Characteristics and controlling factors of volcanic clastic rock reservoirs in Wujiaping Formation of Upper Permian in northern Sichuan Basin

Under the influence of the Emei Taphrogenesis, a set of volcanic clastic rocks were developed in the Wujiapingian age of Permian in the northern Sichuan Basin. The lithology is highly heterogeneous, and the pore structure of reservoir is complex. In order to further clarify the characteristics and formation mechanism of volcanic clastic rock reservoirs in the Wujiaping Formation and confirm the exploration potential, this paper analyzes the reservoir characteristics and formation mechanism of volcanic clastic rock reservoirs in the Wujiaping Formation in the northern Sichuan Basin based on outcrop and actual drilling data and combining with core section observation, geochemistry analysis, argon ion buffing scanning electron microscope, combined determination of cap micro pores and other methods. It is concluded that the volcanic clastic rocks can be divided into three rock types: tuffaceous mudstone, argillaceous tuff and sedimentary tuff. The reservoir space is dominated by nano-scale clay mineral shrinkage pore and organic matter pore and micron-scale organic acid dissolution pore and micro fracture. The reservoir physical property is characterized by medium to high porosity and ultra-low permeability, and the tuffaceous mudstone has the best physical property among the three rock types. The analysis of the controlling factors of the reservoir shows that the near-crater slope and shelf environment is favorable for the development of volcanic clastic rock reservoirs, and the devitrification of volcanic ash and secondary organic acid dissolution are the main causes for the formation of pores in the reservoir.