Heavy crude oil and extra heavy crude oil is any type of crude oil which does not flow easily. Physically heavy crude oil have higher viscosity and density or specific gravity with an heavier molecular composition
Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.
Extra Heavy crude oil have an API gravity below 10°.
Heavy crude oil is closely related to natural bitumen from oil sands. Heavy crude oil and Extra heavy crude oil 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 normal conditions, meaning sea level and 25°C ambient temperature.
The largest reserves of heavy crude oil in the world are located north of the Orinoco river, in the so called Orinoco belt in Venezuela.
These reserves are equivalent to the light crude oil reserves of Saudi Arabia.
More than thirty countries are known to have heavy crude oil 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.