The complexity of the shale pore structure, which can be assessed by the fractal dimension, will affect the percolation and reservoir capability of a shale; thus, the pore structure is important for shale reservoir evaluation. For the pore structure and fractal characteristics of lacustrine shale to be investigated, a combination of X-ray diffraction (XRD), total organic carbon (TOC), scanning electron microscopy (SEM), mercury-injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and Nano-CT experiments were performed on shale samples from the lower submember of the third member of the Eocene Shahejie Formation (Es₃^L) in the Zhanhua Depression, Bohai Bay Basin. On the basis of fractal theory, the NMR fractal dimensions of the analyzed shale samples were determined by the transversal relaxation time (T₂) spectrum from the NMR experiment. The relationships between the NMR fractal dimension and mineral content, TOC, and pore structure parameter were discussed. The results indicate that the pore structure of the lacustrine shale in the study area is complex and exhibits strong heterogeneity. The pore types mainly include intergranular pores, intragranular pores, and some dissolved pores and microfractures. Calcite and clay are the dominant minerals, ranging from 9 to 91% (average 52.23%) and from 1 to 48% (average 18.63%), respectively. The TOC contents are relatively high with values from 0.06 to 9.32%. The calculated NMR fractal dimension (D) values are between 2.2544 and 2.439, which exhibit positive correlations with TOC content, quartz content, and clay mineral content. In contrast, a negative relationship occurs between the NMR fractal dimension and calcite content, indicating that the development of large, dissolved pores in shale samples could reduce the heterogeneity of the pore size distribution. Negative correlations are observed between the NMR fractal dimension and the T₂ cutoff value, T₂ geometric mean, porosity, and average pore throat radius, whereas the NMR fractal dimension exhibits a positive correlation with the displacement pressure (P_d) and has no obvious relationship with permeability. The different relationships suggest that the NMR fractal dimension is closely related to the pore structure; namely, the smaller the NMR fractal dimension is, the better the pore structure is in the shale samples. Four typical samples were chosen to verify the relationship between the NMR fractal dimensions and the shale pore structure in the logging profile of Well L69. Excellent application results were obtained, suggesting that the NMR fractal dimension can be used to indicate the effectiveness of the reservoir in the study area.

中文翻译：

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应用分形理论表征渤海湾盆地沾化De陷湖相页岩孔隙结构

页岩孔隙结构的复杂性可以通过分形维数来评估，它将影响页岩的渗透和储层能力。因此，孔隙结构对于评价页岩储层很重要。为了研究湖相页岩的孔隙结构和分形特征，结合了X射线衍射（XRD），总有机碳（TOC），扫描电子显微镜（SEM），注汞毛细管压力（MICP），核磁核磁共振（NMR）和Nano-CT实验是针对始新世第三系（Es ₃ ^L）在渤海湾盆地沾化De陷。根据分形理论，通过横向弛豫时间（T ₂NMR实验得到的光谱）。讨论了NMR分形维数与矿物含量，TOC和孔隙结构参数之间的关系。结果表明，研究区湖相页岩的孔隙结构复杂，具有很强的非均质性。孔类型主要包括粒间孔，粒内孔以及一些溶解的孔和微裂缝。方解石和粘土是主要矿物，分别占9％至91％（平均52.23％）和1％至48％（平均18.63％）。TOC含量相对较高，值为0.06％至9.32％。计算的NMR分形维数（D）值在2.2544和2.439之间，与TOC含量，石英含量和粘土矿物含量呈正相关。相反，NMR分形维数与方解石含量之间存在负相关关系，这表明在页岩样品中形成较大的溶解孔隙可能会降低孔径分布的不均匀性。NMR分形维数与T ₂截止值，T ₂几何平均值，孔隙率和平均孔喉半径之间呈负相关，而NMR分形维数与位移压力呈正相关（P _d），并且与渗透率没有明显关系。不同的关系表明NMR的分形维数与孔结构密切相关。也就是说，NMR分形维数越小，页岩样品中的孔隙结构越好。选择了四个典型样品，以验证L69井测井剖面中NMR分形维数与页岩孔隙结构之间的关系。获得了出色的应用结果，表明NMR分形维数可用于指示研究区储层的有效性。