Although natural gases may contain up to 10.0 per cent helium in rocks ranging in age from Cambrian to Cretaceous in scattered parts of the United States and Canada, the greatest concentration of rich gas is in Permian carbonate rocks in the Panhandles of Texas and Oklahoma and in southwestern Kansas. At Pinta dome, just northeast of Holbrook in northeastern Arizona, a Permian sandstone yields commercial quantities of gas containing about 8.0 per cent helium. In Canada, the richest known helium gas--1.0-2.0 per cent--is in Cambrian and Ordovician sandy beds just north-northwest of Swift Current, in southwestern Saskatchewan.

The presence of most of the rich helium gas in Paleozoic strata close to the ancient basement rocks suggests that much of it was derived originally from the decay and subsequent vertical diffusion of radioactive elements, such as uranium and thorium, in those rocks. Helium-rich gas from deep wells generally is rich in nitrogen, but not in every case. Seemingly, the origin of rich nitrogen and helium gas is largely a problem for bacteriologists, chemists, and biochemists to solve.

Deep wells yielding natural gas rich in carbon dioxide are chiefly in the western part of the United States and in the Panuco district, Mexico. Most of the rich gas is in beds ranging in age from Mississippian to Cretaceous.

Natural gas rich in hydrogen sulfide is produced chiefly from carbonate rocks in many areas, especially in western Texas.