Abstract: The characteristics of Ordovician carbonate weathered crust reservoirs (Megakarst) were studied at two localities: the Zhaogezhuang section, Tangshan, Hebei province and the Wennan section, Xintai, Shandong province. They are located in the northern and southern margin of Bohai Bay basin, respectively. The former is of buriedhill type, and the major reservoir is composed of vertical vadose (infiltration & percolation zone) about 200 m thick and horizontal phreatic (saturation) zone. The latter is of plateau type, and the major reservoir is an interlayer paleokarst about 380 m thick. The reservoir space is carbonate solution cave, pore, fracture and their combination in both sections. These different genetic patterns provide a direct and visual model for the petroleum exploration of Ordovician carbonate in the basin. As to the petroleum exploration potential of ancient weathered crust reservoirs, it is usually determined by direct overlying source rock strata with high quality. The plateau paleokarst type is easier than the buriedhill type to be predicted.