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Seismic Characterisation of “Prospect B”: A Possible Carbonate Build-Up From the Late Syn-Rift (Barremian) of the Lüderitz Basin, Namibia

Abstract

The rift-drift transition or sag phase during passive margin formation remains relatively poorly understood in terms of dynamics and sedimentary response. Seismic, magnetic and gravity data are key tools for reducing uncertainty during exploration and are substantially less costly than drilling wells; this is particularly true of deepwater projects. Seismic attribute analysis has been carried out on a 3D seismic survey from the Lüderitz Basin, offshore Namibia to better understand the basin's sag phase evolution. Prospect B is a recently identified isolated build-up in the Lüderitz Basin; it has an aerial extent of around 570km2 and in the absence of well control is believed to be Barremian in age. The body has a high amplitude top surface, a distinct stepped geometry and a maximum thickness of 350ms TWT (700-1000m). The facies present within Prospect B include steep clinoforms, strong continuous reflectors and mounded geometries. The Lüderitz Basin sits within the Etendeka igneous province and the syn-rift sediments below the prospect are locally intruded with saucer-shaped sills. Several of the sills are associated with small vents within Prospect B that could be hydrothermal or volcanic in origin. Prospect B shows a weak response on gravity and magnetic data, in contrast to several igneous bodies lower in the sequence. Based on the prospect's internal geometries and seismic attributes, it is believed to represent either a carbonate or volcanically dominated system. The presence of vents and steep clinoforms with a lack of magnetic/gravitational signature could suggest a hyaloclastite mound. Prospect B could also be an isolated carbonate build-up with hydrothermal/volcanic elements. Given its tectonostratigraphic position and proximity to the pre-salt basins of Brazil and Angola, at least a portion of the build-up could be composed of microbialite. In a carbonate model, potential hydrothermal fluids could both enhance and reduce reservoir properties, whereas a hyaloclastite mound is likely to be essentially non-reservoir. The similarity in seismic response between carbonates and volcanics makes it difficult to distinguish the prospect's lithology. Based on geometry and seismic attribute analysis, it is believed that Prospect B is more likely to be a volcanically influenced carbonate build-up; in the absence of a well this cannot be verified. Given its size, Prospect B also has the potential to be a world-class reservoir.