Based upon high resolvable 3D seismic data as well as seismic attributes, a great many gas chimneys are discovered in deepwater Baiyun Sag, the Pearl River Mouth Basin. A gas chimneys can be divided into three components: the root, the leakage zone and the top of the leakage zone. Every one may be of different seismic reflection anomaly including lateral changes of events, local amplitude, frequency and continuity. The roots of gas chimneys are the initial parts of releasing gas pressure and also the source of supplying gas. The leakage zones are the main parts of whole gas chimneys for fluid to pass through. Gas reservoirs are commonly present at the tops of the leakage zones. Two models related with gas accumulation are built. One is the model of gas accumulation at the flanks and the other is the one at the tops of gas chimneys. The hydrocarbon in those chimneys is all derived from Paleogecene to Neogene lacustrine sediments.