Research On The Formation Mechanism Of High-quality Reservoirs In The Middle Permian Qixia Formation In Western Sichuan

Based on the principles of carbonate sedimentology and reservoir geology,result of well drilling,well logging,test and seismic prediction,sedimentary characters,formation mechanism,control factors and distribution of carbonate reservoir of Qixia Formation(Middle Permian)in western part of Sichuan basin are docunmeted by detailed petrographic investigations and integrated outcrop,core,thin section,SEM,cathodoluminescence,fluid inclusions with homogenization temperature and stable isotope,trace elements and rare-earth element analysis.The main understandings and progresses are summarized as follows:Strata of Qixia formation in western Sichuan are commonly stable and lateral continous except the absent of the top of Maokou Formation in southwestern Sichuan.Depositional environments of middle Permian are NE direction dip and destabilization carbonate ramp to basin,in which high energy bioclastic banks of shallow ramp are favorable area.Carbonate reservoir of Qixai Formation are affected by complex diagenesis environments and evolutionary sequence,Diagenetic types are main conclude compaction,pressolution,cementation,dolomitizaion and dissolution,in which compaction,pressolution,and cementation are dominant factor of primary pore damaged,dolomitizaion and dissolution are key of secondary pores formed.Reservoir space formed by rock-fluid-tectonic combined action are characterized by intercrystal pore,intercrystal dissolved pore,secondly are cave and fracture,ensemble pertain to pore-fractured reservoir and fractured reservoir.Rock Physical property is bad and lower pore and lower permiablity.Dolomites of the Middle Permian of the Qixia Formation in western Sichuan Basin are significant hydrocarbon reservoirs.Leopard dolomite are distributes parallel with LongMen Moutain faults,which generated by shallow dolomitization are in the upper of the Qixia Formation.Sucrosic dolomites and grained-dolomites which generated by burial dolomitization are in the middle of the Qixia Formation.The dolomites that formed by composite and multiphase occur as both burial dolomitization and are partly affected with fault-related hydrothermal alteration.Three main factors are occurred that affect reservoir in Qixia Formation:Sedimentary facies are essential for generation of reservoir,dolomitization and dissolution in anadiagenetic stage are critical for expand of reservoir space,tectonic break are necessary for connection of reservoir space.Integrated study shows JiuLongShan-HeWanChang,LaoGuanMiao areas are excellent part in northwestern part for hydrocarbon exploration,secondly are KuangShangLiang of GuangYuan to TongKou of BeiChuan.ZhouGongShan-HanWangChang areas are favorable area in southwestern part,secondly is YouGuanDing area.Meanwhile,crust of weathering in the top of Maokou Formation is widely distributed by the DongWu Movement.Small-size dolines have good exploration potentianl of karstic reservoir,which developed in JiuLongShan and KuangShanLiang area of northwestern Sichuan.