<p>Permanent storage of gigatons of carbon in basalt requires thick, uncompartmentalized sequences that allow lateral dissipation of CO₂ away from injection sites. Here we assess the presence of flow barriers by evaluating the variations in ⁸⁷Sr/⁸⁶Sr patterns in pore water, a method used extensively in sedimentary successions. Present-day pore water ⁸⁷Sr/⁸⁶Sr was measured from residual salts in core samples, while past water compositions were reconstructed from carbonate cements of different ages. We reveal smooth and uninterrupted trends in strontium isotopes through a 100 m thick basalt section indicating a common fluid reservoir unaffected by low-permeability massive lava flow interiors. The data further indicate that unconsolidated overburden sediments act as a good seal preventing seawater to mix with in situ pore water, and buoyant supercritical CO₂ to leak. Finally, basaltic sequences form viable targets for CO₂ storage and suggests that fractures may bypass low-permeability units to create a well-connected reservoir system.</p>