Investigation of hydropyrolysis released aromatic hydrocarbons from Permian kerogens at different maturities in the Sichuan Basin, China

•HyPy was used to release aromatic hydrocarbons from Permian kerogen.•The thermal influence on the aromatic hydrocarbons released by HyPy is small.•Most of the covalently bound aromatic thermal maturity parameters are invalid.•Perylene was detected in the hydropyrolysates of P2d kerogens.•The HyPy released aromatic hydrocarbons can be used in petroleum geochemistry.

Catalytic hydropyrolysis (HyPy) was used to release covalently bound aromatic hydrocarbons from both mature and overmature Permian source rocks in the Sichuan Basin, China. The released covalently bound aromatic hydrocarbons were discussed and compared with their free counterparts obtained by Soxhlet extraction. Furthermore, a mature Permian Dalong kerogen was selected for artificial simulation experiments. The HyPy experiments of the above thermal altered kerogens were subsequently conducted. The results from the artificial simulation sequence were also discussed and compared with those from the natural maturity sequence. For both natural maturity sequence and artificial simulation sequence, the distributions of covalently bound methylphenanthrenes and methyldibenzothiophenes are quite similar at different maturity stages. Triaromatic and monoaromatic steroids which are the aromatization products of steroids were not detected in covalently bound aromatics of all samples. The above results indicate that the rearrangement of methyl group on aromatic nucleus and the aromatization of steroids within kerogen was retarded to some extent or even prohibited, due to the protection of macromolecular structure below a vitrinite reflectance of 2.4%. It implies that aromatic maturity-related biomarker parameters based on the rearrangement and aromatization reactions within kerogens are probably not appropriate for maturity assessment. Meanwhile, perylene which is easily altered by thermal alteration was still existent in covalently bound aromatics released from the Dalong Formation even at over mature stage (Ro: 1.8%) and from the thermal altered kerogen until the temperature below of 400 °C (H/C: 0.49). It means the covalently bound perylene can be protected by macromolecular structure from the destruction of thermal stress at least below Ro of 1.8% (or H/C > 0.5). The results obtained also permitted the geochemical characterization of the source rocks.