Facies and Diagenesis of the Austin Chalk and Controls on Fracture Intensity-A Case Study from North-Central Texas

GC9802. Facies and Diagenesis of the Austin Chalk and Controls on Fracture Intensity--A Case Study from North-Central Texas, by S. D. Hovorka. 47 p., 34 figs., 6 tables, 1998. ISSN: 0082-3309: Print.


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About This Publication
Using geologic and engineering data, the author documented Austin Chalk depositional environments and diagenetic alterations and investigated the factors that have influenced fracture development. Research was funded by the Texas National Research Laboratory.


ABSTRACT
The urban corridors associated with the Dallas, Austin, and San Antonio metropolitan areas lie on Austin Chalk outcrop. In these areas of rapid urbanization, ground-water information is needed to resolve environmental issues. The Austin Chalk also produces hydrocarbons in the deep subsurface. Fluid migration in both oil-field and ground-water environments is strongly controlled by fractures. In each geologic setting, a conceptual model of fracture intensity is required to model or predict flow. Using geologic and engineering data collected during development of the U.S. Department of Energy's Superconducting Super Collider (SSC) in Ellis County, Texas, I document Austin Chalk depositional environments and diagenetic alterations and investigate the factors that have influenced fracture development. Key findings are: (1) the Austin Chalk is laterally and vertically relatively homogeneous, reflecting deposition in deep water on high-productivity, flooded, continental shelves during Late Cretaceous worldwide sea-level highstand; (2) although rhythmically interbedded chalk and marl record intermediate-frequency sedimentological variability, no lower-order cyclic patterns could be identified; (3) facies within the Austin Chalk defined by variations in character of the cycles record marine flooding, deepening, and shallowing; (4) sedimentary structures in excellent exposures in SSC excavations and tunnels provide evidence of sediment transport through channels during Austin Chalk deposition; (5) fracture intensity is low in the middle Austin Chalk as compared with that in the upper and lower Austin Chalk; and (6) ductility, porosity, and permeability are genetically linked to chalk microfabrics. These properties are controlled, not by depositional facies, but by diagenetic alteration of minor amounts of volcanic ash co-deposited with chalk.

Keywords: Austin Chalk, Ellis County, facies, fractures, stratigraphy, SSC, Superconducting Super Collider, Texas


CONTENTS

ABSTRACT

INTRODUCTION

Scope and Purpose

Regional Setting of Austin Chalk Deposition

Faulting and Fracturing in the Austin Chalk

Methods

STRATIGRAPHIC VARIATION IN THE AUSTIN CHALK

Stratigraphic Units

Eagle Ford Formation

Lower Austin Chalk

Middle Austin Chalk

Upper Austin Chalk

Ozan Formation

Stratigraphic Distribution of Fractures

Patterns of Chalk-Marl Cycles

Petrographic Description

Chalk

Marl

Porosity and Permeability

Insoluble Residue

Clay Composition

Sand-Sized Material

Total Organic Carbon

Stable Isotopes

STRATIGRAPHIC RELATIONSHIPS AND ROCK PROPERTIES

Microfabric Control on Rock Properties

Sources of Volcanic Ash

Chalk-Marl Cyclicity

Diagenesis

Vertical Facies Variation in the Austin Chalk

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

Figures

1. Location of the Austin Chalk outcrop; major hydrocarbon-producing fields: Balcones, Luling, Mexia, and Talco fault zones; and the Ellis County study area

2. Sedimentation on the Texas shelf and its relationship to sea-level curves

3. Upper Cretaceous paleogeography

4. Generalized geologic map of part of Ellis County in north Texas, outline of SSC project area, location of boreholes used for stratigraphic and hydrogeologic data and outcrop locations

5. Composite section of Austin Chalk

6. Commercial wireline log showing typical SP log character of the Austin Chalk in Ellis County

7. Characteristic lithologies of unit T

8. Characteristic lithologies of unit A

9. Unit A channels, well exposed in the SSC tunnel

10. Southwest-northeast cross section, showing representative gamma-ray-log stratigraphic correlations of units A, B, C, and D along strike, northwest part of the SSC ring

11. Characteristic lithologies of units B and D

12. Characteristic lithologies of units E through I

13. Characteristic lithologies of units K and L

14. Stratigraphic subdivision of Austin Chalk. Ellis County, Texas, and regional fracture intensity of individual chalk units

15. Characteristics of chalk-marl cycles observed in outcrop and core

16. Plot showing cycles defined by percent insoluble residue and total organic carbon

17. Average chalk- and marl-bed thickness of grouped informal subsurface units

18. Petrography of typical chalk

19. Typical SEM views of chalk

20. Photographs of microstructures in typical chalk

21. SEM photographs of clay coats obscuring details of nannoplankton tests and filling pores in most chalk samples from the middle Austin Chalk

22. Petrography of typical marl

23. Trend with depth in porosity and permeability

24. Graph showing relationship between porosity and permeability

25. Vertical distribution of acid-insoluble material in chalk and marl

26. Average percent clay-sized material in chalk and marl

27. XRD charts showing clay composition of representative samples of acid-insoluble residues from several stratigraphic intervals

28. Vertical distribution of percent of sand-sized materials from acid-insoluble residue

29. Photomicrographs of sand-size materials from acid-insoluble residues

30. Vertical distribution of δ¹⁸O and δ¹³C in calcite through the Austin Chalk

31. Graph showing relationship between δ¹³C and δ¹⁸O in chalk and marl samples

32. Microdistribution of clay, drawn on the basis of SEM examination of typical samples from various Austin Chalk units

33. Graph showing relationship between chalk permeability and δ¹⁸O of calcite and δ¹³C of calcite, both showing a weak negative correlation

34. Block diagrams showing regional paleogeographic reconstruction of the Austin Chalk

Tables

1. Literature on stratigraphy and biozonation of the Austin Chalk

2. Dominant lithologies within the Austin Chalk

3. Characteristics of Austin Chalk facies

4. Petrologic characteristics of informal Austin Chalk units

5. Porosity and permeability data from analysis of core plugs

6. Stable isotopes from representative chalk and marl in the Austin Chalk


Citation
Hovorka, S. D., 1998, Facies and Diagenesis of the Austin Chalk and Controls on Fracture Intensity--A Case Study from North-Central Texas: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 98-2, 47 p.