ABSTRACT

The Gulf slope is unique in that larger volumes of gas hydrate are concentrated in smaller volumes of sediment near the sea floor than in any other known basinal setting. Gas hydrates are ice-like crystalline solids (minerals) in which hydrocarbon and nonhydrocarbon gases are held within rigid cages of water molecules. Most thermogenic gas hydrate occurrences in the central slope are linked to vertical migration of oil and gas from deep Upper Jurassic source rock facies. Migration focuses the formation of thermogenic gas hydrates at the edges of actively charged salt-withdrawal mini-basins, over salt ridges, and near the Sigsbee Escarpment. Bacterial methane hydrates also occur, and are differentiated from thermogenic gas hydrates by molecular and isotopic properties. Isotopic fractionation of hydrocarbons does not occur during gas hydrate crystallization from vent gas. The detailed isotopic properties of individual hydrocarbons from gas hydrate samples could provide insight to subsurface accumulations of either thermogenic hydrocarbons or bacterial methane. At high pressures and low temperatures of the deep sea floor, rapid crystallization of gas hydrates creates sea-floor mounds and tabular vein-fillings at vent sites, deforming hemipelagic sediments. Gas hydrates exist in a relatively unstable environment at the sea floor, and hydrate decomposition further contributes to sediment instability. The Gulf slope is atypically prone to gas hydrate hazards.