Nobody knows or can know how much oil exists under the earth’s surface or how much it will be possible to produce in the future. All numbers are, at best, informed estimates. Within the broad concept of oil reserves there are several key distinctions: ultimately recoverable resource (URR) is an estimate of the total amount of oil or gas that will ever be recovered and produced. Estimated Ultimate Recovery (EUR) is not a resource category as such, but a term which may be applied to an individual accumulation of any status/maturity (discovered or undiscovered). Estimated Ultimate Recovery is defined as those quantities of petroleum which are estimated, on a given date, to be potentially recoverable from an accumulation, plus those quantities already produced therefrom. It is a subjective estimate in the face of only partial information. Whilst some consider URR to be fixed by geology and the laws of physics, in practice estimates of URR continue to be increased as knowledge grows, technology advances and economics change. Economists often deny the validity of the concept of ultimately recoverable reserves as they consider that the recoverability of resources depends upon changing and unpredictable economics and evolving technologies. The ultimately recoverable resource is typically broken down into three main categories: cumulative production, discovered reserves and undiscovered resource. Cumulative production is an estimate of all of the oil produced up to a given date. Discovered reserves are an estimate of future cumulative production from known fields and are typically defined in terms of a probability distribution. Discovered reserves are typically broken down into proved, probable and possible reserves.