If the Bakken formation is around 350 million years old, it only required an inch of new sediment every 2762 years to build up to a depth of two miles in all that time.
As I understand it, the standard theory is that oil was formed from algae, not trees and animals.
Well, maybe not the algae themselves, but algae poop.
Even more likely is that oil is formed from bacteria. After all, SOMETHING ate all the oil from the BP leaking seafloor well. Maybe oil is produced from algae and bacteria poop.