Compaction dis equilibrium and hydrocarbon generation especially gas generation are the two mechanisms that could generate alone considerable overpressures. Based on the three dimensional shape of the top of overpressured systems, the main cause of overpressure and the inner structure of the overpressured systems, two types of overpressured systems could be classified: compartmentalized overpressured systems and dynamic overpressured ones. Overpressured fluid expulsions occur in two levels: primary expulsion from overpressured shales to nearby sandstones, and secondary expulsion from the whole overpressured system to overlying normal pressured system. In compartmentalized overpressured systems, the fluid drains mainly through episodically opened hydraulic fractures in top seals. In dynamic overpressured systems, secondary fluid expulsions occur along faults or fractures at the highs of the tops of overpressured system or structural highs within the overpressured systems. Petroleum in overpressured basins tends to accumulate preferentially in the normal pressure systems due to natural fracture of sediments and resultant cross formation migration. However, both commercial oil accumulation and large gas reservoirs could be formed in overpressured system.