Sedimentary Facies And Sand Body Distribution Of Xishanyao Formation In Yongjin Area Of Junggar Basin

This paper focus on Xishanyao Formation in Yongjin area,Junggar Basin.The study area is analyzed by using loggings,mud loggings,cores,seismic and regional geological datas.Combining sedimentology,high-resolution sequence stratigraphy,logging geology,seismic sedimentology and other related theories and methods to establish an isochronous framework of stratigraphic correlation in the whole area.Accordingly,the stratigraphic development rule of the whole area is analyzed.Moreover,the identification of sedimentary facies signs and microfacies are discussed.The sand body distribution rule was determineed and the sedimentary facies model was established.It is an important reference to oil and gas exploration of Xishanyao Formation in Yongjin area of Junggar basin.The study shows that one third-order depositional sequences and four fourth-order depositional sequences are mainly developed in the Xishangyao Formation in Yongjin area.From bottom to top,the fourth-order depositional sequences are MSC1,MSC2,MSC3 and MSC4 respectively.Due to the influence of Chemo paleouplift,the strata in the study area was eroded,then,the number of fourth-order depositional sequences that can be preserved is different.Three types of fourth-order depositional sequences preservation are summarized:firstly,MSC1 is retained intact,and MSC2 is partially retained,such as Well Y 2 and Y 8;secondly,MSC1 and MSC2 are retained intact,and MSC3 is partially retained,such as Well Y1,Y 3,Y 6;thirdly,MSC1,MSC2 and MSC3 are retained intact,and MSC4 is partially retained,such as Well Y 7.The Xishanyao Formation mainly develops gravelly sandstone,medium-fine sandstone,siltstone and mudstone,The color of the core is brown and gray,and gray is the dominant color,in addition,the coal seam is developed,which represents that the palaeogeographic environment of Xishanyao Formation is underwater.Sedimentary structures are well developed,such as erosion surface,horizontal bedding,trough cross bedding and plate cross bedding.The lithofacies are mainly positive-cycling sediments and anti-cycling sedimentation.The shoreshallow lacustrine subfacies was mainly developed in MSC1 and braided river deltaic front subfacies was developed in MSC2,MSC3 and MSC4.The sand bodies of the Xishanyao Formation are contiguously distributed on the plane.In the vertical direction,they are staggered and superimposed by multi-stage underwater distributary channel sand bodies.The sand bodies are mainly distributed in underwater distributary channels and river mouth bars.By using the technique of stratum slicing,the following understanding was obtained with the combined argumentation of conventional well data and seismic data.It is found that due to the proximity of the northern part to the provenance,the northern amplitude attribute value is higher and the sand body is more densely distributed in the northern part.In the north,the sand body is most developed in the shape of stripes.In the south,Sheet sand is developed in the far end of the study area.In summary,provenance direction,sedimentary microfacies,tectonic movement and lake level fluctuation have a comprehensive influence on the distribution of sand bodies.The braided river delta-lacustrine sedimentary model is developed in Xishanyao Formation.