Heavy crude oil:

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°, 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.