Dolomitization and crystallochemical controls on pore system development in carbonate microfacies: examples from São Luís, Potiguar, and Santos basins (Brazil)

This study investigates carbonates from the São Luís, Potiguar and Santos (Pre-Salt) basins. Dolomitization is key process enhancing reservoir quality in carbonate systems; however, dolomite crystallochemistry studies remain limited in the literature. This work integrates X-rays powder analyses (Rietveld method), microscopy, microCT imaging and porosity/permeability measurements, to correlate mineralogy and petrophysics. Alcântara dolostones (up to 95% dolomite) display high porosity (22.90–27.59%), dominated by intercrystalline and fenestral pores. These values correspond to low Unit Cell Volume-UCV (321 ų) and stoichiometric dolomite characterized by d104 = 2.88 Å and Ca/Mg ≈ 1.6. Pre-Salt dolomitized samples show moderate porosity (8.05–15.05%) associated with intercrystalline pores and UCV around 323 ų, reflecting non-stoichiometric Ca-rich dolomite (d104 = 2.89 Å; Ca/Mg ≈ 1.9). Jandaíra limestones exhibit very low porosity (0.36–1.27%), linked to higher UCV (326 ų) and dolomite cementation composed of highly non-stoichiometric crystals (d104 = 2.90 Å; Ca/Mg ≈ 2.2). These trends demonstrate that increasing dolomitization – and the associated reduction of calcite’s unit cell toward that of dolomite – plays a partial role in generating pore space. Permeability (k) shows weaker mineralogical control. Alcântara (1.91–2.37 mD) and Jandaíra (1.54–1.90 mD) present low k values correlated with higher Crystallite Size-CS (1,215–1,970 Å) and low Micro Strain-MS (0.05–0.07%). Pre-Salt display higher permeability (6.45–13.72 mD), associated with smaller CS (≈ 722 Å) and higher MS (0.13–0.22%), suggesting that diagenetic microstructural features largely govern fluid flow. Dolomite crystallochemistry can highlight potential applicability for rapid petrophysical prediction in drill cuttings.