I think part of the problem is that science tries to establish ‘one’ source for coal, or oil.
There are many different types of coal, and oil.
The reason for that is that they were made from different ‘things’, and different ‘processes’.
All life on Earth is carbon based, and all things ‘return’ to the Earth.
Coal may be formed by the layering of dead animals and plants, after being subjected to heat and pressure.
It is also likely that a stratifying process may take place deeper in the Earth, where elements are subject to unimaginable pressure and temperature.
So, oil, and coal, may come from both sources.
We just don’t know enough about the Earth, to say for sure.
If we did know for sure, then finding it would be a lot more predictable.
(just my thoughts. Not expecting anyone to agree)
It’s pretty well understood that coal is formed from the carboniferous forests that existed onshore. We don’t find coal beds that are formed in marine environments ever.
Oil is bit trickier, but it’s nearly 100% marine. The reason it’s trickier is that oil is not usually found in the sediments where it was formed, but has migrated toward the surface to a completely different rock formation which has a seal above it.
Coal can’t do that.