Heavy Crude Oil

Heavy crude oil or Extra Heavy 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 of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°,meaning that its specific gravity is greater than 0.933.
Production, transportation, and refining of heavy crude oil present special challenges compared to light crude oil. The largest reserves of heavy 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. Heavy crude oil is closely related to tar sands, the main difference being that tar sands generally do not flow at all. Canada has large reserves of tar sands, located north and northeast of Edmonton, Alberta.
Physical properties that distinguish heavy crudes from lighter ones include higher viscosity and specific gravity, as well as heavier molecular composition. Extra heavy oil from the Orinoco region has a viscosity of over 10,000 centipoise and 10° API gravity Generally a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.
Some petroleum geologists categorize bitumen from tar sands as extra heavy oil although bitumen does not flow at ambient conditions.