Is the Tuscaloosa Marine Shale Self-Sourced? Preliminary Oil-Oil and Oil-Source Rock Correlation Studies in the Upper Cretaceous Tuscaloosa Group, Southern Mississippi, USA

Abstract

The U.S. Geological Survey will assess undiscovered hydrocarbon resources contained in the Upper Cretaceous Tuscaloosa marine shale (TMS) of southern Mississippi and adjacent Louisiana. Operators produce light (38-45° API), sweet oil from a ‘high-resistivity’ (>5 ohm-m) zone (HRZ) at the base of the TMS. Our characterization of TMS samples indicates low organic carbon (avg. TOC ~0.75%) and dominance of gas-prone Type III kerogen, potentially inconsistent with a self-sourced model (e.g., Lu et al. 2015, AAPG Bulletin 99, 333-357) for TMS-produced oils. Our work also includes characterization of thin (<1 ft), organic-rich (3-9 wt.% TOC), thermally mature (>0.6% Ro) shales in the underlying lower Tuscaloosa (LT), which do contain an oil-prone mixed Type II/III kerogen. To test if these thin LT shales could generate oils which are trapped at the base of the clay-rich (>50% total clays) TMS HRZ, we are evaluating oil-oil and oil-source rock correlations throughout the Tuscaloosa Group (TG, including LT; TMS; and upper Tuscaloosa, UT). Preliminary observations from a limited data set show close correspondence in δ13C isotope composition between all analyzed TG oils (n=39). SARA proportions, Pr/Ph, Gam/Hop and C29 normal steranes also are similar for TG oils, and suggest all oils are from a common mixed marine-terrestrial source according to discriminant plots. LT shale bitumen extract (n=3) properties indicate a different paralic-deltaic source of organic matter, containing higher concentrations of C29 normal steranes, higher Pr/Ph ratios, greater proportions of long-chain n-alkanes, and dissimilar diasterane/sterane, C30 sterane index, and tetracyclic and tricyclic terpane ratios. However, correspondence in these properties is observed between TMS bitumen extracts (n=3) and TG oils. Excluding the potential for a nearby un-sampled oil-prone TMS source, or long-distance lateral migration from a similar source downdip, these observations suggest: 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 do not contribute substantially to any oil accumulations in the TG. Though the TMS displays poor present-day source rock properties, these data suggest it is the best candidate for a local source rock and has apparently expelled oils to conventional reservoirs and retained oils for production by horizontal drilling and artificial stimulation.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017