Abstract CO 2 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of CO 2 in the reservoir, image the CO 2 , plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of CO 2 trapped beneath intra-reservoir shales. A prominent vertical ‘chimney’ of CO 2 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude-thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected CO 2 is concentrated in the layers. The remainder is interpreted to occupy the feeder ‘chimneys’ and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the CO 2 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by CO 2 present only at high saturations in the thin layers; a minor proportion of low saturation CO 2 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.

Abstract Evidence is presented for the widespread presence of sand injectites in mudstone- prone units in the deep-water sandstones of the Ty formation (Paleocene). Pressure communication throughout the field during production demonstrates that the laterally extensive shale layers in the reservoir are not barriers to cross-flow. Centimeter- scale sand injections are recognized in core, and sedimentary features commonly associated with meter-scale sand injectites are also identified. These remobilized sandstones are interpreted to facilitate fieldwide cross-flow, possibly augmented by subseismic- resolution faulting. Evidence for fault-induced or erosional dissection of the mudstone units is lacking. These injection features are similar in scale and appearance to sand injectites known from other reservoirs.