基于NMR和SEM技术研究陆相页岩孔隙结构与分形维数特征--以松辽盆地长岭断陷沙河子组页岩为例

为了表征陆相页岩的孔隙结构和分形特征,选取松辽盆地长岭断陷沙河子组陆相页岩作为研究对象,通过X射线衍射(XRD)分析、扫描电镜(SEM)图像分析、核磁共振(NMR)实验,运用分形维数理论,讨论NMR分形维数与矿物组成、地球化学参数、物性参数之间的相互关系,并且通过SEM图像提取技术定量化研究页岩储层孔隙特征。结果表明:沙河子组陆相页岩具有多种孔隙类型、孔径分布复杂、非均质性较强的特点。基于弛豫时间截止值,可将NMR分形维数划分为束缚流体孔隙分形维数(0.0609~1.4204)和可动流体孔隙分形维数(2.9640~2.9869)。矿物组成与分形维数的关系显示石英含量与束缚流体孔隙分形维数成负相关关系,黏土矿物含量与束缚流体孔隙分形维数成正相关关系;NMR分形维数与有机质含量呈线性相关,与成熟度不存在明显相关性;这说明矿物组成和有机质含量对NMR分形维数起明显的控制作用。储层物性方面,NMR分形维数与孔隙率成线性负相关关系,而与渗透率成正相关关系,说明NMR分形维数能够作为衡量物性的重要指标。总体来说,SEM图像分形维数可以用来反映孔隙形态的多样性和页岩孔隙的发育程度;NMR分形维数与储层物性之间的关系可用于评价页岩储层质量。

Characteristics of Pore Structure and Fractal Dimension in Continental Shale Based on NMR Experiments and SEM Image Analyses--A Case Study of Shahezi Formation Shale in Changling Fault Depression of Songliao Basin,China

In order to characterize pore structure and fractal characteristics of the continental shale, the Shahezi Formation continental shales in Changling fault depression of Songliao Basin were selected as research object. Based on the fractal dimension theory, X-ray diffraction(XRD) analysis, scanning electron microscope(SEM) image analysis, nuclear magnetic resonance(NMR) experiment were applied to discuss the correlation relationship between NMR fractal dimension and mineral compositions, geochemical parameters, physical parameters. In addition, the pore characteristics of shale reservoir were studied quantitatively with SEM image extraction technology. The results show that Shahezi Formation continental shale is characterized by diverse pore types, complex pore size distribution and strong heterogeneity. Based on the cutoff value of relaxation time, NMR fractal dimension has two segments, which represent the fractal characteristics of bound fluid pore(0.060 9-1.420 4) and seepage pore(2.964 0-2.986 9), respectively. The relationships between mineral compositions and fractal dimensions show that the quartz content is negatively correlated with the fractal dimension of bound fluid pore, while the clay mineral content is positive correlation with it;NMR fractal dimensions show a linear correlation with organic matter content, but no obvious correlation with maturity, which indicates that mineral composition and organic matter content play a significant role in controlling NMR fractal dimensions. In terms of reservoir physical properties, NMR fractal dimensions have a linear negative correlation with porosity, while a positive correlation with permeability, indicating that NMR fractal dimension can be used as an important index to measure physical properties. In all, SEM image fractal dimension can be used to reflect the diversity of pore morphology and the development of various pores in shale;and the relationship between NMR fractal dimension and reservoir physical properties can be used to evaluate the quality of shale reservoirs.