SCIENTIFIC BENEFITS OF USGS CORING ACTIVITIES IN TEXAS

Since 2015, the U.S. Geological Survey Energy Resources Program (ERP) has collected five drill cores in Texas to study mudstones deposited in the southern part of the Cretaceous Western Interior Seaway. All boreholes were cored and logged to depths of less than 300 m from ground level through stratigraphic intervals that include: (1) Cenomanian–Turonian mudstones (Eagle Ford Group, Pepper Shale, Woodbine Formation); (2) over- and under-lying carbonate units (Austin Chalk Group, Buda Limestone); and (3) other units that provide information on the regional stratigraphy and paleoenvironmental conditions (Del Rio Clay Formation). These shallow, mudstone-focused drilling projects have delivered great scientific value by providing thermally immature examples of petroleum source rocks, facilitating studies of stratigraphy, sedimentology and paleoenvironmental conditions, and providing a testbed to develop and improve geochemical proxies. Of particular interest is the variable presence of the Cenomanian–Turonian carbon isotope excursion related to Oceanic Anoxic Event-2. Detailed lithologic and petrographic descriptions have been conducted along with collection of extensive geochemical, mineralogical, and spectroscopic datasets on samples from these drill cores. Collaborative studies are also being conducted with academic and industry partners. The datasets highlight mudstone variability, showing a range of lithologies and petrophysical properties and organic matter from multiple sources deposited under variable conditions. For example, variability in organic matter content and quality in the Eagle Ford indicates shifting conditions in the water column with time and location due to localized differences in redox and detrital inputs. Additionally, we interpret that the Woodbine Delta in northeast Texas influences the cores collected near Waco and Dallas based on sedimentologic, mineralogical, and organic properties. Missing strata in the cores is attributed to localized hiatus or erosion and scouring events at or around the time of deposition. These localized instances of missing strata can complicate attempts to correlate stratal units across the region. This challenge reinforces the need for detailed chemo-, bio-, and chrono-stratigraphic controls in sedimentological studies.