珠江口盆地海底峡谷群侵蚀—沉积作用与天然气水合物动态成藏

Erosion and sedimentation of submarine canyons could cause the leakage of hydrocarbon-baring fluids, and furthermore make the hydrates suffering dynamic adjustment from the destruction to reformation to re-stabilization. The relationships between submarine canyons and marine hydrates have been documented around the world. The submarine canyons system (SCS) in the Pearl River Mouth Basin as the first area for marine hydrate exploration in China was drilled in 2007. However, the distribution and dynamic accumulation of hydrates affected by submarine canyons in the Pearl River Mouth Basin are still unclear. Resorted to geological-geophysical analysis and numerical simulation on the basis of core samples, logging and seismic data, though describing the morphological features and internal architectures of the SCS, this study will reveal the erosion and sedimentation of the SCS. By coupling the distribution of the SCS, favorable depositional system and hydrates, the destruction exerted by the SCS on the hydrates and the resulting leakage of hydrocarbon-bearing fluids would be discussed. Through simulating hydrates accumulation and comparing with the drilling results, this study will find out the key parameters including migrations of the base of hydrate stability zones and the variations of the thickness of hydrate-bearing layer and hydrates saturation. Eventually, the dynamic accumulation of hydrates dominated by the erosion and sedimentation of the SCS would be built up, helping deepen the understanding of the accumulation mechanism of hydrates in the study area. The results will not only enrich the researches on accumulation of marine hydrates developed in the canyons context, but also provide definite references for the prospective evaluation of hydrate in the northern continental slope of the South China Sea.

海底峡谷的侵蚀—沉积作用将诱发含烃流体渗漏，并导致水合物矿藏经历破坏-改造-再稳定的动态过程，二者的相关性是一个较为普遍的现象。珠江口盆地海底峡谷群的发育区域是我国首个海域水合物钻探区，然而峡谷群形成演化影响下的水合物分布及动态成藏过程还有待深入研究。基于岩心-测井-地震资料，采用地质-地球物理-数值模拟综合研究的方法，以峡谷群的形态和沉积建造为切入点，刻画峡谷群与有利沉积体分布和水合物产出的配置关系，分析峡谷群对水合物矿藏的破坏；解剖峡谷群与含烃流体渗漏之间的耦合关联和响应机制。将不同尺度下数值模拟结果与水合物钻探结果进行对比，通过查明稳定带底界的迁移，讨论含水合物层厚度和水合物饱和度的变化，揭示峡谷群侵蚀—沉积作用控制下的水合物动态成藏过程，深化该区水合物的成藏机理。研究成果对于南海北部陆坡相似背景下的水合物勘探具有借鉴和指导意义，同时也将为南海北部水合物资源的远景评价提供科学依据。

南海北部陆坡神狐海域开展的一系列水合物钻探（GMGS1 & GMGS3）结果表明，该区域具有丰富的水合物资源，但水合物藏分布呈斑点状散布，其分布特征可能受海底峡谷群的侵蚀—沉积演化过程控制。为厘清神狐海域海底峡谷群侵蚀-沉积演化、水合物运聚体系（GHMAS）与水合物成藏分布之间的关系，本项目主要开展了3个方面的研究：①海底峡谷群侵蚀—沉积作用精细刻画及对水合物矿藏的破坏；②海底峡谷群侵蚀—沉积作用与含烃流体渗漏的相互作用机制；③海底峡谷群侵蚀—沉积作用与水合物动态成藏。基于上述研究，本项目主要在5个方面取得主要认识：.①对比前人针对神狐海域晚中新世地层提出的两种层位划分方法，基于高精度3D地震数据以小型侵蚀水道为重要界面特征重新厘定了第四纪界面（T1），厘清了神狐海域晚中新世以来的沉积演化过程；.②对3D地震工区内限制型海底峡谷（C9-11）海底地形展开半定量-定量研究与特征刻画，将海底峡谷划分为4段，并分别描述了各区段的峡谷充填类型与地震反射特征；.③识别出3D地震工区内一共存在59个气烟囱构造，气烟囱大致沿海底峡谷脊部分布且平面长轴方向一致，根据气烟囱内部反射特征及其与断层发育的耦合关系进行了将气烟囱划分为3类，其中断层、峡谷形态对气烟囱类型有较大影响；.④识别出GMGS1 & GMGS3区块含水合物层均为细粒浊积体，两个区块含水合物细粒浊积体分别对应于北部小型水道成因和海底峡谷浊积充填，在具有相同的物源的情况下，尽管沉积过程存在差异，其沉积物物性条件仍然相似；.⑤通过ANSYS—Fluent软件建立了流体运移数值模拟模型，初步认识了热量随流体运移过程的扩散效应及冷却效应，当粒度小于0.1 mm时降温效应较为明显，而当粒度增大（0.5 mm）时降温效果不明显。.截至目前，在本项目的支持下，共发表SCI论文3篇，EI论文1篇，中文核心论文7篇，另有4篇论文在投SCI；参加国内外相关领域学术会议5次；培养硕士研究生2名。