Noble gases in CH 4 -rich gas fields, Alberta, Canada

Details Noble gases in CH 4 -rich gas fields, Alberta, Canada Noble gases in CH 4 -rich gas fields, Alberta, Canada

Apr 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.1569h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 4, pp.1569-1589

Computer Science

12

Scientific paper

The elemental and isotopic compositions of helium, neon, argon, and xenon in twenty-one CH 4 -rich natural gas samples from Cretaceous and Devonian reservoirs in the Alberta, Canada, sedimentary basin were measured. In all but a few cases, radiogenic ( 4 He, 40 Ar, and 131-136 Xe) and nucleogenic ( 21,22 Ne) 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: 3 He / 4 He = 1.5 ± 0.5 × 10 -8 , 40 Ar / 36 Ar = 5000-6500, 40* Ar / 4 He = 0.10, 136* Xe / 4 He ~ 0.7 × 10 -9 , and 21* Ne / 22* Ne = 0.452 ± 0.041 (* denotes radiogenic or nucleogenic origin; all 4 He is radiogenic). High nitrogen content with 4 He / N 2 ~ 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: 3 He / 4 He = 10-70 × 10 -8 , 40 Ar / 36 Ar < 1550, and 40* Ar / 4 He ~ 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 40* Ar / 136* Xe 4 He 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 3 He / 4 He ratios; ~ 10-70 times greater than expected if derived from average crust. The mantle, surface cosmogenic 3 He production, cosmic dust, or production in a lithium-enriched environment as potential sources for the 3 He excesses are evaluated. The present data set would seem to rule out cosmogenic 3 He. 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°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.

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