Abstract: In studying the threshold temperature(depth)of oil generation of the source rocks in Qijia-Gulong and Sanzhao depressions in the Songliao Basin with J, Cannan time-temperature formula, a correction factor of 18℃ should be applied to negate the error caused by the geothermal differences between the ancient and recent ages. It has been found that oil generation in the Qijia-Gulong depression started at the time of deposition of Sifangtai formation in late Cretaceous, earlier than that in Sanzhao depression which began at the time of deposition of Mingshui formation in late Cretaceous. This is the reason why oil of Daqing field has its origin from the former, and few oil pools occur in the later.