Overpressure characteristics and effects on hydrocarbon distribution in the Bonan Sag, Bohai Bay Basin, China

Overpressure is widespread in the Bonan Sag of the Bohai Bay Basin, East China, and has a strong influence on the migration and accumulation of hydrocarbons sourced from the Es3 and Es4 formations. Pressure measurements in drill stem tests and estimates from sonic logs yield overpressures among different reservoirs and mudstone beds. The fluid pressure in the Bonan Sag increases with depth and is divided into the upper, middle and lower overpressure units that correspond to the Es1, Es3 and Es4 formations, respectively. Oil-source correlation suggests that the distribution of hydrocarbons generated from Es3 and Es4 is controlled by the three pressure units. Hydrocarbons generated from Es4 source rock may laterally migrate within sandstones and accumulate in reservoirs in the Es4 Formation due to the gypsum seal in the lower overpressure unit. A small amount of hydrocarbons vertically migrated along active faults into a shallow formation during charging. However, the effects from faults were more significant on the migration and accumulation of hydrocarbons from the Es3 shale source in the middle overpressure unit. Hydrocarbons generated from the Es3 source rock can vertically migrate into the Es2 or Es1 reservoirs; however, most of them accumulated in Es3 reservoirs. Therefore, the exploration interval should be within the middle and lower overpressure units in the Bonan Sag or along the active fault within the upper overpressure unit.