In multi layered reservoirs the fluid composition may differ from one layer to another. The knowledge of the lateral and vertical fluid heterogeneities may help to better understand the fluid distribution in the field and consequently contribute to enlighten production. Several mechanisms can affect the fluid composition, like origin, maturity, alteration or physical conditions. The objective of this study is to present a methodology aiming at characterizing and quantifying the compositional differences between fluids from different layers in an oil reservoir. It allows mapping the vertical and lateral heterogeneities in fluid composition. In order to measure the hydrocarbon composition, samples are analyzed by HRGC (High Resolution Gas Chromatography). In a first step this method aims at finding the peak ratios discriminating the oils from different reservoirs. Data are processed for peak recognition and statistical comparison is performed to compare samples. This statistical interpretation allows identifying similarities and differences between chromatograms e.g. oils. The results allowed identification of two end members. All the other samples of the field could be described as a mixture of these two fluids. Different cross-sections have been analyzed aiming at better understanding the field by linking geochemical data with geological features. By coupling the result with geological (lithology, cross-sections, logs…) and reservoir data (porosity, permeability, saturation…), different explanations could be proposed: Possibility of having different end members originating from a same fluid.Water washing as a possible process affecting fluids from different layers in similar and continuous way.“Structural heights” susceptible to reduce the water washing effect for some wells.The dip of the layers might increase water washing effect on fluids. This study showed the possibility to allocate the production in a carbonate field and allowed a better understanding of the fluids distribution. The molecular fingerprinting made it possible to understand the origin of fluids in comingled wells and particularly to identify the most producing part of the well in horizontal drains.


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