Heavy Crude Oil, def. (Wiki)for interest :)
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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°,[1] meaning that its specific gravity is greater than 0.933. This mostly results from crude oil getting degraded by being exposed to bacteria, water or air resulting in the loss of its lighter fractions while leaving behind its heavier fractions.
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[2], the same amount as the conventional oil reserves of Saudi Arabia[3], but 30 or more countries are known to have reserves. Heavy crude oil is closely related to oil sands, the main difference being that oil sands generally do not flow at all. Canada has large reserves of oil 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 (10 Pa·s)[citation needed] and 10° API gravity [4]. Generally a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow.
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https://en.wikipedia.org/wiki/Heavy_crude_oil