摘要
浅水曲流河三角洲是近年来沉积学研究的热点和油气勘探的重点对象,国内外学者基于地下储层、露头、现代沉积进行了大量研究,关于浅水曲流河三角洲的沉积特征与沉积模式,不同学者往往持不同观点,在缺乏沉积过程与演化特征观察、分析的现状下,很难统一认识。基于此,本文选取鄱阳湖赣江三角洲这一典型的浅水曲流河三角洲,根据其水动力、水深、沉积物供给等数据,采用基于泥沙水动力学的沉积过程数值模拟软件(Delft3D)再现浅水曲流河三角洲的生长与演化过程,并与现代浅水曲流河三角洲对比验证,建立浅水曲流河三角洲沉积模式。研究表明:(1)在浅水环境下,曲流河三角洲快速向湖生长,三角洲前缘持续向前延伸并快速平原化,具有典型的宽平原-窄前缘的特征。三角洲垂向沉积厚度较小,但底积-前积-顶积三层结构完整。(2)浅水曲流河三角洲平原发育数条同期活跃的高弯度分流河道,在平原近端,分流河道存在频繁的侧向迁移现象,可对先期三角洲前缘、前三角洲沉积侵蚀改造,形成串珠状点坝,经长期侧向迁移可形成大规模的长条宽带状分流河道复合体,分流河道对先期沉积体的长期改造使得大部分厚层砂体呈正韵律,导致先期河口坝保存程度较低;在前缘远端,由于堤岸不稳定,分流河道往往存在决口改道现象,废弃的水道缺乏砂质沉积物供给而成为高泥质充填水道,多条废弃分流河道在三角洲平原远端形成高泥质充填河网系统。(3)浅水曲流河三角洲前缘水下分流河道是平原分流河道在水体中的短程延伸,水道长约300m,由于水体较浅,河口处水流冲刷能力较强,水携沉积物多在水下分流河道侧缘堆积形成指状坝。由于分流河道建设性极强,水下分流河道往往频繁发生决口或侧向摆动,经多期复合,可形成多分叉复合体指状坝,而前缘水下分流河道在废弃后往往被泥质充填。(4)受三角洲平原远端水道决口改道的控制,前缘多分叉指状复合坝沿岸线迁移,可形成大面积近等厚砂体。
Meandering-river shallow water delta is a hotspot in the study of sedimentology and an objective in oil and gas exploration.Scholars home and abroad have carried out a lot of studies based on the subsurface reservoir,outcrop and modern deposits.Regarding the depositional process and mode of meandering-river shallow water delta,there has been no consensus because of lacking the observation and analysis of deposition process and evolution features.Based on this account,this study selected a typical meanderingriver shallow water delta in Poyang lake,Ganjiang delta,as an example.Combined with hydrodynamic force,water depth and sediments,the growth and evolution process of the Ganjiang delta was reproduced using the sediment hydrodynamic principle-based software（Delft 3D）and the sedimentary mode of meandering-river shallow water delta was established in comparison with modern shallow water delta.Research shows that：（I）In the shallow water environment,meandering river delta grows lakeward rapidly.Delta front extends forward and transforms to delta plain rapidly and is characterized by the wide plain and the relatively narrow front.Although the vertical thickness of sediment is thin,the ternary structure consisting of bottom set,forward set and top set is integrated.（II）Several active high-sinuosity distributary channels were developed simultaneously in delta plain and lateral migration occurred frequently in the proximal part to distal part of distributary channels.Previous delta front and prodelta underwent avulsion reformation,resulting in the formation of moniliform point bar.Long-term lateral migration formed a wide compound channel belts.Long-term reformation of distributary channels to the previous deposition body produced a positive rhythm in most thick sand bodies,resulting in poor preservation of the previous channel dams.In the distal part of delta plain,unstable levee causes distributary channels avulsing and rechanneling frequently.The abandoned channel was filled with argillaceous sediment,and multiple abandoned channels can construct a channel network filled with argillaceous sediment.（III）In front delta,underwater distributary channel is regarded as the extension of meandering channels of delta plain,with a length up to 300 m.Shallow water resulted in that estuarine current has a strong ability to erode,and the sediments carried by water deposited at the channel side edge,and finally grew into a finger bar.Controlled by the lateral migration and avulsion of distributary channel,subsurface distributary channel happened avulsion or lateral migration.Multiple repeating formed a multiple branched complex while the underwater channels were filled with argillaceous sediment after abandonment.（IV）Controlled by the distributary channels＇ avulsion in the distal part of delta plain,the branched mouth bar complexes migrated laterally along the shoreline,finally forming a large sand body with approximately equal thickness in delta front.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2017年第9期2047-2064,共18页
Acta Geologica Sinica
基金
国家自然科学基金项目(编号41372116)
国家重大科技专项(编号2011ZX05009-003)联合资助成果
关键词
曲流河浅水三角洲
沉积过程数值模拟
沉积特征
演化过程
沉积模式
meandering-river shallow water delta
numerical simulation of depositional process
sedimentary characteristics
evolution features
sedimentary model
