ORGANIC PETROLOGY OF THE APTIAN SECTION IN THE DOWNDIP MISSISSIPPI INTERIOR SALT BASIN, MISSISSIPPI, USA
Due to an overlap of maceral-type reflectance ranges, macerals were identified as either 1) secondary solid bitumen (Ro generally <0.8 %; recently migrated and cracked oil?); 2) primary solid bitumen (Ro 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, Ro values increase regionally across the MISB from the southeast to the northwest. Thermal maturity in the eastern half of the basin (Ro 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 (Ro 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.