Heavy oil is defined by the U.S. Department of Energy as having API (American Petroleum Institute) gravities that fall between 10.0° and 22.3° (Nehring et al., 1983). Extra-heavy oils are defined as having API gravities less than 10.0° API. Heavy oils are classified as such using API gravity rather than viscosity values. Two important distinctions must be made between API gravity and viscosity. First, viscosity determines how well oil will flow while API gravity typically determines the yield from distillation. Additionally, temperature and paraffin content can have a large effect on viscosity values while API gravity is not affected by these parameters.

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