American Scientist Online - Old Gas, New Gas

American Scientist Online - Old Gas, New Gas:

"In a hurry to get to methane hydrates, I began by writing, 'Along with petroleum and coal, methane is a fossil fuel, of plant origin…'—at which point I got corrected by the president of Sigma Xi. When I changed what I wrote, geologists gave me more trouble. I had stumbled right into a nest of controversies. Or, an area of current research.

It appears that methane on Earth has not one source, but many. Most (but not all) of the commercial methane in natural gas is thermogenic—thought to derive from petroleum (originally from plants) that is heated and processed deep underground. It's old.

A great deal of additional methane, however, is sequestered in sediments, at sea bottom and in permafrost, in a remarkable set of structures I will soon describe. And its origins are controversial. Much (some think all) is made by archaeans—the neither-bacterial-nor-eukaryotic microorganisms that were only distinguished in recent decades.

But there is an abiogenic source of sequestered methane too. Mantle rocks that contain the mineral olivine (which describes a range of minerals from Mg2SiO4 to Fe2SiO4) are often altered to serpentine ((Mg,Fe)3Si2O5(OH)4), a change that also produces brucite ((Mg,Fe)(OH)2) and magnetite (Fe3O4). "