Diagenetic Evolution Sequence of Lower Triassic Feixianguan Excellent Oolitic Beach Reservoir in Northern Sichuan Basin

A platform/trough in-between sedimentary pattern developed in northern Sichuan Basin during Early Triassic Feixianguan age.Thick carbonate oolitic beach reservoirs of platform-marginal facies are developed in the transition zones between platforms and troughs.The reservoirs underwent both destructive and constructive diagenesis.The destructive diagenesis includes compaction,pressolution and cementation and the constructive diagenesis are micritization,neomorphism and recrystallization,dolomitization,dissolution.The main characteristics of these diagenesis and the influence to reservoirs are discoursed.Combined with every diagenetic stage,a diagenetic evolution sequence of reservoirs is set up.Cementation can be divided into three phases,all of which acted as heavy destruction,especially after the second phase of cementation,the porosity in reservoirs decreased to about 5%.Dolomitization includes mixed-water dolomitization and burial dolomitization,which not only increased porosity by about 5 percent but also made a great number of intercrystalline pores form to create favorable rooms for the post-burial dissolution.Burial dolomitization can enlarge intercrystalline pores in dolostone to form intercrystalline dissolution porosity that can increase about 5% to 20% of additional porosity,which is a necessary condition to be high quality dolostone reservoirs.