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Classifying Reservoir Carbonates When the Status Quo Simply Does Not Work: A Case Study From the Cretaceous of the South Atlantic


The limitations of current limestone and porosity classifications are brought into sharp focus with the discoveries of unusual Aptian lacustrine carbonates, offshore South Atlantic. A range of textures and pore types are found which have no direct analogue with other carbonate successions and reservoirs, including such idiosyncrasies as highly labile matrices. By constraining these unusual rock types into existing classifications, the interpreter is led into false interpretations by the conceptual “baggage” associated with many existing terms and concepts. For example, one critical problem is that existing classifications are based on the concept of textural maturity whereby the presence or absence of a matrix is seen as an indicator of energy levels. What happens when the carbonate grains grew in a matrix which later dissolved producing what appears to be primary intergranular porosity? In existing classifications boundstones are textures which show signs of having been bound biologically during deposition, but what if these features indicating binding are in fact diagenetic? A pragmatic approach is needed to identify textural and pore types and to do this for these important Cretaceous carbonates involves identifying ghost matrices by comparing the same grain frameworks between units with preserved matrices and those lacking it. A robust, practical and tested classification of these lacustrine carbonates has been developed based on distinguishing in situ precipitates (biotic and abiotic) from reworked material. A crucial step was to identify and refine criteria for ghost matrices which have created much of the porosity and to relate these criteria to a precursor silicate. The application of the approach has facilitated the formulation of a successful depositional model, now substantiated by geochemical modelling, and a practical means for rock fabric determination and for interpreting diagenetic pathways and petrophysical properties.