An Evaluation of Source Rocks of the Kwanza Basin: A review
Kwanza Basin (KB) has been, recently, submitted to more accurate petroleum system evaluations, since the well-known Congo Basin has reached a mature production stage. The understanding of KB petroleum systems was provided by geological analogue studies with Congo Basin (Angola) and Santos Basin (Brazil).
The KB, located in the central part of the Angolan coast, is part of the West-Central Coastal Geological Province of the Sub-Saharan Africa region. Was formed during the Early Cretaceous opening of the South Atlantic Ocean, when the Congo–San Francisco craton was rifted along the grain of the Proterozoic west Congolian–Braziliano orogen. The KB presents features typical of the Atlantic-type marginal sag basins filled with Meso-Cenozoic deposits. The geological history of Angolan coastal sedimentary basins can be divided into three main stages: Pre-rift stage (Late Proterozoic to Late Jurassic); Syn-rift stage (Late Jurassic to Early Cretaceous), and Post-rift stage (Late Cretaceous to Holocene). The KB is separated into inner (onshore) and outer (offshore) basins and these in turn are split into small-basins by basement structural highs, produced by the major transform faults. The KB is known by the occurrence of multiple and hybrid petroleum systems based on at least six source formations (Infra-Cuvo, Upper Cuvo, Binga, Teba, Cunga and Quifangindo) of Neocomian to Eocene age. Other formations (Rio Dande, Cabo Ledo and Quissonde) are also good to very good potential sources but are not deep enough to have been submitted to sufficient maturation. The Infra-Cuvo formation (Neocomian – Barremian) contains thick, organic-rich lacustrine shales. It displays Type I kerogen characteristics with an average total organic carbon (TOC) content of 2% (some > 20%) and hydrogen index (HI) values varying from 600 to 800 mg HC/g TOC. This source rock is limited in regional extent because it was deposited within grabens and is mature for hydrocarbon generation. The Upper Cuvo formation (Early Aptian) contains lacustrine and marginal marine evaporitic shales. It displays Type I and II kerogens characteristics with an average TOC content of 3% and HI values of 300 to 800 mg HC/g TOC. The Upper Cuvo formation is more widespread than the Infra-Cuvo formation, and was deposited during a regional sag phase and is mature for hydrocarbon generation. The Binga formation (Aptian – Albian) contains shallow marine organic-rich micritic shales, which are believed to be developed regionally and are mature for oil generation in the deeper parts of the Kwanza basin. It shows Type II kerogen characteristics with an average TOC content of 6% and HI values up to 600 mg HC/g TOC. The Teba formation (Campanian – Maastrichtian) contains marine shales and marls which can be mature for oil generation in the deeper salt-withdrawal troughs. It presents Type II kerogen characteristics with an average TOC content of 4% and HI values more than 400 mg HC/g TOC. Cunga and Quifangondo formations (Eocene – Miocene) contain similar marine black carbonaceous shales due to regional subsidence. They present Type II and II/III kerogens with an average TOC contents of 2 to 10% and HI values of 200 to 500 mg HC/g TOC.
AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015