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Organofacies Variability as a Function of Provenance and Process—Heterogeneity Within the Mowry Shale, Wyoming

Abstract

The Lower Cretaceous Mowry Shale, exposed in the Wind River and Hanna Basins in southern Wyoming, was deposited in the foreland basin of the Sevier orogeny along the western margin of the Cretaceous Interior Seaway. Previous studies have characterized the broad changes in organofacies throughout the basin, with total organic carbon (TOC) and Hydrogen Index (HI) increasing to the southwest on a regional scale. However, finer-scale changes based on autogenic drivers could have important impact on organofacies distribution on a finer scale. Deltaic input points, and delta lobe switching through time, should determine the distribution of low HI, gas-prone source facies within the basin, and understanding their effects on organofacies distribution on a local to sub-regional scale has important implications for unconventional exploration, as well as providing an analog for other similar systems. Through collection of hundreds of samples, we characterize geochemical trends in the Mowry Shale relative to fluviodeltaic input along the southwestern margin of the seaway through organic variation and provenance. Outcrops are up to 200 km apart, showing spatial variation, and multiple bentonite layers two to five feet thick dissect the shale, providing important stratigraphic control across the study area. Rock-Eval Pyrolysis and X-Ray Fluorescence are used to constrain paleoenvironment through organic and geochemical variations. Initial tests confirm a significant increase in total organic carbon moving toward the East (into the basin), as well as a higher marine organic input. Understanding these variations at a finer scale as sampling progresses will allow greater predictability of facies and organic quality within the Mowry as a hydrocarbon source.