WARNE, ALEX¹ and COLIN P. NORTH^2
1National Petroleum Corporation, Namibia
²University of Aberdeen, Scotland.

Abstract: Lava Compartmentalization of an Aeolian Reservoir

Gas in the giant Kudu Field, offshore Namibia, is hosted in aeolian sandstones interbedded with contemporaneous basalts extruded during the initial break up of Gondwana. An analogous succession is exposed in the Huab Basin of NW Namibia, an extension of the Parana Basin of South America. The aeolian Etjo sandstone is interbedded syn-depositionally with the first extrusions of the Etendeka continental flood basalts (1 33+/-1 Ma, equivalent to the Serra Geral basalts in the Parana). More extensive lavas eventually cover the whole basin and turn off aeolian sand accumulation. Part of the outcrop has been studied in close detail to understand and quantify the 3D architecture and fluid flow pathways, and assess the impact of the lavas on reservoir quality.

Perfectly preserved wind ripples at the contact with lavas, basalt fragments incorporated at the base of sand bodies, deep lava cracks filled with sand, and pahoehoe imprints on sandstone, demonstrate that the lava erupted into an active aeolian system. The initially laterally extensive main Etjo erg passes upwards into smaller compound dune complexes interbedded with basalts, then to small isolated simple duneforms. Communication between sandbodies deteriorates progressively as sand supply diminishes and sand body formation becomes erratic. Dune topography controlled ponding of lava and resulted in variable depths of alteration of the sands. The degree and depth of induration determines the permeability of the sandstones and also affects connectivity. Although the main factors contributing to sand-body distribution and 3D geometry, such as sand supply, lava viscosity and topography, are locally unique, the general controls on deposition are predictable throughout the basin.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas