this invention relates in general to characterizing hydrocarbon reservoirs and / or determining flow properties of fluids associated with the reservoir - including fluids introduced into the reservoir to provide for hydrocarbon extraction - using biological tags and real - time polymerase chain reactions for tag detection . in embodiments of the present invention , one or more biological tags may be added to one or more liquids associated with the hydrocarbon reservoir and subsequently one or more liquid samples may be taken from locations associated with the hydrocarbon and the presence of the one or more biological tag may be qualitatively and / or quantitatively tested for in the samples using real - time pcr .