The multistage separation of associated gas from reservoir oil is based on a gradual decrease in pressure and sequential selection of light components into the gas pipeline. The final tasks of the separation process are: the pressure at the first stage and the quality of commercial oil according to the pressure of saturated oil vapor. The separation of phases of borehole products is performed by material balance method, first in molar terms, then in mass terms. The thermobaric conditions for the oil and gas separation stages are selected based on the production need for heating the oil-water emulsion and stabilizing the oil at the final stage of the technological process of the final product treatment.The article considers the features of differential separation, proposes a method for express pressure evaluation at the first separation stage and reveals a linear relationship between this parameter and the methane content in reservoir oil for a sample of 41 oil production facilities. The issues of assessing of degassed oil stability after multistage separation are considered.