Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°. Physical properties that differ between heavy crudes lighter grades include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil is defined with a gravity of less than 10° API (i.e. with density greater than 1000 kg/m3 or, equivalently, a specific gravity greater than 1) and a reservoir viscosity of no more than 10,000 centipoises. With a specific gravity of greater than 1, extra heavy crude is present as a dense non-aqueous phase liquid in ambient conditions.
Heavy crude oil is closely related to natural bitumen from oil sands. Some petroleum geologists categorize bitumen from oil sands as extra heavy crude oil due to the density of less than 10 °API. Other classifications label this as bitumen differing it from extra-heavy oil. They differ in the degree by which they have been degraded from the original crude oil by bacteria and erosion. Often, bitumen is present as a solid and does not flow at ambient conditions.
The largest reserves of heavy crude oil in the world are located north of the Orinoco river in Venezuela, the same amount as the conventional oil reserves of Saudi Arabia, but 30 or more countries are known to have reserves.
Production, transportation, and refining of heavy crude oil present special challenges compared to light crude oil. Generally, a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.
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