MODELING GAS HYDRATE OCCURRENCES IN THE NORTHERN GULF OF MEXICO FROM GIANT PISTON CORES

The seafloor of the northern Gulf of Mexico contains many occurrences of gas hydrate. The topography and recent stratigraphy of the seafloor is complicated by salt-tectonic structures, faulting, sediment failure scars, and high sedimentation rates. The abundant natural oil and gas seeps are often capped by gas hydrate within the hydrate stability zone. While gas hydrate is relatively common at the seafloor, seismic reflection profiles lack bottom simulating reflections (BSR’S). Thus, it is unknown if there are significant gas hydrate accumulations within minibasins away from the seep localities.

To address this question, a cruise was conducted on the R/VMarion Dufresne in July 2002. Seventeen giant piston cores, up to 38-m long, and four giant box cores up to 9-m long, were recovered along seismic reflection transects in water depths ranging from about 600-1,300 m in widely different geologic environments. Transects up to 7 kilometers long were cored from known seafloor gas hydrate occurrences or fluid seep sites into the adjacent basin. Gas hydrate was recovered in four cores near two previously known seep sites in subbottom depths of about 3 to 9-m, but was not found in adjacent basins.

Modeling of gas hydrate stability from porewater geochemistry, hydrocarbon gas composition, and measured geothermal gradients demonstrate that high salt concentrations and spatially variable geothermal gradients greatly affect the possible distribution of gas hydrate. Gas hydrate found at thermogenically-sourced fluid seep sites are likely limited to near seafloor occurrences by high salinity and warm fluid temperatures. Away from seep sites, only significant quantities of methane were measured, suggesting that only Structure I gas hydrate formation is possible with its requisite thinner stability field. Gas hydrate is possible in basin sediments away from seafloor vents where inferred methane flux, measured by sulfate depletion in pore water is high. For example, high methane flux occurs in the small basin adjacent to Bush Hill. In contrast, sites in and near a large landslide east of Mississippi Canyon show no potential to form gas hydrate. Our results confirm the presence of gas hydrate in near-seabed sediments in the northern Gulf of Mexico, and extend our knowledge on where significant gas hydrate accumulations may occur.