Abstract

Rifting, volcanism and hydrothermal alteration produced one of, if not the most unusual carbonate successions seen in the geological record. During the Aptian, in a transitional interval of South Atlantic opening, from rift to “sag”, vast shallow hyper-alkaline lakes formed in the central South Atlantic, extending from both margins and covering at least 100,000km2.. These lakes generated unique carbonate-silicate successions (the Barra Velha Formation and equivalents), up to >500m thick, which now act as the host for the multi-billion-barrel oil fields off Brazil and Angola. The very limited catchment geology and hydrology of these lakes precluded the formation of typical evaporite deposits and instead Mg-silicate gels formed associated with a very small range of calcite morphologies, in part growing in the gels. The gels converted to stevensite clays which later underwent congruent dissolution to produce much of the porosity in the reservoirs. The resulting rock fabrics, often visually stunning, present a dilemma for current classifications. Thermo-dynamic modelling and C & O stable isotopes support the sedimentological models, for what was an extreme system lacking in analogues. However, the biggest challenges have not been the limited amount of data from these deep reservoirs and their unique composition, but the effects of “word magic”, that is the (mis)use of terms in the hope their application brings understanding. The misuse of just two or three terms during the discovery phase has resulted in an almost catastrophic misinterpretation of the reservoirs at all scales. On a positive note, numerous, often very specific university research projects have been supported improving our knowledge of a range of non-marine carbonates, but largely irrelevant to understanding the Aptian lakes.