Proved 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.
Note that some statistics on this page are disputed and controversial. 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 very expensive and difficult to recover. Canada includes in their proven reserves the oil sands of Alberta, Venezuela has started in 2011 to include the heavy oils of Orinoco. The USA have not included in their statistics their shale oil, but it is believed that those reserves are relevant, with exclusion of the Green River Formation in Colorado.
Even if this changes according to the source, it is estimated that at least 70% of the proven oil reserves are currently held by OPEC countries.