富有机质泥岩孔隙结构和孔隙连通性研究

Identification of "sweet spots" for gas shale production is a research area that is attracting vigorous interest and is inspiring examination of shale rock properties across a wide range of scales. Characterization of the pore size distributions in gas shales is a vital aspect of this research. Pore size in organic-rich is between several to hundreds of nanometers dominantly, and due to the lithology of mudstones is changing in distance and the reservoir is heterogeneity, the current pore analysis methods such as optical microscopy and mercury injection are insufficient to accurately monitor these characteristics within the micropore (< 2 nm) range, and optical techniques lack the resolution to image such small pores. This program will study the marine mudstones of Lower Paleozoic in Sichuan Basin by the analysis of organic geochemistry, petrology and mineralogy, and use the method of low temperature adsorption-desorption isotherms, which can yield important information regarding the pore structure of the shales, to monitor network effects and connectivity, as the hysteresis loop yields information as to the mechanism of desorption and its width can yield information on both the pore length and diameter of the pores. Physisorption isotherms which will be combined with the SEM pore imaging and mercury injection are one of the best methods to describe the micropores (<2nm), mesopores (2-50nm) and macropores (> 50nm) quantitatively in organic-rich mudrocks of different composition. The research conclusion of pore structure and connectivity could give some reference and scientific basis for the resource evaluation, fluid retention, and flow pattern in gas shales.

富有机质泥岩孔隙度、孔隙结构和孔隙连通性是页岩含气性评价的核心研究内容之一。由于富有机质泥岩孔径小，以几个到上百个纳米级孔为主，岩性变化大，非均质性强等特点，常规压汞法和近年来新开发的氩离子抛光薄片SEM成像技术，都不能完整描述富有机质泥岩的孔隙分布（<2nm）。本项目拟以四川盆地下古生界海相泥岩为研究对象，通过有机地球化学、岩石学和矿物学分析，采用低温下N2气体吸附/解吸等温线孔隙描述方法，结合SEM孔隙成像及压汞方法，建立不同物质组成的页岩中微孔（<2nm）、中孔（2-50nm）和大孔（>50nm）孔隙定量描述的方法，研究富有机质泥岩孔隙结构和孔隙连通性，为页岩气资源的评价、流体保存和流动方式提供方法和科学基础。

项目“富有机质泥岩孔隙结构和孔隙连通性研究”在3年的实施过程中，按照课题任务书的要求，对四川盆地下古生界富有机质泥岩孔隙特征进行了较为细致的研究。项目组成员三次赴四川泸州、宜宾、重庆等地进行野外考察，共采集野外露头剖面3个，钻井剖面1个，系统采集上奥陶统五峰组和下志留统龙马溪组泥页岩露头新鲜样品158件，钻井样品44件。完成各种分析化验260多项次，发表与项目相关论文5篇，其中EI检索论文4篇，核心期刊论文1篇，待发表文章1篇。共有4名研究生参与了课题的研究，圆满地完成了课题设计的任务，取得了以下3方面的成果与认识：（1）通过野外观测、矿物学、有机地球化学、碳同位素地球化学，以及页岩含气量分析测量，建立了四川盆地古生界上奥陶统五峰组—下志留统龙马溪组泥岩有机质富集模式，发现页岩含气量与有机质丰度呈正相关。（2）通过SEM孔隙成像观察，认为龙马溪组页岩有机孔隙的形成与有机质类型、有机质热演化程度、有机质共存的矿物基质的表面特征、几何形状等有关系，提出龙马溪组页岩的有机孔隙演化可分为三个阶段：1）干酪根没降解之前，以矿物格架支撑的粒间孔为主；2）干酪根开始降解，形成干酪根内部孔隙及有机质和粘土矿物复合体孔隙；3）石油开始二次裂解生气，大多数有机质孔变为纳米尺度，并表现为海绵状有机孔的大量存在。（3）利用氮气吸附法测定页岩样品的比表面积和孔径分布，并结合TOC、矿物组成和含气量的变化，认为有机质对页岩中纳米孔隙的发育起着至关重要的作用，是影响页岩储层性质的重要因素。其次，粘土矿物也不同程度的影响了孔隙的发育。这些认识对中国南方古生界高成熟度海相页岩气的基础理论研究和勘探实际都具有一定的意义。