Isotopic fractionation and identification of abiogenic alkane gases in China

Based on geochemical data from natural gas samples across spring water systems and sedimentary basins, including Songliao, Bohai Bay, Sanshui, Sichuan, Ordos, Tarim and Ying-Qiong, this paper systematically compares the geochemical compositions of abiogenic versus biogenic gases. Emphasis is placed on the diagnostic signatures of abiogenic gases in terms of gas composition, and carbon, hydrogen and helium isotopes. The main findings are as follows. (1) In hydrothermal spring systems, abiogenic alkane gases are extremely scarce. Methane concentrations are typically less than 1%, with almost no detectable C₂₊ hydrocarbons. The gas is dominantly composed of CO₂, while N₂ is the major component in a few samples. (2) Abiogenic alkane gases display distinct isotopic signatures, including enriched methane carbon isotopic compositions (δ¹³C₁>–25‰ generally), complete carbon isotopic reversal (δ¹³C₁>δ¹³C₂>δ¹³C₃>δ¹³C₄), and enriched helium isotope (R/R_a>0.5, CH₄/³He≤10⁶ generally). (3) The hydrogen isotopic composition of abiogenic alkane gases may be characterized by a positive sequence (δD₁<δD₂<δD₃), or a complete reversal (δD₁>δD₂>δD₃), or a V-shaped distribution (δD₁>δD₂, δD₂<δD₃). The hydrogen isotopic compositions of methane generally show limited variation (about 9‰), possibly due to hydrogen isotopic exchange with connate water. (4) In terms of identifying gas origin, CH₄/³He-R/R_a and δ¹³C_CO2–R/R_a charts are more effective than CO₂/³He-R/R_a chart. These new geological insights provide theoretical clues and diagnostic charts for the genetic identification of natural gas and further research on abiogenic gases.