Electrical Imaging Catalog: Microresistivity Images and Core Photos from Fractured, Karsted, and Brecciated Carbonates

GC9702. Electrical Imaging Catalog: Microresistivity Images and Core Photos from Fractured, Karsted, and Brecciated Carbonate Rocks, by Ursula Hammes. 40 p., 32 figs., 1997. ISSN: 0082-3309. Print.


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About This Publication
This study provides a comprehensive comparison of carbonate features in cores with a suite of electrical imaging logs. More than 20 Lower Ordovician Ellenburger Group cores were logged, described, and photographed, and more than 15 electrical imaging logs were interpreted. Funded by the Gas Research Institute and the U.S. Department of Energy.

ABSTRACT
Electrical images in boreholes are becoming increasingly important in interpreting the rock record. In addition to identifying fractures and faults, borehole imaging tools are used in a variety of other applications, such as horizontal drilling; environmental studies; stress-orientation-measurement studies; sequence stratigraphy; and paleotransport, facies, and diagenetic analyses. These tools produce electrical microconductivity images of the well bore, which are interpreted at an interactive graphics workstation, High-resolution (-2.5-mm) and nearly complete borehole coverage can greatly increase the detail and precision of geological interpretations. Yet, to be fully useful, borehole images should be calibrated with core. This study provides the first comprehensive comparison of carbonate features in cores with a suite of all currently available electrical imaging logs. The Lower Ordovician Ellenburger Group, West Texas, serves as a model of a dolomitized, fractured, karsted, and brecciated carbonate reservoir.

Characteristic reservoir features, including fracture breccia, chaotic breccia, laminated mudstones, grainstones, and bioturbation, are identified both on electrical imaging logs and in cores. Electrical images provide more complete information than do cores in cavernous and highly fractured zones because cores either commonly show no recovery or occur as rubble in these zones, which are the most productive in the Ellenburger reservoir. Borehole imaging therefore not only provides in situ visualization of cavernous porosity, chaotic breccias or conglomerates, and highly fractured intervals, but also provides other key insights into karst stratigraphy.

Keywords: breccias, carbonates, core images, electrical microresistivity images, Ellenburger Group, fractures, karst, West Texas

CONTENTS

Abstract

Introduction

Overview of Electrical Imaging Tools

History

FMI-FMS Logging Tools

EMI Logging Tool

Western Atlas Imaging Tool

Imaging Principles

Workstation Procedures

Images of the Ellenburger Group

Geology of the Ellenburger Group

Carbonate Features in Cores and Imaging Logs

Fractures

Depositional fabrics

Postdepositional fabrics

Quality-Control Measures

Summary

Acknowledgments

References

Figures

1. Location map of study area in West Texas

2. Schematic representation of FMI tool

3. Schematic representation of EMI tool

4. Schematic representation of STAR Imager

5. Electrical images derived from microresistivity measurements of an array of electrodes aligned on a pad

6. Interpretation guidelines for differentiating features on electrical images in carbonates

7. Schematic representation of breccia types in Ellenburger paleokarst deposits

8. Images characteristic of artificial fractures

9. Illustration featuring a fracture plane that intersects the cylindrical borehole surface at an inclined angle to the borehole axis

10. FMI image and core photos of a dolomudstone

11. FMI image and core photos of a mottled dolomudstone

12. EMI image and core photos of a dolomudstone

13. FMI image and core photos of a laminated dolomudstone facies

14. FMS image and cores of a laminated dolomudstone

15. FMI image and core photos of a bioturbated dolomudstone

16. FMI image and core photos of a bioturbated dolomudstone displaying resistivity patterns on the electrical images slightly different from those in figure 15

17. EMI image and core photos of a dolograinstone

18. FMI image and core photos of vuggy and solution-enlarged porosity in a dolograinstone

19. EMI image and core photos of a fracture breccia

20. FMI image and core photos of a fracture breccia

21. EMI images and core photo of a fracture breccia

22. EMI image and core photos of a mosaic breccia

23. FMI image and core photos of a mosaic breccia

24. EMI image and core photos of a carbonate-matrix-supported chaotic breccia
25. FMI image and core photos of a carbonate-matrix-supported chaotic breccia

26. FMS image and core photos of a carbonate-matrix-supported chaotic breccia

27. FMI image and core photos of a chaotic breccia

28. FMS image and matching core photo of carbonate-matrix-supported chaotic breccia

29. FMS image and matching core photo of highly altered dolomite host rock and carbonate-matrix-supported chaotic breccia

30. FMS image and matching core photos of a carbonate-matrix-supported breccia

31. EMI image and core photo displaying unconformity on top of the Ellenburger group

32. EMI image and core photos displaying cave-fill sequence having a shale layer on top reflected as a highly conductive, slightly sinusoidal, 6-inch-thick (1 5-cm) layer underlain by resistive, large carbonate and shale clasts in a low-resistivity, siliciclastic matrix

Citation
Hammes, Ursula, 1997, Electrical Imaging Catalog: Microresistivity Images and Core Photos from Fractured, Karsted, and Brecciated Carbonate Rocks: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 97-2, 40 p.