Mechanism of linear covariations between isotopic compositions of natural gaseous hydrocarbons

Abstract

Linear covariations between stable isotopic compositions of methane, ethane and propane have long been observed in natural gas accumulations; e.g., a plot of δ13Cpropane versus δ13Cethane of natural gas fall along a straight line. This work investigates physicochemical mechanisms behind the correlations and derives the theoretical slope value for each correlation line. Most slopes are close to the values of statistical distribution, which are 2/3 (0.67) for δ13Cpropane vs. δ13Cethane, 2 for δ13Cmethane vs. δ13Cethane, and 3/4 (0.75) for δ2Hpropane vs. δ2Hethane. When detailed kinetic isotopic effects are considered, the theoretical slope values of δ13Cpropane vs. δ13Cethane and δ2Hpropane vs. δ2Hethane change slightly to 0.68 and 0.73, respectively; the slope of δ13Cmethane vs. δ13Cethane is about 2.4. These slopes are consistent to field data of natural gas generated from kerogen before dry gas window. An exception is the slope of the δ2Hmethane vs. δ2Hethane correlation line; the deviation of field data from theoretical slope suggests that the reaction path of methane generation differs from that of ethane and propane generation; proton shift is probably involved in methane generation. The physical meanings of intra-molecular isotopic fractionation in propane and clumped isotopic fractionation in methane are discussed based on statistical and kinetic analysis.