Petroleum is found in conventional oil fields, such as those in Texas, Saudi Arabia and the North Sea. Petroleum is also found in oil shale deposits, such as the Bakken formation in North Dakota.
Oil and natural gas are found in places where organisms died millions of years ago and had their remains buried under sedimentary rock. Heat and pressure gradually decompose and modify peat into usable fossil fuels. In a conventional oil field, petroleum is in liquid and gas form. Most of it is easy to pump to the surface, as it is initially pressurized enough to lift itself against the force of gravity. Most of these oil fields have been found in the Middle East, although much oil has been discovered in Texas, Canada, Alaska, Russia, Venezuela, the North Sea and many other regions.
With the depletion of conventional oil, however, oil shale is being exploited as a replacement. Oil shale is a solid, kerogen-bearing rock that is found in wide formations. This petroleum is generally more difficult to refine than conventional oil because it is thicker, even when liquefied, and contains more impurities. Shale oil is found in the Bakken formation in Montana and North Dakota, the Green River formation in Utah and Colorado and the Boyne and Favel formations in Canada.