Good old Wikipedia...Underground coal gasification (UCG) is an...

Good old Wikipedia...

Underground coal gasification (UCG) is an industrial process which converts coal into product gas. UCG is an in-situgasification process carried out in non-mined coal seams using injection of oxidants, and bringing the product gas to surface through production wells drilled from the surface.^[1]

The predominant product gases are methane, hydrogen, carbon monoxide and carbon dioxide. Ratios vary depending upon formation pressure, depth of coal and oxidant balance. Gas output may be combusted for electricity production. Alternatively gas can be used to produce synthetic natural gas or hydrogen and carbon monoxide can be used as a chemical feedstock for the production of fuels (e.g. diesel), fertilizer, explosives and other products. The technique can be applied to coal resources that are otherwise unprofitable or technically complicated to extract by traditional mining methods. UCG offers an alternative to conventional coal mining methods for some resources. It has been linked to a number of concerns from environmental campaigners.^[2]

Thing is, it seems the proportions of gases produced (from the same feedstock) can be fine tuned depending on varying the technique and parameters.

This PDF is quite comprehensive and shows 10.8% CH4 from North Dakota lignite.

https://www.researchgate.net/publication/265687040_Cost_comparison_of_syngas_production_from_natural_gas_conversion_and_underground_coal_gasification/download