There are two types of layers sealing the oil and gas. One is generally known as the caprock，and the other is the cap shell discussed in this paper. The cap shell is an interface of a certain level of stratigraphic sequence. It is known that the sequence boundary is a sedimentary discontinuity. Before new deposits，rock was lithified when the discontinuous sedimentary surface was exposed or near the surface. This resulted in calcium shells or the formation of seepage barriers，which blocked the pores and throat of the surface stratum and made the permeability reduced. For example，the permeability of the calcareous siltstone at the sequence interface between the Minghuazhen Formation and the Guantao Formation of Well Gang-225 in Dagang Oilfield is 0.63 mD，but the permeability of the reservoirs above and below the interface are hundreds or thousands millidarcy. The top boundaries of many reservoirs，such as the main oil-reservoirs of Gangdong and Liaohe Oilfields in China，and the main gas-reservoirs of Elmorth Gasfield in Canada and Naimat Gasfield in Pakistan，are all stratigraphic sequence interfaces. The rock covering on the top of the oil reservoir or gas reservoir is not a regular cap rock such as mudstone or shale，but a layer of permeable sandstone that is interpreted as an aquifer. Oil-water inversion or gaswater inversion seems like a strange phenomenon. In fact，the upper layer of water and the lower layer of oil（gas）are separated by the impermeable sequence interface，named as cap shell，sealing the oil and gas in reservoirs. Although the thickness of a cap shell is thin，it has great significance in sealing oil and gas. A number of domestic and foreign large oil（gas） reservoirs are sealed by the cap shell.