Distribution and causes of present-day overpressure of Shahejie Formation in Linnan Subsag, Huimin Sag, Bohai Bay Basin

The Linnan Subsag is a main hydrocarbon generating area in the Huimin Sag of the Jiyang Depression, Bohai Bay Basin. Oilfields mainly are in the subsag or on the southern and northern faults. Overpressure is found in the Shahejie Formation. Drilling, drill stem test (DST), logging and seismic data as well as the Eaton formula were applied to study the measured pressure characteristics in sandstones, the correspondence between logging and overpressure in both sandstones and shales, and the plane and profile distributions and causes of overpressure. The overpressure depth from DST ranges 3 005 to 4 355 m in sandstones of the Shahejie Formation, the residual pressure is 7.95 to 30.45 MPa, and the pressure coefficient is 1.21 to 1.78. Logged acoustic velocity of shale and sandstone in the overpressure zone is higher than that in the normal pressure zone, and the logged resistance of the overpressure zone is also higher than that of the normal pressure zone. The upper section of the fourth member and the middle and lower sections of the third member of Shahejie Formation mainly develop low overpressure, while medium and strong overpressure also exist regionally. Vertically, overpressure zones mainly occur from 3 000 to 4 500 m depth. There are several medium and strong overpressure zones, mainly in the deep sag and fault zone. The top depth of overpre-ssure zone is 2 500-3 700 m. The high percentage of sandstone leads to the limited distribution of overpressure in the Linnan Subsag. The overpressured sandstone reservoirs in this sag are mainly oil-bearing layers. Hydrocarbon-bearingfluid charging is the main reason for the overpressure of sandstones in the third and fourth members of Shahejie Formation in the Linnan Subsag. The high-quality source rocks are deeply buried. The vitrinite reflectance of the overpressured source rocks is about 0.5% to 1.5%. It is in the oil generation stage and does not have low density characteristics, indicating that oil generation is the main reason for the pressurization of source rocks.