Global map of where gas hydrate reserves have been found; compiled by using data in Kvenvolden & Lorenson (2001), Ginsburg & Soloviev (1998) and Lorenson & Kvenvolden (2007). Solid points are locations where the presence of gas hydrates has been inferred, e.g. by the presence of a BSR, while the open points are location where actual samples of gas hydrates have been recovered.
This shows the two controls on the location of the gas hydrate layer, temperature and pressure. Temperature is controlled by the ocean bottom water temperature and the geothermal gradient at any given location. Pressure is controlled by sea level and sediment failures. Many gas hydrate reserves have been identified by their BSR. This is the free gas trapped below the solid gas hydrate layer which shows up clearly on seismics and follows the sediment surface not the structures, as the gas hydrate layer is controlled by pressure and temperature and not lithology (permission grant for use by M. Maslin).
Phase diagram for permafrost sediments (redrawn from Kvenvolden & Lorenson (2001) and http://www.gashydrate.de), showing that the temperature gradients are considerably lower than in the ocean (figure 6). For example, the temperature can be expected to change by 1.3°C per 100 m within the permafrost zone, compared with 2°C per 100 m in layers below the permafrost zone. The ambient temperature and the thickness of the frozen layer are of paramount importance for the stability of gas hydrate. If the permafrost base is located at a depth of 100 m or less (case 1), the physical conditions will not be adequate for a formation of gas hydrate. The situation is different in case 2, where the permafrost basis is located at greater depth. In polar regions, methane hydrate can occur at depths ranging from 150 to 1650 m.
Phase diagram for ocean sediments (redrawn from Kvenvolden & Lorenson (2001) and http://www.gashydrate.de), showing the physical conditions (temperature and pressure) required for the stability of methane hydrate in a marine environment. Assuming a constant temperature of 0°C, e.g. in polar regions, methane hydrate cannot be stable at a water depth of 100 m. It may occur in a seafloor, which is more than 400 m below sea level. The thickness of the hydrate zone will depend on the temperature gradient. However, with an increasing depth below the seafloor, temperatures get too high for a formation of gas hydrate, so that one can find free gas and water. Given an average temperature increase of 3°C per 100 m sediment depth, when drilling at a water depth of 300 m, we can expect to find a 300 m thick hydrate layer. At 1000 m water depth, the layer will be 600 m thick. If, however, sediments are characterized by a stronger increase in temperature, which can be the case, e.g. at active continental margins (4–6°C per 100 m depth), the hydrate zone will generally be thinner. Gas hydrate has been found in sediments up to 1100 m below the seafloor.
GISP2 methane concentration (Brook et al. 1996; Blunier & Brook 2001) with North Atlantic submarine mass movement occurrence during the last 45 ka plotted as a histogram in 3 ka periods. (Adapted from Owen et al. (2007).)
GISP2 δD-CH4 record (Sowers 2006) with occurrence of North Atlantic submarine mass movements in 1 ka periods for the period 10–20 ka cal BP, the last glacial to de-glacial transition. (Adapted from Owen et al. (2007).)