Ning Zhang and Carl E. Jacobson
Iowa State University
Extraction of hydrocarbons from unconventional reservoirs can be aided by an understanding of natural fracture systems. Here we report fracture characteristics for a potential unconventional target - the Upper Cretaceous Mowry Shale in the Bighorn basin, north-central Wyoming.
Analysis was conducted on Mowry outcrops in Late Cretaceous-early Eocene Laramide uplifts along the northeast margin of the Bighorn basin. Bedding in this area ranges from strongly folded (Alkali Anticline and Sheep Mountain, northwest of Greybull) to nearly flat-lying (Greybull platform, east of Greybull). Measurements were made in the various lithologies of the Mowry Shale ranging from bentonitic mudstone to fine-grained sandstone.
Four primary fracture sets occur throughout the study area. After rotating bedding to horizontal, fracture sets are steeply dipping with average strikes of 0°, 45°, 90°, and 135°. Sets are dominantly extensional, and locally filled with quartz and, less commonly, gypsum. 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 the latter 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 at depth.