In old oil-bearing areas, the acute issue is to replenish the depleted resource base through additional exploration of already developed fields, when additional reserves can quickly be brought into development using the existing infrastructure. For example, in the Volga-Ural province, at the initial stages of exploration and development of fields, the main attention was paid to highly productive Lower Carboniferous and Devonian formations. The formations and deposits of the Middle and Upper Carboniferous and Permian sediments were explored concurrently by the residual principle. This was partly determined by the low productivity of wells in these formations. This led to uncertain identification of genuine contacts and associated deposits and the adoption of a reference initial fluid contact level for an indefinite period. The authors give an example how reference contact levels lead to fragmentation of the main structure into domes with different contacts, and consequent decrease in reserves. A graphical representation of the dynamics of changes in deposit areas was proposed in previous publications. In general, graphs of this kind reflect changes in areas as exploration progresses. The closer the dependence on the graph to ordinate axis, the more identified the upper horizons can be considered. With the help of such graphs, it is possible, without unnecessary labor costs, to analyze the underexploration and outline measures for the priority of additional exploration of the upper sections. From the experience of analyzing the movement of reserves of multi-formations fields at the first stages, the fact of insufficient knowledge on the middle part of the oil and gas section, mainly carbonate (Okskian, Lower Bashkirian deposits), has been established. It is shown that proposed approach can speed up the identification of productive intervals and deposits of different sizes.
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