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Seychelles - Geography

Seychelles is located in the Indian Ocean about 1,600 kilometers (1,000 miles) east of Kenya. The nation is an archipelago of 115 tropical islands with two distinct collections of islands, some comprised of granite and others of coral. The Mahe Group consists of 42 granite islands, all within a 56-kilometer (35-mi.) radius of the main island of Mahe. These islands are rocky, and most have a narrow coastal strip and a central range of hills rising as high as 914 meters (3,000 ft.). Mahe is the largest island and is the site of Victoria, the capital. The coral islands are flat with elevated coral reefs at different stages of formation. They have no fresh water; human life can be sustained on them only with difficulty.

The group may be divided into two clusters; one to the westward, round Mahi, and the other to the eastward, round the Island of Praslin, which is next in size to Mahi. The general aspect of both clusters is green and cheerful; but that of Mahi is the loftiest, for the peaks along the backbone of Mahi itself may be nearly 2000 feet above the water-line, while Silhouette, a woody, conical isle, lying about 17 miles to the northward of Mahi, rises into one central peak of from 2000 to 3000 feet in height.

These islands rest on an immense bank of sand and coral, which is said to extend from north-west to south-east more than 240 miles, and in width between 30 and 90 miles. It is a kind of vast platform in the sea, on which the superstructure of the islands has been raised. The general depth of water on the bank varies between 12 and 40 fathoms. It is free from dangers, but the ground-swell is very great on it. The number of islands, including the small islands, is nearly thirty, of which however only fifteen are of any importance from their size or produce.

The Seychelle Islands are a granite formation cropping up in the center of a vast bowl of coral. This bowl may have a diameter of some 120 miles: its rim rising nearest to the surface of the water. Approaching the rim of this coral bowl from any point oceanwards, there are soundings in from 7 to 8 or 9 fathoms. Here and there, especially on the western quadrant, there are 3-fathom patches. And on the northern, as also on the southern edge, there are low sandy islands, sprinkled with scrubby brushwood, and differing altogether in appearance, as perhaps in structure also, from the Seychelle group proper.

Mahe, the seat of government at the Seychelles, and principal island in the group, is 16 miles long, and from three to five broad, with a very steep and rugged granite mountain running through the center. Although the bank on which this archipelago is situate is of coral formation, yet all the Seychelles Islands, except two, are of granite, huge blocks of which, generally piled up as it were in a confused mass, form their peaks, which are covered with verdure.

Although it presents a singularly steep and precipitous appearance as viewed from the harbor, the luxuriant tropical vegetation which descends to the very water's edge, and its dark forest-clad heights, far from suggesting a forbidding aspect, can but evoke admiration little short of fascinating in the minds of all true lovers of Nature. In these islands, Nature reigns supreme and runs wild at her own sweet will over towering rocks and heights; amidst deep, dark gorges and chasms; through forests of capucin and cedar here the orchid and the pitcher plant olimb the mountain sides and wild cinnamon springs up in greatest luxuriance.

Considered as a port, Victoria offers sheltered anchorage during the south-east trade and south-west monsoon; but is open to the force of that from the north-east. Mahi itself protects the harbour to the westward, and a chain of islets to the eastward. The southernmost link of this chain is separated only by a narrow and reef-strewn channel from the main island.

A system of marine plateaus occurs in the western equatorial Indian Ocean, forming an arcuate series of wide and shallow banks with small islands in places. The oceanic basins that surround the Seychelles - Amirante region are of various ages and reflect a complex seafloor spreading pattern. Due to the seafloor spreading during a tectonic stage, the Seychelles continental block drifted southwestwards to collide with the oceanic crust of the Mascarene Basin, forming an elongated folded structure at first, and then a subduction zone.

The Seychelles Province is directly related to the breakup of Gondwana in the late Paleozoic and Mesozoic. Oil and gas is postulated to have been generated from Permian to Jurassic lacustrine and continental source rocks (rift stage) with total organic carbon ranging from 2.38 to 6.7 weight percent. Oil and gas may also have been generated by Early to Middle Jurassic restricted-marine, marginalmarine, and deltaic rocks containing as much as 1.7 weight percent organic carbon, and by Maastrichtian to Paleocene marine rocks containing as much as 7.82 weight percent organic carbon. Hydrocarbons might have migrated into Mesozoic and Paleogene reservoirs and traps. Traps are mostly structural within the syn-rift rock units and both structural and stratigraphic within the post-rift units. The primary reservoir seals are Mesozoic and Cenozoic mudstones and shales. Rifted passive margin geologic analog was used for the assessment because of similar source, reservoirs, and traps.

The U.S. Geological Survey estimated mean volumes of undiscovered, technically recoverable conventional oil and gas resources for the Seychelles Rifts Assessment Unit in the Seychelles Province. The mean volumes are estimated at 2,394 million barrels of oil, 20,376 billion cubic feet of gas, and 739 million barrels of natural gas liquids. The estimated mean size of the largest oil field that is expected to be discovered is 793 million barrels of oil and the estimated mean size of the expected largest gas field is 4,765 billion cubic feet of gas. For this assessment, a minimum undiscovered field size of 5 million barrels of oil equivalent was used. No attempt was made to estimate economically recoverable reserves.

That various source rocks are mature and generating hydrocarbons, which are actively migrating, is evidenced by the persistent presence of tarballs on some beaches of Seychelles, by live shows in the three exploration wells, and the presence of anomalies on various geosurveys. Indeed, some of these indications have been interpreted as indicative of hydrocarbon accumulations being present in the offshore of the western shelf and Coetivy land. Such results indicate the Seychelles offshore to be a very attractive, albeit frontier oil exploration province.





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