Geologic CO2 sequestration in basalt reservoirs is predicated on permanent CO2 isolation via rapid mineralization reactions. This process is supported by a substantial body of evidence, including laboratory experiments documenting rapid mineralization rates, regional storage estimates indicating large, accessible storage reservoirs, and two successful pilot‐scale studies. Nevertheless, there remains significant uncertainty in the behavior of CO2 flow within basalt fracture networks, particularly in the context estimating physical trapping potential in early time and as CO2 undergoes phase change. In this study, a Monte Carlo numerical model is designed to simulate a supercritical CO2 plume infiltrating a low‐permeability flood basalt entablature. The fracture network model is based on outcrop‐scale LiDAR mapping of Columbia River Basalt, and CO2 flow is simulated within fifty equally probable realizations of the fracture network. The spatial distribution of fracture permeability for each realization is randomly drawn from a basalt aperture distribution, and ensemble results are analyzed with e‐type estimates to compute mean and standard deviation of fluid pressure and CO2 saturation. Results of this model after 10 years of simulation suggest that (1) CO2 flow converges on a single dominant flow path, (2) CO2 accumulates at fracture intersections, and (3) variability in permeability can account for a 1.6 m depth interval within which free CO2 may change phase from supercritical fluid to subcritical liquid or gas. In the context of CO2 sequestration in basalt, these results suggest that physical CO2 trapping may be substantially enhanced as carbonate minerals precipitate within the basalt fracture network.

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