Sequence Stratigraphy And Sedimentary Facies Of Middle-Lower Ordovician In Shunbei Area,Tarim Basin

Based on the seismic data,thin section,outcrops,and logging data,I have analyzed the sequence stratigraphy and sedimentary facies types of the Yingshan-Yijianfang Formation in the Shunbei area,and restored the sedimentary evolution within the fourth-order sequence framework,using the basic theories of sequence stratigraphy and sedimentary petrology,and comprehensively considering the regional geological background.One second-order sequence interface,three third-order sequence interfaces and two fourth-order sequence interfaces have been identified in the Yingshan-Yijianfang Formation.The Yingshan-Yijianfang Formation in the study area has been divided into three third-order sequences,and the Yingshan Formation has been divided into four fourth-order sequences.Hence,a sequence stratigraphic framework for continuous wells and a seismic sequence stratigraphic framework have been established.The sedimentary thickness of the second member of the Yingshan Formation in the study area is the largest,and the thickness of each sequence varies slightly along horizontal direction without obvious thickness variation.Through observation of cores and thin section,eight lithologic types have been identified.Among them,the micrite(Mf1)and wackestone(Mf2)are deposited in beach interact sea and lagoon,indicating a low hydrodynamic condition.The sedimentary environment of the packstone(Mf3)and algae-bonded limestone(Mf5)is mostly medium-low energy shoal facies.Bioclastic limestone(Mf4)and sparry limestone(Mf6)are deposited in high-energy shoal facies with high hydrodyramic condition.Under the constraints of the sedimentary framework,the Yingshan-Yijianfang Formation in the study area is divided into two types of sedimentary facies: restricted platform and open platform,which are further divided into more sedimentary subfacies(intra-platform shoal,beach interact sea,and lagoon).The intra-platform shoal can be divided into intraclast shoal,algae debris shoal and bioclastic shoal.In the Shunbei area,the high-energy shoal with high hydrodyramic condition shows mound-like and progradation architecture on seismic profiles.The beach interact sea and lagoon with low and stable hydrodyramic condition show parallel and sub-parallel architecture on seismic profiles.Based on the analysis of sedimentary facies,development characteristics of each sequence,seismic facies and plane distribution of sedimentary facies,the sedimentary model of Yingshan-Yijianfang Formation in Shunbei area was finally established.Ysq1(the first member of the Yingshan Formation)and Ysq2(the second member of the Yingshan Formation)have been interpreted as restricted platform facies,including lagoon and barrie shoal subfacies.The shoals are characterized by crystal powder dolomite and have small scale with small sediment thickness.As the sea level rises,the Ysq3(the third member of the Yingshan Formation),Ysq4(the fourth member of the Yingshan Formation)and SQ3(Yijianfang Formations)have been interpreted open platform facies,including beach interact sea and intra-platform shoal subfacies.The distribution of shoals has obviously expanded and the scale of shoal bodies has become larger.The types of shoals in SQ3(Yijianfang Formations)is diverse and there are a lot of biological species.The distribution of carbonate reservoir is controlled by the extended range of shoals,and they are mainly distributed in the northern,northwest,and central-southern regions of the study area.The larger the thickness of the shoals,the better the formation of carbonate reservoirs.