Extracting oil is not easy. Many people think that oil is just in a hole and you just drill and it will come up. Drilling toward reservoir rocks, means drilling through rocks that contain substances that may contaminate the oil. For instance, there might be water in a layer of rock. If you just drilled with an open hole, the water would come up.

Drilling requires a casing around the hole that seals off the other substances. This casing allows drilling mud to continuously circulate down the hole and back. Usually cement can be pumped in to help protect from contamination.