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Differences in CH4 and C2H6 carbon isotopic compositions from open and closed pores in coal: Implications for understanding the two-stage δ13C shift during canister desorption
International Journal of Coal Geology ( IF 5.6 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.coal.2020.103586 Yilin Chen , Bo Zhang , Yong Qin , Zhongping Li , Zhaobiao Yang , Caifang Wu , Chunhui Cao
International Journal of Coal Geology ( IF 5.6 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.coal.2020.103586 Yilin Chen , Bo Zhang , Yong Qin , Zhongping Li , Zhaobiao Yang , Caifang Wu , Chunhui Cao
Abstract Canister desorption of freshly drilled coal cores is routinely employed for exploratory assessments of coalbed gas. Coals are molecular sieves containing both open and closed pores. However, little attention has been paid to the differences between the carbon isotopic compositions of CH4 and C2H6 from open and closed pores in coal and their influence on isotopic fractionation during desorption. In this study, canister desorption and pulverization–degassing experiments were conducted to study the carbon isotopic shifts of CH4 and C2H6 during desorption. The δ13C values of CH4 and C2H6 desorbed from three coal samples from the Zhina Coalfield in southwest China displayed a two-stage shift over time during canister desorption (that is, an isotopic decrease followed by an increase), ranging from −38.4 to −30.6‰ and − 32.1 to −28.5‰, respectively. The δ13C values of CH4 and C2H6 desorbed from the coal samples during pulverization ranged from −29.3 to −24.1‰ and − 31.5 to −27.2‰, respectively, approaching the isotopic ranges of coal-derived gas. Low-pressure N2 adsorption measurements showed that many closed pores in coal were opened during progressive pulverization. Gases trapped in closed pores may be considered to be of their original nature, unaffected by subsequent geological processes (i.e., mixing of secondary biogenic gas). Thus, CH4 and C2H6 were more depleted in 13C in open pores than in closed pores. Some open pores on the surface of the coal core were previously closed when in situ but were opened during drilling. Therefore, the initial gas released from the cores was a mixture of gases desorbed from the open and closed pores. The 13C-enriched CH4 and C2H6 desorbed from the pores on the coal core surfaces may migrate into canister headspaces preferentially, thereby providing insight into the two-stage δ13C shift observed during canister desorption.
更新日期：2020-10-01
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