Composition

The proportion of hydrocarbons in the mixture is highly variable and ranges from as much as 97% by weight in the lighter oils to as little as 50% in the heavier oils and bitumens.
The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulfur, and trace amounts of metals such as iron, nickel, copper and vanadium. The 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
Element
Percent range
Carbon
83 to 87%
Hydrogen
10 to 14%
Nitrogen
0.1 to 2%
Oxygen
0.1 to 1.5%
Sulfur
0.5 to 6%
Metals
less than 1000 ppm