玻利维亚乌尤尼盐湖干旱河流末端体系的定量三维沉积构型研究

Salt lakes are not uncommon in the endorheic Meso-Cenozic basins in China and sedimentary systems at edge of these salt lakes have been proved as potential hydrocarbon reservoirs. The uncertainty tends to increase for inter-well stratigraphic correlations of borehole logs and cores, which are based on inaccurate sedimentary models derived from low-resolution seismic data, limited inter-well drilling data and lack of true three-dimensional outcrops or not complete outcrops to expose all sections of sedimentary units (e.g., channel belts). Located at the edge of salt lakes, modern dryland river terminus systems is a sedimentary system, which is regarded as a natural laboratory for establishing quantitative and accurate sedimentary models as well as investigating sedimentary processes. However, due to desert environment, the three-dimensional sedimentary architecture model of dryland river terminus systems to date has been rarely studied. To establish a quantitative sedimentary architecture model, the present project is aimed at investigating a river terminal system at the edge of the world largest salt lake Salar de Uyuni, Bolivia, which has been proved as a typical dryland river terminus system. Along with precipitation data and field campaigns, morphodynamics (e.g., channel morphology and the formation and expansion of crevasse splays) of floodplain and lake-shoreline areas in the study area will be visualized with time-series satellite imagery. In addition, using ground penetrating radar (GPR) and optically stimulated luminescence (OSL), together with lithofacies description and laboratory analyses including particle size analysis and mineral composition analysis, the internal structures of sedimentary units (overbank deposits, abandoned channel fills and point-bar deposits) will be characterized and detailed sedimentary sequence will be established in order to construct a quantitative 3D sedimentary architecture model of the river terminus system. Along with satellite imagery-based visualization of morphodynamics, the quantitative characterization of internal structures and sedimentary sequence of the river terminus system significantly improves the accuracy of the sedimentary architecture models compared with those derived from traditional borehole data. These results of the dryland river terminus system not only provide a new perspective for investigating the heterogeneity of hydrocarbon reservoirs and enhancing oil recovery in the ancient salt-lake sedimentary system, but also provide implications for better understanding the sedimentary systems of shallow-water deltas and alluvial fans in ancient endorheic basins in China.

我国中新生代盆地发育大量的干盐湖，其周边衍生的沉积体系是存储丰富油气资源的主要载体。基于分辨率有限的地震数据、相对匮乏的井间钻井资料以及不完整的岩石露头而建立的精确度低的储层沉积模型，增加了井间小层对比的不确定性。以干盐湖边缘为主的现代干旱河流末端体系，为建立准确的沉积模型和定量研究沉积过程提供得天独厚的条件，然而研究区地处沙漠环境，研究相对困难，目前该体系的三维沉积构型模型的研究仍然缺少。本项目以玻利维亚乌尤尼干盐湖边缘的干旱河流末端体系为研究区，利用时间系列遥感影像再现自然条件下河流改道和决口扇的演化过程，明确环境响应对沉积砂层的控制因素；结合探地雷达和释光测年法，精细解剖沉积物内部结构并划分期次，旨在建立精确的三维沉积构型模型，大幅提高模型的准确性。本项目为解决古盐湖盆地储层非均质性研究和剩余油挖潜等难点上提供全新思路和方法，并对我国湖盆演化形成的浅水三角洲和冲积扇的研究有启示作用。

现代旱区陆相湖盆边缘河流末端体系具有无（少）植被、坡降极低的细粒曲流河体系，由于缺乏对该体系的地貌演化和沉积过程的研究，河道决口及改道机制不明确，致使无法建立该体系精确的沉积构型，难以指导高含水阶段剩余油挖潜。本项目以典型的旱区陆相湖盆边缘河流末端体系-玻利维亚Uyuni盐湖边缘的河流末端体系作为研究对象，得出了以下几点研究成果：①首次利用遥感技术和水文资料，开展了陆相湖盆边缘河流末端体系主要沉积地貌单元对极端洪水事件（单次）响应的研究。利用了水体指数及独立成分分析技术，定量刻画了极端洪水后洪泛平原水淹的规模及其地貌的响应特征。研究揭示极端洪水事件是造成堤岸冲刷进而形成决口扇和河流末端的改道的主要因素。结合高分辨率遥感影像，漫溢洪水造成堤岸决口及洪泛平原沉积地貌单元显著的变化。②结合遥感技术和野外考察，开展了低缓无植被干旱河流末端体系十年尺度河道-洪泛平原地貌演化和沉积特征的研究。研究不仅揭示了洪泛平原地貌单元的发育与演变对河流末端体系河道演变及沉积空间分布具有重要的影响，而且指出高曲率和高分叉角度是引起流槽取直的的主要因素。理清了无植被河流体系侵蚀单元的形成机制，即频繁的溢岸洪水和局部坡降变化诱发极其低缓洪泛平原上裂点的形成共同促使侵蚀单元的形成，指出侵蚀单元的演变对该沉积体系的洪泛平原沉积物的输移和河道迁移的具有重要的影响。还明确了曲流河弯道截弯取与气候变化（尤其是拉尼娜事件）密切相关。③围绕陆相湖盆边缘干旱河流末端体系沉积模式，理清了旱区河流末端沉积体系的形成条件及沉积特征，提出了该体系的沉积模式。④围绕细粒曲流河末端沉积体系点坝沉积开展了水槽实验和矿物学特征研究，揭示了干旱曲流河末端体系的点坝垂向上反旋回的特征，并创新性提出了点坝垂向上反旋回模型。本项目揭示了陆相湖盆边缘河流末端体系沉积过程和沉积特征，为解译地下油气储层及植被出现前地球上（或外星球）岩石地层提供理论依据。