Gas hydrates are crystalline substances composed of water and gas, in which a solid water-lattice accommodates gas molecules in a cage-like structure, or clathrate. Gas hydrates are widespread in permafrost regions and beneath the sea in sediment of outer continental margins. The amount of methane sequestered in gas hydrates is probably enormous, but estimates of the amounts are speculative and range over three orders-of- magnitude, from about 100,000 to 270,000,000 trillion cubic feet. The estimated amount of gas in the hydrate reservoirs of the world greatly exceeds the volume of known conventional gas reserves.

The major goal of this resource appraisal is to estimate the gas hydrate resources in the United States, both onshore and offshore. This appraisal of gas hydrates is based on a play-analysis scheme, which was conducted on a province-by-province basis. The gas hydrate-plays in the United States are regardless of their current economic or technological status. In a play analysis method, prospects (potential hydrocarbon accumulations) are grouped according to their geologic characteristics into plays.

11 gas-hydrate plays were identified within four offshore and one onshore petroleum provinces; for each play. Estimates for each of the 11 plays were aggregated to produce the estimate of total gas hydrate resources in the United States. The offshore petroleum provinces assessed consist of the U.S. Exclusive Economic Zone (EEZ) adjacent to the lower 48 States and Alaska. The only onshore province assessed was the North Slope of Alaska, which included State water areas and some offshore Federal waters. The provinces are geographic in character; however, their formation represents an attempt to group the individual petroleum provinces along broad geologic lines.