Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic

Abstract

Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here. The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remained within the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflected wavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismic velocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well. The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported. Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layered medium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth of the well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence.

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

“…However, Maresh et al (2006) showed that rough surface effects appear to be small compared to one-dimensional (1D) impedance scattering losses for the basalts penetrated in Well 164/07-1 in the nearby Rockall Trough, suggesting that a good estimate of scattering loss can be calculated with a 1D earth model. VSP-derived Q values of layered basalts have been reported from various igneous provinces (Rutledge and Winkler 1989;Pujol and Smithson 1991;Christie, Gollifer and Cowper 2006;Maresh et al 2006;Shaw et al 2008) with values of Q eff,P in the range of 15-48. The subscripts P and S for Q indicate compressional (P-) and shear (S-) wave measurements.…”

“…The subscripts P and S for Q indicate compressional (P-) and shear (S-) wave measurements. In addition to their Q eff,P estimates, Rutledge and Winkler (1989), Christie et al (2006) and Maresh et al (2006) also calculated Q scat,P by elastic modelling, using a point source in a plane-layered medium characterized by well-log velocities and densities. They found that the dominant attenuation mechanism was caused by impedance scattering because their Q scat,P matched Q eff,P , within experimental error.…”

Seismic‐stratigraphic constraints on the age of the Faroe Islands Basalt Group, Faroe–Shetland region, Northeast Atlantic Ocean

“…a decrease in the acoustic impedance with depth (Christie et al, 2006). Furthermore, approximately 700 m of lower FIBG strata has been estimated to have been eroded at this location (Andersen et al, 2002;Jørgensen, 2006) (Figure 7a), which implies an original total thickness of at least 4.2 km of lower FIBG.…”

“…At present, the only regional seismic horizon identified within the FIBG is the A-horizon, an erosional and eruption hiatus which separates the Beinisvørð and Malinstindur formations (Petersen, Brown, & Andersen, 2015) ( Figure 2). The base of the basalt lava pile can also be mapped on seismic-reflection profiles locally in the Faroese sector of the Faroe-Shetland Basin (Ellefsen, Boldreel, & Larsen, 2010) and is observed in the Lopra-1 well that terminated in hyaloclastic rocks of the Lopra Formation (Boldreel, 2006;Christie, Gollifer, & Cowper, 2006). Although refraction-seismic interpretations suggest that sedimentary rocks might be present below the FIBG in the western part of the Faroe-Shetland Basin and beneath the Faroe Platform, including Lower Paleocene rocks (Kiørboe & Petersen, 1995;Ólavsdóttir et al, 2017;Pálmason, 1965;Raum et al, 2005;Richardson, Smallwood, White, Snyder, & Maguire, 1998;Richardson, White, England, & Fruehn, 1999;Smallwood & Maresh, 2002;Smallwood & White, 2002;White et al, 2008), their correlation with the rock record in the eastern part of the Faroe-Shetland Basin remains ambiguous.…”

“…A key issue is the fact that the base of the volcanic succession is probably not a simple reflector. In the Lopra-1A well (Figure 1), the seismic response at the transition from subaerial flows (Beinisvørð Formation) to hyaloclastites (Lopra Formation) changes significantly towards lower amplitudes (Christie et al, 2006). To date, no well has managed to drill through the entire volcanic sequence; thus, it is difficult to estimate the maximum thickness of the lower FIBG.…”