Characteristics and Controlling Factors of Pore Structure in Coal-Measure Shale Reservoirs: Taking Longtan Formation in Western Guizhou as an Example

doi:10.7657/XJPG20250501

Abstract

Abstract:

Pore structure affects gas storage performance of shale and is an important parameter for evaluating shale gas resource potential. Taking the coal-measure shale of Upper Permian Longtan formation in western Guizhou as an example, micro-pores and micro-fractures were qualitatively observed using scanning electron microscopy (SEM) and classified, and the microscopic pore structure and pore size distribution were quantitatively characterized through high-pressure mercury injection and low-temperature nitrogen adsorption experiments. Combining with organic geochemical parameters and mineral composition distribution characteristics, the factors controlling the pore structures of coal-measure shale reservoirs were identified. The results show that the matrix pores in coal-measure shale of the Longtan formation can be divided into six occurrence types: residual primary intergranular pores, mineral moldic pores, clay mineral intergranular pores, intergranular pores, intragranular dissolution pores, and organic pores, and the micro-fractures are mainly extensional, shear, bedding, and diagenetic shrinkage micro-fractures. Micro-pores (especially those with diameter <5 nm) and transitional pores provide the main pore space. The pore space types are dominated by ink bottle holes and V-shaped holes, with a certain amount of parallel slits, and the connectivity between pores is relatively good. Total organic carbon content (TOC), maturity of organic matter, and mineral composition are the main factors controlling the pore structure of the coal-measure shale reservoirs of Longtan formation in western Guizhou. The single-point pore volume and specific surface area of the shale increase with the increase of TOC. The degree of thermal evolution contributes positively to the increase of micro-pore and transitional pore volume. Clay minerals have complex impacts on the pore structure. High brittleness index has a positive effect on the development of meso-pores, macro-pores and micro-fractures, being conducive to shale gas flow.

Key words: western Guizhou, Longtan formation, coal-measure shale, shale gas, pore structure, controlling factor

CLC Number:

TE121

LI Juhao, HE Jinxian, YANG Zhaobiao, ZHANG Xiaoli, WU Meng, MA Li, YUAN Yuan, WEN Mingzhong. Characteristics and Controlling Factors of Pore Structure in Coal-Measure Shale Reservoirs: Taking Longtan Formation in Western Guizhou as an Example[J]. Xinjiang Petroleum Geology, 2025, 46(5): 521-530.

Figures/Tables 12

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Table 1.

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Table 2.

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Table 4.

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储集空间类型	产状分类	亚类
孔隙	粒间孔	残余原生粒间孔
		矿物铸模孔
		黏土矿物间微孔
	粒内孔	晶间孔
	粒内孔	粒内溶蚀孔
	有机质孔	有机质孔
微裂缝	构造缝	张性微裂缝
	构造缝	剪性微裂缝
	成岩缝	层理缝
	成岩缝	成岩收缩缝

样品编号	孔隙度/ %	平均孔隙直径/nm	孔容/（mL·g^-1）				孔容占比/%
样品编号	孔隙度/ %	平均孔隙直径/nm	大孔	中孔	过渡孔	微孔	大孔	中孔	过渡孔	微孔
CY1-1	3.47	15.1	0.004 14	0.000 78	0.003 34	0.005 29	30.55	5.76	24.64	39.05
CY1-2	4.20	12.3	0.003 35	0.000 74	0.003 71	0.008 50	20.57	4.52	22.77	52.14
CY1-3	5.22	17.9	0.004 08	0.001 28	0.010 54	0.004 96	19.57	6.15	50.51	23.77
CY1-4	3.98	12.8	0.003 15	0.000 72	0.003 50	0.007 61	21.01	4.80	23.37	50.82
CY1-5	4.54	13.2	0.003 60	0.000 85	0.004 41	0.008 57	20.65	4.88	25.31	49.16
CY2-1	4.39	14.6	0.004 19	0.001 60	0.004 15	0.007 37	24.22	9.24	23.96	42.58
CY2-2	3.82	15.8	0.005 32	0.001 02	0.003 31	0.005 82	34.39	6.58	21.42	37.61
CY2-3	3.75	14.7	0.004 30	0.000 82	0.002 66	0.005 83	31.57	6.05	19.56	42.82
CY2-4	5.56	13.0	0.005 21	0.001 11	0.006 29	0.010 62	22.41	4.80	27.06	45.73
CY2-5	5.41	12.1	0.004 50	0.000 91	0.005 92	0.011 14	20.03	4.03	26.35	49.59
CY2-6	3.93	14.1	0.004 51	0.000 80	0.004 28	0.006 87	27.39	4.88	26.02	41.71
CY2-7	5.65	13.7	0.005 86	0.001 41	0.006 14	0.010 21	24.80	5.98	25.99	43.23
CY2-8	6.20	12.4	0.005 07	0.001 37	0.006 96	0.012 66	19.45	5.26	26.71	48.58
CY2-9	6.67	12.0	0.005 14	0.001 05	0.007 81	0.014 46	18.05	3.70	27.43	50.82
CY2-10	5.30	13.4	0.004 55	0.001 05	0.006 91	0.009 57	20.61	4.76	31.30	43.33

样品编号	比表面积/ （m²·g^-1）	单点孔体积/ （cm³·g^-1）	平均孔隙直径/nm
CY1-1	22.652	0.032 90	5.809 99
CY1-2	21.566	0.031 54	5.849 57
CY1-3	12.603	0.028 50	9.046 74
CY1-4	12.970	0.026 04	8.031 87
CY1-5	15.383	0.027 99	7.278 61

样品编号	脆性指数/%	样品编号	脆性指数/%
CY1-1	16.08	CY2-4	6.59
CY1-2	33.94	CY2-5	4.68
CY1-3	24.95	CY2-6	6.12
CY1-4	38.24	CY2-7	6.15
CY1-5	28.42	CY2-8	3.65
CY2-1	21.63	CY2-9	3.90
CY2-2	9.50	CY2-10	3.11
CY2-3	9.51