Can There Be Two Independent Sources of Commercial Hydrocarbon Deposits, One Derived from Biological Materials, the Other from Primordial Carbon and Hydrogen, Incorporated into the Earth at its Formation?
Thomas Gold
November 1996
In any discussion of the ultimate origin of hydrocarbons (oil and natural gas) that are commercially extracted, this is a major question: are there two pathways for generating these deposits, or is there evidence that there is only one? If it can be shown that there is only one, then a proof of the derivation of any one hydrocarbon deposit would constitute a proof that this represents the derivation of all.
The two pathways that are under discussion are a derivation from biological materials deposited in the sediments, or a derivation from carbonaceous materials incorporated into the Earth at its formation.
For the biological origin we have the evidence of the (unquestionably) biological origin of some sets of molecules found in all commercial oils.
For the second, the primordial origin of commercial hydrocarbons, we have the comparison with the abundance of similar hydrocarbons on many other planetary bodies, also in interplanetary grains, in comets, and also in the interstellar gas clouds thought to be similar to the cloud that formed the solar system. We also see abundance of methane in the volcanic ocean vents where there are no substantial biological sediments. Furthermore the common association of hydrocarbons with the inert gas helium has no explanation in a formation process from biological materials, but is readily understood as a consequence of the sweeping up of the trace gases by hydrocarbon fluids, if those have migrated up from a deep level, far below the sedimentary blanket.
The evidence for a biological origin, given by the presence of biological molecules in all oils, can be explained not only by a biological origin of the oils themselves, but equally well or better by a contamination with microbial materials in all oil wells. Microbial life at depth in the rocks was predicted on the basis that it would account for the biological molecules in oils, and such life has now been found to be widespread.
It has often been suggested that both modes of derivation may have occurred. There have been suggestions that the two different modes can be identified, and it has been said that "most" commercial oil and gas have come from biological materials, though there is some oil and gas, though commercially insignificant, that derived from deep sources in the mantle. Are there common features in all commercial crude oils that would rule out the dual origin theories?
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.
The depth range indicated is also that of the derivation of diamonds, whose formation required the presence of unoxidized carbon under a pressure in excess of 30 kb. Violent gas-driven eruptions from upper mantle depths were required to deliver the diamonds and other deep source materials to the surface.
3.) The common association of hydrocarbons with helium has no explanation in a biological origin theory; in a theory involving the ascent of hydrocarbons from deep levels, the physical process of sweeping up the gases in the pores of the rock, would be expected; and helium is a significant component of those. This also points to a derivation of the majority of oils from deep levels.
4.) The arguments of Robert Robinson still stand, that any biological debris would be quite unlikely to produce hydrogen-saturated hydrocarbons. The hydrogen/carbon ratio of biological materials is too low in the first place, and slow chemical processing in geologic settings would lead to a further loss of hydrogen. Yet most commercial hydrocarbon deposits contain methane and other high hydrogen components. The average hydrogen proportion is greater, the deeper the level from which the hydrocarbons are withdrawn, corresponding to a hydrogen loss during the upward migration of the fluids.
The overall conclusion is therefore that natural petroleum has detailed chemical features that are common to all, and that we must therefore consider that all derived from the same process. Moreover, most oils bear clear evidence of having had a deep origin, and a high temperature of formation.
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