Substantial maturity influence on carbon and hydrogen isotopic composition of n-alkanes in sedimentary rocks

Abstract

Carbon (δ13C) and hydrogen (δ2H) compound-specific isotope analyses on sedimentary hydrocarbons are widely used for ecological reconstructions and oil-source rock or oil-oil correlations. However, the effects of thermal alteration on isotopic composition are not fully understood, potentially imparting a bias on interpretation of older and more mature sedimentary sequences. We measured δ13C and δ2H of n-alkanes in 23 extracted bitumens from the 1.64 Ga Paleoproterozoic Barney Creek Formation in the southern McArthur Basin, Australia. The samples cover a wide range of thermal maturities with calculated vitrinite reflectance (Rc) values from 0.4% to 1.3%. Our results illustrate that while δ13C of kerogen remains relatively constant, the δ13C and δ2H of n-alkanes have a strong positive correlation with thermal maturity. Average δ13Calk increase by 6.8‰ and δ2Halk by 69‰ among the samples in the analysed maturity range. At the same time, the carbon isotopic offset between n-alkanes and kerogen (Δδ13Calk–ker) climbs from 1.3‰ to 8.5‰ with increasing maturity. Therefore, the substantial maturity influence on stable carbon and hydrogen isotopes of n-alkanes must be considered in palaeoecological and petroleum correlation studies. In the initial stages of maturation, n-alkanes from the Barney Creek Formation display increasingly positive “isotope slopes” in plots of δ13Calk against carbon number. However, with further maturation, the slopes became increasingly negative. The isotope slope inversion indicates that the dominant mechanisms for n-alkane generation and degradation changed during the progression from early diagenesis to metagenesis. Numerical models suggest that the formation of positive and negative isotope slopes may be driven by the balance of the formation of n-alkanes from kerogen and their subsequent degradation, and by dependence of the degradation rate constant k on n-alkane chain length.