The total content, chemical attributes and pattern features of REE are analyzed for the soluble carbonate rock and sulfate rock and the insoluble clastic rock samples from the Ordovician Majiagou weathering crust reservoirs as well as the correlating clastic rock samples from the overlying Carboniferous Benxi Fm. in the middle and east parts of Ordos basin. It is shown that the REE is characteristic of obvious fractionation and enrichment and the total REE content and REE enrichment are higher in insoluble rock than in soluble rock. The correlationship between LREE and HREE and the rich or rare δEu and δCe in these rock reveal that they formed either in epidiagenetic or buried karst environments. It is proved that the REE in the Majiagou filling clastic rock was the same source as that in Benxi clastic rock. Development of the paleokarst during buried diagenesis was accompanies with the generation and migration of natural gas.