Abstract: During the Ordovician period, the Dagang Oilfield was a part of the Huabei Epicontinental Sea, in which a set of carbonate deposits was formed. The Lower Ordovician contains the Yeli Formation and Liangjiashan Formation; and the Middle Ordovician consists of the Majiagou Formation and Fengfeng Formation. The Upper Ordovician is missing. The early Odovician paleogeography was characterized by the south-north differentiation. But the pattern of paleogeography was changed in the Middle Ordovician. There was a dolomitic lime flat along north-east border of the open epicontinental sea. The quality of the Ordovician reservoirs in this area changed in a range from relatively good to relatively bad, and were characterized by higher porosity but low permeability due to multiple deformation and other factors such as lithology, sedimentary facies, diagenesis, faulting and cracking. Favorable reservoirs are distributed in Shenqingzhuang -Tanggu, Kongxi, Wangguantun, Nanpi-Wuqiao and Xuhei regions.