四川盆地威远构造震旦系灯影组气烟囱特征及其地质意义

摘要

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 2 D and 3 D 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 coeffi-cient 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 .%四川盆地威远构造震旦系灯影组气藏充满度仅为25％，与早寒武世拉张槽相隔的高石梯－磨溪构造灯影组则为充满度近100％的特大型整装气田。两者的区域背景和生储盖组合条件相近，但气田充满度出现很大差异。威远构造2D和3D地震剖面存在较多震旦系—三叠系管状异常特征，有的出露地表或隐伏浅层之下，分析可能为气烟囱特征。其主要特征包括：①受气烟囱效应影响，威远构造地表三叠系嘉陵江组出露附近河流中有大量气苗；②上覆寒武系遇仙寺组气藏、洗象池组气藏及二叠系阳新统气藏的天然气非烃气体中的氮、氩、氦成分与威远气田震旦系灯影组天然气成分相似，具有灯影组气藏天然气上窜特征；③威远构造震旦系—二叠系压力系数均为1．0左右，高石梯－磨溪构造灯影组压力系数也为1．0左右，但其上覆层位均为超压。威远构造气烟囱表明，震旦系灯影组气藏封闭系统基本失效，保存条件较差，天然气垂向运移，直接影响现今气藏的规模。