Origin And Reservoir Significance Of Oolitic Dolomite

Oolitic dolostone has attracted continuous attention of scholars for its complex genesis and as an important reservoir rock.The study of oolitic dolostone at home and abroad mainly focuses on macroscopic structure,while the study of microscopic structure,their genesis and their effects on reservoir capacity is relatively few.Taking the Lower Triassic Feixianguan Formation in Shangba Township,Yunyang County,Chongqing as the object of study,this paper dissects the dolomite crystal shape,automorphic degree,grain size,distribution and the relationship with calcite cements in oolitic dolostones,clarifies the microscopic structure characteristics of oolitic grains,and analyses the dolomite,calcite cements,cathodoluminescence and trace elements analysis,carbon and oxygen isotope analysis,order degree analysis,fluid inclusion of residual components of oolitic grains in and between oolitic grains,to explore the origin of oolitic dolomitization,establish diagenetic evolution sequence and evolution model,divide the types of reservoir space in oolitic dolostones,analyze the influence of dolomitization,cementation and dissolution on reservoir space,and establish pore evolution model.The oolitic dolostone of the Feixianguan Formation in the research profile is mainly developed in the 2nd section of Feixianguan Formation.According to oolitic content and dolomitization degree,the oolitic dolostone is divided into 37-48 layers with 12 layers,with a total thickness of 60.25m.The dolomitized ooids can be divided into two types:complete dolomitized ooids and incomplete dolomitized ooids.The latter can be further divided into shell dolomitized ooids,core dolomitized ooids,shell dolomitized ooids and core dolomitized ooids.The crystal type,automorphic degree,grain size and intergranular characteristics of dolomite in different types were analyzed.The corresponding relationships among ooids characteristics,dolomite crystal shape,automorphic degree,grain size,intergranular characteristics,oolitic contact relationship and undolomitized component characteristics were established,and the identification criteria for each subclass were established.In terms of genesis,the complete dolomitized ooids in the lower 2nd Member of Feixianguan(37-43 layers)are similar to those in the shell,core and shell dolomitized ooids.The ooids are mainly round and elliptical,with a little deformation and no fragmentation,and the dolomite in the ooids is semi-automorphic.Under cathodoluminescence,the dolomite grains emit very faint maroon cathodoluminescence,and with weak maroon rims,and trace elements have the same characteristics.It has lower Mn,Fe and higher Sr content,which is the result of dolomitization mainly by reflux seepage and buried dolomitization in a small amount.The microscopic characteristics of complete dolomitized ooids in the upper 2nd Member of Feixianguan(44-48 layers)are similar to those in the upper part.Under cathodoluminescence,most of the dolomite grains have very faint maroon cathodoluminescence,a few of them are weak maroon,have weak maroon cathodoluminescence ring edge,wide ring edge width,higher Mn,Fe and lower Sr content,and are mainly dolomitized by reflux seepage.During burial period,dolomitization is strongly reformed.Result.The core dolomitized ooids are mainly elliptic and almost all deformed,most of which are broken.The dolomite in oolitic grains is self-shaped,and the shell is not dolomitized.Under cathodoluminescence,the dolomite grains emit very weak maroon light with a weak maroon rim.The trace elements have lower Mn,Fe and higher Sr content,which may provide the core of Mg for the dark grain layer of ooids.Later,it experienced the result of a small amount of transformation of buried dolomitization.Reservoir space of oolitic dolostone in the 2nd Member of Feixianguan includes macroscopic reservoir types such as karst cave,structural fracture and structural dissolution,and microscopic reservoir types such as intergranular pore,intragranular pore,intercrystalline pore,corrosion pore and micro-fractures.The formation of reservoir mainly experienced fibrillary calcite cementation in seabed diagenetic environment,atmospheric fresh water and mixed water erosion,dolomitization,cementation,dolomitization,tectonic rupture,dissolution and cementation in burial period.Among them,Due to the difference of radius Ca and Mg ion,the volume in the process of dolomization decreases and intercrystalline pores are formed.Meanwhile,the supporting and stable chemical action of dolomite makes the early pores of oolitic dolomite well preserved after entering the burial period.During the dolomitization stage of burial period,a large number of intercrystalline pores were formed due to the modification of dolomitized fluids to dolomite formed in the early stage,and some of the migration channels of fluids were well preserved.