A comparison of sandstone cores from Trestakk Field was carried out to better understand the difference in permeability between a well near the oil-filled crest (average 26 mD) and a second well located mainly within the water zone (average 4 mD). Plug samples distributed throughout similar 80-90 m cored intervals in each well were analyzed by X-ray diffraction, bulk chemical analysis, point counting of thin sections, and SEM image analysis of mineralogy and porosity. No differences were found between the two wells in either primary sand quality or the degree of diagenetic alteration, including illitization. However, permeability is strongly correlated with both pore-size distribution and abundance of intergranular macroporosity, which in turn appear to be controlled by the volume occupied by illitic clay. Despite the similar degree of illitization in the two wells, the illite may be more widely distributed in the intergranular pore system in the lower-permeability well, thus reducing intergranular macroporosity and permeability. This difference is suggested to result from growth of illite in completely water-filled porosity in the flank position, as opposed to illite growth only in residual-water-saturated porosity on the oil-filled structural crest.