# 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

## TL;DR

This study uses Raman mapping and SEM to analyze silica sphere-like structures in a Brazilian carbonate reservoir, suggesting they may be preserved microfossils from ancient organic matter.

## Contribution

The study introduces Raman spectroscopy mapping as a novel method to identify biosignatures in silica structures from deep-water carbonate reservoirs.

## Key findings

- Raman spectroscopy identified organic matter within silica spheres, showing D- and G-bands indicative of thermal maturity.
- SEM analysis revealed microsphere morphologies consistent with microfossils in the chert samples.
- Excitation wavelength differences in Raman spectra highlighted preserved chemical signatures in the silica structures.

## Abstract

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.