Changing perceptions of United States natural-gas resources as shown by successive U. S. Department of the Interior assessments

Trends in four successive estimates of United States technically
recoverable natural gas resources are examined in this report. The effective dates of these assessments were January 1 of 1975, 1980, 1987, and 1994.
The 1994 estimate of the U.S. total gas endowment increased significantly over the previous three estimates, indicating
that the technically recoverable endowment of gas is not an absolute volume, but rather is a quantity that can increase through time in response to advances in technology and in geologic
understanding. Much of this increase was in the category of reserve growth. Reserve growth refers to additions to the estimated
ultimate recovery of fields that typically occur as discovered
fields are developed and produced. The potential for U.S. reserve growth, rather than being rapidly used up, appears to be sustainable for many years by intensive engineering efforts coupled
with improving technology. Potential additions to reserves in continuous (unconventional) accumulations also represent a type of reserve growth, and were estimated (for the first time) in the 1994 assessment at 358 trillion cubic feet of gas. This resource category provides a significant new contribution to the estimated U.S. total gas endowment.