Title: Origin of nitrogen-rich natural gases in the California Great Valley: Evidence from helium, carbon and nitrogen isotope ratios
Abstract: Dry natural gases produced from the Sacramento and northern San Joaquin basins contain up to 87% nitrogen. δ15N measurements for gases with N2 ≥ 8% vary from 0.9%. to 3.5%.; methane δ13C values vary from −61%. to −15%. 3He4He ratios indicate the presence of mantle-derived helium in addition to radiogenic crustal helium and suggest that the gases may originate from mixing between three principal components: a hydrocarbon-rich gas with 3He4He ≤ 0.1RA derived from the microbial and thermogenic alteration of sedimentary organic matter, a hydrocarbonrich gas with 3He4He ≥ 2.75RA derived from the localized pyrolysis of sediments by magmatic volatiles, and a nitrogenrich gas with 3He4He ≥ 1RA The nitrogen-rich component is characterized by methane δ13C less negative than −25%. and N2Ar ratios as high as 22,000. Nitrogen concentrations tend to increase towards the basement along the shallow eastern margin of the valley suggesting that the nitrogen-rich component is derived from metasedimentary rocks deep within the crust. The Coast Ranges Fraciscan complex, which was thrust well beneath the Great Valley during Late Mesozoic and Early Cenozoic subduction, appears to be the most likely source.
Publication Year: 1988
Publication Date: 1988-04-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 231
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