Two hydrothermal experiments were performed using sandstone core material from the Norwegian North Sea with synthetic brines reacted at approximately 150°C and 450 bars, temperature and pressure calculated to simulate a depth of burial of approximately 4 km. The results of the experiments were analyzed with geochemical modeling and with chemical and petrographic analyses. Geochemical modeling with several computer programs indicated that the experimental fluid was undersaturated with respect to K-feldspar, kaolinite, and illite, but supersaturated with respect to muscovite. Chemical analysis with inductively-coupled plasma mass spectrometry indicated that the fluid reached saturation with respect to K-feldspar. Petrographic analysis with scanning electron microscopy and energy-dispersive scanning indicated that changes took place over the course of the experiments in both the clay and non-clay mineral fractions, and this result was verified by X-ray diffraction analysis that indicated dissolution of both K-feldspar and illite and formation of muscovite. These converging lines of evidence indicate that significant changes took place in the clay mineral fraction of the experimental sandstone core material, reacted at realistic basin temperature, pressure and geochemical conditions, over the course of several weeks.