Porosity in shale reservoirs is profoundly influenced by diagenetic processes that shape the evolution of pore space. In the Songliao Basin, the first member of the Qingshankou Formation (K2qn1) shale serves as a typical lacustrine clay-rich shale oil reservoir. It exhibits variable mineralogy and thermal maturity, with its reservoir quality strongly influenced by these variations. This study integrates microscopic observations, including field emission scanning electron microscopy (FE-SEM), cathodoluminescence (CL), and organic petrology, with bulk rock investigations employing X-ray diffraction (XRD), Rock-Eval pyrolysis, and elemental analysis, to reconstruct the diagenetic pathways leading to changes in porosity. The findings reveal that K2qn1 shale is primarily composed of extrabasinal grain assemblages, such as detrital quartz and clay minerals, with sedimentary organic matter mainly as lamalginite. As maturity exceeds 0.8 %Ro, the presence of authigenic clay minerals, primarily characterized by chlorite, becomes widespread, often associated with bitumen impregnation. Additionally, authigenic dolomite and pyrite form during the synsedimentary to early diagenetic stages. In immature to marginally mature stages (<0.6 % Ro), mechanical compaction is the primary factor leading to porosity reduction. Pores associated with organic matter become visible when maturity reaches the oil window (>0.7 % Ro). At the peak of hydrocarbon expulsion (approximately 1.2 % Ro), a high frequency of organic matter shrinkage pores at interfaces to clay minerals is present, likely playing a significant role in enhancing permeability. Although localized occurrences of dissolution and cementation can be observed, the development of these diagenetic processes is limited and exerts minimal influence on the physical properties of the bulk rock. This study significantly contributes to our understanding of the mechanisms governing diagenetic processes and reservoir evolution in markedly heterogeneous lacustrine shale, considering the diverse origins of grain assemblages.