Sometimes we see the term API or degrees API. The API degree refers to the specific gravity of the oil. Unlike density values it is inversely defined. So a high API value means a low density and vice versa.
It goes back to the Baume scale which is a hydrometer scale to measure densities of liquids lighter than water.
The original instruments were slightly off because of errors caused by salinity and temperature. The modulus of the scales were off. 141.5 instead of 140. But too many were out in use to easily fix the problem.
The American Petroleum Institute introduced the definition of API gravity with the modulus which was used by most hydrometers to compensate. The constants used in these hydrometers are reflected in the conversion formula.
API is defined as:
The lower the value, the higher its density and the higher (usually) the viscosity. Composition, density and viscosity directly translate into economics. A light oil is easier to produce than a heavy oil. The distribution of different qualities is a result of different geology in the major oil producing basins in the world and has led to different oil products that are being sold and traded differently. The most known “brands” are the North Sea Brent and and West Texas Intermediate (WTI). Since oil quality can vary locally as well the products of many oilfields are blended together to create a constant quality output for a region. Almost like blended Whisky…
E.g. the Brent blend has an API of 38 which gives it a density of approx. 0.835 g/ccm.
Heavy oils are defined as oil with an API degree of less than 20. This translates into a density of 0.934 g/ccm. Almost water.
Worldwide distribution of oil qualities
The EIA((http://www.eia.gov/todayinenergy/detail.cfm?id=7110)) published a good overview over different oil-qualities and their regional distribution.