Stable carbon and hydrogen isotopic compositions of Paleozoic marine crude oils from the Canning Basin; comparison with other west Australian crude oils

This study focuses on the stable carbon (13C) and hydrogen (D) isotopic compositions of bulk, saturated and aromatic hydrocarbons in 17 Paleozoic marine crude oils from the Canning Basin, Western Australia. The stable carbon isotopic composition of crude oils is primarily dependent upon the source of the organic matter. Comparison to other Australian and global marine oils and source rocks demonstrates systematic changes in the bulk stable carbon isotopic composition throughout the Paleozoic. From the Early to the Late Paleozoic, Australian oils have become isotopically more enriched in 13C. The most depleted 13Csat value (-32.0') is from the saturated hydrocarbon fraction of a Cambrian oil-stain in the Arafura Basin whereas in the Canning Basin the oldest oils are Ordovician with 13Csat values of about -31'. Late Devonian marine oils from this basin exhibit slightly more enriched values (mean 13Csat = -29.2'), and Mississippian (early Carboniferous) marine oils from both the Canning and Bonaparte basins have mean 13Csat values in the order of -28'. Carbon and hydrogen isotopic compositions of individual C7+ n-alkanes obtained for the three major oil families from the Canning Basin, as determined by biomarker analyses, corroborate previous findings and emphasise both facies variations and differences in the level of thermal maturation attained by their source rocks. The n-alkane-specific ?13C isotopic profiles of the Paleozoic marine oils from the Canning and Bonaparte basins characteristically follow the same trend as the bulk 13C isotopic values. The n-alkane-specific D isotopic profiles of the oils typically complement those of the carbon isotopic profiles; however, there are differences because organic matter produced by photosynthetic organisms in the source rocks is influenced by both water chemistry and thermal maturity, and as a result the oils show a greater range of values than the carbon isotopic data. The isotopic data has been used to refine the characterisation of oil families and petroleum systems of the Canning Basin. The similarities of the n-alkane-specific D isotopic profiles of the Ordovician-sourced Canning Basin oils with those of Early Triassic-sourced oils of the Perth Basin demonstrates that the typing of oil families should not be undertaken exclusively on a single parameter.

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