Abstract Tabular igneous intrusions (sills) are common features in sedimentary basins and have the potential to be useful seals for fluids (e.g. CO 2 or H 2 ) in geological storage scenarios, and may be important as the need for geological carbon sequestration and alternative fuel storage increases. This is advantageous in regions without ready access to large-volume reservoirs in depleted hydrocarbon plays and saline aquifers, such as the northeastern USA. Moreover, geological H 2 storage requires more demanding conditions than those typically associated with oil and gas. An enhanced seal will have the properties of a traditional seal – competent, low permeability and laterally extensive – with the addition of being able to self-seal pre-existing and induced fractures. Self-sealing will occur along fluid pathways like fractures where CO 2 , water and minerals like plagioclase, olivine and pyroxene react together. Dolerite sills from the Gettysburg Basin, Pennsylvania, have remarkably low permeability and homogeneous compositions that include minerals that will readily react with CO 2 dissolved in water. Here, we characterize the physical properties and chemical gradients within several mafic sills cored in five boreholes. In addition, preliminary CO 2 -reaction experiments on dolerite samples demonstrated rapid carbonate mineralization.