Sandy Braided River Architecture Characteristics and Evolution:A case study from outcrops in the Middle Jurassic Yungang Formation,Datong,Shanxi province
[Objective]Braided rivers are a common type of river that can form large-scale oil and gas reservoirs. However,their sedimentary architecture is complex. To gain a deeper understanding of the sedimentary architecture patterns of sandy braided rivers,this study focuses on the sedimentary architecture analysis and evolutionary process of a typical gravelly braided river deposits in the Jurassic Yungang Formation in Datong,Shanxi.[Methods]Obser-vations,measurements,detailed dissections,and analytical tests were conducted on eight sections of the Yungang Formation where braided river deposits are well exposed. This systematic summary covers the geometric morphology,scale,lithological structure and sedimentary structures,lithofacies types,and lithofacies associations of the gravelly braided river depositional bodies.[Results]In the study area,primarily 10 types of lithofacies,8 lithofacies associa-tions,and 5 fourth-level architecture elements (including point bars,braided channels,abandoned channels,flood-plains,and crevasse splays) as well as fall-out beds have been identified.[Conclusion]Ten main phases of positive cyclical deposition can be identified in the outcrops of the Yungang Formation. Within each phase,the braided rivers experienced periodic alternation between flood and dry periods. Braid bars interoccur with braiding channels,aban-doned channels,floodplains,and crevasse splays,developing and waning cyclically to form "laterally connected" sandy bodies. The braid bar complexes and main braiding channels constitute the main body of the gravelly braided river deposits. Based on this,a sedimentary architecture model for sandy braided rivers has been established. The re-search findings can provide important references for the analysis of the evolution of gravelly braided river deposits,reservoir characterization and modeling,as well as the selection of quantitative parameters.
Yungang Formationsandy braided riverarchitectural elementbraided channellithofacies association sequences
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