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Oil reserves are the estimated quantities of crude oil that are claimed to be recoverable under existing economic and operating conditions. Many oil producing nations do not reveal their reservoir engineering field data, and instead provide unsubstantiated claims for their oil reserves.

In most cases, oil refers to conventional oil and excludes oil from coal and oil shale. Depending on the source, bitumen and extra-heavy oil (tar sands) may also be excluded. The exact definition varies from country to country and national statistics are not always comparable. The numbers disclosed by national governments are also often manipulated for political reasons.

The total amount of oil in an oil reservoir is known as oil in place. However, because of reservoir characteristics and limitations in petroleum production technology, 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 oil in place for a given field is often referred to as the recovery factor. The recovery factor of a field may change over time based on operating history and in response to changes in technology and economics. The recovery factor may also rise over time if additional investment is made in enhanced oil recovery techniques such as gas injection or water-flooding.

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.

All reserve estimates involve some degree of uncertainty depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principle classifications - proved and unproved. Unproved reserves can further be divided into two subcategories - probable and possible to indicate the relative degree of uncertainty about their existence. The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997.