Prior work has suggested that potassium mobility is an important control on the formation of late-stage illite in deep reservoirs of the North Sea. We developed a one-dimensional finite-difference reactive transport model to simulate the concurrent dissolution of K-feldspar and illitization of kaolinite in Statfjord Formation reservoirs. The kinetics of K-feldspar dissolution is set to be faster than diffusion, consistent with the observed behavior and the proposition that the reaction was transport-limited. K-feldspar dissolution produced dissolved potassium and silica that diffused and then precipitated to form illite and quartz. Areas that had higher initial reactant (K-feldspar) produced higher solute concentrations and formed chemical gradients having areas that had lower initial feldspar content. In typical North Sea cases this means mass transfer would be from sandstone to shale. The diffusive transport moved significant amounts of potassium and silica over distances of several meters and created observable diagenetic alteration. Simulations of an oil-filled reservoir showed that oil emplacement did not stop illitization but did limit potassium mobility and maximized illite formation within the sandstone. Preservation of preillitization permeability was favored in water-filled reservoirs that had slower illite formation where diffusion could export potassium from the sandstone before precipitation. Similar patterns of transport and precipitation were predicted for silica from K-feldspar dissolution. The results of the model showed good agreement with field data from other North Sea reservoirs.