Argillaceous caprocks do not have the sealing ability when they begin deposition. This ability is obtained when their burial depth increases, the degree of compaction and diagenesis increases , clay minerals are transformed and dehydrated and the evolution of organic material to hydrocarbon preceeds to a certain stage. On the basis of the geohistorical reconstruction of various kinds of sealing ability of caprocks, the authors have distinguished three periods of initiation of the sealing ability of the capillaries, fluid pressure and hydrocarbon concentration of the mudstone caprocks of the Qingshankou Formation in the Sanzhao depression, i. e. : the terminal period of deposition of the Qingshankou Formation, the terminal period of deposition of the Qingshankou Formation and the terminal period of deposition of the fourth member of the Nengjiang Formation, which are all far later than the gas expulsion period of the Upper Jurassic gas source rocks (the terminal period of Jurassic deposition. Therefore, the loss of substantial natural gas in the Upper Jurassic might be the important cause for the fact that only some medium- and small -sized gas fields have been found in the study area, while large gas fields cannot be found.