Pre- and Post-Salt Non-Marine Carbonates of the Namibe Basin, Angola
The Namibe Basin is the southern-most of the Angolan Atlantic margin salt basins; an oblique rifted margin, essentially the conjugate to the hydrocarbon prolific Santos Basin of Brazil. Onshore outcrops are limited to a narrow (20 km wide) elongate (150 km long) coastal strip, occurring at the very edge of the South Atlantic rift. Incised drainage and an arid climate afford good outcrop conditions, allowing a unique insight into Pre- and Post-Salt successions. In this contribution we present the results of a collaborative Sonangol-Statoil study addressing the Namibe Basin. Specifically, we show 2 case studies on fault-related carbonates; 1 Pre-Salt and 1 Post-Salt.
The first rift-related deposits are interbedded basalts (dated 133ma), rhyolites and aeolian-alluvial clastics exposed in a series of tilted fault blocks. Overlying the volcanics are an unusual set of outcrops including fissure ridge and “dam and cascade” carbonates (travertine’s) and associated lacustrine sediments. The travertines are spatially related to extensional-transtensional faults, and typically have “cascade” morphologies, draping underlying topography. Lacustrine units are spatially confined to topographic lows, typically hangingwall basins, and are spatially limited in extent (5-15m thick, 200m to 2 km laterally). Hangingwall basin fills are mixed carbonate-clastic. At 2 locations a lateral passage from travertine to lacustrine facies can be mapped. Secondary diagenesis is complex, including dolomitisation, silicification and exotic hydrothermal cements.
Post-salt non-marine carbonates are associated with a second period of igneous activity (dated 88ma). Multiple volcanic centres can be mapped, again spatially related to faults. Both sub areal and submarine flows are exposed. In the sub areal settings spring mound and fissure ridge carbonates are spectacularly developed, associated with extensional faults. Individual fracture-fed cascades can be mapped into hangingwall basins, where they pass into mixed carbonate-clastic sediments, including locally well developed coquina facies. Secondary diagenesis is again marked, with silicification spatially linked to vents.
The two case studies give an insight into the important link between igneous activity, faulting and non-marine carbonate deposition; analogy can be drawn to the enigmatic Pre-Salt reservoirs of the South Atlantic. Volumetrically however, the described examples do not form significant reservoir bodies.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California