海相页岩中生物来源硅质特征、含量及与页岩气富集的关系研究

Silica minerals, such as quartz, are mainly from terrigenous detritus in most marine environments. But there are abundant of biogenic silica in black marine shales of Southern China, especially for Wufeng-Lungmachi Formation. The sections with high biogenic silica are also organic matter and shale gas enriched sections, implied that there exist inner links between biogenic silica, organic matter and shale gas. Biogenic silica is biogenic amorphous Silica, i.e. opal, which is consisted mainly of various Siliceous bioclastic. Siliceous bioclastic not only have micro-pore structures, but also deposit with organic matter contained in debris, which may result in organic matter and shale gas enriched in the same sections. For high evolved marine shale, diagenetic evolution of minerals is very complicated and biogenic silica may occur in various crystalline state. In order to obtain morphology features, diagenetic level and content information of biogenic silica in black shale, and study relationship between biogenic silica, organic matter and shale gas amount, multiple methods are employed, including SEM, FIB, FTIR spectrum, Raman spectrum, XRD, Rock-Eval, TDG/DTA and Chemical analysis, which has a extremely important significance in shale gas accumulation in high maturation stage of marine black shale in Southern China.

页岩中石英等硅质矿物大多为陆源碎屑成因，但我国南方海相页岩，特别是奥陶-志留系五峰-龙马溪组页岩中含有大量生物成因硅质。这些硅质碎屑丰富的层段，同时也是有机质丰度很高和页岩气富集的层段，暗示三者间可能存在着重要的内在联系。生物来源硅质是指生物来源的无定型硅（蛋白石），主要由各种生物来源的不同组分组成，不仅可与自身和其他生物体一同沉积，而且又具有连通的微观孔隙网络系统，可能为页岩气的形成、富集提供了重要的物质基础。我国海相高演化页岩中生物来源的硅质可能以多种晶态形式同时存在，采用扫描电镜、X射线衍射、红外光谱、拉曼光谱等多种手段，并配合湿法碱化学提取处理和全化学分析，全面获取生物来源硅质的微观形貌和产出特征信息，研究生物硅质的晶态、成岩演化特征，进行生物硅质的地球化学判识和含量分析，揭示生物来源硅质含量与有机质丰度和页岩气富集的关系，对页岩气形成与富集机理研究具有重要意义。

以宏观与微观相结合的研究思路，以超显微结构为主线，以四川盆地上奥陶统-下志留统五峰组-龙马溪组黑色富有机质页岩为主要研究对象，深入开展了富有机质页岩的岩石学、矿物学和地球化学综合研究，明确了上奥陶统-下志留统五峰组-龙马溪组底部黑色富有机质硅质页岩的成因，分析了硅质页岩成岩演化过程中硅质矿物的物相变化特征和孔隙演化特征，研究了生物成因硅质与有机碳含量和孔隙度发育特征的内在关系，确定了五峰组-龙马溪组生物成因硅质与页岩气的富集有良好的对应关系，认为五峰组-龙马溪生物成因硅质的富集是渝东南地区页岩气源储一体、原位滞留式成藏的重要物质基础。具体包括：①通过光学显微-电子显微和氩离子抛光、能谱结合矿物及元素组成的综合岩石学分析，揭示出五峰-龙马溪组优质烃源层段中硅质主要为生物成因的证据。②通过矿物学和地球化学分析手段，明确了生物成因硅质岩的成岩过程，揭示了纳米级孔隙的变化特征，建立了硅质页岩生物硅质的半定量分析方法系列。③基于放射虫富集层对应于浮游藻类大量繁盛和放射虫成因的硅质页岩有利于页岩中原生无机孔隙和次生有机孔隙的保持和保存的特征，提出生物成因硅质为页岩气的富集提供了气源和赋存空间两方面的极为重要物质基础。