MESOZOIC SOURCE ROCKS AND HYDROCARBON POTENTIAL OF THE SEYCHELLES OFFSHORE

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 Gondwa na into Eastern and Western blocks - namely, Middle Triassic lacustrin e mudstones, and Early/Middle Jurassic deltaic-lagoonal mudstones. The other two source-rock intervals were deposited on passive marine shel ves during continental drift phases - namely, Late Jurassic to Early C retaceous 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 yi eld 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, whi le the oldest lies in the gas ''window''. The Jurassic/Cretaceous sour ce rocks, on the other hand, lie within the oil ''window''. Analyses o f numerous beach-stranded tarballs that are believed to be of indigeno us origin reveal, in addition to a source dominated by terrestrial org anic matter, the presence of a source rock dominated by marine algal o rganic matter. Such a source rock may have developed during a Middle J urassic phase of shallow-marine carbonate deposition, which shows some affinity to source-rock quality, and is characterized by an oolitic m arker limestone in each well. This oolitic limestone is also a compone nt facies of the carbonate succession that contains the prolific oil-p rone source-rockfacies of the Middle East.