Abstract: The Ordovician weathering crust in the area of No.7 drill well can be divided intothree karst beIts in the Ordos Basin,namely vertical,horizontal,and deep karst belts,Theinhomogeneity of reservoirs in the weathering crust is dominantly controlled by karstifica-tion and the karst porous belt in the weathering crust is rated as the major reservoir bedsof natural gas,Based on the characteristic study of each karst belt,No.7 drill well is locat-ed in the lower part of the Caledonian palaeo-karstificated slope,toward the west whereOrdos No,5 and No.8 wells are positioned could be more promising prospects for oil/gas.