A clathrate is a chemical compound in which molecules of one material (the “host”) form a solid lattice that encloses molecules of another material (the “guest”). Methane hydrate is a naturally-occurring clathrate in which a host lattice of water-ice encloses guest molecules of methane.
Methane, made of one carbon atom and four hydrogen atoms, is the simplest hydrocarbon molecule and the primary component of natural gas.
In methane hydrate, the gas molecules are not chemically bound to the water molecules but instead are trapped within their crystalline lattice.
The resulting substance looks remarkably like white ice, but it does not behave like ice.
The methane that is captured in the methane hydrate may have been formed by biogenic or thermogenic processes. Biogenic methane is the common by-product of bacterial ingestion of organic matter. This is the same process that produces methane in swamps, and it occurs continually within buried sediments all around the globe. Biogenic processes are capable of producing vast amounts of methane and are considered to be the dominant source of the methane trapped in hydrate accumulations in shallow seafloor sediments.
Thermogenic methane is produced by the combined action of heat and pressure over a long period of time on buried organic material. Over time and with deep burial, organic-rich source beds are literally pressurecooked, with the result being the production of large quantities of oil and natural gas. Along with the oil, the natural gas (largely methane, but also larger molecules like ethane, propane, and others) slowly migrates upward due to its relative buoyancy. Where sufficient quantities of gas reach the zone of hydrate stability, this gas is able to combine with water in the sediments to form methane hydrate.
Given the pressure-temperature relationship found in deep marine environments, it might seem that hydrate could accumulate anywhere in ocean-bottom sediments where water depth exceeds about 400 meters.
However, very deep sediments are generally not thought to contain large quantities of hydrate. The reason is that very deep oceans lack the high biologic productivity needed to create the organic matter that generates methane, and they lack the rapid sedimentation rates needed to deeply bury the organic matter.
Much more info at:
http://www.netl.doe.gov/technologies/oil-gas/publications/Hydrates/2011Reports/MH_Primer2011.pdf
Thanks! You’ll make a chemist out of me yet!