STRATIGRAPHY AND SEDIMENTOLOGY OF THE CRETACEOUS MOWRY SHALE IN THE NORTHERN BIGHORN BASIN OF WYOMING: IMPLICATIONS FOR UNCONVENTIONAL RESOURCE EXPLORATION AND DEVELOPMENT

The Cretaceous Mowry Shale is an extensive mudrock dominated unit and proven source rock historically thought to represent a single, long-term deep water depositional sequence within the Western Cordilleran Foreland Basin in the U.S. and Canada. 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 north-central Wyoming. Detailed measured sections were compiled at outcrop, including acquisition of 0.5 meter resolution spectral gamma radioactivity data by use of a portable gamma scintillometer. This data enables comparison between outcrop and wireline tools, allowing depositional trends to be tracked through the subsurface. Several coarsening upwards cycles are preserved, in places capped by sandy units which 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 where organic-rich shales and mudrocks are prevalent. 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. The results of this study will be a detailed assessment of the sedimentology, stratigraphy, and reservoir properties of the Mowry Shale across a transect spanning the Bighorn and Northern Powder River Basins. This study will enable the formation of a clear nomenclature scheme based on sedimentological and geomechanical components and the interpretation and calibration of wireline data. This facies-log calibration will then be applied to wells where only wireline logs are available to provide a more detailed regional assessment of petroleum system elements which will help aid future development of the Mowry Shale.