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Fracture Systems in the Mowry Shale, Bighorn Basin, Wyoming: Implications for Unconventional Energy Resources


Extraction of hydrocarbons from unconventional reservoirs can be aided by an understanding of both regional and local natural fracture systems. Here we report fracture characteristics for a potential unconventional target – the Upper Cretaceous organic-rich siliceous Mowry Shale of the Bighorn basin, north-central Wyoming. Mowry outcrops in Late Cretaceous-early Eocene Laramide uplifts were investigated along the northeast margin of the Bighorn basin. Bedding in this area ranges from strongly folded (Alkali Anticline and Sheep Mountain, northwest of Greybull, WY) to nearly flat-lying (Greybull Platform, east of Greybull). The upper part of the Mowry Shale typically includes three 1- to 10-foot-thick, very fine- to fine-grained sandstone layers interbedded with bentonitic mudstones and siltstones. Petrographic estimates of porosity range from approximately 0% to 15% throughout the formation. Fracture measurements were recorded in the various lithologies of the Mowry Shale. Four primary fracture sets occur throughout the study area. After rotating bedding to horizontal, fracture sets are steeply dipping with average strikes of approximately 0°, 45°, 90°, and 135°. Sets are dominantly extensional, and some are partly to completely filled with growths of quartz and, less commonly, gypsum. Quartz is also the dominant fill in micro-fractures observed in some thin sections. Fracture spacing, length, and intersection relationships are complex, and may relate to fold geometry, lithology, and degree of weathering. The 45° and 135° sets are best developed in areas of tight folding. These sets are respectively parallel and perpendicular to the Laramide compression direction and are considered to be directly related to folding. In contrast, where folding is weak (Greybull Platform), the 0° and 90° sets are dominant. The origin of these two sets is not clear. Nonetheless, results from the Greybull Platform may serve as an analogue for those in the interior of the Bighorn basin, where gently folded Mowry is present in the subsurface and is a possible exploration target.