Different sources (OPEC, CIA World Factbook, oil companies) give different figures and there are different types of oil, ranging from cheap and easy to recover oils to shale oil or oil sands, which are more expensive and difficult to recover. Reserves may be for a well, for a reservoir, for a field, for a nation, or for the world. Different classifications of reserves are related to their degree of certainty as reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of reserves to the total amount of oil in a particular reservoir is called the recovery factor. Determining a recovery factor for a given field depends on several features of the operation, including method of oil recovery used and technological developments.
OPEC covers 1,112,448 - 1,199,707 in MMbbl oil reserve, where Venezwela, KSA, Canada, Iran Iran and Iraq comes from top in the Que
