Abstract: Different clay mineral assemblages and their replacement have been formed in different sedimentary and diagenetic environments in the western depression ( Huan-xiling, Xinglongtai and Maquanzhi ): 1.Montmorillonite-I/M-illite-kaolinite mineral assemblage 2.1/M-authigenic illite-atlthigenic chiorite mineral assemblage The weak acidic and weak alkaline environments were transformed into slight alkaine-weak reductive ones with the increase of depth and geothermal and mont-morillonite and I/M turned into illite and trioctahedron chlorite. In the north region of the western depression ( Gaosheng and the local area of Shuguang)t 1. I/M-illie mineral assemblage 2. Glauconite-trioctahedron smectite mineral assemblage 3. Fe-saponite mineral The Sha-4 stage in the area was a slight alkaline-weak reductive sedimentary environment without kaolinite mineral and Ca, Fe3 + Fe2+and Mg were rich in the environment due to the degeneration of the secondary saponite from basalt into trio-dioctahedron smectite. Together with Fe and Mg, K and Al supplied by solubilized K and Al-bearing terrigenous minerals under the higher geothermal moved into I/M crystal lattices and thus forming glauconite. The evidence of mineralogy has shown that the geochemical properties of the sedimentary and diagenetic environments are the principal factors which control low-temperature geochemial behaviors of clay minerals during the diagenesis, and secondly, the temperature also plays an important part during the diagenesis of clay minerals.