Pitfalls of Using Entrenched Fracture Relationships: Fracture System within Bedded Carbonates of the Hidden Valley Fault Zone, Canyon Lake Gorge, Comal County, Texas

McGinnis, Ronald N.; Ferrill, David A.; Smart, Kevin J.; Morris, Alan P.

Characterizing fracture systems involves understanding fracture orientations, spacings, and sizes. Traditionally, observation-based relationships, such as lithology, mechanical stratigraphy, bed thickness, structural position, failure mode, and stress history, have been proposed for predicting fracture spacing as well as the relative abundance of joints versus faults in fractured rocks. Developing a conceptual fracture model from these relationships can be a useful process to help predict deformation in a fractured reservoir or other fractured rock systems of interest. A major pitfall, however, when developing these models, is using assumptions based on general relationships rather than site-specific observational data. In this paper, we examine a mixed carbonate-shale sequence in and adjacent to a seismic-scale normal fault where the fracture system does not follow several (or most) established fracture relationships. Specifically, we find that (i) there is no clear relationship between frequency of joints and proximity to the main fault trace, (ii) there is no detectable relationship between fracture spacing and bed thickness, and (iii) joint/fault ratios are far smaller than values typically reported for deformed rocks. However, we did find that (i) the frequency of small-displacement faults is strongly and positively correlated with proximity to the main fault trace, (ii) fracture networks change pattern and failure mode (extension versus shear fracture) from pavement to pavement in vertically adjacent beds, and (iii) faults are more abundant than joints in many areas within the fracture network. We conclude that a different set of fracture network rules apply in rocks where shear failure dominates.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013