PRELIMINARY INVESTIGATION OF THE ORGANIC PETROLOGY OF THE UPPER CRETACEOUS TUSCALOOSA FORMATION, MISSISSIPPI, USA

As part of an upcoming USGS energy resource assessment, we have conducted a preliminary study of the thermal maturity of the lower Tuscaloosa Formation within the downdip portion of the Mississippi Interior Salt Basin in southern Mississippi, USA. The Tuscaloosa Marine Shale, which overlies our sample interval, is an industry target as an unconventional shale oil play in southern Louisiana and southwestern Mississippi with a potential of 7 billion barrels, according to a 1997 study conducted by Louisiana State University’s Basin Research Institute. Shale core samples (depths 9,312 -12,884 ft) collected from six wells were examined for mineralogy, organic geochemistry, and organic petrographic analysis.

XRD mineralogy analysis showed that our samples (n = 19) are abundant in quartz (Mdn. 46 wt.%) and clay (Mdn. 46 wt.%) with only one sample containing greater than 15% carbonate (53 wt.%). Geochemical analysis (n = 9) found that samples were organic lean with TOC content ranging from 0.26-0.95 wt.% (Mdn. 0.60 wt.%). S2 ranged 0.06-0.49 mg HC/g rock (Mdn. 0.29 mg HC/g rock), indicating little present-day hydrocarbon generative potential, but this result is presumed to be unreliable due to the low TOC content of the samples. Six of the samples gave reasonable T_max values (430-439°C, cal. R_o ~ 0.58-0.72%) indicating thermal maturity is in the oil window. Organic petrographic analysis (n = 14), yielded solid bitumen (SB), vitrinite, and inertinite in all but two of the samples. Solid bitumen (R_o values ~0.3-1.0%) tended to be slightly fluorescent, wispy, void-filling, homogenous macerals compared to vitrinite (R_o values ~0.7-1.2%), which had higher relief with subangular edges. Based on SB mean R_o, samples ranged from 0.49-0.93% (Mdn. R_o = 0.71%) indicating a maturity in the oil-early wet gas/condensate window. No conclusive geographic trends could be determined from our limited data set, but a sample from our westernmost well (sample depth = 10,943 ft) on the Adams County High is dominated by lower reflecting SB (mean R_o = 0.58%; s.d. = 0.13; n = 21) while other samples at similar depths have R_o values of ~ 0.70-0.85%. This could suggest more pronounced episodic charging in the western part of the study area but further analysis is needed to better constrain thermal maturity and petroleum system processes in the Upper Cretaceous.