What is it?

From Wikipedia: 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: [5]

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 <0.1%

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. [4]

Composition by weight
Hydrocarbon Average Range
Paraffins 30% 15 to 60%
Naphthenes 49% 30 to 60%
Aromatics 15% 3 to 30%
Asphaltics 6% remainder