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Early cretaceous ostracoda of the santos basin, Brazil: biostratigraphic analysis and palaeoenvironmental interpretation of the pre-salt strata
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posted on 22.03.2017by Poropat, Stephen Francis
The opening of the proto-South Atlantic Ocean during the Early Cretaceous facilitated the formation of some of the largest non-marine hydrocarbon systems on the planet. One basin in particular, the Brazilian Santos Basin, has been shown to contain vast hydrocarbon reserves, sourced from and stored within sediments underlying thick layers of evaporites. The evaporites are known as the “Salt”; thus, the underlying sediments constitute the “Pre-Salt”. Fossilised ostracods from two Santos Basin exploration wells, Bem-Te-Vi and Taquari, were studied to determine the nature of any significant Pre-Salt bioevents. Ostracods were relatively rare throughout the Bem-Te-Vi samples, with only 1,086 specimens recovered from a 771m stratigraphic section, though were more common in Taquari, from which 6,969 ostracods were extracted in a 318m interval. Analysis of cutting samples from these wells, drilled 155km apart, led to the observation of numerous stratigraphic and biological events common to both: 1) the base of the Salt (i.e. the top of the Pre-Salt); 2) an interval bereft of ostracods; 3) the first downhole occurrence of the ostracod species Kroemmelbeincypris symmetrica and Kroemmelbeincypris angulata; 4) a second, minor barren interval; 5) an interval preserving abundant specimens of Kroemmelbeincypris spp.; and 6) the first downhole occurrence of Pattersoncypris salitrensis. Taquari did not penetrate much further below the first downhole occurrence of P. salitrensis, meaning that further correlation between the wells was not possible in this study. However, much of the Bem-Te-Vi well below this event was dominated by igneous rocks; thus, few fossils were recovered. The identification of Kroemmelbeincypris (a new genus described herein) and Pattersoncypris in the Santos Basin Pre-Salt allowed an Aptian age to be applied to the majority of the sediments in the wells studied. This was determined through extrapolation (with modification) of biostratigraphic zonation schemes used in other proto-South Atlantic basins to the Santos Basin. The fact that this was possible demonstrates the interconnectedness of the South Atlantic basins during the deposition of the Pre-Salt, as well as the persistence of non-marine conditions north of the Rio Grande Rise – Walvis Ridge Barrier during the Aptian. The presence of low diversity, smooth-shelled ostracod assemblages in association with microbialites and low diversity palynofloras immediately below layers of evaporites indicate that the Santos Basin Pre-Salt sediments were deposited in a hypersaline lake, located in an arid setting. Ostracod abundance increased during flooding events, suggesting that the drilled sections of the lake system were shallow. Most ostracods were preserved as complete carapaces, and juvenile specimens were common throughout both wells: this suggests an environment characterised by infrequent, rapid deposition. This study represents the first attempt to comprehensively utilise ostracods to determine the age and environmental setting of the Pre-Salt sequence of the Santos Basin, demonstrating strong ties between the Pre-Salt ostracod faunas of the Brazilian Santos and Campos basins with those of the African Kwanza and Congo basins. The biostratigraphic zonation scheme proposed, and the palaeoenvironmental interpretations made herein, represent a valuable tool to enhance the efficiency and effectiveness of future Pre-Salt petroleum exploration in the Santos Basin