In Western Siberia, the main oil-producing region of Russia, all functioning regulated marketsof associated petroleum gas (APG) have been liberalized. Because of the monopoly-monopsony structure there is a threat of market prices deviation from socially optimal levels, corresponding to the maximum of public good. The analysis of this threat and assessment of the factors that support it is an urgent problem, which has not yet been covered in the scientific literature. The purpose of the study is to assess the consequences of the liberalization of APG markets. The tool for solving the problems of the study is the economic theory of formation of market equilibrium prices in the joint production of APG and oil. On a liberalized APG market, the maximum public welfare corresponds to a set of market prices, which are determined when considering a virtual competitive market. The actual price is formed under the influence of non-market factors. The liberalized market has no mechanism for forming a socially optimalcomposition of non-market factors, and the parties have no information allowing them to determine the corresponding socially optimal APG price. Therefore, it must be set by the regulator. The algorithms for calculation of marginal costs of joint production of oil and APG and socially optimal price of APG are constructed.
