Influencing factors of hydrogen isotopic fractionation of light hydrocarbons during evaporation and implications

Abstract

The isotopic fractionation of organic compounds during evaporation is one of the most critical issues in the field of stable isotopes. The hydrogen (H) isotopic compositions of light hydrocarbons (LHs) in oil have gained increasing interest in the research of oil genesis in recent years. However, compared to carbon (C) isotopes, our understanding of H isotopic fractionation patterns and influences for various types of LHs during evaporation is limited. In this study, we performed evaporation experiments at a constant temperature on LHs in three systems: single compound, alkane mixture, and light oil. We assessed the contents and H isotopic compositions of individual LHs in the residual liquid phase. The degree of H isotopic fractionation and influencing factors are studied combined with C isotopes. The H isotopic fractionation of LHs exhibits “inverse isotope fractionation” characteristics (a depletion in D in the residual LHs) with progressive evaporation, which is opposite to the C isotopic fractionation. The degree of isotopic fractionation is influenced by the evaporation system, the compounds’ molecular weight, and their structure. (1) The isotopic fractionation degree of H and C decreases in the following order: light oil > alkane mixture > single compound system. This difference may be related to the unsaturation level of the evaporative system and the evaporation matrix. (2) For LHs with the similar structure in the same evaporation system, the degree of H isotopic fractionation increases with increasing molecular weight due to the buffering effect of the sample pool, while the magnitude of C isotopic fractionation decreases. (3) For LHs with the same C number, methylcyclohexane (MCH) has a lower evaporation rate and less isotopic fractionation than the other C₇ compounds like 3-methylhexane (3-MH), n-heptane (nC₇), and toluene (Tol).

The distinctive fractionation characteristics of H isotopes make them very useful in geological applications. Combining H and C isotopic compositions of the same compound with the commonly used molecular ratios of LHs (e.g., nC₇/MCH, Tol/nC₇) enables full differentiation of three critical processes controlling oil formation: evaporation, thermal maturation, and biodegradation. Furthermore, the differences in H isotopic compositions between C₇ LHs (3-MH, nC₇, and Tol) may be changed by evaporation, but the variation range is much smaller than that caused by source differences. Therefore, combining H and C isotopes may have great potential for oil source characterizations.