FACIES CHARACTERISTICS AND SEQUENCE STRATIGRAPHY OF THE UPPER CRETACEOUS FRONTIER FORMATION IN THE WESTERN POWDER RIVER BASIN, WY

The Upper Cretaceous Frontier Formation in the Powder River Basin in Wyoming is a well-known, prolific hydrocarbon reservoir. Recent technology advances in oil exploration and extraction resurrected the Frontier Formation as a prominent target interval for hydrocarbon development. In the Powder River Basin the 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 detailed facies analysis of thirteen sediment cores in the western Powder River Basin. The Wall Creek member is up to 60m thick and contains a complex assembly of facies, ranging from fine-grained mudrocks to coarse grained sandstones, with the latter forming the traditional reservoir facies. We identified a total of twelve distinct facies mainly based on their mineralogic and petrographic characteristics, and physical and biogenic sedimentary features. 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.

The results of our 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. This facies scheme provides new insights into the recognition and interpretation of the stratigraphic architecture and sequence stratigraphy in a depositional setting with complex mechanisms of sediment dispersal.