A kinetic model for thermally induced hydrogen and carbon isotope fractionation of individual n-alkanes in crude oil

A quantitative kinetic model has been proposed to simulate the large D and C-13 isotope enrichments observed in individual n-alkanes (C-13-C-21) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane delta C-13 values increase by similar to 4 parts per thousand and delta D values increase by similar to 50 parts per thousand at an equivalent vitrinite reflectance value of 1.5%. While the observed 13 C-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 (Delta Delta H-double dagger) and entropy (Delta Delta S-double dagger) 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-C-20 (n-icosane), the changes in enthalpy are calculated to be similar to 1340 J mol(-1) (320 cal mol(-1)) and 230 J mol(-1) (55 cal mol(-1)) for D-H and C-13-C-12, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in delta D 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 in C-13 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. Copyright (c) 2005 Elsevier Ltd.