Natural gas hydrate is found worldwide in sediments of outer continental margins of all oceans and in polar areas with continuous permafrost. There are currently 77 localities identified globally where geophysical, geochemical and/or geological evidence indicates the presence of gas hydrate. Details concerning individual gas-hydrate occurrences are compiled at a new world-wide-web (www) site (http://walrus.wr.usgs.gov/globalhydrate). This site has been created to facilitate global gas-hydrate research by providing information on each of the localities where there is evidence for gas hydrate. Also considered are the implications of gas hydrate as a potential (1) energy resource, (2) factor in global climate change, and (3) geohazard.
Interest in naturally occurring gas hydrate has increased steadily since about 1969 when Vasil''ev et al. (1970) recognized that natural gas could form gas-hydrate deposits that should occur globally wherever the pressure and temperature conditions are favorable. Recognition of the potential for natural gas-hydrate occurrence was followed by its discovery, first in permafrost regions of northern Russia (Makogon et al., 1971, 1972). It was then observed in sediment of the Black Sea (Yefremova and Zhizhchenko, 1974) and inferred by seismic surveys to be present in sediment of the Blake Ridge, in the western Atlantic Ocean (Stoll et al., 1971; Ewing and Hollister, 1972). By the early 1980s, gas hydrate had been found in outer continental margin sediment of the Middle America Trench offshore from Mexico (Shipley and Didyk, 1982) and Guatemala (Harrison and Curiale, 1982). Since then, the rate of discovery of evidence for gas hydrate has accelerated. The early history of discovery of gas hydrate has been summarized by Kvenvolden (2000). EVIDENCE FOR GAS HYDRATE Three kinds of evidence have been used to identify the presence of natural gas hydrate--geological, geochemical, and geophysical.