In-situ saturation monitoring during a coreflood with X-ray CT scanner confirmed that the waterflood performance of a mixed-wet core is totally different from that of a water-wet core. Namely, it was observed that the brine injected in the mixed-wet core preferentially invaded into the most porous lamina, causing early water breakthrough at an outlet end. This result gives a significant insight into understanding the mechanism of unexpected early water production that has frequently been reported from some giant carbonate reservoirs in the Middle East. Many efforts have been made to grasp how such phenomenon took place. One of the most widely accepted explanations at present ascribes it to reservoir heterogeneities such as the existence of super permeability streaks. A drawback of this super-k concept is, however, that neither cores nor conventional logs could detect their locations. Thus, based upon the experimental result above, this study aims at associating the localized wettability conditions of the core sample with the early water breakthrough. For this purpose, a numerical study was carried out, using 3-D images of the in-situ saturation distribution obtained from X-ray CT scanning. Then, the waterflood performance of the mixed-wet core was successfully reproduced in the computer model. It was concluded from this numerical study that in spite of the small variation in porosity and permeability, the local variation in wettability can occur within the Berea sandstone core after it has been in contact with crude oil. As a result, the brine is preferentially imbibed in and runs through the more water-wet laminae, causing the finger-shaped water encroachment and, finally, the early water breakthrough.