The occurrence of natural gas hydrate is controlled by an interrelation among the factors of temperature, pressure, gas composition, and ionic strength of water (Kvenvolden, 1993). Presently the upper depth limit for methane hydrate is about 150 m below the surface in continental polar regions, where surface temperatures are below 0° C. In oceanic sediment gas hydrate occurs where bottom-water temperatures approach 0° C and the water depths exceed about 300 m (Chapter 1, Fig. 2). The lower limit of methane hydrate occurrence is determined by the geothermal gradient; the maximum lower limit is about 2000 m below the solid surface, although the lower depth limit is typically much less (<1000 m), depending on local conditions. The occurrence of gas hydrate is restricted to the shallow geosphere where the amount of methane exceeds its solubility in water and thus is present in large enough quantities to form methane hydrate.

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