The Study Of Sequence Stratigraphy And Sedimentary Characteristics Of Pinghu Formationin In Xihu Sag Of East China Sea Basin

The study area in this paper is located in Xihu sag in East China Sea Basin,and the target layer is Pinghu Formation of Paleogene.On the basis of regional geological background,according to research papers to multidisciplinary theory,such as sedimentary petrology,sequence stratigraphy,seismic stratigraphy,structural geology and so on,the author combines well drilling,well logging,paleontology and the seismic to study the petrology features,judge the sequence interface and then analyze depositional environment,built up the framework of the sequence stratigraphy.On the basis of above,analysing the sedimentary facies spread in stratigraphic framework,establish depositional model of Pinghu Formation.Directing by the theory of reservoir geology,reservoir physics,combined with the core and rock flakes analysis,analyze lithology,physical properties of reservoir and the impact of depositional environment and sequence to reservoir.East China Sea Basin is a composite hydrocarbon basin borned during the Cenozoic,Mesozoic in the Proterozoic,Paleozoic basement.The Philippine plate dived to the Eurasian plate from south to north during the Eocene-Oligocene era.East China Sea Basin is in the state of back-arc spreading,NE rifting activity significantly,forming the structure pattern of EW zonation and NS block.The Jilong movement,Yandang movement,Oujiang movement,Yuquan movement and Huagang movement has significant influence for Pingbei and Yuquan area.The Jilong movement in late Cretaceous-early Paleocene has formed regional angular unconformities,which build the cenozoic sedimentary basement in Xihu sag.The Yandang and Oujiang movement leda the development of downfaulted area.The Yuquan movement which occurred in late Eocene-early Oligocene is the borderlineof rifting stage to depression stage in East China sea shelf basin.The Huagang movement in late Oligocene-early Miocene has made regional angular unconformities in the basin and lead to stratigraphic gap.Through the study of the core,logging,rock flakes,paleontology and seismic,the Pinghu developed braided river delta affected by tidal action-lagoon facies,include: braided river delta plain,braided river delta front,front braided river delta and lagoon facies.The nearshore subaqueous fan that provenance come from the Diaoyu Long fold belt developed small-scale in The eastern edge of the study area.Based on the Vail sequence stratigraphy theory,combined with the deposition background,Pinghu Formation were divided into 3 third-order sequences(SQ1?SQ2?SQ3)and 6 system tracts,each sequence including the transgressive system tract(TST)and high-stand system tract(HST).On the basis of above,establish the framework of sequence stratigraphy and study sedimentary facies spread in stratigraphic framework,then establish depositional model.Finally,analyze the controlling factors of Pinghu group third-grade sequences and its internal systems tracts,they are tectonic movement,paleoclimate,eustatic movement and provenance,tectonic movement and eustatic movement among them play an important role.By Research on key wells,the author finds that the braided river delta front subphase is favorable reservoir.Pinghu reservoir rock types are mainly lithic feldspathic sandstone and feldspathic lithic sandstone.Reservoir space including primary porosity,secondary porosity and micro-cracks,primary porosity is mainly intergranular pores;secondary porosity includes intergranular dissolved pore,intragranular dissolved pore,intergranular holes.Diagenesis includes dissolution,compaction.Comprehensive analysis of development of sequence,sedimentary facies distribution and diagenesis factors summarize that reservoir mainly developes in SQ3-HST.