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[Display omitted] • Evolution of asphaltene-adsorbed aromatics is retarded compared to free ones. • Asphaltene-occluded aromatics profiles respond insignificantly to thermal stress. • Distributions of occluded aromatics are probably thermodynamically controlled. • Steric hindrance imposes a kinetic barrier on the reaction of trapped molecules. The thermal evolution of components trapped in matrix of asphaltenes is supposed to be retarded, as compared to the free ones. Studying the geochemical evolution of trapped polycyclic aromatic hydrocarbons (PAHs) may provide insight for the characterization of high-maturity organic matter. The discrepancies in the thermal evolution between free and trapped (asphaltene-adsorbed/occluded) PAHs (phenanthrene, chrysene, pyrene, and their methylated isomers) were studied by thermal maturation (gold tubes, 300 ∼ 400 °C) experiments on a low-maturity solid bitumen from the Sichuan Basin, southwestern China. The results show that the thermal evolution of asphaltene-adsorbed PAHs is retarded compared to that of free ones. This is attributed to slight differences in reaction kinetics due to steric hindrance by the asphaltene structure. The asphaltene matrix appears to act as a reaction inhibitor leading to a retardation of the thermal evolution of the methylphenanthrene index (MPI) in asphaltene-adsorbed hydrocarbons. This could provide a possibility for maturity assessment of high-maturity organic matter. The thermal evolution of asphaltene-occluded PAHs in the experiments resulted in thermodynamically controlled isomer distributions of occluded methylphenanthrenes and methylchrysenes. However, the occluded methylpyrene isomers were hardly affected by thermal stress. They probably retained their original distributions and may serve as source indicators. [ABSTRACT FROM AUTHOR]

• Thermal evolution of biomarkers trapped by asphaltene structure is distinctly delayed compared with that of free biomarkers. • The potential chemical conversion between asphaltene-occluded n -alkanes and n -alkenes may be related to the different metastable equilibrium of the system. • Steranes and terpanes occluded by asphaltenes can preserve the original characteristics of the diagenesis stage during thermal maturation. • The occlusion space formed by asphaltene aggregates presumably restricts the transformation of active biomarkers during the thermal evolution. Asphaltene-trapped (adsorbed or occluded) biomarkers are considered a valid source of information for crude oil with severe secondary alterations. However, thermal stress might change asphaltene-trapped hydrocarbons to some extent. Therefore, it is indispensable to investigate the thermal evolution behavior of asphaltene-trapped biomarkers. In this study, low-maturity bitumen from the Kuangshanliang area of the Sichuan Basin (China) was selected as a sample. The thermal evolution behavior of free and asphaltene-trapped saturated biomarkers was investigated by thermal simulation experiments. The results revealed, in addition to normal biomarkers, also a series of even carbon number n -alk-(1)-enes in asphaltene-occluded hydrocarbons. All of them were hardly influenced by secondary alterations. Moreover, due to the restriction of macromolecular structure, the thermal evolution of asphaltene-trapped biomarkers lags behind that of free biomarkers. Most biomarker parameters of occluded hydrocarbons changed little with increasing maturity. In particular, the distributions of occluded terpanes or steranes retained the characteristics of early diagenesis of organic matter, even if they suffered from strong thermal alteration. [ABSTRACT FROM AUTHOR]