The Kela-2 gas field in the Kuqa Depression of the Tarim Basin is one of the greatest fields recently discovered in the deep overpressure reservoir in the world. The play conditions of Kela-2 field and Ya21-1 structure features in the Qiongdongnan Basin on the South China Sea shelf has been studied, and a gas accumulation under deep overpressure must have the following two conditions (besides other favourable conditions for gas accumulation): one is that the time of gas charge matches with overpressure occurrence time, and the other is a good quality caprock. If the charge was early and the overpressure occurrence was late, and a fine caprock of the evaporite bed existed, the accumulation of gas is favourable; conversely, it is difficult to form a gas reservoir.