Fluid segregation and retention in deep‑seated rocks near percolation thresholds

Fluid segregation in deep-seated rocks has profound implications for their physical and chemical properties. Gravity drives the segregation of fluids interconnected through grain edges and corners, along with the compaction of the rock matrix, whereas isolated fluids are retained in the rocks. For wetting fluids, the critical volume fraction (i.e., percolation threshold) separating these two cases is principally determined by the balance and anisotropy of solid-fluid interfacial tensions (i.e., dihedral angle and faceting effect); however, the processes controlling the percolation threshold for non-wetting fluids are unclear, despite their critical importance, especially in the amount of pore fluids down-dragged in subducting slabs to the Earth’s interior. Hence, we implemented a combined approach involving high-pressure rock synthesis, high-resolution synchrotron radiation X-ray computed microtomography (CT) imaging, and numerical permeability computation to better understand how the permeability decreases and fluids are retained at low fluid fractions. We chose quartzite as a well-studied natural rock analog that is simplified but does not lose its essence as a silicate polycrystalline aggregate. A mixture of finely ground quartz and amorphous silica powders was sealed in Pt-lined Ni capsules with C-O-H fluid sources at different fractions and compositions and hot-pressed using a piston-cylinder apparatus. The dihedral angles of the experimental systems were 52° and 61–71° for the wetting and non-wetting systems, respectively.

In the wetting system, fluid connectivity rapidly decreased to approximately zero when the total fluid fraction decreased to 3.0–3.7 vol. %, mainly due to the grain faceting effect, consistent with the results of the previous study. In the non-wetting systems bearing CO₂-rich fluids, the cutoff of fluid tubules isolated 4.8–6.2 vol. % of the fluid. A streamline computation based on the X-ray CT images of the experimental products revealed that the fluid flow just above this threshold focused on a few channels, establishing efficient channelized fluid pathways. These retained fluid fractions are higher than those in the previous assessment based solely on the dihedral angle, that is, the pinch-off condition for ideal (isotropic and homogeneous) fluid geometry and the equilibrium fluid fraction that minimizes the total interfacial energy of the fluid-rock system. Hence, the amount of aqueous fluids dragged down to the Earth’s interior could be higher than previously estimated, although the specific volume fraction depends on the anisotropy and heterogeneity of the system of interest.