Gas (Methane) Hydrates -- A New Frontier

Methane trapped in marine sediments as a hydrate represents such an immense carbon reservoir that itmust be considered a dominant factor in estimating unconventional energy resources; the role ofmethane as a 'greenhouse' gas also must be carefully assessed.


Dr. William Dillon,U.S. Geological Survey

Hydrates store immense amounts of methane, with majorimplications for energy resources and climate, but the naturalcontrols on hydrates and their impacts on the environment arevery poorly understood.Gas hydrates occur abundantly in nature, both in Arctic regionsand in marine sediments. Gas hydrate is a crystalline solid consistingof gas molecules, usually methane, each surrounded by a cage ofwater molecules. It looks very much like water ice. Methanehydrate is stable in ocean floor sediments at water depths greaterthan 300 meters, and where it occurs, it is known to cement loosesediments in a surface layer several hundred meters thick.The worldwide amounts of carbon bound in gas hydrates isconservatively estimated to total twice the amount of carbon tobe found in all known fossil fuels on Earth.This estimate is made with minimal information from U.S.Geological Survey (USGS) and other studies. Extraction of methanefrom hydrates could provide an enormous energy and petroleumfeedstock resource. Additionally, conventional gas resources appearto be trapped beneath methane hydrate layers in ocean sediments. Recent mapping conducted by the USGS off North Carolina andSouth Carolina shows large accumulations of methane hydrates.A pair of relatively small areas, each about the size of the Stateof Rhode Island, shows intense concentrations of gas hydrates.USGS scientists estimate that these areas contain more than1,300 trillion cubic feet of methane gas, an amount representingmore than 70 times the 1989 gas consumption of the United States.Some of the gas was formed by bacteria in the sediments, but somemay be derived from deep strata of the Carolina Trough. TheCarolina Trough is a significant offshore oil and gas frontier areawhere no wells have been drilled. It is a very large basin, about thesize of the State of South Carolina, that has accumulated a greatthickness of sediment, perhaps more than 13 kilometers. Saltdiapirs, reefs, and faults, in addition to hydrate gas, may providegreater potential for conventional oil and gas traps than is present inother east coast basins.The immense volumes of gas and the richness of the depositsmay make methane hydrates a strong candidate for developmentas an energy resource.Because the gas is held in a crystal structure, gas molecules aremore densely packed than in conventional or other unconventionalgas traps. Gas-hydrate-cemented strata also act as seals for trappedfree gas. These traps provide potential resources, but they can alsorepresent hazards to drilling, and therefore must be well understood.Production of gas from hydrate-sealed traps may be an easy way toextract hydrate gas because the reduction of pressure caused byproduction can initiate a breakdown of hydrates and a recharging ofthe trap with gas. USGS investigations indicate that gas hydrates may causelandslides on the continental slope.Seafloor slopes of 5 degreesand less should be stable on the Atlanticcontinental margin, yet many landslide scars are present. The depthof the top of these scars is near the top of the hydrate zone, andseismic profiles indicate less hydrate in the sediment beneath slidescars. Evidence available suggests a link between hydrate instabilityand occurrence of landslides on the continental margin. A likelymechanism for initiation of landsliding involves a breakdown ofhydrates at the base of the hydrate layer. The effect would be achange from a semi-cemented zone to one that is gas-charged andhas little strength, thus facilitating sliding. The cause of thebreakdown might be a reduction in pressure on the hydrates due to asea-level drop, such as occurred during glacial periods when oceanwater 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 3,000times the volume of methane in the atmosphere. There is insufficientinformation to judge what geological processes might most affect thestability of hydrates in sediments and the possible release of methaneinto the atmosphere. Methane released as a result of landslidescaused by a sea-level fall would warm the Earth, as would methanereleased from gas hydrates in Arctic sediments as they becomewarmed during a sea-level rise. This global warming might counteractcooling trends and thereby stabilize climatic fluctuation, or it couldexacerbate climatic warming and thereby destabilize the climate. Results of USGS investigations indicate that methane hydratespossess unique acoustic properties.The velocity of sound in hydrate is very high, and therefore thevelocity of sound in the surface layer of hydrate-cemented sedimentsalso is high. Specific acoustic characteristics of hydrate-cementedsediments are not well known and require further study. Suchinformation has significant implications in the use of sonar devicesfor defense, seismic exploration, and research.Realizing the importance of methane hydrates in marine sediments,the USGS has focused work on selected areas where hydrates areknown to be common, and where the influences of hydrates onenergy resources, climate, and seafloor stability can be analyzed.At this stage, it is important for USGS scientists to learn howthe hydrates form, evolve, and break down, how they affectsediments, and what factors control their concentration at certainlocations, as well as to explore for new hydrate accumulations.Cooperation with other Federal agencies, such as the NationalOceanic and Atmospheric Administration for bathymetry studies, theDepartment of Energy for application of hydrate gas extractiontechnology, and the U.S. Navy for acoustic studies, will enhance thesuccess of future work.