THERMAL MATURITY IN THE TUSCALOOSA MARINE SHALE PLAY AREA: PRELIMINARY OBSERVATIONS FROM STUDIES TO SUPPORT OIL AND NATURAL GAS RESOURCES

The U.S. Geological Survey (USGS) is conducting an assessment of undiscovered, technically recoverable, continuous hydrocarbon resources in the Tuscaloosa marine shale (TMS) play area of Mississippi (MS) and Louisiana (LA). In support of this assessment, this study focuses on comparing vitrinite reflectance (VR_o) trends to geothermal gradient (GTG) and overpressure trends in southwestern MS and southeastern LA to provide potential insight into thermal maturity boundary delineation of TMS assessment units.

Previous work by the authors 1) revealed a northwestward-trending present-day area of higher GTG from southwestern MS to northeastern LA and 2) provided VR_o data in the TMS. An updated GTG map, which includes data only from wells that penetrate the lower Tuscaloosa or deeper, reveals that there are no obvious relationships between VR_o and GTG or VR_o and present-day burial depth in our study area. Similar VR_o values are present in areas with lower GTG in southern MS in Wilkinson and Amite counties as well as areas farther updip that have higher present-day GTG. Although the depth to burial of the TMS is deeper in southern MS, a significant increase in VR_o values is not observed relative to VR_o updip at shallower burial depths.

However, preliminary relationships are observed between major sediment pathways of the Upper Cretaceous in MS and previous USGS work on overpressure in the Gulf of Mexico. Generally, the depth at which 0.60 psi/ft overpressure occurs corresponds with the location of sediment pathways and the depth of the lower Tuscaloosa top. Overpressure, which can be caused by rapid sediment loading, is known to retard the maturation of organic matter and, subsequently, VR_o. This could offer a potential explanation regarding the lack of correlation between VR_o and present-day burial depth. However, additional research into the effects of these mechanisms on thermal maturity is required. VR_o analyses of core and cutting samples recently collected from the TMS will also provide further insight into these preliminary observations.