In this paper, the characteristics, source of minerals and genetic mechanism of the calcareous interbeds are analyzed in Member 2 clastic reservoirs in Jiaowei Formation of W1-4 oilfield. The results show that the lithology of the interbeds is dense, their physical properties are poor and there are obvious features on well logs ; the mineral source needed for their genesis mainly results from the following four actions:intraformational dissolution, clay mineral transformation, clastic mineral hydration and the pyrolysis of organic matter; the calcareous interbeds are mainly distributed near the argillaceous beds in the study area, its genetic mechanism is duo to the coupling process between the transference and chemical reaction of sandstone reservoirs and the carbonate nearby the interface of mudstone.