I learned it nearly 30 years ago in school. I just did a search and found a link about it, though.

Also, seems that either I remembered wrongly, or my teacher made a mistake, but it seems it was most of the worlds coal; not oil, that came from all the piles of trees from that period.

https://www.thorogood.co.uk/treevolution-how-trees-came-first-and-rot-came-later-in-earths-deep-past/