Abstract: The Songliao Basin is the largest non-marine oil-bearing basin in China. Due to the absence of sufficient evidence, the hypothesis of seawater incursion(s) into the Songliao Basin remains controversial. Marine fossil materials can provide direct explana-tions. More recently, a few calcareous nannofossils were discovered from units 1 and 2 of the Nenjiang Formation in drill hole SK-1. In these fossils, some taxa have been positively identified, namely Calculites obscurus, Calculites ovalis, Quadrum sp., and Micula sp. They were distributed in marine environment of the Cretaceous. Based on comparisons of the calcareous nannofossil assemblages from global records, the age of units 1 of the Nenjiang Formation seems to have spanned from the latest Turonian to Maastrichtian. The nannofossils from the Songliao Basin have the marine origin, characterized by common diversity and abundance. The distribu-tion and paleoecology of the calcareous nannofossils as well as the co-existing foraminifera in the SK-1 provide further evidence for seawater incursion events in the Songliao Basin. The authors hold that periodical seawater incursion(s) brought calcareous nannofossils and foraminifera into the Songliao Basin during sedimentation of units 1 and 2 of the Nenjiang Formation, when both lake and sea were at high level.