Basal shear zones of recurrent mass transport deposits serve as potential reservoirs for gas hydrates in the Central Canyon area, South China Sea
Basal shear zones of recurrent mass transport deposits serve as potential reservoirs for gas hydrates in the Central Canyon area, South China Sea
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DOI：
10.1016/j.margeo.2021.106631
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发表时间：
2021-11
期刊：
影响因子：
2.9
通讯作者：
C. Liang;Chiyang Liu;Xinong Xie;Xiaohang Yu;Yunlong He;M. Su;Hui Chen;Zhan Zhou;D. Tian-
中科院分区：
文献类型：
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作者：
C. Liang;Chiyang Liu;Xinong Xie;Xiaohang Yu;Yunlong He;M. Su;Hui Chen;Zhan Zhou;D. Tian-
循环传质沉积物（MTD）不仅充当捕获流体的封闭层，而且其由管道连接的基底剪切带也充当天然气水合物稳定带内天然气水合物的潜在储集层。然而，这在以前的研究中很少涉及。利用南海西北部中部峡谷地区常规三维地震反射资料和钻井资料，发现天然气水合物主要赋存于海底裂缝中。这些骨折可分为两组。第一类为水平和低角度斜裂缝，以伪SC构造为特征，大部分发生在MTDs基底剪切带内。它们可能是物质输运过程中产生的强剪切应力的结果。第二组包括管道1和2内的几乎垂直的裂缝。这是由于在管道生成过程中水力压裂的结果。值得注意的是，MTD 2-4的基底剪切带在垂直方向上由管道连接，并且具有良好的横向连续性（面积为数百平方公里），因此与仅限于管道1和2内的水力裂缝（面积小于1平方公里）相比，它们可能为天然气水合物提供更大的储集空间。因此，在未来的天然气水合物勘探中，应更加重视在垂直方向上由管道连接的重复MTDs的基底剪切带。与水分子形成天然气水合物的碳氢化合物气体主要由基底高点上方的三个大型气烟囱提供。我们认为，单一的超压配置和有效的封闭提供了MTD 1的深部是负责连续的天然气聚集以上的基底高点。因此，发生了水力压裂，形成了这些气烟囱。我们还提出了一个三维概念模型，以显示如何MTDs影响流体运移和聚集，并防止甲烷渗漏。
Recurrent mass transport deposits (MTD) not only act as competent seals trapping fluids, but their basal shear zones connected by pipes also serve as potential reservoirs for gas hydrate within the gas hydrate stability zone. However, this has seldom been addressed in previous studies. Using conventional 3D seismic reflection data and well data collected from the Central Canyon area of the northwestern South China Sea, we find that gas hydrates mainly lie in the fractures within MTDs. These fractures can be subdivided into two groups. The first group is composed of the horizontal and low-angle oblique fractures characterized by pseudo-SC structures, the majority of which occurred within the basal shear zones of MTDs. They probably resulted from the strong shear stresses during mass transport. The second group comprises the nearly vertical fractures within pipes 1 and 2. They are attributed to the result of hydraulic fracturing during the generation of pipes. It is important to note that the basal shear zones of MTDs 2–4 were connected by the pipes in the vertical direction and have good lateral continuities (with the area of hundreds of km2), and thus they probably provide larger reservoir spaces for gas hydrates, compared with the hydraulic fractures merely confined within pipes 1 and 2 (with the area less than 1 km2). Therefore, in the future gas hydrate exploration, much more attention should be paid to basal shear zones of recurrent MTDs that are connected by pipes in the vertical direction. Hydrocarbon gas that formed gas hydrates with water molecular, were mainly supplied by three large-scale gas chimneys above the basement highs. We suggest that the single overpressure configuration and effective seals provided by the deep-seated MTD1 were responsible for the continuous gas accumulations above the basement highs. Hence, hydraulic fracturing took place and these gas chimneys were formed. We also propose a 3D conceptual model to show how MTDs influence fluid migration and accumulation, and prevent methane seepages.