Fossil fuels without the fossils

Methane gas can be formed from inorganic components.

For countless years, chemistry students have been taught that fossil fuels, such as natural gas and oil, are derived from prehistoric organisms - hence the name. A number of scientists have attempted to question this accepted scientific truth by arguing that fossil fuels could be produced inorganically, but this theory has never been given much credence. Now, however, US researchers could be beginning to change all that.

The team, composed of physicists from Indiana University and the Carnegie Institution and chemists from Harvard University and the Lawrence Livermore National Laboratory, has successfully generated methane, which is the main component of natural gas, from inorganic materials exposed to high pressures and temperatures. This result will not only force scientists to re-evaluate their theories of fossil fuel formation, but also raises the possibility that large reservoirs of fossil fuels could exist deep inside the Earth.

Using a diamond anvil cell, the researchers exposed calcite (CaCO₃), an iron oxide (FeO) and water to pressures of 5-11 gigapascals (GPa) - 50 000 to 110 000 times the pressure at sea level - and temperatures from 500?C to 1500?C. These conditions mimic those thought to occur in the Earth’s upper mantle, which lies between 50km and 250km below the surface and where CaCO₃, FeO and water are thought to be present.

Using Raman spectroscopy, the researchers detected the production of methane in the form of bubbles at a wide range of pressures and temperatures, although the process was most efficient at temperatures of around 500?C and pressures of around 7 GPa. The researchers propose that the methane is formed by the reduction of calcite and suggest that it could be similarly produced from any carbonate species. They also raise the possibility that heavier hydrocarbons could be generated in the mantle using methane as a precursor.

’Although it is well-established that commercial petroleum originates from the decay of once-living organisms, these results support the possibility that the deep Earth may produce abiogenic hydrocarbons of its own,’ explains lead researcher Henry Scott of Indiana University.

Jon Evans