The amount of remaining recoverable natural gas reserves is affected by the pinching of gas reserves by infiltrating water and degassing of formation water in the Cenomanian water-pressure complex. It is necessary to reliably determine the volume of gas-saturated water in order to estimate the volume of gas dissolved in produced water. The area of the gas-oil contact is determined by known methods, but the height of the reservoir zone, represented by gas-saturated water (transition zone), must be specified.The fluid distribution and the structure of the transition zone depend on gravitational and capillary forces. These forces depend on the physical and chemical properties of the gas and formation water, as well as the composition and filtration properties of the reservoir. To calculate the thickness of the reservoir in which the process of gas dissolution in water takes place, it is proposed to consider it from the point of view of molecular diffusion processes.The authors of the article propose an adapted VNIIGAZ method for determining the transition zone and the volume of water in which gas from underground gas storages is dissolved (a case study of the Medvezhye oil and gas condensate field). The proposed method for calculating the volume of gas dissolved in formation waters of Cenomanian gas deposits, taking into account diffusion, showed an acceptable result. The proposed method for calculating the volume of gas dissolved in formation waters of Cenomanian gas deposits, taking into account diffusion, gave an acceptable result.