BURIAL HISTORY AND HEAT FLOW IN THE TUSCALOOSA MARINE SHALE PLAY AREA: PRELIMINARY INSIGHTS FROM PETROGRAPHIC APPROACHES

The U.S. Geological Survey is assessing undiscovered hydrocarbon resources in the Upper Cretaceous Tuscaloosa marine shale (TMS), a shale-oil play in southwest Mississippi and adjacent Louisiana. Burial history and heat flow in the Cretaceous section are being evaluated through an integrated rock analysis approach. Maximum temperature (Tmax) determined from fluid inclusion microthermometry and calculated from vitrinite (VR_o) and solid bitumen (BR_o) reflectance are here compared to corrected bottom-hole temperatures (BHT).

Results from Lower Cretaceous rocks (James Ls, Rodessa and Paluxy Ss) downdip show strong correlation between BHT, aqueous fluid inclusion homogenization temperatures (Th) and BR_o, suggesting rocks are at maximum burial and BR_o is a good proxy for Tmax. Updip, BHT is ~30°C lower than Tmax from Th, suggesting significant cooling (uplift?) of the Lower Cretaceous section which may correspond to the regional mid-Cenomanian unconformity. In the Upper Cretaceous section (studied downdip only), BR_o consistently underestimates corrected BHT, suggesting BR_o is a poor proxy for Tmax, perhaps due to recent solid bitumen charge or misapplication of the BR_o geothermometer, i.e., the empirical calibration is invalid in the TMS petroleum system. Assuming validity of the BR_o geothermometer, results suggest more recent charge downdip, where BR_o underestimates corrected BHT by up to 60-70°C. VR_o is a better proxy for Tmax in the Upper Cretaceous but still underestimates corrected BHT by 15-25°C, again suggesting possible misapplication of the VR_o geothermometer. Or, assuming valid VR_o geothermometry, results may indicate present-day temperatures reflect a transient anomalous thermal high and VR_o disequilibrium. In support of the latter inference, corrected BHT in the area of a north-northwest trending present-day thermal high (Lohr et al. GSA 2015 abs) shows 15-20°C cooling from Tmax recorded by Th, suggesting cooling by uplift or variation in heat flux.

Results will inform 1-D burial history models and help delineate hydrocarbon assessment units in the TMS, and may provide insight to previous petroleum system models, which invoked long-distance lateral hydrocarbon migration. Results also may have broader application to geodynamics including origin and evolution of regional structural highs.