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

The Cretaceous (Cenomanian) Mowry Shale is of significant current interest to petroleum explorationists in the western USA. There is as yet, however, no detailed geological framework or predictive model of the Mowry petroleum system to guide exploration and development. In this paper, we summarize initial results of ongoing research into the stratigraphy, sedimentology, and reservoir characteristics of the Mowry Shale in the northern part of the Bighorn Basin in Wyoming and Montana. The study encompasses the interval from the Muddy Sandstone at the base, through the fine-grained mudrock-dominated Shell Creek Shale to the somewhat coarser-grained (siltstone-rich) Mowry Shale, and upwards into the shale-dominated, basal Frontier Formation. This succession has previously been interpreted as a single, long-term depositional sequence. We have measured a series of detailed vertical sections at outcrop including collection of spectral gamma data for some sections, facilitating integration of outcrop data with wireline log suites from the subsurface. The Shell Creek Shale comprises a series of mainly fine-grained mudrocks with several coarsening-upward cycles each <20 m thick interpreted as offshore shelf to distal delta front deposits. Gamma readings are typically high (~5000 cpm) throughout. The overlying Mowry Shale is siltstone-dominated with minor interbedded sandstone and at least one sharp-based sandstone body, and is represented by an upward decline in gamma readings. Interpreted depositional environments range from prodelta to delta front. Outcrops near Cody, on the western side of the basin, preserve more and thicker sandstones within the Mowry Shale, indicating dispersal into the basin from the west. The contact between the Mowry Shale and overlying Frontier Formation is an abrupt fining trend into black shale, recording a major flooding event. Bentonites within both Shell Creek and Mowry Shales show high total gamma counts (up to 9000 cpm), high U and Th, but low K counts. Our data suggest a complex stacking pattern within the study interval, with multiple high-frequency sequences recorded. Mapping of these component intervals will facilitate an improved understanding of mechanical stratigraphy and reservoir distribution within the Mowry Shale.