Authors / CoAuthors
Boreham, C.J. | Hope, J.M. | Edwards, D.S. | Logan, G.A.
Abstract
To date, compositional information and compound specific isotope analysis (CSIA) of stable carbon isotopes for individual C1 to C5 gaseous hydrocarbons has been the primary data for the interpretation on Australian natural gases (Boreham et al., 2001). Here we report for the first time the stable hydrogen isotopic composition (D/H ratio) of the C1 to C5 gaseous hydrocarbons in Australian natural gases. The influence of source, maturity and in-reservoir alteration (biodegradation) is documented, and in combination with complementary carbon isotope data, this provides a powerful tool for the study of the origin and correlation of the natural gas. Source influences in Australian natural gases from Australian sedimentary basins show a wide range in hydrogen isotopes with ?D ca. 160 ? for both methane (?D -290 to -135 ?) and iso-butane (?D -255 to -94 ?). On the other hand, the isotopic range for carbon isotopes is an order of magnitude less, ?13C of 17 ? and 13 ? for methane (?13C -48.5 to 31.5 ?) and iso-butane (?13C -35.4 to -22.5 ?), respectively (Boreham et al., 2001). The source rock ages of the natural gases cover most of the Phanerozoic, from Ordovician in the Amadeus Basin to Early Eocene in the Bass Basin. Gases generated from older marine source rocks are most depleted in deuterium whereas gases sourced from the younger terrestrial coals are amongst the most enriched in D; carbon isotopes also show a similar response to age and source organic facies. Biodegradation of natural gas from the Carnarvon Basin produces a drier gas, due to the addition of biogenic methane and selective removal of wet gas components in the order propane > n-butane ? n?pentane > i-pentane > ethane ? i-butane. The addition of isotopically light biogenic methane leads to an overall isotopic shift of ?13C = ?11.5 ? compared to the non-biodegraded thermogenic gas, whereas the hydrogen isotopes remain unchanged. This, coupled with the enrichment in 13C of the associated CO2 suggests a role for anaerobic methanogenic bacteria. For the wet gas components maximum isotopic enrichments of ?13C = 18.2 ? (Boreham et al., 2001) and in ?D of 225 ? occur for those components that have been almost completely biodegraded. The strong positive correlation between carbon and hydrogen isotopes for the individual wet gas components implies a kinetic control on the isotopic composition, consistent with a biological-mediated process. The response of ?D to maturity is less attenuated compared to source and biodegradation effects. A maturation sequence from mature oil-associated wet gas to highly overmature dry gas from the Cooper Basin shows a ?13C enrichment of 15 ? for methane, with less isotopic enrichment in the wet gas components (Boreham et al., 2001). Such a maturity range in carbon isotopes for methane relates to a vitrinite reflectance range between 0.9 to 7.0% (Schoell, 1983), which is consistent with measured source rock maturities in the Cooper Basin (Boreham and Hill, 1998). On the other hand, ?D varies by ca. 50 ? for methane (?D -162 to -116 ?), with a lower isotopic enrichment observed for the wet gas components. The strong correlation shown between hydrogen and carbon isotopes in natural gas components suggests that isotopic exchange with external hydrogen sources (eg. water) is not a significant process. This contrasts with liquid hydrocarbon components where it appears that scrambling of the hydrogen isotopes occurs during oil generation (Schimmelman et al., in press). Furthermore, the relative insensitivity in ?D to maturity effects enhances the potential of CSIA for D/H ratios becoming an important isotopic tool in gas-gas (and gas-oil) correlation where the influence of source is of primary interest.
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nonGeographicDataset
eCat Id
60931
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
ACT
2601
Australia
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- External PublicationAbstract
- ( Theme )
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- organic geochemistry
- ( Theme )
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- hydrocarbons
- ( Theme )
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- petroleum exploration
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2004-01-01T00:00:00
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