Effects of synthetic maturation on phenanthrenes and dibenzothiophenes over a maturity range of 0.6 to 4.7% EASY%Ro

As the exploration target of the shale gas industry is shifting to more deeply buried strata, it is of great application value to study the distribution of molecular maturity ratios in the late dry gas zone and the mechanisms behind them for a robust thermal maturity evaluation. This study investigates the continuous molecular evolution of parent and alkylated phenanthrenes (P, A-Ps), anthracenes (An, A-An), and dibenzothiophenes (DBT, A-DBTs) during the synthetic maturation (Gold tube hydrous pyrolysis) from 0.6% to 4.7% EASY%Ro of two marine shales. The two untreated samples show different elemental composition, bulk geochemistry, and molecular geochemistry characteristics. The distribution sequence of classes with different degrees of alkylation indicates that higher reaction energy is needed for the demethylation of compounds with a lower degree of alkylation. However, the occurrence of trimethyl- and dimethyl-classes of A-Ps and A-DBTs at 550 °C (EASY%Ro: 4.4%) and 600 °C (EASY%Ro: 4.7%) is proposed to result from the breakdown of pyrobitumen at high pyrolysis temperature. The inclusion of An with P when calculating maturity ratios has an influence on their correlation with EASY%Ro at very high maturity. Common molecular thermal ratios including MPI-1, MPI-3, MPDF, MPR, and DMPR were calculated and compared with previous works. All proxies show general trends of firstly increasing and then decreasing along with thermal evolution. The separate trends in the molecular thermal evolution of the two source rocks could be attributed to the difference in organic matter composition and sedimentary facies. Two new thermal molecular maturity ratios applicable in the late gas window, i.e. 2+3-MDBT/4-MDBT and (2,4,6-TMDBT + 1,4,6-TMDBT)/(3,4,6-TMDBT + 2,6,7-TMDBT) are proposed based on the sensitivity of 2+3-MDBT, 2,4,6-TMDBT and 1,4,6-TMDBT isomers towards high maturity.