Magmatically driven hydrocarbon generation and fluid flow in the Namibe Basin of Angola

Abstract

Abstract Magmatic activity can severely alter the thermal structure of a sedimentary basin, with variable effects on the petroleum system. The Namibe Basin of Angola (Cretaceous South Atlantic rift) contains well-exposed magmatic and petroleum system elements and allows integrated assessment of how magmatic activity can modify the petroleum system. The basin was affected by syn-rift and post-rift magmatic events, and bitumen is observed within both the Pre- and Post-Salt stratigraphical sections. In the Pre-Salt, fluorescent bitumen has a lacustrine signature and is associated with calcite and quartz cements. Onshore Pre-Salt units are thermally immature, and therefore the source rock that generated the Pre-Salt bitumen is likely located offshore. Hydrocarbons migrated or re-migrated via magmatically driven fluids, reaching the present-day onshore. Closer to magmatic units, non-fluorescent pyrobitumen was instead observed, evidencing hydrocarbon cracking processes following emplacement. In the Post-Salt, bitumen is in situ and shows marine-like signatures compatible with an immediate Post-Salt source rock depositional environment. In the immediate Post-Salt, units with very high total organic carbon values (TOC; up to 13.8%) and excellent source rock properties (hydrogen index >600 mgHC g −1 TOC) have reached thermal maturation. Within the Namibe Basin these Post-Salt source units lie in proximity to major Turonian–Coniacian–Santonian volcanic centres and associated shallow intrusions, which are likely to have caused thermally forced maturation processes and generation of the Post-Salt hydrocarbons. This paper demonstrates the importance of an integrated field, petrographic and geochemical approach in unravelling the influence of magmatic activity on basin thermal structure and petroleum systems.