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Weathered clastic crust can be subdivided into weathered clay and leached zone in terms of variable weathering of different minerals and mobility of weathered products. On the basis of clastic outcrops and well cores in the Junggar Basin, the dark red Fe-rich weathered clay is formed in an arid environment, whereas the light blue Al-rich weathered clay under humid conditions. According to the geochemical analysis, a new weathering index for weathered clastic crust is built mainly on Fe and Al contents, accurately indicating the weathered clay, sandy leached zone, and muddy leached zone in the Junggar Basin. The breaking pressure of weathered clay is rather large, the same as that of normal muddy cap, effectively to seal oil or gas. The porosity of underlying leached zone is greatly enhanced by weathering and leaching, but its permeability is a function of clay mineral content, i.e., the higher the clay content, the worse the permeability. Weathered crust provides effective sealing conditions for both top and bottom layers of a petroleum reservoir, and is important in the clastic hydrocarbon exploration.