GEOCHEMICAL MODELING OF THE ROLE OF SULFATE IN THE PRECIPITATION OF CALCITE CAP ROCKS FOR GULF OF MEXICO SALT DOMES

Gulf Coast salt domes host a wide variety of geologic resources such as reserves of crude oil and natural gas as well as cap rock-associated mineral resources such as limestone, rock salt, sulfur and metallic sulfides. However, the specific mechanisms that facilitate the precipitation of these cap rocks are still largely unidentified. Insight into the mineralization mechanism(s) can be obtained from the specific geochemical signatures recorded in these structures as well as the geochemistry of the formation waters in the Gulf Coast. Carbon and sulfur isotope compositions of calcite cap rock indicate precipitation via microbial sulfate reduction under closed-system conditions. In some cases geochemical signatures provide evidence for sulfate-dependent anaerobic oxidation of methane (AOM). High calcite-bound trace sulfate concentrations agree well with high aqueous sulfate concentrations (up to 10,000 ppm) in modern seafloor brine pools associated with active Gulf of Mexico salt diapirs. Using PHREEQC, Gulf of Mexico seafloor brines were modeled to provide insight into the processes and chemical reactions that could account for the significant sulfate concentrations and calcite mineralization in the cap rock environment. These models suggest that sulfate is not an abundant component of these solutions. Therefore sulfate is most likely added to the solution during the dissolution of anhydrite cap-rock or the oxidation of reduced sulfur species followed by the quick reduction by local microbial ecosystems.