Thermal evolution characteristics of dibenzothiophene, dibenzofuran and fluorene and their homologues and their geochemical implications

• Dibenzothiophenes (DBT), dibenzofurans (DBF), and fluorenes (F) isomers investigated. • Distinct patterns in methylation-demethylation and isomer stability revealed. • DBT/P, a depositional proxy, is strongly affected at advanced thermal maturity. • A new depositional proxy is proposed based on DBF, DBT and F isomers. • The 3Fs parameters minimize thermal effects at high maturity. The dibenzothiophene (DBT or SF), dibenzofuran (DBF or OF) and fluorene (F) series compounds are characterized by high structural stability and are widely used in sedimentary environment discrimination and maturity assessment. However, their diagnostic parameters can be overprinted by thermal effects at high maturity. To investigate this, we conducted closed-system pyrolysis experiments (275–450℃) on a saline lacustrine source rock from the Paleogene Shahejie Formation and analyzed the products using gas chromatography-mass spectrometry (GC–MS). Specific results show that: (1) The DBT series dominated throughout, with reaction pathways shifting from methylation below 325℃ to demethylation above this threshold. (2) The DBF series exhibited lower thermal stability with a “decrease–increase–decrease” pattern, whereas the F series was more stable at medium to high temperatures. (3) Traditional proxies like the DBT/P–Pr/Ph cross-plot showed an environment trend from marine to coal-derived with increasing temperature, which misrepresents the constant high-salinity, reducing conditions of the source rock, highlighting a significant maturity overprint. In contrast, a new discrimination diagram based on parameters derived from the combined DBT, DBF, and F series effectively minimized thermal interference, consistently and correctly indicating the original depositional environment. This demonstrates the greater robustness of the ‘3Fs’ series over individual compounds for paleoenvironment reconstruction, especially in high-maturity samples.