<p>This study focused on the overpressured low-permeability gas reservoirs of the Huangliu formation in the Dongfang area of Yinggehai Basin, northern South China Sea, with an emphasis on the controlling mechanisms of the reservoir’s physical properties under high-temperature, high-pressure geological conditions. A suite of integrated experimental techniques, including laser particle size analysis, X-ray diffraction, high-pressure mercury intrusion, and nuclear magnetic resonance, was used to comprehensively characterize the rock fabric and pore structure of fine-grained clastic reservoirs. Results indicated that depositional processes constituted the fundamental control on reservoir quality. Grain size and clay content jointly determined permeability variations, with the best reservoir properties developed in high-energy channelized depositional microfacies. The overpressure regime significantly altered diagenetic pathways: on the one hand, it effectively inhibited destructive processes such as mechanical compaction and carbonate cementation, thereby preserving primary intergranular pores; on the other hand, high-temperature, high-pressure conditions enhanced dissolution, leading to the development of secondary pore space. The study further demonstrated that the transformation and enrichment of clay minerals within the fine-grained matrix, particularly authigenic chlorite, represented the key microscopic mechanism responsible for the sharp reduction in reservoir permeability. Overall, this research elucidated a complex genetic mechanism for fine-grained reservoirs under overpressure conditions, characterized by ‘deposition-controlled framework, overpressure-driven pore preservation, and clay-related permeability degradation’. Herein, a geology-driven reservoir quality evaluation concept is proposed, providing an important theoretical basis for favorable reservoir prediction and the economically efficient development of similar high-temperature, overpressured, low-permeability offshore gas reservoirs.</p>