During the operation of multilayer oil fields, there is a likelihood of the occurrence of interstratal oil flows due to the occurrence of secondary technogenic channels of migration along vertical or lateral fill-spill chains. This is due to numerous perforations of fluid-tight and fluid-insulating layers, intensive oil withdrawal, the use of intensive oil recovery technologies (hydraulic fracturing, reservoir pressure maintenance systems and the other physicochemical effects in the reservoir). Thus, the original geological structure of the oil deposit may change during its operation. Besides, for some oil deposits in the later stages of their exploitation, the oil inflows from deeper geological structures can be detected. In this paper, the detection capability of oil deposit reformation and deep feeding on extracted oil composition is shown. The geochemical composition indices are calculated based on the GC/MS analysis of the isomeric composition of paraffins and some classes of aromatic hydrocarbons in oil samples recovered from different wells. The possibility of identifying the source of oil by their values is shown. When comparing the distributions of the values of geochemical indices in different samples of oil from one field, it was found that some of the wells extract oil from one horizon, while the composition of the extracted oil from other wells is of a mixed nature. The composition of oil from one well with a long service life is significantly differ from all others and cannot be explained as the result of mixing oil from two productive horizons. The composition of this oil is highly likely influenced by deep feeding or other technogenic factors.