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Gas Hydrate: What is it?

A gas hydrate is a crystalline solid; its building blocks consist of a gas molecule surrounded by a cage of water molecules. Thus it is similar to ice, except that the crystalline structure is stabilized by the guest gas molecule within the cage of water molecules. Many gases have molecular sizes suitable to form hydrate, including such naturally occurring gases as carbon dioxide, hydrogen sulfide, and several low-carbon-number hydrocarbons, but most marine gas hydrates that have been analyzed are methane hydrates.

Where Hydrate is Happy

Gas Hydrate Stability Curve To the above is a curve representing the stability of Gas Hydrate in sea water. Pressure and temperature are two of the major factors controlling where the hydrate(solid) or methane gas will be stable. Whether or not gas hydrate actually forms depends on the amount of gas available.

Gas Hydrate Stability in Ocean Sediments The diagram to the above shows where the same stability curve above crosses the temperatures of ocean sediments.

Capacity to Trap Gas

Hydrate forms as cement in the pore spaces of sediment as well as in layers and nodules of pure hydrate. Hydrates also seem to have the capacity to fill sediment pore space and reduce permeability, so that hydrate-cemented sediments act as seals for gas traps.

Gas hydrates are stable at the temperatures and pressures that occur in ocean-floor sediments at water depths greater than about 500 meters, and at these pressures they are stable at temperatures above those for ice stability. Gas hydrates also are stable in association with permafrost in the polar regions, both in offshore and onshore sediments. Gas hydrates bind immense amounts of methane in sea-floor sediments. Hydrate is a gas concentrator; the breakdown of a unit volume of methane hydrate at a pressure of one atmosphere produces about 160 unit volumes of gas. The worldwide amount of methane in gas hydrates is considered to contain at least 1x104 gigatons of carbon in a very conservative estimate). This is about twice the amount of carbon held in all fossil fuels on earth.

Gas hydrate concentration occurs at depocenters, probably because most gas in hydrate is from biogenic methane, and therefore it is concentrated where there is a rapid accumulation of organic detritus (from which bacteria generate methane) and also where there is a rapid accumulation of sediments (which protect detritus from oxidation).

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