Exploring for saucer-shaped igneous intrusions on Spitsbergen, Svalbard

Kim Senger*, Srikumar Roy, Kei Ogata, Karoline Bælum, Alvar Braathen, Riko Noormets, Snorre Olaussen, Jan Tveranger

Summary

Igneous intrusions characterize the Svalbard archipelago, reflecting activity within a Large Igneous Province (LIP) of basically Early to mid-Cretaceous age (summary in Maher, 2001). These bodies, largely composed of dolerites and generally classified as part of the Diabasodden suite (Harland and Kelly, 1997), regionally intrude the Permian to Jurassic siliciclastic sedimentary succession deposited prior to the onset of the mainly Eocene West Spitsbergen fold-and-thrust belt in the west (Bergh et al, 1997) and the East Spitsbergen plateau deposits. Volcanic deposits occur in the extreme NE of the archipelago. In the study area of Inner Isfjorden (Figure 1), these dolerites are emplaced in the Triassic De Geerdalen Formation of the Kapp Toscana Group. This approximately 300 m thick shallow-marine to paralic, siliciclastic section is currently being extensively studied by the Longyearbyen CO2 laboratory project (http://co2-ccs.unis.no) due to its potential as a target interval for carbon dioxide storage. One of the 4 boreholes drilled in the vicinity of the Longyearbyen settlement penetrate nearly the whole reservoir section (LYB_CO2_DH4), highlighting the presence of thin igneous intrusions, with the thickest one (2 m thick) occurring at 950 m depth. Furthermore, seismic data show high-amplitude reflectors approximately 50-100 ms twt beneath the borehole. Based on correlation with outcrop data, as well as regional onshore and offshore seismic data sets, these high-amplitude reflectors are interpreted as dolerite sills. We postulate that these intrusions possibly have a significant effect on the compartmentalization of the reservoir.

 

Figure 1: Location map  illustrating the  Inner  Isfjorden study area, on the high Arctic archipelago of Svalbard. Outcrops to the south, 

east and north of the study site indicate the presence of intrusions of the Diabasodden suite (grey on geological map). An example of 

the cliff‐forming doleritic sill is shown in the inset photograph, taken across the fjord from Longyearbyen airport. The black thin lines 

reveal the onshore and offshore seismic database.

Saucer-shaped intrusions on Svalbard

2nd International Geosciences Student Conference 9‐12 July 2011, Krakow Poland 

Introduction

The presence of Cretaceous igneous intrusions, and synchronous lava flows at Kong Karls Land (Grogan et al., 1998), is well documented throughout the Svalbard archipelago. Published K-Ar ages (Birkenmajer et al., 2010; Burov et al., 1977; Firshov and Livshits, 1967; Gayer, 1966) range from approximately 149 Ma to 85 Ma, with an average around 125 Ma. Recent dating, using the more reliable U-Pb dating of zircons as well as Ar-Ar dating, by Polteau et al. (2011) suggests magmatism at approximately 125 Ma. Several thermal events have subsequently led to the thermal resetting of the K-Ar system, giving apparently younger ages. Conventional hydrocarbon exploration wells have penetrated dolerite bodies both, at Tromsøbreen (Polar Gas Prospektering AB, 1988), in southern Spitsbergen, and Colesbukta on the southern shore of Isfjorden (Skola et al, 1980). Velocity analysis of 2D multichannel seismic data has furthermore indicated a high-velocity layer within the Permian section, interpreted as a dolerite intrusion (Digranes and Kristoffersen, 1995).

Their presence in the potential CO2 storage reservoir is fundamental for three reasons:

1) The non-permeable intrusive rock could be directing fluid flow through the reservoir if not fractured during cooling and later Tertiary contractional tectonics.

2) The reservoir may be compartmentalized by the igneous intrusions, especially discordant dykes, leading to differential pressure compartments in the reservoir section.

3) Increased fracturing caused during the intrusion of the dolerite may have resulted in additional permeability pathways in fractures around the intrusion, with enhanced directional flow characteristics.

Methods Here we present an overview on the occurrence of the Diabasodden suite dolerites on Svalbard, with focus on the Inner Isfjorden field area (Figure 1). Utilizing an integration of outcrop, seismic, borehole, LiDAR and multibeam bathymetry data we propose that at least part of the dolerites may be intruded as saucer-shaped intrusions. These characteristic saucer-shaped intrusions are well documented in analogous volcanic basins elsewhere (Polteau et al., 2008b), including the South African Karoo Basin and the Møre and Vøring basins offshore mid-Norway. Their symmetric shape is thought to be related to the interaction of the injected fluid with a growing hydraulic fracture caused by the magmatic intrusion (Polteau et al., 2008a). The complex geometry of inter-linked and nested saucer-shaped intrusions results in the development of a compartmentalizing network. On Svalbard, we postulate that the development of such a complex network, in conjunction with the geological development of the area, may assist in the development of several reservoir compartments of non-equal pressures. Underpressure in the order of 50 Bars is observed at the reservoir interval penetrated with the Longyearbyen CO2 project borehole LYB_CO2_DH4 (Braathen et al, submitted). Intrusion-related compartments could potentially be critical components of the reservoir. Apart from playing a vital role in directing fluid flow, the size of such compartments would impact the exploitable volume for possible future CO2 injection. Our approach explores the igneous intrusions, combined with proposed cap rock shales of the overlying Agardhfjellet Formation in a model in which the compartment size may be restricting the total storage capacity at any given location. This model infers that intruded sills and dykes act as flow barriers, hence they are not offering through-going, open fractures.

Preliminary results

Recent interpretation conducted as part of the Longyearbyen CO2 project has identified several high-amplitude reflectors on both onshore and offshore seismic 2D lines that are likely to be related to igneous intrusions (Bælum et al, submitted). These reflectors are particularly extensive in the Inner Isfjorden study area, where surface outcrops indicate an abundance of dolerite bodies. Their regional geometry can further be studied on one of the outcrops using a 7 km long LiDAR model (Figure 2). Interpretations of both the LiDAR model and the digital elevation model (DEM) draped with a geologic map illustrate the overall stratigraphically-concordant nature of the sills, with several steeply-dipping discordant dykes. Image-based interpretation of dolerites illustrates this aspect for both the Diabasodden and Kapp Thordsen localities. These dykes may represent the steeply-dipping edges and/or the feeder systems of saucer-shaped intrusions.

Saucer-shaped intrusions on Svalbard

2nd International Geosciences Student Conference 9‐12 July 2011, Krakow Poland 

 

Figure 2: Examples of  interpreting  the onshore dolerite bodies using a digital elevation model with a draped geologic map  (A), and 

using a LiDAR scan (B) on the Botneheia mountain. On the LiDAR scan, note particularly the presence of both  layer‐parallel sills and 

discordant dykes. The LiDAR scan is approximately 7 km long.

High-resolution multibeam bathymetry data from Inner Isfjorden indicates the presence of sinuous ridges, approximately 5-30 m high (Figure 3). Seismic interpretation of conventional hydrocarbon exploration 2D lines from the late 1980s suggests that these ridges are in the bedrock, and not related to the glacial erosion prevalent in this high Arctic environment during the Quaternary (e.g. moraines). Differential erosion of weaker sediments in contrast to resistant dolerites may be responsible for the surface expression of these ridges. Alignment of these ridges with onshore exposure of doleritic dykes further supports the onshore-offshore correlation. Due to the regional, gentle south-westerly dip of the host succession, the multibeam data may only highlight one half of a saucer-shaped intrusion. Cross-cutting, high-amplitude reflectors recognizable on several conventional 2D lines may, however, define the full extent of the intrusive bodies. Further, planned, detailed velocity analysis is envisioned to confirm whether the high-amplitude anomalies are actually related to high P-wave velocity intrusions.

 

Figure 3: Composite of multibeam bathymetry and a conventional 2D seismic line (NH8509‐204). A) 2D view showing the location of 

the seismic line in relation to the NW‐SE aligned ridges. B) 3D view of 5m resolution multibeam data set showing the ridges within the 

Triassic section. White points mark the  interpretation of the main  feature, while black points  indicate other parallel and transverse 

ridges. Note  the  green  arrow  indicates  north. Note  that  the  bathymetry  is  shown with  5*  vertical  exaggeration.  C)  Conventional 

seismic line showing the data quality. Note particularly the increased amplitudes associated with the ridge shown in A) and B).  

Saucer-shaped intrusions on Svalbard

2nd International Geosciences Student Conference 9‐12 July 2011, Krakow Poland 

The intrusions are mapped in Permian to mid-Jurassic sedimentary units that, based on regional reconstructions would suggest an emplacement depth of approximately 500-1200 m. Therefore, we predict the inner sill width to be approximately 2.5 to 5 km in accordance with the expected overburden depth at the time of magmatism, using the relationship established by Polteau et al. (2008b). Furthermore, a stacked pattern of saucer-shaped intrusions may be envisioned given the range of sedimentary units intruded.

Conclusions

Based on preliminary results of ongoing work, we conclude that the Cretaceous igneous intrusions of Spitsbergen may be represented by a complex network of vertically stacked saucer-shaped sills. Furthermore, the presence of such intrusions within the potential CO2 sequestration interval of the Kapp Toscana Group is confirmed on the basis of seismic, borehole, outcrop, LiDAR and multibeam bathymetry data. Fieldwork in the summer of 2011 will collect further data in order to better constrain the geometry and nature of the igneous intrusions, which will develop our understanding of their role in the Longyearbyen CO2 storage reservoir model.

Acknowledgements

This work is conducted as part of the “Geological input to Carbon storage” project financed by the Norwegian Research Council. The multibeam data from the Norwegian Hydrographic Service is presented in accordance with permission number 08/620. Fieldwork is partly financed by an Arctic Field Grant from the Svalbard Science Forum. NPI-Geonet of the Norwegian Polar Institute, provided the shape files of the geologic maps. Furthermore, the project is run in very close co-operation with the Longyearbyen CO2 Lab. project (http://co2-ccs.unis.no), which is financially supported by Gassnova, ConocoPhillips, Statoil, Statkraft, Store Norske Spitsbergen Gruvekompaniet, Lundin, LNS and BJ Services.

References

Bergh, S. G., Braathen, A., and Andresen, A., 1997, Interaction of basement‐involved and thin‐skinned tectonism in the Tertiary fold‐thrust belt of central Spitsbergen, Svalbard: AAPG Bulletin, v. 81, no. 4, p. 637‐661. 

Braathen, A., and co‐authors, submitted. Longyearbyen CO2 lab of Svalbard, Norway – first assessment of the sedimentary succession for CO2 storage. Norwegian Journal of Geology. 

Birkenmajer, K., Krajewski, K., Pécskay, Z., and Lorenc, M., 2010, K‐Ar dating of basic intrusions at Bellsund, Spitsbergen, Svalbard: Polish Polar Research, v. 31, no. 1, p. 3‐16. 

Burov, P., Krasil'scikov, A. A., Firsov, L. V., and Klubov, B. A., 1977, The age of Spitsbergen Dolerites (from isotopic dating): Norsk polarinstitutt Årbok 1975, p. 101‐108. 

Baelum, K., Johansen, T.A., Johnsen, H., Røed, K., Ruud, B.O., Mikkelsen, E. and Braathen, A. submitted, Subsurface geometries of the Longyearbyen CO2 lab in Central Spitsbergen, as mapped by reflection seismic data. Norwegian Journal of Geology. 

Digranes, P., and Kristoffersen, Y., 1995, Use of mode‐converted waves in marine seismic data to investigate the lithology of the sub‐bottom sediments in Isfjorden, Svalbard: Pure and Applied Geophysics, v. 145, no. 2, p. 313‐325. 

Firshov, L. V., and Livshits, 1967, Potassium‐Argon dating of dolerites from the region of Sassenfjorden, in Sokolov, V. N., ed., Materialy po stratigrafii Spicbergena: Leningrad. 

Gayer, R. A., 1966, Radiometric age determinations on rocks from Spitsbergen: Skrifter, v. 137. Grogan, P., Nyberg, K., Fotland, B., Myklebust, R., Dahlgren, S., and Riis, F., 1998, Cretaceous Magmatism South and East of Svalbard: Evidence from Seismic 

Reflection and Magnetic Data: Polarforschung, v. 68, p. 25‐34. Harland, W. B., and Kelly, S. R. A., 1997, Chapter 19 Jurassic‐Cretaceous history, The Geology of Svalbard, p. 363‐387. Harmon D. Maher, Jr., 2001, Manifestations of the Cretaceous High Arctic Large Igneous Province in Svalbard: The Journal of Geology, v. 109, no. 1, p. 91‐

104. Polar Gas Prospektering AB, 1988, Preliminary Stratigraphic Column Tromsøbreen II. State Archive, Tromsø.  Polteau, S., Ferre, E. C., Planke, S., Neumann, E. R., and Chevallier, L., 2008a, How are saucer‐shaped sills emplaced? Constraints from the Golden Valley Sill, 

South Africa: Journal of Geophysical Research, v. 113, no. B12104. Polteau, S., Mazzini, A., Galland, O., Planke, S., and Malthe‐Sørenssen, A., 2008b, Saucer‐shaped intrusions: Occurrences, emplacement and implications: 

Earth and Planetary Science Letters, v. 266, p. 195‐204. Polteau, S., Planke, S., Planke, E. E., Faleide, J. I., Svensen, H., Corfu, F., Midtkandal, I., and Myklebust, R., 2011, The Barents Sea Large Igneous Province: 

Extent, Age & Global Implications, NGF Winter Meeting: Stavanger. Skola, I. V., Pcelina, T. M., Mazur, V. B., and Alter, S. M., 1980, New data on the composition and structure of the sedimentary platform cover on the basis of 

materials from the drilling of a parametric hole at Grumantbyen, Geology of the sedimentary platform cover of the archipelago of Svalbard. Collection of scientific papers.: Leningrad, NIIGA, p. 13‐24.