The elemental and isotopic compositions of helium, neon, argon, and xenon in twenty-one CH4-rich natural gas samples from Cretaceous and Devonian reservoirs in the Alberta, Canada, sedimentary basin were measured. In all but a few cases, radiogenic (He-4, Ar-40, and Xe-131-136) and nucleogenic (Ne-21,22) isotopes dominated. Based solely on the noble gas composition, two types of natural gas reservoirs are identified. One (Group B) is highly enriched in radiogenic-nucleogenic noble gases and varies little in composition: He-3/He-4 = 1.5 +/- 0.5 x 10(-8), Ar-40/Ar-36 = 5000-6500, Ar-40*/He-4 = 0.10, Xe-136*/He-4 approximately 0.7 x 10(-9), and Ne-21*/Ne-22* = 0.452 +/- 0.41 (* denotes radiogenic or nucleogenic origin; all He-4 is radiogenic). High nitrogen content with He-4/N2 approximately 0.06 is also characteristic of Group B samples. The remaining samples (Group A) contain a radiogenic-nucleogenic component with a different composition and, relative to Group B samples, the extent of enrichment in this component is less and more variable: He-3/He-4 = 10-70 x 10(-8), Ar-40/Ar-36 < 1550, and Ar-40*/He-4 approximately 0.25. The composition of Group B radiogenic-nucleogenic noble gases is consistent with production in crust of average composition. Enrichment in Group B noble gases and nitrogen increases with proximity to the underlying Precambrian basement, consistent with a present-day mass flux into the overlying sedimentary basin. Inferred Ar-40*/Xe-136*/He-4 ratios imply a basement source enriched in thorium relative to uranium and potassium (Th/U > 20). Combined, the overall lower total radiogenic-nucleogenic content of Group A reservoirs, the greater variability in composition, and the appearance of Group A noble gases in reservoirs higher in the sedimentary sequence relative to the underlying basement implies that the Group A radiogenic-nucleogenic noble gases are indigenous to the sediments. The most interesting aspect of the Group A noble gases are the very high He-3/He-4 ratios; approximately 10-70 times greater than expected if derived from average crust. The mantle, surface cosmogenic He-3 production, cosmic dust, or production in a lithium-enriched environment as potential sources for the He-3 excesses are evaluated. The present data set would seem to rule out cosmogenic He-3. The mantle, cosmic dust, or high Li, however, remain viable candidates. The relative abundances of the nonradiogenic, non-nucleogenic noble gases show no correlation with the Group A-B reservoir classification. Compositional variations indicate three-component mixing between air or an air-like component, 10-degrees-C air-saturated water, and a third component enriched in xenon. Apparently, the latter cannot be derived from equilibrium solubility degassing of air-saturated water or oil-water mixtures, and may have been derived from devolatilization of C-rich petroleum source sediments.
