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Oil-source rock correlation studies in the unconventional Upper Cretaceous Tuscaloosa marine shale petroleum system, Mississippi and Louisiana, USA (2019)

October 23, 2019

The U.S. Geological Survey assessed undiscovered unconventional hydrocarbon resources reservoired in the Upper Cretaceous Tuscaloosa marine shale (TMS) of southern Mississippi and adjacent Louisiana in 2018. As part of the assessment, oil- source rock correlations were examined in the study area where operators produce light (38-45 degrees API), sweet oil from horizontal, hydraulically-fractured wells in an overpressured 'high-resistivity' (greater than 5 ohm-m) zone (HRZ) at the base of the TMS. Our initial characterization of TMS samples indicated overall low organic carbon (avg. TOC ~1.0%) and dominance of a gas-prone Type III or mixed Type II/III kerogen, potentially inconsistent with a self-sourced petroleum system model for TMS-produced oils. Our work also documented thin (less than 1 ft), organic-rich (3-9 wt.% TOC), thermally mature (greater than 0.6% Ro) shales in the underlying Lower Tuscaloosa (LT), which did contain an oil-prone Type II kerogen. To test if these thin LT shales could generate oils which are then trapped and produced from the base of the overlying clay-rich (greater than 50% total clays) TMS HRZ, we evaluated oil-oil and oil-source rock correlations throughout the Tuscaloosa Group (TG, including LT; TMS; and upper Tuscaloosa, UT) in the TMS play area. Geochemical data from 39 oil samples and 17 source rock solvent extracts collected from the TMS play area indicate close correspondence for TG oils (from LT, TMS and UT reservoirs) in thermal maturity (computed from MPI), SARA proportions, n-alkane distributions, isoprenoid and DBT/P ratios, monoaromatic steroids, and delta-13 C isotopic compositions. Other parameters (normal steranes, extended homohopanes, C31R/C30 hopane, norhopane/hopane and tricyclic terpane ratios, Ga/Hop) show most oil samples have similar values, suggesting all TG oils are from a common mixed marine-terrigenous source rock. Tighter distributions for triaromatic steroid (TAS) and delta-13 C isotopic composition for conventional oils in LT and UT reservoirs may indicate charge occurred in a single or shorter pulse relative to TMS oils which show broader TAS and delta-13 C properties, possibly from their generation over an extended period of burial maturation. Dissimilarity in geochemical properties between LT source rock solvent extracts and TG oils indicates LT source rocks did not contribute significantly to conventional and unconventional TG hydrocarbon accumulations. Whereas, TMS solvent extracts are similar to TG oils, suggesting an oil-source rock correlation. Excluding the possibility for long-distance lateral migration from a similar source downdip (which is unnecessary given thermal maturity considerations), the observations indicate: 1. the TMS is a self-sourced reservoir; 2. the TMS is the source of oils accumulated in nearby conventional TG reservoirs; and 3. thin organic-rich shales in the LT did not contribute substantially to any oil accumulations in the TG.

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