Adam Porter for the BBC discusses the reliability of statistics on known oil reserves in How much oil do we really have?.
He reports that the statistics on known oil reserves are not very reliable. These statistics cannot be verified by independent third parties: many countries do not allow outsiders to audit the size of their oil fields. Moreover there are incentives to misrepresent the size of oil reserves. For instance for within OPEC extraction quotas proportional to their declared reserves give an incentive to declare large reserves.
Read the whole story at How much oil do we really have?.