Hydrocarbon Generating Potential: Jurassic Cotton Valley - Bossier Group, North Louisiana Salt Basin

Geological/geochemical evaluation of Upper Jurassic Cotton Valley-Bossier core samples from the North Louisiana Salt Basin (NLSB) indicates that fine-grained rocks associated with these units are thermally mature and represent petroleum source rocks that generated and expelled mostly gas and some oil. These findings are based on source rock characterization of samples from wells within the NLSB and Vernon field, Jackson Parish, using total organic carbon (TOC), Rock-Eval pyrolisis, and visual kerogen data.

The data indicate that these rocks at their present maturity level have mostly low to moderate TOC contents and Type III kerogen. Original kerogen types in the immature stage, as assessed by kerogen petrography, were mainly gas prone Type III and some oil-and-gas prone Type II/III. The principal macerals are partly oxidized, unstructured amorphous organic matter (liptinite) and vitrinite in varying proportions. Amorphous material was derived from degraded marine algal and humic matter (higher plant material). Visual kerogen data support the predominantly gas prone nature of the source rocks. Vitrinite reflectance (Ro) values (0.94 % to 2.62%) and thermal alteration indices (TAI) (2.8 to 3.7) suggest that these source rocks entered the late oil window to main gas maturity window and thus have generated mostly gas with some oil.

Thin section petrography of geochemically analyzed intervals documents the following rock types: muddy fine-grained sandstone, laminated fine-grained sandstone, sandy mudstone, and silty mudstone. These combined analytical results indicate that abundant woody organic material of continental origin was deposited in offshore areas in association with fine siliciclastic sediments in a marine prodelta environment during Jurassic time. The thickness and widespread deposition of predominantly gas-prone source rocks within the NLSB and their high thermal maturity led to sourcing of mainly gas with some oil in overlying Jurassic and Cretaceous reservoirs, particularly in the Bossier and Cotton Valley.