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Mudrock Depositional Environments and Their Significance in Unconventional Resource Plays: An Example From The Cenomanian to Turonian Eagle Ford Group in South and West Texas

Abstract

Mudrocks generally are characterized as homogeneous and monotonously uniform sedimentary rocks that formed from fine-grained (<62.5 µm) sediments settling out of the water column under low energy conditions. Datasets from five petroleum industry wells, two outcrops (Lozier Canyon and Antonio Creek) and a research well drilled behind the Lozier Canyon outcrop provide a robust chronostratigraphic framework for the Cenomanian to Turonian Eagle Ford Group strata and give a better understanding of the stratigraphic variations related to heterogeneities in organic-rich mudrock depositional settings. High-resolution outcrop studies including petrographic analyses were correlated with subsurface core descriptions from Webb, McMullen, Live Oak and Karnes County, Texas. Biostratigraphic data, ash bed uranium - lead (U-Pb) zircon geochronology, carbon isotope chemostratigraphy, energy dispersive X-ray fluorescence (ED-XRF), X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) data were integrated from outcrops and cores to make regional well log correlations. The biostratigraphy and U-Pb zircon geochronology provide chronostratigraphic constraints for detailed outcrop to subsurface correlation. Total organic Carbon (TOC), molybdenum (Mo), uranium (U) and vanadium (V) enrichment factors indicate that deposition of the Lower Eagle Ford Formation occurred under anoxic to sulfidic (euxinic) conditions with variations in paleo-hydrographic restriction, whereas the Upper Eagle Ford Formation was deposited mostly under oxic conditions and records varying levels of bioturbation index (BI). Sedimentological data from outcrops and subsurface cores indicate that the Lower Eagle Ford Formation depositional setting was affected by storm waves and currents across the seafloor, even during periods of widespread anoxia and episodic photic zone euxinia. Bioturbation in the Upper Eagle Ford Formation varies from small chondrites burrows to large (> 0.5 cm diameter) vertical to subvertical burrows. Although bioturbation in the Upper Eagle Ford Formation is common, cores from wells located in a more distal setting record low BI with persistently high TOC values (> 3.5%). The data presented here indicate that organic-rich mudrock depositional settings are complex and heterogeneous and that they require a multidisciplinary approach to unravel subtle variations that are critical to successfully develop mudrock resource plays.