Abstract: The entry pressure is the minimum driving force which is responsible for oil and gas secondary migration and also major index that measures the secondary migration and the blocking capability of a fault.Opening and closing of a fault depends on the difference of the entry pressure in the fault zone and that in its two sides.This paper puts emphasis on the study of closing of southern Keping fault,Yinggan fault and Kulukejumo fault in the north Tarim basin with reference to the pressure-closed lower limit of mudstone cover and r_c30 lower limit of oil-gas migration.The result is consistent with what were observed in the field.