Types, Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area, Northern Continental Slope of the South China Sea
Types, Characteristics and Significances of Migrating Pathways of Gas-bearing Fluids in the Shenhu Area, Northern Continental Slope of the South China Sea
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南海北部陆坡神狐海域含气流体运移通道类型、特征及意义
DOI：
10.1111/1755-6724.13073
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Liu Jie
中科院分区：
文献类型：
--
作者：
Su Ming;Sha Zhibin;Zhang Cuimei;Wang Hongbin;Wu Nengyou;Yang Rui;Liang Jinqiang;Qiao Shaohua;Cong Xiaorong;Liu Jie
广州海洋地质调查局在南海北方大陆坡神狐海域进行了我国首次海洋天然气水合物考察。以往的研究分析了这些天然气水合物的P-T条件、地球物理异常和饱和度计算，但没有详细记录研究区含气流体的运移情况。根据二维/三维地震资料的解释，确定了神狐地区含气流体的两种运移通道，即：垂直和横向路径。垂直通道（大型断层、气烟囱和泥底辟）表现为陡峭的地震反射异常，可向下追踪到始新世烃源岩，并可能穿透到晚中新世地层。较深的气体/流体可以通过这些垂直管道迁移到较浅的地层中。然而，这些途径之间的分布表现出明显的差异。大型断层仅在神狐地区北部和东北部发育，而在钻井区，气烟囱是唯一的垂直运移通道。上新世以来，正断层、拆离断层和有利沉积物构成了神湖天然气水合物钻探区的侧向通道。虽然这些侧向通道与气烟囱相连，但它们对水合物的形成和聚集具有不同的作用。含气流体沿沿着气烟囱向上运移，由于正断层的存在，可能进一步侧向运移，从而扩大烟囱的范围。拆离断层将气烟囱与海底连接起来，可能充当气体/流体逸出的管道。第四纪早期发育的再沉积沉积物位于天然气水合物稳定带内，因此水合物将在这些有利的沉积物中富集。与荔湾3 - 1深海油气田的运移通道（大型断层和泥底辟）相比，天然气水合物钻探区的垂直通道（由气烟囱组成）的运移效率可能相对较低。通过对神湖天然气水合物钻探区运移通道的描述和定性判别，有助于进一步认识优质深层烃源岩与浅层（主要为生物成因）水合物之间的关系。白云凹陷文昌组和恩平组湖相泥岩是白云凹陷的主要烃源岩，可提供热成因甲烷。运移效率相对较低的气烟囱形成了垂向通道。东沙构造运动引起的超压流体的释放可能降低热成因甲烷的垂向运移速率。晚中新世以来的厚的半深海/深海细粒沉积物提供了低渗透性的迁移介质。这些先决条件可能会导致长距离垂直迁移过程中热成因甲烷的碳同位素分馏。因此，尽管地球化学分析表明深沪地区形成水合物的甲烷主要是生物成因的，或者是以微生物成因为主的混合甲烷，但热成因甲烷仍有重要贡献。
The first marine gas hydrate expedition in China has been conducted by Guangzhou Marine Geological Survey in the Shenhu Area, northern continental slope of the South China Sea. Previous study has analyzed theP‐Tconditions, geophysical anomalies and saturation calculations of these gas hydrates, but has not documented in detail the migration of gas‐bearing fluids in the study area. Based on the interpretations of 2D/3D seismic data, this work identified two types of migration pathways for gas‐bearing fluids in the Shenhu area, i.e., vertical and lateral pathways. The vertical pathways (large‐scale faults, gas chimneys and mud diapirs) presented as steep seismic reflection anomalies, which could be traced downward to the Eocene source rocks and may penetrate into the Late Miocene strata. The deeper gases/fluids might be allowed migrating into the shallower strata through these vertical conduits. However, the distributions showed distinct differences between these pathways. Large‐scale faults developed only in the north and northeast of the Shenhu area, while in the drilling area gas chimneys were the sole vertical migration pathways. Since the Pliocene, normal faults, detachment faults and favorable sediments have constituted the lateral pathways in the Shenhu gas hydrate drilling area. Although these lateral pathways were connected with gas chimneys, they exerted different effects on hydrate formation and accumulation. Gas‐bearing fluids migrated upward along gas chimneys might further migrate laterally because of the normal faults, thereby enlarging the range of the chimneys. Linking gas chimneys with the seafloor, the detachment faults might act as conduits for escaping gases/fluids. Re‐deposited sediments developed at the early stage of the Quaternary were located within the gas hydrate stability zone, so hydrates would be enriched in these favorable sediments. Compared with the migration pathways (large‐scale faults and mud diapirs) in the LW3‐1 deep‐sea oil/gas field, the migration efficiency of the vertical pathways (composed of gas chimneys) in the gas hydrate drilling area might be relatively low. Description and qualitative discrimination of migration pathways in the Shenhu gas hydrate drilling area are helpful to further understand the relationship between good‐quality deep source rocks and shallow, mainly biogenically‐produced, hydrates. As the main source rocks of the Baiyun sag, lacustrine mudstones in the Wenchang and Enping Formations may provide thermogenic methane. Gas chimneys with relatively low migration efficiency created the vertical pathways. Caused by the Dongsha tectonic movement, the release of overpressured fluids might reduce the vertical migration rates of the thermogenic methane. The thick bathyal/abyssal fine‐grained sediments since the Late Miocene provided migration media with low permeability. These preconditions may cause carbon isotopic fractionation of thermogenic methane during long‐distance vertical migrations. Therefore, although geochemical analyses indicate that the methane forming gas hydrate in the Shenhu area was mainly produced biogenically, or was mixed methane primarily of microbial origin, thermogenic methane still contribute significantly.