A number of years ago I read The Deep Hot Biosphere by Thomas Gold because I was working on a novel which included extremophile bacteria. Gold's premise was that some/many/most underground hydrocarbons were actually produced underground from methane deep in the earth that is converted by underground bacteria to longer-chain hydrocarbons as they move toward the surface. Many thought gold to be a quack, including most in the oil industry, but I thought his hypothesis at least intriguing enough to test. Which someone apparently has:
An article in Science today seems to suggest that the abiotic theory is correct. In a fairly dense article entitled "Abiogenic Hydrocarbon Production at Lost City Hydrothermal Field,"
researchers Proskurowski et al., find evidence of the abiogenic
formation of short-hydrocarbon chains in an area where hydrocarbons
would not otherwise be able to form by the biogenic theory. What
Proskurowski et al. identified was the formation of carbon chains 1 to
4 carbon atoms in length, with shorter chains forming deeper, and with
isotopic signatures ruling out biogenic origins. The conclusion of the
article is as follows: "Our findings illustrate that the abiotic
synthesis of hydrocarbons in nature may occur in the presence of
ultramafic rocks, water, and moderate amounts of heat."
My sense is that we may now say a fraction of oil is abiogenic, but are a long way from saying that any serious percentage is of non-fossil sources. But it is interesting.