Geological and Geochemical Evidence for Jurassic and Cretaceous Source Rocks within the Southern and Western Seychelles Rift Sequence
JOSEPH, PATRICK, and PATRICK SAMSON
Prolific source rocks produced during the Jurassic and Cretaceous are responsible for 70% of the world's petroleum provinces. The Tethys Seaway produced abundant source rocks in carbonate environments in the Middle East and as marine shales from the Indus Basin of Pakistan to the Australian coast.
Situated at the head of the Jurassic Tethyan rift that cleaved Gondwana, the Seychelles microcontinent received its quota of such Jurassic and Cretaceous source rocks. Within the well Owen Bank A-1, the lower 1990 m comprises black, organic rich shales and limestones deposited in Early Jurassic deltaic, through Middle Jurassic coastal carbonate shelf, to Early Cretaceous open marine environments. These source rocks are known to have generated hydrocarbons.
A later phase of rift/drift tectonics separated Seychelles from Madagascar during the Late Cretaceous along a failed arm of the earlier rift. Although no wells penetrated this sequence, beach-stranded tar geochemistry indicates that two depositional environments locally produced source rocks from the Tethys during this time, namely deltaic (rift) shales and paralic (drift) shales and limestones.
Seismic data indicate the Late Cretaceous rift clastics to be at least 6 km thick, within which deltaic source rocks occur locally and capped regionally by paralic source rocks. Trapping configurations comprise tilted fault blocks and stratigraphic pinchouts. To date, only three wells have penetrated the Jurassic rift/drift play. The equally prospective Late Cretaceous play remains untested.