High Quality Content by WIKIPEDIA articles! 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°. Extra heavy oil is defined with API gravity below 10.0 °API (i.e. with density greater than 1000 kg/m3 or, equivalently, a specific gravity greater than 1). With a specific gravity of greater than 1, extra heavy crude is present as a dense non-aqueous phase liquid in ambient conditions.