Just because it's black, doesn't mean it's oil.
Just saying.
Plus, with what I was taught in school regarding this subject, which was only about a weeks worth of chapter study.........
so it wasn't "in depth" by any means............
Oil came from decaying dinosaurs (and other life forms) that died because of the meter impact that created the "death cloud" over the planet and killed off most life.
The flesh of these dead and decaying lifeforms were dissipated by top crust organisms, because the ash and debris from the meteor impact that spread across the globe, covered the dead lifeforms like a blanket. So this caused the decaying matter to collect and seep into the ground.
Thus the ground surface acted as a filter for the decaying matter, which was carried down to levels where the bacteria could breed and multiply with what was left of the dead lifeforms. It all collected into pockets in the earth, where the bacteria had room to grow, as well as enough food to consume and create what we have today.
Whether thats actually all true or not is another issue. But thats what was taught when I was in school.