南海北部天然气水合物赋存区沉积物中自生矿物特征及其硫酸盐—甲烷转换带指示意义

发布时间：2018-03-07 05:15

本文选题：南海北部　切入点：天然气水合物　出处：《中国地质大学》2016年博士论文　论文类型：学位论文

【摘要】：天然气水合物广泛地分布在大陆边缘的海洋沉积物中,在长期的地质演化过程中下伏天然气水合物藏的失稳可以导致大量甲烷流体的上涌,持续地消耗沉积物孔隙水中的硫酸根离子,并影响硫酸盐-甲烷转换带(SMTZ)的深度位置。硫酸盐-甲烷转换带内发生的甲烷厌氧氧化作用(AOM)不仅可以导致多种孔隙水地球化学指标(例如H2S含量和pH值)的改变,更会影响到多种自生矿物(例如,黄铁矿、重晶石和碳酸盐岩)的形成。相应地,现代海洋天然气水合物赋存区沉积物中的多种自生矿物的研究可以用以指示硫酸盐-甲烷转换带的演化历史。近些年,我国在南海北部陆续开展了多个围绕天然气水合物资源勘探的调查航次,获得了大量天然气水合物赋存区的沉积物。这些珍贵的样品为研究天然气水合物背景下硫酸盐-甲烷转换带内自生矿物的形成提供了绝佳的机会。本论文对南海北部9个站位岩芯柱沉积物中的自生矿物特征进行了详细地研究,通过对黄铁矿、单质硫、石膏等自生矿物的形貌特征和硫同位素组成的综合研究,阐明了沉积物中的硫循环及其矿物形成作用,揭示了硫酸盐-甲烷转换带的演化历史,继而指示了深部天然气水合物藏的演化。主要结论如下：(1)通过铬还原处理和体视镜挑选两种不同方法获得的自生黄铁矿的含量及其硫同位素组成具有相似的变化规律,综合利用自生黄铁矿的含量富集及其硫同位值的正偏的耦合关系可以指示沉积物中的古硫酸盐-甲烷转换带的位置。(2)研究站位中单质硫颗粒集中分布在硫酸盐-甲烷转换带内及其附近,其形成很可能是部分H2S厌氧氧化的结果,相关的过程与硫酸盐-甲烷转换带的演化密切相关。(3)硫酸盐-甲烷转换带内加强的甲烷厌氧氧化作用不仅导致了自生黄铁矿的富集,持续的反应和转化过程伴随充足的物质供应也促进了黄铁矿莓球的进一步生长,最终导致形成的黄铁矿莓球具有异常的粒径分布特征(例如,平均粒径20岬、标准偏差3.0 μm)。(4)硫酸盐还原带内(SRZ)和硫酸盐-甲烷转换带内形成的莓球状黄铁矿在形貌特征、粒径分布以及硫同位素组成上具有显著的不同,并且形成于硫酸盐-甲烷转换带内的黄铁矿莓球的粒径特征不能有效地反映底层水沉积环境的氧化还原条件。(5)自生石膏的SO42-几乎等量地来源于海水硫酸盐和黄铁矿的氧化,而Ca2+可能来源于局部海水酸化导致的钙质壳体的溶解和天然气水合物形成过程中的排离子效应;自生石膏的形成与下伏天然气水合物藏的演化关系密切,其沉淀的位置可能指示了硫酸盐-甲烷转换带的位置。
[Abstract]:Natural gas hydrates are widely distributed in marine sediments on the continental margin. The instability of underlying gas hydrate reservoirs during the long-term geological evolution can lead to the upwelling of a large number of methane fluids. Continue to consume sulfate ions in sediment pore water, The depth position of SMTZ is also affected. The anaerobic oxidation of methane (AOM) in the sulphate-methane transition zone can not only lead to the change of various geochemical indexes of pore water (such as H _ 2S content and pH value), but also affect the depth of SMTZ. It also affects the formation of a variety of authigenic minerals (e.g. pyrite, barite and carbonate rock). The study of a variety of authigenic minerals in sediments from the modern marine gas hydrate deposits can be used to indicate the evolution of the sulphate-methane transition zone. In the northern part of the South China Sea, China has carried out many surveys and voyages around the exploration of natural gas hydrate resources. A large number of sediments in the gas hydrate occurrence area have been obtained. These precious samples provide an excellent opportunity for the study of the formation of authigenic minerals in the sulphate-methane transition zone in the context of natural gas hydrate. This paper provides an excellent opportunity for the South China Sea. The characteristics of authigenic minerals in core column sediments of 9 northern stations were studied in detail. Based on a comprehensive study of the morphology and sulfur isotopic composition of pyrite, elemental sulfur, gypsum and other authigenic minerals, the sulfur cycle and its mineral formation in sediments are clarified, and the evolution history of the sulphate-methane transition zone is revealed. The main conclusions are as follows: (1) the contents and sulfur isotopic compositions of authigenic pyrite obtained by chromium reduction and stereoscopic selection have a similar variation rule. Comprehensive use of the content enrichment of authigenic pyrite and the positive and skewed coupling relationship of sulfur isomorphism can indicate the location of paleo-sulphate-methane transition zone in sediments. In and around the methane conversion zone, The formation of H2S is probably the result of partial anaerobic oxidation of H _ 2S, and the related process is closely related to the evolution of the sulphate-methane transition zone. 3) the enhanced anaerobic oxidation of methane in the conversion zone of sulphate-methane not only leads to the enrichment of authigenic pyrite, but also results in the accumulation of pyrite. The continuous reaction and transformation process, along with sufficient material supply, also promoted the further growth of pyrite berry balls, resulting in the formation of pyrite berry balls with abnormal particle size distribution characteristics (for example, average diameter of 20 headland). The standard deviation (3.0 渭 m) of sulfate-reduction zone (SRZ) and raspberry globular pyrite formed in the sulfate-methane transition zone are significantly different in morphology, particle size distribution and sulfur isotopic composition. Moreover, the particle size characteristics of pyrite raspberry balls formed in the sulphate-methane transition zone can not effectively reflect the redox conditions in the bottom water depositional environment. 5) the so _ 42- of authigenic gypsum originates almost equally from the oxidation of sulfate and pyrite in seawater. Ca2 may be derived from the dissolution of calcareous shell caused by local seawater acidification and the effect of ion expulsion during the formation of natural gas hydrate, and the formation of in-situ gypsum is closely related to the evolution of underlying gas hydrate reservoir. The location of the precipitate may indicate the position of the sulfate-methane transition zone.
【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P618.13

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