In the late Triassic Yanchang period

the Ordos Basin was a continental depression basin.The deposits of lake-delta facies are prevalent

and hydrocarbon reservoirs are mainly low-and ultra-low-permeability litholo-gic type.A research into the area reveals that the abnormally high pressure resulted from hydrocarbon generation in the Chang-7 Formation serves as the major dynamic force for the episodic migration of oil toward low-permeability reservoirs.Physical simulation was performed to analyze the process and features of hydrocarbon accumulation in low-permeability reservoirs under different pressures and to compare the characteristics of episodic hydrocarbon accumulation and steady hydrocarbon accumulation under different charging pressures and at different charging direction.The results show that

regardless the direction of hydrocarbon expulsion from source rocks

the higher the pressure in the low-permeability lithology reservoirs is

the higher the hydrocarbon accumulation efficiency and the oil saturation are.