Thomas Gold wrote about this 20 years ago in Deep Hot Biosphere.

The evidence for oil being biotic is due to the dead organic particles found in it, at pressures and depths where, at the time, were deemed impossible. There was no other explanation.

It wasn't until the mid-70s, when the Soviet Union was doing deep drilling in the Arctic, that they found microbial life at depths previously unknown, thereby removing the only evidence of biotic origin for these hydrocarbons.