Mesozoic Volcanic Rock Series in Bonan Area of Bohai Bay Basin Structure-stratum Cycle and Reservoir Development Characteristics

[Objective] The Bonan area is located in the southern part of the Bohai Sea, which is penetrated by the Tanlu fault from south to north, and is accompanied by a series of NNE strike-slip faults and E-W and NWW normal faults.[Methods] In this paper, using high-resolution 3D seismic data, combined with drilling, logging and core thin section data, the Mesozoic stratigraphic units, key interfaces and fault systems in the study area are identified and tracked in detail, and the volcanic lithofacies and reservoir characteristics are summarized and analyzed.[Results] It is found that there are two groups of Mesozoic faults in Bonan area, which are NWW-trending Indosinian thrust faults and NNE-trending Yanshanian strike-slip faults. Both of them are reversed and transformed into normal faults under the extension of early Cretaceous. The formation of tensile faults causes deep magmatic materials to erupt upward along the faults to form a large number of volcanic strata on the surface. According to the lithologic difference and seismic reflection characteristics, the volcanic activity in Bonan area can be divided into three sub-cycles. Cycle 1 mainly generates overflow facies basic basalt; the second cycle mainly generates neutral andesite and trachyte; cycle 3 mainly generates intermediate-acid andesite and tuff. [Conclusion] The distribution of volcanic rock strata is related to the fault system. On the whole, Bonan No.1 and Dongzhi No.2 Mesozoic faults are used as the boundary, forming a huge Mesozoic volcanic basin. The volcanic facies in Bonan area are mainly divided into five types: volcanic channel facies, explosive facies, overflow facies, effusive facies and volcanic sedimentary facies. The reservoir space in volcanic rocks is rich, mainly including primary pores, secondary pores and fractures. Based on the differences in physical properties, volcanic reservoirs can be divided into three categories, namely, high porosity-high permeability reservoirs, medium porosity-medium permeability reservoirs and low porosity-low permeability reservoirs. The development of volcanic reservoir is mainly controlled by lithology and lithofacies, weathering, structure, fluid dissolution and other factors. According to the difference of main controlling factors, volcanic reservoirs can be divided into fracture-secondary pore reservoirs controlled by weathering and secondary pore-fracture reservoirs controlled by weathering-structure.