南海北部陆坡东沙海域天然气水合物分解与释放甲烷的地质分配研究

The project proposed to study the dissociation process of gas hydrates and the reaction process of the released methane at Dongsha area, the northern South China Sea (SCS). Plenty of authigenic carbonates including Jiulong carbonate-mound that is composed of authigenic carbonates, which covers an area of about 430 km2 in 700-800 meters water depth, was discovered in the SCS expedition in 2004 during the joint Chinese-German RV SONNE Cruise 177. The data of stable C-and O-isotopes of Dongsha carbonates confirm that they were formed as a result of microbiological anaerobic oxidation of methane (AOM) where mostly methane is supplied as a fluid component from below the seafloor. Laboratory studies of the carbonate samples of this and nearby areas imply increased methane release from the dissociation of marine gas hydrate. The paleoenvironmental records from the northern SCS demonstrate that the oscillation of calcium carbonate deposition corresponds to the climatic fluctuations. Local carbonates can be traced back to middle Pleistocene (330 ka B.P.), and a chimney-like carbonate continuously recorded 144-12 ka B.P. according to the U/Th dating. This continuous time period matches the Marine Isotopic Stage (MIS) 6 to the boundary of MIS 1 and 2 (MIS 1/2). The temperature and sea level of SCS underwent a circle of increase (to MIS 5e) -decrease (till Last Glacial Maximum) -increase (MIS 1/2). The first step of this project will be the estimation of the gas hydrate distribution from MIS 6 to MIS 1/2. Then a model will be developed to simulate the dissociation process. Various processes and factors affect the dissociation have been studied independently. A few estimated turn over rates of the AOM and formation of authigenic carbonate and less coupled the gas hydrates dissociation process with AOM, neither with carbonate formation. None studied hydrate dissociation and carbon sequestration process coupling sea level and temperature change for 100 ka in the site of gas hydrate-bearing sediment. The numerical simulation planned in this study will estimate the overall dissociation rates and subsequently AOM, and precipitation of carbonate in Dongsha area. The proposed model will be established as refers to the geological condition at MIS 6. The boundary condition will be temperature, salinity and sea level changed with time. All of the information will be retrieved from the cruise reports, subsequent publication and local paleoenvironmental records. The model will be applied to estimate the speed of hydrate dissociation, biogeochemical turn over rate, carbonate precipitation rates, and the amount of methane propagating by advective process of pore water through the sediment column. The results of this project will elucidate the history of gas hydrate dissociation, AOM and carbonate formation at Dongsha area, estimate the fractions of methane consumed via AOM, precipitated in carbonate and released into water column.

天然气水合物是潜在的能源，而水合物的分解也可能造成环境灾难，这使得水合物的研究备受关注。水合物分解涉及系列相变-运移-反应-沉淀过程，这些过程跨万年尺度并发生于不同深度的地层层位，可是国际上对于这一系列过程仍缺乏系统而深刻的理解。东沙海域大量存在的碳酸盐岩形成于中更新世至全新世，是富甲烷流体渗漏和甲烷厌氧氧化共同作用的产物。富甲烷流体来源于下伏水合物分解，因此自生碳酸盐岩保存了甲烷迁移和碳循环的踪迹，纪录了地质历史时期水合物分解事件。为了揭示东沙海域水合物分解、释放甲烷在地层中的迁移、反应和地质分配特征，研究水合物分解与酸盐岩形成之间的关系，本项目拟根据实际地质、地球化学资料，构建海洋沉积物中天然气水合物分解-反应过程的数学模型，运用数值模拟手段研究东沙海域碳酸盐岩的形成演化历史。成果将为南海天然气水合物开采、水合物分解在碳循环中的作用及对区域尺度气候变化的影响等问题的研究提供依据。

天然气水合物是重要的潜在能源也是重要的潜在环境风险因素，2017年神狐海域水合物试采成功将我国天然气水合物的勘探开采研究提升到世界顶尖水平。四年前资助的基金项目瞄准南海北部陆坡天然气水合物分解过程及释放甲烷的地质分配。首先搭建包含相变过程的传热-传质-渗流-反应数值模拟工具。其次，参照2007年神狐海域和2013年东沙海域水合物勘探揭示的海域水合物储层分布形貌，依据勘探结果设置地质模型，定量研究末次盛冰期（LGM）至今南海北部陆坡水合物蕴藏区天然气水合物藏的演化（形成/分解）历史；同时还对影响水合物演化的环境、地质（水深、粒度、地温梯度）因素对演化过程影响进行了定量研究。最后，对于甲烷在海域环境下参与的地球化学反应（甲烷厌氧氧化-AOM）及最终的地质分配进行了定量研究。研究结果表明从LGM至今，随着海平面和温度增加南海北部典型水合物藏水合物含量略有增长，但是增长数量微弱（<7%）。水合物埋藏深度对于水合物分解过程及产气量的影响受分布形态影响，高斯分布的水合物影响比阶梯状分布水合物影响尤其显著。当南海北部陆坡的上升甲烷通量维持在现有水平时，四分之三上升甲烷会以液态和气态形式留存在沉积物中。微生物种群数量较低条件下，AOM过程仍然可以消耗约四分之一甲烷通量。随着甲烷通量和种群数量的增加，AOM过程可以消耗更多的甲烷，并将上升甲烷以自生碳酸盐岩的形式封存于海底沉积物中。模拟结果与实测自生碳酸盐岩对比显示过去年代甲烷渗漏的强度远高于当今水平，而且过去5万年内南海北部陆坡曾发生多次甲烷强烈喷溢事件。本项目首次在南海海域开展生物地球化学及水合物演化的定量研究，搭建了模拟水合物动态过程的软件工具，积累了区域地球化学反应速率、化合物通量等数据，将这个领域的研究工作由定性分析提升到定量描述阶段，具有里程碑式的意义。