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Geochemistry of a Thermally Immature Eagle Ford Group Drill Core in Central Texas


The Upper Cretaceous Eagle Ford Group displays significant lateral and vertical geochemical variability. Much of the detailed work in the Eagle Ford has focused on areas southwest of the San Marcos Arch. To more fully characterize the Eagle Ford across the entire region, the U.S. Geological Survey acquired a thermally immature drill core north of the San Marcos Arch near Waco, Texas. The Pepper Shale, lower Eagle Ford, and upper Eagle Ford were recovered in the core. Molecular and isotopic analyses of rock extracts were combined with bulk organic and trace element geochemistry and mineralogy to track the variability in organic matter source and depositional environment, as well as to identify mechanisms of organic enrichment.

The biomarker results demonstrate that the Pepper Shale received significant terrestrial organic matter input in its prodeltaic setting compared to the more distally-deposited Eagle Ford that primarily hosts marine organic matter. The upper Eagle Ford contains two chemofacies. The older upper Eagle Ford chemofacies (UEF 1) has similar mineralogy and organic matter to the underlying lower Eagle Ford. In contrast, the younger upper Eagle Ford chemofacies (UEF 2) has a different sterane assemblage, a larger terrestrial component, and a higher clay mineral abundance. While biomarkers show that the lower Eagle Ford and UEF 1 have the same organic matter source, 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 the deposition of the Eagle Ford. Differences in organic facies, rather than thermal maturity, explain observed Tmax variability. Finally, our results show that depositional redox conditions and preservation governed organic enrichment in the Eagle Ford in central Texas.