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 centreor core overlain by a thick purpleto red rubbly crust and redtuffaceous clay.

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 (rw@geus.dk)(2) Geological Institute, Copenhagen University, Denmark (lob@geo.geol.ku.dk)(3) Geological Survey of Faroe Islands (claus@jfs.fo)

Flood basalt covered basins exist world-wide both along continental margins and present day stable continental areas and areincreasingly coming into the focus of the hydrocarbon industry as more accessible fields are being depleted. Also the non-hydrocarbon interest of the pre-basalt geological column is increasing.

Generally, it is difficult to look through the basalt cover by conventional seismic reflection methods. However, in the Faroe Islandregion 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 recentlyacquired profiles, whereas on neighbouring profiles sub-basalt imaging is unsuccessful. This stresses the need for a betterunderstanding of the acoustic and other physical properties of basalt.

In order to approach the problem of sub-basalt imaging the Seismic Faroese Basalt Project (SeiFaBa) was established in 2002 as amulti-disclipinary study of basalts.

Three major Paleogene volcanic units are present in the FaroeIslands: the lower, middle and the upper formations (Figs. 2, 3 &4).

From outcrops in the Faroe Islands a distinct difference betweenthe lower and middle and the middle and upper formations isapparent, whereas the lower and upper formations show features incommon.

The lower and upper formations mainly consist of rather thicksimple flows generally of aa type. The flows are often separated bythin tuffaceous layers (Figs. 4 & 5).

The middle formation consists mainly of thin-bedded compoundflows of pahoe-hoe type (Figs. 4 & 5). A coal-bearing sequence (Ahorizon Figs. 3 & 4) separates the lower and the middleformations.

Investigations of the rock physics of the Faroese basalt are sparseand it is poorly known how the physical properties of the basaltvary with the thickness, morphology and composition of the lavaflows, depth of burial, alteration etc.

In Oct.-Nov. 2002 a 700-m slim borehole was drilled with wire-line coring technique around themiddle-upper basalt formation boundary on the shore of Glyvursnes (Figs. 2 & 3). The Finnish drillingcompany Suomen Malmi (SMOY) made the drilling.

In addition, an old 660-m borehole (Vestmanna-1) partly blocked by tufa (calcite) was reopened to 590m using the same equipment. This hole is located 30 km northwest of Glyvursnes-1 in the lower part ofthe 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 Geologgingincluding optical televiewer, caliper, natural gamma, resistivity, neutron porosity, density, full wavesonic, 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 Suduroycover a major part of the Faroes basalt succession representing a wide range of lava compositions andmorphologies.

.Full core exists from the Glyvurnes-1 and Vestmanna-1 borehole, but only a few core samples from theLopra-1/1A well.

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 reflectionseismic 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.

SeiFaBa

SeiFaBa

is an integrated study of the sub-basalt image problem.

It involves 9 scientific institutions:

Geological Survey of Denmark and Greenland Geological Survey of Faroe IslandsCambridge University, UK Oxford University, UK

Geological Institute, Aarhus University, Denmark Stanford University, USA Oedegaard, Copenhagen, Denmark

is funded collectively by all oilcompanies operating in the faroese sector (the Sindri Group).

DONG Efterforskning og produktion A/S;P/F Atlantic Petroleum; Statoil Færøyene AS; Veba Oil & Gas Gmbh (m

University of the Faroe IslandsGeological Institute, Copenhagen University, Denmark

Agip Denmark; Amerada Hess (Faroes) Limited; Anadarko Faroese Company; BG International; BP Amoco Exploration (Faroes) Limited;Enterprise Oil Exploration Limited; Føroya Kolvetni P/K; Philips Petroleum Europe Exploration Limited; Shell UK Limited; entioned in alphabetic order)

Figure 2. Geological map of the Faroe Islandsshowing the location of deep boreholes andthe surface distribution of the three basalt series

or formations

Lopra-1/1A

Glyvursnes-1

Vestmanna-1

Upper series

Middle series

Lower series

Figure 3. Stratigraphical position of deep boreholes in the Faroe Islands(adapted from Waagstein 1988).

Figure 4. Contrasting lava morphologies, Prestfjall,Suderoy (from Waagstein 1998).west coast of

Unit DUnit C

Unit B

Shoreline trajectory

Unit A

Unit E

Unit F

Unit GTruncation

Unit D'

10

00

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2500 m

Regional top volcanics

Internal volcanic reflections

Base volcanics

Apparent coastline trajectory insigmoid aggradational facies

Disconformity at top of obliqueor sigmoid progradational facies

Parallel bedded platform facies

Oblique progradational facies

Sigmoid aggradational facies

Basement

Passage zone

Unit C'

NW SE

Figure 1. Multi-channel reflection seismic profile from the basalt-covered Faroesregion with seismic stratigraphic principles applied (Andersen, M. S., Egerton, P.D., Hitchen, K. & Boldreel, L.O.: Seismic Facies Analysis of volcanic rocks. Inpreparation).

Figure 6. Two contrasting lava morphologies of the Faroes (from Waagstein 1998)

have rough tops and are mostly simple, consisting of a thicknon-vesicular core grading into a rubbly top and base.

have smooth undulating tops and are typically compoundconsisting of thin beds of variable thickness, vesicles abundance andmineralisation.

Aa flows

Pahoehoe flows

Figure 5. Bands of large empty vesicles atthe base of the crust of a pahoehoe flow lobe(from Waagstein 1998) Figure 7. Vertical transition between two pahoehoe

flow lobes. The lower part of the photograph showsthe highly vesicular crust of the lower lobe. The upperlobe shows pipe vesicles at the base beneath massivelava core almost devoid of vesicles.(From Waagstein 1998)

Figure 8. Glyvursnes-1 core

Highly-vesicular lava crust filledwith zeolites

Former glass skin completelyaltered to dark grey clay

Moderately-highly-vesicularbasal zone with zeolites andgreen clay

Figure 9. New 160-m composite log section from the Faroes.

Figure 10. New 150-m composite log section from the Faroes.

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 chemistryand 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.

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).

6 18

6 18

Caliper (in)

Bitsize (in)

10DT-P ( ec/ft)µs130

DT-S ( sec/ft)µ170 500.5 NPHI

RHOB (g/cc)1.8 3.3

0.0 0 0.03Potasium

0 3Thorium

-1 2Uranium

0 80CGR

F 54 Flow unitSediment/tuff layer Comp 13 compound flow Massive core Porous crust

Acknowledgement: Thanks are due to the Sindri group for permission to publish results from the Glyvursnes-1 well

megavesicles

horizontalvesicular veins

0 m

2

4

6

8

10

12

14

16

18

20 m

rubblylava

isolatedvesicles

vesicularlava

pipe vesicles

pahoehoe toes

tuff

rubblylava

tuff

simpleaa flow

compoundpahoehoe flow

vesiclelayering

LA

VA

CR

US

T

LA

VA

CR

US

TLA

VA

CO

RE

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vesiclecylinders

megavesicles

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Basalt lava SedimentFlow-unit boundary

Rubbly crust TuffMassive core Porous crust

De th

1m:1000m

RESISTIVITY

0 20000Ohm

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50 0LPUDENSITY

1.8 3.3m/cc

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0 50PI Cs.

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0 1%

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140 40s/ft

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LITHOLOGY

BEDDING

MAGMA UNITS

Basalt lava SedimentFlow-unitboundary

Rubbly crust TuffBasalt

conglom.Massive core Porous crust

Compoundpahoehoe flow

Simple aa flow

AN INTEGRATED PETROPHYSICAL APPROACHTO THE SUB-BASALT IMAGING PROBLEMUSING WELL LOGGING DATA TO LINKMEASUREMENTS FROM CORES AND SEISMICSURFACE EXPERIMENTSR. Waagstein (1), L.O. Boldreel (2), C. Andersen (3)(1) Geological Survey of Denmark and Greenland (rw@geus.dk), (2) Geological Institute,University of Copenhagen, Denmark, (3) Geological Survey of Faroe Islands.

Flood basalt covered basins exist world wide along continental margins and areincreasingly coming into the focus of the hydrocarbon industry as more accessiblefields are being depleted. However, it has proved difficult in many places to lookthrough the basalt cover by conventional seismic reflection methods. This stressesthe need for a better understanding of the acoustic properties of basalt. The SeismicFaroes Basalt Project (SeiFaBa) was established in 2002 as an integrated study ofthe sub-basalt image problem. It involves 9 scientific institutions and individualsand is funded collectively by all oil companies operating in the Faroes sector (theSindri group). The planned fieldwork includes core drilling, wire-line logging,multi-azimuth VSP and surface seismic experiments at land and sea. It is mainlycentred around Glyvursnes, a relatively flat promontory near Tórshavn that allowsoptimal 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-linecoring technique and an old 660 m borehole (Vestmanna-1) 30 km farther northwestpartly blocked by calcite fillings was reopened using the same equipment. Anextensive wire-line logging program was subsequently run in both holes. These twoholes, together with the existing 3.65-km Lopra-1/1A hole in the southernmost islandSuduroy, cover all three Faroes basalt formations and a range of lava compositionsand 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 ofthe petrography, rock chemistry and petrophysical properties of core samples with theaim of establishing a log stratigraphy and scale core data to seismic scales.