Clay cements can occur pervasively throughout larger volumes of sandstone, thereby affecting the reservoir properties significantly. They affect irreducible water content and pore surface roughness. Moreover, any induced fluid or heat flow, as a consequence of hydrocarbon and geothermal production or CO2 sequestration, may have unwanted effects because of the clay minerals present. The effects will be dependent on the mineralogy, texture and distribution of the clay minerals. Clay minerals replacing detrital components (feldspars and rock fragments) have limited effect because of their dispersed and isolated occurrence. In this study diagenesis of intragranular clay in siliciclastic sandstones is evaluated, using Rotliegend deposits in the Southern Permian Basin and Lower Triassic Buntsandstein deposits as examples. This study clearly shows that large-scale fluid flow does not play a significant role and that much of the mass involved in diagenesis is retained more or less in situ. In the sandstone proper, clay occurs in various ways: as detrital laminae and beds, as patches related to burrows, and as grain coatings through clay infiltration. In addition, clay occurs as cements rimming grains and replacing detrital feldspars and rock fragments. The apparent detrital clay is partly or largely modified during burial diagenesis and much of the clay is authigenic. Not only the mineralogy is changed but also the location and distribution of the authigenic clays. In conclusion: authigenic clays in reservoir sandstones, including clay rim cement, are genetically associated with and directly linked to infiltrated or bioturbated clay.