Stable carbon isotopic compositions of individual light hydrocarbons in the C5–C7 range in natural gas from the Qaidam Basin, China

Abstract

C5–C7 light hydrocarbons are important components in petroleum and are extensively employed as an auxiliary approach in petroleum research. Consensus on the mechanism of light hydrocarbon generation has not yet been achieved. In this study, 20 gas samples were collected from the western part and northern margin of the Qaidam Basin. The molecular and carbon isotopic compositions of C1–C3 gaseous hydrocarbons and C5–C7 light hydrocarbons, as well as the carbon isotopes of oils were analyzed. Based on the relationships between carbon isotopes (13C) of individual light hydrocarbons and calculated vitrinite reflectance, it suggests that organic matters play a fundamental role in the carbon isotopes of light hydrocarbons, and maturity mainly affects the carbon isotopes of n-alkanes in light hydrocarbons. In addition, carbon isotopic differences between n-, iso- and cyclo-alkanes indicate that light hydrocarbons with different structures are produced from various biological precursors. Besides, carbon isotopes of individual light hydrocarbons can be used to estimate the δ13Ckerogen and distinguish cracked gas. On the one hand, small isotope fractionation between iso-alkanes and kerogen is observed. Average carbon isotopic compositions of iso-pentane ( i-C5), 2-methylpentane (2-MC5) and 3-methylpentane (3-MC5) can provide a similar δ13Ckerogen with actual values. On the other hand, the carbon isotopic difference between 2-MC5 and n-C6 generally decreases with increasing maturity. Combined with the parameter (2-MC6 + 3-MC6)/ n-C6, kerogen-cracked gases are characterized by high δ13C2MC5–δ13CnC6 and low (2-MC6 + 3-MC6)/ n-C6 values, whereas oil-cracked gases exhibit the opposite features.