Gas hydrate occurs worldwide, but because of the pressure-temperature and gas volume requirements, it is restricted to two regions: polar and oceanic outer continental margins. In polar regions, gas hydrate is normally found where there is permafrost both onshore in continental sediments and offshore in sediment of the polar continental shelves. In oceanic outer continental margins, gas hydrate is found in sediment of the continental slope and rise where cold bottom water is present. The worldwide occurrence of known and inferred gas hydrate is shown in Figure 1.
Samples of gas hydrate have been recovered on land in the western Prudhoe Bay oil field in Alaska [reviewed by Collett (1993)] and in the Mackenzie Delta of Canada (Dallimore et al., 1999). Thus far, gas hydrate has been found at about 20 oceanic locations (Kvenvolden et al., 1993; Booth et al., 1996; Ginsburg and Soloviev, 1998), providing irrefutable evidence for natural gas-hydrate occurrence. Deep-ocean drilling has recovered gas hydrate at locations offshore from Peru, Costa Rica, Guatemala, Mexico, western and eastern United States, Japan, and in the Gulf of Mexico. Gas hydrate at or near the seafloor has been recovered at locations in the Black Sea, Caspian Sea, Sea of Okhotsk, offshore from northern California, Oregon, Nigeria, in the northern Gulf of Mexico, offshore Norway (Ginsburg et al., 1999) and offshore Oregon (Suess et al., 1999).
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