Here's a breakdown:

* Source Rock: Oil is actually formed from the decomposition of tiny sea creatures and plants over millions of years in a layer of rock called source rock.

* Migration: The oil then migrates upwards through porous rocks, like sandstone and limestone.

* Reservoir Rock: This migration continues until the oil reaches a layer of rock that is porous enough to hold it but impermeable enough to prevent it from escaping. This is the reservoir rock.

* Cap Rock: A layer of non-porous rock, called cap rock, sits on top of the reservoir rock, trapping the oil beneath.

So, while oil isn't found directly in one specific layer of the ground, it's extracted from the reservoir rock.