ORGANIC PETROLOGY OF THE APTIAN SECTION IN THE DOWNDIP MISSISSIPPI INTERIOR SALT BASIN, MISSISSIPPI, USA

We have investigated a thermal maturity anomaly within the downdip Mississippi Interior Salt Basin (MISB) of southern Mississippi, USA, by examining the organic petrology of Aptian shales (Pearsall-equivalent Rodessa and Pine Island Formations). A stratigraphically equivalent section in south Texas contains an unconventional shale gas play with a U.S. Geological Survey (USGS) estimated mean undiscovered gas resource of 8.8TCF. USGS reconnaissance conducted in 2011-2012 examined Aptian thermal maturity across the onshore Gulf Coast Region and indicated the downdip MISB section was approaching the wet gas/condensate window (R_o~1.2%). A more focused study in 2012-2013 used 6 core samples and 46 high-graded cuttings samples (depth range of 12,000-15,000 ft below surface) collected from 15 wells in the downdip MISB for pyrolysis, TOC, and organic petrography analysis.

Due to an overlap of maceral-type reflectance ranges, macerals were identified as either 1) secondary solid bitumen (R_o generally <0.8 %; recently migrated and cracked oil?); 2) primary solid bitumen (R_o generally >0.8%; indigenous hydrocarbons matured in situ?); 3) solid bitumen/inert (macerals could not be identified with absolute certainty), 4) vitrinite; or 5) inertinite. Based on primary solid bitumen reflectance, R_o values increase regionally across the MISB from the southeast to the northwest. Thermal maturity in the eastern half of the basin (R_o range 1.0 - 1.25%) appears to be related to present-day burial depth and shows a gradual increase with respect to depth. However, thermal maturity continues to increase even as the Aptian section shallows structurally on the Adams County High towards the western margin of the basin (R_o range 1.4 - >1.8%). After evaluating the possible thermal agents responsible for increasing maturity at shallower depths (i.e., igneous activity (>35 mi), salt structures, potential changes in regional heat flux), we propose that significant differential uplift and erosion associated with the Monroe Uplift at the western end of the MISB during development of the mid-Cenomanian unconformity is the most likely cause. Further sampling and analysis in 2013-2014 will examine if source rocks of Oxfordian and Late Cretaceous age in the MISB exhibit a similar east-west thermal maturity trend as in the Aptian section.