The mature lower 48 states continue to yield substantial quantities of natural gas. Onshore reserve additions are dominated by reserve growth, indicating the increasing efficiency of conversion of infield resources to reserves. Total natural gas reserve additions for the 1980's will nearly equal the additions of the best decades of the 1950's and 1960's. Although significantly more wells were required in the 1980's, total additions have been stable for the last two decades at about 1.2 Bcf (34.0 MMcm) per well, and in 1986-88, additions averaged 2.1 Bcf (28.3 MMcm) per well. Disaggregation of 11 years (1979-89) of data for onshore less Appalachian (seven major producing states), Appalachian, and offshore areas shows that average onshore nonassociated gas per reserve growth completion has doubled from 1979-85 (0.9Bcf [25.5 MMcm]) to 1986-89 (2.2 Bcf [62.3 MMcm]). Appalachian region reserve growth has increased in these same periods from 0.07 to 0.13 Bcf/completion (2.0 to 3.7 MMcm/completion), but overall additions are not in proportion to the one-quarter of onshore reserve growth drilling in that region. Onshore reserve growth (adjustments, net revisions, extensions, and new reservoirs in old fields) has increased from an average 81 percent of production in 1979-85 to 92 percent in 1986-89.

Onshore net revisions became positive in 1983 and have steadily increased over seven years, the greatest gains occurring in 1986-89 even after discounting major positive revisions in Kansas in 1986 and in New Mexico in 1988. This discounting actually took place in 1989 data for western New Mexico (-2.133 Tcf [-60.4 Bcm]), but otherwise net revisions for major onshore producing states remained strong at +2.733 Tcf (+77.4 Bcm), or 98 percent of the 1988 level. Net revisions per onshore development completion reached 0.8 Bcf (22.7 MMcm) in 1986-89 from one-fifth or less of that level in 1979-85. It is these net positive revisions that probably most closely reflect recompletion and infill drilling of bypassed gas zones and incompletely drained compartments as mechanisms for extended nonassociated gas reserve growth based on depositional heterogeneity. The resource associated with this development is nearly 120 Tcf (3.40 Tcm) in the lower 48 states, about one-half of which (56 Tcf [1.59 Tcm]) is judged accessible at a wellhead price of <$3.24/Mcf (1989$). Near-term supply-demand balance resulting in annual average wellhead prices of $2.25 to $2.50/Mcf (1990$) and realization that a substantial resource remains accessible, although in smaller increments, would allow continued efficiencies in extraction to meet market requirements into the 1990's.

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