Abstract:  The volcanic rocks of Yixian Formation in Bozhong sag of Bohai Bay basin are widely distributed, but the stratigraphic structure is complex and the lateral variation is fast. The division of volcanic eruption cycles is of great significance for studying the spatial distribution characteristics of volcanic rocks and guiding oil and gas development. Based on the geological data such as core, wall-core thin section, zircon U-Pb dating and logging and seismic data of volcanic rock strata in 38 wells, the volcanic eruption cycle, volcanic rock distribution and its relationship with faults, as well as the oil and gas geological significance of volcanic eruption cycle of Yixian Formation in Bozhong sag of Bohai Bay basin were studied by using volcanic stratigraphy method and seismic volcanic stratigraphy theory. The results show that:1) The volcanic rocks of Yixian Formation in Bozhong sag of Bohai Bay basin can be divided into three cycles. The first cycle is a basic cycle, which mainly develops basalt and basaltic tuff. The average drilling thickness is 189 m, and the eruption time of volcanic rocks is 128-122 Ma. The second cycle is a intermediate cycle, which mainly develops andesite and trachyandesite. The average drilling thickness is 232 m, and the eruption time of volcanic rock is 122-115 Ma. The third cycle is an acidic cycle, mainly developed rhyolite and crypto-explosive breccia rocks. The average drilling thickness is 241 m, and the volcanic eruption time is 115-114 Ma. 2) Volcanic rocks of Cycle 1 are the most widely distributed in the Bozhong sag, and only the Bonan low uplift, Shijiutuo uplift, and the northern part of the Bodong low uplift lack the strata. Their distribution is mainly controlled by the Yanshanian strike-slip faults and is dominated by fissure eruption. Volcanic rocks of Cycle 2 are widely distributed in Bozhong sag, only Bonan low uplift, Shijiutuo uplift and Bodong sag are lack of this stratum. Its distribution is mainly controlled by strike-slip faults in Yanshanian and pre-existing faults in Indosinian  in NWW direction, which is dominated by central eruption. The distribution area of the Cycle 3 acid volcanic rocks in the Bozhong sag is significantly reduced, and the formation is missing in a large area in the northern part of the Bozhong sag. Its distribution is mainly controlled by the strike-slip faults in the Yanshanian  and some pre-existing faults in the NWW Indosinian. 3) The volcanic rocks of Cycle 2 and Cycle 3 in this area have good reservoir physical properties, and the oil and gas display effect is good, which can be used as a favorable horizon for the exploration of volcanic oil and gas reservoirs in Yixian Formation of Bozhong sag in Bohai basin.

Key words: Bohai Bay basin, Bozhong sag, Cretaceous, volcanic rocks of the Yixian Formation, cycle division, stratigraphic distribution, zircon U-Pb dating, volcanic oil and gas reservoirs