Stable carbon isotope compositions of individual light hydrocarbons in oils: New indicator of source facies and maturity

Abstract

Light hydrocarbons (LHs) are key components of petroleum, and the carbon isotopes composition (δ¹³C) of individual LHs contains a wealth of geochemical information. Forty-four oil samples from five different basins were analyzed using gas chromatography (GC), gas chromatography-mass spectrometry (GC–MS), and gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The δ¹³C values of forty-three LHs were recognized and determined by comparing the GC and GC-IRMS methods. The results revealed significant differences in δ¹³C distribution characteristics among different LH compounds. The δ¹³C variation of individual LHs in iso-paraffins showed the widest range, followed by cycloalkanes and aromatics, whereas the δ¹³C variation in n-paraffins showed the narrowest range. The δ¹³C values of most individual LHs are primarily affected by the source facies and thermal evolution. Among them, c-1, 3-dimethylcyclohexane (c-1, 3DMCH) is mainly sourced from higher plants but may also form through abiotic mechanisms such as catalysis or cyclization. The δ¹³C values of c-1, 3DMCH (δ¹³C_{c-1, 3DMCH}) primarily exhibit parental genetic characteristics, enabling effective distinction of oil from different source facies. Specifically, the δ¹³C_{c-1, 3DMCH} in marine oils, lacustrine oils, terrigenous oils, and coal-formed oils are < –22‰, from –22‰ to −20.2‰, from −20.2‰ to −18.4‰, and > −18.4‰, respectively. Moreover, maturity is the primary controlling factor for δ¹³C values of 3MC₇ (δ¹³C_3MC7, 3MC₇: 3-methylheptane), while the source facies serve as a secondary influence. The plot of δ¹³C_{c-1, 3DMCH} and δ¹³C_3MC7 was introduced to classify source facies. As δ¹³C_{c-1, 3DMCH} and δ¹³C_3MC7 increase, the source facies transits from marine to lacustrine, then terrigenous, and finally coal facies. Additionally, increasing δ¹³C_3MC7 indicates a relative increase in maturity. Therefore, the δ¹³C_{c-1, 3DMCH} vs. δ¹³C_3MC7 plot serves as an effective tool for distinguishing source facies and assessing relative maturity.