Abnormal occurrences of dibenzothiophenes biomarker and their methyl derivatives in secondary altered condensates: Insights from GC–MS and Orbitrap MS

Dibenzothiophene (DBT) and methylated dibenzothiophenes (MDBTs) are important parameters for maturity assessment and oil–oil/source correlations of high-mature oils and condensates, in which conventional biomarkers are commonly depleted or modified by secondary alterations. However, the effects and mechanisms of thermochemical sulfate reduction (TSR), thermal cracking, gas invasion, and migration fractionation on DBT and MDBT distributions remain poorly constrained. In this study, representative TSR-altered and thermally cracked condensates from the Tarim Basin, as well as condensates affected by gas invasion and migration fractionation from the Baiyun Sag, were investigated using GC–MS and high-resolution mass spectrometry. The results show that TSR significantly increases DBT/P ratios and S1 species abundance while decreasing CH₂S species, likely due to sulfur incorporation during sulfate–hydrocarbon reactions. TSR also causes abnormally low 4-/1-MDBT ratios, probably related to the depletion of 3-methylbiphenyl, the precursor of 4-MDBT. Therefore, DBT/P and 4-/1-MDBT ratios are unreliable for depositional environment and maturity assessment in TSR-altered condensates. Thermal cracking results in slight increases in DBT/P ratios and decreases in 4-/1-MDBT ratios, S1 species, and CH 2 S (hydrocarbons, mainly aromatic hydrocarbons detected under a positive-ion ESI mode) species due to the breakdown of macromolecular compounds and aromatization of thermally labile compounds. The increase in DBT is also influenced by the original organic matter input; however, DBT/P ratios remain relatively low and can still be used as a reliable indicator for depositional environment interpretation. In contrast, the 4-/1-MDBT ratio is significantly altered during thermal cracking and therefore becomes unreliable for maturity assessment in thermally cracked condensates. Gas invasion and migration fractionation exert different controls on DBT and MDBT distributions. Gas-invaded condensates exhibit relatively low 4-/1-MDBT ratios due to differential petroleum entrapment during gas displacement, whereas migration-fractionated condensates show higher 4-/1-MDBT ratios, reflecting higher maturity and gas-associated migration processes. In contrast, DBT- and MDBT-based parameters remain applicable for condensates affected by evaporative fractionation. These results provide new insights into the applicability and limitations of DBT and MDBT parameters in highly altered petroleum systems.