Diagenesis and reservoir potential of continental carbonates: Cretaceous of the South Atlantic rift

Abstract

The Pre-salt discovery in the central segment of South Atlantic has been the largest hydrocarbon reservoir province discovered in the last 15 years. In the West African basins, an analogous petroleum system to the Brazilian Pre-Salt discoveries has been recognized in the Kwanza, Gabon and Namibe basins. The latter represents the conjugate to the Santos Basin, the most prolific Pre-Salt basin. The Namibe Basin contains pre-salt outcrops, and therefore provides an important constraint on stratigraphy, sedimentology and basin evolution. Carbonates deposits in the Namibe basin are associated with the hydrothermal fluids on fault systems and basement volcanic complex and the studied deposits a combine most of these elements within their Pre-Salt section. As most of the outcrops of basement rocks were largely peneplained during Upper Cretaceous transgressions, Maraquita is one of the few places where the top of the Bero Volcanic Complex is exposed. In the Maraquita Valley, a lake fill succession above the Bero Volcanic Complex occurs within a half graben system is represented by carbonate-clastic ephemeral pools. The Maluca pool and Chapeu Armado deposits are the main focus of study. The deposits show a close relationship to the volcanics and to fault and fracture zones, as the pools locally develop along fault trends associated with hot fluid expulsion and subaerial carbonate precipitation (travertine), passing into lacustrine carbonates. Chapeu Armado depostis within an area interpreted as a major transtensional relay zone. The NW-SE trending faults are linked by a series of NE-SW faults. Palaeo and modern drainage enters the basin at the interpreted fault linkage point and hydrothermal. The Chapeu Armado Nephelinite plug is also located in this zone of fault linkage. Sedimentology, petrography, and geochemistry were analysed in order to understand the geological evolution of the system. Dolomite is the most common diagenetic constituent replacing completely the original fabric and filling interstitial pores, appearing in variable blocky, microcrystalline and more rarely mosaic habits. Additional diagenetic minerals include iron oxide, silica cements and sulphates. The diagenesis was a multi-phase process fluid derived from hydrothermal to meteoric sources. The Maluca and Chapeu Armado shows a complex sedimentary environment with an interplay of multiple factors including a hydrothermal system associated to the Bero volcanics and an active fault system, followed by a multi-phase diagenetic history. The interpretation and evolution of this system provides an understanding of how these carbonate pools/lakes systems formed associated with the igneous basement and also an applicable comparison to the lake systems for the South America Pre-Salt Reservoir.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Catherine Hollis (Supervisor) & Stefan Schroeder (Supervisor)