Abstract: The Chagan sag is one of sags that are highly explored in the Yingen-Ejina Banner Basin. Affected by volcanic rock and multi-stage tectonic activities, its geological conditions are complex, and its reservoir sands are thin and quickly change in lateral direction, which make it difficult to find high-quality reserves. Through comprehensive analysis of the petroleum geologic features in the Chagan sag, it is believed that the source rock distribution controls the oil and gas distribution – oil and gas are mainly distributed near source and around depression. The structure of the sag controls the dominant migration direction of oil and gas, the long-active faults provide effective channels for oil and gas migration, and the effective reservoir controls the oil and gas enrichment degree. Two hydrocarbon accumulation patterns are formed, i.e. self-generating and self-preserving type, and lower-generating and upper-preserving type. On this basis, fruitful exploration results have been achieved in the Chagan sag. For example, 3 hydrocarbon accumulation associations (upper, middle and lower) and 5 oil-bearing series are found and the Jixiang and Ruyi oilfields are proven.

Key words: Chagan sag, petroleum geologic features, hydrocarbon accumulation characteristics, exploration discovery