Reservoir Characteristics Of Carbonate Rocks Of Cambrian Longwangmiao Formation In Zhenba Area,Northeastern Sichuan Basin

Longwangmiao formation is one of the important strata for natural gas exploration in Sichuan basin.In this research,the outcrops of Longwangmiao formation in Zhenba area,northeast sichuan province,is taken as the basic data of the research.Under the thin sections,cathodoluminescence,and porosity/permeability analysis,the sedimentation,reservoir characteristics,diagenesis and their influencing factors of Longwangmiao formation are studied.The results of thin section study indicate that the main rock types of Longwangmiao formation in Zhenba area are grainstone,Crystalline limestone,grain dolostone and crystalline dolostone.Outcrops observation shows that the study area has developed wavy bedding,horizontal bedding,biological boreholes,microbial induced sedimentary structures(MISS)and erosion surface,and these sedimentary structures are generally shown to be relatively high-energy environment.Combined petrology with sedimentary structure analysis shows that the Longwangmiao formation in Zhenba area is in the open platform environment,the main sedimentary subfacies are granular beach and inter-beach sea,and the corresponding microfacies are sandy bioclastic beach,inter-beach depression and calcareous mudstone of quiet water environment.From the north to the south of the study area,sandy bioclastic bank gradually decreased,still water mortar gradually increased.The analysis of reservoir characteristics shows that the main types of reservoir rocks are grain dolomite/limestone and crystalline dolomite/limestone.Intergranular dissolution pore,inter-crystalline dissolution pore and inter-crystalline pore are the main reservoir spaces.The porosity and permeability analysis show that the porosity ranges from 0.46% to 2.75%,and the permeability ranges from 0.0016 to 4.73 m D.The general performance is low—altitude low—permeability characteristics.The diagenesis types of Longwangmiao formation in Zhenba area are complex and varied.Micritization plays a preserving role in reservoir space development,and dissolution,early dolomitization,compaction,recrystallization,and fracture are mainly positive effects.Compaction,filling and cementation play a destructive role in the development of reservoir space.According to the time sequence,the above diagenesis can be divided into three stages:syngenetic diagenetic stage,shallow(early)burial diagenetic stage and middle-late burial diagenetic stage.The controlling factors of reservoir are analyzed in terms of sedimentary facies and diagenesis.The results show that:(1)in the comprehensive comparison of sand beach,inter-beach sea and depression three sedimentary microfacies,we find that sand beach is the most dominant reservoir;(2)seepage-reflux dolomitization and micritization have certain significance for reservoir pore preservation;Fresh water shower and burial karst are the most favorable diagenesis,although the mold holes formed by fresh water shower are filled by later cementation.Recrystallization optimizes reservoir quality as a whole;(3)cementation and compaction and compaction and dissolution have produced serious damage to the reservoir.