AN INTEGRATED PETROPHYSICAL APPROACH TO THE SUB-BASALT IMAGING PROBLEM USING WELL
LOGGING DATA TO LINK MEASUREMENTS FROM CORES AND SEISMIC SURFACE EXPERIMENTS
R. WAAGSTEIN (1), L. O. BOLDREEL (2) & C. ANDERSEN (3)
(1) Geological Survey of Denmark and Greenland ([email protected])
(2) Geological Institute, Copenhagen University, Denmark ([email protected])
(3) Geological Survey of Faroe Islands ([email protected])
Flood basalt covered basins exist world-wide both along continental margins and present day stable continental areas and are
increasingly coming into the focus of the hydrocarbon industry as more accessible fields are being depleted. Also the nonhydrocarbon interest of the pre-basalt geological column is increasing.
NW
1000 ms
Generally, it is difficult to look through the basalt cover by conventional seismic reflection methods. However, in the Faroe Island
region it is possible in some places to image through km-thick basalts (e.g. Andersen et. al. 1991; Boldreel & Andersen 1993, 1994;
Boldreel et al. 1996; Roberts et al. 1999) both on 30 year old reprocessed reflection seismic profiles (Fig. 1) and on recently
acquired profiles, whereas on neighbouring profiles sub-basalt imaging is unsuccessful. This stresses the need for a better
understanding of the acoustic and other physical properties of basalt.
Vestmanna-1
Glyvursnes-1
Upper series
Middle series
Lower series
Passage zone
Shoreline trajectory
Truncation
Unit D'
Unit C'
Unit G
Unit F
Unit B
Unit C
Unit E
Unit D
Unit A
In order to approach the problem of sub-basalt imaging the Seismic Faroese Basalt Project (SeiFaBa) was established in 2002 as a
multi-disclipinary study of basalts.
Three major Paleogene volcanic units are present in the Faroe
Islands: the lower, middle and the upper formations (Figs. 2, 3 &
4).
SE
Regional top volcanics
Internal volcanic reflections
Base volcanics
Disconformity at top of oblique
or sigmoid progradational facies
Apparent coastline trajectory in
sigmoid aggradational facies
2500 m
Parallel bedded platform facies
Oblique progradational facies
Sigmoid aggradational facies
Basement
Figure 1. Multi-channel reflection seismic profile from the basalt-covered Faroes
region with seismic stratigraphic principles applied (Andersen, M. S., Egerton, P.
D., Hitchen, K. & Boldreel, L.O.: Seismic Facies Analysis of volcanic rocks. In
preparation).
From outcrops in the Faroe Islands a distinct difference between
the lower and middle and the middle and upper formations is
apparent, whereas the lower and upper formations show features in
common.
The lower and upper formations mainly consist of rather thick
simple flows generally of aa type. The flows are often separated by
thin tuffaceous layers (Figs. 4 & 5).
The middle formation consists mainly of thin-bedded compound
flows of pahoe-hoe type (Figs. 4 & 5). A coal-bearing sequence (A
horizon Figs. 3 & 4) separates the lower and the middle
formations.
Lopra-1/1A
Figure 2. Geological map of the Faroe Islands
showing the location of deep boreholes and
the surface distribution of the three basalt series
or formations
Investigations of the rock physics of the Faroese basalt are sparse
and it is poorly known how the physical properties of the basalt
vary with the thickness, morphology and composition of the lava
flows, depth of burial, alteration etc.
Figure 3. Stratigraphical position of deep boreholes in the Faroe Islands
(adapted from Waagstein 1988).
The basalt cover continues offshore the Faroe Islands and most of the Faroe-Rockall Area is covered by Paleogene flood basalt (e.g. Boldreel & Andersen 1994). Seismic stratigraphical principles can be applied in reflection
seismic interpretation of the basalt succession and various units can be mapped. However, correlation between the seismic profiles of thick basalt offshore and the basalt formations on-shore has proven difficult.
In order to cover the full range of basalt properties it was decided during SeiFaBa to investigate all three basalt formations:
1) On-shore the Faroe Islands by means of drilling, core description, wire-line logging and measurement of physical properties of core samples.
Thin-bedded pahoe compound flows,
Faroese middle basalt formation
Coal-bearing A-horizon
Thick aa flows of simple type,
Faroese lower basalt formation.
The flows have a massive centre
or core overlain by a thick purple
to red rubbly crust and red
tuffaceous clay.
In Oct.-Nov. 2002 a 700-m slim borehole was drilled with wire-line coring technique around the
middle-upper basalt formation boundary on the shore of Glyvursnes (Figs. 2 & 3). The Finnish drilling
company Suomen Malmi (SMOY) made the drilling.
In addition, an old 660-m borehole (Vestmanna-1) partly blocked by tufa (calcite) was reopened to 590
m using the same equipment. This hole is located 30 km northwest of Glyvursnes-1 in the lower part of
the middle basalt formation and extends 100 m into the lower formation (Figs 2 & 3).
An extensive wire-line logging program was subsequently run in both holes by Robertson Geologging
including optical televiewer, caliper, natural gamma, resistivity, neutron porosity, density, full wave
sonic, spectral gamma (poor) and temperature/conductivity.
The Glyvurnes and Vestmanna holes, together with the existing 3.65 km Lopra-1/1A re-entry hole
((Hald & Waagstein 1984; Nielsen et al. 1984; Boldreel 2002; 2003) in the southernmost island Suduroy
cover a major part of the Faroes basalt succession representing a wide range of lava compositions and
morphologies.
.Full core exists from the Glyvurnes-1 and Vestmanna-1 borehole, but only a few core samples from the
Lopra-1/1A well.
Figure 4. Contrasting lava morphologies, Prestfjall,
west coast of Suderoy (from Waagstein 1998).
SeiFaBa is an integrated study of the sub-basalt image problem.
It involves 9 scientific institutions:
Geological Survey of Denmark and Greenland
Geological Institute, Copenhagen University, Denmark
Geological Institute, Aarhus University, Denmark
Geological Survey of Faroe Islands
Cambridge University, UK
Stanford University, USA
University of the Faroe Islands
Oxford University, UK
Oedegaard, Copenhagen, Denmark
SeiFaBa is funded collectively by all oilcompanies operating in the faroese sector (the Sindri Group).
Agip Denmark; Amerada Hess (Faroes) Limited; Anadarko Faroese Company; BG International; BP Amoco Exploration (Faroes) Limited; DONG Efterforskning og produktion A/S;
Enterprise Oil Exploration Limited; Føroya Kolvetni P/K; P/F Atlantic Petroleum; Philips Petroleum Europe Exploration Limited; Shell UK Limited; Statoil Færøyene AS; Veba Oil & Gas Gmbh (mentioned in alphabetic order)
simple
aa flow
compound
pahoehoe flow
20 m
tuff
16
LAVA CRUST
tuff
18
rubbly
lava
vesicular
lava
pahoehoe toes
14
megavesicles
12
isolated
vesicles
LAVA
CORE
8
LAVA CRUST
10
vesicle
layering
megavesicles
horizontal
vesicular veins
LAVA
CORE
6
vesicle
cylinders
BZ
4
pipe vesicles
Figure 5. Bands of large empty vesicles at
the base of the crust of a pahoehoe flow lobe
(from Waagstein 1998)
0m
BZ
2
rubbly
lava
Figure 7. Vertical transition between two pahoehoe
flow lobes. The lower part of the photograph shows
the highly vesicular crust of the lower lobe. The upper
lobe shows pipe vesicles at the base beneath massive
lava core almost devoid of vesicles.
(From Waagstein 1998)
Figure 6. Two contrasting lava morphologies of the Faroes (from Waagstein 1998)
Aa flows have rough tops and are mostly simple, consisting of a thick
non-vesicular core grading into a rubbly top and base.
Pahoehoe flows have smooth undulating tops and are typically compound
consisting of thin beds of variable thickness, vesicles abundance and
mineralisation.
Moderately-highly-vesicular
basal zone with zeolites and
green clay
Thick simple flows
Former glass skin completely
altered to dark grey clay
Highly-vesicular lava crust filled
with zeolites
Compound flows
Flow-unit boundary
Sediment
Basalt lava
Rubbly crust
Porous crust
Massive core
Tuff
Figure 8. Glyvursnes-1 core
Figure 9. New 160-m composite log section from the Faroes.
6
6
De th
1m:1000m
DT-C
CALIPER
3
Inch
4 140
s/ft
NPHI
40 50
1.8
m/cc
RESISTIVITY
LITHOLOGY
LPU
DENSITY
0
0
Ohm
Caliper (in)
Bitsize (in)
130 DT-P (µsec/ft) 10
18
0.5 NPHI
0.0
170 DT-S (µsec/ft) 50
18
1.8 RHOB (g/cc) 3.3
0
Potasium 0.03
0
Thorium
0
CGR
-1
Uranium
3
GAMMA RAY
20000 0
BEDDING
PI Cs.
50
POTASSIUM
3.3
MAGMA UNITS
0
%
1
Compound
pahoehoe flow
Simple aa flow
Flow-unit
boundary
Sediment
Basalt lava
Massive core
Porous crust
Rubbly crust
Tuff
Figure 10. New 150-m composite log section from the Faroes.
Basalt
conglom.
Sediment/tuff layer Comp 13 compound flow
F 54 Flow unit
Massive core
Porous crust
Figure 11. Example of wire-line logs from the Lopra-1/1A well representing lower basalt formation aa lava flow.
Variations in the physical properties within the individual lava flows and between the different flows exit.
The physical properties of compound flows are different from aa flows. (After Boldreel 2001 and 2002).
As seen from the Figures 4-8 large variations exist between the three basalt formations of the Faroe Islands and from Figures 9-11- their differences in log response are shown.
Future work will include detailed studies of cores and wire-line logs. The wire-line logs will be comparred with lithostratigraphy and correlated with petrography, rock chemistry
and rock physical lab measurements of core samples for scaling of the core and log data to a seismic scale.
References:
Andersen, M. S., Egerton, P. D., Hitchen, K. & Boldreel, L. O.: Seismic Facies Analysis of Volcanic Rocks. To be submitted.
Andesen, M. S., Boldreel, L. O., Gunnarsson, K., Kjartanasson, E., Ewing, J., Talwani, M. & Saywer, D. 1990: A seismic investigation of the Rockall Plateau. AnnalesGeophysicae, 1990, p. 63 EGS XV General Assembly, Copenhagen.
Boldreel, L. O.: A stratigraphical division of the plateaubasalt based on interpretation of wireline logging from the Lopra well, Faroe Islands, Northatlantic. Resubmitted to GEUS special publication 2001.
Boldreel, L. O. 2002: Identification and characterization of basalt and sediment units based on wireline logs from the Lopra deep well, Faroe Islands, NE-Atlantic Ocean. EGS General Assembly XXVII Nice, France abstract EGS02-A-05330.
Boldreel, L. O. & Andersen, M. S. 1993: Late Paleocene to Miocene compression in the Faroe-Rockall area. In: Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (ed. By Parker, J. R.) Pp. 1025-1034.
Boldreel, L. O. & Andersen, M. S. 1994: Tertiary development of the Faeroe-Rockall based on reflection seismic data. Bulletin of the Geological Society of Denmark, 41 (2), pp. 162-180.
Hald, N. & Waagstein 1984: Lithology and chemistry of a 2-km sequence of Lower Tertiary tholeiitic lavas drilled on Suderoy, Faroe Islands (Lopra-1). In Berthelsen, O. Et al. (Eds.) The deep drilling project 1980-1981 in the Faeroe Islands. Føroya Frodskaparfelag,
Torshavn.
Nielsen, P. H., Stefánsson, V. & Tulinius, H. 1984: Geophysical logs from Lopra-1 and Vestmanna-1. In Berthelsen, O. Et al. (Eds.) The deep drilling project 1980-1981 in the Faeroe Islands. Føroya Frodskaparfelag, Torshavn.
Roberts, D. G., Thompson, M., Mitchener, B., Hossack, J., Carmichael, S. M. M. & Bjørnseth, H. M. 1999: Palaeozoic to Tertiary rift and basin dynamics; mid-Norway to Bay of Biscay; a new context for hydrocarbon prospectivity in deep water frontier. In Fleet, A. J.
& Boldy, S. A. R. (Eds) Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference, 7-40.
Waagstein R. 1988: Structure, composition and age of the Faeroe basalt plateau. Geological Society, London, Special Publications, 39, 225-238.
Waagstein, R. 1998: A geological field guide to the Palaeogene flood basalts of Suderoy, Faroe Islands. Danmarks og Grønlands Geologiske Undersøgelse Rapport 1998/30.
Acknowledgement: Thanks are due to the Sindri group for permission to publish results from the Glyvursnes-1 well
80
2
AN INTEGRATED PETROPHYSICAL APPROACH
TO THE SUB-BASALT IMAGING PROBLEM
USING WELL LOGGING DATA TO LINK
MEASUREMENTS FROM CORES AND SEISMIC
SURFACE EXPERIMENTS
R. Waagstein (1), L.O. Boldreel (2), C. Andersen (3)
(1) Geological Survey of Denmark and Greenland ([email protected]), (2) Geological Institute,
University of Copenhagen, Denmark, (3) Geological Survey of Faroe Islands.
Flood basalt covered basins exist world wide along continental margins and are
increasingly coming into the focus of the hydrocarbon industry as more accessible
fields are being depleted. However, it has proved difficult in many places to look
through the basalt cover by conventional seismic reflection methods. This stresses
the need for a better understanding of the acoustic properties of basalt. The Seismic
Faroes Basalt Project (SeiFaBa) was established in 2002 as an integrated study of
the sub-basalt image problem. It involves 9 scientific institutions and individuals
and is funded collectively by all oil companies operating in the Faroes sector (the
Sindri group). The planned fieldwork includes core drilling, wire-line logging,
multi-azimuth VSP and surface seismic experiments at land and sea. It is mainly
centred around Glyvursnes, a relatively flat promontory near Tórshavn that allows
optimal layout of seismic lines and integration of core, log and seismic data.
We present the initial task of drilling and logging, which was performed in Oct.-Nov.
2002. A 700 m slim borehole was drilled on the shore of Glyvursnes with wire-line
coring technique and an old 660 m borehole (Vestmanna-1) 30 km farther northwest
partly blocked by calcite fillings was reopened using the same equipment. An
extensive wire-line logging program was subsequently run in both holes. These two
holes, together with the existing 3.65-km Lopra-1/1A hole in the southernmost island
Suduroy, cover all three Faroes basalt formations and a range of lava compositions
and morphologies. We show examples of the correlation of lava flow sequences and
wire-logging measurements.
Detailed analysis of the new logs is being planned together with laboratory studies of
the petrography, rock chemistry and petrophysical properties of core samples with the
aim of establishing a log stratigraphy and scale core data to seismic scales.