Early diagenetic evolution based on petrography and stable isotope analysis in the Barra Velha Formation of the Brazilian Pre-salt

Abstract

Mineralogy and texture of diagenetic phases in the Aptian Pre-Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study examined data from eight wells from one oil-field in the Santos Basin. Calcite is the most abundant mineral and occurs with fibro-radial texture as spherulites and shrubs and sparse microcrystalline mudstone. The δ¹⁸O values from calcite spherulites and shrubs suggest water of similar composition and temperature, but they have different δ¹³C values. Mudstones show lower δ¹⁸O, suggesting warmer lake water and/or lower evaporation, whereas δ¹³C values indicate a variable, but commonly strong influence of biogenic CO₂. Dolomite with rhombohedral habit was the first to precipitate, followed by lamellar, saddle and anhedral varieties. Rhombohedral dolomites show a positive δ¹³C-δ¹⁸O correlation and a similar range in values to spherulites and shrubs, suggesting similar lake water. The lamellar dolomite is related to biofilms, whereas anhedral dolomite is attributed to mixing of pore and meteoric waters. Lamellar and anhedral dolomites have similar isotopic values, precipitating after rhombohedral dolomite in slightly warmer and/or less evaporatively concentrated pore water. Saddle dolomite is related to hydrothermal fluids that percolated the formation during early diagenesis. Silica occurs as replacement of primary calcite and Mg-clay, but also as a cement and more rarely as a depositional chert. Both dolomite and silica display a complex range of petrographic textures, many of which are facies dependent. This study focusses on the most important phases of the paragenetic sequence that took place pre-burial and are (1) formation of Mg-clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica.