Because shale cap rocks can be clearly found for some heavy oil reservoirs, the top-water reservoir concept was proposed to explain the phenomena. The concept of top-water reservoir is lack of scientific principle, which is easy to make reservoir engineering misunderstood. In order to correct the concept, a study was made to analyze the sealing mechanism of cap rocks, which concludes that cap rocks are not necessarily shale rocks, and all rocks with poor physical properties can be cap rocks. Top-water reservoirs are not without cap rocks, they are only with no shale cap rocks, but they have physical property cap rocks to seal oil and gas. The oil and water distribution in this kind of reservoirs is still normal. In order to prevent misunderstanding reservoir engineering, the top-water reservoir concept should not be used any more.