Contribution of abiotic methane polymerization of C2+ hydrocarbons in highly mature natural gas reservoirs

The formation and evolution of thermogenic gases were investigated using a combination of intermolecular and intramolecular isotope analyses of 32 natural gas samples collected from the Sichuan and Tarim basins (China) and the Arkoma Basin (USA). Three evolution stages (I–III) were identified: 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 δ¹³C 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. C₂₊ hydrocarbons in the Changning shale gases are all generated from abiotic methane polymerization, and the contribution ratio in the Weiyuan shale gases is about 85 %, while, the contribution of C₂₊ hydrocarbons in dry gases from the Tarim Basin formed by this way is no more than 55 %. High methane abundance, high temperature, and abundant catalyst are beneficial to abiotic methane polymerization.