Operando 4D synchrotron tomography resolves multiphase hydrothermal dolomitization in a natural carbonate rock

Hydrothermal dolomitization is a critical process in carbonate diagenesis, capable of nonlinearly and heterogeneously restructuring pore networks, thereby fundamentally affecting permeability and fluid pathways in carbonate-hosted geothermal systems. Reaction rates and mechanisms in natural rocks remain poorly constrained, as few experimental setups permit direct observation of the process. Here, we present early analyses of operando (4D) µCT data acquired at the PSICHÉ beamline of Synchrotron SOLEIL (France) that document hydrothermal dolomitization in a fractured limestone from the Terwagne Formation of the Lower Carboniferous Kohlenkalk sequence (North Rhine-Westphalia, Germany). Our data provide mechanistic insights that cannot be obtained from conventional experimental approaches.

The fine-grained oosparitic limestone contains microstylolites, which are likely to be diagenetic. A cylindrical core (20.08 × 9.76 mm) was drilled sub-parallel to bedding and axially fractured ex situ (UCS = 98.1 MPa) to promote fluid flow in an otherwise low-porosity (<2%) rock. The initial permeability at experimental conditions was 1.1–2.9 × 10^-10 m². The experiment was conducted using the X-ray transparent Heitt Mjölnir triaxial flow-through rig (Freitas et al., 2024), with continuous injection of a 2.05 M NaCl–MgCl₂–CaCl₂ brine at 1.5 µL min^-1, at 260 °C, 20 MPa confining pressure, and 15 MPa pore fluid pressure. Reaction progress was documented in 62 three-dimensional volumes at a 5.8 µm voxel size over 128 h. Each tomography volume is based on 1,400 projections acquired over 180° using a pink beam with a peak energy of ~81 keV. Fluid samples collected after 49, 79, 105, and 128 h were analysed by ICP-OES for Na, Ca, and Mg concentrations, and post-mortem SEM/EDX analyses corroborated the µCT-based interpretations.

Our 4DµCT data resolve the spatiotemporal evolution of reaction products, allowing observation of phase formation sequences, quantification of local reaction rates, and identification of rate-limiting transport mechanisms controlling phase growth within a fractured carbonate rock. Early analyses show that calcite reacts with brine and forms several distinct phases nearly simultaneously, including magnesite, dolomite-type carbonate, and locally brucite where carbonate availability is limited. Post-mortem SEM/EDX reveals that the dolomite-type phase comprises both Ca-dolomite and stoichiometric dolomite, which cannot be distinguished in our 4DµCT data. Magnesite and brucite remain largely confined to the inlet region, whereas dolomite-type carbonate nucleates preferentially along hydraulically active fractures and stylolites with apertures exceeding ~32 µm, reflecting the evolving fluid pathways during reaction. Our observations indicate that magnesite precipitation generates macro-porosity (10–100 µm), facilitating advective fluid transport, whereas dolomite-type carbonate develops sub-micron to micron-scale porosity, likely resulting in transport dominated by grain-boundary diffusion. Brucite locally reduces porosity, but its metastable nature likely limits its impact on bulk fluid flow. Porosity generation associated with dolomite-type replacement enhances fracture and stylolite connectivity, establishing preferential fluid pathways in the process. These spatially and temporally heterogeneous transport regimes reflect local chemical-hydraulic feedbacks, producing differential growth rates among phases and exerting first-order control on the overall rate of dolomitization. ICP-OES data are consistent with bulk mineralogical evolution, while 4DµCT uniquely resolves a spatiotemporal coupling between fluid flow and reaction progress.