PRELIMINARY THERMAL MATURITY AND ORGANIC PETROLOGY OF THE UPPER CRETACEOUS TUSCALOOSA MARINE SHALE, MISSISSIPPI, USA

The Upper Cretaceous Tuscaloosa Marine Shale (TMS), a shale oil play in central Louisiana and southwestern Mississippi, is the focus of a planned USGS unconventional hydrocarbon resource assessment. To determine the thermal maturity and organic composition of the TMS in southwestern Mississippi, cuttings samples were collected from the Mississippi Office of Geology’s Core and Sample Library (depth range 9,850-13,956 ft).

The TMS samples were medium dark to dark gray with minor olive to brown gray coloration. TOC values ranged from 0.48-1.6 wt. % (mean TOC=0.77 wt. %, n=82). All of the organic petrographic samples (n=30) contained vitrinite, alginite, solid bitumen and inertinite macerals. The majority of the dispersed organics indicate a mix of Type II (marine) and Type III kerogen (terrestrial), but the majority of the organics were Type III gas-prone kerogen. TMS vitrinite reflectance (R_o) ranged from 0.57-0.99% R_o (mean vitrinite R_o 0.71%) placing thermal maturity in the early oil window. Vitrinite R_o values tend to increase from 0.57% R_o in the east to 0.80% R_o in the west, with a decrease close to the Louisiana and Mississippi state line (0.62% R_o). The only organic matter visible using blue light fluorescence was solid bitumen (red) and wispy, narrow lamalginite macerals (bright yellow). Solid bitumen R_o was lower than co-occurring vitrinite at 0.33-0.70% R_o (mean solid bitumen R_o 0.52%; n=29) but lower than inertinite (>1.0% R_o). The Jacob (1989) and Landis and Castaño (1994) equations overestimated vitrinite R_o placing estimated thermal maturity closer to the early condensate-wet gas window (vitrinite R_o 1.0-1.4%). Results showed 57% of values from the Jacob equation (0.65-0.91% R_o; mean 0.79% R_o) were within a standard deviation of measured vitrinite R_o (std. dev. 0.06-0.13) compared to only 13% using Landis and Castaño equation (0.73-1.06% R_o; mean 0.90% R_o). Even though some samples in the dataset are updip of the current TMS shale oil play, TOC and organic petrology show that the TMS in southwestern Mississippi has a fair to poor source rock potential (<1 wt. % TOC). Additional samples and further analysis are needed to determine where significant hydrocarbons are being generated in the TMS.