each characterized by a variety of midfan facies, are productive. Paleodip-oriented channels display a transition from braided in the updip parts of the SPWU to meandering downdip. Laterally, channels are flanked by levees that grade into upward-coarsening, unconfined distal-fan sediment. Investigation of facies-defined hydrocarbon saturations clearly shows that facies boundaries compartmentalize the reservoir, providing for interwell, stratigraphic entrapment of oil.

Fieldwide heterogeneity is pronounced. Stacking of channels in the upper Spraberry in the eastern half of the SPWU results in a dip-oriented belt of better reservoir quality. Wells completed in this axis have produced two to as much as six times the amount of oil produced from wells located off of the depositional axis. This swath of superior wells trends obliquely to the natural fracture orientation, suggesting that although fractures are important in early production, the contribution of matrix porosity is critical throughout the life of the reservoir.

Current economics dictates that reserve growth might best be attained by siting new strategic infill wells in the depositional axis and by selectively recompleting existing wells to produce bypassed oil in undrained reservoir compartments in areas of poorer reservoir quality.

Keywords: Midland Basin, Permian (Leonardian), Spraberry Trend, submarine fans, reservoir architecture, oil reserve growth, Texas, West Texas

CONTENTS

Abstract

Introduction

Reservoir characterization for improved recovery

Stratigraphic setting of the Spraberry Formation

Submarine fan depositional systems

Sedimentary Character of Spraberry Reservoirs

Shackelford and Preston Units, Central Spraberry Trend

Sandstone distribution and architecture of the upper Spraberry

Upper Spraberry operational unit 1

Upper Spraberry operational unit 2

Upper Spraberry operational unit 3

Upper Spraberry operational unit 4

Interpretation: upper Spraberry operational units 1, 2, 3,and 4-the Floyd submarine fan

Upper Spraberry operational units 5 and 6

Interpretation: upper Spraberry operational units 5 and 6-the Driver submarine fan

Sandstone distribution and architecture of the lower Spraberry

Lower Spraberry operational unit 1

Interpretation: lower Spraberry operational units 1 and 2-the Jo Mill submarine fan

Facies composition of Spraberry genetic reservoir units

Spraberry lithofacies

Nonreservoir lithofacies

Black, organic-rich claystone

Laminated siltstone

Reservoir lithofacies

Massive and laminated sandstone

Relation between lithofacies, log facies, porosity, and permeability

Spraberry log facies

Upper Spraberry unit 1b

Interpretation

Upper Spraberry unit 2.

Interpretation

Upper Spraberry unit 5

Interpretation.

Sedimentological Constraints on Oil Recovery from the Shackelford and Preston Waterflood Units

Development history of Spraberry waterflood units

Facies architecture and reservoir continuity

Vertical variability in oil saturation

Lateral variability in oil saturation between wells

Waterflood-unit-wide variability in reservoir architecture and relation to production

Sedimentary patterns

Production patterns

Fractures and production

Reservoir Architecture and the Potential for Reserve Growth

Conclusion

Acknowledgments

References .

Figures

1. Structure of the Permian Basin and location of the major Spraberry-Dean oil fields in the Midland Basin

2. Pie diagrams illustrating the relation between depositional systems and patterns of oil accumulation in and ultimate recovery from sandstone reservoirs

3. Spectrum of recovery efficiencies in typical sandstone reservoirs

4. Principal Spraberry waterflood units, central Midland Basin

5. Paleozoic stratigraphy of the Midland Basin

6. Submarine fan environmental model and hypothetical vertical profile of a prograded fan system

7. Divisions of the Spraberry Formation in the central trend area, TXL Oil Corporation Midland Fee B No. 1 well

8. Isopach and percent-sand maps of operational units 1 through 6 of the upper Spraberry

9. Net-sand and percent-sand maps of operational unit 1, upper Spraberry

10. Percent-sand maps of operational units 2 and 3

11. Net-sand map of operational unit 5, upper Spraberry

12. Percent-sand map of operational unit 1, lower Spraberry

13. Mudstone and fine-grained siltstone lithofacies of the Spraberry Formation

14. Reservoir lithofacies: massive sandstone with numerous vertical water escape pipes and massive sandstone grading vertically into laminated sandstone with dish structures

15. Relation between sediment character including vertical grain-size trends and corresponding gamma-ray log response, Magnolia Petroleum Company D. T. Bowles No. 5 DD well

16. Relation between gamma-ray log character, sediment character, and porosity and permeability, Mobil Preston No. 1 well

17. Log facies map and typical logs of unit l b of operational unit 1, upper Spraberry

18. Log facies map and typical logs of operational unit 2, upper Spraberry

19. Log facies maps and typical logs of lower and upper intervals of operational unit 5, upper Spraberry

20. Resistivity cross section A-A' showing the pronounced stratification of hydrocarbon-bearing intervals in the upper Spraberry

21. Detailed resistivity cross section B-B' of unit 1b of operational unit 1, upper Spraberry, showing strong interwell (macroscopic) heterogeneity

22. Sequential distribution of high-percent-sand axes, upper Spraberry

23. Upper Spraberry trunk axes determined by superposing channel-axis maps shown in figure 22

24. Cumulative production from all initial development (pre-1955) wells in the Shackelford and Preston waterflood units

25. Cumulative water-cut map showing percent water of all fluids produced to 1972

26. Naturally fractured fine-grained sandstone, upper Spraberry

27. Initial potential map showing the importance of fracture porosity in early production

28. Percent-sandstone cross sections of the upper Spraberry showing the pronounced vertical stratification and the strong lateral heterogeneity of Spraberry reservoirs

29. Schematic architecture of upper Spraberry submarine fan reservoirs in the central Spraberry Trend

Tables

1. Subdivisions of the Spraberry Formation

2. Morphology and dimensions of upper Spraberry submarine fan depositional axes and component channels, Spraberry Trend area

3. Shackelford and Preston Spraberry unit reservoir characteristics



Citation
Tyler, Noel, and Gholston, J. C., 1988, Heterogeneous Deep-Sea Fan Reservoirs, Shackelford and Preston Waterflood Units, Spraberry Trend, West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 171, 38 p.