Three wells in the Seychelles offshore indicate the existence of four potential source-rock intervals within the Mesozoic succession. Two of these originated during the rift phase that eventually cleaved Gondwana into Eastern and Western blocks -namely, Middle Triassic lacustrine mudstones, and Early/Middle Jurassic deltaic-lagoonal mudstones. The other two source-rock intervals were deposited on passive marine shelves during continental drift phases - namely, Late Jurassic to Early Cretaceous mudstones and siltstones during the East-West Gondwana drift, and Maastrichtian to Paleocene mudstones during the later Seychelles-India drift. These source rocks are dominated by terrestrial organic matter. Although TOCs are generally good (greater than 1.0%) and range to excellent (7.82%), potential hydrocarbon yields are generally only poor to fair (less than 6 kg HC/tonne of rock). One good potential yield of 10 kg/tonne has been measured. Maturity data (R(o) and T(max)) indicate that, in the wells, the youngest source rock is immature, while the oldest lies in the gas ＇＇window＇＇. The Jurassic/Cretaceous source rocks, on the other hand, lie within the oil ＇＇window＇＇. Analyses of numerous beach-stranded tarballs that are believed to be of indigenous origin reveal, in addition to a source dominated by terrestrial organic matter, the presence of a source rock dominated by marine algal organic matter. Such a source rock may have developed during a Middle Jurassic phase of shallow-marine carbonate deposition, which shows some affinity to source-rock quality, and is characterized by an oolitic marker limestone in each well. This oolitic limestone is also a component facies of the carbonate succession that contains the prolific oil-prone source-rockfacies of the Middle East.