Post-Rift Potential Source Rock Correlations and Prospectivity of the Deep Atlantic Conjugate Margins, South of the Walvis Ridge

Abstract

Several isolated small half graben (5-10 km wide and 1-2 km deep) are present along the conjugate margins of the South Atlantic, south of the Walvis Ridge which were produced by Early Cretaceous rifting. These can contain source rocks (e.g. AJ-1 well in South Africa). However, the areal extent of these grabens is restricted. The main accommodation space created by Early Cretaceous extension is filled by magma which has produced seaward dipping reflectors (SDRs). The upper part of the SDRs has been drilled in the Walvis and Pelotas basins and consists of sub-aerial reddened basalts. No source rock potential has been encountered, nor is any expected in such a sub-aerial sequence. Hence, the hydrocarbon prospectivity of the deepwater margins will largely rely on the presence of post-rift source rocks. Early drift, Barremian to Aptian Age, marine source rocks were identified in several wells including the recent Murombe-1, Wingat -1 and Moosehead-1 wells, where residual TOC levels up to 6% in source packages of up to 150 m were measured. The deposition of this source rock is interpreted to be widespread in the deep water and modelled to be oil mature in several large kitchen areas, situated in present day water depths from <500m to over 3000m. Additionally, good quality Cenomanian to Santonian age source rocks have also been identified in several wells offshore Namibia, such as the 1911/15-1 well with TOC levels reaching up to 10 % and a gross thickness of 170m, and the Kabeljou-1 well with average TOC of 5.3% and approximately 50m thickness. The Barremian to Aptian age source rocks on the Namibian margin are now recognised to be the principal source rock for future exploration drilling and they are characterised by parallel-bedded, low amplitude, low frequency, continuous low-impedance reflectors, which lie above the SDRs. The Aptian section above the SDRs in the deepwater (>2 km) Pelotas-Argentine margin has never been drilled, but seismic data indicates a similar stratal package. Recent 2D/3D seismic data from Namibia will be used to characterise the source rock package and compare with recently acquired seismic from Argentina and Uruguay.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018