Thermodynamic stability of methyldibenzothiophenes in sedimentary rock extracts: Based on molecular simulation and geochemical data

Abstract The thermodynamic stabilities of methyldibenzothiophene (MDBT) isomers and the mechanisms of 1,2-methyl shift, methylation and demethylation have been systematically investigated by molecular modelling. Combining with the geochemical data from a set of lacustrine mudstone samples in the Liaohe Basin (East China), this study reveals the possible reaction mechanisms governing the formation and occurrence of MDBT isomers in source rocks. The Boltzmann distribution of MDBTs in gas, water and cyclohexane was calculated to show the relative thermal stability with the increasing of temperature (burial depth). A total of eight reaction pathways involving in 1,2-methyl shift, methylation and demethylation chemical processes via ionic and free radical reactions, and the transition states and energy barriers are calculated. The results show that three main reaction pathways (P1, P3 and P6) are three possible pathways for the transformation reactions between DBT and MDBTs during the onset of the oil generation window to high maturity stages of sedimentary organic matter. The 1,2-methyl shift isomerization (P1) and low concentration of the possible precursor (2-methylbiphenyl) for 1-MDBT, cause the relative low abundance of 1-MDBT isomer during the oil generative window. Methylation of DBT with methyl radical attack (P3), the formation of MDBTs with TMAB as methyl donor (P6), the prevailing degradation reactions and much higher thermodynamic stability of 4-MDBT relative to 1-MDBT, makes the MDR maturity indicator regularly increase during the oil generative window to high maturity stages. Therefore, a previously proposed MDR maturity indicator was further confirmed by the theoretical calculations, molecular simulations and the geochemical data of the lacustrine mudstone samples in this study.