Hydrocarbon Generation Potential and Expulsion Efficiency in Shales and Coals: Example from the Washakie Basin, Wyoming

Abstract The present study compares two source lithologies, shale and coal, that could have generated oil and gas in the Washakie Basin of Wyoming. Organic petrographic, geochemical, and clay diagenetic studies were performed on both shales and coals in order to understand the oil and gas generation in, and the expulsion of hydrocarbons from, these source rocks. The results reveal that shale generates oil in the liptinite layers. As depth of burial increases, a portion of the generated oil migrates out from the liptinite layers into the clay matrix. With further burial, some of the oil impregnating the clay matrix migrates out, whereas the oil residue is thermally degraded to gas. In coal, on the other hand, the hydrocarbons are generated and subsequently stored in pores and vesicles. These hydrocarbons cannot migrate out until microfracturing of the coal matrix takes place. Liquid hydrocarbon expulsion was determined to be much more efficient in shale than in coal. Hydrous pyrolysis experiments performed on shale and coal in the Almond Formation indicate that the coal may generate up to ten times more oil and gas than the shale. Thus, Almond coal can be an important source of hydrocarbons, especially when highly fractured.