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RI0277. Wolfberry (Wolfcampian-Leonardian) Deep-Water Depositional Systems in the Midland Basin

RI0277. Wolfberry (Wolfcampian-Leonardian) Deep-Water Depositional Systems in the Midland Basin

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RI0277

RI0277. Wolfberry (Wolfcampian-Leonardian) Deep-Water Depositional Systems in the Midland Basin: Stratigraphy, Lithofacies, Reservoirs, and Source Rocks, by H. S. Hamlin and R. W. Baumgardner. 61 p., 38 figs., 4 plates in pocket, 1 appendix, 2012.  RI0277, $25.00.

 

About This Publication

The Wolfberry play in West Texas combines favorable geology with innovative completion practices to form one of the largest unconventional oil plays in the United States. Wolfberry wells produced almost 55 million barrels of oil in 2011, and that figure could double in a few years. In this report, the authors used wireline logs to correlate and map stratigraphic intervals and drill cores to characterize lithofacies and calibrate wireline logs for lithofacies identification and mapping beyond cored wells. On the basis of lithofacies composition, rock-body geometries, and bedding architecture, they interpreted depositional facies and elements within the sequence stratigraphic and paleogeographic framework. Wolfberry basinal deposits are oil rich, but most lithofacies are relatively impermeable. Mudrocks are organic rich, thermally mature, and oil prone. Sandstones and carbonates are mostly thin and of poor reservoir quality. The Wolfberry reservoir-source-rock system, however, is more than 2,000 ft (610 m) thick, and by means of massive, multistage, hydraulic-fracture stimulation treatments, large volumes of marginal reservoirs are accessed and produced.



 

ABSTRACT

The Wolfberry play combines favorable geology with innovative completion practices to form one of the largest unconventional oil plays in the United States. Wolfberry wells produced almost 55 million barrels of oil in 2011, and potential exists for that figure to double in a few years. Abundant organic carbon, brittle calcareous mudrock, and thin permeable beds form the geologic basis for the play. The Wolfberry concept grew out of preexisting plays in low-permeability sandstones (Spraberry Formation) and detrital carbonates (Wolfcamp interval) and developed in the early 2000’s through the application of modern hydraulic-fracture stimulation technology and refinement of geologic understanding of the reservoir-source-rock system. This report describes Wolfberry geology at regional and local scales and is intended to provide a context and reference for exploration and development.

Lower Permian (Wolfcampian and Leonardian Series) stratigraphy in the Midland Basin records deposition in an intracratonic, deep-water basin surrounded by shallow-water carbonate platforms. On the basin floor, siliciclastic, turbidite depositional systems alternate with calcareous, hemipelagic depositional systems in horizontal, laterally persistent layers. Turbidite sandstones form important reservoirs in basin-floor settings. Along the platform margins, slope depositional systems comprise carbonate-dominated clinoforms. Nearslope (periplatform) detrital carbonates (primarily debris flows and turbidites) form important Wolfberry reservoirs. By flooding or exposing the wide platforms, sea-level fluctuation controlled sediment input into the basin. During sea-level lowstands, platforms were exposed, and siliciclastic sediment was transported directly into the basin. During sea-level highstands, flooded platforms became carbonate factories, and sediment input to the basin comprised platform-derived carbonate and hemipelagic (windblown) silt and clay. The hemipelagic depositional system was active throughout the sea-level cycle, and organic matter and siliciclastic silt are abundant in all basinal intervals.

We used wireline logs to correlate and map stratigraphic intervals and drill cores to characterize lithofacies and calibrate wireline logs for lithofacies identification and mapping beyond cored wells. On the basis of lithofacies composition, rock-body geometries, and bedding architecture, we interpreted depositional facies and elements within the sequence stratigraphic and paleogeographic framework.

Siliciclastic intervals include the lower Wolfcamp interval, the Dean Formation, and the lower and upper intervals of the Spraberry Formation. These inferred lowstand intervals comprise submarine fans that extend over 150 mi (241 km) north-south and cover the basin floor. Spraberry and Dean sandstone turbidites are composed of very fine grained sand and coarse silt derived from source areas in the north. Permeable turbidite channel sandstones thin southward, grading into low-permeability turbidite lobes and sheets having widespread lateral continuity. The lower Wolfcamp interval forms a west- and north-thinning wedge of siliciclastics derived from tectonic source areas to the east and south.

Calcareous intervals include the upper Wolfcamp interval, lower and middle Leonard intervals, and middle interval of the Spraberry Formation. These inferred highstand intervals, which form equally widespread layers on the basin floor, are composed of hemipelagic deposits (mudrocks and calcareous mudrocks) and detrital carbonate mass-flow deposits. Basinal calcareous intervals are typically thicker, coarser grained, and more permeable near the platforms that supplied the carbonate detritus. In basin-center areas, calcareous intervals are mudrock dominated but include numerous thin, permeable interbeds.

Wolfberry basinal deposits are oil rich, but most lithofacies are relatively impermeable. Mudrocks are organic rich, thermally mature, and oil prone. Sandstones and carbonates are mostly thin and of poor reservoir quality. The Wolfberry reservoir source-rock system, however, is more than 2,000 ft (610 m) thick, and by means of massive, multistage, hydraulic-fracture stimulation treatments, large volumes of marginal reservoirs are accessed and produced.

 

Keywords: deep-water depositional systems, Leonardian, lithofacies, Midland Basin, Permian, production statistics, reservoir properties, West Texas

 

 

TABLE OF CONTENTS

Abstract

 

Introduction

 

            Geologic Setting

            Previous Geologic Studies

            Methods and Data

 

Wolfberry Play

 

Regional Stratigraphy

 

            Lithology from Wireline Logs

 

            Sequence Stratigraphic Framework

 

            Wolfcamp Interval

 

            Lower Leonard Interval

 

            Dean Formation

 

            Middle Leonard Interval

 

            Spraberry Formation

 

Lithofacies and Depositional Systems

 

            Sandstone Lithofacies

 

            Conglomeratic Lithofacies

 

            Laminated Siltstone Lithofacies

 

            Mudrock Lithofacies

 

            Carbonate Packstone/Grainstone Lithofacies

 

            Mineral Composition of Lithofacies

 

            Wireline-Log Recognition of Lithofacies

 

Reservoir Development

 

            Sandstone Turbidites

 

            Sandstone Reservoir Properties

 

            Detrital Carbonates

 

            Calcareous Mudrocks

 

            Source Rocks

 

Summary and Discussion

 

Acknowledgments

 

References

 

Appendix A. Cross-section wells: identification and interval boundaries

 

Figures

 

1. Map of Midland Basin and surrounding platforms and basins, showing oil-producing areas in Wolfcampian and Leonardian basinal lithofacies

 

2. Stratigraphic chart of Wolfcampian and Leonardian Series in Midland Basin and correlative units in Delaware Basin and on Central Basin Platform

 

3. Map showing subsurface data used in this study

 

4. Average volume of fluid used in hydraulic-fracture stimulation treatments for 24,252 wells in Spraberry Trend field

 

5. Average thickness of gross perforated interval for 24,252 wells in Spraberry Trend field

 

6. Number of actively producing Wolfberry wells in each year from 1998 through 2011

 

7. Total Wolfberry oil and gas production in each year from 1998 through 2011

 

8. Average-best-month oil production for Wolfberry wells completed in each year from 1998 through 2011

 

9. Map showing best-month oil production for 9,640 Wolfberry wells completed between 1998 and 2011

10. Typical modern wireline-log suite through Wolfcampian and Leonardian basinal intervals in
basin-center location

11. Cross section showing typical gamma-ray and resistivity logs from widely separated parts of basin

12. Cross section showing selected gamma-ray and resistivity logs in profile across northern basin margin

13. Regional north-south cross section A-A´, showing Upper Pennsylvanian and Lower Permian stratigraphy and lithology in Midland Basin and northern Val Verde Basin

 

14. Regional west-east cross section B-B´, showing Upper Pennsylvanian and Lower Permian stratigraphy and lithology in north part of Midland Basin

 

15. Regional west-east cross section C-C´, showing Upper Pennsylvanian and Lower Permian stratigraphy and lithology in central part of Midland Basin

16. Regional west-east cross section D-D´, showing Upper Pennsylvanian and Lower Permian stratigraphy and lithology in south part of Midland Basin

17. Isopach map of Wolfcamp interval

 

18. Isopach map of lower Leonard interval

 

19. Isopach map of Dean Formation

 

20. Isopach map of middle Leonard interval

 

21. North-south cross section showing individual Spraberry sandstones

 

22. Isopach map of lower Spraberry interval

 

23. Net-sandstone map of lower Spraberry 1L sandstone

 

24. Isopach map of middle Spraberry interval

 

25. Isopach map of upper Spraberry interval

 

26. Net-sandstone map of upper Spraberry 1U sandstone

 

27. Photographs of sandstone lithofacies in core

 

28. Thin-section photomicrographs of lithofacies

 

29. Photographs of conglomeratic lithofacies

 

30. Photographs of laminated siltstone lithofacies

 

31. Photographs of mudrock lithofacies

 

32. Ternary diagram of X-ray diffraction mineral composition data from 182 core samples

 

33. Wireline logs compared with core description, core analysis, and lithofacies, upper Spraberry

 

34. Cross section showing wireline-log responses and lithofacies composition in widely separated cores, lower Spraberry

35. Gamma-ray and resistivity logs through cored interval in lower Spraberry, also showing total organic carbon curve based on 78 samples

36. Gamma-ray and resistivity logs compared with lithofacies identified in core, TOC measurements, and foot-by-foot X-ray fluorescence measurements of elemental calcium and aluminum, upper Wolfcamp and lower Leonard

37. Maps showing regional distribution of oil-productive lithofacies in Spraberry and Dean sandstones and Wolfcampian and Leonardian detrital carbonates and calcareous mudrocks

 

38. Porosity and permeability measurements from conventional core-plug analysis from 21 cores used in this study

 

Tables

 

1. Basinal lithofacies in Wolfcampian and Leonardian cores

 

2. Total organic carbon in Wolfcampian and Leonardian lithofacies, Midland Basin

 

Plates (in pocket)

 

1. Wolfberry cross section A-A´ showing regional stratigraphy and “quick-look” lithology from

 gamma-ray and resistivity logs

 

2. Wolfberry cross section B-B´ showing regional stratigraphy and “quick-look” lithology from gamma-ray and resistivity logs

 

3. Wolfberry cross section C-C´ showing regional stratigraphy and “quick-look” lithology from gamma-ray and resistivity logs

 

4. Wolfberry cross section D-D´ showing regional stratigraphy and “quick-look” lithology from
gamma-ray and resistivity logs