Gas hydrate is a solid phase of water and low-molecular-weight gases, mostly methane, that forms in marine sediments when gas concentrations are adequate, temperature is low, and pressure is high. Although gas hydrate may be a common phase in the shallow geobiosphere, little is known about it because it is unstable under normal surface conditions.
Leg 164 of the Ocean Drilling Program (ODP) studied natural gas hydrate in marine sediment at the Blake Ridge on the continental margin off southeastern North America and found that it occupies more than 1% of the sedimentary section from 200 to 450 m below seafloor (mbsf). At the Blake Ridge, free gas is dispersed throughout a region a few hundred meters thick beneath the gas hydrate—bearing zone. Coupled with geophysical data indicating that gas hydrate occurs throughout a laterally extensive portion of the Blake Ridge, the drilling confirmed that these sediments contain enormous amounts of methane.