砂质辫状河心滩坝沉积构型模式与沉积动力学机制研究

The sandy braided river sand bodies can form large-scale oil-gas and subsurface water reservoir. Channel bar is the main architecture element that constitutes the sandy braided river reservoir. It can be divided into many types and its formation mechanism is very complex. However, to date, the sedimentary architecture models and the corresponding sedimentary dynamic formation mechanism are not yet clear. In this project, the various types of channel bars are taken as object of study. Firstly, the hydrological data of typical sandy braided river will be collected; the initial formation, accretion, migration and erosion-transformation process and final preservation status will be reproduced by numerical sedimentary process simulation method based on 3D hydrodynamic filed; the data of water depth, velocity, sedimentary topography and grain size distributary in the whole depositional process will be collected to clarify the sedimentary evolution rules and architecture characteristics of various types of channel bars. Secondly, a sedimentary architecture model construction method based on depositional process analysis will be developed and quantitative architecture models for various kinds of channel bar will be constructed. Finally, seven groups of single-factor controlled experiments will be carried out to simulate the channel bar formation and evolution process under the control of seven kinds of geological factors. Based on the experiments, the geological conditions that required for the formation of various types of channel bars could be determined. And the sedimentary dynamic mechanism of channel bar could be identified based on the combined study of 3D hydrological field and sediment transport data.

砂质辫状河砂体可形成大规模油气和地下水储集体，心滩坝是构成砂质辫状河储层的主体构型要素，其成因复杂，类型多样。然而，截至目前，各类心滩坝的沉积构型模式与沉积动力学机制尚不明确。本项目以砂质辫状河多种类型心滩坝为研究对象，首先，搜集典型砂质辫状河水文数据并通过基于三维水动力场的沉积过程数值模拟再现心滩坝初始形成过程、增生方式、迁移规律、受侵蚀改造过程及最终保存状态，获取演化过程中的水深、流速、沉积地貌及沉积物粒度分布数据，明确各类心滩坝的沉积演化规律与构型特征；在此基础上开发基于沉积过程解析的心滩坝三维构型模式构建方法，建立心滩坝定量化三维构型模式；最后，设定7组单因素对照实验，模拟水深、流量、底床坡度等7种主要地质因素控制下心滩坝的形成与演化过程，确定各类心滩坝形成所需的地质条件，并结合三维水动力场、沉积物搬运数据定量分析，明确不同类型心滩坝沉积水动力学机制。

本项目通过砂质辫状河沉积数值模拟与大数据解析，取得了如下认识：①砂质辫状河的普遍演化过程包括6个阶段，分别为心滩雏形形成、菱形坝沉积、舌形坝沉积、单元坝沉积、复合坝沉积及改造复合坝沉积，在这一过程中心滩坝的规模首先快速增大，随后缓慢增大，最后维持稳定；②各类心滩的连续性演化受到水流样式及其变化的主导。在平坦底床上，水流受河岸阻滞产生二次流并向对岸传导形成水波干涉，导致心滩雏形形成并逐步生长为菱形坝。菱形坝约束水流并导致沉积物与水流先汇聚后发散，从而形成舌形坝。舌形坝生长至相互接触后使得水流变得不规律且逐步聚集形成辫状水道并进一步导致单元坝形成。单元坝在不同辫状水道内不同强度的水流驱动下发生差异化顺流迁移，不同单元坝的迁移速率差异导致独立的单元坝在河床相互“追赶”并最终发生“碰撞”，从而形成复合坝。复合坝形成后辫状水道逐步趋于稳定，部分水流漫过复合坝顶部并改造其地貌，形成顶部发育诸多冲沟的改造复合坝；③菱形坝、舌形坝及单元坝构型样式较为简单，在顺流方向上由多个向上变薄的前积体组合而成，而在横切水流方向上呈底平顶凸状并由多个向上变薄的加积体叠置而成。复合坝、改造复合坝构型样式较为复杂，在顺流和横切水流方向上由自下而上逐步变薄的前积体、侧积体反复切割叠置而成并偶见辫状水道充填沉积；④砂质辫状河沉积演化过程对沉积物粒度、流量较为敏感。沉积物粒度主要影响河流演化速率，粒度越大，演化速率越快，反之则越慢。此外，心滩坝规模与沉积物粒度关联较小。流量不仅控制砂质辫状河演化速率，而且影响心滩坝规模。流量越大，演化速率越快、心滩坝规模越大，反之则演化速率变慢，心滩坝规模变小。