Fractal characterization of pore structure in Cambrian Niutitang shale in northern Guizhou, southwestern China

doi:10.13745/j.esf.sf.2022.5.36

Abstract

Abstract:

Pore structure is the key factor affecting the distribution and flow of shale gas, and fractal analysis can be used to quantitatively characterize the complex pore structures in shale. To better understand the porous structure of the organic-rich shale from the Lower Cambrian Niutitang Formation in northern Guizhou Province, southeastern China, shale samples from Well ZK, Songtao area, are analyzed to determine the pore parameters using high-pressure mercury injection and low-temperature nitrogen adsorption methods, combined with scanning electron microscope observation, shale geochemistry and mineral composition analysis. The pore fractal dimensions are calculated by FHH model, and the influencing factors for pore structure are discussed. To summarize: (1) the quartz content in the organic-rich shale ranges between 39%-68.4%; clay content, 11.5%-28.2%; organic carbon content, 2.77%-5.81% (average 3.81%); while kerogen is mainly of type I, with high thermal maturity. (2) The BET specific surface area ranges between 11.954-21.744 m²/g (average 14.572 m²/g); total pore volume, 0.0186-0.0259 cm³/g (average 0.0214 cm³/g); and average pore size, 4.773-7.025 nm (average 5.967 nm). Micropores contribute largely to the total specific surface area; while mesopores and macropores account for a large proportion of pore volume. (3) The organic-rich shale has complex pore structure dominated by micropores, with high heterogeneity and multifractal characteristics. The pore fractal dimensions D₁ and D₂ obtained from low-temperature nitrogen adsorption data have narrow pore-size distributions (D₁, 2.65-2.71; D₂, 2.79-2.85), while mercury injection data yield a wider pore-size distribution for macropores (D_Hg between 2.21-2.81). (4) D₂ is positively correlated with TOC content and micropore volume, and mineral composition has no significant effect on pore fractal dimensions. The fractal dimension of the Niutitang shale in the study area is similar to that of the gas-producing Longmaxi shale, indicating shales of this area have good pore structure.

Key words: pore structure, fractal dimension, Niutitang Formation, high pressure mercury injection, northern Guizhou Province

CLC Number:

TANG Xuan, ZHENG Fengzan, LIANG Guodong, MA Zijie, ZHANG Jiazheng, WANG Yufang, ZHANG Tongwei. Fractal characterization of pore structure in Cambrian Niutitang shale in northern Guizhou, southwestern China[J]. Earth Science Frontiers, 2023, 30(3): 110-123.

Figures/Tables 15

Table 1 Organic matter parameters and mineral compositions of the Lower Cambrian Liutitang Formation shale samples from Well ZK in Songtao, northern Guizhou

样品号	深度/m	TOC含量/%	TOS含量/%	有机质元素比		δ¹³C_PDB/‰	矿物组成含量/%
样品号	深度/m	TOC含量/%	TOS含量/%	H/C	O/C	δ¹³C_PDB/‰	石英	钾长石	斜长石	方解石	(铁)白云石	黄铁矿	黏土矿物
ZK1	1 322.40	3.43	4.08	0.16	0.06	-31.6	46.2	4.0	17.0	1.2	2.7	3.8	25.1
ZK2	1 323.85	3.97	3.47	—	—	—	57.0	4.9	12.2	1.2	0	3.3	21.4
ZK 3	1 324.35	3.13	2.95	0.22	0.04	-31.6	45.9	—	16.3	1.1	5.8	3.5	27.4
ZK 4	1 325.58	4.26	3.17	—	—	—	55.7	2.6	15.6	1.4	2.0	6.1	16.6
ZK 5	1 326.37	3.19	2.59	0.11	0.03	-31.2	57.0	—	9.2	—	1.4	2.9	29.5
ZK 6	1 328.45	2.8	2.74	—	—	—	51.3	4.6	10.6	0.8	—	5.9	26.8
ZK 7	1 330.82	3.22	2.77	0.14	0.04	-32.2	59.9	4.3	9.9	2.2	—	3.5	20.2
ZK 8	1 331.65	2.85	2.47	—	—	—	63.9	8.5	11.6	2.0	—	2.5	11.5
ZK 9	1 332.45	2.77	2.04	0.16	0.03	-32.1	57.9	3.9	12.1	1.4	—	2.5	22.2
ZK 10	1 334.38	3.00	2.29	—	—	—	68.4	—	11.0	1.1	—	2.7	16.8
ZK 11	1 336.55	4.19	2.78	0.11	0.03	-31.8	52.5	8.8	11.4	—	1.4	6.1	19.8
ZK 12	1 337.35	5.65	3.33	—	—	—	48.0	9.0	12.9	—	1.4	3.4	25.3
ZK 13	1 338.86	4.53	2.12	0.16	0.03	-32.1	56.3	—	12.9	—	0	2.5	28.3
ZK 14	1 339.03	4.32	2.66	—	—	—	49.4	2.5	14.7	—	3.0	3.4	27.0
ZK 15	1 339.48	5.81	5.40	0.21	0.05	-31.8	39.0	4.1	15.2	0.5	1.9	19.9	19.4

Fig.1 Mineral compositions of shale samples from the Niutitang Formation, Songtao, northern Guizhou

Fig.2 (a) Plot of TOC vs. TOS contents at increasing depths showing the variation trends of TOC/TOS ratio with depth, and (b) plot of H/C vs. O/C atomic ratios for thermal maturity assessment of the Liutitang Formation shales in Songtao, northern Guizhou

Fig.3 SEM images of shale samples from Well ZK in Songtao, northern Guizhou for pore structural characterization

Table 2 Pore structural parameters and pore fractal dimensions in shale samples based on N2 adsorption experiment

样品编号	比表面积/ (m²·g^-1)	总孔体积/ (cm³·g^-1)	平均孔径/ nm	孔径/ nm	p/p₀<0.45情况下			p/p₀>0.45情况下
样品编号	比表面积/ (m²·g^-1)	总孔体积/ (cm³·g^-1)	平均孔径/ nm	孔径/ nm	拟合系数 $R 2$	分形维数 $D 1$	分形维数D $' 1$	拟合系数 $R 2$	分形维数 $D 2$
ZK1	13.650	0.021 9	6.427	281.7	0.99	2.702 3	2.106 9	0.99	2.808 4
ZK2	14.765	0.019 0	5.148	284.8	0.99	2.703 7	2.111 1	0.99	2.834 2
ZK3	13.021	0.020 3	6.246	286.2	0.99	2.690 0	2.070 0	0.99	2.806 2
ZK4	14.715	0.019 5	5.292	289.6	0.99	2.678 1	2.034 3	0.99	2.822 7
ZK5	12.696	0.021 0	6.604	286.5	0.99	2.650 8	1.952 4	0.99	2.787 8
ZK6	12.139	0.021 3	7.025	294.4	0.99	2.677 6	2.032 8	0.99	2.799 5
ZK7	13.195	0.021 6	6.542	284.6	0.99	2.690 9	2.072 7	0.99	2.809 9
ZK8	11.954	0.019 8	6.631	279.4	0.99	2.691 6	2.074 8	0.99	2.814 1
ZK9	12.326	0.018 6	6.025	286.1	0.99	2.694 6	2.083 8	0.99	2.812 6
ZK10	13.041	0.020 7	6.360	283.1	0.99	2.687 4	2.062 2	0.99	2.804 9
ZK11	15.459	0.022 8	5.891	272.7	0.99	2.698 7	2.096 1	0.99	2.829 4
ZK12	14.491	0.021 8	6.020	279.8	0.99	2.688 3	2.064 9	0.99	2.814 1
ZK13	18.034	0.025 2	5.601	273.5	0.99	2.706 1	2.118 3	0.99	2.832 6
ZK14	17.348	0.021 4	4.923	352.1	0.99	2.700 7	2.102 1	0.99	2.842 7
ZK15	21.744	0.025 9	4.773	252.7	0.99	2.699 8	2.099 4	0.99	2.850 3

Table 2 Pore structural parameters and pore fractal dimensions in shale samples based on N2 adsorption experiment

样品编号	比表面积/ (m²·g^-1)	总孔体积/ (cm³·g^-1)	平均孔径/ nm	孔径/ nm	p/p₀<0.45情况下			p/p₀>0.45情况下
样品编号	比表面积/ (m²·g^-1)	总孔体积/ (cm³·g^-1)	平均孔径/ nm	孔径/ nm	拟合系数 $R 2$	分形维数 $D 1$	分形维数D $' 1$	拟合系数 $R 2$	分形维数 $D 2$
ZK1	13.650	0.021 9	6.427	281.7	0.99	2.702 3	2.106 9	0.99	2.808 4
ZK2	14.765	0.019 0	5.148	284.8	0.99	2.703 7	2.111 1	0.99	2.834 2
ZK3	13.021	0.020 3	6.246	286.2	0.99	2.690 0	2.070 0	0.99	2.806 2
ZK4	14.715	0.019 5	5.292	289.6	0.99	2.678 1	2.034 3	0.99	2.822 7
ZK5	12.696	0.021 0	6.604	286.5	0.99	2.650 8	1.952 4	0.99	2.787 8
ZK6	12.139	0.021 3	7.025	294.4	0.99	2.677 6	2.032 8	0.99	2.799 5
ZK7	13.195	0.021 6	6.542	284.6	0.99	2.690 9	2.072 7	0.99	2.809 9
ZK8	11.954	0.019 8	6.631	279.4	0.99	2.691 6	2.074 8	0.99	2.814 1
ZK9	12.326	0.018 6	6.025	286.1	0.99	2.694 6	2.083 8	0.99	2.812 6
ZK10	13.041	0.020 7	6.360	283.1	0.99	2.687 4	2.062 2	0.99	2.804 9
ZK11	15.459	0.022 8	5.891	272.7	0.99	2.698 7	2.096 1	0.99	2.829 4
ZK12	14.491	0.021 8	6.020	279.8	0.99	2.688 3	2.064 9	0.99	2.814 1
ZK13	18.034	0.025 2	5.601	273.5	0.99	2.706 1	2.118 3	0.99	2.832 6
ZK14	17.348	0.021 4	4.923	352.1	0.99	2.700 7	2.102 1	0.99	2.842 7
ZK15	21.744	0.025 9	4.773	252.7	0.99	2.699 8	2.099 4	0.99	2.850 3

Fig.4 Nitrogen adsorption-desorption isotherms for selected Liutitang Formation shale samples in Songtao, northern Guizhou

Table 3 Pore structural parameters for selected Liutitang Formation shale samples in Songtao, northern Guizhou based on mercury intrusion experiment

样品编号	深度/m	孔体积/(cm³·g^-1)			孔体积比例/%			孔隙度/ %		拟合系数 R²		分形维数 D_Hg
样品编号	深度/m	中孔 (50~200 nm)	大孔 (>200 nm)	总孔	中孔 (50~200 nm)	大孔 (>200 nm)		孔隙度/ %		拟合系数 R²		分形维数 D_Hg
ZK11	1 336.55	0.000 0	0.000 7	0.000 7	0	100	0.13		0.96		2.813 1
ZK12	1 337.35	0.000 0	0.000 5	0.000 5	0	100	0.12		0.95		2.474 7
ZK13	1 338.86	0.000 0	0.000 5	0.000 5	0	100	0.09		0.94		2.690 2
ZK14	1 339.03	0.000 4	0.000 4	0.000 8	50	50	0.16		0.87		2.216 1
ZK15	1 339.48	0.000 9	0.000 4	0.001 3	69	31	0.31		0.96		2.506 9

Fig.5 Pore size distribution plots for selected Liutitang Formation shale samples (ZK11-ZK15) in Songtao, northern Guizhou based on low-temperature nitrogen adsorption (a) and mercury injection data (b)

Fig.6 FHH model fitting results for selected shale samples in nitrogen adsorption experiment, revealing the change of pore fractal dimensions in different pore-size ranges

Fig.7 Fractal analysis plots for selected shale samples in high-pressure mercury injection experiment, showing the multifractal characteristics

Fig.8 Correlation relationships between pore structural parameters of the Liutitang Formation shales in Songtao, northern Guizhou

Fig.9 Relationship between fractal dimension and pore structural parameters

Table 4 Correlation matrix for mineral components and pore fractal dimensions of the Liutitang Formation shales in Songtao, northern Guizhou

参数	各参数间的相关系数
	矿物含量							TOC含量	D₁	D₂
	石英	钾长石	斜长石	方解石	白云石	黄铁矿	黏土矿物	TOC含量	D₁	D₂
矿物含量
石英	1
钾长石	-0.102	1
斜长石	-0.644	-0.128	1
方解石	0.423	0.085	-0.029	1
白云石	-0.639	-0.257	0.721	-0.202	1
黄铁矿	-0.609	0.106	0.296	-0.157	0.14	1
黏土矿物	-0.446	-0.418	0.079	-0.606	0.35	-0.179	1
TOC含量	-0.58	0.244	0.368	-0.555	0.156	0.573	0.076	1
D₁	-0.215	0.247	0.442	0.062	0.004	0.131	-0.162	0.317	1
D₂	-0.371	0.188	0.412	-0.247	0.05	0.521	-0.185	0.696	0.713	1

Fig.10 Relationship between TOC and pore structural parameters of the Liutitang Formation shales in Songtao, northern Guizhou

Fig.11 Comparison of pore fractal dimensions between different shale formations

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