Abiotic origin (A-biogenic hydrocarbons)

Over many decades, the origin of oil and gas was the subject of a heated discussion: Is it biotic or abiotic. When the petroleum system was much less understood than today, and, geochemistry of source rocks and petroleum was less advanced, it seemed that one might give equal attention to both possible origins. The biotic school maintains that the bulk of accumulated hydrocarbons are formed from organic matter as described in the petroleum system logic. The abiotic school suggests that oil and gas are formed by outgassing of methane from the mantle, and further synthesis to more complex hydrocarbons by non-biological, chemical reactions.

The major advances in geochemical and isotope analysis to link source rocks to accumulated oil, as well as the ability to predict quantities and qualities of generated hydrocarbons on the basis of the biotic petroleum system, puts a heavy weight on the biotic origin hypothesis.(Nederlof,1988).

Quite often, the proponents of abiotic origins, come up with arguments and examples which unfortunately are not convincing from the view of petroleum geology. For instance "Oil is found in basement" . Such examples are invariably clearly related to basement (even wheathered basesment) in juxtaposition with source rocks or at least sediments from which the oil has migrated. Or "the deepest fossil found was at 16,000', but oil is found to a depth of 30,000'. That precludes a biotic origin". Unfortunately such reasoning displays a misunderstanding of even basic geology! (Mrgheb, 2013, Gold,1988, Mahmoud et al., 1997). Detecting traces of hydrocarbons in igneous or metamorphic rocks can be explained by plate tectonics, as eventually sourcerocks, including whole accumulations will be subducted and then again emerge. Most complex hydrocarbons will then be decomposed during their millions of years travel at high temperatures, but at least some methane will survive. So the hydrocarbon presence in such rocks will not necessarily prove that vast amount of abiotic HC are stored in the mantle, or being formed there. Extraterrestial methane on planets and their satellites do not prove that the inner earth is a rich HC source. Some proponents of abiotic origin suggest that oil is continually replenished by abiotic processes! King-Hubbert surely would disagree.

However, giving a heavy weight to the organic, biotic hypothesis, is not to say that some abiotic processes take place. With the present evidence, the contribution of abiotic HC, if any, is of minor importance.