Fracture and Small Fault Networks in Basement-Involved, Fault-Related Folds Above Oblique-Slip Faults: Insights from Physical Models

David P. Keating and Mark P. Fischer
Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL

Networks of fractures and small faults play an important role in determining reservoir produceability and trap viability in basement-involved, fault-related folds. Because many basement-involved, fault-related folds originate by reactivation and/or inversion of pre-existing normal faults, their formation often involves a component of oblique-slip on the underlying basement fault. Under oblique-slip conditions, fold-related fracture and small fault networks may have significantly different characteristics than in folds formed above pure dip-slip faults. This study uses scaled physical models to investigate how varying degrees of oblique slip affect fracture network properties in basement-involved, fault-related fold. The models consist of a single homogenous clay layer overlying a rigid basement with a single master reverse-fault. We systematically vary the degree of obliquity in each model and use close-range photogrammetry to quantify and examine the patterns of strain that develop in each model.

In our models shear strains are accommodated on the limb of the fold, while extension occurs at the top of the fold, and shortening occurs at the bottom of the fold. Our results show that with increasing obliquity there is an increase in extension in the upper region of the fold, and hence an increased likelihood of opening mode fracturing. Because shear strains are localized on the fold limb, it is important for any structural analysis of natural structures attempting to interpret oblique slip that the analysis is performed at the appropriate vertical position on the fold. Higher shear strains can lead to increased faulting in the limb of fold that can compartmentalize and act as a barrier to fluid flow. In summary, the results of our experiments suggest that increased oblique slip leads to increased secondary porosity via fracturing in the upper fold region, while at the same time permeability is decreased in the fold limb, decreasing reservoir quality while at the same time increasing sealing potential.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado