Methane hydrates occur in polar permafrost regions and marine outer continental margins and represent a potentially enormous energy resource.Hydrates are naturally occurring crystalline compounds of natural gas enclosed within a cage-like lattice of water ice. Chemists call such structures clathrates. In methane hydrates, water crystallizes in the cubic system, rather than in the hexagonal structure of normal ice.
The resulting compound packs a lot of methane in its dense organization. One cubic foot of hydrate contains about 164 cubic feet of methane gas. With adequate gas concentrations, methane hydrates form and are stable under moderate- to high pressure, low temperature conditions. This Methane Hydrate Stability Zone (MHSZ) typically occurs: 1) on continental margins at water depths greater than about 300 m and bottom water temperatures close to 0° C, where gas hydrate is found from the sediment surface to depths of about 1100 m below the seafloor, and 2) in polar continental regions, where gas hydrate can be present in sediment and permafrost at depths between about 150 and 2000 m.Early estimates of the total resource were as speculative as they were impressive. Current work using geology-based Total Petroleum System (TPS) assessments still yields very large numbers.