Geological Survey of Denmark and Greenland Bulletin 26, 2012, 65-68 65 New evidence for possible generation of oil off south-western Greenland Troels Laier and Hans Peter Nytoft In 2011, traces of bitumen in the 1160 Ma old Ilímaussaq in- trusion in South Greenland have been examined in order to determine their origin. The investigation was prompted by the recent interest in hydrocarbon exploration off western Greenland, an interest expressed in the form of four new li- cences in the region (Christiansen 2011). The hydrocarbon potential in the region was realised after reinterpretation of seismic profiles across the Labrador Sea, and this indicates the presence of a sedimentary basin off south-western Green- land (Fig. 1; Chalmers & Pulvertaft 2001). However, the main problem in petroleum exploration off south-western Greenland is that no prolific marine source rocks have been demonstrated (Christiansen 2011). Therefore, any trace of hydrocarbons, however small that may help demonstrate the occurrence of source rocks in the region, deserves careful ex- amination. Recently, bitumen biomarkers have been used to question the presumed abiogenic origin of hydrocarbons in crystalline rocks of the Ilímaussaq intrusion (Laier & Nytoft 2012). In this paper, we focus on the origin of the bitumen and com- pare it with previous finds in central West Greenland. The presence of hydrocarbons in the Ilímaussaq intrusion has been known since 1970 (Petersilie & Sørensen 1970) but unlike the discovery of oil seeps in the Nuussuaq region in central West Greenland twenty years later, which had a posi- tive impact on petroleum exploration (Christiansen 2011), the hydrocarbons in the Ilímaussaq intrusion were largely ignored in the context of offshore exploration. The reason for this is twofold: (1) hydrocarbons in the Ilímaussaq in- trusion are much more difficult to recognise than on Nuus- suaq, and (2) analytical results are confusing with respect to the origin of the hydrocarbons. Discrete millimetre-size hydrocarbon accumulations have only been observed twice, and samples of this material were unfortunately not avail- able for analysis in the present investigation. The material, which is a waxy paraffinic hydrocarbon of C28H56, was lo- cated in vugs of pegmatite veins and labelled as an evenkite- like mineral by Konnerup-Madsen et al. (1979). Otherwise hydrocarbons in the Ilímaussaq intrusion only exist in fluid inclusions, mainly as C1–C5, and as dispersed bitumen invis- ible to the naked eye. The stable carbon isotopic ratio of me- thane (δ13C = –7‰) released from the inclusions by crushing (Petersilie & Sørensen 1970) differed from that of associated methane in most oil and gas reservoirs, which has δ13C values of –30 to –50‰. The ratio was closer to the isotopic ratio of primodial carbon of the Earth’s mantle, which has δ13C values around –5‰. The paraffinic hydrocarbons of ‘even- kite’ on the other hand had a δ13C value of –29‰, which is within the expected range for hydrocarbons generated by thermal maturation of organic matter (Konnerup-Madsen et al. 1988). Fig. 1. Map of south-western Greenland showing the distribution of Mes- ozoic/Cenozoic rift basins offshore (green). Modified from Chalmers & Pulvertaft (2001). Oil seeps occur on Nuussuaq. 100 km Greenland 48°W 60°N 66°N 66°N Nuussuaq Ilímaussaq intrusion Marraat © 2012 GEUS. Geological Survey of Denmark and Greenland Bulletin 26, 65–68. Open access: www.geus.dk/publications/bull 6666 Dispersed bitumen in crystalline rocks The distribution of bitumen was examined by microscopy using ultraviolet light, which causes the aromatic constitu- ents of bitumen to fluoresce. Unfortunately, a number of the rather common minerals, e.g. sodalite, in different rock types of the Ilímaussaq intrusion also fluoresce strongly making it almost impossible to indentify traces of bitumen with cer- tainty in some samples. Lujavrite contains only little soda- lite and offers the best possibility to study the distribution of bitumen (Fig. 2). The bitumen occurs along crystal edges and as trails of tiny inclusions within single crystals of, for example, eudialyte. Trails of tiny inclusions are usually taken as evidence of a secondary origin formed in healed fractures. Thus hydrocarbons probably migrated through the rock and were trapped in certain minerals. Bitumen content and composition It was possible to extract bitumen from naujaite, kakortokite and lujavrite, which are the three major rock types in the Ilí- maussaq intrusion, by using a 7:1 mixture of dichlorometh- ane and methanol. The bitumen content varied from 110 to 300 mg per kg rock and consisted of paraffins (20%), aro- mates (20%), NSO compounds (compounds with nitrogen, sulphur and oxygen; 50%) and asphaltenes (10%). The non- polar fraction from the different rock extracts was analysed by gas chromatography with a flame-ionisation detector for total composition and mass spectrometry for biomarker characterisation (Fig. 3). All samples gave very similar m/z 217 chromatograms showing a typical marine sterane distri- bution, although the presence of oleananes in the m/z 191 chromatogram suggests some input from land plants. Olean- anes are derived from angiosperms, which appeared in the Late Cretaceous, and hence the presence of oleananes pro- vides a maximum age for the source of the bitumen. Since other triterpanes are known to co-elute with oleananes, gas chromatograph mass spectrometry (GC-MS-MS) was also conducted to confirm their presence (Fig. 4). The GC-MS- MS analysis not only confirmed the presence of oleananes but also showed the existence of bicadinanes, which is a less common group of biomarkers from land plants from Late Cretaceous or Tertiary. Oleananes and bicadinanes were also observed in oil seeps from the Nuussuaq region (Fig. 4; Bojesen-Koefoed et al. 1999; Nytoft et al. 2002). Migration and entrapment of hydrocarbons Not only is bitumen much more difficult to recognise in rocks from the Ilímaussaq intrusion than in basalts from Nuussuaq, but the migration route of hydrocarbons to the Ilímaussaq intrusion is also less evident. In Nuussuaq the oil seeps are found in Tertiary plateau basalts overlying Creta- ceous and Tertiary sediments, some of which are potential A B DC 100 μm Fig. 2. Photomicrographs of lujavrite (GGU 57033) viewed under visible light (A, B) and in ultraviolet light (C, D) showing hydrocarbons along the edges of eudialyte crystals and as traces of tiny inclusions within the crystals. Fig. 3. Chromatograms of the aliphatic fraction of extract from Ilímaussaq kakortokite m/z 191 (hopanes+tricyclic) and m/z 217 (steranes). FID: flame ionisation detector, GC-MS: gas chromatography - mass spectrometry. GC-FID GC-MS m/z 191 Pr Phy nC15 nC25 Ts Tm H29 H30 H31 H32 H33 H34 H35 nC30 m/z 217 C29 αα S αα R GC-MS 67 source rocks. In the Ilímaussaq intrusion, the bitumen is found in Proterozoic crystalline rocks which are much older than the source rock, which is not older than Late Creta- ceous as shown by the presence of oleanes and bicadinanes. In contrast to the Nuussuaq region, no potential source rock for hydrocarbons has been reported in South Green- land, where Proterozoic gneisses and granites, igneous and sedimentary rocks are found. The youngest rocks in South Greenland are those of the Gardar Province (c. 1300–1100 Ma), which is dominated by continental sandstones and lavas with numerous dykes and large intrusions (Poulsen 1964), one of which is the Ilímaussaq peralkaline intrusion. The Ilí- maussaq intrusion solidified 3–4 km below the surface but is now exposed as a result of erosion. The latest uplift and ero- sion started c. 35 Ma ago according to thermo-chronometric investigations (Japsen et al. 2006). However, the latest uplift phase was preceded by subsidence during Late Cretaceous to Eocene. During this subsidence phase the region was prob- Fig. 4. Pentacyclic C30 triterpanes in an Ilímaussaq bitumen (412 → 191 and 412 → 369). Numbered peaks: oleananes (Ol + Lup) and similar land-plant components (I – III). Peaks H1–H6: hopanoids. Peaks T, T1 and R: bicadinanes. 48 H6412 191 100 Marraatoil Nuussuaq GGU 314654 H1 H2 H3 H4 H5 Ol+ Lup T I II III + ? 43 44 45 4846 47 43 44 45 4846 47 H1 H2 H3 H5 H6 T 412 191 100 Ol+ Lup H4 H4 H2 T T1 R 412 369 11 43 44 45 4846 47 Ol+ Lup Ol+ Lup H1 H2 H3 H4 H5 H6 T 412 191 100 Ilímaussaq naujaite GGU 154344 43 44 45 4846 47 Ol+ Lup H4 T H2 T1 R 412 369 5 43 44 45 46 47 Ol+ Lup H6412 191 100 412 369 10 43 43 44 44 45 45 48 48 46 46 47 47 H1 H2 H3 H5 T H4 H2 T T1 R H6 GGU 414869A H4 Nuussuaq 43 44 45 4846 47 412 369 10 Ol+ Lup H2 H4 T T1 R I H6 Ilímaussaq lujavrite GGU 57033 Retention time (min) Retention time (min) Ol + Lup Oil seep Retention time (min) Retention time (min) 6868 ably covered by up to 2 km of marine sediments (Chalmers & Pulvertaft 2001; Japsen et al. 2006). The reburial and uplift history in South Greenland was probably similar to that of West Greenland, including the Nuussuaq region (Bonow et al. 2007). Thus it is likely that hydrocarbons migrated from marine sediments into the rocks of the Ilímaussaq intrusion during the period of reburial (Fig. 5). Hydrocarbons have only been reported from the Ilímaus- saq intrusion and not from other rocks in South Green- land. There may be two reasons for this: (1) rocks from the Ilímaussaq intrusion have been studied in much more detail than other rocks due to the occurrence of rare minerals, some of which have economic potential, and (2) the Ilímaussaq in- trusion may be more deeply weathered than other rocks in the region because it contains water-soluble minerals (Rose- Hansen & Sørensen 2002). From the biomarkers found in the bitumen, there is lit- tle doubt that the hydrocarbons were generated by thermal maturation of a marine source rock and migrated into the rocks of the Ilímaussaq intrusion at a later stage. It is unlikely that the bitumen formed by condensation of lighter hydro- carbons of abiogenic origin as hypothesised by Petersilie & Sørensen (1970). Hydrocarbon gases in fluid inclusions in rocks from the Ilímaussaq intrusion were considered to be abiogenic in origin on the basis of the heavy isotopic value of methane (Petersilie & Sørensen 1970; Konnerup-Madsen et al. 1988). However, they may also be of organic origin if the isotopic ratio of the gases was altered by fractionation due to diffusion of gases from the rocks (Laier & Nytoft 2012). References Bojesen-Koefoed, J.A., Christiansen, F.G., Nytoft, H.P. & Pedersen, A.K. 1999: Oil seepage onshore West Greenland: evidence of multiple source rocks and oil mixing. In: Fleet, A.J. & Boldy, S.A.R. (eds): Petroleum geology of Northwest Europe. Proceedings of the 5th Conference. Pe- troleum Geology Conference Series 5, 305–314. London: Geological Society. 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Danmarks og Grønlands Geologiske Undersøgelse Rapport 2006/63, 77 pp. Konnerup-Madsen, J., Larsen, E. & Rose-Hansen, J. 1979: Hydrocarbon- rich fluid inclusions in minerals from the alkaline Ilímaussaq intrusion, South Greenland. Bulletin de Minéralogie 102, 642–653. Konnerup-Madsen J., Kreulen R. & Rose-Hansen J. 1988: Stable isotopic characteristics of hydrocarbon gases in the alkaline Ilímaussaq complex, South Greenland. Bulletin de Minéralogie 111, 567–576. Laier, T. & Nytoft, H.P. 2012: Bitumen biomarkers in the mid-Proterozoic Ilímaussaq intrusion, Southwest Greenland – a challenge to the mantle gas theory. Marine and Petroleum Geology 30, 50–65. Nytoft, H.P., Bojesen-Koefoed, J.A., Christiansen, F.G. & Fowler, M.G. 2002: Oleanane or lupane? Reappraisal of the presence of oleanane in Cretaceous–Tertiary oils and sediments. Organic Geochemistry 33, 1225–1240. Poulsen, V. 1964: The sandstones of the Precambrian Eriksfjord Formation in South Greenland. Rapport Grønlands Geologiske Undersøgelse 2, 16 pp. Petersilie, I.A. & Sørensen, H. 1970: Hydrocarbon gases and bituminous substances in rocks from the Ilímaussaq alkaline intrusion, South Greenland. Lithos 3, 59–76. Rose-Hansen, J. & Sørensen, H. 2002: Geology of the lujavrites from the Ilímaussaq alkaline complex, South Greenland, with information from seven bore holes. Meddelelser om Grønland, Geoscience 40, 58 pp. Fig. 5. Sketch of the geological evolution of the Ilímaussaq area. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: tl@geus.dk including hydrocarbons Granite Rift deposits Lavas Intrusion Cretaceous Palaeogene Migrating fluids, A B C D 1150 Ma Mesozoic Palaeogene Present