干旱湖盆曲流河末端细粒沉积体系及沉积模式

Sedimentary Model of Fine-Grained Dryland Meandering River Terminus Systems in a Semi-Arid or Arid Endorheic Basin

掌握沉积特征并建立准确的沉积模式对解译古代岩石的沉积环境和构建地下油气储层构成模型起到至关重要的作用.在梳理内陆湖盆干旱河流体系沉积模式形成和发展的基础上,综述玻利维亚乌尤尼盐湖边缘现代Colorado曲流河末端体系的研究成果.明确干旱曲流河末端沉积体系通常发育在低缓的湖岸线附近,仅有一条活跃河道且为曲流河,并具有向下游沿程的河道横截面积减小的特征,其余河道均废弃并已充填或者部分充填.指出干旱曲流河末端沉积体系河道演化的主控因素,低频、高强度的雷暴引发的洪水导致漫溢和改道,漫溢促使大量决口扇沿河道两侧形成与演化,决口扇砂体叠加形成席状砂;而河流改道促使河道砂体相互叠加切割,形成假分枝状的形态.细粒曲流河末端沉积体系主要包括河道沉积、漫溢沉积和洪泛平原沉积,其中大量发育的决口扇沉积为该体系最显著的特征之一,提出了干旱湖盆曲流河末端细粒沉积体系的沉积模式.最后,讨论了该体系与浅水三角洲的形成条件,如气候和地貌(坡度)方面存在的显著差异.干旱曲流河末端体沉积模式不仅丰富当前内陆湖盆边缘河流沉积理论,同时有助于精细刻画类似油气储层构型,也为油气的勘探开发提供思路.

Understanding sedimentary features and establishing accurate sedimentary models play an important role in deciphering sedimentary environments of ancient sedimentary rocks as well as establishing sedimentary architecture models of hydrocarbon reservoirs. This study reviews the theoretic development of dryland fluvial systems in arid endorheic basins and recent results of modern Bolivian Río Colorado River meandering river terminus system.On the low-gradient coastal regions dryland river terminus systems are generally characterized by only one active meandering channel with a downstream reduction in cross-sectional area while other channels are abandoned and completely（or partially） filled. This study also points out controls on channel morphology in the dryland meandering river terminus systems. Infrequent but high magnitude thunderstorm-induced floods enhanceriver bank bleaches, whereby abundance of crevasse splays are triggered along the channel and amalgamations of crevasse splays lead to‘sheet sand＇on floodplain. Avulsions are generally initiated as crevasse channels and new river channels cut through or superimpose on previous channels over time forming an apparently distributive pattern. In addition, this study proposes the sedimentary model of fine-grained dryland meandering river terminus systems, including channel deposits, overbank deposits and floodplain deposits. Crevasse splays deposits are the major component of overbank deposits in this meandering river terminus system. Distinctions in terms of sedimentary settings were also discussed between shallow-water deltas and dryland meandering river terminus systems, indicating significant differences in climate and slope for the two sedimentary systems. The proposed sedimentary model of dryland river terminus systems not only complements the current theory of sedimentary systems in endorheic basins, but also helps to accurately characterize hydrocarbon reservoirs as well as provides a new perspective on the exploration and developments of hydrocarbon resources.