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HODGES, SEAN, Shell (UK), CHARLES LINE, Mobil (UK), and BOB EVANS, Statoil (UK)

Abstract: Ben Nevis - An Exploration Giant in the Atlantic Margin

In 1996 Shell/BP re-evaluated the prospectivity of a large sub-basalt domal structure on 2D seismic, which they dubbed "Ben Nevis". The structure lies in the far north of the UKCS, and was being offered as part of the 17th UK Licensing Round. In 1997 Shell/BP were awarded Tranche 65 (219/16, 17 & 18) over the northern part of the structure, while Tranche 66, covering the southern part of the structure, was awarded to Statoil, as operator for the Atlantic Margin Group (AMG - including Mobil and Enterprise).

Sub-basalt imaging - various novel seismic techniques are being used to crack the difficult problem of imaging beneath basalt, but in Tranche 65 a structure was already visible on 2D seismic, although poorly imaged. It was decided to acquire a high-quality 3D survey to improve this imaging. Shell/BP and the AMG jointly acquired the 1300 sq. km survey required to cover the entire structure, using conventional acquisition parameters: 4600 m streamer with 100 m cross-line spacing, towed at a depth of 10 m, and 4000 cubic inch airguns, fired at 25 m intervals and towed at a depth of 6 m, resulting in 25 m x 25 m bins. The processing sequence included a constant velocity PSTM with particular care paid to multiple removal and velocity-picking. The prime objective was to obtain a good structural picture; it was found that this could be achieved with reasonable amplitude preservation.

The resulting high-quality 3D seismic volume has much better focusing of energy and vastly reduced multiple interference and noise compared to the 2D, allowing not only coherent interpretation of the sub-basalt dome, but also giving a clear picture of intra-basalt structure. The structures within the basalt indicate how the plateau was formed, such as individual feeder volcanoes and some of the flows that they fed (illustrated in Figure 1), and volcaniclastic foresets formed as the plateau built out into deeper water.

Ben Nevis - this sub-basalt dome has been recognized since at least 1975 from early 2D seismic. It is closely associated with large gravity and magnetic anomalies (confusingly known as "Brendan's Dome"), and has consequently long been regarded as being of igneous origin. The Ben Nevis structure is visible on 2D seismic, despite the 300 ms of overlying basalt, and covers an area of 800 sq. km. New interested was sparked in the area by the acquisition and 3D-modelling of modern aerogravity and aeromagnetic data. This modelling demonstrated that the anomalies were generated by a two-phase igneous intrusion offset some 20 km from the dome. Furthermore, these data showed that the Ben Nevis structure seemed to be composed of low density, low magnetic susceptibility material, suggesting a sedimentary origin.

Several models are being considered for the formation of the Ben Nevis dome. One possibility, in-keeping with the largely extensional regime prevailing in the area since Permian times, is a ramp-anticline, similar to those seen in the Jeanne d'Arc Basin of Newfoundland. Complex keystone faulting at the crest of the anticline could account for the relatively poor imaging of this part of the structure.Another model is that the structure was formed at a restraining bend along a transpressive fault, causing localised compression and uplift. There is evidence for late (Oligo-Miocene) movement through the area but little evidence to suggest that the Ben Nevis structure was created by earlier such movement.

Currently, the preferred Shell/BP model is that the Brendan's Dome igneous intrusion caused the uplift of a Jurassic fault block, inverting the Cretaceous fill of the adjacent half-graben. The resulting inverted domal structure ("Ben Nevis") was then eroded, before being.covered by the expanding basalt plateau as it built out from the north-west. This is illustrated in Figure 2.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England