The results of a comprehensive analysis of geological and field information are presented in order to assess changes in the hydrogeochemical field of the oil reservoir of the U horizon of the Verkh-Tarskoye oil field during its development in the period from 1994 to 2021. The main production facility is at the IV stage of declining production. The water cut of the production wells stock reaches 98% with cumulative production of 14.86 million tons (as of May 1, 2021). Since 2015, there has been an increase in the TDS of produced water, which is explained by the processes of their mixing with circuit waters along the periphery of the reservoir with a decrease in reservoir pressure and more saline waters of the U horizon, supplied during joint operation.