Mobility is an important parameter of tight sandstone in the process of oil and gas development, prediction, and evaluation. The complex diagenesis process and the lack of knowledge about the control mechanism of the evolution process on mobility restrict the understanding of the hydrocarbon accumulation mechanism and "sweet spot "optimization of tight sandstone.In this study, the tight sandstone of the Fuyu oil layer of the Quantou Formation in the northern Songliao Basin was taken as the research object. Utilizing a combination of high-pressure mercury injection, nuclear magnetic resonance, scanning electron microscopy, and thin section observation, the mechanism of diagenesis on the mobility of tight sandstone in the diagenetic evolution process was explored. The results show that compaction mainly occurs in the early diagenesis stage. The pore types are mainly primary pores and residual intergranular pores. Macropores and mesopores are mainly developed. Macropores are the main occurrence space of movable fluid, and the saturation of movable fluid is high. In the middle diagenesis stage A, compaction and dissolution mainly occur, and a certain degree of cementation occurs at the same time. The pore types are mainly residual intergranular pores and dissolution pores. The pore structure is gradually complicated and the connectivity is reduced.The proportion of mesopores gradually increases, and the saturation of movable fluid continues to decrease. In the B stage of the middle diagenetic stage, cementation mainly occurred. The precipitation of many cements occupied the effective reservoir space and throats such as residual intergranular pores and dissolution pores. The macropores almost disappeared, and the mesopores became the main occurrence space of the fluid, and the mobility was poor. The control of diagenesis on reservoir pore structure in different diagenesis stages determines the mobility of tight sandstone reservoirs. Finally, the mobility evolution model of the tight sandstone reservoirs under the constraint of diagenetic evolution is established. The potential relationship between diagenetic evolution, pore structure, and free fluid occurrence is clarified. All these can help a better understanding of the spatial distribution law and mobility difference of sweet spots in the tight sandstone reservoirs, and also provide a conceptual basis for further optimization of development means and technical schemes.

Despite the significant progress made in tight gas exploration and development in recent years, the understanding of the dynamic mechanisms of tight gas accumulation is still limited, and numerical simulation methods are lacking. In fact, the gap between theory and field application has become an obstacle to the development of tight gas exploration and development. This work sheds light on the dynamic mechanisms of hydrocarbon accumulation in tight formations from the aspect of capillary self-sealing theory by embedding calculation of pressure- and temperature-dependent capillary force in a pore network model. The microscale dynamic mechanisms are scaled up to the reservoir level by geological simulation, and the quantitative evaluation of reserves based on real geological sections is realized. From the results, several considerations are made to assist with resource assessment and sweet spot prediction. Firstly, the self-sealing effect of capillary in the micro-nano pore-throat system is at the core of tight sandstone gas accumulation theory; the hydrocarbon-generated expansion force is the driving force, and capillary force comprises the resistance. Furthermore, microscopic capillary force studies can be embedded into a pore network model and scaled up to a geological model using relative permeability curve and capillary force curve. Field application can be achieved by geological numerical simulations at the reservoir scale. Finally, high temperature and high pressure can reduce capillary pressure, which increases gas saturation and reserves.