南方下寒武统牛蹄塘组页岩有机来源氮气形成机理及模式

Recently, the shale gas exploration and exploitation has been rapidly developed in China. The yield of high content nitrogen in shale gradually attracted great attention. As an important type of nitrogen in shale, organic nitrogen has key influence on the formation, enrichment and evolution of shale gas, but the current research is relatively insufficient. This project takes the lower Cambrian Niutitang shale as an example in the southeastern Chongqing and northern Guizhou area. The organic nitrogen source material of shale is studied, and its occurrence mode, content distribution and isotope composition are obtained. Several sets of simulation experiments with different parameters are designed to obtain the kinetic parameters of organic nitrogen generation in shale. The variation of organic nitrogen yield during thermal evolution is analyzed to research the formation process of organic nitrogen. The forming conditions and main influencing factors of organic nitrogen in shale are systematically combed, and the formation mechanism and geological model are discussed. The formation pattern of organic nitrogen in shale is established under actual geological condition. The research result of this project is beneficial to reveal the nitrogen genetic mechanism, optimize the geological theory of natural gas formation and provide reference basis for predicting nitrogen-enriched shale gas distribution and reducing the exploration risk. It has scientific and practical significance.

近年来，我国页岩气勘探和开发获得快速发展，页岩中高含量氮气的产出逐渐引起了高度重视。作为页岩中氮气的一种重要成因类型，有机来源氮气是页岩气形成、富集和演化的关键影响因素，但目前研究程度相对薄弱。本项目以南方渝东南-黔北地区下寒武统牛蹄塘组页岩为例，对页岩有机来源氮气母质展开研究，获得其赋存形态、含量分布及氮同位素组成；设计多组不同参数条件的模拟实验，获取页岩有机来源氮气生成的动力学参数，分析热演化过程中有机来源氮气产率变化，研究页岩有机来源氮气形成过程；系统梳理页岩有机来源氮气的形成条件与主要影响因素，探讨其形成机理与地质模型，建立实际地质条件下页岩有机来源氮气形成模式。本项目研究成果有利于揭示氮气成因机制和完善天然气形成地质理论，同时可为预测富氮页岩气分布和降低勘探风险提供参考依据，具有科学和实践意义。

油气成藏机理研究表明天然气与氮气同生或伴生的现象普遍存在，对于页岩气中的氮气成因目前没有统一的认识。项目以渝东南-黔北地区下寒武统牛蹄塘组页岩为例，精细解剖了含氮气的下寒武统页岩元素地球化学特征，探讨了页岩有机质富集的约束条件及影响因素，评价了下寒武统页岩热演化过程中氮气与甲烷的产生规律，预测了热成因氮气的产量。下寒武统的黑色页岩层系在沉积时水体环境经历了厌氧化-贫氧化-氧化一系列变化，厌氧化的水体条件、较高的古生产力以及较快的沉积速率，共同导致了下寒武统页岩底部高丰度有机质富集的现象。下寒武统页岩含气变化复杂，页岩有机质在高温下发生了转化为非烃类产物的反应，尤其是在实验温度700℃以后，甲烷含量快速降低，氮气开始大量生成。在成熟度3.0%之后，页岩样品的氮气产率开始明显增加，结合页岩的成熟度、转化率差异等因素，预测下寒武统牛蹄塘组页岩热成因氮气产量为0.5-1.5方/吨，部分甚至可达2.0方/吨。通过傅里叶变换离子回旋共振质谱（FT-ICR-MS）测定了页岩有机质元素含量和分子组成，采用“元素-分子-化合物”的高分辨率分析技术，建立了有机质氮化合物表征成熟度的评价参数与方法。刻画了下寒武统页岩的复杂含气特征，结合含氮气页岩地层的测井响应特征，提出了不同气体性质的测井识别分类标准与方法，揭示了富氮气的页岩储层含气量、含气特点及其变化规律，探讨了不同条件下的下寒武统页岩氮气来源模式。