Origin of organic matter in the Eagle Ford Formation

The Upper Cretaceous Eagle Ford Formation is an organic-rich mudrock of economic significance for oil and gas exploration. In order to facilitate a better understanding of paleoceanographic conditions during Eagle Ford deposition, this study integrates the isotope chemistry of bulk organic matter with inorganic geochemical data. Measurements of total organic carbon (TOC), total N, delta(13)Corg, delta N-15 and inorganic major, and trace elements were taken from 166 Eagle Ford and Pepper Formation outcrop samples from McLennan County, central Texas. These data reveal the chemostratigraphic character and the evolution of Cretaceous seawater chemistry on the Texas shelf and allowed the identification of six distinct chemofacies that are useful for correlation purposes. Based on these data, changing paleoredox conditions were documented ranging from normal marine (oxic) conditions associated with the Pepper Formation, anoxic conditions associated with the Lower Eagle Ford Formation, suboxic conditions associated with most of the upper Eagle Ford, and then a return to normal marine conditions at the top of the Eagle Ford Formation. The high TOC content of the Lower Eagle Ford was most likely caused by high productivity that in turn drove conditions to anoxia. Geochemical data that correlate well with TOC were used to identify intervals of potential organic enrichment.