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.
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 more expensive and difficult to recover.
Because proven reserves includes oil recoverable under current economic conditions, nations may see large increases in proved 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.