Oil is a mixture of hydrocarbons usually existing in the liquid state in natural underground pools or reservoirs. In its natural state, called crude oil, it remains liquid at atmospheric pressure after passing through surface separating facilities. Depending upon the characteristics of the crude stream, it may also include:
Liquids produced at natural gas processing plants are excluded. Crude oil is refined to produce a wide array of petroleum products, including heating oils; gasoline, diesel and jet fuels; lubricants; asphalt; ethane, propane, and butane; and many other products used for their energy or chemical content.
Natural gas is often found in association with oil.