Conventional crude oil is classified as light, medium, or heavy according to its measured API gravity.
  • Light crude oil has an API gravity higher than 31.1 (i.e., less than 870 kg/m3)
  • Medium oil has an API gravity between 22.3 and 31.1 (i.e., 870 to 920 kg/m3)
  • Heavy crude oil has an API gravity below 22.3 (i.e., 920 to 1000 kg/m3)

Extra heavy crude oil with API gravity less than 10 ( >1000 kg/m3) is referred to as bitumen. Bitumen derived from oil sands in Alberta has an API gravity of around 8. It can be diluted with lighter hydrocarbons to produce diluted bitumen, which has an API gravity of less than 22.3 (equivalent to conventional heavy oil), or further upgraded to an API gravity of 31 to 33 as synthetic crude (equivalent to conventional light oil).

Many of the world's heavy oil fields have been in production for more than 60 years, in places such as California, Lake Maracaibo, Alberta, Saskatchewan, China, and former Soviet Union Republics. More are being expanded and exploited with new technology today, based on expected rises in oil prices, erratic as this may be.