Massive vein-filling gas hydrate: relation to ongoing gas migration from the deep subsurface in the Gulf of Mexico

A gas hydrate mound that contains massive, vein-filling, structure II gas h ydrate occurs on the upper continental slope (similar to 540 m water depth) of the Gulf of Mexico, southwest of the Mississippi Delta. The mound is lo cated in the Green Canyon (GC) Block 185, adjacent to Jolliet Field in GC 1 84. Jolliet Field contains oil and gas that filled fault traps caused by sa lt deformation during late Pleistocene-Holocene time. In contrast to reserv oir oil in Jolliet Field, which shows bacterial oxidation effects, the C-1- C-5 reservoir gas is unaltered by bacterial oxidation. Disassociated gas is assumed to have recently entered from the subsurface hydrocarbon system. V ertical migration of gas along faults is ongoing, manifested on the sea flo or by gas vents, gas hydrate, complex chemosynthetic communities, and by a large gas plume that extends from the vents to the sea surface. The isotopi c properties of C-1-C-5 hydrocarbons from reservoirs, gas vents, and gas hy drate correlate closely. Although outcropping gas hydrate is transiently st able because of variations in seawater temperature, the bulk of buried gas hydrate at GC 185 is stable and perhaps increasing in volume because of the copious gas flux, The massive accumulation of gas hydrate at the GC 185 si te is attributed to the gas that has recently entered the vents, largely fr om Jolliet Field, and to the synchronous activation of fault conduits allow ing gas migration to the sea floor. Synchronous late gas charge and faultin g could also explain the wide distribution of gas hydrate across the upper Gulf slope. (C) 2001 Elsevier Science Ltd. All rights reserved.