Heavy crude oil or extra heavy crude oil is any type of crude oil which does not flow easily. 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 (g/ml).
Production, transportation, and refining of heavy crude oil presents special challenges compared to light crude oil.
Physical properties that distinguish heavy crudes from lighter ones include higher viscosity and specific gravity as well as heavier molecular composition. Generally a diluent is added at regular distances in a pipeline carrying heavy crude to facilitate its flow. EXHEAT electric heaters can be used to reduce viscosity in order to improve flow of heavy oils in a pipeline or within a storage tank.