Abstract: The volcanic gas reservoir of Yingcheng formation in Xushen gas field in Xingcheng region is dominated by acidic volcanic rocks with reservoir spaces like primary pore, secondary pore and fissure. The reformation effect of hydrocarbon alkali fluid on the volcanic rocks appears in the formation and development of secondary pore zone, which lies on the scale of material inflow and outflow in the reservoir caused by this fluid. The volcanic reservoir reformation by the fluid under open system includes primary mineral metasomatic alteration and dissolution, which leads to damage of inner structure of the rocks, such as mechanical strength reducing, porosity increasing. The structure is very favorable for structural fragmentation, attracting hydrothermal liquids and mineral enrichment. The hydrocarbon alkali fluid can not only cause porosity and permeability increasing by the alteration and dissolution of primary minerals, but also make porosity and permeability decreasing by packing or filling pores. The study shows that the particular lithology and lithofacies as well as structural factors in this region lead to that the outflow rate is larger than the inflow rate by hydrocarbon alkali fluid, the developed secondary pores could be connected into secondary pore zones, which can be as good migration channels and reservoir spaces.

Key words: Xushen gas field, hydrocarbon alkali fluid, secondary pore, metasomatism, volcanic rock