Plant cell walls were apparently a tough nut for them to crack. Coal seams exist the world over for this reason. These trees, mostly fern palms, would die and fall over, and just lay there for millennia until they got buried by normal geological processes. Take the same trees and subject them to the much greater pressures and lack of oxygen at the bottom of an ocean and you get oil. This is why crude oil is found in ancient river deltas and inland seas. It's always locked in sedimentary deposits that date from the era.