Abstract: Taking first member of lower Ganchaigou Formation(E₃¹) in Shaxi oilfield as study case,based on the analytic data of casting thin section,scanning electron microscope,electron probe energy spectrometry,cathodoluminescence microscope,x-ray diffraction of clay minerals,vitrinite reflectivity of well core samples,the diagenesis,diagenetic sequences and stages of the formation have been studied.Through analyses of effect on the reservoir property of diagenesis and its phases,it indicated that the genetic mechanism of the low permeability of this area is the filling and cementation of autogenetic mineral during diagenesis.According to the diagenesis and characteristic pore space combination,the reservoir rocks of E₃¹ can be divided into 4 types of diagenetic reservoir facies(DRF).Facies A: DRF with middle to strong dissolution of unstable ingredients to form secondary pores;facies B: DRF with middle to strong compaction and middle cementation with residual intergranular pores;facies C: DRF with strong carbonate cementation;facies D: DRF with strong cementation of early anhydrite.The characteristics and origin of each diagenetic reservoir facies have been described in detail.Finally the types and pore space evolution have been discussed.