Petroleum occurs naturally in the earth in porous rock, found anywhere from thousands of meters underground all the way to the earth's surface. The porous layer of rock normally lies between two nonporous layers, so oil does not flow out of its reservoir.In most cases water and/or natural gas occur along with petroleum in the rock. Oil can be extracted from the rock by sinking a pipe into the earth until it penetrates the saturated porous rock. In most cases, oil will then begin to flow of its own accord out of the rock and into the pipe.

The upward flow of oil can be caused by a number of different factors. In some cases, the porous layer of rock is covered by a hollow cap filled with natural gas. Pressure of the gas forces petroleum out of the rock into the pipe. In...