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AAPG Bulletin

Abstract

AAPG Bulletin, V. 99, No. 12 (December 2015), PP. 22212245

Copyright copy2015. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/07061513012

Pitfalls of using entrenched Previous HitfractureNext Hit relationships: Fractures in bedded carbonates of the Hidden Valley Fault Zone, Canyon Lake Gorge, Comal County, Texas

Ronald N. McGinnis,1 David A. Ferrill,2 Kevin J. Smart,3 Alan P. Morris,4 Camilo Higuera-Diaz,5 and Daniel Prawica6

1Department of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; [email protected]
2Department of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; [email protected]
3Department of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; [email protected]
4Department of Earth, Material, and Planetary Sciences, Geosciences and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; [email protected]
5Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; ECOPETROL, Calle 37 8-37, Bogotá, Colombia; [email protected]
6Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166; Daniel Prawica Photography, 758 Concord Lane, Barrington, Chicago, Illinois 60010; http://www.danielprawica.com

ABSTRACT

Characterizing natural Previous HitfractureNext Hit systems involves understanding Previous HitfractureNext Hit types (faults, joints, and veins), patterns (orientations, sets, and spacing within sets), size distributions (penetration across layering, aperture, and trace length), and timing relationships. Traditionally, observation-based relationships to lithology, mechanical stratigraphy, bed thickness, structural position, failure mode, and stress history have been proposed for predicting Previous HitfractureNext Hit spacing along with the relative abundance of opening-mode Previous HitfractureNext Hit versus faults in fractured rocks. Developing a conceptual Previous HitfractureNext Hit model from these relationships can be a useful process to help predict deformation in a fractured reservoir or other fractured rock systems. A major pitfall when developing these models is using assumptions based on general relationships that are often site specific rather than universal. In this paper, we examine a mixed carbonate-shale sequence that is cut by a seismic-scale normal fault where Previous HitfractureNext Hit attributes do not follow commonly reported Previous HitfractureNext Hit relationships. Specifically, we find (1) no clear relationship between frequency (or spacing) of opening-mode fractures (joints and veins) and proximity to the main fault trace and (2) no detectable relationship between Previous HitfractureNext Hit spacing and bed thickness. However, we did find that (1) the frequency of small-displacement faults is strongly and positively correlated with proximity to the main fault trace, (2) Previous HitfractureNext Hit networks change pattern and failure mode (extension versus shear Previous HitfractureNext Hit) from pavement to pavement through the mechanically layered stratigraphic section, and (3) faults are more abundant than opening-mode fractures in many areas within the Previous HitfractureNext Hit network. We interpret that the major fracturing initiated near maximum burial under relatively high-differential stress conditions where shear failure dominated and that mode-1 extension fracturing occurred later under lower differential stress conditions, filling in between earlier formed shear fractures. We conclude that whenever possible, site-specific observations need to be carefully analyzed prior to developing Previous HitfractureNext Hit models and perhaps a different set of Previous HitfractureTop network rules apply in rocks where shear failure dominates and mechanical stratigraphy influences deformation.

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