We know the planet consists of a large amount of hydrocarbons.
We know that 'gravity' and 'heat' from the 'core' of the Earth make the depths of the planet a giant pressure cooker.
That liquid hydrocarbons would be produced and force their way 'up' through the more solid matter (running away from the heat?) seems to make a lot of sense.
When I was a kid in school and even when I was in college, we were taught that oil and coal came fromt he same source, biomass conversion. The fossilized leaves, etc. found in coal were shown as proof positive. Interesting how such myths take hold and are so difficult to shake off.