Petroleum occurs at a depth ranging from a few hundred meters to about 2-3 kilometers. So, its mining is very expensive process. The possible locations are first identified by aerial surveys and ground tests (this is called oil exploration). Then, holes are drilled into the earth’s crust and pipes are inserted until the pipes reach the petroleum bearing rocks (this is called oil drilling). When pipes are lowered to slightly greater depths, then the crude oil gushes out on its own due to high internal pressure of natural gas. When the gas pressure decreases, the crude oil is pumped out with the help of pumps.
The crude oil from the oil field is taken to the refineries with the help of underground pipelines. The drill-hole from which crude oil are mined are called oil-wells. Now –a day, petroleum is also mined by drilling holes into the seabed. Such a drilling is called offshore drilling. The first oil-well was drilled in Pennsylvania, U.S.A., in 1859.
How is petroleum refined ?
Petroleum (crude oil) consists of a large number of organic compounds mostly saturated hydrocarbons mixed with sea water, and silt (fine clay). Petroleum (crude oil) cannot be used as such .However; Petroleum (crude oil) can be separated into several useful products by fractional distillation.