Geological Survey of Denmark and Greenland Bulletin 26 ... - Geus

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A B GC-FID Pr Phy nC25 nC15 nC30 GC-MS m/z 191 H29 H30 C D Tm Ts H31 H32 H33 H34 H35 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. Dispersed bitumen in crystalline rocks The distribution of bitumen was examined by microscopy using ultraviolet light, which causes the aromatic constituents 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 certainty in some samples. Lujavrite contains only little sodalite 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 Ilímaussaq intrusion, by using a 7:1 mixture of dichloromethane and methanol. The bitumen content varied from 110 to 300 mg per kg rock and consisted of paraffins (20%), aromates (20%), NSO compounds (compounds with nitrogen, sulphur and oxygen; 50%) and asphaltenes (10%). The nonpolar fraction from the different rock extracts was analysed by gas chromatography with a flame-ionisation detector for total composition and mass spectrometry for biomarker GC-MS m/z 217 αα S 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. characterisation (Fig. 3). All samples gave very similar m/z 217 chromatograms showing a typical marine sterane distribution, although the presence of oleananes in the m/z 191 chromatogram suggests some input from land plants. Oleananes are derived from angiosperms, which appeared in the Late Cretaceous, and hence the presence of oleananes provides 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 Cretaceous and Tertiary sediments, some of which are potential C29 αα R 66
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 Cretaceous 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 Greenland, 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 Ilímaussaq intrusion solidified 3–4 km below the surface but is now exposed as a result of erosion. The latest uplift and erosion 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- Marraat oil Nuussuaq GGU 314654 II H4 Ilímaussaq lujavrite GGU 57033 H4 III + ? 100 412 191 H1 T I H2 H3 Ol+ Lup H5 H6 100 412 191 H1 T H2 H3 Ol+ Lup H5 H6 43 44 45 46 47 48 43 44 45 46 47 48 I Ol+ Lup H4 T H2 H4 10 412 369 T T1 R H2 H6 11 412 369 T1 R Ol+ Lup 43 44 45 46 47 48 Retention time (min) Oil seep H4 Nuussuaq GGU 414869A 43 44 45 46 47 48 Retention time (min) Ilímaussaq naujaite GGU 154344 H4 100 412 191 H1 T H2 H3 Ol + Lup H5 H6 100 412 191 T H1 H2 H3 Ol+ Lup H5 H6 43 T 44 45 46 47 48 43 44 45 46 47 48 H4 H4 H2 10 412 369 T1 R H2 Ol+ Lup H6 5 412 369 T Ol+ Lup T1 R 43 44 45 46 47 Retention time (min) 48 43 44 45 46 47 Retention time (min) 48 Fig. 4. Pentacyclic C 30 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. 67
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Geological Survey of Denmark and Greenland Bulletin 26 ... - Geus

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