Microleakage is one of the most important pathways in which the natural gas diffuses through caprocks. When natural gas enters the pore surfaces of caprocks, the phenomenon of adsorption appears. When adsorbed layer of the solid surface has reached a certain thickness, the condensation of capillary void happens, and the gas phase becomes “liquidoidlike”, filling the hole. The liquidoidlike matter exists in the form of micellar. When this kind of condensation happens, the quantity of gas condensation will increase unceasingly, and then the gas becomes something whose nature is similar to the liquid. Its viscous force between moleculars increases and approaches the viscosity of water. Two different kinds of liquids may be mutually soluble when their viscosity is similar. In the form of micell, light hydrocarbons in the state of air bubble migrate vertically upward through leakage, this phenomenon is called microleakage. Based on the analysis of the microleakage mechanism, this paper establishes a quantitative model of microleakage diffusion, and calculates how much nature gas leaks through the period of geological history by a case study.