Petroleum, often referred to as "black gold", is a viscous, dark brown, or greenish flammable liquid that occurs in the upper layers of certain areas of the earth's crust.

Petroleum contains only crude oil, but in daily use it is also used in the form of other solid, liquid and gaseous hydrocarbons. Under standard temperature and pressure conditions, light hydrocarbons such as methane, ethane, propane, and butane are gases which boil at -161.6 °C, -88.6 °C, -42 °C, and -0.5 °C, respectively (- 258.9°, -127.5°, -43.6°, and +31.1 °F), while the higher carbons, from pentane upwards, are either solids or liquids. However, in underground oil sources, the proportions of gases, liquids, and solids depend on surface conditions and the phase diagram of the petroleum mixture.[5]

Oil wells produce mostly crude oil, and sometimes contain natural gas in it. Because the pressure at Earth's surface is lower than underground, some of the gas will escape in mixed form. Gas wells mostly produce gas. However, because the temperature and pressure underground are greater than those on the surface, the escaping gas sometimes also contains larger hydrocarbons, such as pentane, hexane, and heptane in gaseous form. On the surface, this gas will condense to form natural gas condensate. The physical form of condensate is similar to gasoline.

The percentage of light hydrocarbons in crude oil varies depending on the oil field, the maximum content can be up to 97% of the gross weight and the minimum is 50%.

The types of hydrocarbons found in petroleum mostly consist of alkanes, cycloalkanes, and various types of aromatic hydrocarbons, plus a small number of other elements such as nitrogen, oxygen and sulfur, plus several types of metals such as iron, nickel, copper, and vanadium. . The amount of molecular composition varies from one oil to another, but the percentage proportions of the chemical elements can be seen below:

Elemental composition by weight