Gas hydrate is a gas-bearing, ice-like crystalline solid. The substance's build ing blocks consist of a gas molecule (generally methane) sur-rounded by a cage of water molecules. The total amount of methane in hydrate in the world is immense - the most recent speculative estimate centers on values of 21x1015 cu meters. Thus, it may represent a future energy resource. This estimate was presented by Keith Kvenvolden at the International Symposium on Methane Hydrates, Resources in the Near Future, sponsor ed by Japanese National Oil Company (Tokyo, October, 1998).

But, as with any natural resource, there is a need to find naturally occurring concentrations in order to effectively extract gas. We need to answer four basic questions:

• Do methane hydrate concentrations suitable for methane extraction exist?
  
• How can we recognize these concentrations?
  
• Where are concentrations located?
  
• What processes control methane hydrate concentrations?
  

Gas hydrate occurs naturally at the pressure/ temperature/chemical conditions that are present within ocean floor sediments at water depths greater than about 500 meters. The gas hydrate stability zone (GHSZ) extends from the sea bottom downward to a depth where the natural increase in temperature causes the hydrate to melt (dissociate), even though the downward pressure increase is working to increase gas hydrate stability.

Thus, the base of the GHSZ tends to parallel the seafloor at any given water depth (pressure), because the sub-seafloor isotherms (depths of constant temperature) generally parallel the seafloor. The layer at which gas hydrate is stable commonly extends from the sea floor to several hundred meters below it. The gas in most gas hydrates is methane, generated by bacteria in the sediments. In some cases, it can be higher carbon-number, thermogenic hydrocarbon gases that rise from greater depths.