Two Karroo basins in the coastal area of Tanzania were selected for source‐rockn analysis i.e. for analysis of potential or probable oil and gas generation. A several hundred ft thick middle section, of a mostly terrestrial, fluviatile‐lacustrine nature up to a deltaic sequence with a minor marine intercalation, showed many laminated fine, pelitic‐lutitic strata in a generally fine sedimentary cycle with large quantities of organic residues. Source‐rock evaluation and geochemical analysis were carried out and yielded moderate values of organic carbon content (up to 2.4%), with maceral composition of mainly structureless organic matter and some vitrinite. They probably represent marginal source rocks for oil and gas which have been “overcooked” during organic metamorphism, and are now