On-line pyrolysis-GC-IRMS: isotope fractionation of thermally generated gases from coals

Abstract

The isotopic composition of reservoir gases is frequently used for source rock to gas correlations. The prime control on the isotopic signature of gas results from kinetic isotope fractionation during thermal gas generation. A new on-line pyrolysis-GC-IRMS system has been developed as an analytical tool to study the kinetic isotope fractionation during thermal gas generation from sedimentary organic matter. The method is based on open-system non-isothermal pyrolysis in the temperature range from 100 to 1100°C and allows simultaneous monitoring of the isotopic composition of seven individual pyrolysis product (methane, ethane, ethene, propane, propane, carbon monoxide and carbon dioxide) at a high sampling frequency. Due to the high data density achieved by this procedure a detailed reaction kinetic interpretation of the pyrolysis curves in terms not only of compositional but also isotope specific reaction kinetics is feasible. This method and the data presented encourage both the re-evaluation of existing concepts for source-rock to gas correlations and also offer new insights into controls on isotope fractionation during thermal gas generation. Current results indicate a high variability of the isotopic composition of the seven pyrolysis products. Although a general trend of enrichment in the 13C species with increasing pyrolysis temperature is observed, characteristic reversals of this isotopic trend are commonly occurring for methane, CO, and CO 2. These variations reflect differences in the isotopic composition of the precursor structures, which are progressively decomposed during pyrolysis. The analytical data obtained from the approach presented here open up the perspective of developing new kinetic fractionation models taking into account both the isotopic variability of precursor structures and their thermal stability.