Typical disequilibrium compaction caused overpressure of Paleocene Dongying Formation in northwest Liaodongwan Depression, Bohai Bay Basin, China

Bohai Bay Basin is an important petroliferous basin in China. Petroleum potential in Liaodongwan Depression of offshore Bohai Bay Basin has been proved by previous exploration and production, and overpressures were encountered. Summarized by the DST measurements and mud weight data of over 60 wells in Liaodongwan Depression, the abnormally high pressures preserved in Paleocene Dongying (Ed) Formation distribute over the entire depression and could reach as high as 64.2 MPa around the depth of 3420 m. Through the well logs analysis, typical corresponding characteristics to disequilibrium compaction can be found: higher in acoustic transit time and lower in density than normal compaction trend. Based on these corresponding features the pore pressure prediction was carried out for all the wells in the area. Sonic velocity-effective stress relationship and sonic velocity-density cross plots were employed to investigate the origin of overpressure in Ed Formation. Sedimentation rates and geothermal features were studied as well. Combining all the analysis, the overpressure of Ed Formation in northwest Liaodongwan Depression was dominantly generated by disequilibrium compaction. With this widely distributed overpressure, thick layers of mudstone of Ed Formation serve as a regional overpressured caprock.