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Stratigraphy and Sedimentology of the Cretaceous Mowry Shale in the Northern Bighorn Basin of Wyoming: Implications for Unconventional Resource Exploration and Development


The Mowry Shale (Cenomanian/Albian) is an extensive mudrock dominated unit historically believed to represent a single, long-term deep water depositional sequence within the Western Cordilleran Foreland Basin of North America. While the Mowry Shale has received study for almost a century, detailed stratigraphic context is still lacking. Such an understanding will be required as the formation is increasingly explored as an unconventional resource play. This study examines the stratigraphy of the interval between the Muddy Sandstone and the Frontier Formation, focusing on the Mowry Shale both at outcrop and in the subsurface of the Bighorn Basin of northern Wyoming. Detailed measured sections were compiled at outcrop, including acquisition of spectral gamma radioactivity data at 0.5 m spacing to emulate the responses of the formation to subsurface wireline gamma logs. Together, these data sets inform a detailed correlation of proximal to distal stratigraphic architectures across the Bighorn Basin. Bentonite beds throughout the succession show high total gamma counts (up to 9000 cpm), with high spectrographic signatures from Uranium and Thorium, but relatively low Potassium counts. Basin floor organic-rich shales and mudrocks are prevalent in the lower parts of the succession, grading upward into platy to blocky, often silicified siltstones with subordinate interbedded sandstone of interpreted prodelta to distal delta front origin. The upper part of the Mowry Shale preserves several sandstone bodies each up to 3 meters thick, of interpreted delta front origin. Several coarsening upwards cycles are preserved, in places capped by sandy units. These bodies are expected to contain the primary reservoir within the Mowry Shale. Our data suggest that these sandy zones thicken westward towards the emerging Rocky Mountain Highlands and thin distally to the southeast into the Mowry Sea. Our data suggest a complex stacking pattern within the study interval with multiple high-frequency sequences recorded, some of which are up to 50 meters thick. Mapping these component intervals facilitates an improved understanding of mechanical stratigraphy and reservoir distribution within the Mowry Shale.