Gas hydrates are naturally ocurring, crystalline, ice-like substances composed of gas molecules (methane, ethane, propane, etc.) held in a cage-like ice structure. (clathrate).
The formation and stability in the subsurface of these structures are constrained by a relatively narrow range of high pressure and low temperature and depend on the influx of free gas and the amount of gas dissolved in the pore fluid.
Hydrates are a concentrated form of natural gas compared with compressed gas, but less concentrated than liquefied natural gas. It is estimated that a significant part of the Earth's fossil fuel is stored as gas hydrates, but as yet there is no agreement as to how large these reserves are.