Effect of maturation on the indigenous δD signatures of individual hydrocarbons in sediments and crude oils from the Perth Basin (Western Australia)

Abstract

We measured the stable hydrogen isotopic composition ( δD) of selected aliphatic hydrocarbons in nine sediments of various maturity and two crude oils, all from the northern Perth Basin (on-shore, Western Australia). The sediments and crude oils are from the Lower Triassic Hovea Member (Sapropelic Interval) of the Kockatea Shale. The n-alkanes, pristane and phytane from two immature sediments have δD values that represent the expected isotopic compositions of their precursors. Phytane is enriched in deuterium (D) relative to pristane in all sediment extracts and crude oils; this is attributed to either different sources, or to different isotopic effects during their derivation from phytol. With increasing maturity, pristane and phytane become enriched in D while the n-alkanes generally remain at a constant isotopic composition. A more rapid enrichment of D in isoprenoids relative to n-alkanes with increasing maturity suggests that hydrogen isotopic exchange is occurring via a carbocation mechanism. The phytane diastereomer ratio for each of the sediments correlates linearly with the progressive enrichment of D in phytane and therefore maturity, suggesting that significant hydrogen exchange occurs at the chiral carbons of phytane. The δD values of pristane and phytane from the crude oils are similar or more positive relative to the δD values of the n-alkanes. The average δD values of pristane and phytane for the sediments correlate well with equivalent vitrinite reflectance values, as does the conventional biomarker maturity parameter T s/ T m, indicating that δD values may be useful for establishing maturity over a wider range than conventional biomarker parameters. The results provide further evidence that care must be taken not to over-interpret δD values of sedimentary hydrocarbons in samples of high thermal maturity.