东太平洋水合物海岭沉积物中黏土矿物与水合物饱和度相关性研究

为了探讨水合物储层中不同黏土矿物与水合物饱和度的关系,选取位于东太平洋水合物海岭国际大洋钻探计划204航次中的3个钻孔(1245B孔、1244C孔和1251B),开展储层沉积物黏土矿物的测试和综合分析。结果表明,蒙脱石、伊利石、绿泥石为主要黏土矿物(平均含量分别为40.3%、33.4%、21.4%);高岭石为次要黏土矿物(平均4.9%)。伊利石、绿泥石、高岭石三种矿物含量的垂向变化规律相似,但它们与蒙脱石的垂向变化趋势相反。该区水合物饱和度与蒙脱石含量呈正相关,且正相关程度在“细粒岩性”层段较高(R=0.55~0.97);水合物饱和度与伊利石、绿泥石、高岭石则显示负相关。3个钻孔中水合物储层厚度、水合物饱和度在各钻孔中分布的差异性表明水合物饱和度的受控因素复杂,首先是气源和流体迁移与供给,其次为沉积物岩性;而“细粒岩性”层段较高含量蒙脱石与水合物饱和度正相关关系可能代表了更次一级的沉积影响因素。为检验其它海区是否也存在黏土矿物对水合物饱和度的影响,将上述3个钻孔与印度外海Krishna-Godavari盆地17-07P钻孔记录进行对比。结果显示,17-07P孔的砂含量高,在粗砂层蒙脱石含量与水合物饱和度呈正相关,反映两个海域岩性对水合物饱和度控制机理不同。上述“细粒岩性”层段蒙脱石含量和伊利石、绿泥石、高岭石3种黏土矿物含量与水合物饱和度之间相关性的差异,可能因为蒙脱石具有特殊层状结构和表面化学性质(层间表面带负电荷、具有可交换的层间水合阳离子等),有利于促进水合物的形成和富集,因而表现为正相关;而伊利石、绿泥石、高岭石自身属性与蒙脱石不同(吸水膨胀能力、对甲烷的吸附能力较蒙脱石弱),不利于水合物的聚集,表现为负相关。

Correlation of Clay Minerals and Gas Hydrate Saturation in Sediments from the Hydrate Ridge,Eastern Pacific Ocean

To understand the correlation between clay minerals and gas hydrate saturation in hydrate reservoir strata, we analyzed the clay minerals in sediments from three drillholes (1245B, 1244C and 1251B) from the Ocean Drilling Program (ODP) Leg 204 on the Hydrate Ridge, Eastern Pacific Ocean. Montmorillonite, illite and chlorite are the main clay mineral components (average 40.3%, 33.4% and 21.4%, respectively), and kaolinite is minor (average 4.9%). The vertical distribution patterns of illite, chlorite and kaolinite are similar, opposite to that of montmorillonite. The data show positive correlation between hydrate saturation and montmorillonite content in the three holes, and the degree of positive correlation increases with depth, which is dominated by fine-grained sediments (R = 0.55 - 0.97). Meanwhile, gas hydrate saturation is negatively correlated with illite, chlorite and kaolinite. The differences of hydrate reservoir thickness and distribution patterns of hydrate saturation in the three holes suggest complex controlling factors for hydrate saturation, including mainly (1) gas-fluid supply and migration and (2) lithology. Correlation between montmorillonite content and hydrate saturation in the fine-sediment interval might belong to subordinate influence. In order to test any relationship between montmorillonite and hydrate saturation in other regions, we used the records from drillhole 17-07P in the Krishna-Godavari Basin (offshore India) for comparison with our data. Results indicate that the values of sand components in reservoir strata of this region are significantly higher than that on the Hydrate Ridge, showing a positive correlation between hydrate saturation and montmorillonite content in the sand reservoir strata. The comparison results indicate that the hydrate saturation controlling factors vary between two areas. In view of the difference between two types of correlation (i.e., between the montmorillonite content and hydrate saturation, and between the contents of illite, chlorite and kaolinite and hydrate saturation) in the fine-grained sediments, we inferred that montmorillonite is conducive to the hydrate formation due to its special layered crystal structure and surface chemical properties (negative charge and exchangeable hydrate cation in the interlayer surface), which benefits natural gas hydrate formation and accumulation. The properties of illite, chlorite and kaolinite are different from montmorillonite (weaker water absorption, and swelling and methane adsorption capacity than montmorillonite), and is inconducive to natural gas hydrate accumulation.