DISTRIBUTION AND EVOLUTION CHARACIERISTICS OF ABNORMAL PRESSURE AND ITS RELATIONSHIP WITH HYDROCARBON ACCUMULATION IN THE DONGYING SAG OF BOHAI BAY BASIN

Numerical simulation method is applied to analyze distribution and evolution characteristics of abnormal pressure system in the Cenozoic Dongying Sag of the Jiyang Depression,the Bohai Bay Basin.The results show that there are extensive abnormal pressure systems in the 3rd and 4th members of Paleocene Shahejie Formation in the Dongying Sag,and they are mainly overpressure ones.Overpressure system has begun to develop since the Shahejie Formation deposits,and increases gradually until the terminal of Dongying episode.In the late Oligocene strata,uplifting and denudation which result from the 2nd episode of Himalaya Movement change the overpressure system into minus-pressure system.After this,fluid pressure of overpressure system increases once more along with the sedimentary period of Guantao Formation in Miocene,and achieves maximal area and maximum in the period of Minghuazhen Formation and Quaternary.At present the normal pressure system locates at the formation above 2 200 m in depth,and the overpressure system is formed at the formation below 2 200 m.High sedimentary rate of the Shahejie Formation makes the development of overpressure system during the early stage,whereas both clay mineral dehydration and hydrocarbon generation result in dramatically rising of fluid pressure of overpressure system since the Late Oligocene.Combined with research conclusion of hydrocarbon accumulation stages,overpressure systems which develop in the terminal of Dongying period,the terminal of Guantao period,and the Minghuazhen-Quaternary period have good relationship with the 3 hydrocarbon accumulation stages of the Dongying Sag.It suggests that hydrocarbon generation is an important reason for overpressure development;meanwhile,overpressure is a favorable power for hydrocarbon migration.