A kinetic model for thermally induced hydrogen and carbon isotope fractionation of individual n-alkanes in crude oil
A kinetic model for thermally induced hydrogen and carbon isotope fractionation of individual n-alkanes in crude oil
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DOI：
10.1016/j.gca.2004.12.026
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发表时间：
2005-09
影响因子：
5
通讯作者：
Yongchun Tang;Y. Huang;Geoffrey S. Ellis;Yi Wang;P. G. Kralert;B. Gillaizeau;Qisheng Ma;R. Hwang-R.-Hwa
中科院分区：
文献类型：
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作者：
Yongchun Tang;Y. Huang;Geoffrey S. Ellis;Yi Wang;P. G. Kralert;B. Gillaizeau;Qisheng Ma;R. Hwang-R.-Hwa
本文提出了一个定量动力学模型，用以模拟在无水封闭系统条件下，北海原油人工热成熟过程中，单个正构烷烃（C13-C21）中D和13 C同位素的大量富集。在本实验条件下，当镜质体反射率为1.5%时，正构烷烃δ 13 C平均值增加了1.4 ‰，δD平均值增加了1.50 ‰。虽然13 C-富集没有表现出显着的依赖于烃链长度，热诱导D-富集增加正构烷烃碳数。这种差异分馏效应被推测是由于与较低分子量同系物相比，较高分子量正构烷烃的热裂解程度更大，以及产生同位素更轻的较低分子量化合物的综合效应。这种与碳数相关的氢同位素分馏行为可以形成一种新的成熟度指标的基础，以定量评估石油储层中石油裂解的程度。通过对正构烷烃均裂C-C键时同位素取代反应的平均焓变（ΔΔH <$）和熵变（ΔΔS <$）的量子力学计算，得到了与实验结果相当吻合的同位素分馏预测.对于正二十烷，D-H和13 C-12 C的焓变分别为1340 J mol-1（320 cal mol-1）和230 J mol-1（55 cal mol-1）。由于与氢同位素分馏相关的焓项大约是碳的六倍，因此单个长链烃的δD值的变化提供了对石油所经历的热蚀变程度的高度敏感的测量。外推的动力学模型，以典型的地质加热条件下预测显着富集在13 C和D的正二十烷在等效镜质体反射率值对应的正构烷烃的热裂解的开始。本研究的实验和理论结果对利用化合物氢同位素数据进行石油地球化学和古气候研究具有重要意义。然而，还有许多其他地球化学过程会显著影响观测到的氢同位素组成（例如，生物降解、水洗、与水和矿物质的同位素交换）也必须考虑在内。
A quantitative kinetic model has been proposed to simulate the large D and13C isotope enrichments observed in individual n-alkanes (C13–C21) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane δ13C values increase by ∼4‰ and δD values increase by ∼50‰ at an equivalent vitrinite reflectance value of 1.5%. While the observed13C-enrichment shows no significant dependence on hydrocarbon chain length, thermally induced D-enrichment increases with increasing n-alkane carbon number. This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight, n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This carbon-number-linked hydrogen isotopic fractionation behavior could form the basis of a new maturity indicator to quantitatively assess the extent of oil cracking in petroleum reservoirs. Quantum mechanical calculations of the average change in enthalpy (ΔΔH‡) and entropy (ΔΔS‡) as a result of isotopic substitution in n-alkanes undergoing homolytic cleavage of C-C bonds lead to predictions of isotopic fractionation that agree quite well with our experimental results. For n-C20(n-icosane), the changes in enthalpy are calculated to be ∼1340 J mol-1(320 cal mol-1) and 230 J mol-1(55 cal mol-1) for D-H and13C-12C, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in δD values for individual long-chain hydrocarbons provide a highly sensitive measure of the extent of thermal alteration experienced by the oil. Extrapolation of the kinetic model to typical geological heating conditions predicts significant enrichment in13C and D for n-icosane at equivalent vitrinite reflectance values corresponding to the onset of thermal cracking of normal alkanes. The experimental and theoretical results of this study have significant implications for the use of compound-specific hydrogen isotope data in petroleum geochemical and paleoclimatological studies. However, there are many other geochemical processes that will significantly affect observed hydrogen isotopic compositions (e.g., biodegradation, water washing, isotopic exchange with water and minerals) that must also be taken into consideration.