Future world energy consumption is projected to increase along with the high economic growth of emerging countries. This will make it necessary to expand the development of unconventional resources such as shale oil and gas and coalbed methane. The production of secondary methane gas using microorganisms at oil fields and coal mines is another area of great interest in the development of unconventional resources. This paper describes a feasibility study involving a new method to convert unused organic matter contained in sedimentary rock in northern Hokkaido into biogenic methane. The new method, subsurface cultivation and gasification (SCG), is proposed for the production of biogenic methane gas in the subsurface environment. Our approach uses hydrogen peroxide (H2O2) to decompose organic matter rapidly into usable substrates for methanogens. Two wells were drilled through several lignite seams in the Soya coal-bearing formations in the Tempoku coalfield in northern Hokkaido, Japan, and lignite cores were recovered. Batch tests using a 1 wt% H2O2 solution were performed on the lignite to estimate the amount of low-molecular-weight organic acids as substrates for methanogens. Oxidative decomposition of the lignite produced a high yield of low-molecular-weight organic acids. Moreover, it was confirmed that methanogens converted the H2O2 reaction solution containing organic acids into methane. H2O2 would be useful for effective SCG at lignite seams, and conversion of organic matter from lignite into biogenic methane with the help of microorganisms is expected to be highly profitable. The results reveal that using SCG to produce biogenic methane in this coalfield shows great commercial promise.