Characteristics Of Dolomite Reservoirs In Buqu Formation Of Middle Jurassic In Shuanghu Area,Qiangtang Basin Of Northern Tibet

Qiangtang basin is located in northern Tibet Plateau which is one of the key basins of oil and gas exploration in our country,and is the most potential basin of hydrocarbon exploration in the Tibet Plateau.These years,the localities shown hydrocarbon in this basin with industrial significance are almost discovered in the grain dolomites from Buqu Formation in middle Jurassic strata.It is significant that about hydrocarbon exploration and development by study on physical properties and rock cause of oil-bearing dolomite in Buqu Formation.Therefore,this research makes a detailed study on the Middle Jurassic dolomites in Shuanghu area,we research the characteristics of dolomite by its microstructure observation,cathodoluminescence,fluorescence luminescence,we also reseach the causes of dolomite through carbon and oxygen isotopes,fluid inclusions temperature and salinity analysis.There are two main sedimentary systems of Jurassic stratum in study area including platform system and deepsea-shelf system.The Middle Jurassic Buqu Formation dolomites are mainly developed in the northern platform system.In the study area,4 types of vertical sequences of Buqu Formation dolomite can be classified into 4 kinds of depositional sequences,the deposition sequences are transgressive sequence,-sequence,regressive-transgressive sequence and regressive sequence.Through outcrops and microscopic observations,the dolomites in the study area can be divide into 5 types according to grain size,which are powdery dolomite,fine dolomite,medium dolomite,coarse dolomite and unequal crystal dolomite,and it also can be divided into 5 types through the dolomite crystal structure which are powdery-finely planar dolomite,fine planar dolomite,finely-coarsely nonplanar dolomite,coarsely saddle dolomite and coarsely saddle dolomite cement.Cathodoluminescence and fluorescence microscopic investigations revealed that these dolomites had ring or ring structures,and different colors of the ring means they suffered different diagenetic processes.In addition,combined with the microstructure,carbon and oxygen isotopes,salt water inclusions temperature measurement and salinity analysis,burial history of Qiangtang basin,we believed that the dolomite in the study area are mainly burial-hydrothermal genesis.We speculated that the dolomite has experienced three dolomitization,respectively,quasi-synchronic dolomitization,buried dolomitization and hydrothermal dolomitizationThe pore types of dolomite in the study area can be divided into intergranular pores,intergranular pores,interplanar pores,dissolution holes,intergranular cracks and fissures,and the main reservoir space is the primary intergranular pores and intergranular dissolved pores.Generally speaking,the quality of the reservoir in study is medium-better,the porosity of the high quality reservoir is mainly dolomite with coarse grains.The development of dolomite reservoirs is controlled by the sedimentary facies and diagenesis.The karstification of syngenetic,epiphytic and burial periods has a good constructive effect on the physical properties of reservoirs.Compaction,pressure-dissolving,cementation and recrystallization act as destructive effects on reservoir properties.Only when the pores dissolved by the hydrothermal action are not filled it is possible to form a high quality reservoir.