Since 1960, the Potential Gas Committee (PGC) has prepared periodic estimates of the undeveloped and undiscovered natural gas resource potential remaining in the United States. In preparing its estimates, the PGC uses available information regarding past discoveries and production of natural gas as well as geologic and engineering data about prospective geologic provinces. Three categories of estimates are made. First, probable supply is associated with discovered fields and represents the remaining gas that could be developed and produced with additional drilling and field development. The estimate of probable supply is based on extrapolation of characteristics of the previously developed parts of the field. Second, possible supply is related to projections of additional new fields that are anticipated to be amenable to discovery during the continued exploration within provinces and trends that are already established as productive. The estimate of possible supply is based on analysis of the geologic characteristics of the undeveloped part of the province or trend and application of the geologic and engineering characteristics that have been established in the fields discovered to date. Finally, speculative supply is any additional supply that is anticipated might be discovered and developed in formations and geologic provinces that are not presently productive; this estimate is based on the known geologic characteristics of the speculative province and analogy with known productive areas. The estimates of speculative potential resources are subject to m ch greater uncertainty than the estimates of either possible or probable potential.