Abstract

Gas hydrate is a crystalline substance composed of water and gas,in which solid lattices of water molecules trap gas molecules in a cage-like structure, or clathrate.Gas hydrate occur in sedimentary deposits under conditions of pressure and temperature present in permafrost regions and beneath the sea in outer continental margins.The combined information from Arctic gas-hydrate studies shows that, in permafrost regions,gas hydrate may exist at subsurface depths ranging from about 130 to 2,000 m. The presence of gas hydrate in offshore continental margins has been inferred mainly from anomalous seismic reflectors known as bottom-simulating reflectors (i.e., BSRs),which have been mapped at depths below the sea floor ranging from about 100 to 1,100 m. While methane,propane,and other gases can be included in the clathrate structure,methane hydrate appear to be the most common in nature.The amount of methane sequestered in gas hydrate is probably enormous,but estimates of the amounts are speculative and range over three orders-of-magnitude from about 3,114 to 7,634,000 trillion cubic meters,which represents a very large potential energy resource. Gas hydrate also represent a significant drilling and production hazard.Russian,Canadian,and American drilling engineers have described numerous problems associated with gas hydrate,including blowouts and well-bore casing failures.Seafloor stability is also an important issue related to gas hydrate.Seafloor stability refers to the susceptibility of the seafloor to collapse and slide as the result of gas hydrate disassociation.It is possible that both natural and human induced changes can contribute to in-situ gas hydrate destabilization,which may convert a gas-hydrate-bearing sediment to a gassy water- rich fluid,triggering seafloor subsidence and catastrophic landslides.Evidence implicating gas hydrate in triggering seafloor landslides has been found along the Atlantic Ocean margin of the United States. However,the mechanisms controlling gas hydrate induced seafloor subsidence and landslides are not well known. Recent studies indicate that atmospheric methane,a greenhouse gas,is increasing at a rate such that the current concentration will probably double in the next 50 years.It is predicted that methane will surpass carbon dioxide as the predominant atmospheric greenhouse gas in the second half of this century.The source of this atmospheric methane is uncertain;however,numerous researchers have suggested that destabilized natural-gas hydrate may be contributing to the build-up of atmospheric methane.