Oil is a hydrocarbon. It is a black, brownish, or amber liquid that is a complex mix of elements including carbon, hydrogen, sulphur, nitrogen, oxygen and metals. Oil can be classified as light, medium, heavy, or extra heavy based on how thick (viscous) it is. “Crude” oil is oil that has not been refined or upgraded. Oil can also be classified as “sweet” or “sour” depending on how much sulphur is in the oil.