Computer-assisted image analysis of back-scattered electron (BSE) micrographs of petrographic sections is useful for the quantification of clay mineral microporosity in sandstones. Authigenic clay minerals from North Sea clastic reservoirs generally have higher microporosities than detrital clay minerals. Diagenetic kaolinite has microporosities varying from 25 to 50%. Diagenetic chlorite has a generally uniform grain-coating texture and microporosities of about 5 0%. Dispersed diagenetic illitic clays are difficult to characterize using this method, because their delicate morphology is susceptible to damage during sample preparation. Despite this limitation, the analytical data indicate a minimum microporosity of 63% for dispersed illitic clays. Analytical data for clay-rich detrital clasts have much lower microporosity values of about 10%. Interpretation of images from detrital clasts is complicated by the effects of sample preparation.