PRELIMINARY THERMAL GRADIENT EVALUATION IN THE TUSCALOOSA MARINE SHALE PLAY AREA

The U.S. Geological Survey (USGS) is conducting an assessment of undiscovered, technically recoverable continuous (unconventional) hydrocarbon resources in the Tuscaloosa Marine Shale (TMS), a shale oil play in southwestern Mississippi (MS) and southern Louisiana (LA). As part of the assessment, this study focused on systematic thermal gradient evaluation in the TMS play area to provide constraints on burial history and hydrocarbon generation.

Bottom hole temperatures (BHT) were obtained from the IHS Inc. Energy database for 261 wells over 24,444 km² for seven parishes in LA and nine counties in MS. Wells with BHT, log total depth, and time since circulation were used to calculate a corrected BHT (via Waples et al. [2004, Petroleum Geoscience v. 10, p. 239-245] method with average surface temperature). Thermal gradients were calculated using corrected BHT and range from 21.8 to 50.0 °C/km. The corrected BHT data were interpolated into geographic information system software to create a thermal gradient map.

The thermal gradient map reveals a robust north-northwest to south-southeast trending thermal high from Adams to Amite counties in southwest MS ranging from 31.0 to 50.0 °C/km. Lower gradients from 23.0 to 31.0 °C/km are seen in southwest Wilkinson County and to the east in Amite and Pike counties, MS. The north-northwest to south-southeast trending thermal maximum crosses the Adams County High (ACH), a poorly documented structure lying between the MS and LA Salt Basins which contains a large proportion of Mississippi’s conventional hydrocarbon production. Previous work by our team attributed anomalous thermal maturity in the lower Cretaceous section (determined by solid bitumen vitrinite reflectance [R_o]) on the ACH to paleoheat flux, consistent with higher present-day thermal gradients. Thermal gradient spatial trends are less defined in other portions of our study area with limited corrected BHT data. Thus, future work will include comparison of thermal gradients to R_o values from samples collected in the TMS play area. Additionally, expanding thermal gradient evaluation towards the Jackson Dome and Monroe Uplift where buried igneous intrusions are documented, and possibly higher thermal gradients, would assist in better understanding geology and burial history of the ACH and the TMS.