Geology and Petroleum Potential of Seychelles

Surya N. Khanna, George E. Pillay

Seychelles archipelago represents a microcontinent in the western Indian Ocean with an exclusive economic zone of over 1.15 million km².

Seychelles lies within the Somali plate, which is attached to the African plate along the Mozambique fracture zone and rift valley system on the mainland. Until the Late Jurassic, Seychelles had been associated with the depositional regime of east Africa, Madagascar, and thereafter with the latter until the Late Cretaceous when a portion of the continental crust separated from Madagascar and moved counterclockwise, representing the present-day Seychelles Banks and possibly the western flank of the Mascarene Plateau north of Cargados Carjos.

Over 8 km of sediments are expected in the deeper basinal area. The Karroo (possibly pre-Karroo) group of rocks of Permian-Jurassic age are the oldest sediments deposited on the granitic basement. These are overlain by Bajocian transgressive mudstones and limestones followed by rifting. Marine sedimentation in places interrupted by block faulting, tilting, and differential erosion continued thereafter until the outpour of Maestrichtian submarine lavas.

Following Late Cretaceous regression and possible erosion of the high structures, transgression again occurred during the Paleocene-lower Eocene, overstepping the earlier sedimentation. After the middle Eocene unconformity, rapid differential shelf carbonate growth continued during the rest of the Tertiary, with development of reefs at places.

Three wells drilled on the western shelf indicated hydrocarbon shows of 1.1% (average) organic carbon and maturity (vitrinite reflectance of 0.5 R_o at 2,430 m), pointing to source potential in the drilled Upper Jurassic-Lower Cretaceous marine clastics, which are expected to be preserved in grabens, depressions, and fault blocks and may be over 1,000 m thick.

With structural and combination traps, reservoir potential is noticed in Karroo sandstones, limestones, and sandstones developed at various levels in the Mesozoic section along with Tertiary reefs and carbonate buildups.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.