The paper aims to draw some generic conclusions on reservoir characterization based on the Sleipner operation where CO2 is being injected into the Utsira Sand. Regional mapping and petrophysical characterization of the reservoir, based on 2D seismic and well data, enable gross storage potential to be evaluated. Site-specific injection studies, and longer-term migration prediction, require precision depth mapping based on 3D seismic data and detailed knowledge of reservoir stratigraphy. Stratigraphical and structural permeability barriers, difficult to detect prior to CO2 injection, can radically affect CO2 migration within the aquifer. (C) 2004 NERC. Published by Elsevier Ltd. All rights reserved.