Methane clathrate (CH4·5.75H2O) or (4CH4·23H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, methane gas, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Originally thought to occur only in the outer regions of the Solar System, where temperatures are low and water ice is common, significant deposits of methane clathrate have been found under sediments on the ocean floors of the Earth (around 1100 m below the sea surface). Methane hydrate is formed when hydrogen-bonded water and methane gas come into contact at high pressures and low temperatures in oceans. Methane clathrates are common constituents of the shallow marine geosphere and they occur in deep sedimentary structures and form outcrops on the ocean floor. Methane hydrates are believed to form by the precipitation or crystallisation of methane migrating from deep along geological faults. Precipitation occurs when the methane comes in contact with water within the sea bed subject to temperature and pressure. In 2008, research on Antarctic Vostok Station and EPICA Dome C ice cores revealed that methane clathrates were also present in deep Antarctic ice cores and record a history of atmospheric methane concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for global warming research, along with oxygen and carbon dioxide. Methane clathrates used to be considered as a potential source of abrupt climate change, following the clathrate gun hypothesis. In this scenario, heating causes catastrophic melting and breakdown of primarily undersea hydrates, leading to a massive release of methane and accelerating warming. A 2020 review of the climate tipping points research concludes that for methane hydrate release the "likelihood of occurrence remains unclear". The tipping point cascades (such as Arctic ice loss causing further warming and a release of methane from hydrates) have only been studied starting from 2018. The IPCC Sixth Assessment Report ignores the uncertainty, claiming that no "detectable" impact on the global temperatures will occur in this century through this mechanism.
General Methane hydrates were discovered in Russia in the 1960s, and studies for extracting gas from it emerged at the beginning of the 21st century.
Structure and composition
The nominal methane clathrate hydrate composition is (CH4)4(H2O)23, or 1 mole of methane for every 5.75 moles of water, corresponding to 13.4% methane by mass, although the actual composition is dependent on how many methane molecules fit into the various cage structures of the water lattice. The observed density is around 0.9 g/cm3, which means that methane hydrate will float to the surface of the sea or of a lake unless it is bound in place by being formed in or anchored to sediment. One litre of fully saturated methane clathrate solid would therefore contain about 120 grams of methane (or around 169 litres of methane gas at 0 °C and 1 atm), or one cubic metre of methane clathrate releases about 160 cubic metres of gas. Methane forms a "structure-I" hydrate with two dodecahedral (20 vertices, thus 20 water molecules) and six tetradecahedral (24 water molecules) water cages per unit cell. (Due to sharing of each water molecule between four cages, there are 46 water molecules and eight methane molecules per unit cell.) This compares with a hydration number of 20 for methane in aqueous solution. A methane clathrate MAS NMR spectrum recorded at 275 K and 3.1 MPa shows a peak for each cage type and a separate peak for gas phase methane. In 2003, a clay-methane hydrate intercalate was synthesized in which a methane hydrate complex was introduced at the interlayer of a sodium-rich montmorillonite clay. The upper temperature stability of this phase is similar to that of structure-I hydrate.
Under high pressure, methane hydrate can also form as a coexisting mixture of structure I and structure II, and neutron scattering experiments have shown that methane at the interface between the two structures diffuses several orders of magnitude faster than methane trapped within a single clathrate cage, a phenomenon attributed to confinement effects or transient nano-bubble formation at the structural interface. Above roughly 2 gigapascals, methane clathrate transforms from the cage-based structure-I form into a denser "filled ice" phase (MH-III), in which methane molecules occupy channels within a water framework resembling ice Ih rather than discrete cages. Raman spectroscopy and ab initio molecular dynamics simulations have shown that a further high-pressure phase (MH-IV), with a water skeleton more closely isomorphic to ice Ih, forms near 40 GPa and remains stable to at least 150 GPa, the highest pressure at which any gas hydrate has been observed to persist — a regime relevant to the deep interiors of giant planets such as Uranus and Neptune.
Natural deposits
Methane clathrates are restricted to the shallow lithosphere (i.e. < 2,000 m depth). Furthermore, necessary conditions are found only in either continental sedimentary rocks in polar regions where average surface temperatures are less than 0 °C; or in oceanic sediment at water depths greater than 300 m where the bottom water temperature is around 2 °C. In addition, deep fresh water lakes may host gas hydrates as well, e.g. the fresh water Lake Baikal, Siberia. Continental deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less than 800 m depth. Oceanic deposits seem to be widespread in the continental shelf (see Fig.) and can occur within the sediments at depth or close to the sediment–water interface. They may cap even larger deposits of gaseous methane.
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