When simulating borehole resistivity measurements in a reservoir, it is common to consider an oilwater contact (OWC) planar interface. However, this consideration can lead to an unrealistic model since in the presence of capillary actions, the mix of two immiscible fluids (oil and water) often appears as an oil-water transition (OWT) zone. These transition zones may be signi cant in the vertical direction (20 meters or above), and in context of geosteering, an e cient method to simulate the OWT zone can maximize the production of an oil reservoir. Herein, we propose an e cient one and a half dimensional (1.5D) numerical solver to accurately simulate the OWT zone in an oil reservoir. Using this method, we can easily consider arbitrary resistivity distributions in the vertical direction, as it occurs in an OWT zone. Numerical results on synthetic examples demonstrate signi cant di erences between the results recorded by a geosteering device when considering a realistic OWT zone vs an OWC sharp interface.