The Western Sichuan foreland basin is not only a typical foreland basin in west China, but also an important tight sandstone gas area in China. Fluid cross-formation flows took place in the foreland basin. Fluids within the permeable layers of the Second and Forth Members of the Upper Triassic Xujiahe Formation have flowed cross the impermeable layers of the Third and Fifth members of the Upper Triassic Xujiahe Formation to the permeable layers of the Jurassic Ziliujing Formation, Jurassic Qianfoyan Formation, Shaximiao Formation, Suining Formation and Penglaizhen Formation. The fluid cross-formation flow has made Jurassic sandstone be secondarily dissolved by acidic water and become better reservoir rocks. Besides, it has afforded a natural gas source for the formation of Jurassic gas pools. The gas reservoirs in the foreland basin are characterized by chimney effect of gas activity, early accumulation and late formation of natural gas, and abnormal high pressure. Deep-basin gas is the basic state of the early accumulation of natural gas in the Upper Triassic (mainly the Second and Forth Members of the Xujiahe Formation) in the foreland basin and the basis for late-stage formation of gas reservoirs. Such areas are called rich-in-gas areas in this paper. They are the requisite for the forming of late-stage gas pools. Natural gas reservoirs were formed because early-stage rich-in-gas areas were broken due to uplifting of the crust to produce fluid cross-formation flows, and as a result natural gas was migrated and cumulated at a very high speed and in a large quantity through relatively narrow paths. Such formation of natural gas is called explosion accumulation. This type of gas accumulation generated Upper Triassic fracture-readjusted gas pools and Jurassic gas pools, both of which are not secondary gas reservoirs. Identification and evaluation of the early-stage gas accumulation and related rich-in-gas areas are the key issue and also a difficult problem in the future as regard to exploration of gas pools in the foreland basin of western Sichuan.