Fine characterization and lithologic trap distribution patterns of delta-beach bar sedimentary system under high-precision sequence constraints:A case study of the upper submember of the 4th member of the Shahejie Formation in the eastern segment of the so
The study aims at revealing the development of lithologic traps in the lowstand systems tract(LST)in the slope zone in a downfaulted basin,and investigates the LST in the upper submember of the 4th member of the Shahejie Formation(the Sha 4 Member)in the eastern segment of the southern gentle slope zone of the Dongying Sag,Jiyang Depression,in the Bohai Bay Basin(the study area).Using data from cores and logs,we examine the high-precision division,comparison,and dynamic evolution of sequences and perform the fine characterization of sedimentary microfacies in the planar view.The results indicate that the LST in the upper submember of the Sha 4 Member can be divided into three parasequence sets composed of nine parasequences,presenting six,four,and five stratigraphic superposition patterns at the parasequence,parasequence set,and systems tract levels,respectively.The LST in the study area experienced nine relative lake-level fluctuations at the parasequence level,receiving sufficient sediment supply during the relative lake level falls.This led to a planar distribution characterized by predominant basin-margin progradation transitioning to intrabasinal progradation.The LST contains a delta-beach bar sedimentary system comprising three sedimentary facies,namely the fan delta,delta,and beach bar facies,which can be further divided into seven sedimentary microfacies.Lithologic traps have been formed within the LST,with sand bodies pinching out in beach bars'main bodies and beach ridge microfacies as reservoirs,mudstones formed by lacustrine transgression at the bottom of parasequences and parasequence sets as local seals,and mudstones in the transgressive systems tract(TST)as regional seals.
lithologic trapdelta-beach bar sedimentary systemparasequencelowstand systems tract(LST)slope zone of a downfaulted basinDongying SagBohai Bay Basin
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