Development Characteristics and Main Controlling Factors of Oil and Gas Reservoirs in Ultra Deep Mesozoic Volcanic Reservoirs in Bohai Sea: Taking Bozhong A Structure as an Example

Mesozoic volcanic rocks are widely developed in the Bohai Bay basin. However, the exploration of large-scale volcanic hydrocarbon reservoirs has remained a persistent challenge in deep buried-hill plays in the Bohai area. To investigate the primary controls on substantial hydrocarbon accumulation, this study focuses on the Bohai A Mesozoic volcanic rocks, which have recently achieved significant exploration breakthroughs in the Bohai Sea area, as the research target area. drilling and logging data, sidewall core and thin-section observations, petrochemical analyses combined with 3D seismic, it characterizes the lithology-facies associations, volcanic edifice types, reservoir space characteristics, and key factors governing hydrocarbon accumulation. Research has shown that the BZ-A structure develops three volcanic rock cycles from base to top: basic, intermediate, and acidic. The late stage acidic volcanic rock cycle is composed of multiple large and medium-sized acidic lava volcanic structures stacked together, and is a thick and stable distribution of rhyolite. It mainly develops three types of rock types: effusive facies including vesicular and massive rhyolites, and volcanic conduit facies represented by rhyolitic cryptoexplosive breccia, which have the lithological and lithological basis for developing high-quality volcanic rock reservoirs. Although primary porosity is limited, extensive secondary modification-driven by intense tectonic activity and multi-phase fluid dissolution—has developed abundant structural fractures and dissolution pores/fissures.. The reservoir is vertically divided into a surface weathering dissolution zone and an inner crack dissolution zone with a double-layer reservoir structure, and the overall structure is a large-scale high-quality fracture solution with high porosity and permeability. The favorable factors for oil and gas accumulation in BZ-A include high porosity and permeability acidic volcanic rock fracture solution bodies, ancient uplifted anticline traps, proximity to the Bozhong hydrocarbon rich depression, overlying thick layer overpressure mudstone, and late stage weak tectonic activity. Based on these findings, a comprehensive hydrocarbon accumulation model for ultra deep, large-scale Mesozoic volcanic reservoirs is established, which offers valuable insights for volcanic hydrocarbon exploration in the Bohai Bay basin and analogous settings worldwide.