A new methodology for locating fluid-flow zones in a rock formation is proposed and applied to the Buntsandstein sandstone reservoir penetrated by borehole EPSI, Soultz-sous-Forets, France. The method is based on analyzing thermal gradients obtained by two different methods for the same cored borehole, (1) from temperature log and (2) from thermal conductivity measurements. Comparing the two thermal-gradient datasets reveals the fluid-circulation zones in the borehole. Using this approach, we identified three main hot-fluid circulation zones within the Buntsandstein reservoir, two controlled by sedimentary facies and one controlled by tectonic structures. (C) 2012 Elsevier Ltd. All rights reserved.