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

Abstract

ABSTRACTA 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.