A maturation scale for molecular simulation of kerogen thermal degradation

Abstract

Correlating molecular dynamics (MD) simulations of thermal degradation of organic matter with natural processes in ancient sediments is vital for revealing the mechanisms of hydrocarbon generation at molecular and atomic levels. This study establishes a practical maturation scale for molecular modeling of the thermal degradation of kerogen. ReaxFF reactive MD simulations of the thermal evolution of typical kerogen types (Type I, II, and III) were carried out at temperatures ranging from 1200 K to 2600 K. The results demonstrate that the H/C and O/C atomic ratios of kerogen molecules decrease as simulation temperatures increase and that the evolutionary paths of the three kerogen types fit well with those shown in the van Krevelen diagram. The corrected activation energies of pyrolysis for these three typical kerogen types were found to be 61.1 kcal/mol, 53.5 kcal/mol and 52.8 kcal/mol, respectively, consistent with those obtained from both laboratory simulations and basin modeling. By comparison with the relationships between measured vitrinite reflectance values and the H/C atomic ratios of kerogen samples reported in the literature, a quantitative relationship was established between the vitrinite reflectance (%Ro) values of kerogen and simulation temperatures (T). The cross-plots of T vs %Ro for kerogen types II and III show that vitrinite reflectance exponentially increases with increasing simulation temperature. Two logarithmic correlation equations were constructed to match geological maturation levels to the corresponding levels in MD simulations of thermal degradation of types II and III kerogen. These maturation scales are generally in line with the simulation temperatures set for thermal degradation in kerogen pyrolysis in previous studies. This work has promising practical implications for research on the thermal evolution behavior of ancient sedimentary organic matter using ReaxFF MD simulation.