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Petroleum can be included only in crude oil, but in common usage it includes both crude oil and natural gas. Petroleum or crude oil is a naturally occurring, flammable liquid. It is a complex mixture of hydrocarbons of various molecular weights, and other organic compounds, that is found in rock formations beneath the earth's surface.

Under surface pressure and temperature conditions, the lighter hydrocarbons methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10) occur as gases, while the heavier ones from pentane (pentane - C5H12) and up are in the form of liquids or solids. However, in the underground oil reservoir the proportion which is gas or liquid varies depending on the subsurface conditions, and on the phase diagram of the petroleum mixture.

An oil well produces predominantly crude oil, with some natural gas dissolved in it. Because the pressure is lower at the surface than underground, some of the gas will come out of solution and be recovered (or burned) as associated gas or solution gas. A gas well produces predominately natural gas. However, because the underground temperature and pressure are higher than at the surface, the gas may contain heavier hydrocarbons such as pentane, hexane, and heptane in the gaseous state. Under surface conditions these will condense out of the gas and form natural gas condensate, often shortened to condensate. Condensate resembles gasoline in appearance and is similar in composition to some volatile light crude oils.

The proportion of hydrocarbons in the petroleum mixture is highly variable between different oil fields and ranges from as much as 97% by weight in the lighter oils to as little as 50% in the heavier oils and bitumens.

Petroleum is a mixture of a very large number of different hydrocarbons; the most commonly found molecules are alkanes (linear or branched), cycloalkanes, aromatic hydrocarbons, or more complicated chemicals like asphaltenes. Each petroleum variety has a unique mix of molecules, which define its physical and chemical properties, like colour and viscosity.

The true or exact molecular composition varies widely from formation to formation but the proportion of chemical elements vary over fairly narrow limits as follows.

Composition by weight:


Percent range
83 to 87%
10 to 14%
0.1 to 2%
0.1 to 1.5%
0.5 to 6%
less than 1000 ppm

Four different types of hydrocarbon molecules appear in crude oil. The relative percentage of each varies from oil to oil, determining the properties of each oil.

Composition by weigh:



15 to 60%
30 to 60%
3 to 30%