Since its founding in the early 1960s, the sole purpose of the Potential Gas Committee (PGC) has been to organize and train geoscientists, engineers and others for the timely preparation and dissemination of biennial assessments of the technically recoverable natural gas resource base of the United States.

The PGC assessments are “base-line” estimates in that they attempt to provide a reasonable appraisal of total U.S. natural gas resource potential–an appraisal that is not subject to assumptions regarding time of development of the resource, life span of the natural gas industry, or specific prices to be paid for the produced gas. No consideration is given to whether or not this resource will be developed; rather, the assessments are of resources that could be developed if a need for them and adequate price/cost relationships exist. Furthermore, they assume that there are no governmental or regulatory restraints on development and production of the resource and they do not consider the effects of access factors such as availability of pipeline connections.

PGC’s assessments do not include, and are distinct and separate from, the volumes of “proved reserves” contained within the nation’s discovered fields. Annual estimates of proved gas reserves have been prepared independently by the U.S. Department of Energy’s Energy Information Administration (EIA) since 1977. Their proved reserves estimates are, by definition, limited to known volumes of natural gas that have been established by drilling and are believed to be recoverable in the future with a 90 percent or higher probability. The PGC does, however, use the EIA’s proved reserves estimates by summing them with the PGC’s assessment of “potential resources” to derive “Future Gas Supply.” That volume, in turn, is summed with cumulative historical production to yield “Ultimately Recoverable Resources.”

The Potential Gas Committee reports its assessments of potential resources in three categories of decreasing geological certainty:

1. Probable resources (discovered but unconfirmed resources associated with known fields and field extensions; also undiscovered resources in new pools in both productive and nonproductive areas of known fields);
2. Possible resources (undiscovered resources associated with new field/pool discoveries in known productive formations in known productive areas); and
3. Speculative resources (undiscovered resources associated with new field/pool discoveries in as-yet nonproductive areas).

For each category, a “minimum,” “most likely” and “maximum” volume is assessed (with no implied numerical quartiles or other quantification of certainty, only the assessor’s best judgment). The three respective “most likely” values are summed arithmetically to yield the Total “most likely” resource at the province, area and national levels.

PGC’s members assess resources using a variety of published and online information, as well as unpublished but publicly available data. Because members also have access to certain unpublished data that are considered confidential in nature, the Committee maintains a long-established policy of protecting these confidential sources.

The following brief timeline helps to illustrate how the PGC has refined its procedures and developed increasingly detailed assessments as more geological, geophysical, drilling and production information has become available through the years.

Initially, assessments were made for eleven areas (Regions A through L) across the Lower 48 States, but only the totals for three larger encompassing supply regions (East, Central, West) were published. In 1968 Alaska was added, and area totals (including offshore Gulf of Mexico) were reported separately.

In 1970 onshore resources were first assessed separately for two vertical drilling-depth intervals, 0–15,000 ft and 15,000–30,000 ft. Offshore resources were assessed for two water-depth intervals, 0–200 m (continental shelf) and 200–1,000 m (continental slope). A third offshore interval, >1,000 m, representing the outer continental slope, was added for the southwest Bering Sea, Alaska, in 1984 and for the deepwater Gulf of Mexico in 1988.

In 1984 all study area boundaries in the Lower 48 State were reconfigured and renamed to correspond closely to geologic province boundaries established by the American Association of Petroleum Geologists (AAPG). Assessments were made then for 58 onshore and offshore provinces within six areas (Atlantic, North Central, Gulf Coast, Mid-Continent, Rocky Mountain and Pacific) and for 31 onshore and offshore Alaska provinces that PGC delineated independently. Several province boundaries have been revised since then, most notably the separation of the formerly combined Uinta and Piceance basins in 2008, resulting in 90 total provinces.

Another fundamental change arising in the 1980s was the Committee’s treatment of gas resources in “unconventional” reservoirs, specifically coal seams, fractured shales and low-permeability (“tight”) sandstones. These had been included in the PGC’s assessments to varying degrees but not reported separately from resources in conventional reservoirs. In 1986 a new work committee was created to address technically recoverable coalbed gas in selected coal basins, and its first separate assessment appeared in the 1988 report. Coalbed gas continues to be assessed at the province level. All other noncoal resources were referred to as “Traditional” resources, which were intended to include conventional clastic, carbonate, tight sand and shale reservoirs.

In light of the growing importance of shale gas, the PGC in 2008 elected to compile a separate tabulation of its shale-gas resources assessments, by province, while still incorporating them as part of Traditional resources. The 2010 report now lists shales as a separate category of Traditional resources in the area-level assessment tables.

While many continue to use the “most likely” or modal resource values for planning purposes, the PGC began reporting “mean” values in 1988. Mean values are computed by statistical aggregation of the minimum/”most likely”/maximum value ranges assessed for each resource category. Coalbed gas resources are aggregated separately and only at the national level. Shale gas resources are not aggregated separately because they are part of Traditional resources.

In contrast to the tabulated totals of the “most likely” values, all mean-value totals (Area, Lower 48 States’ and national Probable, Possible, Speculative, Total, onshore and offshore) reported in the PGC’s official summary tables are derived by  statistical aggregation, not by arithmetic summation. This procedure imparts greater statistical validity to the results and allows for more direct comparison of PGC’s assessments with those made by other organizations. The only mean values that are summed arithmetically are Total U.S. Traditional Resources (Probable, Possible, Speculative, Total) and the corresponding values for Total Coalbed Gas Resources, to yield Grand Total U.S. Resources.

Note: for readers who undertake the process of arithmetically summing individual Area mean values and find that their calculated total does not derive the national total shown by PGC, please be aware that the PGC national total is derived by statistical aggregation. It is not a mathematically correct exercise to arithmetically sum the results of individually aggregated mean values.