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

摘要

Albian carbonate reservoirs are prominent in the subsurface of the South Atlantic. Equivalent exposures in south- ern Angola (Benguela and Namibe basins) have received relatively little detailed sedimentological work. In the Namibe Basin, carbonates form metre-thick beds interbedded with shallow-marine and continental alluvial fan siliciclastics. Characteristic carbonate mounds (5 m high, 1-2 m in diameter) rise above a basal carbonate bed, which consists of oncoid-peloidal rud-grainstones with oysters and echinoderms. Thrombolite mound microfacies include red algae and microbial-algal crusts. The microfacies are marine, and compare with documented occurrences of algal-microbial-oncoidal textures in Albian carbonates of the Congo and Angola. Burial processes dominated diagenesis and have reset carbonate geochemistry from marine values, with the probable exception of Mg concentrations. Up to 22% of primary (intergranular) and secondary (microporosity, mouldic, vuggy, fracture) porosity developed as a consequence of important dissolution and partial cementation. Two depositional models for the localized mound occurrence are discussed: (1) marine ingression into a coastal embay- ment and the formation of shallow-water microbial bioherms; and (2) a submarine groundwater spring discharging in coastal areas downdip from alluvial siliciclastics. Marine fauna, similarity with marine Albian strata elsewhere and a partly pre- served marine Mg geochemical signature favour a marine ingression. Environmental conditions were likely to have been stressed on account of the siliciclastic input, variable salinity and elevated nutrients, all of which are consistent with the observed microfacies. A submarine spring is conceptually feasible, but is considered to be less likely owing to the absence of a clear meteoric signature and the low likelihood of bicarbonate-rich groundwater in the region. Using the discussion of depositional models for the studied outcrop, and incorporating a literature review, the study pro- poses a set of criteria to distinguish various marine and non-marine carbonate mounds in the subsurface. The most diagnostic criteria are: (1) marine or continental fauna; (2) sediment geochemistry, in particular Mg, Sr, and δC and δO isotopes where preserved through diagenesis; and (3) carbonate fabrics, such as crystalline shrubs, that are diagnostic of thermogenic continental mounds. The scale of geobodies and the mineralogy of mounds can sometimes be used as additional criteria. This set of criteria can help exploration and production geologists who need to devise exploration and development strategies in unconventional carbonate reservoirs of the South Atlantic and other rift basins.