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MESOZOIC SOURCE ROCKS AND HYDROCARBON POTENTIAL OF THE SEYCHELLES OFFSHORE
Author(s) -
Plummer Ph. S.
Publication year - 1994
Publication title -
journal of petroleum geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.725
H-Index - 42
eISSN - 1747-5457
pISSN - 0141-6421
DOI - 10.1111/j.1747-5457.1994.tb00124.x
Subject(s) - source rock , geology , cretaceous , gondwana , paleontology , mesozoic , sedimentary rock , maturity (psychological) , rift , window (computing) , continental margin , submarine pipeline , geochemistry , tectonics , oceanography , structural basin , psychology , developmental psychology , computer science , operating system
Three wells in the Seychelles offshore indicate the existence of four potential source‐rock intervals within the Mesozoic succession. Two of these originated during the rift phase that eventually cleaved Gondwana into Eastern and Western blocks — namely, Middle Triassic lacustrine mudstones, and Early/Middle Jurassic deltaic‐lagoonal mudstones. The other two source‐rock intervals were deposited on passive marine shelves during continental drift phases — namely, Late Jurassic to Early Cretaceous mudstones and siltstones during the East‐West Gondwana drift, and Maastrichtian to Paleocene mudstones during the later Seychelles‐India drift. These source rocks are dominated by terrestrial organic matter. Although TOCs are generally good (greater than 1.0%) and range to excellent (7.82%), potential hydrocarbon yields are generally only poor to fair (less than 6 kg HC/tonne of rock). One good potential yield of 10 kg/tonne has been measured. Maturity data (R 0 and T max ) indicate that, in the wells, the youngest source rock is immature, while the oldest lies in the gas “window”. The Jurassic/Cretaceous source rocks, on the other hand, lie within the oil “window”. Analyses of numerous beach‐stranded tarballs that are believed to be of indigenous origin reveal, in addition to a source dominated by terrestrial organic matter, the presence of a source rock dominated by marine algal organic matter. Such a source rock may have developed during a Middle Jurassic phase of shallow‐marine carbonate deposition, which shows some affinity to source‐rock quality, and is characterized by an oolitic marker limestone in each well. This oolitic limestone is also a component fades of the carbonate succession that contains the prolific oil‐prone source‐rock fades of the Middle East.