In order to further the course of petroleum exploration and development in the Chagan (查干) depression in the Yingen(银根)-Ejinaqi (额济纳旗) basin, Northwest China, the major character istics of the petroleum systems were comprehensively analyzed, including the essential geological elements, processes, and oil characteristics. Petroleum system theory and quantitative simulation techniques were used. The dolomitic mudstone and shale of the Second Member of the Bayingebi (巴音戈壁)Formation are the major source rocks, with higher organic matter content and greater intensities of hydrocarbon generated and expelled. The sandstone in the First Member of the Suhongtu (苏红图) Formation is the main reservoir rock. The mudstone in the Second Member of the Suhongtu Formation is the most important seal. The crude oil discovered so far has a close relationship with the source rock in the Second Member of the Bayingebi Formation. There are three petroleum systems in the Chagan depression. The primary system is the Second Member of the Bayingebi Formation-the First Member of the Su hongtu Formation, which is the most important target for hydrocarbon exploration in the depression.