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

Abstract Dolomitization is widely recognized as a key diagenetic process in carbonate systems, yet its crystallochemical controls on pore system evolution remain incompletely understood. This study integrates petrography, powder X-ray diffraction with Rietveld refinement, wavelength-dispersive X-ray fluorescence, scanning electron microscopy, X-ray microtomography, and petrophysical measurements to investigate the relationship between dolomite crystallochemistry and porosity-permeability development in carbonate microfacies from the Alcântara, Jandaíra, and Barra Velha formations, Brazil, representing contrasting diagenetic settings along the Brazilian Equatorial and Southeast margins. Alcântara dolostones preserve near-stoichiometric dolomite (Ca/Mg approximately 1.6; unit cell volume 320.65-321.06 ų) interpreted as syn-depositional to early diagenetic in origin, associated with microbially mediated precipitation in a restricted evaporitic lagoon, and exhibit the highest porosity values in the dataset (22.9–27.6%) with moderate permeability (1.91–2.37 mD) reflecting fine-crystalline pore throat geometry. Jandaíra Formation samples contain Ca-enriched, non-stoichiometric dolomite (Ca/Mg approximately 2.2; unit cell volume 325.85-326.23 ų) formed under meteoric fluid circulation, with near-zero matrix porosity resulting from pervasive early cementation and residual permeability (1.54–1.90 mD) governed by fracture networks. Pre-Salt Barra Velha carbonates exhibit intermediate compositions (Ca/Mg approximately 1.9; unit cell volume 322.09-324.26 ų) reflecting mesodiagenetic overprinting at burial depths exceeding 3,000 m, with moderate porosity and the highest permeability values (up to 13.72 mD) controlled by burial-related dissolution and structural discontinuities. Dolomite d-spacing and unit cell volume show strong correlations with porosity (R² approximately 0.90) across formations, interpreted as crystallochemical proxies for diagenetic equilibration rather than direct causal controls. Permeability shows no meaningful correlation with crystallographic parameters, confirming that fluid transmissivity is governed by pore connectivity rather than crystal chemistry. These results demonstrate that XRD-derived crystallochemical parameters provide a complementary fingerprint for diagenetic pathway reconstruction in dolomitized carbonate successions, with practical implications for hydrocarbon reservoir characterization and carbon capture and storage assessment.