Calcareous sandstones that are almost dense layers or dry beds are frequently seen in Paleogene reservoir in Bohai oilfield.Geological engineers try all means to avoid this formation. So, finding the reason of forming calcareous sandstone and predicting the distribution of calcareous sandstone is the first work for avoiding inefficient well. This article takes the 3rd member of Shahejie formation in well area 3 of Jinzhou 25-1 oilfield as example, and the result is that there are two conditions for forming calcareous sandstone. One of them is the material condition, the other is the environment condition. The mudstone in the lacustrine facies provides the carbonate flow which is the material condition. The sand which belongs to fluvial facies deposits in the mud, and the sand lens fully contacts with mud. In this situation, the pressure of generating hydrocarbon pulls the flow of carbonate into the sand, and then the flow deposits. The pressure and the facies are the environment condition. The material condition and environment condition are contacted and influenced each other. At the same time, forming calcareous sandstone is affected by facies, contact area and palaeo-geomorphology. At last, considering distribution of sand by seismic facies, this article predicts the area of calcareous sandstone which is located in the western side and beside the depositional center, the eastern side of the smaller depositional center. Geological engineers modified five wells locations to avoid the drilling risk in order not to drill the calcareous sandstone.