Effect of Maturation on the Distribution of Fluorenes, Dibenzothiophenes, and Dibenzofurans in Source Rock: Laboratory Investigation by Hydrous Pyrolysis of Coal Samples in the Erlian Basin

An immature brown coal sample from the Damoguaihe Formation in the Huolinhe Sag of the Erlian Basin was subjected to hydrous pyrolysis. The thermal evolution characteristics of fluorenes (Fs), dibenzothiophenes (DBTs, SFs), and dibenzofurans (DBFs, OFs) were systematically investigated across a simulated temperature range of 250 to 475°C (equivalent to RO = 0.33–2.17%). The results demonstrate regular variations in the bitumen yield, bulk composition, and conventional maturity parameters of aromatics (e.g., MNR, MPI) with increasing temperature. Notably, the relative content of OFs and the OFs/SFs ratio exhibit a triphasic evolution: initial increase, plateau stabilization (RO = 0.69–1.45%), and then rapid decline at higher maturity levels. Maturity-sensitive parameters, including MDR and the ratios of 4,6-/1,4-DMDBT and 2,4-/1,4-DMDBT, gradually increase to their maximum values at RO = 1.72% and then decrease. Similarly, the (2-+3-)/(1-+4-) MF and (2-+3-)/1-MF ratios follow this unimodal trend, with the inflection point at RO = 1.45%. In contrast, potential oil migration indicators (1-/4-MDBF and 4,6-/2,4-DMDBT ratios) show minimal sensitivity to maturation within the oil window. These findings confirm the universal impact of thermal maturation on the distributions of Fs, DBTs, and DBFs in source rocks. The application of these parameters requires careful consideration of the thermal maturity range to ensure accurate interpretations.