Abstract In sedimetary basins not currently undergoing primary compaction (e.g., Rocky Mountain Basins), p-wave velocities noticeably vary with azimuth, yet the mechanism(s) controlling the anisotropy remain uncertain. Possible geologic causes for azimuthal anisotropy include but are not limited to sedimentary fabrics, steep bedding, changes in local in-situ or residual stress, and open or mineralized fractures. To test these hypotheses, P-wave velocity azimuths (Vfast) from a proprietary seismic survey of a NNW-trending Laramide Anticline on Casper Arch in central Wyoming were compared to image log data from the seismic coverage area and fracture orientations from nearby analog structures.

Abstract The Pennsylvanian-age Tensleep Formation in south-central Wyoming is comprised of repeated limestones, sandy limestones, and sandstones. Strata of these varied lithologic units are folded over Beer Mug Anticline and cut by numerous intersecting fractures. The anticline, with a near-vertical forelimb and backlimb dip up to 50 degrees, provides an ideal analog for fracture systems in tightly folded Paleozoic hydrocarbon reservoirs. Fracture type and degree of development vary systematically with lithology, structural position, and degree of folding. Fracturing is most intense towards the core of the anticline, which locally consists of folding. Fracturing is most intense towards the core of the anticline, which locally consists of brecciated, oil-stained rock with large-scale vuggy porosity. Most of these strata exhibit inherited (F0) fracture patterns that predate folding, as well as fold-related extension fractures that trend approximately normal (F1) and parallel (F2) to the axis of folding.