Abstract: The author describes the characteristics of the Caledonian erosional surface in the North China, i. e the top surface of the Ordovician marine carbonate system (Tg reflection wave group), and its controlling over the formation of the oil/gas pools in the marine carbonate sequences. The sedimentation hiatus for such an erosional surface lasted as long as 130Ma with the ommission of the Silurian,Devonian and the lower Carboniferous systems. The stratigraphic contact along the surface is an unconformity. Since the prolonged erosions with weathering, leaching,dissolution and denudation, the dissoluted pores,caves and fissures have been well developed in the Ordovician carbonate sequences, thus they can be excellent reservoir rocks for oil/gas accumulations. However, the Tertiary sequences are of most significant source rocks for large oil/gas pools in the Paleozoic carbonate reservoir, except for the source rocks of the carbonate rocks themselves. Through the faults and the uncomfornities, the generated hydrocarbons would accumulate and migrate into the traps located above or beneath the Caledonian erosional surface and prolific oil/gas pools would be developed.