Non-Marine Carbonates – Facies, Diagenesis and Porosity Development

The discovery of major non-marine carbonate reservoirs in the South Atlantic offers new challenges for carbonate sedimentologists and reservoir geologists. Non-marine carbonate precipitation originates in a wide spectrum of settings, e.g. ambient temperature freshwater fluvial and lacustrine systems, alkaline and saline in lakes, and hydrothermal water springs. Such variability has limited the understanding of non-marine carbonate facies models and little information is available about their diagenetic pathways. This review aims to emphasize the specific properties of non-marine carbonate reservoirs. Their depositional environments include river cascades, barrages and channels, lake shorelines and epilimnion zones, sub-lacustrine groundwater springs, palustrine settings, and thermal water springs. Main external controls are basin morphology and hydrology, tectonic setting and climate. As for marine carbonates, non marine precipitation can be: 1) an abiotic process, driven by CO2 degassing in cooling, flowing or wave agitated water or by mixing of bicarbonate-rich and Ca-rich waters at sub-lacustrine springs; 2) a biologically-induced process via biomineralization or organomineralization mechanisms associated to cyanobacteria, heterotrophic bacteria and EPS; 3) a biologically-controlled process in skeletal biota, e.g. molluscs, ostracods and algae. Unlike marine carbonate systems, where environmental and temporal controls on the carbonate factory, its original mineral composition, primary and secondary porosity are relatively well understood, non-marine carbonates have setting-specific mineral suites, build-up morphologies, microfabrics and porosity developments. Water chemistry, in particular its Mg/Ca ratio (and eventually temperature), play a key role in determining the primary carbonate mineralogy and the associated minerals, including clays. The primary mineralogy affects the diagenetic potentials of the carbonates, producing different diagenetic pathways and porosity development. Non-marine carbonates also exhibit a wider range of primary pore systems reflecting complex interactions between abiotic and biologically-induced precipitation processes: novel framework porosity, i.e. within dendritic and shrub-like growth forms, and unusual forms such as coated-bubble porosity. Secondary porosity includes biomoldic (after degradation of vegetation and aragonitic shell dissolution), and vuggy porosity due to meteoric dissolution.