Buried Gas Hydrates in the Deepwater of the South Caspian Sea, Azerbaijan: Implications for Geo-Hazards

Abstract

Two multi-channel seismic reflection profiles in the deepwater of the South Caspian Sea, offshore Azerbaijan, document one of the first examples of buried gas hydrates. Based on their geophysical signature, these clathrates are characterized by (1) a depth-restricted, lenticular body well beneath the seafloor, (2) the apparent accumulation of free gas within the underlying sediment, and (3) evidence of associated recent slope failure in the overlying strata. The interpreted thickness and depth of gas hydrates in the South Caspian Basin fall within the hydrate stability field predicted from the rmobaric modelling of gas compositions identified from coring at the seafloor. Predicted minimum water depths (∼150 m) and maximum thicknesses (1,300 m) for hydrate stability are much shallower and considerably thicker in the South Caspian Sea than for other known hydrate occurrences. Accumulation of these hydrates near the base of the continental rise appears to control a large region (> 200 km2) of shallow deformation, here named the Absheron allochthon. Such attributes make these gas hydrates important, and perhaps previously underestimated, geo-hazards of the South Caspian region. Primary among these hazards are (1) uncontrolled release of free gas trapped beneath the hydrate seal, (2) disruption of the gas hydrate stability field leading to either explosive dissociation of the gas hydrate, or (3) reduction in sediment strength, slope instability, and mass sediment transport. Documentation of the presence and distribution of gas hydrates, especially when concealed at depth in the subsurface, is a clear pre-requisite for exploration activities in the deepwater region of the South Caspian Sea.