Geophysical methods to quantify gas hydrates and free gas in the shallow subsurface: Review and Outlook

Gas hydrates and free gas linked to gas hydrate systems account for 500 to 12000 Gt of carbon – most likely around 3500 Gt (Buffett und Archer, 2004; Kvenvolden, 1993; Milkov und Sassen, 2002). Gas hydrate is an ice-like compound consisting of water-captured gas molecules. Predominantly, natural gas hydrate contains methane that is produced by biological degradation of organic matter. With sufficient water- and of free gas-supply gas hydrate forms in sediment basins all over the world. As gas hydrate is stable only at high pressure and low temperature it occurs at water depth of more than 300 m. The exact depth depends on bottom water temperature and other environmental circumstances such as pore water salinity and the precise composition of the captured gas. Within the sediment the depth of the gas hydrate stability field is further controlled by the geothermal gradient.