Reaction kinetics of stable carbon isotopes in natural gas-insights from dry, open system pyrolysis experiments

Open system nonisothermal pyrolysis:with on-line compound-specific (13)C/(12)C stable-isotope analysis (Py-GC/IRMS) has been performed on three carbonaceous sediments from NW Germany (Carboniferous, Westphalian coal, HI = 286 mg(HC)/g(TOC), R(0) = 0.72%), West Siberia (retaceous; Cenomanian shale, HI = 192 mg(HC)/g(TOC), R(0) = 0.43%, and Malaysia (Tertiary, Miocene coal, HI 190 mg(HC)/g(TOC), R(0) = 0.36%). The study was focused on the generation of methane, ethane; and propane + propene. Measured delta (13)C-values of pyrolytically generated light hydrocarbons were in the range of delta (13)C-values commonly observed in thermogenic natural gas (-20 to -40%, PDB). While the isotopic composition of the pyrolysis products showed a general enrichment; in (13)C species with increasing temperature, the isotopic trends of methane displayed characteristic structures involving reversals in certain temperature intervals. On the basis of the experimental data, reaction kinetic parameters have been derived for each isotopic species of the hydrocarbon gases assuming parallel first-order reactions and an Arrhenius-type temperature dependence. The resulting kinetic parameter sets for the Westphalian coal were then tentatively applied to geologic temperature histories to model the chemical and isotopic composition of natural gas generated and accumulated in reservoirs of the NW German Basin. The isotopic compositions (delta (13)C-values) of methane computed in this simulation show-a good agreement with actual isotopic compositions of the natural gases in NW German gas fields. It is demonstrated that the combination of isotope-specific reaction kinetics with the regional thermal history provides a useful tool to account for variations in the isotopic, composition of reservoir gases in the course of the accumulation history. These results indicate that, despite the undisputed differences between laboratory and natural conditions for gas-generation, open system nonisothermal pyrolysis provides isotope-specific reaction kinetic parameters that satisfactorily describe the isotope effects associated with thermogenic natural gas generation in geologic systems. Application of these parameters in basin modeling studies permits prediction/reconstruction of isotopic compositions of natural gases with the same level of confidence as commonly applied; bulk and compound-specific kinetic parameters.