Abstract: Microfossils of Ortonella,Hedstromia, Bevocastria, Cayeuxia, Sepulidea and Foraminifera and Foraminifera-like are fouad in carbonate samples from the Yuan Jiang formation of the Dongting depression for the first time. According to the characteristics of the carbonate, it is believed that sea water transgressed from the northeastern and northern parts of the Dongting depression during Paleogene. This brought the depression an environment of brackish water linking with the maritime basins. On this basis, the oil prospects of each part of the Dongting depression are discussed.