Abstract Knowledge of fluid flow processes in the subsurface is important for CO 2 storage operations as well as for hydrocarbon exploration. Repeated seismic surveys for more than 10 years of CO 2 injection into the Utsira Formation, in the Sleipner area, offer a unique dataset. This dataset holds information on fluid migration processes that can be analysed for the benefit of hydrocarbon exploration and CO 2 storage considerations alike. Thorough analyses of these datasets reveal several features that give useful information of subsurface fluid flow processes. The CO 2 in the Utsira Formation has flowed laterally beneath thin, intra-formational shales. At the same time, CO 2 has flowed vertically through shaly horizons that would normally be considered as barriers to fluid flow. This flow has apparently taken place through vertically stacked flow conduits through the shales. These conduits may to some extent have existed prior to the start of CO 2 injection, but may also have been augmented by the CO 2 injection process. The calculated pushdown of seismic reflectors below the CO 2 plume is less than that observed, which may point to the presence of hitherto unrecognized flow paths for the CO 2 . Hydrocarbon migration pathways are in general not recognizable in seismic data. This implies that such avenues are significantly thinner than those of the CO 2 migration in the Utsira Formation. This result points to the presence of mixed-wet migration pathways, in which capillary flow resistance may not control the (sub-horizontal) flow path thickness. A circular depression at the top of the Utsira Formation that existed prior to the injection may be interpreted as a result of a deeper seated sand remobilization feature. Such features will also promote vertical hydrocarbon migration where they are present. A more widespread occurrence of such features may explain why hydrocarbons are generally found beneath thick shales, but are less likely to be found below thin intra-formational shales below the structural spillpoint of the top seal. These observations suggest that seal thickness is an important parameter, even if the capillary entry pressure of the sealing rock is sufficiently high to preserve significant hydrocarbon columns.

Abstract The growing emissions of greenhouse gases, especially CO 2 , are seen worldwide as one of the major causes of climate change. International treaties like the Kyoto Protocol are supposed to contribute to reducing the emission of greenhouse gases. Underground sequestration has the potential to play an important role in keeping large volumes of CO 2 from escaping into the atmosphere in the short term. The first case of industrial scale CO 2 storage in the world (close to one million tonnes per year since 1996) is taking place at the Sleipner underground CO 2 storage site in the North Sea offshore Norway. Careful monitoring of the behaviour of the storage facility is required to establish its safety. To this end, two time-lapse seismic surveys have been acquired; the first repeat survey was completed in October 1999 and the second in October 2001. The presence of CO 2 beneath thin intra-shale layers within the reservoir has caused significant changes both in reflection amplitudes (up to a factor 10) and in travel time (more than 40ms) through the CO 2 plume (the velocity push-down effect). Some aspects of the interpretation of these time-lapse seismic surveys will be presented here.