Crude oil is a color less or a black liquid. Which are customarily characterized by the type of hydrocarbon compound that is most prevalent in them: paraffin’s, naphthenic, and aromatics. Paraffin’s are the most common hydrocarbons in crude oil, certain liquid paraffin’s are the major constituents of gasoline (petrol) and are therefore highly valued. Naphthenic are an important part of all liquid refinery products, but they also form some of the heavy asphalt like residues of refinery processes. The most common aromatic in crude oil is benzene, a popular building block in the petrochemical industry.

The petroleum industry, uses the American Petroleum Institute (API) gravity scale, in which pure water has been arbitrarily assigned an API gravity of 10°. Liquids lighter than water, such as oil, have API gravities numerically greater than 10. On the basis of their API gravity, crude oils can be classified as heavy, medium, and light as follows:

• Light crude oil has an API gravity higher than 31.1° (i.e., less than 870 kg/m3)
• Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m3)
• Heavy crude oil has an API gravity below 22.3° (i.e., 920 to 1000 kg/m3)
• Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m3)

Crude oil with API gravity less than 10° is referred to as extra heavy oil or bitumen. The crude which has an API gravity of less than 22.3°, or further “upgraded” to an API gravity of 31 to 33° as synthetic crude. Crude oil with an API gravity between 40 and 45° commands the highest prices. Above 45°, the molecular chains become shorter not valuable to refineries.