Member 8 sandstone of the Upper Triassic Yanchang Formation is the major exploration target in Daijiaping area

Ordos Basin.Member 8 sandstone of the Yanchang Formation mainly consists of feldspathic litharenite and lithic arkose

currently is at the A phase of middle diagenesis stage

and is categorized into low-porosity and extremely low-permeability reservoir.The intensive diagenesis is main cause of reservoir densification.Compaction and carbonate cementation largely reduce the primary porosity

while chlorite cementation and feldspar dissolution improve the reservoir quality

and the alkaline-acidic-alkaline change of digenetic environment control the formation of clay mineral

the precipitation of carbonate cements

and the dissolution of feldspar.Five types of diagenetic facies are identified in the Member 8 sandstone of the Yanchang Formation according to multiple qualitative and quantitative parameters.Quantitative calculation of porosity evolution in different digenesis facies shows that the differences of primitive compositions among different diagenetic facies as a result of sedimentation is the material basis of the cementation types and the dissolution strength

while the differences of diagenesis types and intensity is the main cause of the differences in porosity evolution and reservoir quality.The distribution of high quality reservoirs is closely related to the diagenesis facies.The weakly-compacted chlorite cement facies and modera-tely-compacted secondary dissolution facies in subaqueous distributary channel sandstone show the best reservoir quality

thus are the most favorable diagenesis facies in the Member 8 sandstone of the Yanchang Formation reservoir.