A global assessment of effective fracture porosity and density in crystalline rocks

Josse Maria van den Berg, Elco Luijendijk, Grant Ferguson, Oliver Warr

Knowledge of effective fracture porosity in crystalline rocks is important for water resources, geological storage and subsurface biogeochemistry, but remains poorly constrained due to limited and regionally biased data. Here, we use global permeability datasets and equations that relate effective porosity and permeability to estimate effective porosity and hydraulically active fracture density for 27,966 points globally. The resulting global effective porosity distributions are right-skewed and show a power-law decrease with depth. The median and mean effective porosity values are much lower than previously assumed. The natural variability in effective porosity is much larger than parameter uncertainty, underscoring the robustness of our global trends. Low effective porosity implies enhanced fluid isolation in the crystalline crust, with implications for hydrogen and helium accumulation and the deep biosphere.