Gas hydrates occur abundantly in nature, both in Arctic regions and in marine sediments. Gas hydrate is a crystalline solid consisting of gas molecules, usually methane, each surrounded by a cage of water molecules. It looks very much like water ice. Methane hydrate is stable in ocean floor sediments at water depths greater than 300 meters, and where it occurs, it is known to cement loose sediments in a surface layer several hundred meters thick.
Hydrates store immense amounts of methane, with major implications for energy resources and climate, but the natural controls on hydrates and their impacts on the environment are very poorly understood.
This estimate is made with minimal information from U.S. Geological Survey (USGS) and other studies. Extraction of methane from hydrates could provide an enormous energy and petroleum feedstock resource. Additionally, conventional gas resources appear to be trapped beneath methane hydrate layers in ocean sediments.
The immense volumes of gas and the richness of the deposits may make methane hydrates a strong candidate for development as an energy resource.
Because the gas is held in a crystal structure, gas molecules are more densely packed than in conventional or other unconventional gas traps. Gas-hydrate-cemented strata also act as seals for trapped free gas. These traps provide potential resources, but they can also represent hazards to drilling, and therefore must be well understood. Production of gas from hydrate-sealed traps may be an easy way to extract hydrate gas because the reduction of pressure caused by production can initiate a breakdown of hydrates and a recharging of the trap with gas.
USGS investigations indicate that gas hydrates may cause landslides on the continental slope.
Seafloor slopes of 5 degrees and less should be stable on the Atlantic continental margin, yet many landslide scars are present. The depth of the top of these scars is near the top of the hydrate zone, and seismic profiles indicate less hydrate in the sediment beneath slide scars. Evidence available suggests a link between hydrate instability and occurrence of landslides on the continental margin. A likely mechanism for initiation of landsliding involves a breakdown of hydrates at the base of the hydrate layer. The effect would be a change from a semi-cemented zone to one that is gas-charged and has little strength, thus facilitating sliding. The cause of the breakdown might be a reduction in pressure on the hydrates due to a sea-level drop, such as occurred during glacial periods when ocean water became isolated on land in great ice sheets.
Methane, a "greenhouse" gas, is 10 times more effective than carbon dioxide in causing climate warming.
Methane bound in hydrates amounts to approximately 3,000 times the volume of methane in the atmosphere. There is insufficient information to judge what geological processes might most affect the stability of hydrates in sediments and the possible release of methane into the atmosphere. Methane released as a result of landslides caused by a sea-level fall would warm the Earth, as would methane released from gas hydrates in Arctic sediments as they become warmed during a sea-level rise. This global warming might counteract cooling trends and thereby stabilize climatic fluctuation, or it could exacerbate climatic warming and thereby destabilize the climate.
Results of USGS investigations indicate that methane hydrates possess unique acoustic properties.
The velocity of sound in hydrate is very high, and therefore the velocity of sound in the surface layer of hydrate-cemented sediments also is high. Specific acoustic characteristics of hydrate-cemented sediments are not well known and require further study. Such information has significant implications in the use of sonar devices for defense, seismic exploration, and research.