Reservoir sedimentological characteristics of the Upper Permian Changxing Formation in Kaijiang-Liangping area

Based on field section measurement,well core observation and thin section identification in combination with n(Mg)/n(Ca)ratio,the degree of order and C,O,Sr isotope characteristics of dolomites with different textures and genetic types,the authors made a synthetic study of the diagenesis,porosity development and evolution regularity of carbonate reservoirs of Changxing Formation in Kaijiang-Liangping area.The destructive diagenetic processes in the reservoirs of the study area are mainly cementation,compaction-pressure solution,neomorphism and secondary mineral filling,whereas the constructive diagenetic processes are mainly burial dolomitization,recrystallization,dissolution and fracturing,which effectively improve the reservoir porosity and permeability and constitute the basic characteristics of the reef and beach reservoir that has the combination of a variety of secondary porosity types such as intergranular pores,intragranular pores,moldic pores,intercrystal pores,superlarge dissolved pores,dissolved caverns and fractures.Dolomitization,as the most important and common diagenetic action,went through penecontemporaneous dolomitization in early diagenetic stage,burial dolomitization in early diagenetic stage,burial dolomitization in middle diagenetic stage,burial dolomitization and the formation of tectonoclastic dolostone in late diagenetic stage.Further studies suggest that carbonate reservoirs of Chang-xing Formation experienced a very complex diagenetic and evolutionary history,which can be divided into five phases:penecontemporaneous diagenesis,early diagenesis,middle diagenesis,late diagenesis and tectonic period.On the basis of the reconstruction of diagenetic sequence and the restoration of porosity evolution history,the authors have reached some conclusions:① favorable reef and beach facies determine the locations and spatial distribution of the reservoirs;② the multistage burial dolomitization of reef and beach complexes constitutes the basis of reservoir formation;③ fracturing and dissolution constitute the key to the formation of high quality reservoirs.