Petroleum: animal, vegetable or mineral?
Petroleum - the archetypal fossil fuel - couldn't have formed from the remains of dead animals and plants, claim US and Russian researchers1. They argue that petroleum originated from minerals at extreme temperatures and pressures.
Other geochemists say that the work resurrects a scientific debate that is almost a fossil itself, and criticize the team's conclusions.
The team, led by J. F. Kenney of the Gas Resources Corporation in Houston, Texas, mimicked conditions more than 100 kilometres below the Earth's surface by heating marble, iron oxide and water to around 1,500 °C and 50,000 times atmospheric pressure.
They produced traces of methane, the main constituent of natural gas, and octane, the hydrocarbon molecule that makes petrol. A mathematical model of the process suggests that, apart from methane, none of the ingredients of petroleum could form at depths less than 100 kilometres.
Yet petroleum is found far shallower than this in sedimentary rocks. The conventional view is that oil, from which petroleum is derived, forms just a few kilometres below the surface at temperatures of 50-150 °C. This process can also be recreated in the laboratory.
Showing that oil can also form without a biological origin does not disprove this hypothesis. "It doesn't discredit anything," said one geochemist who asked not to be named.
The theoretical case for a non-biological origin of oil was first made over 100 years ago. Later experiments showed that it should be possible to make oil from minerals alone.
"No one disputes that hydrocarbons can form this way," says Mark McCaffrey, a geochemist with OilTracers LLC, a petroleum-prospecting consultancy in Dallas, Texas. A tiny percentage of natural oil deposits are known to be non-biological2- but this doesn't mean that petrol isn't a fossil fuel, he says.
"I don't know anyone in the petroleum community who really takes this prospect seriously," says Walter Michaelis a geochemist at the University of Hamburg in Germany.
Petroleum that forms inorganically at the high temperatures and massive pressures close to the Earth's mantle layer could be forced towards the surface by water, which is denser than oil. It can then be trapped by sedimentary rocks that are impermeable to oil, geochemists concede.
But a wealth of chemical evidence points to a biological origin for petroleum. Petroleum and biological molecules contain the same type of carbon and have the same molecular structure - hinting at a common origin.
Moreover, these chemical signatures are used to find oil. If oil was formed from simple minerals, "these wouldn't have been such a good predictive tool", says McCaffery.
Kenney and his team were unavailable to comment on their research.
Kenney, J. F., Kutchenov, V.A., Bendeliani, N. A. & Alekseev, V. A. The evolution of multicomponent systems at high pressures: VI. The thermodynamic stability of the hydrogen-carbon system, the genesis of hydrocarbons, and the origin of petroleum.. Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.172376899 (published online August 2002
Sherwood Lollar, B., Ward, J. A., Slater, G. F. & Lacrampe-Couloume, G. Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs. Nature 416, 522 - 524 (2002).
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Clarke, T. Fossil fuel without the fossils. Nature (2002). https://doi.org/10.1038/news020812-3