Recent work carried out at Carnegie suggests that substantial amounts of the hydrocarbon material could exist in the deep Earth and be formed by inorganic (or abiogenic) processes. These hydrocarbons, which include natural gas and petroleum, could migrate from the mantle to the near-surface region and contribute to 'fossil fuel' reserves. Read more about the work of Alexander Goncharov and colleagues from Sweden and Russia here.
<a data-cke-saved-href="http://cdac.carnegiescience.edu/images/stories/kolesnikov nat geosci 2009.pdf" href="http://cdac.carnegiescience.edu/images/stories/kolesnikov nat geosci 2009.pdf" "="">[A. Kolesnikov, et al., Nature Geosci., 2, 566-570 (2009)].