Acid Dissolution of Magnesian-Calcite Stromatolites from Lagoa Salgada: An Experimental Evaluation Using X‑ray Microtomography.

The dissolution behavior of carbonate rocks in acidic environments is a fundamental process in reactive flow through porous media, with key applications in reservoir stimulation and carbon storage. Although many studies have examined dissolution kinetics in homogeneous rocks dominated by calcite or dolomite, the influence of compositional and structural heterogeneity remains poorly constrained, particularly in microbial carbonates typical of Brazil's Pre-Salt reservoirs. This study investigates the acid dissolution of a magnesian-calcite stromatolite, considered an analog of the Barra Velha Formation, and compares it with two reference materials: Indiana Limestone and Silurian Dolomite. Static dissolution experiments were performed using 0.5 M hydrochloric acid. Dissolution behavior was quantified through average reaction rates and complemented by high-resolution X-ray microtomography, which enabled visualization and quantification of internal morphological evolution during the experiments. Results reveal substantial variation among stromatolitic facies, with dissolution rates increasing from Layer A to Layer C despite an opposite trend in magnesian-calcite content. Layer C exhibited the most extensive structural alteration and greatest mass loss, whereas Layer A showed slower kinetics and minimal modification. Indiana Limestone displayed intermediate behavior, while Silurian Dolomite was the least reactive. These findings demonstrate that, under the experimental conditions investigated and considering the stromatolite samples and fluid system used, microstructure and pore architecture exert a stronger control on carbonate dissolution rates than mineralogical composition alone under conditions of negligible fluid movement.