Seismic characterization of a buildup in the late syn-rift succession of the Lüderitz Basin, Namibia: volcanic edifice or carbonate platform?

The transition from ‘rift’ to ‘drift’ phases during passive margin formation is not fully understood in terms of structural evolution and sedimentary response. ‘Rift–drift’ sequences have become prominent in... [ view full abstract ]

The transition from ‘rift’ to ‘drift’ phases during passive margin formation is not fully understood in terms of structural evolution and sedimentary response. ‘Rift–drift’ sequences have become prominent in academia and hydrocarbon exploration after the discovery of prolific Aptian ‘Pre-Salt’ sag plays in the South Atlantic. High drilling costs for deep-water projects make seismic, magnetic and gravity data the key preliminary tools for understanding margin evolution and related sedimentary successions. In this paper seismic facies analysis is applied to characterise a late syn-rift geobody in the South Atlantic.

The structure ‘Prospect B’ has recently been identified in the Lüderitz Basin, offshore Namibia. The structure is believed to be Barremian in age and forms part of the late syn-rift succession. It is likely that lacustrine/restricted marine conditions prevailed during deposition, with restricted marine conditions coming to an end possibly in the late Aptian (Torsvik et al., 2009; Lentini et al., 2010).

The top of ‘Prospect B’ is characterised by a high amplitude reflector and has a distinct stepped geometry. The structure has an aerial extent of around 570km2 and reaches a maximum thickness of about 350ms TWT (700-1000m). Within the structure steep clinoforms, strong layering, mounded horizons and vents are visible. The sequence overlies a sag basin, into which igneous bodies (that are probably associated with the vents) appear to be locally intruded. Given its size, ‘Prospect B’ could potentially be a world-class hydrocarbon reservoir.

Three models to explain the observations will be discussed:
• Hyaloclastite mound: the steep aggrading and prograding clinoforms, coupled with the high amplitude topsets, could be explained by basaltic lava flows and hyaloclastic delta foresets.
• Isolated carbonate platform: ‘Prospect B’ bears many structural similarities to isolated carbonate platforms, such as the Miocene platforms of Southeast Asia.
• Travertine: Given the structure’s age and position, it is possible that it shares similarities with the thermogenic/microbial ‘Pre-Salt’ carbonates of Brazil and Angola.

References
Lentini, M. R., Fraser, S. I., Sumner, H. S., & Davies, R. J. (2010). Geodynamics of the central South Atlantic conjugate margins: implications for hydrocarbon potential. Petroleum Geoscience, 16, 217-229.
Torsvik, T. H., Rousse, S., Labalis, C., & Smethurst, M. A. (2009). A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin. Geophysical Journal International, 177, 1315-1333.