He said in an interview:
It's interesting that the same biomarkers found in oil are present in coal and represent, of course, parts of prokaryotic archaea that re-worked primordial hydrocarbons.
There are some occurrences of coal in Precambrian, Neoproterozoic. According to fossil record of planet Earth there's no superior plant at that time, then the Proterozoic coal is surely abiotic and represents probably ancient oil accumulation with high hydrogen loss and biodegradation of primordial hydrocarbons.
It is also common association of coal over oil and gas production areas. See below a comparison between maps of oil and coal occurrences in the United States.