Abstract: The gas reservoirs were mainly accumulated in lower member of the Neogene Minghuazhen Formation (lower Minghuazhen member) in Shijiutou bulge, Bozhong Sag, Bohai Bay Basin. However, the exploration results were unsatisfactory due to the consequent faults in the slope zone in the lower part of the bulge and the fact of “hydrocarbon passing through without retention” in Guantao Formation transport layer. Based on the previous study, and combined with abundant seismic, well drilling and geochemical data, the hydrocarbon migration path in the eastern Shijiutuo bulge has been analyzed in detail, clarifying the migration mechanism in the slope zone of the bulge.The study results indicate that Shinan No.1 Fault, a boundary fault on the south side of the bulge, had a greater opening coefficient in Guantao Formation than in Dongying and Minghuazhen formations. The hydrocarbon filling degree in traps in Guantao Formation in the bulge slope zone in the upper fault plate was much higher than the steep slope zone in the lower fault plate, controlling the upward hydrocarbon migration from sag area to bulge zone and the lateral diversion in Guantao Formation, with the advantageous diversion direction of the bulge zone. The heterogeneous transport layer in Guantao Formation and slope gradient controlled the oil accumulation in the slope zone, and the microscopic bed plane “lithologic traps” were formed by sandstones with different physical properties. On this basis, when the slope angle of convergent ridge in Guantao Formation was less than 1°, a large amount of reservoir space was formed for oil accumulation. Affected by tectonic activity of consequent faults in the late stage, oil and gas in Guantao Formation in the slope zone migrated to the lower Minghuazhen member sand reservoir and accumulated on a large scale. The above new migration pattern of “hydrocarbon convergence in gentle slope–transit by fault” has broken through the traditional understanding that hydrocarbon “passing through without retention” in the transport layer in the consequent slope zone, and successfully discovered Qinhuangdao 27-3 Oilfield in the lower Minghuazhen member in the slope zone in the eastern Shijiutuo bulge with reserve level of 100 million tons, providing a reference for the subsequent exploration outside source rock in the slope background in Bohai Bay Basin.

Key words: Bozhong Depression, Shijiutuo uplift, shallow reservoir of Minghuazhen Formation, Guantao Formation passage, accumulation model, exploration practices