Abstract: Depositional settings of the Upper Triassic in the western Sichuan Basin are favorable for gas accumulation in the deep basin.The delta deposit constitutes the dominante reservoir in the deep basin.The numerous and mature coal-beds and organic-rich shales associated intimately with reservoir rocks throughout the region provide a prerequisite for gas accumulation in deep basin.Gas was trapped in the deepest part of the basin and occupies low-permeability reservoirs extensively.Gas was generated in adjacent source rocks,and moved into the sand layer, and then slowly migrated updip. This special form of gas trap in deep basin defies conventional concepts of gas entrapment by turning them virtually upside down.Gas is located downdip of a narrow aquifer which outcrops in basin margin.The gas/water contact is really a wide belt,which marks the transition from downdip gas to updip water. Original gas accumulation pressures lie above the regional formation water pressure gradient.Gas-productive units in the deep basin are not controlled by local structure.

Key words: deep basin trap, tight sand, gas/water inversion, abnormal pressure, trapping mechanism, Upper Triassic, Sichuan Basin

摘要： 川西前陆盆地的盆地演化、盆地结构和相对稳定的斜坡构造有利于深盆气的形成和聚集.上三叠统为海陆过渡相和陆相湖盆沉积,以三角洲沉积为主的砂体十分发育,晚侏罗世中期至早白垩世末期储层开始致密化,早白垩世以后储层进入超致密阶段,但对于深盆气仍是十分有效的储层.砂体的上覆和下伏层均为暗色泥岩夹薄煤层的气源岩,因此对深盆气的聚集和富集成藏十分有利.煤系气源岩分布广、厚度大、生气潜力高、热演化程度高,从而为川西坳陷深盆气的形成提供了充足的气源.在早白垩世-晚白垩世中期,源岩进入生气高峰,为深盆气形成的重要时期,造成孔隙中流体体积急剧膨胀,引起川西区处于超压环境.喜山运动后盆地大范围仍处于水动力交替停滞带,上三叠统深盆气藏大面积具有整体封存条件.因此,上三叠统形成了一个几乎覆盖全盆地的特大型深盆气藏.通过对深盆气藏的主控地质因素的分析,可将川西坳陷划分为深盆气分布区、气水过渡带和上倾含水区(常规气藏分布区)等三个区带.从全盆地看,气水分布具有明显的倒置关系,深盆区内气水分布不受构造控制.

关键词: 深盆气藏, 致密砂岩, 气水倒置, 运聚机理, 上三叠统, 四川盆地