Algal–microbial carbonates of the Namibe Basin (Albian, Angola): implications for microbial carbonate mound development in the South Atlantic

Abstract

Albian carbonate reservoirs are prominent in the subsurface of Angola and the Republic of Congo. Equivalent deposits are well exposed in southern Angola (Benguela and Namibe basins), but have received relatively little detailed sedimentological work. In the Namibe Basin, carbonates form m-thick beds interbedded with shallow-marine and continental alluvial fan siliciclastics. Characteristic carbonate mounds or pillars (≤5m high, 1-2m in diameter) rise above a basal carbonate bed, which consists of oncoid-peloidal rud-grainstones with oysters and echinoderms. Thrombolitic microfacies in the mounds include red algae and microbial-algal crusts. Microfacies are marine, but formed in a stressed environment as a result of detrital input and/or brackish conditions. Shallow burial processes dominated diagenesis; meteoric diagenesis is possible, but no conclusive evidence was found in this study. Primary (intergranular) and secondary (microporosity, mouldic, vuggy, fracture) porosity developed in the carbonates as a consequence of important dissolution and only partial cementation.Up to 22% porosity were estimated visually, suggesting good reservoir potential, but contingent on offshore carbonate development and presence of comparable diagenetic processes. Depositional models for these carbonates need to explain the localized occurrence and bioherm nature. A marine ingression into a coastal embayment and formation of shallow-water microbial

What do we really know about early diagenesis of non-marine carbonates?

“…This is in part because they form important continental paleoclimate archives (Andrews, 2006;Fairchild et al, 2006;Fairchild and Treble, 2009;Tanner, 2010;Frantz et al, 2014). Several non-marine carbonate fabrics have been recognized in subsurface lithologies of economic interest (Harris, 2000;Shiraishi et al, 2008b;Della Porta, 2015;Ionescu et al, 2015;Wright and Barnett, 2015;Schroeder et al, 2016;Claes et al, 2017a;Mercedes-Martín et al, 2017). Furthermore, many carbonate factories in non-marine environments have been studied to understand close interactions between micro-and macrobiota (bacteria, algae, bryophytes, plants) that influence mineral precipitation processes, pore fluids and the resulting carbonate fabric (Freytet and Verrecchia, 1998;Benzerara et al, 2006;Bissett et al, 2008;Takashima and Kano, 2008;Bontognali et al, 2010;Jones, 2010;Peng and Jones, 2013;Pace et al, 2016), offering researchers the chance to improve approaches to detection of ancient and extraterrestrial life.…”

Lattice Boltzmann Simulations of Fluid Flow in Continental Carbonate Reservoir Rocks and in Upscaled Rock Models Generated with Multiple-Point Geostatistics

“…The latter processes influence the size and shape of pores, producing some of the most complex pore networks recorded in sedimentary rocks. Recent hydrocarbon discoveries highlighted the continental carbonate reservoir potential in the presalt exploration, offshore Brazil [2,3] and in the Namibe basin, Angola [4][5][6]. Quantitative data about lithofacies' occurrence, distributions, and their related porosity and permeability are key to understanding the reservoir behavior.…”

Modelling the impact of depositional and diagenetic processes on reservoir properties of the crystal-shrub limestones in the ‘Pre-Salt’ Barra Velha Formation, Santos Basin, Brazil

“…The Barra Velha Formation comprises a unique Cretaceous (Aptian), non-marine, Pre-Salt sedimentary succession of continental carbonate and non-carbonate rocks intercalated with volcanic basaltic rocks, which formed along the Brazilian margin of the South Atlantic during rifting and the breakup of Gondwana (Figure 1A and B; Abrahao and Warme, 1990;Karner and Gamboa, 2000). Broadly similar litholgies are known from the Kwanza and Namibe basins in the conjugate margin of Angola 4 ( Figure 1A; Shroeder et al, 2015;Saller et al 2016). The Barra Velha Formation sequence is mostly composed of lacustrine autochthonous and allochthonous sedimentary rocks, where the autochthonous component formed by both abiogenic and biogenic processes within a hydrothermal evaporitic environment (Rezende and Pope, 2015;Saller et al, 2016).…”