Introduction

Examination of over 1700 carbonate and siliciclastic cores in the Western Canada Sedimentary Basin (WCSB – including northern Montana) and to a lesser extent the Williston Basin by the first author over the last 35 years has revealed the common occurrence of linear structural fabrics along bedding planes in mudrocks (particularly where overpressured), and a related ‘ridge and groove’ fabric in limestones. In mudstones, these fabrics are manifested as polished slip faces (PSF), typically but not always with slickensides/striae, and cleavage. The same or similar fabrics have been observed in outcrop in the Utica shale of Quebec (Denis Lavoie, pers. com., 2011), and in core and outcrop in the Marcellus shale in New York State (Fronterra Geosciences). All of these fabrics are attributed to layer-parallel shortening (LPS).

Increasing exploration for oil and gas in fine-textured unconventional ‘mudrock’ reservoirs has brought focus on the potential role of these structural fabrics as an expression of subsurface stress conditions, in orientation and behavior of hydraulic fractures, and for anisotropic reservoir permeability implications. Release of paleomag oriented core data in one of these reservoirs in the WCSB adds many insights to the orientation and significance of the structural fabrics.

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