The distribution and origin of dimethyldibenzothiophenes in sediment extracts from the Liaohe Basin, East China

The distributions of dimethyldibenzothiophenes (DMDBTs), the relationship between DMDBTs and dimethylbiphenyls (DMBPs), and the applications of DMDBTs as maturity indicators in source rocks have been investigated in a set of 21 lacustrine shales from the Eocene Shahejie Formation (Well SG1) in the Western Depression, Liaohe Basin, China. All source rock samples are characterized by total organic carbon contents of 1.37-3.27% and Type II (with minor Type III) kerogen. They were deposited in suboxic and brackish lacustrine environments and have maturities ranging from immature to mid-mature. The 3,3′-DMBP isomer can potentially react to yield 4,6-DMDBT; 2,6-DMDBT and 2,8-DMDBT by incorporating sulfur into biphenyl, which may be supported by a strong positive correlation between the absolute concentration changes of 3,3′-DMBP and those of 4,6-DMDBT, 2,6-DMDBT plus 2,8-DMDBT. The relative abundance of DMDBT isomers may be explained by the sulfur radical mechanism and are also controlled by steric hindrance and thermodynamic stability. The 4,6-/(1,4- + 1,6)-DMDBT ratio shows no regular trend with increasing maturity at the low stage, and it should be used with caution as a maturity indicator for immature sediments. However, within the oil generation window, the relative concentration of 4,6-DMDBT progressively increases with increasing maturity, which can be explained by its higher thermodynamic stability relative to the 1,4-DMDBT isomer. The 4,6-/(1,4- + 1,6)-DMDBT ratio exhibits a linear increase with increasing thermal maturity of the sediments. Thus, this ratio can be applied as an effective maturity indicator for source rocks within oil generation window.