<P> Oil reserves denote the amount of crude oil that can be technically recovered at a cost that is financially feasible at the present price of oil . Hence reserves will change with the price, unlike oil resources, which include all oil that can be technically recovered at any price . 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 . </P> <P> The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place . However, because of 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 . </P> <P> Based on data from OPEC at the beginning of 2013 the highest proved oil reserves including non-conventional oil deposits are in Venezuela (20% of global reserves), Saudi Arabia (18% of global reserves), Canada (13% of global reserves), and Iran (9%). </P> <P> Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource . While new technologies have increased the accuracy of these techniques, significant uncertainties still remain . In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time . This phenomenon is called reserves growth . </P>

Who owns most of the world oil supply