Special Oil & Gas Reservoirs ›› 2025, Vol. 32 ›› Issue (2): 22-32.DOI: 10.3969/j.issn.1006-6535.2025.02.003

• Geologic Exploration • Previous Articles     Next Articles

Microporous structure of the Longmaxi Formation shale reservoirs in southern Sichuan Basin and its effect on adsorption capacity

LI Shuaizhi1,2, LIU Chenglin1,2, LIU Wenping1,2,3, HE Yubo1,2, LIU Jia4, XU Liang4   

  1. 1. State Key Laboratory of Oil and Gas Resources and Engineering, China University of Petroleum (Beijing), Beijing 102249, China;
    2. College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China;
    3. PetroChina Southwest Oil & Gasfield Company, Chengdu, Sichuan 610051, China;
    4. Shale Gas Research Institute, PetroChina Southwest Oil & Gasfield Company, Chengdu, Sichuan 610051, China
  • Received:2024-05-11 Revised:2025-01-25 Online:2025-04-25 Published:2025-06-16

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Abstract: Shale gas primarily exists in an adsorbed state on the surfaces of organic matter and shale mineral particles. To enhance the understanding of the nanoscale micro-and meso-porous structures within the first sub-bed, first sub-member of the Longmaxi Formation and to compare the differences in the impact of two types of pore structures on shale gas adsorption capacity, this study employed argon ion polishing-field emission scanning electron microscopy, low-pressure N2/CO2 adsorption, and CH4 isothermal adsorption experiments, analyzed the differences in micro porous structure and meso porous structure and pore size distributions among various shale lithofacies, established the relationship between micro porous structure and meso porous structure and shale gas adsorption capacity, and compared the contributions of the two types of pores to shale gas adsorption capacity. The study shows that: the Longmaxi Formation shale predominantly develops micropores, with dominant pore size ranging from 0.4 to 0.6 nm and 0.8 to 1.0 nm, and has a relatively underdeveloped meso porous structure. The argillaceous shale has the largest micropore specific surface area, while the calcareous shale has the largest meso pore specific surface area. The argillaceous shale has both the largest micro porous structure and meso pore volumes. Shales of argillaceous, siliceous, carbonate, and mixed lithofacies all exhibit fractal characteristics in their pore structures, with the siliceous shale having the largest micropore fractal dimension and the calcareous shale having the largest meso pore fractal dimension. As the Total Organic Carbon (TOC) content increases, the fractal dimension, specific surface area, pore volume, and meso pore volume of shale micropores correspondingly increase. The TOC content, micro pore and meso pore specific surface areas, fractal dimensions, and pore volumes all positively contribute to shale gas adsorption capacity, with micropores having a stronger influence. The argillaceous shale with high TOC content, maximum micropore specific surface area, and pore volume is the most favorable lithofacies for shale gas adsorption capacity. The findings of this study have significant implications for the exploration and development of shale gas in the Longmaxi Formation in southern Sichuan Basin.

Key words: shale, pore structure, adsorption capacity, Longmaxi Formation, southern Sichuan Basin

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