Heavy crude oils with an API gravity of less than 10.0°API are considered very heavy crude. This oil is very dark, nearly solid or tar-like, and has a petroliferous odor. Most very heavy crude is a dense, viscous petroleum called bitumen, which is comprised of asphalt-rich heavy hydrocarbons. Waterproofing compounds, asphalt, and mineral waxes are produced from bitumen.
Very heavy crude oils may be found at the bottom of crude oil reservoirs, where heavier compounds have settled. However, they are most commonly found in oil sands (or tar sands), bodies of sand that contain heavy hydrocarbon residue, such as in Venezuela or in Canada’s Athabasca reservoir.