The rapid expansion in exploration and development of fine-textured unconventional hydrocarbon reservoirs (siltstone, mudstone, shale: ‘mudrock’) has increased interest in the significance of several types of structural fabrics revealed by core examination. The two main fabrics described here are designated ‘polished slip faces’ (PSF), typically with slickensides and ‘cleavage.’ These fabrics are being linked to higher production intervals in several formations in western Canada and Montana. PSF and cleavage in variable frequency have been documented in Devonian, Mississippian, Triassic and Cretaceous rocks in the Western Canada Sedimentary Basin (WCSB), mainly in siliciclastic units, but also in limestones. Occurrence of PSF and cleavage in the WCSB appears to be most common within 100 kms of the Foothills Deformed Belt, and to increase in frequency toward that structural margin.
Analysis of these structural fabrics provides insights into the subsurface stress regime, orientation of stresses, temperature exposure, degree of organic maturation, relationship to overpressuring, generation of bitumen/pyrobitumen, horizontal and oblique shear displacement associated fluid types and mineralization, timing of formation, and other products and processes. Interest is driven by the possibility that these fabrics may play a role in decisions regarding orientation of horizontal drilling, and that they may influence orientation and behaviour of hydraulic fractures. This talk summarizes some of the current (but still evoving) observations and interpretations of PSF and cleavage fabrics.
*Graham Davies Geological Consultants (GDGC) Ltd.
**Canadian Discovery Ltd.