I wanted to make sure I was accurate, so I asked my older sister, the paleontologist. She said more-or-less what I thought, but added a few useful details:
First, you need decaying organic material, so the region in question must have had abundant living material at some time. Secondly, it needed an opportunity to decay while being pressed beneath sedimentary layers (volcanic rock would destroy the oil before it finished forming, or burn it off afterward.
Secondly, you need access to the oil. If it's buried so deep you can't affordably get to it or is in shale fields so absorbent that extracting the oil from its surroundings is impractical, then once again it won't be used. There is abundant oil beneath parts of the ocean where the sun cannot desiccate organic material first and it can accumulate at the seafloor, but once again, forces like continental drift and seafloor spreading that goes with it can prevent formation or once again make extraction ridiculously pricey.
