Interesting. I knew how coal was formed but also thought a lot of our oil came from Carboniferous forests. After reading your comment I had to go read a few papers and articles! Apparently the consensus these days is that most of our oil came from microscopic aquatic life (diatoms, plankton, algae) that died and was buried in de-oxygenated water. The sheer amount of them that had to live and die to create these vast reservoirs of oil is mind blowing.