Gas hydrate concentrates methane and sometimes other gases in its crystal lattice and this gas can be released intentionally creating a resource or escape accidentally forming a hazard. The densest accumulations of gas hydrate tend to occur at sites where the base of the gas hydrate stability zone (commonly the upper several hundred m of the sedimentary section) is configured to trap gas, often as a broad arch. The gas may rise from below or form by bacterial activity at shallow depth, but gas commonly is concentrated near the base of the gas hydrate stability zone by recycling. This gas accumulates in presumably leaky traps, then enriches the hydrate above as it migrates upward by diffusion, fluid movement through sedimentary pores, or flow along fracture channelways. Analysis of seismic reflection profiles is beginning to identify such concentrations and the circumstances that create them. The first attempt to explore for gas hydrate off Japan by the Japanese National Oil Corporation produced quite favorable results, showing high gas hydrate contents in permeable sediments. Gas hydrate dissociation can be a safety concern in drilling and production. The volume of water and gas released in dissociation is often greater than the volume of the hydrate, so overpressures can be created. Furthermore, the gas hydrate can provide shallow seals, so the possibility of high-pressure flows or generation of slides is apparent.
The gas-bearing, ice-like form of water known as gas hydrate or gas clathrate occurs in abundance in marine sediments and stores immense amounts of methane, with major implications for future energy resources and seafloor stability. Gas hydrate forms wherever appropriate physical conditions exist - moderately low temperature and moderately high pressure - and the materials are present - water and gas near saturation in the water1. These conditions can be found in the deep sea, commonly at water depths greater than about 500 m or somewhat shallower depths (about 300 m) in the Arctic's colder water. Like ice, crystalline methane hydrate is less dense than water, so if hydrate forms in the water it floats upward and breaks down (dissociates) at lower pressures and warmer temperatures. However, if the gas hydrate forms within sediments, it is bound in place. Gas hydrate can exist in a zone (the gas hydrate stability zone - GHSZ), that extends from the seafloor down to a depth where the temperature has risen sufficiently to make hydrate unstable (even though the pressure increase that comes with greater depth will increase gas hydrate stability). The base of the GHSZ ranges from just below the seafloor to several hundred to a thousand meters below it at greater water depths (higher pressures).
Gas hydrate in ocean sediments is mostly (99%) formed of biogenic methane, but significant amounts of thermogenic gas hydrates that include higher carbon-number hydrocarbons exist in active petroleum areas (e.g. Gulf of Mexico, Caspian Sea).