Evolution characteristics and geological implication of tricyclic aromatic hydrocarbons based on thermal simulation experiments

Key words: 
• Keywords: Polycyclic Aromatic Hydrocarbons /
• Phenanthrene Series Compounds /
• Thermal Simulation Experiment /
• Residual Oil /
• Petroleum Geochemistry

Abstract: Abstract: [Objective] The geochemical significance of phenanthrene series compounds within tricyclic aromatic hydrocarbons (TAHs) remains debated, particularly regarding the relative control of thermal maturity and depositional environment. This study focuses on variations in phenanthrene series parameters during thermal evolution and their controlling mechanisms.[Methods] Lacustrine shale samples from the Shahejie Formation (Es?) of Well X in the Bohai Bay Basin were subjected to closed-system pyrolysis using a DK-III thermal simulation apparatus at seven temperature points (275–450 °C) for 48 h. Aromatic fractions of expelled oil were analyzed via GC–MS to determine the distribution and evolution of polycyclic aromatic hydrocarbons (PAHs).[Results] The original shale extract was dominated by aromatic steroids, followed by tricyclic aromatics. With increasing simulation temperature, bicyclic aromatics first increased then decreased, tricyclic and heteroaromatic hydrocarbons remained relatively stable, and tetracyclic and pentacyclic aromatics increased, while aromatic steroids decreased sharply. In the phenanthrene series, methylphenanthrenes (MPs) were dominant at low temperatures, gradually replaced by dimethylphenanthrenes (DMPs) at 300–400 °C, and phenanthrene (P) became dominant above 400 °C. Anthracene series abundance increased with temperature up to 400 °C, then declined. Bicyclic aromatics showed higher thermal stability below 375 °C, while aromatic steroids transformed into tri- and tetracyclic aromatics between 300–400 °C. Two new maturity parameters, DPR-3 (2,7-DMP/1,2-DMP) and DPR-4 [2,7-DMP/(2,10+1,3+3,10+3,9-DMP)], exhibited strong positive correlation with maturity, supplementing existing F1, F2, DPR-1, and DPR-2 indices.[Conclusions] Methylphenanthrene distributions are suitable for depositional environment assessment in low-maturity samples but are more reliable maturity indicators at mature to overmature stages. Aromatic steroids and bicyclic aromatics are sensitive to early-stage thermal evolution, whereas phenanthrene and anthracene series compounds are better indicators for high maturity. The newly proposed DPR-3 and DPR-4 parameters improve the accuracy and applicability of maturity evaluation, especially in high- to overmature stages. This work refines the understanding of phenanthrene series geochemical significance and provides a robust aromatic hydrocarbon framework for deep hydrocarbon resource assessment and thermal maturity classification.