New stratigraphic and geochemical insights into the Mowry Shale in the Powder River Basin, Wyoming and Montana, USA

The Cretaceous Mowry Shale in the Powder River Basin (PRB) of Wyoming and Montana is an emerging continuous (unconventional) resource. It consists of laminated, organic-rich, siliceous sediments that were deposited in a shallow epicontinental sea. Here we present regional subsurface mapping combined with facies descriptions, as well as geochemical and geomechanical data from 6 PRB Mowry cores. Data include TOC, TC, programmed temperature pyrolysis, ICP-OES/MS, continuous XRF, and Leeb hardness. We mapped over 3000 well logs and analyzed over 1200 feet of core from proximal and distal marine settings and varying thermal maturities.

We divided the Mowry Shale into three informal units based on regional stratigraphy: an upper, middle, and lower unit. Subsurface mapping indicates that the thickness of the Mowry Shale decreases from approximately 325 feet in the northwest PRB to 125 feet in the southeast. Overall, core hardness shows a positive correlation with Si and a negative correlation with Al. The upper and middle Mowry have the highest abundance of total Si and are harder than the lower Mowry. The middle Mowry, however, has the highest Si/Zr and least amount of observed sand and silt, suggesting a biogenic origin of Si. The middle and lower Mowry are enriched in TOC and redox-sensitive trace elements. The upper Mowry has the highest abundance of detrital sediment, as indicated by Zr and Ti. In core, this unit comprises significantly more silt- and sand-rich facies than the middle and lower units. Bentonites occur throughout all cores, ranging in thickness from <0.1–>2.3 feet. Silicified facies, which are hard, gray, and vitreous, often occur immediately above and below many bentonites, but can also occur without any relation to bentonites. The enrichment of silica and the corresponding increase in hardness are attributed to the presence of radiolarians and the diagenesis of bentonite.