Sedimentary Reservoir Characteristics Of Sinian Dengying Formation In Eastern Sichuan Basin

In this paper,petroleum geology,sedimentary petrology,oil and gas reservoir geology and other courses as the theoretical basis,through the field outcrop,core data,thin section data,logging data and seismic data,using scanning electron microscopy,cathode luminescence and other technical means,the Dengying formation,sedimentary facies and reservoir characteristics in the eastern Sichuan area are systematically studied.The following achievements and understandings have been achieved:(1)The Sinian Dengying Formation in the study area is composed of argillite-fine-grained dolomite,silty-fine-grained algal dolomite,sand-gravel dolomite,with a small amount of argillaceous dolomite,sandy dolomite,siliceous dolomite,mudstone,shale,etc.According to lithology and electrical characteristics,the first member of Dengying Formation,the second member of Dengying Formation,the third member of Dengying Formation and the fourth member of Dengying Formation can be divided from bottom to top.the second member of Dengying Formation and the fourth member of Dengying Formation are rich in algal dolomite,while the third member of Dengying Formation develops argillite and mudstone.Under the influence of tectonic activities,stratum thickness thinned to the southeast of the basin in plane.(2)The Sinian Dengying Formation in the study area is mainly carbonate platform deposition,the first member of Dengying Formation,the second member of Dengying Formation,the fourth member of Dengying Formation developed restricted platform facies,the third member of Dengying Formation developed mixed platform facies;The subfacies of granular beach,tidal flat and lagoon in restricted platform can be divided according to the sedimentary facies markers of algae,paste and mud in rocks.In particular,microfacies such as algal dune beach,paste lagoon and mud cloud flat in platform are developed.(3)The Sinian Dengying Formation reservoir in the study area consists of the second member of Dengying Formation and the fourth member of Dengying Formation;The reservoir rocks are silt-fine-grained algal dolomite,silt-fine-grained dolomite and sand-gravel dolomite.The reservoir space is the karst cave,dolomitization,recrystallization,semi-filled small fractures and so on.The average porosity and permeability of the second member of Dengying Formation were 1.92% and 0.01 m D respectively.The average porosity and permeability of the fourth member of Dengying Formation were 0.6% and 0.009 m D respectively.Dengying Formation reservoir is of low porosity and ultra-low permeability type.(4)The development of Sinian Dengying Formation reservoir in the study area was influenced by sedimentation,diagenesis and tectonic movement.The algal dolostone and sand-gravel dolomite formed the inner platform dune beach(algal dolostone,algal cloud flat,etc.),which is rich in organic matter,is easy to be dissolved.The dissolution pore is developed along the algal laminae.The mud-silt dolostone and sand-gravel dolostone are mainly affected by dolomitization and recrystallization to form dolomite intercrystalline pore and intergranular pore,and the reservoir space develops.Epigenetic karstification occurred along the unconformity surface of the second member of Dengying Formation and the fourth member of Dengying Formation,resulting in the formation of solution pores and caves on the top of the strata,and the scale of vertical downward solution pores became smaller.Hydrocarbon charging can fill pores in the early stage to avoid the damage of cementation in the later stage,which is conducive to the preservation of reservoir space.The horizontal stretching movement in Tongwan formed more Doric tension fractures,and the atmospheric fresh water dissolved downward along the tension fractures to form solutionfractures and solutiongullies.The fractures formed by the multi-stage tectonic movement effectively improved the reservoir performance of the reservoir in the study area,which was conducive to oil and gas migration and storage.