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Preliminary Evaluation of Source Rock Potential and Burial History of the Upper Cretaceous Tuscaloosa Marine Shale in Mississippi and Louisiana, USA

Abstract

As part of a planned USGS unconventional hydrocarbon resource assessment, this study focuses on the source potential and burial history of the Upper Cretaceous Tuscaloosa marine shale (TMS), a shale oil Previous HitplayNext Hit in central Louisiana (LA) and southwestern Mississippi (MS). Core and cutting samples were collected from MS, LA, and Texas State core repositories (depth range 8,490-20,030 ft) to analyze organic geochemistry, thermal maturity, and fluid inclusions in and near the current Previous HitplayNext Hit area. Total organic carbon (TOC) content ranged from 0.39-1.6 wt. % (mean TOC 0.80 wt. %, n=99) suggesting Previous HitfairTop-poor source potential. S2 values (0.12-1.85 mg HC/g rock; n=45) indicate very poor hydrocarbon generative potential (<3 mg HC/g rock). Kerogen typing using HI vs. OI showed a mix of Type II (oil-prone, marine), III (gas-prone, terrestrial), and IV (inert) kerogen, with the majority of samples plotting as Type III. Thermal maturity based on PI versus Tmax ranged from immature to early condensate/wet gas. However, vitrinite reflectance (Ro; avg. TMS Ro 0.72%; n=36) indicated early oil (∼0.6% Ro) to early wet gas/condensate (1.1% Ro). Organic petrographic analysis showed samples contained vitrinite, solid bitumen and inertinite macerals, supporting the geochemistry results indicating primarily a Type III kerogen. Based on our data, further analysis is needed to determine where liquid hydrocarbon sources occur in the TMS. Using bottom hole temperatures (BHT) obtained from IHS Energy from LA (7 parishes) and MS (9 counties), thermal gradients were calculated using corrected BHTs for 261 wells. Thermal gradients (total range 21.8-50.0°C/km) showed a robust north-northwest to south-southeast trending thermal high for southwestern MS (range 31.0-50.0°C/km). Lower gradients were also found in southwestern MS to the west-southwest at 23.0-31.0°C/km. Assuming valid Ro geothermometry, results may indicate present-day temperatures reflect a transient anomalous thermal high and Ro disequilibrium. Corrected BHTs in the present-day thermal high showed 15-20°C cooling from maximum temperature recorded by aqueous fluid inclusions, possibly suggesting cooling by uplift or variation in heat flux. These results will be used to inform 1-D burial history models and help delineate hydrocarbon assessment units in the TMS.