The analysis was undertaken to unravel possible reasons of reservoirs separation in hydrocarbon accumulations. Separation zones are practically not recognized by direct evidence such as well correlation, or seismic data. Possible locations of such separation zones can be inferred only from the presence of a combination of indirect indicators including: multi-facies zones, contact of sedimentary bodies from different depositional source areas, zones of reduced net thickness or NTG, increased compartmentalization, continuity, lack of communication between injecting and producing wells, contradictory well test results, different hydrocarbon contacts. The paper considers main reasons of forming the zones of separated reservoirs, which can be associated with a junction of areas formed by different depositional centers, or with reduced NTG of sedimentary sequences separating major sand depositional features, with boundaries of facies where pore space structure may change, with capillary barriers. Separation of reservoirs in the same formation zone can also be confirmed by a different reservoir temperature (when excluding reservoir production effects), presence of faults including “rootless” faults such as, for instance, a clastic elongated dike filled with clastic material. Besides, influence can be exerted by geochemical processes, as well as by lithology change and porosity reduction at facies boundaries, or by interfacing with thin shale barriers. As a result of the analysis, the authors identified main reasons of reservoirs separation in hydrocarbon accumulations
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