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Petroleum (from Latin petra – rock and oleum – oil) or crude oil (also known as black gold) is a black, dark brown or greenish liquid found in formations in the earth. The American Petroleum Institute, in its Manual of Petroleum Measurement Standards (MPMS), defines it as "a substance, generally liquid, occurring naturally in the earth and composed mainly of mixtures of chemical compounds of carbon and hydrogen with or without other nonmetallic elements such as sulfur, oxygen, and nitrogen."

Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. It consists of a complex mixture of hydrocarbons, mostly alkanes, but may vary greatly in appearance and composition. Petroleum is used mostly, by volume, for producing fuel oil and petrol (gasoline), both important "primary energy" sources (IEA Key World Energy Statistics). Petroleum is also the raw material for many chemical products, including solvents, fertilizers, pesticides, and plastics. 84% (37 of 42 gallons in a typical barrel) of all petroleum extracted is processed as fuels, including gasoline, diesel, jet, heating, and other fuel oils, and liquefied petroleum gas [1]; the other 16% is converted into other materials such as plastic.

Known reserves of petroleum are typically estimated at around 1.2ื10^12 barrels[2] with at least one estimate as high as 3.74ื10^12 barrels (3,740,000,000,000)[3]. Consumption is currently around 84ื106 barrels per day, or 31ื10^9 barrels per year. Because of pumping difficulties, usable oil reserves are only about 1/3 of total reserves. At current consumption levels, world oil supply would be gone in about 33 years. However, this ignores any additions to known reserves, changes in demand, etc. As the supply of petroleum becomes more scarce, consumers may look to alternative fuel sources such as ethanol, photovoltaic, or clean-burning hydrogen. Petroleum forms naturally within the earth too slowly to be sustainable for human use.




Octane, a hydrocarbon found in petroleum, lines are single bonds, black spheres are carbon, white spheres are hydrogen

The chemical structure of petroleum is composed of hydrocarbon chains of different lengths. These different hydrocarbon chemicals are separated by distillation at an oil refinery to produce gasoline, jet fuel, kerosene, and other hydrocarbons. The general formula for these hydrocarbons is CnH2n+2. For example 2,2,4-Trimethylpentane, widely used in gasoline, has a chemical formula of C8H18 which reacts with oxygen exothermically.

C8H18(aq) + 12.5O2(g) โ†’ 8CO2(g) + 9H2O(g) + heat

Incomplete combustion of petroleum or gasoline results in emission of poisionous gases such as carbon monoxide and/or nitric oxide. For example:

C8H18(aq) + 12.5O2(g) + N2(g) โ†’ 6CO2(g) + 2CO(g) + 2NO(g) + 9H2O(g) + heat

Formation of petroleum occurs in a variety of mostly endothermic reactions in high temperature and/or pressure. For example, a kerogen may break down into hydrocarbons of different lengths:[5]

CH1.45(s) + heat โ†’ .663CH1.6(aq) + .076CH2(aq) + .04CH2.6(g) + .006CH4(g) + .012CH2.6(s) + .018CH4.0(s) + .185CH.25(s)

Biogenic theory

Most geologists view crude oil and natural gas, as the product of compression and heating of ancient organic materials over geological time. According to this theory, oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea bottom in large quantities under anoxic conditions. (Terrestrial plants tend to form coal) Over geological time this organic matter, mixed with mud, is buried under heavy layers of sediment. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis. Because most hydrocarbons are lighter than rock or water, these sometimes migrate upward through adjacent rock layers until they become trapped beneath impermeable rocks, within porous rocks called reservoirs. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.

Geologists often refer to an "oil window" which is the temperature range that oil forms in—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Though this happens at different depths in different locations around the world, a 'typical' depth for the oil window might be 4–6 km. Note that even if oil is formed at extreme depths, it may be trapped at much shallower depths, even if it is not formed there. (In the case of the Athabasca Oil Sands, it is found right at the surface.) Three conditions must be present for oil reservoirs to form: first, a source rock rich in organic material buried deep enough for subterranean heat to cook it into oil; second, a porous and permeable reservoir rock for it to accumulate in; and last a cap rock (seal) that prevents it from escaping to the surface.

The vast majority of oil that has been produced by the earth has long ago escaped to the surface and been biodegraded by oil-eating bacteria. What oil companies are looking for is the small fraction that has been trapped by this rare combination of circumstances. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping, but contain so much migrating oil that, although most of it has escaped, vast amounts are still present - more than can be found in conventional oil reservoirs. On the other hand, oil shales are source rocks that have never been buried deep enough to convert their trapped kerogen into oil.

The reactions that produce oil and natural gas are often modeled as first order breakdown reactions, where kerogen is broken down to oil and natural gas by a set of parallel reactions, and oil eventually breaks down to natural gas by another set of reactions. The first set was originally patented in 1694 under British Crown Patent No. 330 covering "a way to extract and make great quantityes of pitch, tarr, and oyle out of a sort of stone." The latter set is regularly used in petrochemical plants and oil refineries.

Means of production


The most common method of obtaining petroleum is extracting it from oil wells found in oil fields. After the well has been located, various methods are used to recover the petroleum. Primary recovery methods are used to extract oil that is brought to the surface by underground pressure, and can generally recover about 20% of the oil present. After the oil pressure has depleated to the point that the oil is no longer brought to the surface, secondary recovery methods draw another 5 to 10% of the oil in the well to the surface. Finally, when secondary oil recovery methods are no longer viable, tertiary recovery methods reduce the viscosity of the oil in order to bring more to the surface.

Alternative methods

As oil prices continue to escalate, other alternatives to producing oil have been gaining importance. The best known such methods involve extracting oil from sources such as oil shale or tar sands. These resources are known to exist in large quantities; however, extracting the oil at low cost without negatively impacting the environment remains a challenge. It is also possible to transform natural gas or coal into oil (or, more precisely, the various hydrocarbons found in oil). The best-known such method is the Fischer-Tropsch process. It was a concept pioneered in Nazi Germany when imports of petroleum were restricted due to war and Germany found a method to extract oil from coal. It was known as Ersatz ("substitute" in German), and accounted for nearly half the total oil used in WWII by Germany. However, the process was used only as a last resort as naturally occurring oil was much cheaper. As crude oil prices increase, the cost of coal to oil conversion becomes comparatively cheaper. The method involves converting high ash coal into synthetic oil in a multistage process. Ideally, a ton of coal produces nearly 200 liters (1.25 bbl, 52 US gallons) of crude, with by-products ranging from tar to rare chemicals.


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