Abstract: There exist three types of over-pressured formations of Paleogene in Dongying sag. The undercompaction and hydrocarbon generation are the genetic mechanism of mudstone overpressure, according to the synthetic compaction curve and pressure forward modeling. The contact transmission leads to the overpressure of type-I sandstone contacting over-pressured mudstone, while the connecting transmission is the cause of forming overpressure of type-II sandstone. It is shown that weak overpressure occurs in mudstone in deeper well interval of Middle Sha-3 member, middle overpressure in Lower Sha-3 member and high overpressure in Sha-4 member. The mudstone pressure center depends on the sedimentary center, so the intensity of overpressure is decreasing toward the basin margin and distributed circularly. There is little overpressure in shallow sandstone less than 3 000 m in depth, except for its part distribution near the fault zone and unconformity, and the overpressure intensity of the sandstone is greater than that of the mudstone. In depth of over 3 000 m, such an intensity of the sandstone is smaller than that of the mudstone.

Key words: Dongying sag, Paleogene, sandstone, mudstone, shale, overpressure, genesis, origin