The San Juan basin, northwestern New Mexico, has long produced methane gas which commonly carries sufficient liquids to yield high BTU values. With this production, however, there have been wells which produced only carbon dioxide, or such large quantities of nitrogen as to be of very little economic value.

Early production came largely from the Dakota sandstones (Cretaceous) the gases of which contain an average of 80% methane and 15% higher hydrocarbons. Nitrogen and carbon dioxide average less than 3% and 1% respectively, yielding an average BTU of 1,194.

Later exploration has proved extensive gas reserves in the uppermost sequence of Upper Cretaceous sandstones, where the gases average 85% methane and 12% higher hydrocarbons. Nitrogen and carbon dioxide values are 1% or less and BTU values average 1,133.

Deeper exploration has shown a plethora of problems in the occurrence of gas in this basin. Gas from Permian, Pennsylvanian, and Mississippian reservoirs shows quantities of hydrocarbons much lower than in shallower formations, and nitrogen content as high as 81%. These nitrogen-rich gases carry some of the highest percentages of helium in the entire basin, from 3% to as high as 7.5%, although the production and reserves of Paleozoic rocks are several orders of magnitude smaller than those of the Cretaceous.

Characteristics of the gases in each group or sequence of formations are presented as related to depth of production. Economic factors may be evaluated on the basis of production from different zones at deeper

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levels. Wide variations in analyses appear to be related to the genesis of the reservoir rocks and to the geologic history of these deeper zones. Analyses of gas from individual sequences indicate isolated production from lenses in which gas has had very little opportunity to mingle with other, more widespread, gases from blanket-type sandstones.

Analyses of gases from the Uinta basin, eastern Utah, may be separated into those from Tertiary, Paleocene-Upper Cretaceous, lower Upper Cretaceous; Jurassic; and Pennsylvanian production. Pressures are related approximately to depths of production. The percentages of methane and total hydrocarbons are highest in the Eocene Wasatch gases, although the deeper gases exhibit a much higher content of the higher hydrocarbons, and these yield a high BTU value.

Percentages of nitrogen are low in the Tertiary reservoirs and increase to as much as 19% in the Pennsylvanian. Carbon dioxide values are variable, and helium values increase as the nitrogen values increase. Variations appear to be dependent on geologic environments rather than on depth only.

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