Understanding coalbed gas distribution from longwall mining: Geochemical and isotopic approach

Coalbed methane plays a critical role in both energy resource potential and environmental and mine safety strategies. Despite much research has been done on coalbed gas origin, the combined effects of coal maturity, gas migration processes, and mine ventilation in an active mining environment are still not fully understood. This study combined molecular and stable carbon and hydrogen isotope analyses of coalbed gases collected along the longwall coal face during the early and final stages of operation, with petrographic and geochemical characteristics of freshly mined coal from the Upper Silesian Coal Basin, Poland. Complementary organic petrography and biomarker analysis performed on freshly mined coal revealed that the vitrinite-rich bituminous coal is at peak oil-window maturity. Gas composition from a borehole in the coal seam indicated methane as the dominant component (up to 87 %), with contributions from carbon dioxide (up to 2.3 %) and lower amounts (0.1–1.5 %) of heavier hydrocarbons (C₂ − C₆). Heterocyclic species of sulfur (thiophene) and nitrogen (pyridine), chlorinated (1,2-DCE) compounds, oxygen-bearing organic gases (acetic acid), dimethyl sulfide (DMS), and some portion of ortho-cresol, tetrahydrofuran, formaldehyde, and cumene tracers were also found in the complex gas mixtures from the borehole.

Isotopic data of methane in the examined longwall coal face averaged at −48.1 ± 0.7 ‰ for δ¹³C and − 192.1 ± 7.1 ‰ for δ²H and coal maturity vitrinite reflectance at 0.75 ± 0.02 %. These molecular and isotopic coalbed gas data implied a thermogenic origin of the coalbed methane, with fingerprints of biodegradation in thermogenic system. Slightly depleted isotope values from the borehole under examination (δ¹³C: −50.9 ± 0.2 ‰ and δ²H: −194.2 ± 2.8 ‰) suggested the presence of free gas stored in the coal fractures. Spatial variations in gas composition and isotopic shifts across the longwall panel were found to be driven by ventilating airflow supplied to the working face. Isotope alteration along airflow path from main- to tailgate entries emphasized the influence of secondary processes, such as desorption and diffusion. Preferential loss of isotopically light ¹²CH₄ molecules, especially at gas accumulation zones (tailgate), aligns with findings from laboratory desorption experiment on successively crushed coal samples. In contrast, less systematic δ²H-CH₄ patterns under active mining reflected the combined effects of gas mixing, site-specific conditions, and parameters of coal mine operations.