HR: 09:35h
AN: OS41C-06    [Abstracts]
TI: Impact of Shallow Convection on the Gas Hydrate Reservoir in the Gulf of Mexico Salt Tectonics Province
AU: Wilson, A
EM: awilson@geol.sc.edu
AF: Dept. of Geological Sciences, University of South Carolina, Columbia, 29208
AU: * Ruppel, C
EM: cdr@eas.gatech.edu
AF: Earth and Atmos. Sciences, Georgia Tech, Atlanta, 30332
AB: Previous modeling studies have suggested that subseafloor hydrogeology in the northern Gulf of Mexico could be strongly affected by the presence of salt domes, but these efforts were at the time limited to formulations that decoupled thermal and chemical buoyancy. The earlier studies concluded that downwelling associated with the negative buoyancy of dense briny fluids dominated upwelling associated with positive thermal buoyancy near salt domes. In this study, we use modern hydrologic models that fully couple thermal and chemical effects to re-examine this problem with particular focus on Gulf of Mexico gas hydrate reservoirs. We first demonstrate that even slight variations in seafloor bathymetry lead to the onset of shallow convection in marine sediments and that the existence of such convective patterns is not dependent on the presence of salt or the geometry of the salt body. Bathymetric highs are generally the loci of upwelling, while downwelling is concentrated in bathymetric lows. The length scale of the convective cells depends on the wavelength of seafloor topography but is generally hundreds to less than 2000 m, consistent with observational evidence one of us has earlier reported for the Mississippi Canyon and Garden Banks gas hydrate areas. The model calculations are consistent with the observed pattern of chloride, sulfate, and thermal anomalies, suggesting that the modeling results can be used to estimate the variation in the depth of hydrate stability and hydrate occurrence in these highly dynamic systems. Our simulations of the transient evolution of convective regimes near salt domes show that the near-surface, thermally-driven system eventually separates from the deeper, chemically-driven system dominated by stable, dense brines. In this scenario, the gas hydrate stability zone will change as a function of time due to the changing hydraulic regime in the sediments. Superposed on such hydraulic effects on the hydrate stability zone would be the influence of better understood processes such as sedimentation or erosion of the sedimentary column. Finally, we explicitly consider the role of faults in focusing fluids in these systems and conclude that faults can radically perturb the chemical and thermal conditions near salt domes to the point of entirely dominating the flow field and thus the gas hydrate stability field. The results are applied to zones of focused flux, such as the mud mounds in the Garden Banks and Mississippi Canyon areas and the seeps at Bush Hill, and to a zone of 'diffuse advective flux' characterized by a regional BSR at Keathley Canyon.
DE: 1847 Modeling
DE: 3004 Gas and hydrate systems
DE: 3015 Heat flow (benthic)
DE: 3021 Marine hydrogeology
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005