Overpressure of Deep Jurassic System in the Central Junggar Basin and Its Influence on Petroleum Accumulation

New discoveries and breakthroughs have been made in recent years in the deep parts of the central Junggar Basin, where the Jurassic reservoirs are unconventionally dense with abnormal overpressure development. The development and distribution of overpressure in this basin and the influence of overpressure on petroleum accumulation were analyzed. There are two extremely high overpressure systems in the Jurassic Badaowan and Xishangyao formations, from where the abnormal overpressure in the strata overburdened Jurassic reservoirs was transferred. Paleopressure simulations show that hydrocarbon generation pressurization of the main source rocks in the Badaowan Formation is a process characterized by at least two phases of overpressure increase followed by a phase of overpressure release. Overpressure inhibits the thermal evolution of source rocks in the study area, resulting in lower values of maturity parameter Ro at depths > 4500 m compared with the normal values at depths < 4500 m. The deep reservoirs > 4500 m are very dense, with strong compaction and little retention of primary pore space, indicating the overpressure did not protect the primary pores, while the over-pressured acidic fluid promoted the formation of dissolved pore space. Overpressure and faults are two key factors of petroleum migration, and they jointly control petroleum accumulation in the central Junggar Basin.