DEPOSITIONAL-ENVIRONMENTS AND OIL POTENTIAL OF JURASSIC CRETACEOUS SOURCE ROCKS WITHIN THE SEYCHELLES MICROCONTINENT/

The Seychelles microcontinent became isolated between the Somali, Masc arene and Arabian basins of the Indian Ocean as a result of the Mesozo ic fragmentation of Gondwana. Major rifting events occurred during the Triassic-Middle Jurassic and Late Cretaceous (Cenomanian-Santonian an d 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-E arly Cretaceous, Campanian-Maastrichtian and Paleocene times. Recent g eochemical analysis of cuttings from the Seagull Shoals-1 well has ide ntified an oil-prone liptinitic (Type II) coaly shale within early Mid dle Jurassic abandoned deltaic deposits. This coaly source rock is reg ionally developed, having also been identified in the Majunga and Moro ndava basins of Madagascar. Oil-prone Type II organic matter has also been identified in the Owen Bank A-1 well within restricted marine sha les of late Middle Jurassic age. These shales are part of a thick post -rift source rock sequence that extends into the Early Cretaeous and i s in part correlative with the proven Late Jurassic Uarandab Shale of Somalia. Analysis of Campanian marine shales from Reith Bank-1 well id entified significant dilution of total organic carbon content in compo site, compared to picked, well cuttings samples. This finding supports a published inference that these post-rift shales have source rock po tential. The very limited drilling of the various Jurassic/Cretaceous source rock: sequences in Seychelles (only three penetrations, all wit hin a 13 km radius) indicates that they span the entire maturity windo w and that some have generated hydrocarbons. However, the petroleum sy stem 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 Cretac eous rift/drift sequence remains untested. (C) 1998 Elsevier Science L td. All rights reserved.