Abstract: The Paleogene in the Qikou Sag of Dagang oilfield was divided into three second-level sequences upon unconformities from seismic profiles. The sequence of Es33-Es32 was formed in the period of strong faulting and depression, while the basin of lake was expanding under humid climate. As the result, mudstone was widely developed in the semi-deep and deep lake. The sequence of Es31-Es1z was deposited in the uplifting period of Es31, when denudation prevailed. In the period of Es2, faulting became weak and the water level was dropping due to lake shrinkage. The lake was expanded again in Es1z and Es1x, and thus it was the major stage for deposition. The sequence of Esld and Ed inherited the depositional framework of Es1z, and the region was stable then in sedimentation. In the early stage of Ed, the basin was rather stable with weak subsidence, while in the later stage of Ed, denudation dominated due to uplifting of the basin.