Evaluation of internal geometries within the Miocene Utsira Formation to establish the geological concept of observed CO2 responses on 4D seismic in the Sleipner area, North Sea

Abstract

CO2 has been injected into the Miocene Utsira Formation at the Sleipner field in the Norwegian North Sea since October 1996. Repeat seismic surveying over the injection site in 1999, 2001, 2004 and 2006 have revealed the temporal development of the CO2 plume. However, in order to help better understand future plume development and aid in locating a new injection site the geological evolution of the Utsira Formation and its resultant stratal architecture needs further development in the greater Sleipner area. Combined used of seismic and well data show that the base of the Utsira Formation, the Middle Miocene Unconformity (MMU), is heavily deformed by soft sedimentary deformation. The source for this deformation is mass sand mobilization and injection of Skade Formation sandstones in the otherwise dominantly argillaceous sediments of the Upper Hordaland Group. Skade Formation sandstones are observed thickening in up-folded, and mounded regions of MMU, where seismic data reveal V-shaped amplitude anomalies or ‘chaotic’, noisy areas. Outside the deformed areas the Upper Hordaland Group is an otherwise flat sequence of continuous acoustic reflectors that are offset by a pervasive network of polygonal faults. Onlapping reflection terminations of lower Utsira Formation reflectors onto the deformed surface of the MMU indicate that soft sedimentary deformation occurred at a shallow depth before deposition of the Utsira Formation. Stratal elements within the sand rich (0.98 N:G) Utsira Formation include: i) south westerly dipping clinoforms, ii) erosional scours, and iii) large-scale sand waves, suggesting high depositional energy and potential erosion of (c.1 - 2.5 metre thick) shale interbeds. During deposition of the Utsira Formation differential compaction within the Upper Hordaland Group has down-folded, and rotated intra-Utsira reflectors onto underlying MMU mounded features. Løseth’s et al. (2003) and Jackson’s (2007) models for gas and fluid expulsion from the mobilized sediments during burial, leading to differential compaction, is the preferred hypothesis for this phenomenon. The result of collapsed sediments on reservoir architecture is folding, and the creation of the anticlinal internal geometries where the CO2 is injected today. CO2 reached the top of the reservoir by 1999, via a sequence of small accumulations beneath interpreted as intra-formational shale beds. It appears from this rapid ascent that shale layers are laterally discontinuous, and perhaps eroded by the high-energy depositional model inferred.

Regional Exploration and Characterisation of CO2 Storage Prospects in the Utsira-Skade Aquifer, North Viking Graben, North Sea

“…In map view, the closures are primarily juxtaposed against the mounds (grey shapes; Figure 14B). It is postulated that deflation of the mounds led to localised subsidence and formation of the coeval synforms in the aquifer and shallower stratigraphy (Kennett & Jackson, 2008). This localised subsidence and downward rotation of the strata led to the formation of adjacent antiforms, which characterise the closures here highlighted for CO 2 storage.…”