In reservoirs with fresh brine the effects of clay minerals are much larger than in more saline brine reservoirs. An evaluation of a very large shaly sand reservoir is presented, where the shale conducts up to 30 times more of the electricity than the brine. A reliable interpretation model was found by adjusting the standard Waxman- Smits model. By comparison it was found that the Dual Water model is incorrect. This adapted evaluation model is based on an extensive set of laboratory measurements. The upper and lower Aeolian sands could be evaluated consistent with Capillary Pressure data, whereas the middle Marine sands were more difficult to evaluate, due to different clay distributions and variable Rw values.