Gas seepage from Tokamachi mud volcanoes, onshore Niigata Basin (Japan): Origin, post-genetic alterations and CH4-CO2 fluxes

► Tokamachi gas shows signals of subsurface hydrocarbon biodegradation. ► Hydrocarbon molecular fractionation depends on gas flux. ► Substantial gas emission from mud volcanoes is from invisible diffuse seepage. ► Global mud volcano methane emission is likely higher than 10 Mt a−1.