Formation of large- and medium-sized Cretaceous volcanic reservoirs in the offshore Bohai Bay Basin, East China

Based on the geological and geophysical data of Mesozoic oil-gas exploration in the sea area of Bohai Bay Basin and the discovered high-yield volcanic oil and gas wells since 2019, this paper methodically summarizes the formation conditions of large- and medium-sized Cretaceous volcanic oil and gas reservoirs in the Bohai Sea. Research shows that the Mesozoic large intermediate-felsic lava and intermediate-felsic composite volcanic edifices in the Bohai Sea are the material basis for the formation of large-scale volcanic reservoirs. The upper subfacies of effusive facies and cryptoexplosive breccia subfacies of volcanic conduit facies of volcanic vent-proximal facies belts are favorable for large-scale volcanic reservoir formation. Two types of efficient reservoirs, characterized by high porosity and medium to low permeability, as well as medium porosity and medium to low permeability, are the core of the formation of large- and medium-sized volcanic reservoirs. The reservoir with high porosity and medium to low permeability is formed by intermediate-felsic vesicular lava or the cryptoexplosive breccia superimposed by intensive dissolution. The reservoir with medium porosity and medium to low permeability is formed by intense tectonism superimposed by fluid dissolution. Weathering and tectonic transformation are main formation mechanisms for large and medium-sized volcanic reservoirs in the study area. The low-source “source-reservoir draping type” is the optimum source-reservoir configuration relationship for large- and medium-sized volcanic reservoirs. There exists favorable volcanic facies, efficient reservoirs and source-reservoir draping configuration relationship on the periphery of Bozhong Sag, and the large intermediate-felsic lava and intermediate-felsic composite volcanic edifices close to strike-slip faults and their branch faults are the main directions of future exploration.