12 Mowry Shale–Belle Fourche Shale, Bighorn Basin, Wyoming, USA—A Mesozoic Clastic-Biosiliceous Shelf System: A Prolific Source Rock with Associated Mudstone Reservoir Potential

ABSTRACT The Cretaceous Mowry Shale and associated units in the Bighorn Basin of Wyoming, illustrate the expression of sequence-stratigraphic surfaces and units in a Mesozoic epeiric sea in a proximal to distal, detrital-to-biogenic–dominated shelf setting. Associated units include the Muddy Sandstone, Belle Fourche Shale, and Frontier Formation and are of Albian–Cenomanian age. The Mowry Shale is a highly siliceous mudstone, significantly enriched in marine organic matter that served as a prolific source rock across the Western Interior basin of the United States. It is also a source of bentonite as well as a mudstone reservoir. Vertical and lateral variations of rock properties are predictable within a sequence-stratigraphic framework. The strata of the Mowry Shale and associated units record the interplay of intrabasin organic production (algal blooms) and associated surges in zooplankton population (mainly radiolaria) versus extrabasinal detrital siliciclastic input (a mixture of siliceous sand and clay minerals). Lowstand systems tracts are dominated by siliceous sandstone and minimal biogenic content because of poor primary production and dilution by extrabasinal detritus. The transgressive systems tract shows a marked change from a lower interval dominated by extrabasinal detrital argillaceous-siliceous mudstone to an upper interval dominated by intrabasinal biogenic siliceous and siliceous-carbonaceous mudstone. The intrabasinal biogenic siliceous and siliceous-carbonaceous mudstone continues into the lowermost interval of the highstand systems tract. Another distinct change, back to extrabasinal detrital argillaceous-siliceous mudstone to siliceous sandstone, marks the mid to upper highstand systems tract. This chapter illustrates how the sequence-stratigraphic approach can be scaled practically to the data and time available for investigation and does not depend necessarily on large amounts of original proprietary work. This systematic approach helps make full and appropriate use of published observations to complete a stratigraphic framework. This chapter also illustrates the construction of a provisional sequence-stratigraphic framework based on the type and amount of data commonly available early in the investigation of a basin: outcrop sections, well logs, and some published analyses (similar to our example of the Kimmeridge Clay Formation in Bohacs et al. [2022d, Chapter 11 this Memoir]). Such a preliminary framework is useful to focus more detailed analyses on targeted intervals and rock properties, and to integrate information gained later by biostratigraphy, geochemistry, geomechanics, and petrography.