As defined by the U.S. Geological Survey (USGS), heavy oil is a type of crude oil characterized by an asphaltic, dense, viscous nature (similar to molasses), and its asphaltene (very large molecules incorporating roughly 90 percent of the sulfur and metals in the oil) content. It also contains impurities such as waxes and carbon residue that must be removed before being refined. Although variously defined, the upper limit for heavy oil is 22° API gravity with a viscosity of 100 cp (centipoise).

The American Petroleum Institute's "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.

In comparison with heavy oil, light or "conventional" oil flows naturally and can be pumped without being heated or diluted. Light oil is characterized by an API gravity of at least 22°, and extra-heavy oil has an API gravity of less than 10°. Natural bitumen, also known as oil sands, shares the characteristics of heavy oil but is even more dense and viscous - with a viscosity greater than 10,000 cP.

Heavy oils typically are not recoverable in their natural state through a well or by ordinary production methods. Most require heat or dilution to flow into a well or through a pipeline.

How Does Heavy Oil Form?

The formation of heavy oil and bitumen, like other forms of petroleum, originated with plant life millions of years ago. When the plants and small organisms (plankton) that fed on them died off, the sediments containing their remains were buried at the bottom of inland seas. In a highly simplified explanation, over time, the heat and pressure converted the carbohydrates into hydrocarbons.

Oil formation usually takes place in very fine-grained sedimentary rocks known as black shales. After oil is formed, continued pressure from overlying rocks causes it to migrate through permeable rock layers until it becomes trapped in reservoirs of porous rocks such as sandstone or limestone.

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