Guan Yuanxiao^1,2, Ou Yongliang^1,2, Zhang Luning^1,2, Liu Changjie^1,3

(¹Research Center for Oil & Gas Resources, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China;

²University of Chinese Academy of Sciences, Beijing 100049, China

³Key Laboratory of Petroleum Resources Exploration and Evaluation, Lanzhou 730000, China

Position-specific isotope analysis (PSIA) of propane is an emerging geochemical tracer technique for elucidating the genesis and alteration processes of natural gas. The position-specific isotopic fractionation induced by adsorption-desorption dynamics during chromatographic transport remains insufficiently understood. Using propane whcih is the simplest molecule exhibiting position-specific isotopic characteristics in nature, this study preliminarily investigates the variations in bulk carbon isotope and position-specific carbon isotope during chromatographic transport. This study confirms the existence of chromatographic fractionation effects on propane position-specific isotope and provides a preliminary quantification of fractionation characteristics and fitting parameters. Compared to conditions at 70 °C and 100 °C, the position-specific isotopic fractionation during adsorption-desorption was more pronounced at lower temperatures (e.g., 40 °C) using a highly polar stationary phase. This phenomenon is attributed to the stronger adsorption and delayed elution of the polar molecular configuration ¹³CH₃-¹²CH₂-¹²CH₂ relative to the non-polar ¹²CH₃-¹³CH₂-¹²CH₃, resulting in a gradual depletion of the position-specific carbon isotope ratio with increasing retention time. Comparative analysis of fitting slopes (k_c and k_t) revealed that the magnitude of this chromatographic effect approximates the position-specific isotopic fractionation observed during kerogen thermal cracking. Among the four factors examined—temperature, flow velocity, column polarity, and mixed gas composition—temperature was determined to be the dominant factor governing the intensity of position-specific isotopic fractionation during propane adsorption-desorption in chromatography. Geologically, this work provides a simplified model for understanding isotopic variations during subsurface gas migration and offers potential criteria for distinguishing secondary alteration processes.

Key Words: Propane, Natural gas, Adsorption process, Isotope effect, Position-specific isotope analysis

Author Profile

Guan Yuanxiao, Male, PhD Candidate, mainly engaged in gas geochemistry research. E-mail: guanyuanxiao23@mails.ucas.ac.cn.

Ou Yongliang ,Female, PhD Candidate, mainly engaged in gas geochemistry research. E-mail: ouyongliang@nieer.ac.cn.

Zhang Luning, Male, Master Candidate, mainly engaged in gas geochemistry research. E-mail: zhangluning24@mails.ucas.ac.cn

Liu Changjie, Male, Researcher, PhD, mainly engaged in propane position-specific geochemistry research. E-mail: liuchangjie @nieer.ac.cn.