Heavy oil is a complex amalgam with compounds of high molecular weight
Over millions of years, heavy oil was formed as a result of the hydrocarbon deposits being degraded by algae, which resulted in the loss of its lighter hydrocarbon fractions—with the heavier fractions remaining.
By definition, heavy oil typically has a viscosity of 100 centipoise or greater
Extra heavy oil is generally referred to as heavy oil with a viscosity, a property that defines ease of flow, in excess of 10,000 centipoise. Heavy oil ranges from oil that flows by itself to bitumen—or tar sands “ultra heavy oil”—that is actually embedded in sand. Located at depths less than 75 meters, tar sands are mined as a “solid” rather than extracted in a liquid form.
Heavy crude oil is any liquid petroleum with an API gravity of less than 22.3°
API readings are based on the American Petroleum Industry's method of determining the gravity of crude oil. The lower the API number, the heavier the oil and the higher its specific gravity. In the case of oil sand hydrocarbons, also called extra heavy oil, the API gravity can register as low as 10 (2-4 for ultra heavy oil).
Heavy oil usually contains significantly higher contents of asphaltenes
The presence of asphaltenes, chemically altered fragments of organic chemical compounds, in oil can greatly complicate the production process. Subsequently, certain asphaltene elements require that the heavy oil also undergo a special refining process called deasphalting. The chemical composition of asphaltenes can consist of various amounts of sulphur, oxygen, hydrogen, nitrogen, carbon, and the heavy metals nickel and vanadium and are widely recognized as soluble.