Evolution of abnormal pressure in the Paleogene Es3 formation of the Huimin Depression, Bohai Bay Basin, China

The evolution of abnormal pressure is of importance for the analysis of hydrocarbon migration and accumulation processes. However, paleo-pressure reconstruction is still a challenge with great uncertainty. In this study, the PVT simulation method and homogenization temperature-salinity method were used for abnormal pressure evolution analysis in the Huimin Depression, which is characterized by a complex distribution of abnormal pressure, including overpressure, normal pressure and underpressure. The origins of the abnormal pressure conditions were also analyzed to examine the rationality and reliability of paleo-pressure reconstruction by basin modeling and quantitative calculation. The results revealed different evolutionary processes for the paleo-pressure in the Central Deep-Sag Zone (CDSZ) and the Northern Tectonic Uplift Belt (NTUB). In the CDSZ, the abnormal paleo-pressure exhibited a rising stage during the late Oligocene owing to disequilibrium compaction. From the early to middle Miocene, the paleo-pressure decreased to hydrostatic pressure because of the elastic rebound of rocks and temperature reduction. From the late Miocene to middle Pliocene, the paleo-pressure rose again because of the deposition and hydrocarbon generation. From the late Pliocene to the present, the paleo-pressure decreased again owing to the termination of hydrocarbon generation and secondary migration of hydrocarbons. In the NTUB, the paleo-pressure remained hydrostatic without disequilibrium compaction during the late Oligocene, followed by an intense reduction process due to rock elastic rebound and temperature reduction owing to strong tectonic uplift from the early to middle Miocene. From the late Miocene to middle Pliocene, the paleo-pressure rose because of the deposition and hydrocarbon injection from the CDSZ. From the late Pliocene to the present, the termination of hydrocarbon generation, weakening of hydrocarbon injection from the CDSZ, and the gas diffusion led to the decrease of pressure again in the NTUB. This study reveals that the paleo-pressure experienced two rising and two falling stages in the CDSZ and finally is characterized from normal pressure to overpressure. The paleo-pressure in the NTUB experienced one rising and two falling stages and is currently characterized by underpressure. This study provides a new approach for assessing the paleo-pressure evolution based on multiple analytical methods and further contributes to research on hydrocarbon-accumulating dynamics and hydrocarbon-migrating processes.