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Methodology
Proven reserves are those quantities of petroleum which, by analysis of geological and engineering data, can be estimated, with a high degree of confidence, to be commercially recoverable from a given date forward from known reservoirs and under current economic conditions.
Some statistics on this page are disputed and controversial. Different sources (OPEC, CIA World Factbook, oil companies) give different figures. Some of the differences reflect different types of oil included. Different estimates may or may not include oil shale, mined oil sands or natural gas liquids.
Because proven reserves include oil recoverable under current economic conditions, nations may see large increases in proven reserves when known, but previously uneconomic deposits become economic to develop. In this way, Canada's proven reserves increased suddenly in 2003 when the oil sands of Alberta were seen to be economically viable. Similarly, Venezuela's proven reserves jumped in the late 2000s when the heavy oil of the Orinoco was judged economic.
Countries
Reserves amounts are listed in millions of barrels (MMbbl).