Sedimentary Facies And Sandbodies Distribution Of The Third Member Of The Shahejie Formation In The Gudao West Slope Belt

Guided by the modern sedimentary geology and sequence stratigraphy,based on the data collection,core observation,utilizing data of test analysis,drilling,well logging,seismic explanation,and other technical methods to identify the main types of sedimentary facies,summarize the sedimentary pattern and spatial-temporal evolution in the sand group level.Based on the data of more than 120 exploratry wells and detailed observation of more than 700 meters core of 23 main wells,in most parts of the western slope of Gudao,the sedimentary characteristics was summarized.Through the sedimentary analysis of the core section of key wells,the features of wells,sand bodies and plane sedimentary deposits are described to characterize the sand formations.There were found five sedimentary facies:fan delta facies,braided river delta facies,nearshore subaqueous fan,slump fan and lacustrine facies.The lacustrine facies are composed of shallow lakes,(semi-)deep lakes and beach bar.Nearshore subaqueous fan contains middle-fan mainly,but terminal-fan is rare.Fan delta facies is dominated by fan delta front subfacies,including underwater braided river,braided river channel,mouth bar and sand sheet microfacies.Braided river delta facies compose delta front subfacies mainly,distributary channel,mouth bar and sand sheet are developed,vertical strata are progressive sequences mostly.Slump fan includes fan-root and fan-middle.In the lower part of Member 3 of Shahejie Formation,the subsidence center is located in the northwestern part of the study area,the thickness of the strara is increased from southeast to northwest gradually.In the middle submember of Member 3 of Shahejie Formation,the region is still dominated by deep lake — semi-deep lake subfaces.However,compared with the lower formation,deep lake area reduced,braided river delta front is developed at the eastern and southern,the eastern areas are still developing nearshore subaqueous fan.In the upper submember of Member 3 of Shahejie Formation,the denuded area in the southern strata increased,and the residual area of the lake basin decreased.With the background of shallow palaeowater depth,facies in deep water environment like deep lake facies,nearshore subaqueous fan,slump fan became to shallow water environment like braided river delta facies and fan delta facies in vertical and had progradational trend to the north of research area.There are braided river delta facies in the southwest and nearshore subaqueous fan in the northeast,and the formation is exposed in the upper submember of Member 3 of Shahejie Formation in late.