摘要
[目的]砂质辫状河是一类常见的河流,可形成大规模油气储层,但其沉积构型特征复杂.为了深入理解砂质辫状河沉积模式,以山西大同侏罗系云冈组的典型砂质辫状河沉积为对象,开展了沉积构型解剖与演化过程研究.[方法]针对云冈组辫状河沉积出露良好的8条剖面开展观察、测量、精细解剖及分析测试,系统总结了砂质辫状河沉积体的几何形态、规模、岩石结构与沉积构造、岩相类型与岩相组合等特征.[结果]在研究区主要识别出10种岩相类型、8种岩相组合、5种四级构型单元(心滩、辫流水道、废弃水道、泛滥平原和决口扇)及落淤层.[结论]云冈组露头可识别10期主要的正旋回沉积韵律,各期内发育的辫状河经历周期性的洪水期—枯水期交替.心滩与辫流水道、废弃水道、泛滥平原以及决口扇等间互伴生,周期性发育与消亡,形成"泛连通体"砂体.心滩复合体与主辫流水道构成了砂质辫状河沉积的主体.基于此建立了砂质辫状河沉积模式.研究成果可为砂质辫状河沉积演化的过程分析、储层表征与建模的模式指导和定量参数的选取等提供重要的参考依据.
Abstract
[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.
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