Lithofacies of the Wolfcamp and Lower Leonard Intervals, Southern Midland Basin, Texas

RI0281. Lithofacies of the Wolfcamp and Lower Leonard Intervals, Southern Midland Basin, Texas, by R. W. Baumgardner, Jr., H. S. Hamlin, and H. D. Rowe. 67 p., 41 figs., 5 tables, 1 appendix, 2016. ISBN 978-1-970007-06-0: Print.

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About This Publication
This report, conceived as a follow-up to regional treatment of the Wolfberry depositional systems in Report of Investigations No. 277, focuses on basinal facies of the lower Leonard and Wolfcamp intervals in the southern Midland Basin. The report provides detailed lithologic information about mudrocks and associated facies that were largely ignored before they became prolific producers of hydrocarbons as a result of hydraulic fracturing and horizontal drilling. Readers interested in the regional view presented in RI 277, and those seeking detailed lithologic information about the so-called Wolfcamp A and B zones, should find value in the core-based descriptions of lithofacies in this report, which examines the most continuous basinal cores publicly available from the Lower Permian/Upper Pennsylvanian interval in the Midland Basin. Detailed core description (including thin-section analysis)—combined with total organic carbon (TOC) sampling, handheld XRF scanning, and micro-rebound hammer testing at 1-ft spacing (techniques applied to these cores for the first time)—reveals significant, facies-related differences in TOC content, mineralogy, and rock strength. In addition, programmed pyrolysis (Rock-Eval^®) analysis reveals that most kerogen in the lower Leonard/Wolfcamp interval is in the oil-production window and has matured to Type II–III (“oil-gas prone”). Stratigraphers as well as explorationists will find information here to improve their understanding of Permian-age basinal mudrocks in the Midland Basin.

ABSTRACT
Basinal mudrocks and associated facies assigned to the Wolfcamp and lower Leonard lithostratigraphic units in the Midland Basin are prolific producers of oil and gas. Four facies are recognized in core on the basis of macroscopic examination, XRD analysis, and ED-XRF elemental chemistry: (1) siliceous mudrock, (2) calcareous mudrock, (3) muddy bioclast–lithoclast floatstone, and (4) skeletal wackestone/packstone. Mudrocks are largely hemipelagic deposits of fine-grained sediment delivered by suspension settling. Floatstones were deposited by debris flows that originated on carbonate shelves around the basin. Wackestones/packstones are finer grained sediment density-flow deposits, probably turbidites, reworked by local bottom currents. These sediments were deposited below storm wave base in a basin having limited connection to the open ocean. During deposition, low-oxygen conditions prevailed in bottom waters and sediments, as shown by the presence of agglutinated foraminifera, rarity of burrows, widespread prevalence of small (<6 μm) pyrite framboids, presence of phosphatic nodules, and elevated molybdenum concentrations. Total organic carbon (TOC) content reaches 6.8 percent. TOC is facies-dependent (highest in siliceous mudrock) and varies widely within small vertical distances. Cyclicity is evident in individual meter-thick, upward-fining cycles of floatstone or wackestone/packstone overlain by calcareous and siliceous mudrock. Rock strength increases as calcite content increases, likely as a result of diagenetic cementation. Most kerogen in the lower Leonard–Wolfcamp interval is in the oil-production window and has matured to Type II–III (“oil–gas prone”). On the basis of their TOC and hydrogen content, siliceous mudrocks have the highest potential for hydrocarbon generation.

Keywords: basinal mudrocks, cyclicity, organic matter, Pennsylvanian, Permian, Reagan County, Rock-Eval®, rock strength, stratigraphy


CONTENTS

Abstract

Introduction

     Stratigraphic nomenclature

Previous Work

Regional Setting

     Late Pennsylvanian–Early Permian paleoclimate and sea level

     Tectonics

Methods

Compositional Analysis

Lithofacies

   Siliceous mudrock facies

     Calcareous mudrock facies

         Summary of mudrock facies

     Muddy bioclast–lithoclast floatstone facies

         Description of matrix

         Description of clasts

     Skeletal wackestone/packstone facies

         Description of matrix

         Description of clasts

     Concretions

     Organic matter production, accumulation, and preservation

         Production of organic matter

         Accumulation/dilution of organic matter

         Preservation/decomposition of organic matter

         Biotic indicators

         Phosphatic nodules

         Pyrite framboids

         Molybdenum

Facies Interpretation and Depositional Processes

Depositional Model

Cyclicity

     Meter-scale cycles

     Origins of cyclicity

Reservoir Properties
     Rock strength and fractures

     Programmed pyrolysis analysis.

Discussion

Conclusions

Acknowledgments

References

Appendix

Figures

1. Principal structural elements of Midland Basin area in Permian time

2. Stratigraphic and operational names of formations in the study area

3. Structure map on top of Pennsylvanian rocks underlying the Wolfcamp interval

4. Isopach map of Wolfcamp interval within boundaries of the Midland Basin

5. Isopach map of the lower Leonard interval within boundaries of the Midland Basin

6. Paleogeographic map of southwestern United States during late Wolfcampian time

7. Interpretations of paleoclimate during Wolfcampian–early Leonardian time

8. Schematic cross section of southwestern margin of Midland Basin

9. Map of selected fields and wells in allochthonous carbonate deposits in Wolfcamp and lower Leonard

10. Wireline logs of four study wells showing cored intervals in this report

11. Scatterplots of ED-XRF elemental results and XRD mineralogical data from O. L. Greer 1 core

12. Scatterplots of elemental geochemistry from ED-XRF results

13. Ternary diagrams based on XRF and XRD data

14. Photographs of slabbed core showing interpreted facies

15. Facies interpreted from O. L. Greer 1 core

16. Facies interpreted in six cored intervals in the O. L. Greer 2 well

17. Facies interpreted from four cored intervals in Rupert P. Ricker 1 well

18. Facies in the lower Leonard interpreted from R. Ricker 1 core

19. Photographs of siliceous mudrock facies

20. Photographs of siliceous mudrock features

21. Photographs of calcareous mudrock facies

22. Photographs of calcareous mudrock facies

23. Photomicrographs of muddy bioclast–lithoclast floatstone facies in Wolfcamp

24. Photographs of muddy bioclast–lithoclast floatstone facies

25. Photographs of skeletal wackestone/packstone facies

26. Photographs of skeletal wackestone/packstone facies

27. Northeast–southwest cross section showing distinctive package of three correlative carbonate beds in lower Wolfcamp

28. Map of three distinctive correlative carbonate-rich beds in lower Wolfcamp

29. Photographs of carbonate concretion in calcareous mudrock facies in Wolfcamp

30. Crossplot of Ni and TOC values for O. L. Greer 1 and R. Ricker 1 wells

31. Depth plot of total organic carbon in O. L. Greer 1 core and parts of R. Ricker 1 core

32. Form of pyrite and size of pyrite framboids relative to redox conditions

33. Schematic cross section showing allochthonous carbonate deposits in upper Wolfcamp

34. Depositional model of basinal sediments described in this study

35. Plots showing correlative cycles in the O. L. Greer 1 and R. Ricker 1 wells

36. Graph of estimated rock strength, or unconfined compressive strength (UCS), in MPa

37. Graphs of estimated rock strength (UCS) compared to mineral content of facies

38. Photographs of natural, cement-filled fractures in core

39. Crossplot of hydrogen index vs. T_max

40. Crossplot of S₂ and TOC

41. Crossplot of vitrinite reflectance equivalency (%R_oe) vs. depth

Tables

1. Basinal facies in cores from Wolfcamp and lower Leonard in northern Reagan County, Texas

2. Average weight percent of minerals in matrix (not clasts) of each facies

3. Total organic carbon data in weight percent

4. Whole rock visual maceral estimates (on samples from sample half of O. L. Greer 1 core)

5. Values of estimated unconfined compressive strength (UCS) for facies in this study

Citation
Baumgardner, R. W., Jr., Hamlin, H. S., and Rowe, H. D., 2016, Lithofacies of the Wolfcamp and Lower Leonard Intervals, Southern Midland Basin, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 281, 67 p.