Abstract: The Chang 4+5 reservoir of Yanchang Formation in Zhidan-Danba area are mainly composed of lithic feld spathic and feldspathic sand stones,in which the average content of quart,feld spar and rock fragments are 25%,46% and 8.4% respectively.Metamorphic and igneous fragments are dominant constitute.The major diagenesis of reservoir is mechanical compaction,cementation,replacementation and dissolution.At present,these sand stones are in the lated iagenctic Astage(Ro=0.66～0.83,and T=90～130℃),so they are under the control of acidic diagenetic environment being of abundant secondary dissolution porosity.Base on the statistics of sand stone compositions,it is indicated by the quantitative calculation that compaction is the dominan treason for the loss of the primary porosity in the study a rea and next cementation.Where as the increa singration of porosity caused by the dissolution is relatively high (48%～70%).So it is the dominant part of the porosity in the study aera.