Upper Triassic clasolite reservoirs in West Sichuan are a set of tight sand-mud rocks with low porosity-low permeability

high aqueous saturation and abnormal high pressure.According to their fluid pressures and gas reserves

the reservoirs could be divided into superhigh pressure-low reserve type (Ⅰ)

high pressure-high reserve type (Ⅱ) and normal to transitional pressure-high reserve type(Ⅲ).The formation and evolution of paleopressure controlled the formation of gas reservoirs.Successive pressure increase caused by gas generation and compression resulted in superhigh pressure-low reserve type reservoirs (Ⅰ) ; long-term pressure increase resulted in long distance gas migration and trap area enlargement

and thus forming high pressure-high reserve type reservoirs (Ⅱ); the development of structural fractures and high-pressure enlarged fractures

the early-stage enrichment and late-stage reformation of the reservoirs should be the basic formation model of normal to transitional pressure-high reserve type reservoirs(Ⅲ).