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Stratigraphic and Geochemical Variability of The Cenomanian to Turonian Eagle Ford Group in Southwest Texas: Implications for Identifying Potentially Productive Hydrocarbon Pay Zones

Abstract

Stratigraphic correlation using core data, thin-sections, biostratigraphic data, wire line logs, and geochemical analysis from XRF and XRD analysis, bulk rock carbon and oxygen stable isotope variations and Rock-Eval total organic carbon (TOC) variations document vertical stratigraphic variations and paleo-redox conditions, that may be critical to hydrocarbon exploration and exploitation, in the Cenomanian to Turonian Eagle Ford Group in wells in Karnes, McMullen and Live Oak counties in the South Texas Submarine Plateau. The Eagle Ford Group in these wells is divided into lithological units based on a combination of depositional and diagenetic characteristics such as lithological variations identified in cores and skeletal grain types and abundance observed in thin-sections. The Lower Eagle Ford Formation is mainly composed of dark grey to black mudstone interbedded with light grey skeletal wackestone, packstone and grainstone, with the skeletal grainstone and packstone interbeds occurring more frequently higher up this interval. Most of the skeletal grainstone and packstone are internally massive, well cemented with a concave base and a convex top, suggesting they may have been differentially compacted with respect to the surrounding mudstone due to early cementation. The Upper Eagle Ford Formation is mainly composed of light grey skeletal grainstone and packstone with few interbeds of skeletal wackestone and mudstone. Several beds in this interval have scoured surfaces and soft-sediment deformation structures. Carbon isotope variations, commonly associated with oceanic anoxic events, is used in conjunction with biostratigraphic data and TOC data to determine the timing and magnitude of preservation of organic matter in these wells. Mo and U enrichment factors estimated from XRF data provide insight into the paleo-redox conditions at these well locations during deposition. The U-Mo enrichment suggest that the lower Eagle Ford Formation, reflecting an efficient particulate pump in redox-cline that periodically moved from the sediment up into the water column and efficient resupply Mo from the open ocean, was likely deposited under euxinic conditions, with TOC values ranging from 2 to 6% whereas the upper Eagle Ford Formation was likely deposited under anoxic conditions, with TOC values ranging from 1 to 3 %. Furthermore, Mo/TOC plots indicate that there was moderate water mass restriction at the study location during deposition.