Stylolite-controlled dolomitization and dedolomitization in low-porosity carbonates (Lessini Mountains, Southern Alps, Italy)

Dolomitization is among the most widespread processes affecting carbonate rocks and may significantly overprint carbonate successions during post-diagenetic and deformation-related fluid infilling and circulation. The process is generally hindered in low-porosity/low-permeability carbonates (e.g., micritic limestones). However, primary (e.g., bedding interfaces) and secondary (e.g., fractures) rock planar anisotropies might compensate for this low porosity/permeability, acting as potential pathways for fluid ingress and as loci for the initiation of fluid-rock interaction. Among these anisotropies, burial stylolites are particularly widespread in carbonate successions, forming through progressive chemo-mechanical dissolution-precipitation over time. Their large lateral continuity (>1 km) and potentially high vertical frequency make stylolites key features in governing the syn-to-post diagenetic evolution of sedimentary successions.

Although stylolites have traditionally been considered fluid barriers, recent studies challenge this paradigm, a view that we further stress here. We present petrophysical data from micritic limestones of the Lessini Mountains (Italian Southern Alps), where a large portion of the exposed carbonate Jurassic-Cretaceous succession (>700 m thick) is almost entirely overprinted by a regional dolomitization event, which produced large volumes of massive, sandy, crystalline dolostones. We studied preserved patches of micritic limestone where the progression of dolomitization from initiation to complete overprint is clearly visible. Hg-porosimetry, SEM imaging, μ-CT and cathodoluminescence were combined to constrain petrophysical variations associated with dolomitization. Results show that burial stylolites (Hg injection capillary threshold pressure – HgP c. 4 Psi) affecting the micritic limestones (HgP c. 5140 Psi) were systematically exploited by the dolomitizing Mg-rich fluids, transiently aided by fluid overpressure surges, which locally induced brecciation and further enhanced fluid-rock interaction.

Progressive dolomitization increased rock porosity and density from ~1% to ~20% and from 2.65 g/cm³ to 2.9 g/cm³, respectively (from micritic limestone to massive dolostone). Pore characteristics (pore-size, sphericity, 3D-φ-angle and 3D-Eulerian characteristic - all constrained by μ-CT data and Hg-μporosimetry) indicate a complex evolution characterised by (i) early diffuse dolomitization followed by (ii) localised dedolomitization triggered by the later ingress along the porous stylolites of a Mg-poor fluid, which selectively infiltrated the dolomitized succession and created significant rock porosity. Dedolomitization appears to have been more efficient (and is better preserved) along the dolomitized stylolites than within the massive dolostones, where fluid-rock interaction was inhibited by the larger rock volume.

Spatio-temporal porosity variations related to dolomitization and dedolomitization, guided by- and preserved within stylolites, have significant implications for (i) reservoir quality evaluation, and (ii) the mechanical behaviour of carbonate rock masses during post-fluid-infiltration deformation phases. In these settings, dolomitization and dedolomitization promote long-term fluid ingress and circulation, thus even modulating further deformation localisation.