Oil Prospectivity of Deep Water Turbidites in the Walvis Basin, Magmatic Margin Offshore Namibia
Until 2015 the primary risk element for exploration in the magmatic margin south of the Walvis Ridge was the uncertainty on the presence and effectiveness of source rock and charge. This paper will discuss the evidence for de-risking source rock and focus on the presence and effectiveness of reservoir and trapping mechanism in the Walvis Basin. Exploration wells operated by HRT during 2012 and 2013 offshore Namibia have significantly reduced the uncertainty on distribution of oil mature Aptian source in the Walvis Basin. Both wells, Wingat-1 and Murombe-1, encountered thick (up to 130 m) Aptian source rock, with a low frequency and low amplitude seismic character, above the Seaward Dipping Reflectors. The Wingat well not only penetrated the Aptian source rock, but it also recovered light oil 38o to 42o API in several thin sandy lenses within the Aptian interval. This is the first oil recovered from the Aptian in the deep water and proves the efficiency of this as an oil source. The source rock has been mapped from these penetrations across the Walvis Basin, and the overlying isopach calculated to delineate the area where the Aptian is in the oil window. Having de-risked the source rock for presence and effectiveness in the Walvis Basin, we discuss the evidence both from well and 2D seismic data for the presence of turbidite sand channels and fans in this area. The Murombe well encountered 242 m gross section of coarse clastics with a 15% n/g ratio and average porosity of 19%, proving that Late Cretaceous turbidites on this margin can provide high quality reservoir sands. A number of Late Cretaceous high amplitude sand channels within a comparable interval are clearly identified in the deep water seismic down dip and on-strike from the Morombe-1 well location, interpreted as a lower slope meandering channel system. We will discuss paleotopographic reconstructions to review the provenance for the coarse clastics and propose that the Late Cretaceous channel complexes within the Walvis Basin are composed of clean sands with a high proportion of quartz derived from the Congo Craton and Damara Belt containing metasediments, polymetamorphic gneisses and granites. The presence of mature oil generative source rock below thick clean quartz rich sands is therefore proven in this basin. We will conclude by showing the evidence for an up-dip bypass zone for the non-deposition of sediment, and/or laterally continuous fault systems that will provide up-dip trapping.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018