How do heavy oils differ from lighter conventional crude oils?

Heavy oils and conventional light crude oils differ primarily in their physical properties, mobility, and the complexity required for their extraction and refining.

Physical and Chemical Differences

The classification of crude oil is based on specific gravity and the American Petroleum Institute (API) gravity scale. Light oils typically have an API gravity above 31.1°, while heavy oils fall below 22.3°. As crude oil becomes heavier, it becomes denser, more viscous, and darker in color due to a higher concentration of asphaltene hydrocarbons. Furthermore, heavier crudes generally contain higher levels of sulfur and metallic impurities.123

Mobility and Extraction

Conventional light and medium oils are mobile and can be recovered through standard production wells. In contrast, heavy oils are much more viscous and often require enhanced recovery methods—such as the application of heat, gas, or chemicals—to lower their viscosity and allow them to flow into a well bore. Bitumen, found in tar sands, is an immobile, degraded remnant of ancient petroleum that does not flow at all and must be mined or processed with specialized thermal techniques.12

Economic and Refining Considerations

Heavy oils are generally considered a less desirable energy resource than lighter crudes because they are more costly to extract and process. Refining heavy oil yields fewer high-value distillate products, such as gasoline and kerosene, and produces higher volumes of low-value residues like asphalt and coke. Additionally, the energy-intensive nature of extracting and upgrading heavy hydrocarbons means that a significant portion of the recovered energy is consumed during the production process itself.2