【作者】 邬黛黛；

【导师】 叶瑛；

【作者基本信息】 浙江大学 ， 矿物学、岩石学、矿床学， 2008， 博士

【摘要】 天然气水合物已在近些年成为热点研究领域。根据已有的资料，我国最有希望实现此领域突破的海区是南海陆坡区。在分析了天然气水合物成矿背景和成矿条件的基础上，确定了沉积速率高、沉积物巨厚、地质构造独特的南海存在台西南盆地、珠江口盆地、西沙海槽、琼东南盆地、东沙海域等水合物远景区。本文对琼东南盆地T1、尖峰北盆地T2、台西南盆地GC10、HD196A和HD319的五个站位大型重力活塞柱沉积物样品，及九龙甲烷礁的TVG-1和TVG-11两个站位的电视抓斗样品，通过自生矿物组分，孔隙水化学成分异常，有机碳含量，沉积速率，顶空气甲烷含量以及烃类化合物等分析，对南海天然气水合物的早期成岩作用和地球化学特性做了系统的研究。最后，结合前人的研究成果，指出南海北部陆坡特别是台西南盆地和琼东南盆地是天然气水合物成藏的有利远景区。其中，主要成果和认识有：1．XRD和扫描电镜观察表明，在琼东南盆地、台西南盆地和尖峰北盆地的沉积物样品中普遍存在对天然气水合物有指示意义的自生碳酸盐、硫酸盐、和草莓状黄铁矿。其中文石～高镁方解石的显微结构特征与冷泉沉积物类似，属微生物成因，特别是九龙甲烷礁。2．从九龙甲烷礁的自生矿物组合和显微结构构造，可以确认TVG-1和TVG-11所代表的地质体与冷泉活动有关，流体中的CH₄／Ba²⁺比值高，与墨西哥湾和黑海的冷泉类似。甲烷礁样品的超显微结构表明，碳酸盐沉淀与微生物活动有直接关系。TVG-1形成较早，文石已转化为方解石，但仍保留文石假象，TVG-11形成较迟，文石尚未变化。3．琼东南盆地和台西南盆地的孔隙水成分分析结果显示，孔隙水中的离子浓度随深度有明显的变化。随着深度加深，SO₄^2-含量急剧降低，较浅的硫酸根-甲烷界面（SMI）深度，Ca²⁺、Mg²⁺、Sr²⁺浓度显著降低，Mg²⁺／Ca²⁺、Sr²⁺／Ca²⁺比值急剧增加等地球化学特性与世界上天然气水合物产区的浅表层沉积物孔隙水中离子浓度异常吻合较好，暗示这些地方很可能是深水油气或海底天然气水合物的产出位置。4．台西南盆地沉积物中有机碳含量丰富，HD196A站位大部分在1％以上，可以满足天然气水合物形成的气源条件。台西南盆地在晚更新世以来的沉积速率都较高，在16．7-20．8cm／ka之间。柱状沉积物底部甲烷含量急剧增加，可能和来自深部的烃渗漏有关。因此台西南盆地，特别是HD196A站位，是天然气水合物形成的有利地区。5．台西南盆地HD196A站位沉积物的烃类化合物主要为：正构烷烃、类异戊二烯烃、支链烷烃、多环芳烃等化合物。根据正构烷烃分布曲线的形状、碳数范围、主峰位置以及CPI值和OEP值，说明沉积物中的有机质主要来源是大陆高等植物、海洋浮游生物藻类和细菌源共存，并以陆源高等植物来源占优势。在样品中检测到角鲨烯，验证了这一地区的生物甲烷合成和细菌作用，具有生成油气及天然气水合物可能性。样品中普遍存在菲系列化合物、屈系列化合物、苝和惹烯化合物等，均表明沉积物中的多坏芳烃主要来自陆源高等植物。其中含量丰富的苝指示了陆源有机物质的快速堆积和还原环境。芳烃的甲基化和甲基重排作用，显示沉积物中有机质的热演化程度较高，有机质处于较成熟状态。从柱状样品的中部到底部，S₈相对浓度的急剧增加，结合孔隙水中SO₄^2-浓度异常和顶空气甲烷含量的异常，暗示沉积物底部已接近SMI界面。可以认为HD196A站位是海底油气和天然气水合物勘探的有利地区。6．综合考虑南海尖峰北盆地、琼东南盆地和台西南盆地都是水合物有利地区，巨厚的沉积物和高含量的有机质，有利于天然气水合物形成的气源条件、温压条件和良好的圈闭条件，说明这些海域是天然气水合物的有利勘探远景区。尤其是台西南盆地采样点附近的海域，存在天然气水合物的可能最大，琼东南盆地次之，而尖峰北盆地采样点有待进一步的考察研究。更多还原

【Abstract】 Gas hydrate is becoming one of the most exciting research fields in the world within three decades as its great gas resources. According to the known gas hydrate reserves, the South China Sea （SCS） has the most favourable settings for gas hydrate to occur around offshore of the South China Sea. Based on the geological background and conditions for gas hydrate formation, such as high sedimentation rate, thick sediments, distinctive geological structure in these area, it has indicated that Taixinan Basin, Pearl River Mouth Basin, Xisha Trough, Qiongdongnan Basin and Dongsha Area in the South China Sea are very probable for gas hydrate reserves.In this paper, the marine core sediments of site T1 in Qiongdongnan Basin, site T2 in Jianfengbei Basin, sites CG10, HD196A and HD319 in Taixinan Basin, and other sediments of sites TVG-1 and TVG-11 in Jiulong Methane Reef were collected. We have researched the early diagenesis and the geochemistry characteristics systematically of the gas hydrate in the South China Sea by analyzing the authigenic mineral compositions, chemical compositions in pore water, content of organic carbon, sedimentation rate, methane concentrations in the headspace of sediments, hydrocarbon compounds and so on. Finally, in the light of some indicators inferred from the above methods and some results collected from the former research, some prospective gas hydrates are briefly pointed out in the northern slope of South China Sea, especially in Taixinan Basin and Qiongdongnan Basin. The main achievements are giver as follows:1. Complicated authigenic minerals were identified by XRD and SEM, such as miscellaneous carbonates, sulphates and frambiodal pyrite in the sediment samples from Qiongdongnan Basin, Taixinan Basin and Jianfengbei Basin. These authigenic minerals consistently indicate the gas hydrate to occur in the area. The assemblage and fabric characters of carbonates are similar to what being found in cold-seep sediments, which is thought to be related with microorganisms fueled by dissolved methane, especially found in Jiulong Carbonate Reef. 2. The cold-seep evidences have been found in TVG-1 and TVG-11 samples from the authigenic minerals assemblage and microstructure tectonics in Jiulong Methane Reef. Their high value of CH₄/Ba²⁺ values are very similar to the cold-seeps in Gulf of Mexico and Black Sea. The microstructure in the sediments of Methane Reef shows that carbonate deposition is related with microorganisms. The sample TVG-1 was formed earlier than the TVG-11, because aragonite has been found in TVG-11, and aragonite had transformed into calcite in TVG-1.3. Results of chemical compositions analysis in pore water of sediment samples from Qiongdongnan Basin and Taixinan Basin, show that the concentrations of SO₄^2- decrease sharply in pore water, and the depths of SMI （Sulfate-methane interface） are shallow. The concentrations of Ca²⁺、Mg²⁺、Sr²⁺ decrease clearly, and the ratios of Mg²⁺/Ca²⁺、Sr²⁺/Ca²⁺ increase sharply as the depth increased. These geochemical characteristics are similar to chemical compositions abnormalities in pore water of the shallow sediments where the gas hydrate occurs in the world. Those results strongly indicate there should be gas hydrates or deep water oil （gas） reservoirs underneath.4. Abundant organic carbon content in Taixinan Basin sediments, special in the site HD196A, which is over 1% in most of its sediments. That indicates that there are enough gas resources to form gas hydrate. The deposition rate in the Taixinan Basin is fast, ranges from 16.7 to 20.8 cm/ka since the Late Pleistocene. Methane concentration in the headspace of sediments increases sharply in the bottom of core samplem, which indicates that there is positively a hydrocarbon reserviors underneath the sampling site. Therefore, Taixinan Basin, special the site HD196A, is a positive area for gas hydrate formation.5. The main hydrocarbon compounds in the core sediments of HD196A from Taixinan Basin are: n-alkanes, isoprenoid, branched alkanes, polycyclic aromatic hydrocarbons （PAHs）, etc. Based on the distribution curve of n-alkanes, range of carbon chains, the position of their main peaks and the value of CPI and OEP, it is believed that organic matter in the sediments was a mixture of terrestrial plants, marine phytoplankton, algae and bacteria, but dominated by terrestrial higher plants. Squalene was identified in sample, which show that biological methane synthesizing and bacteria function were occurrence in this area. The ubiquity of phenanthrene series, chrysene series compounds, perylene and retene compounds in the sediment indicate that PAHs came from terrestrial higher plants. Abundance of perylene designates fast accumulation of terrestrial organic matter and reductive condition. The methylation and methyl reset of PAHs indicate that the organisms in the sediment were higher therm-evolvement and were matured in this area. Relative concentration of S₈ increases sharply from the middle to the bottom of the core sample, and the abnormality of SO₄^2- concentrations in pore water and methane concentrations in the headspace of sediments, which all imply that the bottom of the core is close to the sulfate-methane interface （SMI）. These evidences all indicate that the researched area is rather optimistic for oil, gas and/or gas hydrate resources.6. In summary, because of thick sediments, abundance organic matters, the gas resources for gas hydrate, temperature and pressure conditions, entrapped basin and so on, Jianfengbei Basin, Qiongdongnan Basin and Taixinan Basin in the South China Sea all have the most favourable conditions for gas hydrate to occur. Overall Taixinan Basin has the best chance for gas hydrate formation, and then Qiongdongnan Basin. Gas hydrate is a very complicated reservoir system, and there is much investigation and research on gas hydrate need to be carried out.更多还原