High-resolution Sequence Stratigraphy And Reservoir Distribution Of Wutonggou Formation In Jimsar Sag

The paper has investigated the sequence stratigraphic classification and sedimentary system of the Permian Wutonggou Formation of Jimsar sag in the east of Junggar basin based on the cores,well logging and seismic data.The high-resolution sequence stratigraphy is used to analyze the sequence stratigraphic classification in sand group levels and sandbody distribution of the JI7 well block in the southeast area,where developed braided delta system.The vertical evolution model of the braided delta in this area has been built,the control factors of reservoir development and the contradiction of oil/water distribution as well.It is commonly agreed that the Late Permian prototype basin of Eastern Junggar Basin is a Single fault basin with gentle slope in the east and steep slope in the west.During Triassic-Jurassic,the Junggar basin reversed under the regional extrusion action and the End Permian strata was truncated on the slope high part.However,this study shows that the Permian Wutonggou Formation is a Complete third-order sequence,and it has a base level rising cycle and base level falling cycle which are divided by MFS.During the third-order sequence evolution process,the front edge of the west basin-controlling fault was deep-water lacustrine,the north area developed fan-delta system and the southeast area mainly developed braided-delta system.Based on the analysis of core,logging facies,seismic facies and paleogeomorphology,the braided delta system in the Southeast area has been studied particularly.The JI7 well block mainly has developed braided delta plain and the delta front facies belt is narrower.The sedimentary microfacies types,which include filling channel,braided channel,channel bar and between channel of braided delta plain;underwater distributary channel and sheet sand of delta front;semi-deep lacustrine and shore shallow lacustrine,has been recognized.We also have established the identification marks of each microfacies in this study.We show that,in the period of Wutonggou Formation,the original geomorphic of JI7 well block in the southeastern area of Jemsar is very flat and the onlap is unconspicuous.The upper Wutonggou Formation on the high part is eroded in different degrees under the tectonic inversion in late Permian,and the base level falling stratum is almost eroded completely.According to the classification and correlation of the rest strata of Wutonggou Formation,the rest base level rising strata can be divided into 2 fourth-order cycles(roughly equivalent to W1 and W2)and 4 fifth-order cycles.It is shown that the braided delta system of JI7 well block has gone through the retrogradation process under the background of transgression based on the analysis of the sand group level(fifth-order cycle)classification and correlation,and the space-time evolution characteristics of braided delta.The result also shows that the sand body distribution in fifth-order cycle is clear,and the evolutionary pattern of sedimentary system in JI7 well block has been built according to the combination of geological analysis and geophysical prediction.Under the guidance of sedimentary model of braided delta,qualitative and semi-quantitative analysis combined,it is proposed that the horizontal connectivity of interwell sand body is very poor in spite of large-area sheet shape distribution of braided channel,and this is also the reason for the contradiction between oil and water distribution in some well area.