汊口滩沉积特征及沉积模式——以鄱阳湖赣江三角洲汊口滩为例

随着我国许多油田进入高含水开发阶段，剩余油挖潜难度增大，复合分流河道带和单河道的划分已不能满足生产要求，迫切需要进行分流河道内部有利砂体的识别。现代沉积研究是认识分流河道内砂体发育特征的有效手段。通过对鄱阳湖赣江三角洲的现场考察，发现汊口滩是三角洲分流河道中发育的重要砂体类型，以发育在分流河道的分汊口处为典型特征。根据水动力条件、沉积物组合、沉积构造等特征，将汊口滩划分为滩头、滩中、滩尾3个沉积单元。从滩头到滩尾具有水动力条件减弱、沉积物层理规模减小、单砂层厚度减小、粒度变细、泥质夹层增多的特点。汊口滩主要是由于在分汊口处，水流受到汊口的顶托作用流速降低，沉积物按粒度分异堆积形成；堆积方式主要以向上游方向的逆流加积为主。与水道砂体相比，汊口滩发育的层理类型多，而且内部夹层发育，非均质性更强；由于夹层的遮挡作用，砂体不易发生水淹，有利于形成剩余油的富集区。

Sedimentary Characteristics and Model of Branch Mouth Bar:A Case Study of Branch Mouth Bar in Ganjiang Delta of Poyang Lake

Many oilfields in China have entered the stage of high water cut, and the remaining oil production is becoming harder and harder. The division of compound channels and single channels cannot satisfy the demand of oil exploration. It is needed to identify the favorable sand bodies in distributary channels. Research on modern sediment is an effective means to understand the development characteristics of sand-bodies in distributary channels. Branch mouth bar is an important sand type in the distributary channel based on the study of Ganjiang delta in Poyang Lake, which is characterized by its development in the branch site of the distributary channel. According to the hydrodynamic conditions, sediments, and sedimentary structures, the branch mouth bar is divided into bar head, bar middle, and bar tail. From the bar head to the bar tail, the hydrodynamic energy weakens, the bedding size decreases, the single sand body becomes thinner, the sediments become finer, and the mud interlayers increase. Obstructed by the branch site, the velocity of flow decreases, and the sediment deposits to form branch mouth bars mainly stacking in countercurrent accretion. The branch mouth bar develops more beddings, interlayers, and heterogeneity than the waterway. Because of the blocking effect of interlayers, it is not easy to be watered out, so branch mouth bar is the favorable zone for remaining oil.