The co-existence of both Upper Triassic organic-rich sediments and super-tight sand-stone is a very typical petroleum geological condition in Western Sichuan Basin.The oil-generating peak period in the end of Middle Jurassic was just the time when original porosity of the sandstone reduced and secondary porosity increased gradually.Favourable condition,combined with various traps formed earlier,formed the first upsurge of lateral migration and accumulation.After continuous subsidence of more than 100 million years of the basin,the rocks were already getting super-tight (average Φ<7%),hydrocarbon generating was slow down and turned to craking,and superpressure formed in sedimenta-ry rocks,thus resulting in the sluggishness of natural gas.Himalayan tectonic movement resulted in many new structural traps,and fractures connected pore spaces in the super-tight sandstone.This let to the second upsurge of both horizental and lateral migration and accumulation.The porosity of supertight sandstone tended to be“pseudo-limit”of gas migration,this made things more complicated for gas accumulation and recovery:1.Linear gas-pool groups distributed along fracture system;2.Layer-like reservoirs formed by felty sand-stone with fracture system could be good anticlina gas pools with edge water drive,but the gas production would vary remarkably in different wells.However,this type of gas poolsare still the main targets for gas prospecting in West Sichuan.3.Porosity lower than the“pseudo-limit” (Φ≈2.5%,throat T≈10nm)is not valuable,but the gas would flow out from fracture belt suddenly into the wellbore and even make blowout sometime,then the production will decline rapidly.4.Gas fields with certain reserve might form in local places where better porosity of sandstone equivalent to the tight sandstone class (8%—15%)at the shallow formations or favourable facies and petrogenetics.The production is high when fissure developed in the sandstone;otherwise,it is low.This is another target for gas exploration.
