Oil is thought to have formed from the remains of organisms that died millions of years ago. Billions of tiny organisms lived in shallow water or in the surface layers of the oceans. As they died their remains settled to the bottom and were eventually buried. The remains were compressed under the weight of rock. They underwent chemical changes due to the high pressure and eventually formed oil and gas, which remained trapped under the rock layers.

               Geologists locate deposits of oil, and holes are drilled so the oil can be extracted. Usually, water is pumped into an oil well under pressure, forcing the lighter oil up to the surface.

Picture credit: google