Structural-sedimentary Characteristics And Its Coupling Relationship Of Paleogene In Northern Depression,Beibuwan Basin

As one of the most important secondary tectonic units in Beibuwan Basin,Northern Depression is rich in hydrocarbon resources,and currently its Paleogene strata is the important hydrocarbon exploration strata.Based on regional geological background,cores,logging and seismic data,this paper focuses on the structural-sedimentary characteristics of Northern Depression.Fault system development and basin structural framework have been analyzed amply,which helps to understand the structural characteristics of study area.Besides,sequence stratigraphic framework,distributions of sedimentary facies and lithofacies paleogeography have been studied amply,which helps to understand the sedimentary characteristics of study area.In addition,the coupling relationship between structural and sedimentary characteristics of Northern Depression has been studied.The coupling relationship includes structural breakslope belt and distribution of sedimentary sand,source-sink system controlled by structures,basin structural framework and distribution of sedimentary facies,and basin evolution and sedimentary fill.The study results are helpful for further hydrocarbon exploration works in Northern Depression,Beibuwan Basin.Paleogene NE-NEE-trending faults control the structure framework of Northern Depression,while NEE and NW-trending faults control the development of sub-structural units,which displays NEE-trending half-graben basin that faulted in the northwest part and overlapped in the southeast part.The basin structures of Weixinan Sag and Haizhong Sag show complicated half-graben and simple half-graben,respectively,while Weixinan Low Uplift is controlled by a boundary fault.Sub-sags located within sags can be divided into four types,including simple half-graben,complicated half-graben,graben and graben-complicated halfgraben sub-sag.The Paleogene evolution of study area can be divided into the syn-rifting stage(Changliu Formation-Wei-3 Member depositional period)and transition stage from syn-rifting to post-rifting(Wei-2 Member-Wei-1 Member depositional period).Meanwhile,The syn-rifting stage can be subdivided into rifting ? episode(Changliu Formation-Liu-2 Member depositional period)and rifting ? episode(Liu-1 Member-Wei-3 Member depositional period).Ten third-order sequences,included in two second-order sequences,were identified during Paleogene in Northern Depression,and seven types of sedimentary facies developed,including alluvial fan,nearshore subaqueous fan,fan delta,braided river delta,meandering river delta,sublacustrine fan and lacustrine facies.During Changliu Formation-Liu-3 Member depositional period the depression was “narrow basin,shallow water " sedimentary environment,and plenty of fan deltas and alluvial fans facies were developed.During Liu-2 Member depositional period the depression was “wide basin,deep water" sedimentary environment,which mainly developed a large area of semi-deep and deep lacustrine subfacies,and various sedimentary facies including nearshore subaqueous fans,fan deltas and braided river deltas were developed.During Liu-1 Member-Wei-3 Member depositional period the depression was “wide basin,semi-deep water " sedimentary environment,which mainly developed fan deltas,braided river deltas facies,while semi-deep and deep lacustrine sub-facies was only distributed within subsag.During Wei-2 Member-Wei-1 Member depositional period the depression was “narrow basin,shallow water" sedimentary environment,and a large area of shore-shallow lacustrine subfacies and braided river deltas facies were developed.There is obvious coupling relationship between structural and sedimentary characteristics of Paleogene in Northern Depression.During Paleogene seven types of structural break-slope belts were identified in study area,including single-faulted steep slope break-slope belt,singlefaulted gentle slope break-slope belt,fault-stepped steep slope break-slope belt,fault-stepped gentle slope break-slope belt,simple sub-sag break-slope belt,axial sub-sag break-slope belt and fork-shaped faulted break-slope belt.Different types of structural break-slope belt exert different controls on the distribution of sedimentary sand.There were three regional provenance and one local provenance during Paleogene in study area.Four types of sedimentation transportation channels were identified,which were fault plane,eroded channel,intermountain gully and relay ramp sedimentation transportation channels.In addition,four types of sourcesink systems were identified,including basin margin faulted gully,basin margin faulted corner,uplift axial gully and gentle gully source-sink systems.In different evolution stages Northern Depression has different basin structures,which affects the distributions of sedimentary facies.During rifting ? episode,the tectonic activity was intense,and the sediments covered the entire lake basin,which developed various sedimentary facies including fan deltas,nearshore subaqueous fans in northern steep slope belt,braided deltas in southern gentle slope belt,lacustrine facies and sublacustrine fans in central sub-sag belt.During rifting ? episode,the tectonic activity was subdued,and Weixinan Low Uplift exposed to water surface which separated Weixinan Sag from the Haizhong Sag,and various sedimentary facies were formed including nearshore subaqueous fans,fan deltas in northern steep slope belt,large braided deltas in southern gentle slope belt,lacustrine facies and sublacustrine fans in central sub-sag belt.During transition stage,the tectonic activity was weak,which was dominated by shallow lake facies,and plenty of braided deltas were developed in northern steep belt and southern gentle slope belt,whereas the Weixinan Low Uplift still exposed to water surface.