Controls on commercial helium accumulation in coal seams: From fundamentals to an integration of basin geology and gas geochemistry

Coal seams, as a result of the coalification of terrestrial plant matter, were previously regarded as unfavorable for economically significant helium accumulations. However, this viewpoint has been challenged by the increasing discoveries of helium-rich gas reservoirs in such settings. This review investigates the major controls on commercial helium accumulations in coal seams, using an integrated analysis of basin geology and gas geochemistry. Six main findings are summarized as follows. (1) Naturally occurring helium accumulates in nitrogen-, carbon dioxide-, methane-, and mixed-gas reservoirs, with nitrogen and methane reservoirs being the most favorable for commercial helium accumulation. (2) Commercial helium accumulation in coal seams is predominantly (' 70%) of external origin, although these seams are commonly enriched in thorium and uranium. (3) The formation of helium accumulations in coal seams largely depends on an external replenishment system, including source rocks, carrier fluids, migration pathways, and an open system. (4) Two key geological elements contribute to favorable trap conditions: one is coal seams that show high water saturation, effective pore connectivity, and adequate natural gas; and the other is overlying cap rocks that exhibit fine pore throats, high gas pressure, low temperature, and high associated fluid content. (5) An accumulation window at a depth of ca. 100–1250 m for helium in coal seams is proposed. (6) Three main geological patterns, namely basement uplift, basement fault and anticlinal crest patterns, are proposed for commercial helium accumulations in coal seams, and these patterns can be distinguished by variations in He/N₂ ratios and N₂, CO₂, and CH₄ contents. The findings presented herein are expected to provide guidance for helium exploration in coal seams.