Research On The Genesis Of Dolomite In Changxing Formation Of The Permian In Tieshan-shuangjiaba Area,Northeast Sichuan

As the main reservoir rocks of Changxing Formation in the northeast Sichuan,this study,Changxing Formation dolomite in the Tieshan-Shuangjiaba Area was analyzed from perspectives of the structure-genesis,original depositional environment,diagenesis mode and diagenetic fluid properties,and geological data were fully integrated including the dolomite order degree,cathodeluminescence,SEM,trace elements,and stable isotopes.It is demonstrated that: 1)the dolomitization of Changxing Formation microcrystalline dolomicrite took place in the restricted sea basin behind the platform marginal reef that was characterized by the relatively closed environment and strong evaporation.Meanwhile,parent rocks of biogenic dolomite and crystalline dolomite were identified to be reef limestone and bioclastic limestone,which were originally deposited in the platform margin reef flat or intra platform reef flat.2)Geochemical characteristics of the dolomite inheriting properties of parent rocks significantly varied in different facies,which can be attributed to different sedimentary characteristics:?1?The microcrystalline dolomicrite of the restricted sea basin behind the platform marginal reef gets a low dolomite order degree?Average0.5?,high Sr trace element and low Fe and Mn trace elements.Also it gets a positive biased characteristics of ?¹³C and ?¹⁸O?2?biogenic dolomite and crystalline dolomite of the platform margin reef gets a high dolomite order degree?Average0.8?,compared to the biogenic dolomite and crystalline dolomite in the platform higher Mn trace element and lower Fe and Sr trace elements.Also it gets a positive biased characteristics of ?¹³C and?¹⁸O but a lower?¹³C and?¹⁸O.3)dolomicrite formed during the penecontemporaneous period at near-surface and falls in the category of evaporating pump dolomitization with the dolomitization fluid of penecontemporaneous high salinity evaporated seawater,with high salinity and reducible fluid properties and the Mg2+required for dolomitization is presumed to provide a mechanism for the evaporation of brine by means of evaporation.4)The biogenic dolomite and crystalline dolomite were thought to fall in the category of thermal convection dolomitization in the shallow to middle-buried diagenetic environments with the dolomitization fluid of the synsedimentary high salinityevaporated seawater sealed in the stratum.with high salinity and lack of fluid properties of continental fresh water,the mechanism of Mg2+ is presumed to be the thermal convection of the fluid in the platform...