Sequence Stratigraphical Architecture Character And Depositional Evolution Research Of Shahejie Fm At Niuju-Changtan Area Of Eastern Depression

Based on the theory of high resolution sequence stratigraphy and models of terrestrial sequence stratigraphy in rifted lacustrine basin,the paper aims to study the Shahejie Formation sequence architecture and the temporal and space evolution of the sedimentary systems,using seismic,drilling/logging data and the cores comprehensively.And then predicted the better reservoir zones considering with oil/gas migration and congregation.By well-seismic calibration and picking across the all research area,the article recognize five sequence boundaries and four maximum flooding surfaces.The five sequence boundaries are SB1,SB2,SB3,SB4,SB5,and the four maximum flooding surfaces are F1,F2,F3,F4.The article further divided the Shahejie Formation into 4 third-order sequences,they are named as SQ1(corresponding to the low third-member of Shahejie Formation),SQ2(corresponding to the middle third-member of Shahejie Formation),SQ3(corresponding to the upper third-member of Shahejie Formation),SQ4(corresponding to the first-member and second-member of Shahejie Formation).In the sequence framework,we identifyed six kinds of sedimentary facies:the alluvial fan,fan delta,braided delta,subaqueous fans,turbidite fans and lacustrine.After making clear sedimentary facies types and the main source direction,we drawed the plane distribution of sedimentary facies with half a base-level cycle for the unit,and disclosed the sedimentary characteristic and evolutional process.SQ1 consists of fan delta,subaqueous fan in the margin of eastern sag and three larger turbidite fan in the central sag of Niuju-Changtan.During the period of SQ2,the range of lacustrine transgression reaches the maximum,while the scale of sandbody mainly from the brachy-axis of the sag is largest.The main reservoir sandbodys are(1)three-stage fan deltas in the northwest,(2)two turbidite fans in the sag resulted from slumping fronter of fan delta,and(3)subaqueous fans in East.Materials of SQ3 origin from North trend of the long axial,and regionaly covered by braided-delta.Distribution of early SQ4 is limited,and mainly alluvial fan and floodplain;two braided-delta developed in shallow lacustrine setting in mid-and late-period,in which the bigger one is in West.At last by combining with the sequence stratigraphic position,structure map,distribution of sedimentary facies,sandbody thickness,the coditions of forming oil/gas trap,trap types,source-reservoir-cap assemblage features and fault guidance system,the paper evaluates the favorable prospects,proposes that SQ2 is the mainly potential layer,and structure-lithologic hydrocarbon reservoir has been thought of a focal point of exploration.