Factors Controlling The Distribution Of Methyldibenzothiophene In Sedimentary Rocks And Thermal Evolution Mechanism

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 reaction,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 oil generation window to high maturity stages of sedimentary organic matter.The 1,2-methyl shift isomerization(P1)and low concentration of possible precursor(2-mehtylbiphenyl)for 1-MDBT cause the relative low abundance of 1-MDBT isomer during 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 oil generative window to high maturity stages.The distribution characteristics of MDBTs from source rock and pure material samples in each maturation were simulated by thermal simulation experiments,and the reliability of the calculation system was verified.The thermal stability characteristics of MDBTs were confirmed;certain methylation and demethylation reactions were determined in the system,in which demethylation was the main reaction,and both water and isooctane reacted.In the source rocks of the western depression of the Liaohe Basin,MDR increases with depth,although the Boltzmann distribution of 4-MDBT thermal stability decreases with temperature,but it is still much higher than 1-MDBT.In the calculated pathway,methylation of P3 begins to preferentially produce 4-MDBT at the end of the oil window;the demethylation of P6 preferentially produces 2-MDBT from high to overmature,while methyl migration of P1 The function first generates 2-MDBT from the oil-producing window,and then migrates to generate 4-MDBT.2-MBP capable of producing 1-MDBT is low in the sample.The appeal factor led to the lowest enrichment of 1-MDBT in the geological section,while 4-MDBT had the highest enrichment.Therefore,the maturity parameter MDR is a regular growth characteristic change from the oil production window to the high maturity stage and is a reliable maturity parameter.