Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 53-56 |
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53
Flood basalt-covered basins exist worldwide along continen-
tal margins and are now in focus as targets for future hydro-
carbon exploration. It is generally difficult to image through
the basalt cover by conventional seismic reflection methods,
and this is a major challenge to future petroleum exploration
offshore the Faroe Islands. Long-offset profiling has proven
very successful (White et al. 2003). Surprisingly, however, it
is possible to image through kilometre-thick basalt sequences
on some conventional profiles. Details of basalt stratigraphy
are revealed on old, reprocessed seismic profiles as well as on
recently acquired profiles, even though the imaging may be
tal margins and are now in focus as targets for future hydro-
carbon exploration. It is generally difficult to image through
the basalt cover by conventional seismic reflection methods,
and this is a major challenge to future petroleum exploration
offshore the Faroe Islands. Long-offset profiling has proven
very successful (White et al. 2003). Surprisingly, however, it
is possible to image through kilometre-thick basalt sequences
on some conventional profiles. Details of basalt stratigraphy
are revealed on old, reprocessed seismic profiles as well as on
recently acquired profiles, even though the imaging may be
unsuccessful on nearby profiles (e.g. Boldreel & Andersen
1993). This stresses the need for a better understanding of the
acoustic and other physical properties of basalt as well as of
the degree of three-dimensional heterogeneity. The SeiFaBa
project (Seismic and petrophysical properties of Faroes
Basalt, 20022005) is funded by the Sindri Group as part of
the programmes for licensees within the Faroese offshore
area, and addresses these issues with special focus on the sub-
aerially extruded flood basalts of the Faroe Islands (cf. Japsen
et al. in press).
1993). This stresses the need for a better understanding of the
acoustic and other physical properties of basalt as well as of
the degree of three-dimensional heterogeneity. The SeiFaBa
project (Seismic and petrophysical properties of Faroes
Basalt, 20022005) is funded by the Sindri Group as part of
the programmes for licensees within the Faroese offshore
area, and addresses these issues with special focus on the sub-
aerially extruded flood basalts of the Faroe Islands (cf. Japsen
et al. in press).
Seismic and petrophysical properties of Faroe Islands
basalts: the SeiFaBa project
basalts: the SeiFaBa project
Peter Japsen, Morten Sparre Andersen, Lars Ole Boldreel, Regin Waagstein, Robert S. White
and Michael Worthington
and Michael Worthington
Fig. 1. A: Geological map of the Faroe Islands showing the location of deep boreholes and the distribution of the three Palaeogene basalt formations
(modified from Waagstein 1998). B: Location of the Faroe Islands relative to the extent of flood basalts and producing oil fields (modified from Grant
et al. 1999; Sørensen 2003).
Geological Survey of Denmark and Greenland Bulletin 4, 5365 (2004) © GEUS, 2004
Drilling and logging at Glyvursnes and
Vestmanna
Vestmanna
The Glyvursnes-1 well was drilled outside Tórshavn to 700
m as a slim borehole with wire-line coring technique as part
of the SeiFaBa project in 2002 (Fig. 1; Waagstein et al. 2003).
Glyvursnes is well suited for combining vertical seismic pro-
files (VSP) and surface seismic experiments (onshore and off-
shore): the terrain is relatively flat and the seismic effects of a
nearby near-vertical shear zone can be studied in detail (Fig.
2). During the same operations, the existing 660 m deep
Vestmanna-1 well was reamed and logged. Three deep wells
on the Faroe Islands have drilled thick sections of the Lower,
Middle and Upper Basalt Formations (Fig. 3; Rasmussen &
Noe-Nygaard 1970):
m as a slim borehole with wire-line coring technique as part
of the SeiFaBa project in 2002 (Fig. 1; Waagstein et al. 2003).
Glyvursnes is well suited for combining vertical seismic pro-
files (VSP) and surface seismic experiments (onshore and off-
shore): the terrain is relatively flat and the seismic effects of a
nearby near-vertical shear zone can be studied in detail (Fig.
2). During the same operations, the existing 660 m deep
Vestmanna-1 well was reamed and logged. Three deep wells
on the Faroe Islands have drilled thick sections of the Lower,
Middle and Upper Basalt Formations (Fig. 3; Rasmussen &
Noe-Nygaard 1970):
1. The Vestmanna-1 well was drilled to 660 m in 1980 and
reopened to 590 m in 2002 and logged. The hole was dril-
led in the lower part of the Middle Basalt Formation and
extends 100 m into the Lower Basalt Formation. A full
core was taken.
led in the lower part of the Middle Basalt Formation and
extends 100 m into the Lower Basalt Formation. A full
core was taken.
2. The Lopra-1/1A well was drilled through the Lower Basalt
Formation to 2.2 km in 1981, and deepened to 3.6 km in
1996 without reaching the base of the volcanic succession.
One short core of basalt was taken from this well.
1996 without reaching the base of the volcanic succession.
One short core of basalt was taken from this well.
3. The Glyvursnes-1 well was drilled to 700 m in 2002 to
around the boundary between the Middle and Upper Ba-
salt Formations (Fig. 4). A full core was taken.
salt Formations (Fig. 4). A full core was taken.
An extensive logging programme was run in the Glyvursnes
and Vestmanna boreholes, comparable to that previously run
in the Lopra well in 1981 and 1996. The programme
includes optical televiewer, caliper, natural gamma ray, resis-
tivity, neutron porosity, density, full wave sonic, spectral
gamma (of poor quality) and temperature/conductivity mea-
surements.
and Vestmanna boreholes, comparable to that previously run
in the Lopra well in 1981 and 1996. The programme
includes optical televiewer, caliper, natural gamma ray, resis-
tivity, neutron porosity, density, full wave sonic, spectral
gamma (of poor quality) and temperature/conductivity mea-
surements.
The Vestmanna and Glyvursnes boreholes penetrate,
respectively, the lowermost 550 m and the uppermost 450 m
of the 1400 m thick Middle Basalt Formation. In both sec-
tions the Middle Basalt Formation is characterised by the
presence of plagioclase-phyric compound flows composed of
thin flow-units of variable porosity, and by rare thin beds of
tuff. The lowermost part of the Middle Basalt Formation in
the Vestmanna borehole and the lowermost 70 m of the
Upper Basalt Formation in the Glyvursnes borehole are very
similar. The overlying section at about 230 to 285 m in
Glyvursnes-1 consists of a few relatively thick plagioclase-
phyric flow-units possibly forming two compound flows (the
Tórshavn flows), that are morphologically very similar to the
flows of the Lower Basalt Formation (except that the latter are
near-aphyric).
of the 1400 m thick Middle Basalt Formation. In both sec-
tions the Middle Basalt Formation is characterised by the
presence of plagioclase-phyric compound flows composed of
thin flow-units of variable porosity, and by rare thin beds of
tuff. The lowermost part of the Middle Basalt Formation in
the Vestmanna borehole and the lowermost 70 m of the
Upper Basalt Formation in the Glyvursnes borehole are very
similar. The overlying section at about 230 to 285 m in
Glyvursnes-1 consists of a few relatively thick plagioclase-
phyric flow-units possibly forming two compound flows (the
Tórshavn flows), that are morphologically very similar to the
flows of the Lower Basalt Formation (except that the latter are
near-aphyric).
Future work
Ultrasonic velocities and other parameters will be measured
on core samples from selected drill holes. The samples will be
investigated under varying pressure for both dry and satu-
rated samples. Studies of the velocityporosity relationships
of basalts will focus on matrix properties, but will also take
into account variations in magma type, secondary minerali-
sation, pore shapes and fractures.
on core samples from selected drill holes. The samples will be
investigated under varying pressure for both dry and satu-
rated samples. Studies of the velocityporosity relationships
of basalts will focus on matrix properties, but will also take
into account variations in magma type, secondary minerali-
sation, pore shapes and fractures.
A lithostratigraphic interpretation of the borehole logs
will be carried out, and the results will be correlated with pe-
trography, rock chemistry and ultrasonic properties of core
samples. This comparison of data acquired at core and log
scale will be extended to data of seismic scale acquired from
flood basalts of all three formations. Well log and core data
trography, rock chemistry and ultrasonic properties of core
samples. This comparison of data acquired at core and log
scale will be extended to data of seismic scale acquired from
flood basalts of all three formations. Well log and core data
54
Fig. 2. Aerial photograph of Glyvursnes with well location and outline of
seismic data recording on Glyvursnes in 2003. Red crosses indicate the
locations of permanent seismometers during the experiments. Blue dots
mark the positions of the three airgun pits for the VSP experiments. Blue
lines mark the locations of the 14 Hz geophone strings used for the
onshore reflection profiles. Red lines show the locations of onshore
geophone strings and the tethered streamer during the offshoreonshore
experiments and marine reflection experiment. Yellow and green
crosses mark shotpoints for the marine reflection experiment (from
Japsen et al. in press).
will be examined to evaluate how magma type and secondary
mineralisation influence the relationships between velocity
and porosity of basalt. Supplementary analysis of well logs
from exploration wells in the FaroeShetland Basin will be
carried out to provide additional data concerning the distri-
bution of elastic properties of the basalts around the Faroe
Islands.
mineralisation influence the relationships between velocity
and porosity of basalt. Supplementary analysis of well logs
from exploration wells in the FaroeShetland Basin will be
carried out to provide additional data concerning the distri-
bution of elastic properties of the basalts around the Faroe
Islands.
Seismic experiments at Glyvursnes
The extrusive basalt flows in the FaroeShetland Basin are
generally sub-horizontal on a regional scale, although there
are often large physical property variations (largely governed
by porosity) within individual flows, and locally rugged
small-scale relief on the tops of flows. The strong layering and
local relief may cause internal multiples, forward- and back-
scattering of the incident energy, multiple-mode conversion,
anisotropy, absorption and geometric spreading, and low-
pass filtering of the energy that propagates through a stacked
layer of basalt flows.
generally sub-horizontal on a regional scale, although there
are often large physical property variations (largely governed
by porosity) within individual flows, and locally rugged
small-scale relief on the tops of flows. The strong layering and
local relief may cause internal multiples, forward- and back-
scattering of the incident energy, multiple-mode conversion,
anisotropy, absorption and geometric spreading, and low-
pass filtering of the energy that propagates through a stacked
layer of basalt flows.
Since flow thicknesses are at least an order of magnitude
smaller than the seismic wavelength, reflections are rarely
seen from individual flows: the seismic response depends on
the complex interactions of reflections from multiple flow
units. To study these effects, three closely integrated seismic
experiments were carried out around the Glyvursnes-1 bore-
hole in 2003 (Fig. 2).
seen from individual flows: the seismic response depends on
the complex interactions of reflections from multiple flow
units. To study these effects, three closely integrated seismic
experiments were carried out around the Glyvursnes-1 bore-
hole in 2003 (Fig. 2).
Three VSP surveys were undertaken with source offsets of
14, 242 and 415 m. The two offset locations were on either
side of a well-defined shear zone.
side of a well-defined shear zone.
Onshore and offshore high resolution data were acquired
along the lines shown in Fig. 2. Two 625 m lines were shot
with the charges along the line, while short profiles were
acquired with the source displaced laterally. The marine seis-
mic reflection data were acquired on a tethered streamer
using a moving airgun cluster. Additional data were obtained
at longer offsets.
with the charges along the line, while short profiles were
acquired with the source displaced laterally. The marine seis-
mic reflection data were acquired on a tethered streamer
using a moving airgun cluster. Additional data were obtained
at longer offsets.
A dense array of 45 autonomous seismometers was de-
ployed for a six-month period around the site of the borehole.
During periods of controlled source seismic shooting, all sites
maintained a sampling rate of 200 samples/sec. For interven-
ing periods, when recording earthquakes, a sampling rate of
100 samples/sec was used. In addition, three 400 m long
independent temporary land arrays were set up in September
2003 using a mixture of one- and three-component geo-
phones.
During periods of controlled source seismic shooting, all sites
maintained a sampling rate of 200 samples/sec. For interven-
ing periods, when recording earthquakes, a sampling rate of
100 samples/sec was used. In addition, three 400 m long
independent temporary land arrays were set up in September
2003 using a mixture of one- and three-component geo-
phones.
Throughout acquisition of the marine reflection seismic
and the offshoreonshore, wide-angle seismic data, a three-
component borehole seismometer was held clamped at a
depth of 400 m in the Middle Basalt Formation in the Gly-
vursnes borehole.
component borehole seismometer was held clamped at a
depth of 400 m in the Middle Basalt Formation in the Gly-
vursnes borehole.
55
Fig. 3. Stratigraphical position of deep
boreholes in the Faroe Islands
(modified from Waagstein 1988).
See Fig. 1 for location of wells.
Fig. 4. Composite log from the Glyvursnes-1 well. Note the correlation
between P-slowness (P-wave velocity) and density/neutron porosity that
reflects the porosity-contrast between the porous crust and the massive
core of the basalt flows (from Japsen et al. in press).
Future work
A detailed analysis of seismic wave propagation through a
typical Faroese basalt succession will be carried out using the
logging carried out in 2002, ongoing laboratory work on the
core and the seismic data acquired during the summer of
2003. With the sonic logging and the vertical VSP, a detailed
vertical velocity profile will be obtained in the Glyvursnes-1
borehole and determine intrinsic and extrinsic attenua-
tion/scattering of seismic waves in one dimension. By com-
bining the two detailed fixed offset VSPs and variable offset
with data registered at fixed depth, it is planned to address
attenuation/scattering in two (three) dimensions. Using the
multichannel seismic data and the autonomous seismic array
it is planned to obtain two-dimensional and three-dimen-
sional velocity distribution in a small area around the bore-
hole. The multichannel surface seismic experiment is
designed to analyse sub-critical and post-critical reflections
from the basalt succession drilled at Glyvursnes, providing an
image of structural continuity in the area.
typical Faroese basalt succession will be carried out using the
logging carried out in 2002, ongoing laboratory work on the
core and the seismic data acquired during the summer of
2003. With the sonic logging and the vertical VSP, a detailed
vertical velocity profile will be obtained in the Glyvursnes-1
borehole and determine intrinsic and extrinsic attenua-
tion/scattering of seismic waves in one dimension. By com-
bining the two detailed fixed offset VSPs and variable offset
with data registered at fixed depth, it is planned to address
attenuation/scattering in two (three) dimensions. Using the
multichannel seismic data and the autonomous seismic array
it is planned to obtain two-dimensional and three-dimen-
sional velocity distribution in a small area around the bore-
hole. The multichannel surface seismic experiment is
designed to analyse sub-critical and post-critical reflections
from the basalt succession drilled at Glyvursnes, providing an
image of structural continuity in the area.
Perspectives
Drilling of the new borehole at Glyvursnes and re-logging of
Vestmanna-1 in combination with the extensive data set for
the Lopra-1 well will give valuable new stratigraphic and
petrophysical control of the Lower, Middle and Upper Basalt
Formations on the Faroe Islands. The planned experiments
will provide a link from hand-specimen scale, through the
slightly larger averaging of borehole logs, to seismic scales
ranging from high-resolution VSPs and multi-channel land
arrays, to onshoreoffshore shooting using wide-band air-
guns, to the truly long-period response of the basalt flows
using teleseismic arrivals. The investigations will provide a
unique data set and, hopefully, new understanding of the
seismic and petrophysical properties of Faroe Islands basalts.
Vestmanna-1 in combination with the extensive data set for
the Lopra-1 well will give valuable new stratigraphic and
petrophysical control of the Lower, Middle and Upper Basalt
Formations on the Faroe Islands. The planned experiments
will provide a link from hand-specimen scale, through the
slightly larger averaging of borehole logs, to seismic scales
ranging from high-resolution VSPs and multi-channel land
arrays, to onshoreoffshore shooting using wide-band air-
guns, to the truly long-period response of the basalt flows
using teleseismic arrivals. The investigations will provide a
unique data set and, hopefully, new understanding of the
seismic and petrophysical properties of Faroe Islands basalts.
Acknowledgements
Thanks are due to the Sindri Group for permission to publish results
from the Glyvursnes-1 well. SeiFaBa is funded collectively by all oil com-
panies operating in the Faroe Islands sector (the Sindri Group).
from the Glyvursnes-1 well. SeiFaBa is funded collectively by all oil com-
panies operating in the Faroe Islands sector (the Sindri Group).
References
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Authors' addresses
P.J. & R.W., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K. E-mail: pj@geus.dk
M.S.A., University of the Faroe Islands, Noatún 3, FO-100 Tórshavn, Faroe Islands. Present address: Geological Survey of Denmark and
Greenland, Øster Voldgade 10, DK-1350 Copenhagen K.
L.O.B., Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
R.S.W., Bullard Laboratories, University of Cambridge, Madingly Road, Cambridge CB3 0EZ, UK.
M.W., Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.
P.J. & R.W., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K. E-mail: pj@geus.dk
M.S.A., University of the Faroe Islands, Noatún 3, FO-100 Tórshavn, Faroe Islands. Present address: Geological Survey of Denmark and
Greenland, Øster Voldgade 10, DK-1350 Copenhagen K.
L.O.B., Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
R.S.W., Bullard Laboratories, University of Cambridge, Madingly Road, Cambridge CB3 0EZ, UK.
M.W., Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.
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