Sedimentology, Chronostratigraphy, and Reservoir Characterization of the Upper Turonian Wall Creek- Turner System, Powder River Basin, Wyoming
Dr. Patrick Sullivan
Mendenhall Postdoctoral Fellow at the United States Geological Survey
Abstract:
The Upper Turonian Wall Creek–Turner depositional system of the Powder River Basin (PRB), Wyoming, comprising the Wall Creek Member of the Frontier Formation and the coeval Turner Member of the Carlile Shale, is a prolific sandstone hydrocarbon reservoir whose age and depositional origin has long been debated owing to enigmatic "shelf-isolated" subsurface geometries and a lack of age control. Regional sedimentology, high-precision geochronology, and reservoir characterization are integrated into a unified framework for the Wall Creek–Turner system across the 150-mile-wide PRB. Detailed description of 19 outcrop and 37 cored sections defines twelve facies associations recording a low-gradient transition from proximal deltaic sandstones in the western PRB to distal storm-reworked distal lower-shoreface in the east. The apparent shelf-isolation of these sand bodies is shown to be a preservational artifact of a dynamic Cretaceous shoreline by eustatic and structural factors in a period of low sediment accommodation and does not reflect in-situ offshore-bar deposition. Five new CA-ID-TIMS 206Pb/238U ages, tied to ammonite zones and well-log correlations, demonstrate that the Wall Creek and Turner members are broadly coeval, share five progradational episodes within an overall retrogradational sequence, and are draped over a composite Mid-Turonian Unconformity whose greatest hiatus follows a central-basin “backwarp” which may be attributable to transient, long-wavelength lithospheric flexure associated with a documented thrust transfer in the Sevier hinterlands. Placed within this chronostratigraphic framework, over 1,200 routine core-analysis measurements tied to five biozone-defined reservoir units reveal that reservoir quality reflects the interplay of depositional facies, bioturbation, and compaction in this backstepping system. Together these results provide a predictive, genetic framework for the distribution of Wall Creek and Turner reservoirs in the PRB, new insights into the origin of the Mid-Turonian Unconformity, and the associated prevalence of apparent shelf-isolation in Turonian sandstones across the U.S. Western Interior.
Bio:
Patrick Sullivan just received his Ph.D. in geology from Colorado School of Mines, where he also received a geology M.S. in 2021. He spent his undergraduate years at Macalester College in St. Paul, Minnesota, also his hometown. His research at these institutions has spanned from the sedimentology of ancient deltas to the trace element geochemistry of marine mudstones, to documenting crocodile digestion traces on 70 million year-old gar scales. A unifying theme of Patrick’s work is uncovering how ancient landscapes govern the modern distribution of resources (water, oil and gas, dinosaurs, etc.), and he integrates well logs, outcrop and core sedimentology, paleontology, and geochemical datasets in his research. He is currently applying those skills as a Mendenhall Postdoctoral Fellow at the United States Geological Survey to investigate the geologic factors behind orphaned oil and gas well emissions, allowing for more precise regional (and global) emission estimates and cost-effective plugging strategies.
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