Calibrated log facies of the Eagle Ford Shale of the East Texas Basin and a revised regional evaluation of the depositional environments of Cenomanian–Turonian strata

The Eagle Ford Shale in the East Texas Basin contains Cenomanian–Turonian aged deposits that vary significantly in composition, thickness, and sedimentology across the basin. These deposits differ from age‑equivalent strata that form continuous (unconventional) oil and gas reservoirs of southwest Texas. The relatively limited number of studies of the Eagle Ford Shale in the East Texas Basin largely due to the lack of active exploration and development in the area, and the presence of significant gaps in the stratigraphic record across the Cenomanian–Turonian boundary interval. To address these knowledge gaps, the U.S. Geological Survey Energy Resources Program drilled four cores targeting the Eagle Ford Shale along the Cretaceous outcrop belt of the East Texas Basin. Core locations span from Georgetown, Texas in the southwestern part of the study area to Bonham, Texas in the northeast. These cores contain a range of lithologies that are interpreted to represent distal pelagic offshore environments to fluvial‑deltaic influenced systems that vary spatially and stratigraphically across the basin. Each core penetrates the entire Eagle Ford Shale with nearly 100% recovery and is accompanied by a full well log suite.

High resolution (≤2 ft.) geochemical sampling, detailed sedimentologic descriptions from core and thin section observations, and biostratigraphic age determinations were completed for each core. These data and observations provide a basis for calibration with petrophysical properties to develop log‑based facies models of the Eagle Ford Shale. These core‑based models then serve as calibration points for stratigraphic correlations to nearby wells containing relevant geophysical log suites. Once correlations were established, cross‑sections and maps of the interpreted facies were made across the East Texas Basin to evaluate basin‑scale variability of age‑equivalent lithologies, providing key insights into the depositional processes and environments across the study interval.