Coal Petrology Effect on Nanopore Structure of Lignite: Case Study of No. 5 Coal Seam, Shengli Coalfield, Erlian Basin,
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Original Paper
Coal Petrology Effect on Nanopore Structure of Lignite: Case Study of No. 5 Coal Seam, Shengli Coalfield, Erlian Basin, China Jincheng Zhao,1,2 Jian Shen,1,2,4 Yong Qin,1,2 Jinyue Wang,3 Junlong Zhao,1,2 and Chao Li1,2 Received 20 May 2020; accepted 30 August 2020
The pore structure of lignite reservoirs is affected mainly by the maceral composition of coal and depositional conditions due to geothermal effects without thermogenic hydrocarbon generation. Low-temperature nitrogen adsorption (LT-NA) and low-temperature carbon dioxide adsorption (LT-CA) were used to characterize nanopore structure. Sub-maceral identification allowed characterization of coal petrology features. The coal samples were rich in huminite—mainly attrinite and densinite—and had medium inertinite content, dominated by fusinite. The micropore volume varied from 0.036 to 0.077 cm3/g, controlling the pore specific surface area. Three pore types were identified from LT-NA isotherms. Hysteresis coefficients were used to describe quantitatively the pore types. Low tissue preservation index and vegetation index, moderate gelification index, and groundwater influence values indicate that the No. 5 coal seam (Shengli coalfield, Erlian basin, China) was deposited in limnic to limno-telmatic environments with relatively high levels of reduction and supply of marginal aquatic/herbaceous vegetation. Poor preservation of plant structure and hydrodynamic conditions, more developed marginal aquatic/herbaceous vegetation, and high ratio of huminite to inertinite contributed to high micropore content and low mesopore content. Based on sharp variations in coal macerals, coal facies indices, dried ash yield (Ad), and organic sulfur (So,d), three sedimentary cycles were distinguished. Pore structures were controlled by the sedimentary environment. Micropores increased with enhanced reduction and reduced oxidation. When the redox of the sedimentary environment showed little variation, mesopores increased with rising hydrodynamic conditions. KEY WORDS: Lignite, Maceral composition, Pore structure, Coal facies, Coal petrology, Peat-forming environment.
INTRODUCTION 1
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China. 2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China. 3 Henan Province Research Institute of Coal Geological Prospecting, Zhengzhou 450000, China. 4 To whom correspondence should be addressed; e-mail: [email protected]
Resources of low-rank coalbed methane (CBM) occupy an important position in global CBM resources. The USA, Canada, and Australia have commercialized CBM resources in the Black Warriors, Powder River, and Bowen Basin, respectively (Moore 2012). Geological resources of low-rank CBM in China are about 18 trillion cubic meters,
Ó 2020 International Association for Mathematical Geosciences
Zhao et al. accounting for about 47% of total CBM resources in China (Q
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