Abstract: In the Mesozoic petroleum system in Yanqi basin,the Badaowan Formation is the principle source rock with coal-measure mudstone as the main type of source rock.The Sangonghe Formation is the main reservoir with braid ed river and fan delta as the key sandboy types.Reservoir conditions are poor and it belongs to low porosity and low permeability reservoir types.The paleo-traps took shape in J 1s .Two hydrocarbon generation and expulsion events occurred,one at the end of Jurassic and the other from the end of Tertiary till now.Most of the hydrocarbon migrated vertically along the faults.Two petroleum systems,one primary and one secondary,are recognized corresponding to two reservoir-forming events.The evolution of petroleum system went through3stages:formation of paleo-traps at the end of Jurassic;tectonic movement and modification and destruction of plaeo-reservoirs during the middle and late stages of Yanshan period;and overlying of the Tertiary and formation of the current reservoirs in Himalayan period.The reservoirs formed at early stages were destructed by the multistage activation of faults and by uplift and erosion occurred during the middle and late stages of Yanshan period,while the reservoirs formed during Himalayan period are well preserved till present.