Abstract Geological complexities such as variable permeability and structure (folds and faults) exist to a greater or lesser extent in all subsurface environments. In order to identify safe and effective sites in which to inject CO 2 for sequestration, it is necessary to predict the effect of these heterogeneities on the short- and long-term distribution of CO 2 . Sequestration capacity , the volume fraction of the subsurface available for CO 2 storage, can be increased by geological heterogeneity. Numerical models demonstrate that in a homogeneous rock volume, CO 2 flowpaths are dominated by buoyancy, bypassing much of the rock volume. Flow through a more heterogeneous rock volume disperses the flow paths, contacting a larger percentage of the rock volume, and thereby increasing sequestration capacity. Sequestration effectiveness , how much CO 2 will be sequestered for how long in how much space, can also be enhanced by heterogeneity. A given volume of CO 2 distributed over a larger rock volume may decrease leakage risk by shortening the continuous column of buoyant gas acting on a capillary seal and inhibiting seal failure. However, where structural heterogeneity predominates over stratigraphic heterogeneity, large columns of CO 2 may accumulate below a sealing layer, increasing the risk of seal failure and leakage.