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I am curious to see what people think of the abiotic theory of oil deposit formation versus the traditional theory.

I have long wondered how enough organic material became trapped underground to degrade and form oil deposits. Especially when you consider the shear amount of oil that we have found underground over the decades. And considering how much oil we use. If you think about it, when most critters die the tend to decompose mostly or fully above ground. So I have trouble believing that such large amounts of oil, especially in such large concentrations, have formed from dying plants and animals over the centuries.

Recently I heard about the theory of abiotic oil. My understanding of the theory is that oil deposits are actually chemicals and material that is formed at the mantle boundary that seeps up further into the crust where it is consumed by micro-organisms that process it and turn it into what we know as oil. My understanding may be a bit off I realize.

But some evidence of this has been presented in the form of old, capped oil wells that after a few decades, when re-examined, seem to be full once more and are now run at full pumping capacity once more. There are other things I have read as evidence. It seems to me that this would be a better explanation for the large amounts of oil deep underground.

Does anyone know enough about these subjects to say that one theory holds more water than the other? Has there been any new evidence pointing to one or the other theory more?

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closed as off topic by David Z Oct 25 '12 at 16:33

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The large amounts cannot be deposited in a short time. "over the centuries" here really means a time span in the range of 70 million years. I am not sure for oil, but at least in coal you can still find organisms: – Alexander Oct 25 '12 at 15:57
There's a stupendous amount of limestone on the planet (10% of all sedimentary rock), much of which is composed from skeletal fragments of marine organisms. Thats a lot of dead biota. So it isn't surprising that, over millions of years, decomposing organisms can produce very large quantities of oil. – RedGrittyBrick Oct 25 '12 at 20:30
up vote 1 down vote accepted

It seems most experts think the abiotic theory is nonsense


For example

Dr. Jon Clarke: .

The fact remains that the abiotic theory of petroleum genesis has zero credibility for economically interesting accumulations. 99.9999% of the world's liquid hydrocarbons are produced by maturation of organic matter derived from organisms. To deny this means you have to come up with good explanations for the following observations.

  1. The almost universal association of petroleum with sedimentary rocks.
  2. The close link between petroleum reservoirs and source rocks as shown by biomarkers (the source rocks contain the same organic markers as the petroleum, essentially chemically fingerprinting the two).
  3. The consistent variation of biomarkers in petroleum in accordance with the history of life on earth (biomarkers indicative of land plants are found only in Devonian and younger rocks, that formed by marine plankton only in Neoproterozoic and younger rocks, the oldest oils containing only biomarkers of bacteria).
  4. The close link between the biomarkers in source rock and depositional environment (source rocks containing biomarkers of land plants are found only in terrestrial and shallow marine sediments, those indicating marine conditions only in marine sediments, those from hypersaline lakes containing only bacterial biomarkers).
  5. Progressive destruction of oil when heated to over 100 degrees (precluding formation and/or migration at high temperatures as implied by the abiogenic postulate).
  6. The generation of petroleum from kerogen on heating in the laboratory (complete with biomarkers), as suggested by the biogenic theory. The strong enrichment in C12 of petroleum indicative of biological fractionation (no inorganic process can cause anything like the fractionation of light carbon that is seen in petroleum).
  7. The location of petroleum reservoirs down the hydraulic gradient from the source rocks in many cases (those which are not are in areas where there is clear evidence of post migration tectonism).
  8. The almost complete absence of significant petroleum occurrences in igneous and metamorphic rocks (the rare exceptions discussed below).

    The evidence usually cited in favour of abiogenic petroleum can all be better explained by the biogenic hypothesis e.g.:

  9. Rare traces of cooked pyrobitumens in igneous rocks (better explained by reaction with organic rich country rocks, with which the pyrobitumens can usually be tied).

  10. Rare traces of cooked pyrobitumens in metamorphic rocks (better explained by metamorphism of residual hydrocarbons in the protolith).
  11. The very rare occurrence of small hydrocarbon accumulations in igneous or metamorphic rocks (in every case these are adjacent to organic rich sedimentary rocks to which the hydrocarbons can be tied via biomarkers).
  12. The presence of undoubted mantle derived gases (such as He and some CO2) in some natural gas (there is no reason why gas accumulations must be all from one source, given that some petroleum fields are of mixed provenance it is inevitable that some mantle gas contamination of biogenic hydrocarbons will occur under some circumstances).
  13. The presence of traces of hydrocarbons in deep wells in crystalline rock (these can be formed by a range of processes, including metamorphic synthesis by the fischer-tropsch reaction, or from residual organic matter as in 10).
  14. Traces of hydrocarbon gases in magma volatiles (in most cases magmas ascend through sedimentary succession, any organic matter present will be thermally cracked and some will be incorporated into the volatile phase, some fischer-tropsch synthesis can also occur).
  15. Traces of hydrocarbon gases at mid ocean ridges (such traces are not surprising given that the upper mantle has been contaminated with biogenic organic matter through several billion years of subduction, the answer to 14 may be applicable also).

The geological evidence is utterly against the abiogenic postulate.


Abiogenic Origin of Hydrocarbons: An Historical Overview by Dr. Geoffrey Lasby

Abstract: The two theories of abiogenic formation of hydrocarbons, the Russian-Ukrainian theory of deep, abiotic petroleum origins and Thomas Gold's deep gas theory, have been considered in some detail. Whilst the Russian-Ukrainian theorywas portrayed as being scientifically rigorous in contrast to the biogenic theory which was thought to be littered with invalid assumptions, this applies only to the formation of the higher hydrocarbons from methane in the upper mantle. In most other aspects, in particular the influence of the oxidation state of the mantle on the abundance of methane, this rigour is lacking especially when judged against modern criteria as opposed to the level of understanding in the 1950s to 1980s when this theory was at its peak.

Thomas Gold's theory involves degassing of methane from the mantle and the formation of higher hydrocarbons from methane in the upper layers of the Earth's crust. However, formation of higher hydrocarbons in the upper layers of the Earth's crust occurs only as a result of Fischer-Tropsch-type reactions in the presence of hydrogen gas but is otherwise not possible on thermodynamic grounds. This theory is therefore invalid. Both theories have been overtaken by the increasingly sophisticated understanding of the modes of formation of hydrocarbon deposits in nature.

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Numbers 14 and 15 seem to contradict some of the earlier statements in the list. Specifically talking about the progressive breakdown at temperatures over 100 degrees. If the mantle is contaminated with biogenic organic material (which isn't surprising) then why would it not fully breakdown in the mantle before having a chance to rise once more? I may be misunderstanding these statements though. – pthurmond Oct 25 '12 at 19:39
@pthurmond: Here's my understanding: crude oil contains a mix of hundreds of different hydrocarbons, in a refinery, heat is used to separate these, including separating hydrocarbon gases from residuals like coke, asphalt and tar. So (5) heating breaks down crude oil (14&15) into hydrocarbon gases like methane, ethane, propane and butane (which rise) and heavier tars etc (which are subducted further, are heated further so broken down further) – RedGrittyBrick Oct 25 '12 at 20:23

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