The Geochemical Mechanism Of Dibenzothiophenes Formation

Dibenzothiophenes?DBTs?are important sulfur-containing organic compounds in crude oils and sediments.These compounds have great theoretical value and practical significance for determining the origin of oil and gas,indicating the organic maturity,and tracing the migration of oil.Even though dibenzothiophenes have attracted more research attention,the understanding of their precursors and geochemical formation mechanisms is still controversial.This study uses gold tube high-temperature and high-pressure thermal simulation device to conduct experiments under different conditions,aiming to understand the geochemical formation mechanism of the DBTs.Results of thermal simulation experiments show that whether the montmorillonite is present in the reaction system of the 3,3'-dimethylbiphenyl?3,3'-DMBP?and the elemental sulfur or not,that the 3,3'-DMBP can react with the elemental sulfur at 200⁵00?and produce C₀-^C₂-alkyl substituted dibenzothiophenes,biphenyls?BPs?,hydrogen sulfide?H₂S?and other products.According to chemical reaction kinetics,the above reaction will occur when EasyR_o reaches 0.30%.Before EasyR_o reaches 2.04%,the amount of product of increases slowly with the increase of EasyR_o.However,when EasyR_o>2.04%,the amount of product begins to increase rapidly.Therefore,it can be concluded that before the organic maturity reaches the mature stage,DBTs can be generated under actual geological conditions by the reaction of the 3,3'-DMBP and the elemental sulfur and this process will be controlled by thermodynamics.In addition,productive rates of the dibenzothiophene and methyldibenzothiophene in products increase respectively with the increase of the productive rates of the biphenyl?BP?and the methylbiphenyl?MBPs?,which can present positive correlations.This indicates that there is a reactant-product relationship between BP and MBPs as well as between the dibenzothiophene and the methyldibenzothiophene.These experimental results provide new geochemical evidence for the genesis of the DBTs in the Shahejie Formation in the Liaohe Basin and the Upper Indus Basin in Pakistan.In addition,there is a certain positive correlation between the content of BPs and DBTs in geological samples,which indicates that DBTs may be generated through the reaction of BPs and the elemental sulfur.The catalytic effect and thermal stability of DBTs are also studied.With the absence of montmorillonite,no obvious methylic migration or demethylation occurres in the reaction of the 3,3'-DMBP and the elemental sulfur.However,with the presence of montmorillonite,dimethyldibenzothiophenes among reaction products occur methylic migration at 400?,whereas 3,3'-DMBP does not occur of methylic migration all the time.In the reaction system of 4,6-DMDBT and montmorillonite,4,6-DMDBT obviously occurs methylic migration and demethylation after heated,and producing C₁-C₄-alkyl-substituted dibenzothiophenes as well as a small amount of BT,BPs,BN[1,2]T,and H₂S.Results of these experimental studies indicate that the input of montmorillonite leads to a second reaction among products in the simulation process,and other compounds are generated by methylic migration,demethylation,and even the cleavage of C-S bond Moreover,montmorillonite will dehydrate at 200?and transits to the illite,so more research will be done on reactions after the input of montmorillonite.