The origin of trimethyldibenzothiophenes and their application as maturity indicators in sediments from the Liaohe Basin, East China

The distributions of trimethyldibenzothiophenes (TMDBTs), the relationship between TMDBTs and trimethylbiphenyls (TMBPs) and the application of TMDBTs as maturity indicators have been investigated in a set of lacustrine shales from the Eocene Shahejie Formation in the Liaohe Basin, East China. The 3,5,3′-, 3,5,4′- and 3,4,4′-TMBP isomers can react to yield (2,4,6- + 2,4,8)-TMDBT, 2,4,7-TMDBT, and (3,4,7 + 2,3,7)-TMDBT by incorporating a sulfur atom into biphenyl, respectively. The absolute concentrations of (2,4,6 + 2,4,7 + 2,4,8)-TMDBT increase with those of (3,5,3′ + 3,5,4′)-TMBP. Similarly, the absolute concentrations of (3,4,7- + 2,3,7)-TMDBT show a similar increase with those of 3,4,4′-TMBP. The good straight line relationships (R2 = 0.96) indicate the product–precursor links between TMDBTs and TMBPs in the sediments. On the basis of the possible differences in the thermodynamic stability among TMDBT isomers, a new maturity indicator 2,4,6-/(2,4,7 + 2,4,8)-TMDBT ratio is proposed in this study. This ratio has a good correlation with 4-/1-methyldibenzothiophen ratio (MDR) and Tmax (°C), respectively, indicating similar behavior with increasing maturity. Both of the 2,4,6-/(2,4,7 + 2,4,8)-TMDBT ratio and MDR exhibit similar maturation trends. They gradually decrease with increasing maturity at the low maturity stages, and then are followed by a regular increase with the increasing maturity. Therefore, this ratio is an effective maturity indicator for source rocks and an oils maturity assessment (Ro > ≈0.55%), and it must be applied with caution as a maturity indicator for extracts of immature sediments.