Study On Distribution And Connectivity Characteristics Of The Jurassic Sandstones In The East Of Kuqa Depression

The marker beds were identified,and the stratigraphic framework of the Jurassic Yangxia Formation and Ahe Formation in the eastern Kuqa Depression was established based on logging,core and regional geological data.The types of sedimentary facies were identified and the distribution characteristics of sandstones were describe based on description of sedimentary structure and the characteristics of logging.Analysis of the results,a sedimentary evolution model from the Ahe Formation to the Yangxia Formation was proposed.The connectivity distribution and superimposition mode of the subsurface sandstones were determined.By fitting the relationship between the sand-to-ground ratio and the the connected length of sand bodies,the sandstone connectivity probability judgment formulas of the Y3 and Y4 unit in the Yangxia Formation were established.Based on the stratigraphic correlation of the Yangxia Formation and the Ahe Formation,the Yangxia Formation can be divided into four units,which are named as Y1,Y2,Y3,and Y4.The Ahe Formation can be divided into 5 units,which are named as A1,A2,A3,A4,and A5.In general,the thickness of each unit shows a decreasing trend from north to south.The sedimentary subfacies were identified,and divided into braided river delta front,braided river delta plain,and shallow lacustrine facies based on the color,lithology,grain size,sedimentary structure,and logging feature.The three sedimentary subfacies can be subdivided into six microfacies,including braided river channel in delta plain,inter-braided channel in delta plain,flooded swamp,deltaic front underwater distributary channel,subaqueous distributary channel bay in delta front,and sublittoral lake.The Y1 unit of Yangxia Formation is composed of shallow sublittoral lake.The Y2 unit is dominated by braided river delta front subfacies.The Y3 unit mainly develops braided river channel microfacies.The Y4 unit mainly develops flooded swamp microfacies.The A1 unit of the Ahe Formation develops the braided river delta front subfacies.The A2 to A5 units mainly develop the braided river delta plain subfacies.Through a comprehensive study of sedimentary microfacies and sandstone distribution characteristics,it is determined that the braided river delta plain-front-lacustrine depositional system has formed during lake transgression period.The Yangxia Formation is featured by small to medium sized channels developing fine-grained and medium to thin beded sandstones.The sandstone bed in the north of the study area has thick single bed and large vertical superimposed thickness,and it has limited intercalated mudstone and good longitudinal connectivity.In the southern part of study area,the thickness of single bed and vertical superimposition in Ahe formation decreases,mud content and heterogeneity increases,forming vertically aggradated sandstone beds.The width-to-thickness ratio of the connected sandstone beds in the field was used to guide the connectivity analysis of the subsurface sandstones.The width-to-thickness ratio of the sandstone beds in the Yangxia Formation and the Ahe Formation are approximately 90 to1 and 200 to 1,respectively.By delineating the sandstone of field section,four sandstone superposition modes were summarized,which are isolated,lateral stacked,superimposed,and composite distribution.A discriminant model for the connection probability of sandstone in the Y3 and Y4 units of the Yangxia Formation was also established to guide the planar connection probability analysis.