At the top of an oil reservoir, you'll typically find a layer of cap rock.

Here's why:

* Trapping Mechanism: Cap rock acts as a seal, preventing the oil and gas from escaping upwards. It's usually an impermeable rock type, like shale, salt, or even a thick layer of clay.

* Formation: Oil and gas are lighter than water and migrate upwards through porous rock layers. When they encounter an impermeable layer, they get trapped.

* Importance: The cap rock is crucial for forming and maintaining an oil or gas reservoir. Without it, the hydrocarbons would simply escape to the surface.

Let me know if you'd like to know more about the different types of cap rocks or how oil reservoirs form!