南方下古生界页岩氮气生成潜力及其主控因素研究

Shale organic matter and ammonium-clay minerals were both able to generate a certain content nitrogen gas in thermal maturation process. The pyrolysis nitrogen gas was one of the main sources of high content nitrogen gas in shales. Related research of nitrogen content and nitrogen isotope composition of typical shale has not been extensive studied, and the formation mechanism of nitrogen gas in shales also has not been systematically studied. Considering the high content nitrogen gas in the Lower Cambrian in south China, this research intend to study the content distribution and influence factors of nitrogen element in the Lower Paleozoic shale in Upper Yangtze region. Pyrolysis experiment is carried out on the basis of typical lower-matured shale sample to study the evolution characteristic and formation mechanism of nitrogen gas generation in shales and its kerogen. On this basis, the research will evaluate and compare the nitrogen gas generation potential of the Lower Cambrian and the Lower Silurian shales in southern China and investigate the contribution of the pyrolysis nitrogen gas to the high content nitrogen gas. This research provides the basis for further understanding the nitrogen gas generation potential and the source of high content nitrogen gas, these results would beneficial to reduce the exploration risk and has important theoretical and practical significance for shale gas exploration and development.

页岩中的有机质及含氮粘土矿物在热演化过程中均能生成一定量的氮气，这些热成因氮气可能是页岩地层中高含量氮气的主要来源之一。目前，我国尚未大量开展典型页岩中氮元素含量分布及其同位素特征的相关研究，对页岩中氮气的形成机理也缺乏系统的认识。针对我国南方海相页岩气勘探中遇到的高含量氮气这一实际问题，本申请拟首先对南方上扬子地区下古生界两套页岩地层氮元素含量分布特征及其影响因素开展详细研究，结合典型低成熟页岩的热模拟实验，以期建立比较完整的页岩及干酪根生成氮气的热演化模型，明确氮气生成的相关机理及高过成熟阶段页岩的生气特征。在此基础上，对我国南方下古生界两套页岩地层的氮气生成潜力进行评价，探讨热成因氮气对页岩地层中高含量氮气的贡献。该研究有助于认识页岩地层高含量氮气的来源及形成机理，进而降低页岩气的勘探风险，对于我国页岩气勘探开发具有重要的理论和实际意义。

页岩地层中的高含量氮气直接影响页岩气的组成，进而影响页岩气的勘探开发。本项目采用多种地球化学分析手段，系统地研究了上扬子地区下古生界页岩氮元素含量、组成及同位素特征，结合热模拟研究结果，探讨了下寒武统与下志留统页岩地层的氮气生成潜力。项目取得的主要成果与认识包括：1.上扬子地区下寒武统页岩与下志留统页岩的总氮元素含量分别介于0.064-0.567% 和0.056-0.203%，平均为0.238%和0.142%。两套页岩地层的有机氮含量与其TOC含量呈很好的线性关系，而无机氮含量主要与伊利石含量有关；2.热演化过程中氮气的主生气期明显晚于烃类气体，页岩全岩在热解Ro 3.5%之前释放的的氮气主要来自于是矿物中NH4+形式的无机氮，而有机成因氮气的大量生成在相对更高的热解阶段（Ro>3.5%）；3.结合热模拟实验结果及上扬子地区下古生界两套页岩地层的氮元素组分及分布特征，估算得到下寒武统页岩和下志留统页岩的初始氮元素含量分别约0.189-0.953%和0.082-0.352%，两套页岩地层N2生成潜力约为0.25-1.32m3/t和0.07-0.36m3/t。整体上，成熟度大于Ro 3.5%且有机质丰度较高的下寒武统页岩为高氮气风险区。本项目研究成果有助于进一步明确上扬子地区下古生界页岩高含量氮气的来源及主控因素，为我国高过成熟海相页岩气的勘探提供依据。