Abstract Unintended CO2 releases from carbon capture and storage sites could impact water resources or reverse climate mitigation efforts through atmospheric emissions. Comprehensive understanding of how CO2 migrates in the shallow subsurface is required for appropriate monitoring of any CO2 release. Here, 3D meter‐scale realistic sedimentary bedforms were generated to represent a variety of cross‐bedding structures with varied contrasts between the grain size of the bedding and lamina. Gas migration simulations were conducted through each sedimentary structure, demonstrating the sensitivity of gas migration to centimeter‐scale bedding structures. A bedding‐laminae grain size contrast threshold was identified at which mass distributions transition from ganglia‐dominated to pool‐dominated. Furthermore, this threshold coincides with a transition from vertically‐dominated to horizontally‐dominated flow, regardless of bedding structure. This indicates that centimeter‐scale heterogeneity could prevent gas reaching the surface in the event of a release, but simultaneously cause unanticipated lateral migration away from the release site.