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Organic Facies and Reservoir Characterization of Eagle Ford Shale as Determined by Stratigraphy, Source Rocks, and Oil Geochemistry


The source of the Cenomanian to Santonian petroleum systems across East and South Texas has been attributed to the Eagle Ford Shale play. However, little effort has been made to distinguish the relationship between the depositional settings, organic facies, oil families, and lithostratigraphic characteristics of the source rock. This study finds that there are significant variations in stratigraphy, reservoir type, and produced hydrocarbon chemistry between the South Texas Eagle Ford and the so-called East Texas Eagle Ford. The South Texas Lower Eagle Ford Shale reservoir facies is dominated by organic-rich, relatively low clay, foraminfera-rich, coccolith mudstones/marlstones, whereas the superficially equivalent source rocks in East Texas have a much more dominant terrestrial influence. In a regional reservoir modeling study at the confluence of East Texas and South Texas on the San Marcos Arch, the interplay of these depositional systems had to be accounted for to achieve reliable results. The model included analysis of cores from multiple counties combining detailed stratigraphic facies descriptions and petrophysical data from the base of the Austin Chalk to the Buda Formation. Source rock data is available from approximately 118 wells throughout this interval between the Austin Chalk and Buda. The dataset includes TOC, pyrolysis, and vitrinite reflectance data. Based on TOC analyses across the entire trend, the average TOC of the East Texas and South Texas Eagle Ford is 3.43%. Pyrolysis data and visual kerogen descriptions clearly show the South Texas Eagle Ford contains primarily Type II algae-rich oil prone kerogen. In contrast, the East Texas Eagle Ford contains type II kerogen with terrestrially derived mixed kerogen from the northeast. Thermal maturity in the Eagle Ford play area varies systematically with structure independent of the depositional systems from early oil generation to dry gas trending northwest to southeast. Produced oil geochemistry data from 70 oils include bulk molecular compositions, Pristane/nC17, Phytane/nC18, Pristane/Phytane, C13 to C20 isoprenoids, saturate and aromatic carbon isotope compositions, sterane and hopane ratios. The geochemical data suggest that the oils from the South Texas Eagle Ford and East Texas Eagle Ford plays are generated from two distinct types of organofacies. One type is dominantly carbonate mudstone sourced in South Texas, and the other type is siliciclastic marine shale sourced in East Texas.