In the Cantarell field in Mexico oil is produced primarily from naturally fractured carbonate reservoirs. These types of reservoirs are among the most difficult to develop given the uncertainties associated with detection and characterization, both locally in the well and regionally in the field, of the fracture networks that predominantly control fluid flow. Additional complexities result from difficulties understanding matrix-fracture interactions.In the last few years, the oil-water contact in the Cantarell field has been advancing, and this has considerably reduced the oil window zone. One likely explanation is that the natural fracture network (which provides most of the permeability in the field) favors production of water and gas over oil. This new challenge has forced Petróleos Mexicanos (PEMEX E&P) to make new efforts to characterize their naturally fractured carbonate reservoirs and improve their reservoir models. Using the critically-stressed-fault hypothesis, which assumes that the faults and fractures that are hydrologically conductive today are those that are critically stressed (active) in the current stress field, GeoMechanics International (GMI) analyzed image logs of over 20 wells in detail, and constructed a geomechanical model of the Cantarell field, to define the role of the fracture network in fluid flow. In a previous study, GMI used all available data, including seismic data, caliper logs, image logs, electric logs, drilling reports, sedimentological analyses of cores, and regional tectonic studies, to develop a comprehensive geomechanical model of the field. We then used and complement this model in conjunction with all available image logs to predict the orientations of the most and least critically stressed (hence permeable and impermeable) fractures. The results have been effectively used to improve well trajectories both to minimize costs and to optimize production throughout the life of the reservoir. The results have been particularly useful in horizontal wells, where preventing water production requires avoiding steeply dipping fractures that are critically stressed.