Systems and methods for analyzing produced fluids in a mature water flood (or EOR scheme) and determining whether the introduction of an EOR agent, such as a chemical or a gas additive, or some other alteration in treatment, is enhancing the recovery of hydrocarbon from parts of the reservoir otherwise untouched by injected fluids. The monitoring can be used to identify subtle changes in the produced fluid caused by their flow through different pore structures. In a carbonate formation for example, ions and salts from the rock fabric are dissolved into the reservoir fluids, whether they are water or oil. These can be detected by various fluid analysis and particularly water analysis methods. The changes in reservoir fluid paths associated with the injection of an EOR agent are detected in the observation well.