The future of natural gas is a difficult subject because of peculiarities in natural gas occurrence and production. Data from which to infer the magnitude of unknown resources and the various components of future supply are even more scanty than is similar information for petroleum. The assumptions required are therefore less well founded, and it is not surprising to find relatively few public discussions and estimates concerning the subject.
Natural gas has two major modes of occurrence: it is often found by itself, in distinct gas reservoirs, and it can occur together with crude oil. Because the effect of these purely physical circumstances on production characteristics is important, the American Gas Association distinguishes two types of gas occurrence on the basis of production characteristics:
Non-associated gas is free gas not in contact with crude oil in the reservoir; and free gas in contact with oil where the production of such gas is not significantly affected by the production of crude oil.1
Associated gas is free gas in contact with crude oil in the reservoir where the production of such gas is significantly affected by the production of crude oil.1