New insights into secondary gas generation from the thermal cracking of oil: Methylated monoaromatics. A kinetic approach using 1,2,4-trimethylbenzene. Part III: An isotopic fractionation model

In the present work we propose the elaboration of an isotopic fractionation model predicting δ13CCH4 during the thermal degradation of 1,2,4-trimethylbenzene, a synthetic model compound for methylated monoaromatic hydrocarbons present in oil. Optimization of the model was constrained by experimental data above 30% conversion at 425, 450 and 475 °C. The optimized model accurately predicted experimental δ13CCH4 at the three temperatures. A ratio of frequency factors Ω = 1.028 and variations of activation energies between 36 and 79 cal/mol were obtained. The extrapolation of the model prediction at a burial rate of 50 m/Ma and geothermal gradient of 25 °C/km illustrated the greater isotopic fractionation of the CH4 generated under geological conditions than under laboratory conditions. Moreover, CH4 generated from the thermal degradation of 1,2,4-trimethylbenzene was systematically depleted in 13C (−43‰ < δ13CCH4 < −35‰ for 1.6% < R0 < 2.6%) compared with CH4 generated from the late primary cracking of kerogen in the same maturity range. Our results obtained on a synthetic molecule still need to be confirmed on a natural sample. If confirmed, they would suggest that in thermal settings, isotopically 13C depleted CH4 might not be necessarily the result of mixing with microbial CH4 but could constitute evidence of the secondary cracking of the reservoired oil.