These conditions mimic those found 64 to 153 km deep inside the earth. The methane reacted and formed ethane, propane, butane, molecular hydrogen, and graphite.

'The notion that hydrocarbons generated in the mantle migrate into the earth's crust and contribute to oil-and-gas reservoirs was promoted in Russia and Ukraine many years ago,' wrote the authors of the study, which was carried out in association with scientists from Russia and Sweden.

'The synthesis and stability of the compounds studied here as well as heavier hydrocarbons over the full range of conditions within the Earth's mantle now need to be explored.

'These and related questions demonstrate the need for a new experimental and theoretical programme to study the fate of carbon in the deep earth,' the study authors added.

These results were published in the July online issue of Nature Geoscience.