Abstract: In the thrust belt of the southern margin of Junggar Basin, the high-yield oil and gas production has successively been obtained in the deep to ultra-deep tight clastic reservoirs in the lower combination, with effective reservoirs and production capacity closely related to formation overpressure intensity. In order to clarify the mechanism of formation overpressure on high-quality reservoir and production capacity, the influence of formation overpressure on reservoir pore structure, permeability, oil and gas saturation, reservoir permeability,and production pressure difference is studied by integrating with previous studies and using geological, logging, drilling, well testing and petrophysical experimental data. In addition, the petrophysical experiments are conducted with the simulated dynamic pore pressure in reservoir formation conditions. The study results show that the highly–extremely over-pressured strata were widely developed in the study area, with intergranular pores retained, overpressure fractures developed, and reservoir “storage pores” and “connection pores” interconnected with each other, forming a high-quality reservoir with double porosity structure of matrix pores and fractures, which was conducive to the formation of high oil and gas saturation. The pore structure of the reservoir was controlled by lithology and overpressure intensity. Formation overpressure had little influence on the absolute reservoir porosity, but had great influence on permeability. When the pore pressure reached a critical value, the permeability increased abnormally, which was conducive to fracture initiation, enhancement of fluid flow capacity,and formation of high-yield oil and gas layers. Therefore, the formation overpressure showed mechanism of “retaining pores, increasing permeability and enhancing saturation”, and the overpressure intensity was the key factor for forming high-quality reservoir and achieving high-yield oil and gas production, as well as the basis for realizing high-yield and steady oil and gas production. The study results indicate that the favorable exploration area of deep to ultra-deep reservoirs in the southern margin of Junggar Basin is the effective traps with formation pressure coefficient of higher than 2.0.

Key words: southern margin of Junggar Basin, ultra-deep formation, Lower Cretaceous Qingshuihe Formation, tight clastic rock, formation overpressure, pore structure, mechanism of high-yield production