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Petroleum (from Latin petrus–rock and oleum–oil) or mineral oil is a thick, dark brown or greenish inflammable liquid, which, at certain points, exists in the upper strata of the earth. It consists of a complex mixture of various hydrocarbons, largely of the methane series, but may vary much in appearance, composition, and properties. Biological material in rocks starts off largely as a waxy material known as kerogen. Under the influence of heat and pressure, kerogen breaks down first into liquids and to gases. Both the liquid (petroleum) and gas phases (natural gas) tend to migrate through porous rocks until they encounter impermeable beds where packets/pools will tend to collect. After a drilling and pumping process to extract it from the strata, petroleum is refined by distillation. The products include kerosene, benzene, gasoline, paraffin wax, asphalt, etc.

Strictly speaking, petroleum consists entirely of aliphatic hydrocarbons, those composed of nothing but hydrogen and carbon.

The four lightest hydrocarbons -- CH4 (methane), C2H6 (ethane), C3H8 (propane) and C4H10 (butane) -- are all gases, boiling at -107°C, -67°C, -43°C, and -18°C, respectively (-161°, -88°, -46°, and -1° degrees F).

The chains in the C5-7 range are all light, easily vaporized, clear naphthas. They are used as solvents, dry cleaning fluids, and other quick-drying products. The chains from C6H14 through C12H26 are blended together and used for gasoline. Kerosene is made up of chains in the C10 to C15 range, followed by diesel fuel/heating oil (C10 to C20) and heavier fuel oils as the ones used in ship engines. These petroleum compounds are all liquid at room temperature.

Lubricating oils and semi-solid greases (including Vaseline®) range from C16 up to C20. Chains above C20 form solids, starting with paraffin wax, then tar and asphaltic bitumen.

Oil field in California, 1938

Boiling ranges of petroleum atmospheric pressure distillation fractions in degrees centigrade:

Petroleum's worth as a portable, dense energy source (powering the vast majority of automobiles, trucks, trains and ships), and as the base of many industrial chemicals makes it one of the world's most important commodities. Access to it was a major factor in several military conflicts, including World War Two and the Gulf War. Much of the world's readily accessible reserves are located in the Middle East, a politically unstable region.

In the 1970s, belief in an oil crisis, a fear that the world would run out of petroleum, proved unfounded as the application of computer technology caused the amount of accessible petroleum to increase. The future of petroleum as a fuel remains somewhat controversial. Some would argue that because the total amount of petroleum is finite, the dire predictions of the 1970s have merely been postponed. Others argue that technology will continue to allow for the production of cheap hydrocarbons and that the earth has vast sources of unconventional petroleum reserves in the form of tar sands[?], bitumen fields, oil shale, and methyl hydrate that will allow for petroleum use to continue for an extremely long period in the future.

List of Petroleum Companies

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