--> --> Mineralogical and Geochemical Properties of the Cretaceous Mowry Shale in the Wind River Basin, Wyoming Near the Casper Arch

AAPG Rocky Mountain Section Meeting

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Mineralogical and Geochemical Properties of the Cretaceous Mowry Shale in the Wind River Basin, Wyoming Near the Casper Arch


The Cretaceous (Albian-Cenomanian) Mowry Shale in the Wind River Basin of central Wyoming is a potential self-sourced, continuous shale oil play that may be prospective through the application of new drilling technologies. In order to improve the availability of data on the geochemical properties of prospective source rock plays, analyses of new core samples have been integrated with available literature data. New geochemical data presented herein are from three cores drilled south of the Casper arch in Natrona county. The purpose of the data collection was to better understand the depositional environment, source rock potential and mineralogical composition of thermally immature Mowry Shale using standard and multivariate data interpretation tools. Total organic carbon (TOC) and programmed pyrolysis analyses show that the Mowry Shale contains a thick interval (~200 ft) of mostly oil-prone, good-to-excellent source rock (TOC > 2 wt. %; hydrogen index > 300 mg/g; Tmax ~430°C). Major element data were combined with mineralogy results from X-ray diffraction and infrared spectroscopy to examine the major lithologies in the Mowry Shale, which primarily include mudstone with minor amounts of altered volcanic ash and sandstone. The dominant phases are quartz (chert) and clay minerals (illite, muscovite, and kaolinite) with some feldspar and pyrite, as well as trace carbonate abundances. Trace element parameters (Ni/Co, V/Cr and U and Mo enrichment factors) indicate anoxia was common during deposition of the organic-rich interval of the Mowry Shale. Multivariate curve resolution (MCR) applied to the S2 peaks of Rock-Eval type pyrograms permitted delineation of two organic matter types in the Mowry Shale. One is indicative of gas-prone, low hydrogen content kerogen sourced from land plants (Tmax ~440°C) and the other is more oil-prone with higher hydrogen content and likely sourced from marine organisms (Tmax ~420°C). This interpretation of the MCR model is consistent with petrographic results showing two kerogen types in samples from the Oil Mountain core. These results provide geochemical and mineralogical background on the Mowry Shale to support basin modeling, well log correlation, and other exploration and development efforts in the Wind River Basin.