Porous and Permeable Crystalline Basement: Evidence for Large‐Scale Hydraulic Connectivity at Kivetty, Finland

Abstract Porosity and permeability in crystalline shield basement are commonly assumed to be low, yet some fault zones sustain fluid circulation over kilometer scales. Here we quantify multiscale fault‐zone connectivity at Kivetty, central Finland, by integrating legacy deep‐borehole data, including packer tests, downhole logs, and a long‐term multi‐borehole pumping test, with new core logging, laboratory petrophysics, mineral mapping, and X‐ray computed tomography (XCT). We show that the most transmissive intervals occur within the fault core and are dominated by breccia–open and altered–leached granitic rocks with high porosity and permeability. These rocks are characterized by quartz depletion, biotite dissolution and alteration, including replacement by chlorite and epidote, and abundant matrix porosity expressed as microporosity, intercrystalline and intracrystalline pores, and vugular cavities. Core‐plug porosity reaches 30%, XCT shows that up to 94% of the pore space is connected, and the most permeable plugs remain in the ∼10 −13 m 2 range at 50 MPa, corresponding to a confining pressure at ∼2 km depth. At borehole scale, permeability is concentrated in discrete sub‐intervals (10 −14 –10 −12 m 2 ), and hydraulic responses up to ∼1 km from the pumping interval indicate kilometer‐scale connectivity in the fault system. These results show that coupled brittle deformation and hydrothermal alteration can generate connected pore networks through brecciation and leaching, allowing crystalline fault cores to sustain high permeability and large‐scale fluid flow rather than act as seals where reaction‐enhanced porosity is preserved. Our findings provide a basis for assessing analogous fault‐controlled reservoirs in crystalline basement.