Stable isotope-based origin identification and enrichment zone division of medium- to low-rank coalbed methane

Authors

• WANG Qiong, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, ChinaFollow
• XU Fengyin, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, ChinaFollow
• REN Pengfei, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, China
• XU Hao, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
• LU Pengda, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, China
• HU Han, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, China
• ZHOU Siyuan, College of Energy Resources, Chengdu University of Technology, Chengdu 610051, China;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610051, China

Abstract

Objective Medium-to low-rank coalbed methane (CBM) exhibits complex origins with significant vertical differentiation, leading to unclear origin identification and enrichment patterns. This study aims to determine the genetic mechanisms, enrichment patterns, and efficient exploitation pathways of medium- to low-rank CBM. This will help improve the theoretical system for the exploration and exploitation of such CBM. Methods CBM data from 14 typical low- to medium-rank coalfields and coal-bearing basins across the world were comprehensively analyzed, including CBM compositions C₁/(C₂+C₃), carbon and hydrogen isotopes of CH₄ (δ¹³C-CH₄ and δD-CH₄), carbon isotope of CO₂ (δ¹³C-CO₂), and hydrogen and oxygen isotopes of associated coalbed water (δD-H₂O and δ¹⁸O-H₂O). Based on these data, as well as the CBM characteristics of typical blocks along the southern margin of the Junggar Basin, this study systematically investigated the origin identification, vertical zonation, and exploitation strategy of CBM. The proportions of biogenic and thermogenic gases in medium- to low-rank CBM were characterized both qualitatively and quantitatively. Accordingly, the differential genetic mechanism of such CBM was proposed, revealing that medium- to low-rank CBM is dominated by biogenic gas in shallow parts and is supplemented by thermogenic gas in deep parts. Finally, the genetic and enrichment zones of medium- to low-rank CBM were determined. Results and Conclusions The results indicate that the medium- to low-rank CBM across the world exhibit δ¹³C-CH₄ values ranging primarily from –70‰ to –50‰ and C₁/(C₂+C₃) ratios mostly falling within 100-10000. These findings suggest a predominance of biogenic origin, with CO₂ reduction identified as the primary pathway of microbial gas production. Vertically, medium- to low-rank coal seams can be divided into five CBM genetic and enrichment zones: methane weathering zone, biogenic gas enrichment zone, transition zone of gas with mixed origin, in situ thermogenic gas zone, and thermogenic gas enhancement zone mixed with exogenous gas from shallow to deep. Based on the vertical complementary and paragenetic characteristics of gas resources, this study proposes an integrated full-depth exploitation strategy for the efficient production of both shallow biogenic gas and deep thermogenic gas. The results of this study improve the theoretical system for the exploration and exploitation of medium- to low-rank CBM, providing a scientifically robust basis and technical pathway for the full-depth efficient exploitation of similar gas reservoirs.

Keywords

medium- to low-rank coal, coalbed methane (CBM), biogenic gas, thermogenic gas, stable isotope, full-depth exploitation

DOI

10.12363/issn.1001-1986.25.12.0882

Recommended Citation

WANG Qiong, XU Fengyin, REN Pengfei, et al. (2026) "Stable isotope-based origin identification and enrichment zone division of medium- to low-rank coalbed methane," Coal Geology & Exploration: Vol. 54: Iss. 8, Article 9.
DOI: 10.12363/issn.1001-1986.25.12.0882
Available at: https://cge.researchcommons.org/journal/vol54/iss8/9

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