Diagenetic reservoir facies and diagenetic evolutionary sequences of extra-low permeability reservoir in Xu4 Formation of Fenggu Structure in western Sichuan

The reservoir of Xu4 Formation in Fenggu Structure of western Sichuan Depression belongs to the low porosity and extra-low permeability reservoir.Research has been carried out about the reservoir's diagenesis,diagenesis stage,diagenetic reservoir facies and diagenetic evolution sequence on the basis of analysis assay data.The diagenetic-reservoir facies are divided into 4 types,namely,the residual intergranular pores-dissolved pores diagenetic reservoir facies with weak compaction and strong dissolution,the dissolved pores diagenetic reservoir facies with compaction and dissolution,the dissolved pores-microporosity diagenetic reservoir facies with middle to strong compaction and weak dissolution and the tight diagenetic reservoir facies with strong dissolution.The formation conditions of the relic primary intergranular pores and secondary dissolution pores in the reservoirs are different due to their different diagenetic evolutionary sequences.The preservation of pores in reservoir is mainly controlled by the rock components,compaction of pores,the filling of authigenic minerals and the multi-stages dissolution,as well as the injection of acid fluid.