Abstract: Fullness coefficient of the Upper Sinian Dengying Fm gas reservoir in the Weiyuan Structure is only 25%,while that in the Gaoshiti-Moxi Structure separated with the former by the Cambrian intracratonic sag is approximately 100%.Though the Dengying Fm in the two structures have similar regional background and source-reservoir-cap assemblage conditions,they have large differences in the fullness coefficient of natural gas reservoir.Abnormal tubular characteristics can be found in the Sinian-Triassic on 2D and 3D seismic profiles of the Weiyuan Structure.Some of them expose on the surface or are subtle under the shallow layer.They are supposed to be the characteristics of "gas chimney".Their major features are as follows:①Affected by the gas chimney,a large amount of gas seepages can be found in rivers near the Lower Triassic Jialingjiang Formation outcrops of the Weiyuan Structure; ②Non-hydrocarbon gases components such as nitrogen,argon and helium of the gas reservoirs in the Upper Cambrian Yuxiansi Formation,Xixiangchi Formation and Lower Permian Yangxin Formation are similar to the Upper Sinian Dengying Formation in Weiyuan,indicating the upward channeling of gas in the Upper Sinian Dengying Fm; ③ Although the pressure coefficient is 1.0 in the Dengying Formation of Gaoshiti-Moxi Structure,the overlying strata have overpressure.The existence of gas chimneys in Weiyuan shows that the seal system of the Upper Sinian Dengying Fm gas reservoir is basically failed,thus the sealing conditions are poor.The vertical migration of natural gas directly affects the scale of the current gas reservoir of the Weiyuan Structure.

Key words: gas chimney, natural gas, Dengying Formation, Weiyuan Structure, Sichuan Basin