Fracture Development within Partially-Decoupled Basement-Involved Folds

Chris Zahm
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Resurgence in hydrocarbon exploration in the U.S. Rocky Mountains has broadened from traditional structural traps to more subtle secondary structures and compartmentalized reservoirs blocks. Furthermore, the presence of structural heterogeneities (e.g., reservoir-scale faults and fractures) has been shown to have a profound effect on hydrocarbon production in existing field by creating permeability anisotropy within reservoirs. Prediction of the occurrence, geometry and intensity of structural heterogeneities is difficult, but is further complicated in areas like the U.S. Rocky Mountains where significant rock strength contrasts create heterogeneous deformation mechanisms and vary the expected structural geometries.

In this study I propose mixed modes of deformation for basement-involved folds which are strongly dictated by the mechanical properties of the strata being deformed. The heterogeneous deformation styles have profound implications for the creation of fault-bound reservoir compartments in rocks associated with folding or overlying weaker strata which develop secondary faults and fractures. I will compare fault and fracture development of the basement-coupled Tensleep sandstone at Alcova Reservoir Anticline with partially-decoupled, Mesozoic strata exposed at Thermopolis Anticline. Results of this study will highlight potential areas of high deformation that may occur along the crest and forelimb Mesozoic strata within folds that may be prospective for limited hydrocarbon exploitation.

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