Genetic mechanism of the Lower-Middle Ordovician Yingshan Formation reservoir in the Tarim Basin becomes a focus in the current exploration of the basin.Samples were taken from wells in Tahe area and Keping-Bachu outcrops to study the development and evolution of diagenesis in parasequence

parasequence set

third-order sequence stratigraphic framework

through petrology and geochemistry methods.The results show that the sedimentary facies significantly control the diagenesis.Tidal flat sedimentary environment is characterized by three zones:marine phreatic diagenesis in the subtidal

marine diagenetic environment and meteoric vadose environment in the intertidal

and meteoric water seepage cements associated with karst at the top of the parasequence in the supratidal.Regressive parasequence set developed syngenetic meteoric dissolution in early transgression

and marine diagenesis in late transgression stage.During early highstand period

parasequence were still affected by seawater diagenesis until the top of late thinning parasequence is exposed to subaerial environments.In the third-order sequence of the Lower Yingshan Formation

transgression is characterized by paleokarst controlled by parasequences and apparently increasing cementation to the down-dip.Weak karst developed du-ring the late highstand.Intense burial dolomitization appeared in the Lower Yingshan Formation.The remnant connate seawaters could be the main diagenetic fluids of dolomitization.In the third-order sequence of the Upper Yingshan Formation

seawater cementation is widely developed near marine flooding surface.The rapid transgression resulted in the development of weak syngenetic meteoric diagenesis.Epidiagenesis controlled by fractures developed in the Upper Yingshan Formation.