Differentiation of Carbon Isotope Composition and Stratabound Mechanism of Gas Desorption in Shallow-Buried Low-Rank Mul
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Original Paper
Differentiation of Carbon Isotope Composition and Stratabound Mechanism of Gas Desorption in Shallow-Buried Low-Rank Multiple Coal Seams: Case Study of Well DE-A, Northeast Inner Mongolia Geng Li,1,2,4 Yong Qin,1,2,4 Zhongling Yao,3 and Wutao Hu3 Received 15 September 2020; accepted 3 November 2020
The desorbed gas composition and d13C(CH4) and dD(CH4) of five coal groups in Well DE-A of the Labudalin Basin in northeastern Inner Mongolia show that the d13C(CH4) and dD(CH4) are heavy, and significantly heavier in some layers, whereas the overall d13C(CO2) is abnormally light. Based on coalfield exploration data, this study analyzes the characteristics and control factors of methane isotope alienation of shallow-buried low-rank multiple coal seams in a small coal-bearing depression. The maximum buried depth of the coal seam in the Derbu coalfield is less than 600 m, and the coal rank is sub-bituminous coal. The carbon isotope composition (d13C(CH4)) of CBM varies from 56.3 to 39.8&, wherein the arithmetic average is 48.72&. Compared with coal seams of the same metamorphic degree, the d13C(CH4) in Derbu is relatively heavy, wherein different coal groups and different coal seams of the same coal group have different characteristics, and the d13C(CO2) is relatively light. Studies have shown that the existence of thermogenic gas, anaerobic oxidation of methane microorganisms, and desorption-fractionation of methane isotopes in Well DE-A are causes for the heavier d13C(CH4) and dD(CH4). Note that the thermal cause is the basis for the heavier d13C(CH4) of the gas sample, while the carbon isotope fractionation of methane and oxidation of microorganisms further aggravate the d13C(CH4). The preservation conditions, such as lithofacies combination sequence and hydrogeological conditions, of different coal formations are also important factors influencing d13C(CH4) differentiation, and the superposition and coupling of various reasons make this differentiation more obvious. By comparing the geological characteristics of Well DE-A with the vertical variation characteristics of carbon isotopes in the desorbed methane of the Surat Basin, it was concluded that the vertical variation of the desorbed gas sample in shallow-buried low-rank multiple coal seams has complex characteristics. In addition to maturity, local horizons are more obviously controlled by lithologic association and hydrological characteristics. Especially for small coal-bearing depressions or coal-bearing strata near outcrops with shallow-buried depth, the horizontal distance between the outcrop line of different coal formations and target well locations is obviously short. Thus, surface water and surface microorganisms can migrate
1
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, Jiangsu, PeopleÕs Republic of China. 2 School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, Jiangsu, PeopleÕs Re
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