Abstract During geological carbon dioxide storage in deep saline aquifers, buoyant CO 2 tends to float upwards in the reservoir overlaid by a low permeable formation called a caprock. Caprocks should serve as barriers to potential CO 2 leakage that can happen through diffusion and permeation through faults, fractures or pore spaces. The leakage through intact caprock would mainly depend on its permeability and CO 2 breakthrough pressure and is affected by the heterogeneities in the material. Here, we study the sealing potential of a caprock from the Illinois Basin – Eau Claire shale, with sandy and clayey fractions distinguished via electron microscopy, grain/pore size analyses and surface area characterization. The direct measurements of permeability of sandy shale provide the values on the order of 10 −15 m 2 , while clayey specimens are three orders of magnitude less permeable. The CO 2 breakthrough pressure under in situ stress conditions is 0.1 MPa for the sandy shale and 0.4 MPa for the clayey counterpart – these values are higher than those predicted by the porosimetry methods performed on the unconfined specimens. Sandy Eau Claire shale would allow penetration of large CO 2 volumes at low overpressures, while the clayey formation can potentially serve as a caprock in the absence of faults and fractures.

Abstract The Cauvery Basin in East India represents a failed rift zone in the west and transform-related termination in the east. The deformation associated with a westward-propagating rift zone involves stretching and necking-related deformation from west to east. The rift axis and the flanks exhibit maximum and minimum deformation. In this study, we document the increasing role of buoyancy-driven processes and the development of the rift asymmetry during the advanced stages of rifting in a magma-poor setting. We use a series of reflection seismic profiles intersecting the failed rift zone maturing eastward. The onset of buoyancy-controlled extension correlates with the localized extension. It creates a relatively symmetrical axial dome, and exhibits rift flank rotations and central up-warping. This permanent uplift is associated with lower crustal ductile flow. Notably, the deep-seated syn-rift buoyancy forces progressively operate eastward. We deduce the associated transient dome uplift and its subsequent dissipation using a seismic flattening technique. The axial dome formation is associated with an upwelling asthenosphere and lower lithospheric mantle. This correlates with localized contraction within flat-lying fault blocks at its flanks, concurrently forming the typical hanging-wall and footwall geometry. The multiple shallow- to deep-seated mechanisms promote strain acceleration in the uplifted regions along the rift zone.