Defined as gas accumulation in direct contact with source rocks, deep basin gas is characterized by tight reservoir, close connection of reservoir and source rocks, reversion relationships between water and gas, abnormal geo-pressure, etc. It usually occurs within the deeper part of basins with no water driving. Energies of expelled gas initially comes from the chemical conversion from kerogen during the hydrocarbon generation, which gives the gas energies to leave source rocks and migrate in tight reservoirs. The expanding forces derived from gas generation that results in the piston-modeled driving of formation waters by continuous gas supply bring about the symbolic reversion contact of formation water and gas. There are only two possible ways leading to the reversion contact within the porous rock media. One is a shorter gas column when gas is initially expelled into the normal reservoirs, which is affected by the buoyancy of gas. The other is a greater gas column when the conditions (such as the tight properties, homogeneity and scaled distribution of reservoir and so forth) are met to accumulate deep basin gases, which are affected by the pressures of water columns but with no buoyancy of gas. Compared with the normally trapped gas, the accumulation of typical deep basin gas is dominantly controlled by the capillary pressure and burying depth of tight sands. The original geo-pressures of normally trapped gas can only be of the super-pressures whereas the original geo-pressures of deep basin gas accumulations are widely varied. As regards to the preservation states, both the normally trapped and deep basin gas accumulations can steadily exist when their dynamic powers become equipoise.