Three-dimensional seismic characterisation of large-scale sandstone intrusions in the lower Palaeogene of the North Sea: completely injected vs. in situ remobilised sandbodies

A large number of km-scale, saucer-shaped sandstone bodies of enigmatic origin have recently been documented in the North Sea and the Faroe Shetland Basin. This study utilises three-dimensional seismic data, calibrated by well data, to examine two such bodies that exhibit very similar saucer-shaped geometries in cross-section. The Volund and Danica structures, located 250 km apart are interpreted as end members of a spectrum of large-scale remobilised and injected sandstones present in the North Sea Palaeogene. Both are characterised by a central 1–2 km-wide low area surrounded by a discordant, 2–300 m tall inclined dyke complex, that tips out into a bedding concordant body interpreted as a shallow-level sill and/or partly extruded sandstone. The origin of the central concordant sandstone body as either injected (laccolith) or depositional is of key importance to a complete understanding of the origin and prospectivity of these structures. The key criteria for recognising an injected vs. depositional origin for the central concordant sandbody are: (1) a flat, nonerosional base; (2) ‘jack-up’ of the overburden equal to the underlying sand thickness; (3) equally thick layers of encasing mudstones; and (4) paleogeographic context. This study suggests that the Danica structure was deposited as a channel sandstone and remobilised in situ; this led to the formation of wing-like intrusions along the channel margins. In contrast, the Volund structure overburden displays a forced-fold geometry, arguably a diagnostic feature of an intrusive origin. The ability to recognise and differentiate completely injected vs. in situ remobilised sandbodies is important both from a basin analysis, hydrocarbon exploration and rock mechanics points of view. An improved understanding of these aspects will lead to a reduction of risks associated with the exploration and development of such a sandbody and an enhanced understanding of sediment remobilisation and fluid flow on a basin scale.