Geochemical Studies of Natural Gas Part III. Inert Gases in Western Canadian Natural Gases

Abstract

Abstract In natural gases, the stratigraphic and geographic variations in thecontents of both nitrogen and helium are of geochemical interest, although onlyhelium is of commercial importance. Nitrogen may originate from a great varietyof sources, including air, either originally trapped in the sediments orintroduced dissolved in percolating groundwaters, the denitrification ofnitrogenous compounds or the decay of certain radioactive minerals. In contrast, there exist radioactive sources for all natural helium, andthus the problem of the migration and accumulation of helium in rocks differsfrom that of the other components of natural gases. Helium owes its originmainly to the radioactive decay of uranium and thorium, and its accumulation tothe differential rates of upward diffusion through the sedimentary strata fromthe basement rocks. The geochemical evidence gives credence to a mathematicalmodel of the diffusion process, with special reference to helium. Contents of nitrogen ranging from about 8 per cent to 85 per centcharacterize natural gases in southern Alberta from strata ranging in age fromUpper Devonian to Lower Cretaceous. In strata of the Middle Devonian Elk PointGroup, the amount of nitrogen in natural gases increases to a maximum of nearly20 per cent as the depositional edge in the Peace River region is approached.Elsewhere, in other stratigraphic units, concentrations are generally less than10 per cent. Concentrations of helium generally over 0.1 per cent are foundpredominantly in natural gases from the Peace River arch, the Sweetgrass archand the hinge belt of the Alberta basin. Contents in the range of 1.0 to 1.9per cent helium have been found in the commercial gas occurrences in UpperCambrian strata in southern Saskatchewan. Introduction This paper is the third and final part of a series concerned with thegeochemistry of natural gas in Western Canada.