We present the results of assessment for petroleum occurrence in the Jurassic-Cretaceous deposits of Yamal Peninsula based on the water - gas equilibrium. It was identified that the water - gas system within the study area is complex. Reservoirs were assumed to present "relicts" of the previous hydrogeological epoch, and water being the most mobile component of this system, resulting in the slow forward reservoir shift to the equilibrium that corresponds to the qualitatively new condition of geochemical system. Most of the studied territory has favorable conditions for preservation of petroleum pools. The northern and central Yamal (south-east of Kara megasyncline, south-west of South-Kara megasaddle, Bova-nenkovo-Nurminka inclined ridge, southern Pai Khoi-Novaya Zemlya megamonocline, southern and central East Pai Khoi monocline) are notable for a small shift of the phase equilibrium in the water-gas system, suggesting discovery of large hydrocarbon reserves in them. The analysis of the obtained results confirmed the already discovered pools on the studied fields and allowed to predict 13 lost pools. In contrast, the available hydrogeochemical data reduce the opportunity to reveal new hydrocarbon deposits in the Yarudeika megahigh. One can also talk on the low probability of preserving oil and gas condensate accumulations in this area, even if they were formed at the previous stages of the water-driven system development.