Rotliegend sandstones of the Altmark-region (Sachsen Anhalt) are important natural gas reservoirs and thus potential targets for the long-term storage of carbon dioxide. Aims of this study are the investigation of the diagenetic mineral assemblage and the development of a geometric model to characterize the pore-mineral interface of sandstones. For this purpose optical- and scanning electron microscopy, image analysis, as well as electron microprobe- and XRD-investigations are applied, complemented by thermal basin modelling. The authigenic mineral assemblage, basin-modelling and a nitrogen-rich gas composition point to alkaline burial conditions controlled by ammonium-rich fluids from marine source rocks. Chemical bleaching, related to the authigenesis of chlorite, improves the reservoir quality. As a result of their inhibitory effect on the subsequent blocky cementation, chlorite grain coatings amplify the initial contrasts of reservoir quality between the lithofacies types. Major controlling factors of the porosity and permeability are the blocky cementation, mechanical compaction, the authigenic clay mineral assemblage and the pore geometry. The newly developed geometric model for the quantitative description of pore-exposed mineral surfaces is based on the spatial determination of the pore-surface in high-resolution thin-section photographs by image analysis. Mineral-specific roughness factors and a three-dimensional pore model are also taken into account. The frequency of minerals exposed to the open pore space increases with the spatial homogeneity of the mineral and its ratio of surface to volume. These conditions are fulfilled in particular to the authigenic clay minerals illite and chlorite, which account for a significant proportion of the pore-exposed mineral surfaces. In light of the significant differences to previously used methods, geometric surface models are recommended as input parameters for chemical modeling.