Source Rock Aspects of the Deep-Water Potential, Offshore Namibia

Swart, Roger, NAMCOR (Pty) Ltd, Windhoek, Namibia

Recent drilling of exploration wells in Namibia has proved that there are two significant Cretaceous oil-prone source rocks offshore Namibia, namely the Apto-Albian and the Cenomanian. Previously the south-western coast of Africa had long been regarded as being a gas-prone region, based largely on the dry gas and source rock quality found in the first well drilled in the Kudu gas field.

The Cretaceous paleogeography of the proto-South Atlantic and its margins is important in understanding the development, distribution and quality of these Cretaceous source rocks. During the Aptian the proto-South Atlantic was an anoxic basin with restricted ocean­ic circulation. Northwards the Walvis-Rio Grande Ridge formed a formidable barrier to movement of water. The ocean at this stage was only 500 km wide and oceanic circulation was restricted, with the Falkland Plateau still being located to the south of the basin. In the distal, restricted parts of the basin excellent, oil-prone source rocks developed that have hydrocarbon generation potential in excess of 60kgHC/ton.

A younger oil-prone source rock of Cenomanian age has been proven in the Walvis Basin. The environment of deposition of this source sequence is likely to have been anoxic where bottom water circulation was poor or absent. This source rock is capable of generat­ing large amounts of oil but is immature in the well locations.

Recent biomarker work on the condensates associated with the Kudu gas have shown that the condensates were derived from a highly mature, marine, oil-prone source rock of probable Cretaceous age.