Preliminary Assessment of Natural Fracture Patterns in Frontier Formation Sandstones, Southwestern Wyoming

Abstract Sets of fractures having north, west-northwest to east-northeast, and northeast strikes are present in outcrops of Cretaceous Frontier Formation sandstone along the western margin of the Green River Basin in southwestern Wyoming. The oldest fractures strike north and likely result from lateral stretching associated with Cretaceous subsidence of the northtrending foredeep, and possibly indentation of the Uinta Mountains from the south. Fractures with an easterly strike are younger; they cut across or abut north-striking fractures where the two sets occur together. East-striking fractures may reflect east-west shortening during Cretaceous/early Tertiary orogenesis, although they predate local folds and faults. Both within beds and on the scale of several beds, fracture networks have attributes that could govern the way fractures transmit fluids. Two or more fracture sets occur together only locally; more commonly, adjacent beds contain only one or the other set, causing the preferred direction of potential fluid-flow in fractures to switch by about 90° between adjacent beds. Where only one fracture set is present, fracture connectivity is low. Fractures generally terminate within beds or at bed boundaries, so networks are vertically partitioned. Commonly, fractures are arranged in swarms characterized by abrupt variations in fracture density.