The paleokarst zone of Caledonian weathering crust at the top of Ordovician in south Ordos is different from the one in Middle Ordos'but the gypsum cast corroded pore reservoirs of the 5th Member of the Lower Ordovician Majiagou Formation in the northeast part of south Ordos is similar to those in Middle Ordos.Different formations are located in different positions of the weathering crust of south Ordos.Reservoirs of the 3rd and 5th Members of the Majiagou Formation

undergone multi stage interstratum karst corrosion in geohistory

can be induced into four major stages and develop into favourable gypsum cast resorvoirs.The target area of oil and gas exploration should be set northward of Huangling Huanglong area;that of the platform marginal reef and backreef beach of Middle Ordovician should be set between Yongshou-Chunhua-Xunyi areas.