Often the oil contains natural gas. It is dissolved in the
oil because of the high pressure deep underground.
When the drill penetrates the oil-bearing layer this
pressure is released. It provides a tremendous thrust
and may be sufficient to bring all the oil to the surface.
If the gas pressure is not sufficient a pumping station
must be built to bring up the oil. This increases the
cost of the drilling operation. In its natural 'crude' state
oil consists of a mixture of various substances called
hydrocarbons. These substances must be separated
according to their densities, and purified. This process
is called refining.