Abstract: Based on the thin section examination,SEM,X-ray diffraction,this paper studied the diagenetic types and their control on reservoirs in the 8^th Member of Xiashihezi Formation(He8)tight sandstone in Gaoqiao region.The He8 reservoir has experienced complex diagenesis after deposition.Compaction resulted in the loss of primary intergranular pores.And cementation reduces the primary intergranular pores while increasing the strength of the skeleton particles,which is conducive to the preservation of residual primary intergranular pore.Replacement in the study area features in feldspar replacing kaolinite and iron calcite replacing quartzes debris.Dissolution improves the reservoir physical properties by dissolving the unstable detrital grains and cements.Fracturing causes the formation of micro-fractures.Quantitative evaluation of the increase and decrease of the porosity caused by different diagenesis reveals that compaction is the most significant in porosity reduction,being about 22.40%-33.17%,with an average of 27.166%,followed by cementation and replacement,bing about 5.5%-17%,with an average of 10.01%.In contrast,dissolution increases porosity about 6.86%-8.41%,with an ave-rage of 8.586%.The porosity of the tight sandstone is the final result of the different diagenesis.

Key words: porosity evolution, diagenesis, tight sandstone reservoir, Xiashihezi Formation, Gaoqiao area, Ordos Basin