Characteristics and Generation of the Reservoir Overpressure in Bozhong Depression, Bohai Bay Basin

The analysis of fluid migration and rapid hydrocarbon accumulation is essential for exploring distribution of overpressure and probing into its generaion.Based on the RFT of many drills in Bozhong depression, the characteristics and generation of reservoir overpressure have been discussed in this paper.Three reservoir overpressure systems have been identified in the Bozhong depression: an upper reservoir overpressure in Guantao Formation-Minghuazhen Formation, a middle reservoir overpressure in Dongying Formation-Shahejie Formation-Kongdian Formation, and a lower reservoir overpressure in base (Pre-Cenozoic).Detailed investigation of reservoir overpressure in the Bozhong depression shows that overpressure probably is the result of a combination of following three mechanisms: mechanical compaction disequilibrium, overpressure transference and thermal cracking.Transfer overpressure maybe is common in subsurface reservoirs, but recognizable transfer overpressures are infrequent.A few transfer overpressures preserved in reservoirs at shallower depths named "overpressure fluid fossil", resulted from deeper overpressured fluid charging, helps us to understand the rapid petroleum accumulation in overpressured sedimentary basins.