Abstract: The third member of the Oligocene Dongying Formation (E₃d₃) in the eastern Liaohe offshore area is the most important exploration stratum rich in oil and gas resources, and the exploration degree of positive structures is high. However, whether the sag area can provide sediment supply and develop large-scale sand bodies is a key issue that urgently needs to be resolved for further reserve discoveries. Based on the analysis of the paleogeomorphic background during the depositional period and the characteristics of surrounding source rocks, we discussed the sediment source directions and depositional systems through multiple methods such as heavy mineral analysis, sand-to-mud ratio, seismic attributes, and single-well facies analysis. Results show that the paleogeomorphic background of the E₃d₃, characterized by high elevations in the north and low in the south, and high on both the eastern and western sides, which created conditions for the development of multi-directional sediment sourced depositional systems in the area. During the depositional period of the E₃d₃, the source rocks consisted of mainly three types: Archean metamorphic rocks, Mesozoic glutenite, and Mesozoic volcanic rocks. The characteristic assemblages of dominant heavy minerals could be divided into four types: garnet + leucoxene, garnet + tourmaline, titanomagnetite + garnet, and garnet + staurolite. The study area features multi-source supply and multiple types of depositional systems. There were two sediment sources from the east and west in the southern part, primarily forming braided river delta and shallow lacustrine depositional systems. The northern part received sediment sources from the northeast-east, northwest, and northeast directions, mainly forming delta depositional systems, which provided conditions for the development of large-scale sand bodies.