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In this study, hydrous pyrolysis of the Qingshankou Formation shale was employed to simulate the OM transformation process, and the evolution of porosity was analyzed using BIB-SEM and image processing technology. Subsequently, the hydrocarbon expulsion behavior is analized with the occurrence state of solid bitumen in pores at different stages. Image-based porosity revealed a trend of a significant decrease in porosity from 8.9 vol% at 0.8%Ro to 0.7, 1.4, and 2.5 vol% at 1.0, 1.1, and 1.2%Ro, respectively, suggesting clear pore occlusion in clay matrix. A sharp increase in porosity to 13.6 vol% at 1.3%Ro indicates that the primary phase of hydrocarbon expulsion occurs between 1.2 and 1.3%Ro, which is slightly delayed compared to the results of previous research based on geological evolution samples. At the onset of the dry gas window (1.5 %Ro), the porosity of the pyrolyzed sample decreases to 7.2 vol%, which may be caused by the inherent variability of the hydrous pyrolysis experiment, or additional occlusion of pores resulting from ongoing hydrocarbon cracking processes. In summary, the results of this study have significant implications for shale oil extraction behavior, but also for the matrix permeability evolution of organic-rich seal rocks.