Geochemical and Microstructural Characteristics of Clay Minerals and Their Effects on the Pore Structure of Coal-Measure Shale: A Case Study in Qinshui Basin, China

Abstract

As the essential component of shale, clay minerals have a vital influence on the pore structure and the gas content of reservoirs. To investigate the compositional characteristics of coal-measure shale and its effects on pore structure, a total of thirteen Taiyuan formation shale samples were collected from the Qinshui Basin and were analyzed using a combination of X-ray diffraction analysis, X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy (FE-SEM), polarized optical microscopy, and field emission scanning electron microscopy. The results show that the principal minerals of the samples are quartz, kaolinite, and illite. Most of the kaolinite was an original terrigenous detrital material with low crystallinity and a low degree of ordering, whereas the illite was mainly composed of 1Md resulting from diagenesis. Clay minerals developed slits, irregularly-shaped or multisized pores during diagenesis, which can be classed into interlayered pores, intergranular pores, and microfractures. Eight micro-morphological forms of clay minerals were summarized based on FE-SEM observations, such as compacted, parallel, bent, tilted, mutually supporting structures, etc., which are mainly formed by the mechanical compaction of clay minerals with different sizes, shapes, and contact relationships. The diversity and complexity of the micro-morphological forms of clay minerals contribute to the strong heterogeneity, low porosity and high permeability anisotropy of shale.

Pore Characteristics and Gas Preservation of the Lower Cambrian Shale in a Strongly Deformed Zone, Northern Chongqing, China

“…This result is obviously different from previous research on clay minerals. For example, Ji et al [86,87] documented that micropores in chlorite and illite (both of them are present in theYC2 samples, see Table 2) were underdeveloped, and their pores were dominated by pores with a size of 20-100 nm (mesopores and macropores); Li et al [88] suggested that 2-50 nm pores in clay minerals provided their main pore volume and specific surface area. Considering that the porosity of the YC2 samples is mainly controlled by the clay minerals (Figure 6c), it can be further deduced that the tectonic deformation resulted in a significant reduction in mesopores in clay minerals, with the formation of some larger deformation macropores and the transformation into micropores.…”