Composition, Origin, and Distribution of Coalbed Methane in the Huaibei Coalfield, China

This study researched the geochemical origin of coalbed methane (CBM) in three mining areas of the Huaibei coalfield and the distribution of different origin gases. The Rock-Eval data indicate that coal and coaly mudstone in this coalfield are both good gas-source rocks and are within the gas generation window. The large ranges of isotope data (delta C-13(CH4) = -42.8 parts per thousand to -75.5 parts per thousand; delta D-CH4 = -153.2 parts per thousand to -228.1 parts per thousand; delta C-13(CO2) = +8.2 parts per thousand to -24.3 parts per thousand) reflect the complicated formation mechanism and evolutionary process of CBM. Both thermogenic and biogenic gas are present within the mining areas to varying degrees, with the biogenic gas being predominately generated via the CO2 reduction pathway. Gas migration was most prevalent in the Suixiao mining area, low in the Linhuan mining area, and not identified in the Suzhou mining area. An appropriate distribution model for the gases of different origin is, from bottom to top, a thermogenic gas zone, a mixture zone, a secondary biogenic gas zone, and a gas-weathered zone. Spatially, secondary biogenic gas was in the basin margins, thermogenic gas in the central part, and the mixed gas in the transitional zone. The CBM prospectivity is greatest in the Suzhou mining area, moderate in the Linhuan mining area, and worst in the Suixiao mining area.