伸展构造对深水扇沉积的控制机理研究——以东非鲁武马盆地海底深水扇为例

Previous researches on deep-water fans are mostly focused on stable passive continental margin basins. However,studies with regard to marine rift basins where extensional structures are developed were not well documented. Particularly, these studies mainly focused on sedimentary feature descriptions. At present, there are few studies on the control mechanism of extensional structures on deep-water fans, resulting in the unclear of the deep-water fan distribution in marine rift basins. The extensional tectonic activity in the deep-water area of the East African Rovuma Basin is intense, accompanied by developed submarine gravity flow depositions, which makes the basin is an ideal place for the study of the control mechanism of extensional structures on the deep-water fans. This study intends to use 3D seismic data to characterize the distribution of submarine deep-water fans in the Rovuma Basin and restore ancient landforms, so as to clarify the control action of extensional structures on the distribution and evolution of deep-water fans. A geological model will be built on the basis of seismic interpretations. The sedimentary physical simulation experiment based on the model will be conducted to study the control mechanism of extensional structures on gravity flow deposition processes and internal configurations. According to the experimental result, turbulence models suitable for gravity flows in different flow regimes will be built. Three-dimensional numerical simulation based on the models will be performed to determine the fluid transformation mechanism and deposition process of gravity flows under extensional tectonic settings. Finally, the control mechanism of the extensional structures on the deposition of deep-water fans will be revealed. This study can enrich the theory of gravity flow evolution and reveal the distribution characteristic of deep-water fans in marine rift basins, which will provide a new theoretical basis for the sand body prediction in deep-water oil and gas explorations.

深水扇的研究多集中在构造稳定的被动大陆边缘盆地，而在伸展构造发育的海洋裂谷盆地内研究较少，且多为沉积特征描述。目前尚无伸展构造对深水扇沉积的控制机理研究，导致对海洋裂谷盆地内深水扇沉积分布规律认识不明确。东非鲁伍马盆地深水区伸展构造活动强烈，海底重力流沉积发育，是研究伸展构造对深水扇沉积控制作用的理想场所。本研究通过三维地震解释，刻画鲁伍马盆地深水扇沉积展布并恢复古地貌，明确伸展构造对深水扇分布及演化的控制作用。基于地震解释结果，建立地质模型，开展沉积物理模拟实验，研究伸展构造对重力流沉积过程及内部构型的控制作用。在实验基础上，建立不同流态重力流的湍流模型方程，通过三维数值模拟，明确伸展构造背景下重力流流体转化机制及沉积过程。最终揭示伸展构造对深水扇沉积的控制机理。该研究可丰富并细化重力流沉积演化理论，揭示海洋裂谷盆地深水扇分布特征，为深水油气勘探砂体预测提供依据。

过往对深水扇的研究多集中在构造稳定的被动大陆边缘盆地，而在伸展构造发育的海洋裂谷盆地内研究较少，导致对海洋裂谷盆地内深水扇沉积分布规律认识不明确。东非鲁伍马盆地受东非裂谷活动影响，伸展构造活动强烈。本研究通过三维地震解释，确定东非鲁伍马盆地陆坡深水扇沉积单元类型、内部构型及分布规律。对比不同构造阶段的深水扇沉积特征，明确伸展构造对深水扇分布及演化的控制作用，建立相应的沉积模式。鲁伍马盆地新生代深水区在东非裂谷活动前，主要沉积类型为水道-朵体沉积和远洋泥沉积，东非裂谷活动后，引发褶皱推覆带，形成了块体搬运沉积和远洋泥沉积，朵体与水道充填沉积为次要沉积，东非裂谷后期在鲁伍马盆地诱发伸展构造，发育大型深水扇和远洋泥沉积，同时局部发育滑塌沉积，断层上下盘的沉积厚度及沉积相不同。盆地深水区发育多条大型陆坡峡谷，上陆坡较陡，峡谷内以侵蚀作用为主，沉积物主要局限在褶皱推覆带的翼部。下陆坡较缓，峡谷末端发育砂泥混杂的碎屑流沉积，受北大西洋底流影响，在峡谷北侧发育泥质漂积体。在陆坡边缘，东非裂谷活动引发海底滑塌。在峡谷末端发育深水扇，峡谷内重力流在峡谷开口处漫溢开来，在开口处轴向延伸处，重力流流速大，侵蚀作用强，在海底形成纯侵蚀型旋回坎。受北大西洋底流影响，沉积物向北侧偏移，北侧重力流密度低，流速小，侵蚀作用弱，形成纯沉积型旋回坎。而受推覆褶皱带和断裂活动影响，海底坡折发育，在坡折处重力流流速增加，旋回坎突然中断。在峡谷开口轴向区及北侧中间的过渡地带，轴向区溢散的重力流及北侧向下陆坡方向流动的重力流混合，形成形态复杂的旋回坎。在峡谷开口南侧沉积物分布局限。相关研究揭示了东非大陆边缘深水扇沉积过程，对完善全球源-汇系统研究具有重要意义，同时可促进东非深水沉积砂体预测及深水油气勘探理论与技术的进步，为我国能源安全和建设“21世纪海上丝绸之路”的提供重要保障。