When a new reservoir is discovered it is obviously important to determine how much oil and/or gas it contains in order to justify undertaking the expense of development and production. Proved natural gas reserves are the total volume of natural gas estimated to be recoverable from known reservoirs under the economic and operating conditions existing at the time of the estimate. Inferred reserves are those that should eventually be added to proved reserves through extensions of known fields, revisions of earlier reserve estimates resulting from new subsurface and production information, and production from new producing zones in known fields. Past-performance or behavioristic extrapolation methods are based on the extrapolation of past experience with indicators as discovery rates, cumulative production, or productive capacity curves. The reserves/production ratio is a measure of the rate of petroleum production. Mobile Oil Company has employed a probabilistic engineering analysis for partially explored to well-explored plays.