[VadeRetro]

Wikipedia describes both ideas, but the "biogenic" theory is "mainstream."

When it was later discovered that all fossil fuels contain traces of biological debris, the biogenic theory gained further support because the idea that life (even microbial life) could exist at the depths at which petroleum had been found seemed even less plausible.

Still, the idea that tar is not found at all anywhere strikes me as funny.

[js1138]

...because the idea that life (even microbial life) could exist at the depths at which petroleum had been found seemed even less plausible.

Well, excuse my admitted ignorance, but the discovery of thermophyles seems to blow this assumption away. Secondly, the theories don't have to be mutually exclusive. If a Miller process formed petroleum, there is no reason to believe that early life shouldn't be found in the same strata.

So what kind of biological debris is found in petroleum? Anything that would date it by morphology, or is it just generic organic molecules?