Results of the evaluation of petroleum-bearing potential of Yamal's Cretaceous deposits based on the study of water-gas equilibria are presented. It has been established that the "water-gas" system within them has a complex and unequal pattern. Hydrocarbon accumulations serve as "relics" of antecedent hydrogeological epochs, with groundwater being the most mobile component of this system. The sediments composition is therefore subjected to a slow directional change aimed at establishing an equilibrium, which subsequently results in a qualitatively new state of their geochemical framework. Most of the studied area is characterized by very favorable conditions for persistent preservation of oil and gas accumulations. In northern and central regions of Yamal, there is a slight shift in the water - gas system phase equilibrium, suggesting a potential for discovery of new oil/gas pools and significant hydrocarbon reserves within them. The analysis confirmed the presence of earlier discovered hydrocarbon pools within the studied fields and enabled prediction of new thirteen accumulations. The hydrogeochemical analysis and the water-gas equilibria modeling results have shown that the prospects are significantly lower for new discoveries in SW Yamal.