Abstract:  Mesozoic volcanic rocks are widely distributed in the Bozhong depression of Bohai Bay basin. At present, volcanic oil and gas reservoirs have been found in several formations, which have great exploration potential. However, the complexity of the volcanic lithology and lithofacies in the northern part of the Bozhong depression and the non-homogeneity of the reservoirs due to multiple tectonic movements and fluid modification make the study of the mechanism of reservoir formation relatively weak at present. In order to clarify the main controlling factors of Mesozoic volcanic reservoir formation, this study analyses the petrological characteristics of volcanic rocks by core observation, thin section description, physical properties and major element data, classifies the volcanic facies based on the seismic data, and systematically summarizes the formation mechanism of the effective volcanic reservoirs and hydrocarbon accumulation features. The study shows that: There are 7 types of volcanic rocks and 6 kinds of lithofacies, the dominating reservoir space is pore + fracture network type. The intermediate-acid volcanic rock is the basic condition for the formation of high-quality reservoir, and the effusive intermediate-acid lava is the favorable lithology and lithofacies for the formation of high-quality volcanic reservoirs. The tectonic fracture and the dissolution of meteoric fresh water and organic acids in the later period ensure the effectiveness of high-quality reservoirs and greatly improve the porosity and permeability of volcanic reservoirs. The main hydrocarbon source rocks in the study area are directly connected to the volcanic buried hills, or can be effectively transported through faults and unconformities, and can be aggregated into reservoirs by developing multiple anticline trap in the main period of reservoir formation, while the Third Member of the Shahejie Formation and Dongying Formation, with thick layers of super-pressured mudstones, act as an effective cover and are steadily laterally distributed on the whole area. 

Key words: Bozhong sag, Mesozoic, volcanic rocks, reservoirs, reservoir space, volcanic oil and gas reservoirs ,