Mud volcano gas hydrates in the Caspian Sea

Two gas hydrate accumulations associated with mud volcano craters were investigated by means of sparker survey, bottom water and sediment sampling using gravity corer and subsequent chemical and isotopic studies of gas. bottom and porewater and carbonate inclusions. Hydrate contents in sediments were up to 35% per volume. Sometimes hydrates were encountered immediately on the seabed. Correlations have been established between hydrate contents, water contents (after dissociation of hydrates), chlorinity and the oxygen and hydrogen isotope composition of the pore water. The hydrate water is enriched with respect to deuterium. Liquid water in sediments contains higher (18)0 as result of isotopic exchange with carbonates. Hydrates are thought to have formed from the mud volcano brines (from their water and from dissolved light hydrocarbons which are thermogenic in origin). Each accumulation has its own deep source. The developed approach presents a thorough study of the hydrate and water contents in sediments along with the water composition.