Abstract: The Middle Ordovician St. Peter Sandstone (dominantly quartzarenites and feldsarenites) occurs throughout the central Michigan Basin at depths ranging from 1.5 to 3.5 km and produces condensate and/or natural gas from several horizons in more than 36 fields. The primary mineralogy, textures and reservoir characteristics have been dramatically modified by a complex diagenesis. The main diagenetic features, generally observable throughout the basin, are, chronologically: early marine carbonate cement; quartz overgrowth and pressure solution; burial dolomite and anhydrite; dissolution of framework grains and early cements; and pervasive authigenic illite and chlorite cementation. This late clay cement commonly occurs in secondary porosity formed after the dolomite cement. A K-Ar study of the fine-grained authigenic illite indicates that this regionally significant episode of clay cementation in the Michigan Basin occurred during Late Devonian-Mississippian times. Sixteen samples from 11 wells located mainly in the central part of the basin yielded consistent ages ranging from 367 Ma to 327 Ma with an average of 346 ± 11 Ma. Combined with burial-history reconstructions for the central basin, these ages indicate that illite formed at depths of approximately 3 km. Fluid-inclusion temperatures for the dolomite and quartz cements associated with the diagenetic illite suggest temperatures of formation for the latter on the order of 150°C or higher. Combined with the depth estimate, these temperatures imply the existence of elevated geothermal gradients, i.e., 38°C/km or greater, in the central Michigan Basin at the time of illitization. Because illite and/or hydrocarbons typically fill secondary pores in the St. Peter Sandstone, the K-Ar age data also place constraints on the timing of development of secondary porosity and hydrocarbon emplacement.