previous  |  next

What is crude oil?

Crude oil is naturally occurring petroleum. Petroleum is the general term for solid, liquid or gaseous hydrocarbons. Hydrocarbons are a class of organic compounds consisting only of carbon and hydrogen and which are the basis of oil, natural gas and coal.

Crude oil consists of carbon (83 to 87 per cent), hydrogen (10 to 14 per cent), sulphur (up to six per cent), nitrogen (up to two per cent), oxygen (up to 1.5 per cent) and metals (less than 1,000 parts per million). Hydrocarbon molecules range from a single a carbon atom with four hydrogen atoms to simple chains of two or more carbon atoms to complex configurations involving chains and rings of carbon atoms. Hydrocarbon molecules are generally grouped in four categories:


  • also known as alkanes
  • linear carbon chains with no rings, branched chains are known as isoparaffins or isoalkanes
  • single bonds only between carbon atoms
  • formula: CnH(2n+2)


  • also known as cycloalkanes
  • contain one or more single ring structures
  • single bonds only between carbon atoms
  • formula: CnH2(n+1-g) where g is the number of rings


  • contain one or more planar rings of six carbon atoms with what appear to be alternating single and double bonds between carbon atoms (benzene rings)


  • complex hydrocarbons with high molecular weights
  • consist primarily of carbon, hydrogen, nitrogen and oxygen
  • insoluble in light alkanes such as n-pentane and n-heptane, so occur as solids in crude oil
  • carbon to hydrogen ratio of approximately 1:1.2

Crude oil is commonly classified as light, medium or heavy, referring to its gravity as measured on the American Petroleum Institute (API) Scale. The API gravity is measured in degrees and is calculated using the formula (141.5/S.G.) - 131.5 = API Gravity. Conventional crude oil is oil that flows naturally or that can be pumped without being heated or diluted.

Light crude oil is defined as having an API gravity higher than 31.1, and is characterized as having a high paraffin to naphthene and aromatic ratio.

Medium oil has an API gravity between 22.3 and 31.1.

Heavy oil has an API gravity below 22.3. Conventional heavy oil from the Lloydminster, Alberta area has an API gravity ranging from 9 to 18. Heavy oil is characterized by a high ratio of aromatics and naphthenes to paraffins.

Crude oil that does not flow, or cannot be pumped without being heated or diluted is called bitumen, and generally has an API gravity of less than 10. The bitumen mined from the oil sands deposits in the Athabasca area of Alberta, Canada has an API gravity of around 8, but is upgraded to an API gravity of 31 to 33. This upgraded oil is known as synthetic crude oil. Bitumen is composed primarily of polycyclic aromatics and asphaltics.

Crude oil containing sulphur compounds, especially hydrogen sulphide, is known as sour oil.

previous  |  next