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There are many methods and tools for estimating the current water/oil (WOC) or gas/oil contact (GOC) in the reservoir. PLT and RST logs can timely monitor the production of each phase in the well and estimate fluid contacts. Material balance or numerical simulation models allow to estimate or predict the depth fluids contacts. In all cases, these tools involve a large amount of human and financial resources and in most cases their accuracy depends on the time expend to calibrate them. This work proposes a methodology to determine the current depth of the fluid contacts, using surface measurements of the specific gravity of oil (°API) in wells. The proposed methodology is based on the principle of the compositional variation of the fluid in the reservoir and its effect of the properties of the produced fluids. Two diagnostic plots are proposed: (1) for estimating the current depth of the fluids contact at well level and (2) for predicting water breakthrough time. Some real examples of wells showing the behavior outlined in this paper will be presented to support all presented theories. Finally the proposed methodology and diagnostic plots were applied to a reservoir with proven compositional gradient to validate the proposed work.