Depositional environments and oil potential of Jurassic/Cretaceous source rocks within the Seychelles microcontinent

The Seychelles microcontinent became isolated between the Somali, Mascarene and Arabian basins of the Indian Ocean as a result of the Mesozoic fragmentation of Gondwana. Major rifting events occurred during the Triassic-Middle Jurassic and Late Cretaceous (Cenomanian-Santonian and Maastrichtian) during which shaly source rock facies accumulated in principally marginal marine/deltaic environments. Between these times, post-rift passive margin deposition within restricted to open marine environments produced shaly source rocks during late Middle Jurassic-Early Cretaceous, Campanian-Maastrichtian and Paleocene times. Recent geochemical analysis of cuttings from the Seagull Shoals-1 well has identified an oil-prone liptinitic (Type II) coaly shale within early Middle Jurassic abandoned deltaic deposits. This coaly source rock is regionally developed, having also been identified in the Majunga and Morondava basins of Madagascar. Oil-prone Type II organic matter has also been identified in the Owen Bank A-1 well within restricted marine shales of late Middle Jurassic age. These shales are part of a thick post-rift source rock sequence that extends into the Early Cretaeous and is in part correlative with the proven Late Jurassic Uarandab Shale of Somalia. Analysis of Campanian marine shales from Reith Bank-1 well identified significant dilution of total organic carbon content in composite, compared to picked, well cuttings samples. This finding supports a published inference that these post-rift shales have source rock potential. The very limited drilling of the various Jurassic/Cretaceous source rock: sequences in Seychelles (only three penetrations, all within a 13 km radius) indicates that they span the entire maturity window and that some have generated hydrocarbons. However, the petroleum system associated with the Triassic-Early Cretaceous rift/drift sequence has been tested only by three invalid wells at the western extremity of the Seychelles Plateau, whilst that associated with the Late Cretaceous rift/drift sequence remains untested. (C) 1998 Elsevier Science Ltd. All rights reserved.