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

Abstract

The desorbed gas composition and δ13C(CH4) and δD(CH4) of five coal groups in Well DE-A of the Labudalin Basin in northeastern Inner Mongolia show that the δ13C(CH4) and δD(CH4) are heavy, and significantly heavier in some layers, whereas the overall δ13C(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 (δ13C(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 δ13C(CH4) in Derbu is relatively heavy, wherein different coal groups and different coal seams of the same coal group have different characteristics, and the δ13C(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 δ13C(CH4) and δD(CH4). Note that the thermal cause is the basis for the heavier δ13C(CH4) of the gas sample, while the carbon isotope fractionation of methane and oxidation of microorganisms further aggravate the δ13C(CH4). The preservation conditions, such as lithofacies combination sequence and hydrogeological conditions, of different coal formations are also important factors influencing δ13C(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 along the short high permeability layers (coal seam or sandstone layer), and influence the coal-bearing seam. Meanwhile, different groundwater dynamic conditions also result in the transformation of desorbed gas in different coal groups, revealing variation in the methane carbon isotopes.