南苏丹Melut盆地Palogue油田古近系Yabus组河型演化规律及主控因素分析*

针对南苏丹Melut盆地Palogue油田Yabus组独特的辫状河—曲流河沉积体系,利用定量统计和成因动态分析方法,通过引入“微相砂体密度”变量,深入总结了辫状河和曲流河不同的垂向岩相序列,从空间演化角度详细阐述了辫状河和曲流河的差异沉积过程及其河型转化规律,建立了同一物源体系下不同阶段河型转化的沉积模式,并明确了河型转化的主控因素。研究表明: Yabus组河流沉积体系先后经历了3个演化阶段。早期辫状河形成阶段,沉积物顺流加积形成垂向沙坝;中期辫—曲交汇沉积阶段,河流下切作用明显减弱,河流沉积作用转变为侧向加积,在辫状河道中形成斜列砂坝,且辫流坝开始向河道边部迁移形成曲流河点坝;晚期曲流河发育阶段,河道弯度大且砂体沉积规模小。

Evolution of fluvial pattern and analysis of dominated factors in the Paleogene Yabus Formation of Palogue Oilfield in Melut Basin,South Sudan

Unique braided-meandering depositional systems were developed in the Yabus Formation of Palogue Oilfield. The vertical lithofacies sequences of braided and meandering rivers were thoroughly summarized by quantitative statistics and dynamic cause analysis method with introducing the variable index of microfacies sandbody. The differentiated depositional processes and fluvial pattern transformation between the braided and meandering rivers were elaborated from the perspective of spatial evolution. Subsequently,depositional models of the fluvial pattern transformation in different stages under the individual provenance system were established and the dominated factors were further clarified. The study indicates that evolution of the fluvial depositional system in the Yabus Formation experienced three stages. During the early stage,the braided river deposition were dereloped and the downstream accreting longitudinal bars were formed. In the middle stage,there was combined deposition between braided and meandering rivers with significant decrease of fluvial incision. Because of the lateral accretion,the diagonal bars were generated and the braided channel bars migrated to river bank to which transformed into meandering point bars. Eventually,it completely became meandering river with high sinuosity and small-scale depositional sandbody.