Pore geometry and permeability estimation of carbonates from the Barra Velha Formation: Insights from digital image analysis of thin sections

Abstract

The analysis of geological and petrophysical properties is fundamental for evaluating the heterogeneous and complex nature of carbonate reservoirs, such as those of the Brazilian Pre-Salt. While porosity and permeability are routinely quantified at the core scale, the relationship between pore geometry and these properties remains complex and highly variable in carbonate systems. This study investigates how pore-scale geometric attributes derived from thin sections relate to porosity and permeability measured in corresponding core plugs from the Barra Velha Formation, Santos Basin. Digital Image Analysis (DIA) was applied to 355 blue-epoxy–impregnated thin sections to quantify total optical porosity (TOP) and pore geometry parameters, including Aspect Ratio (AR), Gamma (circularity) (γ), Pore structure complexity (PoA), and Dominant Pore Size (DomSize). Facies (F1–F5) and pore types were identified qualitatively to support geological interpretation, and routine core analysis (RCAL) were obtained from 304 core-plug samples. Permeability was initially estimated using the Kozeny equation and subsequently refined through facies-specific Multivariate Linear Regression (MLR) models incorporating TOP and geometric parameters. The results indicate that facies F1, F2, and F4 exhibit higher porosity and permeability associated with interparticle and vuggy pore systems, whereas facies F3 and F5 display lower permeability linked to intraparticle and moldic pores and stronger diagenetic modification. Silicification locally reduces porosity while, in some cases, increasing DomSize through selective dissolution. Overall, permeability is primarily controlled by DomSize and PoA. Facies-specific MLR models significantly outperform Kozeny-based estimates, highlighting the potential of DIA as a cost-effective tool for core-scale reservoir characterization in complex carbonate reservoirs.