Veins recovered from deep sedimentary formations offer insights into mineral precipitates and processes that lead to sealing of underground fractures. These processes are of interest in the context of subsurface negative emissions technologies such as geologic carbon storage, where fractures are potential pathways for unwanted fluid migration. In this study, we characterized the mineralogy and porosity of a syntaxial vein in a mudrock sample from the Wolfcamp formation in Texas. The original fracture had an aperture of 5 mm and is now filled with distinct zones of minerals and vuggy regions. Thin sections from cuts across the vein were examined at micron scale resolution using scanning electron microscopy, energy dispersive X-ray spectroscopy, QEMSCAN, and polarized light microscopy. Larger-scale analyses were done using synchrotron X-ray fluorescence. Collectively, these methods reveal elongated crystals of dolomite as large as 900 μm, overlain with a mixture of smaller crystals including calcite and ferroan dolomite. Silica fills some of the void space. Mineral identification was corroborated using powder X-ray diffraction. Quantitative analysis of a 3D X-ray computed tomography image indicates that the vein volume contains 62% elongate dolomite crystals, 33% mixed ferroan dolomite and calcite, 1% silica, and 4% vuggy void space. Synchrotron small and ultra-small angle X-ray scattering reveals that the vein mineral precipitates have ~1% porosity. This is much smaller than the porosity of the mudrock matrix. The findings in this study suggest that as the formation formed and subsided, fracture fluids migrated vertically and experienced pressure reduction causing exsolution of CO2. A geochemical simulation demonstrated how this could have led to carbonate precipitation in the veins. A fundamental understanding of the sequence of vein mineral precipitation and the associated reduction in porosity may inspire strategies designed to induce fracture sealing, thus preserving the integrity of underground fluid storage. Changes that would lead to CO2 exsolution, carbonate supersaturation, and mineral precipitation include increasing the pH, addition of divalent cations, enabling vertical migration with subsequent depressurization, and heating to reduce carbonate solubility.