Adsorbed gas in a formation may be estimated. Nuclear magnetic resonance (NMR) data for a subsurface geological formation is obtained, and at least a portion of the NMR data is corrected to produce corrected NMR data. A NMR-based estimate of formation porosity is determined using the corrected NMR data. Dielectric permittivity data for the subsurface geological formation is obtained, and a dielectric permittivity-based estimate of the formation water-filled porosity is determined using the dielectric permittivity data. A gas volume is determined using the determined NMR-based estimate of the formation porosity and the determined dielectric permittivity-based estimate of the formation water-filled porosity. The gas volume may be determined by subtracting the determined dielectric permittivity-based estimate of the formation water-filled porosity from the determined NMR-based estimate of the formation porosity. The gas volume per unit volume of the formation may be determined using an equation of state.