Abstract: Based on comprehensive study of source rock, geochemical characteristics of crude oil, correlation of oil and source and hydrocarbon migration pattern of the Eogene in Chexi sag, this paper draws several conclusions. Source rock in the lower-middle Es³ (the third member of Shahejie formation of Eogene) which is buried 2 800-3 000 m below is the major source rock in Chexi sag. Source rock in the upper Es³ and Es¹ which is buried 2 200 m below has some potential of immature oil. At present, most crude oil comes from the high-grade source rock of lower Es³. Sporadic petroleum discoveries are related to source rock of the middle Es³, upper Es³ and Es¹. Petroleum generated from source rock of the lower Es³ at first migrated vertically through faults to reservoir beds in Es². Later, because that the vertical pathway was sealed, petroleum migrated laterally to reservoir beds in the upper Es⁴ or buried hill reservoir. These reservoir beds below source rock of the lower Es³ area the most important area for future exploration.