In porous media, non-wetting phase is trapped upon water saturation due to capillary forces acting in a heterogeneous porous structure. Within the capillary fringe gas phase is trapped and released along with the fluctuation of the water table, creating a highly active zone for biological transformations and mass transport. We conducted column experiments to observe and quantify magnitude and structure of trapped gas phase at the pore-scale using micro-CT. Different grain size distributions of glass beads were used to study the effect of pore structure on trapping at various capillary numbers. Viscous forces were found to have negligible impact on phase trapping compared to capillary and buoyancy forces. Residual gas saturations ranged from 0.5% to 10%, while residual saturation increased with decreasing grain size. Gas phase was trapped by snap-off in single pores but also in pore clusters, while this single-pore trapping was dominant for grains larger than 1mm in diameter. Gas surface area was found to increase linearly with gas volume and increases with decreasing grain size.