The Study Of Sequence Stratigraphy And Depositional Facies Of Ordovician In Bachu-maigaiti Region Of Tarim Basin

With Ordovician layer as the research object,the study area in the thesis is located in Bashu rise-Maigaiti slope of Tarim Basin.Guided by various theories such as sequence stratigraphy,paleontology,sedimentary petrology,reservoir physics,etc,combined with the carbonate sedimentary facies,sequence and reservoir research and based on analysis of drilling,logging and seismic data,this thesis does an in-depth study on rock types,sedimentary environment,sequence stratigraphy and reservoir characteristics of the target layer.The sequence boundaries could identify and determine the sequence stratigraphic framework through analysis of rock types and the determination of depositional environment.On the basis of this research,the distribution of sedimentary facies within the sequence stratigraphic framework is studied,and the sequence evolution and its main controlling factors are explored.Bashu rise-Maigaiti slope belongs to the stable craton carbonate platform over the whole Ordovician and its terrain is flat and deposition is stable.The modern structural pattern is formed after a number of tectonic movements(The Caledonian early II screen tectonic movement,middle Caledonian,late Caledonian and Hercynian early tectonic movement).Because of the Caledonian mid-I screen tectonic movement,Yijianfang group of middle Ordovician system in the study area has been denuded,and the middle and lower strata of Yingshan group have also suffered varying degrees of erosion.Silurian and Devonian in the study area have also been denuded influenced by the Caledonian mid-II screen tectonic movement,middle Caledonian,late Caledonian and Hercynian early tectonic movement.Yingshan group has been found denuded in the northeastern of Bashu rise-Maigaiti slope,and the upper strata of Ordovician layer have also been found denuded in yubei 1-1x,yubei1,yubei1-2,well BK2,BT4,BT6,shenghe2,yubei1,macan1 and shan1.Meanwhile,the upper strata of Ordovician system are covered by the lower strata of Bashu group of Carboniferous.Based on the core observation of 12 wells and the analysis and identification of 512 rock sheets,five sedimentary facies(restricted platform,open platform,gentle slope,platform edge and basin)are identified combined with geophysical features such as logging and seism.Middle and lower Bashu rise-Maigaiti area is bound to the edge of the platform.The lower penglaiba group developed restricted platform in Bashu area,gentle slope in Maigaiti area,the middle and lower yingshan group developed restricted and open platform in Bashu rise-Maigaiti area,gentle slope in the east of Maigaiti slope,qiaerba group was missed,lianglitage group developed open platform which was partially eroded,and sangtamu group developed basin base.On the basis of the Vail sequence stratigraphy theory,the Ordovician sedimentary geological background,as well as a detailed study of the sequence boundary signs,three supersequence boundaries(SSB1,SSB2,SSB3)and eight sequence boundaries(SB2,SB3,SB5 and SB6)are identified.Interface types are the uplift of the unconformity(SSB1,SSB2,and SSB3)and erosional unconformity(SB2,SB3,SB4,SB5,SB6,SB7 and SB8).On this basis,two supersequences(SSQ1~SSQ2)and eight sequences(SQ1~SQ8)are divided.Combined with logging and seism,the sequence stratigraphic framework is established and sequence deposition phase plane is drawn.Finally,through the analysis of sequence development controlling factors and evolution patterns,it is considered that tectonic movement and sea-level fluctuation are the main controlling factors in the Ordovician sequence formation and development in Bashu region.The Ordovician reservoirs of Bashu region mainly developed in the Yingshan group including platform calcarenites,micrite deposition which developed middle Caledonian,late Caledonian and early Hercynian karst.Diagenesis has the main function of compaction,pressolution,cementation,dolomitization,silification and siliceous filling as well as dissolution,among which pressolution and dissolution play constructive roles.With the analysis of reservoir development controlling factors combined with geological data,it is found that the development of reservoir is mainly controlled by tectonic deformation,fracture development and ancient karst topography.