The available experimental material on the abundances of trace elements in naphthides of different phase states was summarized to show their substantial difference allowing differentiation between vanadium oils and secondary gas-condensate deposits. The possibility of using the trace-element content of naphthides for revealing the exlent of influence of individual operated reservoirs during their simultaneous development is shown. The concentration of individual trace elements and their ratios can be used to study the dynamics of motion of the water-oil contact. The criteria used to distinguish between the crude oils and gas condensates underlay the methods proposed for monitoring the advance of the oil-gas contact during the development of gas condensate fields.