There has been speculation, particularly by Thomas Gold in his book The Deep Hot Biosphere, that methane can form without the intervention of organisms.  In Gold’s case, he proposed an origin in the mantle that supported a thriving organic community at great depth in the crust, and that such abiogenic methane is the source of all hydrocarbon and coal deposits.  Not many people believe Gold.  However, there are chemically feasible means of generating simple hydrocarbons in the upper earth, notably the Fischer-Tropsch catalytic process that has been used to synthesise artificial fuels.  The Fischer-Tropsch process hydrogenates a carbon-bearing gas, such as carbon dioxide, and commercially has used chromium oxide as a catalyst.  In hydrothermal systems that permeate olivine- and orthopyroxene-rich ultramafic rocks, those minerals breakdown to serpentines, talc and magnetite, and the reactions generate hydrogen, which is often found dissolved in samples of oceanic hydrothermal fluids and occasionally in onshore springs, where mantle rocks in ophiolites are being weathered.  So there is no shortage of hydrogen for potential reactions in sea-floor hydrothermal systems, and they contain lots of dissolved carbon dioxide.  Ultramafic rocks are rich in chromium generally in the form of Fe-Cr oxide or chromite.  Geochemists from the University of Minnesota simulated a hydrogen-carbon dioxide-chromite hydrothermal system to see if the Fischer-Tropsch process would work (Foustoukos, D.I. & Seyfried, W.E. 2004.  Hydrocarbons in hydrothermal vent fluids: The role of chromium-bearing catalysts.  Science, v. 304, p. 1002-1005).  It did, producing methane, ethane and propane under simulated conditions of sea-floor vents.  They conclude that these simple hydrocarbons help support thriving bacterial communities in “black smokers”.  Their results also support the possibility of such vents having produced “feedstock” for processes that led to the origin of life, but also lend a cautionary note to claims for ancient organic matter (see Early biomarkers in South African pillow lavas in May 2004 EPN)