Abstract: Oil and gas explorations in the Ordos Basin have already entered the peak period of reserves discovery. The reserves of oil resources in the source rocks of Yanchang Formation and natural gas resources in Upper Palaozoic coal measure strata have been basically ascertained. As the explorations of oil and gas in the basin progresses, it is a fairly urgent task to find new replacement exploration areas. According to the previous research results, the ancient marine source rocks are well developed in this region, including the Middle-Upper Ordovician marine carbonate source rock series and the Middle-Upper Proterozoic marine clastic source rock series. The Middle-Upper Ordovician marine source rocks exhibit different characteristics in different waters. The Middle-Upper Proterozoic marine source rocks are developed around the salt depression of Huabei Submarine Plateau in the eastern Basin, dominated by argillaceous limestones, with thin single layer and multiple series of strata. These source rocks have low-moderate abundance of organic matter and high maturity; moreover, they have large-scale hydrocarbon generation potential and make a great contribution to the Ordovician pre-salt self-source oil reserves. The Upper Ordovician marine source rocks are mainly developed in the L-shaped slope facies of the Qin-qi Trough in the western and southern Basin, dominated by shales and argillaceous limestones. These source rocks are characterized by moderate organic matter abundance and great cumulative thickness. They reached the high-postmature gas generation stage in Tianhuan Depression, with a limited distribution range. Additionally, these source rocks entered in a mature oil generation stage on the hanging side of the thrust belt along the western margin of the Basin, exhibiting great hydrocarbon generation potential. The Middle-Upper Proterozoic marine argillaceous source rocks are effective source rocks in the adjacent region, dominated by argillites and featured by moderate organic matter abundance and high postmaturity, which is also partly revealed in some new wells at the Gan-Shan Taphrogenic Trough in the Basin. The Ordovician marine carbonatites and the Middle-Upper Proterozoic deep marine strata in which the above-described two marine source rocks are extensively distributed are two new areas of natural gas exploration in the future, in which five favorable zones exhibit great exploration potentials and are expected to become the replacement zones of natural gas exploration.