Raman Mapping as an Investigative Tool for Understanding the Origin of Silica Sphere-like Structures from a Presalt Carbonate Reservoir of the Aptian Barra Velha Formation in the Santos Basin

Silicification in carbonate deposits refers to a diagenetic process in which silica replaces carbonate minerals, which are typically associated with hydrothermal fluids. During silica deposition under hydrothermal conditions, quartz crystallites often form on the surfaces of microorganisms, acting as nucleation sites. This silica replacement involves the simultaneous chemical transformation of the original skeletal materials, followed by precipitation. Organic matter-filled silica sphere-like structures were identified in the thin sections from the Barra Velha Formation chert (Santos Basin, Brazil), by the use of the Raman spectroscopy mapping technique and Scanning Electron Microscopy (SEM). The remaining organic matter showed two typical bands in the first-order Raman spectrum, i.e., the D-band located at approximately 1350 cm^-1 and the G-band at about 1600 cm^-1. The analysis of Raman parameters such as G-FWHM-(Full Width at Half-Maximum of the G-band) and RBS (Raman Band Separation) led to inference of the thermal maturity of samples by calibration against vitrinite standards. Differences in the spectral profile of organic matter obtained with 532 and 632.8 nm excitation radiations revealed the richness of chemical signatures preserved as sphere-like from the pre-salt deep-water Tupi/Lula Field. The SEM analysis revealed silica microsphere morphologies as indicative of the presence of microfossils in this rock. The chemical composition of silica sphere-like structures in BVF may be interpreted as biosignatures preserved in the chert.