The familiar tale is that “Prehistoric plants died and changed into oil and gas while sandwiched between layers of rock in the hot, high-pressure environment deep below Earth’s surface.”
Where and when did this come from and how is it supported?
It’s what I grew up hearing but it seems to make little sense.
Interesting article:
The Mysterious Origin and Supply of Oil
http://www.livescience.com/environment/051011_oil_origins.html
Back then, trolls lived underground, and they operated the giant presses (made of rock) and squeezed out the oil and gas from the dead plants and dinosaurs.
After all their work was done, they tunneled to the surface, and now live in small caves.
Shales and other organic rich sedimentary rocks are the source rocks. The "oil shales" found in Wyoming, Colorado and Utah don't contain "oil" but "kerogen", a material that with time and heat can become oil. These deposits are basically the intermediate step on the way to becoming free oil. With enough time and enough heat, suitable reservoir rock, a trap in the reservoir with a good seal there might be oil hell of an oilfield in the making ... but only on a geologic timescale.