有机质石墨化对南方下古生界海相页岩有机质孔隙结构的影响

Shales of Lower Paleozoic marine strata in South China have experienced very high levels of thermal maturity. As the main carrier of shale porosity, both the kerogen and bitumen organic matter should have undergone different degrees of graphitization. Graphitization not only can reduce the ability of shale gas generation, more importantly, it will promote the adjustment and transformation of organic matter structure, which leading to the destruction of organic matter-pores, the reduction of reservoir space and the decrease of gas bearing capacity. This project will focus on the high thermal evolution marine organic-rich shales, which is composed of the Lower Cambrian Niutitang (or Shuijingtuo, Qiongzhusi) Formation, the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation. Some advanced electron microscope analyzing techniques, testing techniques and pyrolysis simulation experiment will be applied to carry the comparative study on the structure characteristics of organic matter and porosity among different matured shales. The objectives of this research are to: (1) characterize the graphitization degree of shale organic matter. (2) propose the characteristic parameters and methods for the research of organic matter graphitization. (3) discuss the genesis and coupling relationship between the graphitization degree of organic matter and the evolution of shale pore structure. (4) establish the response relationship between graphitization and resistivity of shale organic matter, and the logging response model of shale graphitization. The results will be significant to enhance the understanding on the evolution rules of organic matter pores and the enrichment theory of shale gas, and provide the theoretical basis and scientific guidance to the reservoir property evaluation, the gas-bearing prediction, and the establishment of exploration and exploitation design of high thermal evolution shales.

中国南方下古生界海相页岩已普遍进入高-极高热演化阶段，作为页岩孔隙重要载体的干酪根和沥青有机质都将发生不同程度的石墨化，石墨化除了会降低页岩的生气能力，更重要的是它会促使有机质结构发生调整与改造，进而导致页岩有机质孔隙遭受破坏、储集空间骤减、含气性能降低。项目重点以下寒武统牛蹄塘组（或水井沱、筇竹寺）和上奥陶统五峰组-下志留统龙马溪组海相页岩为研究对象。旨在通过多种分析测试、显微观测和模拟实验，研究高演化页岩有机质的结构特征；表征页岩有机质的石墨化程度；提出适用于有机质石墨化研究的特征参数与方法；探讨有机质石墨化程度与页岩孔隙结构演化之间的成因联系和耦合关系；试图建立页岩有机质石墨化与电阻率的响应关系，提出页岩石墨化的测井响应模型。所取得成果将有助于深化对页岩有机质孔隙演化规律的认识，丰富页岩气富集理论；为高演化页岩储集性能评价、含气性预测和开发方案制定提供理论依据和科学指导。

中国南方下古生界海相页岩已普遍进入高-极高热演化阶段，作为页岩孔隙重要载体的有机质（干酪根/沥青）发生了不同程度的石墨化。石墨化程度的差异与有机质孔隙的富集、保存与破坏之间存在着必然的成因联系，对页岩的物性、含气性，以及可压裂性具有重要影响。.项目重点以下寒武统、上奥陶统-下志留统高演化海相页岩为主要研究对象，综合应用激光拉曼、红外光谱、场发射透射电镜、X射线衍射、固体核磁共振和X射线光电子能谱等技术手段，对梯级热演化页岩有机质（干酪根）的化学结构进行了系统的对比分析，揭示了有机质进入石墨化阶段的关键证据和节点，明确了不同热演化阶段有机质石墨化程度的差异性，探索出了一套适用于页岩有机质石墨化表征的技术方法与特征参数。利用气体吸附、显微观察等技术手段，系统分析了不同石墨化阶段页岩在有机质孔隙富集程度和孔隙结构上的差异性；从有机质结构演变角度，揭示了有机质石墨化作用与页岩有机质孔隙演变之间的成因联系。.主要成果认识包括：1）明确了页岩有机质发生石墨化的“成熟度窗口”：即“Ro%=3.5%”，有机质化学结构在该“转折点”发生突变，开始由无定形碳转变为结晶石墨；2）初步揭示了有机质石墨化对有机质孔隙保存和破坏的影响机理：随着石墨化程度增加，生烃能力停止、孔隙压力难以维持，无序向有序结构的转变导致有机质的脆性断裂机制加强，有机质孔隙易在压实作用的持续增强下坍塌；3）明确了有机质石墨化是导致海相页岩呈现极低测井电阻率（低于10Ω-1Ω）的主要原因，建立了“极低电阻率页岩成因模式”；4）极低电阻率测井响应是页岩发生“石墨化”的重要标志，预示着页岩储层极可能遭受了相当大的损害，相关区域页岩气勘探存在着极大的风险。.所取得成果有助于深化极高热演化阶段页岩有机质孔隙的富集与保存机理，可为我国南方高演化海相页岩有利区筛选、页岩储层预测，规避勘探风险、提高经济效益提供理论依据和科学指导。