Stratigraphic Controls on CO2 Migration at Sleipner: An Example From a Basin-Floor Fan of the Utsira Formation

Abstract

For nearly three decades, Equinor's Sleipner Carbon Capture and Storage project has demonstrated how the application of geological principles, modelling techniques and analysis of repeated time-lapse (4D) seismic data has helped to characterise the CO₂ plume migration within the late Miocene–early Pliocene Utsira Formation. However, the influence of stratigraphic complexity on fluid migration has been rather poorly understood. This has resulted in a significant degree of uncertainty in the geological characterisation of the storage formation, including the distribution of mudstone-rich elements, which may serve as baffles and barriers for migration of fluid, and elements that allow for their bypass. Our study, utilising high-quality 3D seismic data integrated with wireline-logs, time-lapse seismic and regional contextual information, has shown that the Utsira Formation in the South Viking Graben represents a confined, channelized submarine fan system characterised by a complex stratigraphic architecture. The study has highlighted that the intricate interplay between fan lobes, channel erosion, channel infill and draping of lobes, lobe-complexes and channel incision surfaces by mud-rich layers, provides a first-order control on CO₂ storage compartments and exerts a substantial influence on vertical and lateral fluid flow pathways. The latter is well expressed by the morphology of several mapped CO₂-filled layers. Both generally discontinuous channel-base mud-rich drapes and more continuous lobe-complex and fan mudstone drapes have been locally compromised by processes linked to channel erosion and sand injection, in some cases combined with faulting and fracturing. This complex stratigraphic pattern has probably been exacerbated by post-depositional deformation that triggered fluid and sediment expulsion from the Utsira Formation and the underlying early-Miocene Skade Formation. These factors allowed for increased vertical connectivity between originally disconnected sandstone bodies and fluid migration from deeper to shallower layers, prior to injection of CO₂, thus serving as preferred pathways post-injection.