Carboniferous volcanic weathered crust, whose petrophysical feature is better than that of contemporary primary volcanic reservoirs, is one of the major volcanic reservoirs in Junggar Basin. The observation of casting thin sections and drill cores from Dixi area in Luliang Uplift shows that the improvement by weathering and leaching is more effective at pore space than fracture. The altitude of volcanic rocks,influenced by tectonic settings and palaeogeomorphology,determined the weathering duration. Ages of the strata overlying the Carboniferous vary from the Lower Permian to the Cretaceous,and suggest the duration of weathering changes greatly in different areas. The longest weathering duration is about 150Ma in eastern uplift,100Ma in Luliang uplift and only 55Ma in western uplift. The shortest stratigraphic break in the three uplifts,however, only lacks part of the Upper Carboniferous and Lower Permian. There is a wave corresponding relationship between the weathering duration and the volcanic reservoir porosity. The porosity increases with weathering duration changing from 0 to 30Ma,gets to the peak when weathering duration is about 40Ma,and decreases as the weathering duration gradually increases from about 55 to 100Ma. Concerning the lithological influence on porosity,volcanic lava and volcanic breccia have the largest porosity,andesite comes second and tuff has the smallest porosity.