Oil, or petroleum, is a complicated and diverse mixture of many hydrocarbon compounds, not easily characterized. Different deposits yield significantly different chemical compositions ranging from light crudes to heavy viscous tar sands. In addition, within an oil reservoir and during the life of an oil well, the chemical composition can, and most often does, change. However, in broad terms, petroleum can be divided into six fractions obtained from fractional distillation of the crude oil. With the understanding that the exact percentage and chemical of the fractions is neither fixed nor constant, the following outlines the general composition of each.

The "gas" fraction is the lightest, consisting of the organic gases from methane to the butanes. These compounds, at normal pressure and temperature, have a boiling point below 100° F (40° C). They usually constitute a minor component of an oil deposit although most reservoirs have a gas cap...