Abiotic gas: atypical but not rare

Abiotic gaseous hydrocarbons comprise a fascinating, but poorly understood,
group of Earth fl uids generated by magmatic and gas–water–rock reactions that do not directly involve organic matter. At least
nine different inorganic mechanisms, including Fischer-Tropsch type reactions,
occur over a wide range of temperatures. Trace amounts (typically parts
per million by volume) are formed in volcanic and geothermal fl uids, but
considerable amounts of methane, reaching 80–90 vol%, are now recognized
in an increasing number of sites in Precambrian crystalline shields and
serpentinized ultramafi c rocks. Surface manifestations of abiotic gas related
to serpentinization release gas directly to the atmosphere in ways that are
similar to seepages of ordinary biotic gas from petroliferous areas. Abiotic
methane is more widespread than previously thought. It also likely exists in
sites undergoing active serpentinization and may be present in petroleum
systems in the vicinity of serpentinized rocks.