马里亚纳弧前海底蛇纹岩泥火山无机成因甲烷形成水合物的条件及潜力分析

Serpentinization of ultramafic rocks could generate large amounts of abiotic methane, which would provide necessary gases for methane hydrate formation under the seafloor. Numerous serpentine mud volcanoes in large size occurred in the forearc regions of Mariana trench. The seepage fluids are enriched in abiotic methane on the top of serpentine mud volcanoes, and the pressure and the temperature beneath the seafloor are thermodynamically in methane hydrate stabilization zone. However, little is known about whether methane hydrate could form at the Mariana serpentine mud volcanoes, and its occurrence and resources prospect is not clear. This proposal intends to establish thermodynamic equilibrium model for Mariana serpentine mud volcanoes based on the geochemistry of pore water and seepage fluids, and temperatures, geothermal gradient and water depths by ODP drilling and submersible survey. Methane flux and the oxidation rate of methane would be investigated by geochemical parameters of pore water, such as concentration of sulfate. The dynamic methane hydrate accumulation model at serpentine mud volcanoes would be established and used to simulate the generation of methane hydrate and its occurrence in subsurface. Finally, amount of gas hydrate resourcehs will be evaluated. This research would contribute to basal theoretical support for the investigation and exploration of methane hydrate in ocean seafloor.

超基性岩的蛇纹石化作用生成大量无机成因的甲烷，为大洋海底形成甲烷水合物提供必要的气源，最近在Fram海峡超慢速扩张脊发现有蛇纹石化甲烷水合物的发育。马里亚纳海沟弧前海底发育规模巨大的蛇纹岩泥火山，其顶部海底渗漏进入水体的流体富含甲烷，温度和压力符合甲烷水合物形成的温压条件，蛇纹岩泥火山海域无机成因甲烷是否能形成水合物、潜力如何尚不清楚。本项目以马里亚纳蛇纹岩泥火山已有的深潜、大洋钻探计划和国际大洋发现计划钻探资料为基础，根据渗漏流体和蛇纹岩泥火山沉积物孔隙水的地球化学特征及海底温度、地温梯度与水深等，建立适合蛇纹岩泥火山形成甲烷水合物的热力学相平衡模型；以硫酸根等孔隙水地球化学等参数，反演甲烷的供给通量，建立蛇纹岩泥火山甲烷水合物生成的动力学模型，模拟甲烷水合物生成过程和分布状态，评价甲烷水合物的资源量潜力，为大洋蛇纹岩化形成的甲烷水合物调查和勘探提供理论基础。

一般认为大洋区海底沉积物缺乏丰富的有机质，不具备天然气水合物发育的气源条件。然而最近研究表明洋壳广泛的蛇纹岩化作用可以产生大量的无机成因甲烷，并发现了有蛇纹岩化有关的水合物发育证据。蛇纹岩化过程中生成大量甲烷和氢气，很可能生成甲烷氢气水合物，成为潜在的甲烷和氢气资源。本项目首先开展海洋蛇纹石发育区的甲烷氢气水合物的热力学研究，建立了计算甲烷氢气水合物发育所需的温度、压力和溶解甲烷浓度等关键条件的模型。其中氢气含量主要影响水合物稳定的温压条件，但是在水合物稳定带内，能否生成水合物还取决于甲烷浓度，氢气浓度影响较小。其次项目评估了马里亚纳弧前蛇纹岩泥火山区、Fram海峡超慢速扩张脊、Lostcity慢速扩张脊三种海洋蛇纹石化发育区的水合物稳定带厚度达到约858mbsf-2515mbsf，约200mbsf和水合物不稳定。最后通过马里亚纳弧前蛇纹岩泥火山区ODP1200站位的硫酸根氧化甲烷的生物地球化学研究，确定了大洋钻探1200站位硫酸根氧化甲烷的速率达到约0.13-0.84mol/m2-a，约占深部向海底供给甲烷的56-87%。确定的该站位流体的甲烷浓度约为0.03-0.05mol/kg，其中ODP1200E钻孔的甲烷浓度达到0.05mol/kg，生成水合物所需的甲烷浓度相同，具备了水合物发育的温压条件和气源条件，说明马里亚纳弧前蛇纹岩泥火山区的无机成因甲烷可以生成水合物，为开展天然气水合物的研究和勘察提供的新的思路。