Texture-controlled hydrothermal dolomitization experiments to investigate interface kinetics and pore-scale transport

Dolomitization is a key diagenetic process that reorganizes porosity and permeability in carbonate rocks, yet the coupling between interface kinetics and transport heterogeneity remains poorly constrained. We performed time-resolved hydrothermal dolomitization experiments on oolitic limestone and chalk to test how carbonate texture impacts the dynamics of replacement fronts. Pseudo 4D micro-CT, along with SEM, EBSD and microprobe analyses reveal that the mechanism of dolomitization is depending on the texture and on the chemistry of the fluid. We studied oolitic limestone, chalk and carrara marble, three rocks with different porosity and permeability. In both oostone and chalk, dolomite rims propagate rapidly and produce wavy reaction fronts, witnessing a progressive replacement inward rather than along pore-connected pathways, unlike in the carrara marble. However, mass-balance calculation of the fluid rock interaction returns similar mass and volume loss independently of the texture, suggesting that the chemistry of the fluid controls the reaction. This is consistent with the produced dolomite grain size, who follow a similar distribution law regardless the texture, suggesting some self-organization during the replacement controlled by the fluids. Roughness characterization of the front shows that in oostone and carrara marble, the scaling law of the front follows a Brownian Motion (H=0.5), while it shows a persistent behaviour in the chalk (H=0.8 to 0.6). This suggests that the expression of the replacement process is governed by random distribution of heterogeneities in some cases, following the interface coupled dissolution precipitation model, but that there is a memory effect in other cases. In this study, the memory effect can be related mechanical processes such as local microfracturing, suggesting a potential role of local pressure on replacement. We propose that the shape of the front is governed by the rate of the front propagation, as if the latter is fast like in chalk, the mass transfer becomes less efficient to compensate the volume change, and some local overpressure may appear to drive the reaction propagation. This rate of front propagation appears to be affected by both initial grain size and pore size homogeneity.