Abstract:

Mesozoic volcanic rocks widely developed and show significant hydrocarbon potential in Bozhong Sag, Bohai Bay basin. However, there is still a lack of in-depth understanding of the reservoir-forming mechanism of high-quality Mesozoic volcanic reservoirs. Based on the analysis results of blue dye thin section, scanning electron microscopy, physical property test, high-pressure mercury injection, nuclear magnetic resonance, and nitrogen adsorption, the reservoir spaces, porosity and permeability, microstructure of pores are analyzed, and the forming mechanism and distribution of high-qualitty reservoir are discussed. The Mesozoic volcanic reservoirs were divided into two types, high porosity with mid-low permeability (Class Ⅰ reservoir), and mid porosity with low permeability (Class Ⅱ reservoir). The low porosity with low permeability volcanic rocks can not form effective reservoirs (Class Ⅲ, non-reservoir). The Class Ⅰ reservoir is composed of intermediate-acid vesicular lava altered by cryptoexplosive brecciation and dissolution. The Class Ⅱ reservoir is composed of massive intermediate-acid lava altered by cryptoexplosion brecciation and dissolution, or basic vesicular lava altered by dissolution. The main forming mechanism of favorable volcanic reservoirs in the Bozhong sag benefitted from the primary pore, fracture, dissolution, and devitrification. A reservoir development model is established, which is characterized by vertical multi-layers and lateral multi-centers of high-quality volcanic reservoirs under weathering background. Two types of volcanic bodies, multi-eruption unit, multi-mound superimposed acid composite lava volcano, and intermediate-acid intrusive lava dome, are important exploration targets for high-quality volcanic reservoirs in the Bozhong sag.

Key words: Bohai Bay basin, Bozhong sag, volcanic reservoir, reservoir development model, exploration target