Crude oil is described as a mixture of thousands of chemicals and compounds, primarily hydrocarbons.

Crude oil must be broken down into its various components by distillation before these chemicals and compounds can be used as fuels or converted to more valuable products.

Depending upon the characteristics of the crude stream, it may also include: small amounts of hydrocarbons that exist in gaseous phase in natural underground reservoirs but are liquid at atmospheric pressure after being recovered from oil well (casinghead) gas in lease separators and are subsequently commingled with the crude stream without being separately measured. Lease condensate recovered as a liquid from natural gas Wells in lease or field separation facilities and later mixed into the crude stream is also included; small amounts of nonhydrocarbons produced with the oil, such as sulfur and various metals; and drip gases, and liquid hydrocarbons produced from tar sands, oil sands, gilsonite, and oil shale.