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.

中文翻译：

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成熟度对沉积岩中正构烷烃碳氢同位素组成的实质性影响

摘要 沉积碳氢化合物的碳 (δ13C) 和氢 (δ2H) 化合物特定同位素分析被广泛用于生态重建和油源岩或油油对比。然而，热蚀变对同位素组成的影响尚不完全清楚，这可能会对更古老和更成熟的沉积层序的解释产生偏差。我们测量了来自澳大利亚麦克阿瑟盆地南部 1.64 Ga 古元古代 Barney Creek 地层的 23 种提取沥青中正构烷烃的 δ13C 和 δ2H。样品涵盖了广泛的热成熟度，计算出的镜质体反射率 (Rc) 值从 0.4% 到 1.3%。我们的结果表明，虽然干酪根的 δ13C 保持相对恒定，但正构烷烃的 δ13C 和 δ2H 与热成熟度呈强正相关。在分析的成熟度范围内的样品中，平均δ13Calk增加6.8‰和δ2Halk增加69‰。同时，随着成熟度的增加，正构烷烃和干酪根之间的碳同位素偏移量（Δδ13Calk-ker）从1.3‰攀升至8.5‰。因此，在古生态和石油相关性研究中必须考虑成熟度对正构烷烃稳定碳氢同位素的实质性影响。在成熟的初始阶段，来自 Barney Creek 组的正构烷烃在 δ13Calk 对碳数的图中显示出越来越正的“同位素斜率”。然而，随着进一步成熟，斜率变得越来越负。同位素斜率反演表明，正构烷烃生成和降解的主要机制在早期成岩作用到变质作用的过程中发生了变化。