Study On The Diagenesis And Dolomite Genesis Of The Lower Qiulitage Group In The Central Part Of The Shaya Uplift

Shaya uplift is an important prospect area of the Tarim Basin,and recent data revealed favorable prospect of Cambrian strata on Shaya uplift.Previous study have been made in characteristics of sedimentlogy,reservoir and diagenesis of carbonate rocks of Cambrian Lower Qiulitage Group,but the research about fluid origin,dissolution mechanism and genetic mechanism of dolomitization is not profound.Hence it's necessary to study diagenesis and genetic mechanism of dolomite of Lower Qiulitage Group in Shaya uplift,to make it clear that different diagenetic effect on reservoir and expect to guide prospect of deep oil field in the basin.Based on extensive data collection and utilization of carbonate petrology,geological geochemistry,reservoir geology,diagenesis and relevant theories,the diagenesis and genetic mechanism of dolomite of Lower Qiulitage Group are analyzed with petrographic section,cathodeluminescence,scanning electron microscope,electron probe,microelement,X diffraction analysis,fluid inclusion,isotopic analysis and other technique means in Shaya uplift.The results can be summarized as follow:The lithology of dolomite in study area contains crystalline dolomite,residual granular dolomite,grain dolomite,micrite dolomite,laminar algal dolomite,breccia dolomite and little limestone in reservoir of Lower Qulitage Group.Crystalline dolomite is the primary type while residual granular dolomite,grain dolomite and micrite dolomite are secondary.The major diagenesis types of Lower Qiulitage Group include compaction,cementation,neomorphism,excessive dolomitization,silicification,filling,dolomitization,dissolution and fracture.Dissolution,dolomitization and fracture are most constructive to reservoir.The supergene karst is intensively disturbed in Yaha area and wells XH2,DG1,DG2;And massive pores in well TS1 are originated from contemporaneous dissolution.The burial dissolution can be divided into intergranular dissolution and later hydrothermal non-selective dissolution,which generates massive secondary pores.The secondary pores are filled by bitumen and hydrothermal minerals such as pyrite,quartz,calcite,saddle dolomite,anhydrite,and greatly destroyed by filling of quartz.Contemporaneous dolomitization and shallow burial dolomitization are advantageous to reformation and preservation of reservoirs space;Medium-coarse crystalline dolomite generated from deep burial dolomitization reduces porosity mostly.The dolomite origin models of Lower Qiulitage Group in middle Shaya uplift include seepage reflux dolomitization,shallow burial dolomitization and deeply burial dolomitization and tectonic-hydrothermal dolomitization,with corresponding products as grain dolomite,silty-fine crystalline dolomite,medium-coarse crystalline dolomite and saddle dolomite,respectively.Moreover,various dolomitization has different effects on reservoirs.Through the analysis of diagenesis characteristics,combining with diagenetic sequence and dolomite genesis,we can conclude the pore evolution pattern of two rock types in the study area.The porosity is controlled mainly by contemporaneous dissolution,karstification,submarine cementation,burial dolomitization,hydrothermal dissolution,hydrothermal fluid filling and organic acid dissolution.