Abstract: Drilling results have revealed the occurrence of immature crude oil in the shallow reservoirs of the 3^rd and 4^th members of Shahejie Formation in the Western Depression of Liaohe basin. Proved reserves are impressive. We studied in this paper the process of oil generation,migration and accumulation, and believed that the formation of immature oil requires a specific combination of depositional environment of source rocks,availability of migration channels and tectonic activities. The scale of the accumulation depends upon the constraints of hydrocarbon from source rocks and the physical properties of the reservoir rocks. Compared with the mature accumulation, there are both similarities and differences. With the improvement of exploration technology, more lithology and structure controlled immature accumulations have been recognized. Further study of the oil accumulation of this type will doubtlessly broaden the scope of oil and gas exploration in the basin.