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An Integrated Stratigraphic, Sedimentologic, and Petrophysical Study of the Upper Cretaceous Frontier Formation in the Western Powder River Basin, WY


In the western Powder River Basin in Wyoming, the Upper Cretaceous Frontier Formation is divided into three distinct clastic wedges that reflect episodic shoreline progradation from the west to the east into the Cretaceous seaway. The youngest of those progradational wedges is known as the Wall Creek member and is the focus of this study. We present results from an integrated geologic, petrologic, and petrophysical study based on detailed facies analysis of thirteen sediment cores and over 800 wireline log suites in the western Powder River Basin. The Wall Creek member varies in thickness from <10m to 60m in this area, and contains a complex assembly of facies, ranging from fine-grained mudstones to coarse grained sandstones, with the latter forming the traditional reservoir facies. We identified a total of twelve distinct core facies and three sub-facies based on grain size, physical and biogenic sedimentary features, ichnology, and petrology. A wide variety of cross-stratified and rippled sandstones, bioturbated sandstones and siltstones, and mud rip-up clasts and mud drapes are indicative of a wide range of sedimentary processes and depositional environments, ranging from offshore to mouthbars. Similarly, ichnofacies assemblages range from a diverse and robust infaunal community in some of the facies, suggesting substantial reworking of those facies in a normal marine offshore transition zone environment, to a trace fossil suite recording environmental stresses closely associated with nearshore (tidally influenced) deltaic settings. Propagation of these core facies at wireline log resolution across a geographically significant and well resolved area was accomplished through principle component analysis utilizing wireline log suites. We upscaled the twelve identified core facies to nine uniquely identified log (electro) facies based upon their statistical occurrence. Basin scale isopach maps of log facies distribution offer an added geologic comprehension of the stratigraphic architecture and reservoir distribution in the Wall Creek member. The results from our integrated study provide a new framework of the facies heterogeneity observed in the Wall Creek Member of the Frontier Formation in the western Powder River Basin.