Thermal evolution trend of aromatic hydrocarbons in shales from Permian Dalong Formation in Sichuan Basin and its significance in thermal maturity

The Paleozoic marine shales in southern China hold great significance for hydrocarbon exploration. However, the lack of organic matter (vitrinite) derived from higher plants in marine sediments leads to considerable uncertainties in maturity assessment. In this paper, an thermal simulation experiment of hydrocarbon generation and expulsion for the Permian Dalong Formation shales in Guangyuan, Sichuan Basin was conducted to explore the applicability of aromatic maturity parameters in highly mature to over-mature stages. Analysis of the aromatic hydrocarbon extracts from the solid samples after thermal simulation was performed using gas chromatography-mass spectrometry (GC-MS). The results revealed that the thermal simulation residual oil from Dalong Formation shales in Shangsi section, Guangyuan area, Sichuan, contained abundant aromatic hydrocarbon compounds, including naphthalene, phenanthrene, chrysene, benzo(a)anthracene, dibenzothiophene, biphenyl, dibenzofuran, fluorene, fluoranthene, pyrene, anthracene, and triaromatic steranes. Most aromatic maturity parameters exhibited turning points with increasing thermal simulation temperatures, and the parameter values showed different trends before and after the turning points, indicating that these parameters have specific applicable ranges. Among them, aromatic maturity parameters (such as MNR, DNR, MPI3, DPR, F1, F2, MDR and DBDBT2) can be used to evaluate the maturity of shales at mature to highly mature stages (0.8% < Easy Ro < 2.5%), while PMNr is more suitable for evaluating high to over-mature shales (2.5% < Easy Ro < 4.5%). It was found that the parameters of C-2 DBF-1/MDBF-1 and C-2 DBF-2/MDBF-1 are effective for evaluating shales at mature to over-mature stages (0.8% < Easy Ro < 4.5%), showing a stronger correlation with temperature in the high-temperature evolution stages. This suggests their potential for assessing high to over-mature shales.