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 hydrate 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 located in the Green Canyon (GC) Block 185, adjacent to Jolliet Field in GC 184. Jolliet Field contains oil and gas that filled fault traps caused by salt deformation during late Pleistocene-Holocene time. In contrast to reservoir 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. Vertical migration of gas along faults is ongoing, manifested on the sea floor by gas vents, gas hydrate, complex chemosynthetic communities, and by a large gas plume that extends from the vents to the sea surface. The isotopic properties of C-1-C-5 hydrocarbons from reservoirs, gas vents, and gas hydrate correlate closely. Although outcropping gas hydrate is transiently stable 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 site is attributed to the gas that has recently entered the vents, largely from Jolliet Field, and to the synchronous activation of fault conduits allowing gas migration to the sea floor. Synchronous late gas charge and faulting could also explain the wide distribution of gas hydrate across the upper Gulf slope. (C) 2001 Elsevier Science Ltd. All rights reserved.