Effective fracture porosity in crystalline rock

Crystalline rocks are typically  low in porosity, but they often contain fractures, which provide critical pathways for fluid flow and influence groundwater storage, resource estimation, and safety assessments for nuclear waste repositories. Despite their importance, effective fracture porosity in crystalline rocks remains poorly constrained due to limited and regionally biased measurements. In this study, we used global permeability datasets and modified an existing equation to estimate porosity from permeability, incorporating fracture roughness and aperture. This allowed us to calculate nearly 28,000 porosity values across a wide range of depths and geological settings. The resulting porosity distributions are highly right-skewed and show an exponential decrease with depth. Our findings indicate that porosity values in crystalline rocks are generally lower than previously assumed. Median porosity values in the upper 100 meters are several orders of magnitude lower than the commonly assumed 1% porosity, highlighting a significant discrepancy between our estimates and traditional assumptions. We quantified uncertainty using Monte Carlo simulations, which show that natural variability in porosity dominates over parameter uncertainty, underscoring the robustness of our global trends. These findings imply that groundwater storage in crystalline rocks is far smaller than previously estimated, and groundwater velocities may be higher than predicted by models assuming larger porosity, with implications for contaminant transport and nuclear waste safety.