河控浅水三角洲前缘树枝状沙坝沉积构型与形成机理^*

Depositional architecture and formative mechanism of dendritic bars within river-dominated shallow-water delta front

摘要

河控浅水三角洲前缘砂体可分为分流沙坝型和指状沙坝型2类河-坝组合类型,前人多关注扇状的分流沙坝型与鸟足状的指状沙坝型的前缘砂体,而对多个指状沙坝组成的树枝状沙坝关注不足,其沉积构型与形成机理尚不清楚。以鄱阳湖日帽洲三角洲为例,综合卫星地图、探地雷达、浅钻孔、沉积数值模拟数据,揭示了河控浅水三角洲前缘树枝状沙坝的沉积构型与形成机理。研究认为,树枝状沙坝是由多个相互分叉、交汇的指状沙坝拼接而成,呈现“河在坝上走”的河-坝组合关系,其间发育分流间湾。根据规模差异,树枝状沙坝内的分流河道可分为主支分流河道与侧支分流河道,主支分流河道的宽度大、数量少,多发育于近源端并延伸至沙坝末端,分布于沙坝中部; 侧支分流河道的宽度小、数量多,多分布于前缘两侧与末端。树枝状沙坝的形成与细粒、黏性、高排量河流供给相关,细粒与黏性沉积物促进了稳定天然堤的加积与指状沙坝的形成,高排量导致了分流河道的分流与决口,从而形成树枝状的指状沙坝。河控浅水三角洲前缘树枝状沙坝储集层的侧向连通性较差,优势储集层位于指状沙坝中部,为分流河道砂体与近岸河口坝砂体。

Abstract

River-dominated shallow-water delta front sands can be categorized into distributary-bar and bar-finger types based on combination relationships between distributary channel and mouth bar. Scholars paid more attention to delta front sands of lobate distributary-bar type and bird-foot bar-finger type. However,the dendritic sand bars that composed of multiple bar fingers has been largely overlooked,and its depositional architecture and formative mechanism are still unclear. This paper focuses on the Rimaozhou deposits in Poyang Lake to explore the depositional architecture and formative mechanisms of the dendritic bars in river-dominated shallow-water deltas,utilizing satellite imagery,Ground Penetrating Radar(GPR)profiles,shallow core sampling,and sedimentary numerical simulations. The findings indicate that dendritic bars consist of a complex network of bifurcating and confluent bar fingers. Distributary channels incise mouth bars,as like walking on the bar. Distributary bays nestle among these bar fingers. According to width,distributary channels within dendritic bars can be divided into chief branches and side branches,where the formers are wide,cover a small number,and distributed at central sand bars;instead,the latter are narrow,cover a large number,and distributed at sides and terminal of sand bars. The development of dendritic bars is driven by the supply of fine-grained,cohesive sediments and high discharge levels. Fine-grained and cohesive sediments contribute to the growth of stable levees and the formation of bar fingers,while high discharge encourages the bifurcation and avulsion of distributary channels,facilitating the formation of dendritic bar fingers. Dendritic bars in river-dominated shallow-water delta reservoirs exhibit low lateral connectivity. The most promising reservoir locations are typically found at the central mouth bar,characterized by distributary channel sands and proximal mouth bar sands.

关键词

河控浅水三角洲 / 树枝状沙坝 / 鄱阳湖 / 日帽洲三角洲 / 沉积数值模拟 / 探地雷达 / 沉积构型 / 形成机理

Key words

river-dominated shallow-water delta / dendritic bars / Poyang Lake / Rimaozhou delta / sedimentary numerical simulation / ground penetrating radar / depositional architecture / formative mechanism

基金资助

*国家自然科学基金青年科学基金项目(编号: 42202178)和中国石油大学(北京)科研基金资助(编号: 2462023YJRC034)联合资助