Did you ever notice that in a forest fallen trees that have been there where they fell for many years are just hollow outlines of their former selves?
We can only assume that they slowly “combusted”, turned to water vapor and CO2 and left behind but traces of their former selves over 100s and 1000s of years; how much of the plant growth was supposed to have been trapped underground to be compressed into crude over time?
There are no trees, land plants, or dinosaurs in crude oil, and there's never been a theory to that effect.
Oil comes from ancient dead microscopic plankton (algae and diatoms) that lived in shallow seas.