oa Contribution of Abiotic Methane Polymerization of C2+ Hydrocarbons in Highly Mature Natural Gas Reservoirs, Based on Analysis of Position-Specific Isotopes

The co-occurrence of abnormal intermolecular and intramolecular carbon isotopic compositions is caused by the surface-catalyzed methane polymerization of C2+ hydrocarbons. The formation and evolution of thermogenic gases can be divided into three evolution stages (I–III): In stage I, hydrocarbon gases are produced through thermal decomposition of organic matter, and kinetic isotope effects in C–C bond breakage control their isotopic distributions; In stage II, C2–C5 hydrocarbons crack with increasing thermal maturity, with their formation and decomposition tending toward thermodynamic equilibrium, and at the end of this stage, the intermolecular and intramolecular isotopic compositions of gaseous hydrocarbons are in thermodynamic equilibrium; A remarkable feature of stage III is the surface-catalyzed abiotic polymerization of methane, which provides a critical origin of C2+ hydrocarbons in this stages and leads to isotopic anomalies in C2+ hydrocarbons, including the reversal of d13C distributions of C1–C3 and the reverse evolution trend of SP value of propane (i.e., tending to be positive). The contribution of C2+ hydrocarbons from the abiotic polymerization of methane can be determined based on a two-end member model.