Geochemistry of a thermally immature Eagle Ford Group drill core in central Texas

Abstract

The Upper Cretaceous Eagle Ford Group displays significant lateral and vertical geochemical variability. Much of the work on the Eagle Ford Group has been focused southwest of the San Marcos arch. To more fully characterize the Eagle Ford across the entire region, a thermally immature drill core was acquired north of the San Marcos arch that recovered the Pepper Shale and the Eagle Ford Group. Molecular and isotopic analyses of rock extracts were combined with bulk organic and inorganic geochemistry and mineralogy to track the variability in organic matter source and depositional environment, as well as to identify drivers of organic enrichment.The Pepper Shale received significant terrigenous organic matter in its distal deltaic or prodeltaic setting compared to the more distally deposited Eagle Ford Group that primarily hosts marine organic matter. The upper Eagle Ford contains two chemofacies. The older upper Eagle Ford chemofacies has similar mineralogy and organic matter to the underlying lower Eagle Ford, and both intervals display good to excellent source rock potential. In contrast, the younger upper Eagle Ford chemofacies has a different sterane assemblage, a larger terrigenous component, and a higher clay mineral abundance. However, anoxic to euxinic depositional conditions distinguish the lower Eagle Ford from the upper Eagle Ford, which was deposited under oxic to dysoxic conditions. Redox chemistry and organic matter source are two important variables that determine source rock quality, but they did not change in parallel during Eagle Ford deposition. Differences in organic facies explain the Tmax variability, and depositional redox conditions governed organic-richness in the Eagle Ford Group in central Texas.