Abstract: Carbonate and clastic source rocks generated in continental margin environment at the southwestern margin of the Ordos Basin during the early Paleozoic.Explorations proved that the lower Paleozoic source rocks experienced higher hydrocarbon generation even though their organic matter abundance was low.Good source rocks (usually type Ⅰ and Ⅱ) were found in weak-reducing and reducing bathyal environment with high salinity content.Cambrian source rocks generated hydrocarbon during Triassic and had limited contribution to the present reservoirs.The source rocks in Majiagou and Pingliang Formations of Ordovician generated hydrocarbon during Jurassic and Cretaceous.Ordovician source rocks are in high evolution stage,represented by the carbonate rocks in Majiagou Formation as well as the dark mudstones and carbonate rocks in Pingliang Formation.The hydrocarbon generation quantity of the lower Paleozoic is 172.87×10¹² m³,while in the north it exceeds 40×10⁸ m³/km²,and in the south it amounts to 30×10⁸ m³/km².