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High-Resolution Chemical Facies Analysis of the Cenomanian-Turonian Eagle Ford Formation: Sedimentation and Water Mass Evolution in the Maverick Basin, South Texas

Abstract

The Late Cretaceous Eagle Ford Fm represents marine deposition on the South Texas Shelf during late Cenomanian and Turonian time. The lower Eagle Ford (LEF) is a marl and limestone succession containing elevated %TOC (average TOC ~3.5%, maximum of 6.5%). The upper Eagle Ford (UEF) is a comparatively organic-lean (average TOC ~1%) interval consisting of limestones and marls. A 600-foot-long core containing the Eagle Ford shale was recovered from the Maverick Basin, South Texas. Mineralogical results were generated at a 2-foot interval using X-ray diffraction (XRD), and high-resolution (2-inch sampling interval) chemostratigraphic results were generated using handheld X-ray fluorescence (XRF) unit. The chemostratigraphic record of the Eagle Ford and underlying Buda Fm, supplemented by insights from XRD, TOC, and core description, was evaluated using a hierarchical clustering analysis (HCA) technique in order to (1) quantitatively identify dominant end member geochemical facies compositions, (2) evaluate the chemical facies in terms of their relative elemental rankings, (3) elucidate linkages between lithofacies and chemical facies, and (4) interpret the history of depositional and paleohydrographic conditions during accumulation of the Eagle Ford shale succession. Using a 14-cluster parameterization, HCA identified five dominant clusters that account for 89% of the chemical facies variability in the core. The majority of samples from the Buda Fm, and chemically similar limestones throughout the Eagle Ford succession account for 19% of chemical facies observed, which are best characterized as Mn-enriched limestones. The majority of the UEF limestones account for 26% of the chemical facies identified, which are characterized by enrichments in Sr and U. LEF euxinic marls account for 19% of the chemical facies, and are characterized by enrichments in redox-sensitive trace elements (Mo, V, Ni, Zn, Cu, As, U) and S. This facies also contains the highest %TOC values. Almost 15% of the succession is characterized as oxic marl, with minimal redox-sensitive trace elements, and enrichments in elements associated with siliciclastic detritus (Zr, Al, Ti, K, Rb). Approximately 10% of the succession is characterized as high-Ca oxic marl, with detrital enrichments in Rb, K, and Cr. When integrated with traditional core description, chemical facies provide important insights into vertical and lateral changes in depositional environments and hydrographic conditions in the basin.