Fossil fuel without the fossils

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.

References 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).

Wasn't this Gold's argument?

No, this was not Gold's argument. He said that petroleum (and coal) was formed by cracking of primordial methane by temperature/pressure gradients as it rose from deep (perhaps 100-300 km) in the outer mantle. He wasn't saying that methane itself was formed from other minerals.

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.

Of course, there are more and better reasons to believe that it isn't a fossil fuel than that it is. The remaining petroleum which is assumed to be of biological origin has been assumed to be so on the basis of studies themselves done over a hundred years ago in the infancy of petroluem studies. It was assumed that since the petroleum preferentially rotated light in the same direction that organic substances of known biological origin did, it had to have come from biological sources. The assumption was that if petroleum was abiogenic, there would be an equal mix of enantiomers of these compounds. (This is what happens when enantiomeric chemicals are produced industrially. Although the forms are mirror images, they may have very different biological properties--as seen in the racemic mixture of thalidomide. One enantiomer was a sedative, the other was a teratogen). At the time this theory was locked into place and assumed to be true, they didn't reckon on the discovery of bacteria at rather extreme depth and temperature capable of living off methane and various forms of hydrocarbons (aliphatic, aromatic, halogenated and nitrated compounds). The presence of living organisms would contaminate the petroleum with biogenic organic compounds and impart a rotatory effect characteristic of organics derived from biogenic processes. Although petroleum is levo-rotatory, it is only slightly so, and much, much less than would be expected if it were entirely of biological origin.

In addition to the above, Gold describes the following considerations which militate against a biogenic origin for petroleum:

The similarity of all commercial crude oils encompasses the following factors:

1.) Nickel and vanadium porphyrins are found in varying proportions, but in all petroleum deposits. Porphyrin molecules are complex molecules made up of carbon, hydrogen and nitrogen, together with a metal atom. Their presence in petroleum has been attributed to chlorophyll from photosynthesizing plants, and to the haem of the blood of animals, and both these will indeed produce porphyrin molecules. But those would contain the metal atoms of magnesium and iron. However no single case is known of magnesium or iron porphyrins having been found in petroleum anywhere. An explanation that on every occasion in all oils the original metal atoms had been exchanged for just nickel and vanadium from the rocks in their surroundings, seems extremely improbable. No explanation has been offered how plant debris would have produced the nickel and vanadium molecules, while, in the other explanation, nickel and vanadium complexes may well be expected, since these two metals are particularly prone to make organometallic compounds. This find therefore favors a deep origin, and at the same time a common origin for all oils.

2.) The heavier hydrocarbon molecules have a large variety of isomers (molecules of the same number of hydrogen and carbon atoms, but assembled in different geometrical configurations). The distribution of isomers of aromatic hydrocarbons has been demonstrated to depend upon the temperature range of their formation. Studies of the isomers of 322 oils from various oil fields of the world, have shown that they have a common set of isomers, a set that has been demonstrated to come from a formation temperature of between 700 and 1,100 °C. There is no significant difference in this range between oils coming from different tectonic settings. The overall hydrocarbon composition corresponds to the equilibrium state at temperatures 1,300 to 1,500 °C and pressures of 20 to 40 kb. The estimate is that this is the condition in the upper mantle at depths of 60 to 160 km. Temperatures and pressures in the sedimentary blanket are certainly far from the conditions necessary to account for the isomeric composition characteristic of all natural oils. (This information comes primarily from the publications of two chemists and thermodynamicists from the Ukraine, G.E. Boiko and E.B. Chekaliuk, over the years from 1950 to 1982. Although there is much reference to these publications in the Soviet scientific literature, and I have referred to them in my publications, I have found no other reference to these in the U.S., British, German or French literature.) This universal property of oils thus makes it extremely unlikely that two completely different modes of formation could have been responsible for such complex but similar products; at the same time the temperature range indicated is far too high for a sedimentary origin.

Here in a U.S.G.S. Professional Paper 1570, The Future of Energy Gases, 1993 Gold sums up the primary arguments for and against a biogenic origin of petroleum:

Arguments have been advanced for each viewpoint, and although they conflict with each other, each line of argument sounds strangely convincing. In favor of the biogenic origin of petroleum, the following four observations have been advanced:

(1) Petroleum contains groups of molecules which are clearly identified as the breakdown products of complex, but common, organic molecules that occur in plants, and that could not have been built up in a non-biological process.

(2) Petroleum frequently shows the phenomenon of optical activity, i.e. a rotation of the plane of polarization when polarized light is passed through it. This implies that molecules which can have either a right-handed or a left-handed symmetry are not equally represented, but that one symmetry is preferred. This is normally a characteristic of biological materials and absent in fluids of non-biological origin.

(3) Some petroleums show a clear preference for molecules with an odd number of carbon atoms over those with an even number. Such an odd-even effect can be understood as arising from the breakdown of a class of molecules that are common in biological substances, and may be difficult to account for in other ways.

(4) Petroleum is mostly found in sedimentary deposits and only rarely in the primary rocks of the crust below; even among the sediment, it favors those that are geologically young. In many cases such sediment appears to be rich in carbonaceous materials that were interpreted as of biological origin, and as source material for the petroleum deposit.

On the other side of the argument, in favor of an origin from deeply buried materials incorporated in the Earth when it formed, the following observations have been cited:

(1) Petroleum and methane are found frequently in geographic patterns of long lines or arcs, which are related more to deep-seated large-scale structural features of the crust, than to the smaller scale patchwork of the sedimentary deposits.

(2) Hydrocarbon-rich areas tend to be hydrocarbon-rich at many different levels, corresponding to quite different geological epochs, and extending down to the crystalline basement that underlies the sediment. An invasion of an area by hydrocarbon fluids from below could better account for this than the chance of successive deposition.

(3) Some petroleums from deeper and hotter levels lack almost completely the biological evidence . Optical activity and the odd-even carbon number effect are sometimes totally absent, and it would be difficult to suppose that such a thorough destruction of the biological molecules had occurred as would be required to account for this, yet leaving the bulk substance quite similar to other crude oils.

(4) Methane is found in many locations where a biogenic origin is improbable or where biological deposits seem inadequate: in great ocean rifts in the absence of any substantial sediments; in fissures in igneous and metamorphic rocks, even at great depth; in active volcanic regions, even where there is a minimum of sediments; and there are massive amounts of methane hydrates (methane-water ice combinations) in permafrost and ocean deposits, where it is doubtful that an adequate quantity and distribution of biological source material is present.

(5) The hydrocarbon deposits of a large area often show common chemical or isotopic features, quite independent of the varied composition or the geological ages of the formations in which they are found. Such chemical signatures may be seen in the abundance ratios of some minor constituents such as traces of certain metals that are carried in petroleum; or a common tendency may be seen in the ratio of isotopes of some elements, or in the abundance ratio of some of the different molecules that make up petroleum. Thus a chemical analysis of a sample of petroleum could often allow the general area of its origin to be identified, even though quite different formations in that area may be producing petroleum. For example a crude oil from anywhere in the Middle East can be distinguished from an oil originating in any part of South America, or from the oils of West Africa; almost any of the oils from California can be distinguished from that of other regions by the carbon isotope ratio.

(6) The regional association of hydrocarbons with the inert gas helium, and a higher level of natural helium seepage in petroleum-bearing regions, has no explanation in the theories of biological origin of peroleum.

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