Kazanskoe oil and gas condensate deposit is situated in the Tomsk region. One of the beds of this deposit contains a fluid, which is related to volatile oil according to the composition and properties. It was determined that the initial reservoir conditions had the liquid single-phase near-critical fluid with a high content of dissolved gas. The bubble point pressure of reservoir fluid was equal to 22.96 MPa, which is only by 1.5 MPa below the reservoir pressure. This feature greatly affected the deposit development process. The intense release of gas from the reservoir oil has occurred during the extraction with a decrease of pressure in reservoir below the bubble point pressure. There was a substantial increasing of gas-oil ratio and decreasing of the density of the extracted liquid products, produced in many production wells. Experimentally, it was determined that the density of the stabilized wellhead hydrocarbon liquids over 719 kg/m³ indicates on the liquid or two phase state of a reservoir fluid in a bottomhole formation zone. Whereas the density of the stabilized wellhead hydrocarbon liquid is less than 713 kg/m³ (the gas state). The complex of laboratory tests of reservoir fluid samples made it possible to investigate the phase of fluid behavior during its development.