Water segregation in the course of gas hydrate formation and accumulation in submarine gas-seepage fields

Abstract

Gas hydrate accumulations associated with submarine gas-seepage fields have been detected in the Okhotsk Sea and the Gulf of Mexico. It is possible that gas hydrates known from the continental margin offshore northern California, Oregon and Nigeria are also connected with gas-seepage fields. Gas-hydrated sediments in the accumulations (1) occur at very low subbottom depths, sometimes directly at the sea bottom; (2) have specific structures, resembling cryogenic structures, caused by hydrates (subhorizontal lenticular-bedded and porphyraceous structures); (3) the sediments can be distinguished from overlying non-hydrated sediments by a higher water content; (4) an association of gas hydrates and calcium carbonate concretions occurs. The behaviour of gas and water during gas hydrate formation under bottom gas-seepage conditions is considered. Gas hydrate formation occurs at the upper part of the sedimentary cover, within the limits of the diffusion halo around the up-going flow of gas which saturates the pore water. Water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well. The mechanism of gas transportation to the gas hydrate accumulation sites is diffusion. The way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation. The water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate-bearing sediments in submarine gas-seepage fields.