GAS TRAP IN DEEP BASIN OF THE UPPER TRIASSIC IN SICHUAN BASIN

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