| This study is based on the observation of Zhangcun profile in Hongya andXibeixiang profile in Guangyuan, fully combining with drilling-related profiles, themiddle Permian series of northwestern Sichuan is divided into five three sequences,and thus establishing a Sichuan third-order sequence comparison grid northwest of thebasin. According to previous research results and comprehensive analysis of the studyfor the middle Permian carbonate platform sedimentary system, the mainsedimentary facies are open platform facies and platform margin shoal facies,sedimentary connected well profile is visible on section2,4,5sequence HST systemtract, which is more beneficial to the growth of the beach body on a flat surface areabeach body transgressive systems tract development in the period of relative internalplatform, HST, beach body advantages in developing regional platform edges.Dolomite of middle Permian is divided into powder-slender crystalline anhedraldirty dolomite, slender-medium crystalline automorphism-subhedral dolomite,fine-medium crystalline anhedral dolomite and saddle dolomite. Dolomite holes andcracks can also be found hydrothermal activity related to quartz, illite, fluorite, pyrite,barite, bitumen and other hydrothermal minerals. When the rock was completelydolomitized, the dolomite was produced in bulk, with the waning of thedolomitization degree, dolomite was produced in porphyritic, patch shape andleopard porphyritic.Vertically,in northwest Sichuan,dolomite only develops in theupper part in qixia group,in southwest sichuan dolomite develops not only in qixiagroup, the verticale distribution of dolomite has nothing to do with the sequenceinterface. The distribution of dolomite on the transverse has nothing to do with thesedimentation,is mainly controlled by faulting, from northwest to southeast, withtectonic setting, the thickness of the dolomite also gradually thinning.By CL analysis it can be found that powder-slender crystalline anhedral dirtydolomite is close to shine, slender-medium crystalline automorphism-subhedraldolomite, fine-medium crystalline anhedral dolomite and saddle dolomite shows theglowing red or bright red light. The degree of order of slender-medium crystallinedolomite and saddle dolomite are higher, average value is0.8.The saddle dolomite has the highest homogenization temperature, average value is156℃,250℃of the hightemperature underground buried depth should be about7700m,far above the middlePermian experienced the biggest buried depth in the study area, is the most favorableevidence of hydrothermalism. Slender-medium crystalline automorphism-subhedraldolomite have the lower homogenization temperature, average value is114℃, thehomogenization temperature average value of fine-medium crystalline anhedraldolomite is between114-156℃, as the same time, the salinity of saddle dolomite isaround double of the same period sea water. The δ13C value of the dolomite isgenerally higher, as the same time is similar with the δ13C value of the micrite, theδ18O value of powder-slender crystalline anhedral dirty dolomite is higher, averagevalue is-2.39‰PDB, the δ18O value of slender-medium crystalline dolomite andsaddle dolomite is lower, average value are-6.94‰PDB and-7.04‰PDB,muchhigher than the background value of micrite. Strontium isotopic composition ofSaddle dolomite and slender-medium crystalline dolomite is similarity, range islarger,performance is less than0.70600,0.70600-0.7100and above0.71000.Powder-slender crystalline anhedral dirty dolomite has the highest∑Ree in all typesof dolomite,which is1/3of contemporary micrite,but the distribution model has thecloseest similarity with micrite,the∑Ree of slender-medium crystalline dolomite andsaddle dolomite is broadly similar, which is about1/10of micrite, the distributionmodel has a certain similarity with micrite and igneous rock,the value of δCe and δEuall less then one in all types of dolomite.Crossplots of fluid-inclusion homogenization temperature against oxygenisotopic shows that slender-medium crystalline automorphism-subhedral dolomite,fine-medium crystalline anhedral dolomite and saddle dolomite are formed by hightemperature and high salinity conditions.The Mg2+of hydrothermal dolomite mainlycomes from the deep hydrothermal fluid associated with volcanism, clastic rockformation under the middle Permian and the seawater of same period maybe has acertain contribution to Mg2+of hydrothermal dolomite. According to the distributioncharacteristics of the hydrothermal sedimentary deposits and hydrothermalsedimentary siliceous rock strata in southern China, and the paleo-heat flow evolutioninversion results of Sichuan Basin, the author think that the middle Permian dolomiteemplacement came soon after deposition of the host, specifically,at the end of themiddle Permian to late the early Permian. Hydrothermal process is influenced by theemeishan mantle plume, from southwest to northwest in Sichuan the averagethickness is gradually thinning,the average value of δ18O is gradually ascending, the fluid-inclusion homogenization temperature is gradually reducing, which all aboutdolomite.The middle Permian reservoir space mainly includes intergranular holes,intergranular dissolution pores, caverns, the hydrothermal breaking cracks.The mainreservoir type is dissolution vug dolomite reservoir. Vertically,in northwest Sichuan,reservoir only devolops in the upper part in qixia group,in southwest sichuanreservoir develops not only in qixia group, the distribution of reservoir on thevertically has nothing to do with the sequence interface. Dolomite which ishydrothermal genesis has a high probability to reservoir. Hydrothermal process notonly has a constructive role, but also has a destructive role. Hydrothermal solutionpores has a high possibility to be saved as reservoir space from statistical fillingconditions of hydrothermal solution pores.The distribution of dissolution vugdolomite reservoir is similar to hydrothermal dolomite, controled by tensile fractureand the strength of the emei mantle plume, southwest region is the most favorablearea,the middle of Sichuan is favorable area, northwest region is more favorable area. |
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