Microscopic Pore Structure and Fractal Characteristics From the Carboniferous Xiangbai Formation Shale in Northwestern Guizhou

doi:10.19657/j.geoscience.1000-8527.2022.201

Abstract

Abstract:

To study the microscopic pore characteristics of shale in marine coal-bearing strata, we studied the Carboniferous Xiangbai Formation shale in northwestern Guizhou, and used SEM and N₂ adsorption analyses and fractal theory to study its fractal characteristics. We then explore the influencing factors of microscopic pore structure. The results show that four types of pores can be identified (intragranular, intergranular, organic matter and micro-fracture pores), a large number of microfractures and organic matter pores are developed, which have strong hydrocarbon generating ability and good reservoir performance. The adsorption isotherms of N₂ show an inverse S-type, indicating that the adsorption isotherms were mainly mesopores. The hysteresis loops formed by adsorption isotherms and desorption isotherms were mainly H2-type thin-necked wide-body ink bottle pores. The specific surface area is large (average 13.20 m²/g), and the total pore volume is large (average 0.015 5 cm³/g). The average pore size is 6.22 nm and the massive development of nano-scale micropores would provide rich reservoir space for hydrocarbon gases. The fractal dimension D of microscopic pore structure of shale samples is larger (average 2.723 2), reflecting strong heterogeneity of pore structure. BET specific surface area shows negative correlation with TOC and quartz content, and positive correlation with fractal dimension. The average pore size has good positive correlation with the quartz content, strong negative correlation with fractal dimension, and moderate negative correlation with clay mineral content. Larger BET specific surface area and smaller average pore size correspond to larger fractal dimension and more complex pore structure.

Key words: northwestern Guizhou, Xiangbai Formation, organic-rich shale, microscopic pore structure, fractal theory

CLC Number:

TE132.2

JIANG Bingren, DENG Ende, HAN Minghui, MA Zijie. Microscopic Pore Structure and Fractal Characteristics From the Carboniferous Xiangbai Formation Shale in Northwestern Guizhou[J]. Geoscience, 2022, 36(04): 1065-1073.

Figures/Tables 9

Fig.1 Location map of the study area

Fig.2 Microscopic pore types and characteristics of shale samples from the Carboniferous Xiangbai Formation

Fig.3 Adsorption-desorption curve of typical shale samples from the Carboniferous Xiangbai Formation

Fig.4 Pore size distribution of shale samples from the Carboniferous Xiangbai Formation

Table 1 Fractal dimension of shale micro-pores based on N2 adsorption

样品编号	线性拟合方程	D	R²
XB-1	y=-0.2611x + 1.6284	2.738 9	0.976 8
XB-2	y=-0.1586x + 1.2983	2.841 4	0.991 7
XB-3	y=-0.2440x + 1.6505	2.756 0	0.974 7
XB-4	y=-0.2731x + 1.4773	2.726 9	0.973 9
XB-5	y=-0.3069x + 1.6567	2.693 1	0.977 9
XB-6	y=-0.2868x + 1.5138	2.769 9	0.986 8
XB-7	y=-0.2301x + 1.5559	2.769 9	0.987 0
XB-8	y=-0.5784x - 0.0422	2.421 6	0.972 4
XB-9	y=-0.2730x + 1.4160	2.727 0	0.983 2
XB-10	y=-0.2398x + 1.5872	2.760 2	0.980 0
XB-11	y=-0.2667x + 1.4757	2.733 3	0.979 0
XB-12	y=-0.2586x + 1.4056	2.741 4	0.989 8

Fig.5 Fractal curve of typical shale samples from the Carboniferous Xiangbai Formation

Fig.6 Relationship between pore structural parameters and TOC of the Carboniferous Xiangbai Formation shale

Fig.7 Relationship between pore structural parameters and mineral composition of the Carboniferous Xiangbai Formation shale

Fig.8 Relationship between pore structural parameters and fractal dimension of the Carboniferous Xiangbai Formation shale

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