PRELIMINARY MAXIMUM FLOODING SURFACE DEPTH MODELING OF THE TUSCALOOSA MARINE SHALE IN SOUTHWEST MISSISSIPPI, USA

The Tuscaloosa marine shale (TMS), or middle Tuscaloosa, is an informal unit of the Upper Cretaceous Tuscaloosa Group. The TMS is an unconventional source rock-reservoir in the U.S. Gulf Coast Basin with sweet high-gravity oil production mainly occurring in southwestern Mississippi and in adjacent parts of Louisiana. It was deposited during the late Cenomanian to early Turonian and generally represents a significant marine flooding event. The lower half of the TMS, which is characterized by a basal zone of relatively high resistivity thought to indicate the presence of hydrocarbons (Enomoto et al., 2018), represents a deeper marine depositional environment and is where previous workers identified the presence of the maximum flooding surface (MFS) (Liu, 2005; Lowery et al., 2017). A MFS typically is associated with increased total organic carbon (TOC) of more marine, oil-prone organic matter.

Our study presents preliminary modeling of peak (maximum) TOC to infer a MFS in the lower TMS in southwestern Mississippi to corroborate ongoing geophysical log and core sample-based sequence stratigraphic analysis of this unit. Using Passey’s delta-log-R (ΔlogR) method in BasinMod^® modeling software, the separation between transit-time and resistivity well log curves in organic-rich, fine-grained rocks were used to calculate TOC in the lower TMS from nine wells in Amite County. Calculated TOC averages ~2.0-3.0 weight % (wt. %) in the lower TMS, which is similar to results calculated by previous workers (Berch and Nunn, 2014). Peak TOC values range from ~2.9-6.1 wt. % at depths of ~11,000-12,130 ft. Preliminary results illustrate that the depth of peak TOC, inferred here as the approximate MFS, increases from northeast to southwest in our study area. This corresponds with the increasing northeast to southwest depth trend of the TMS. Future work will include wells with measured TOC data to compare to the calculated TOC values. If corroborated by these tests, the ΔlogR modeling work will be expanded across the TMS play area to provide a more expansive MFS prediction map.