Bruce J. Farrer¹, Gary A. Cole², Z.(Alan) Yu²
(1) BHP Petroleum, London, United Kingdom
(2) BHP Petroleum, Houston, TX

Abstract: Petroleum Systems of the Deep Water Kwanza Basin: pre- and post salt source rocks

The deepwater to ultra deepwater Kwanza Basin, offshore Angola, has the potential to become a significant oil prone province. To date commercial discoveries in the Kwanza Basin have been limited to the present day onshore. Wells drilled in near shore ‘shelfal’ waters have encountered minor oil shows typed to both pre- and post salt sources. The deepwater trend has yet to be tested by a well.

The primary source rocks in this region, as inferred from wells drilled in the deepwater (DSDP sites) and the shelfal/onshore areas are; 1) the synrift pre-salt sequence represented by the Type I lacustrine shales of the Barremian-Aptian (Bucomazi Fm equivalent); and 2) the drift/post salt marine section that can contain significant thickness of organic rich Type II oil prone shales represented by the deepwater Tuenza Fm and the Cabo Ledo Fm.

Positive seepage anomalies recorded during deep water piston core surveys have been typed to a marine marl source. The penetrated pre-salt section is dominated by lacustrine deposition whereas the post salt section is marine.

Source facies variation within the Pinda Grp and basal heat flow remain critical risks within the Kwanza Basin. Initial studies suggest temperature gradients in excess of 35 deg C/Km will be required to mature significant volumes of hydrocarbons. Current thinking links seepage to the post salt marine source rocks. If lower than expected thermal gradients are encountered a pre-salt marine source interval may have to be invoked to explain the current seep to source correlation.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana