Abstract: The Chagan Sag is an important hydrocarbon exploration area in the Yingen-Ejinaqi Basin. Oil has been discovered in multiple formations including the Bayingebi, Suhongtu and Yingen formations in the Lower Cretaceous. Reservoirs were found mainly in the Wuliji and the central structural belts. Hydrocarbons generated from source kitchens migrated for a short-distance and accumulated in surrounding traps. Hydrocarbon generation, charging and accumulation time analysis showed that accumulation can be divided into three stages:the second member of Suhongtu Formation, the Yingen Formation and the Neogene. Different stages of hydrocarbon accumulation resulted in various physical properties, maturities and biomarkers of crude oils.