Kinetic clumped isotope fractionation during the thermal generation and hydrogen exchange of methane

Abstract

Abstract This work investigated the kinetic fractionation of clumped isotopes (13CH3D and 12CH2D2) during thethermal generation and hydrogen exchange of methane. Kinetic reaction networks involving the five most abundant isotopologues of methane are applied. Hydrogen isotope exchange is modeled with a chain reaction network constrained by thermodynamic parameters of methane isotopologues. Analytical solutions under isothermal conditions are obtained for isotope fractionations during methane generation. Both methane generation and isotope exchange are possible to result in “anticlumping” at low conversions and clumped isotope reversal at high conversions. Sensitivity of anticlumping to 13C and deuterium kinetic isotope effects (KIEs) are tested for methane generation. D-D clumping is found to be sensitive to the deuterium KIE of the capping hydrogen, especially when the quantum tunneling effect is present. Clumped isotopic compositions of methane in natural gas accumulations are affected by multiple geochemical and geological factors. Precursor methyl groups are expected to be enriched in clumped isotopes, and the enrichment results in clumped isotopic compositions more positive than the equilibrated ones at the beginning of methane generation. In the oil window to early gas window, clumped isotopes deplete with thermal maturity due to combinatorial anticlumping and gas expulsion. These two factors may bring about clumped isotopic compositions more negative than the equilibrated values. In late wet gas to dry gas window (the same range of ethane and propane decomposition), C-H bonds of methane are activated, and isotope exchange occurs through free radical reactions; as a result, clumped isotopes eventually approach equilibrium. Clumped isotopic composition of methane records the chemical, thermal and flow histories of its source rock, rather than merely the temperature state of a gas accumulation.