Those quantities of petroleum which, by analysis of geological and engineering data, can be estimated with reasonable certainty to be commercially recoverable, from a given date forward, from known reservoirs and under current economic conditions, operating methods, and government regulations. Proven (proved) reserves can be categorised as developed or undeveloped. Where probabilistic methods have been used to estimate reserves, proven reserves are those with a better than 90% chance of being economically recoverable. Sometimes abbreviated as P90. Reserves with a greater than 50% chance but less than 90% chance are defined as Probable, or P50. Reserves with a greater than 10% chance but less than 50% chance are Possible or P10. Reserves may be classified as proved, if facilities to process and transport them to market are operational at the time of the estimate or there is a reasonable expectation that such facilities will be installed. “Reasonable expectation” is usually taken to mean that a developer has taken the decision to build the necessary facilities and the development plan has been approved by the relevant authorities. Thus large resources, such as those known to exist in North Alaska, are not classified as “proven” because there is no route to market. Different countries have adopted many different definitions of reserves, but gas contracts often refer to the Society of Petroleum Engineers (SPE) definitions. These and similar definitions from the World Petroleum Council were harmonised in 1997.