American Petroleum Institute’s gravity “API gravity” is a standard to express the specific weight of oils, computed as (141.5/sp g) – 131.5, where sp is the specific gravity of the oil at 60 degrees Fahrenheit. The lower the specific gravity value, the higher the API gravity will be.
Light Oil, also known as “conventional oil,” has an API gravity of at least 22° and a viscosity less than 100 centipoise (cp). It flows naturally and can be pumped without being heated or diluted.
Heavy Oil is asphaltic, dense (low API gravity), and viscous (thick and sticky) oil that is chemically characterized by its content of asphaltenes (very large molecules incorporating most of the sulfur and perhaps 90% of the metals in the oil). Although variously defined, the upper limit for heavy oils has been set at 22°API gravity and a viscosity of less than 100 cP. Heavy oils typically require heat or dilution to flow into a well or through a pipeline.
Extra Heavy Oil is the portion of heavy oil that has an API gravity of less than 10°.
Extra-Heavy Oil Natural Bitumen is also known as “oil sands”, and it shares the attributes of heavy oil but is even more dense and viscous. Natural bitumen has a viscosity greater than 10,000 cp.
Definitions:
American Petroleum Institute (API) – the largest US trade association for the oil and natural gas industry.