Methyldibenzothiophenes distribution index as a tool for maturity assessments of source rocks

The characteristics for the thermal evolution of dibenzothiophenes maturity parameters, such as 4 MDBT/DBT, 2,3 MDBT/DBT, 4 MDBT/1 MDBT and 4 MDBT/(4 MDBT+1 MDBT), have been discussed on the basis of GC/MS analysis for the two selected suites of source rocks in Kuqa depression, Tarim basin and Yabrai basin, Hexi Zoulang area. It is the first report that methyldibenzothiophene distribution index (MDBI), namely, 4 MDBT/(DBT+4 MDBT+2,3 MDBT+1 MDBT) increasing with the maturation, which primarily follows the principle of chemical thermodynamics, has been proposed as a new effective parameter of maturity in the investigated basin or depression. The results indicate that MDR 4 and MDR 2,3, which are thermodynamically controlled, are better maturity parameters to evaluate the thermal maturation of source rocks in the areas. It is found that the rate of MDR ratio appears a remarkable increase in a little high maturation, and MDR is very reliable for maturity determination of type Ⅱ2 and Ⅲ of organic matters. The correlation matrix for MDBI and other effective maturity indicators, such as Ro, tmax and MPI, further demonstrates that MDBI may accurately provide the information about the history of thermal evolution for source rocks within the vitrinite reflectance (Ro) values varying from 0.67% to 1.22%. Moreover, the new index presented in this paper implies a new access to identify the thermal maturation of oils.