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Recent organic-rich deep sedimentation off Southwest Africa: keys for understanding the deep offshore petroleum source-rock distribution.

Abstract : The Atlantic South-eastern margin is influenced by the presence of broad upwelling cells associated with the Benguela Current System. Sedimentary records of cores taken from 35°S to 10°S between 1000 to 4000m depth show extraordinarily high organic carbon (OM) contents for this deep and oxygenated environment. Thus this margin is different from classically known models of preservation in deep setting by the bottom water anoxia or redepositional processes Temporal variability of organic carbon observed along this margin from the north of Walvis ridge to South Africa, shows an enrichment in total organic carbon (TOC) during the Last Glacial Maximum (20kyrs) with respect to Holocene Climatic Optimum(8kyrs). It is the same for biogenic silica, mainly related to diatoms input, which follows the same climatic logic. Carbonate contents are negatively correlated with TOC and biogenic silica. The Lüderitz upwelling cell is presently the most productive area of the Benguela Current System, during at least 280 kyrs, and abundant OM accumulates on the adjacent slope sediments even at great water depth. Results indicate that OM is more oxidized at 3606 m water depth than on the upper slope sediments (1029 m) although petroleum quality of OM throughout the deep-water core remains surprisingly high for hemipelagic deep-sea sediments (HI = 200 to 400 mg HC/g TOC). The petroleum quality of OM accumulated on the upper slope is consistently high: HI average 450 mg HC/g TOC. Organic sedimentation in front of Lüderitz is modellised with the software DIONISOS (IFP), in the aim of developping a 3D stratigraphic model and predicting petroleum quality and quantity of organic matter along the margin. The first step in modelling the deep organic matter sedimentation is to decipher between the main controlling factors the key parameters to be introduce in the model DIONISOS. This imply to define simple laws for productivity, degradation, advection of sediment, etc. Rock Eval parameters shows that organic sedimentation along the margin can be considered as an important source-rock sedimentation. Source-Rock Potential Index (SPI) calculations are in agreement with a maximum of a potential hydrocarbon formation in front of Lüderitz, from shallow depth (500m), to deeper depth (2000m). We believe that the probability that such organic-rich facies occurred in the past in equivalent oceanographic settings at the edge of large oceanic basins should be carefully considered in deep offshore exploration.
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Conference papers
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https://hal-insu.archives-ouvertes.fr/hal-00084962
Contributor : Nathalie Pothier <>
Submitted on : Tuesday, July 11, 2006 - 11:18:40 AM
Last modification on : Wednesday, October 14, 2020 - 3:44:05 AM

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  • HAL Id : hal-00084962, version 1

Citation

Raphaël Blanke, Laetitia Pichevin, J. Tranier, François Baudin, Philippe Bertrand, et al.. Recent organic-rich deep sedimentation off Southwest Africa: keys for understanding the deep offshore petroleum source-rock distribution.. Congrès Géologique International, 2004, Florence, Italy. ⟨hal-00084962⟩

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