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

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(Hamann et al. 2005). Cretaceous rifting resulted in several kilometres of downfaulting towards the Thetis Basin, and to a lesser extent also along the margins of the Danmarkshavn Basin. Consequently, the Danmarkshavn Ridge became an elevated rift-shoulder subject to erosion during most of the Cretaceous. In contrast, vast amounts of sediment were shed into the adjacent basins and kilometre-thick Cretaceous successions exist in the depocentres. Large fault- and anticlinal structures within Cretaceous and older successions formed in association with the Cretaceous extension. Many of these resemble structures hosting world-class oil and gas fields on the conjugate European Atlantic margin, thus boosting expectations for the North-East Greenland margin. A change from fault-controlled subsidence concentrated along the basins to regional eastward tilting took place close to the Cretaceous/Tertiary boundary. The change resulted in flooding and burial of the Danmarkshavn Ridge during the Early Tertiary and instigated regional eastwards progradation across the basin areas. Only modest volcanism, if any, took place across the basins north of Shannon. However, the southern Danmarkshavn and Thetis Basins were heavily intruded far north of the edge of the East Greenland basalt province, which degrades seismic imaging of the deepest stratigraphy over large areas. During mid-Tertiary time, sediment supply increased resulting in build-up and eastward progradation of a steep shelf-slope. This was likely caused by denudation of the East Greenland margin and the inner basin areas. As a result, the Lower Tertiary and Mesozoic deposits lie beneath the Plio/ Pleistocene sediments along the inner half of the North-East Greenland shelf. Implications for the petroleum prospectivity offshore East Greenland Assessment of the petroleum prospectivity along the South- East and central East Greenland margin is associated with considerable uncertainties. A sub-basaltic source rock seems to be a prerequisite for a working petroleum system along the margin, but the thick volcanic cover complicates analysis of the deeper geology. Regional presence of sub-basaltic basins is at present uncertain offshore South-East Greenland, which also makes the prediction of an effective petroleum system along this part of the margin uncertain. Farther north between Liverpool Land and Shannon the chances of sub-basaltic basins containing significant source-rock intervals are high. However, interpretation of the sub-basaltic structural style is problematic at present and the maturation history of potential source-rock intervals is uncertain, which complicates the prospectivity assesment. Farther north, extreme sea-ice conditions are a major impediment to future exploration. In addition, Tertiary uplift and erosion as well as past glacial advances across the shelf may have affected the regional petroleum potential. However, the presence of large structures and the good chance of adequately matured source-rock intervals urge for further investigation. The highest density of structures is located along the flanks of the basins and across the Danmarkshavn Ridge, which are also the areas covered in the coming licence rounds (Fig. 2). Many of these structures are associated with significant potential direct hydrocarbon indicators, which may reflect the presence of significant petroleum accumulations offshore North-East Greenland. This is highly encouraging for further exploration in one of the last frontier areas on Earth. Acknowledgement The Bureau of Minerals and Petroleum, Nuuk, is acknowledged for funding. References Gaina, C., Werner, S.C. & the CAMP-GM group 2009: Circum-Arctic mapping project – gravity and magnetic maps. Report 2009.010, 21 pp. Trondheim: Geological Survey of Norway. Gautier, D.L. et al. 2011: Assessment of NE Greenland: prototype for development of Circum-Arctic resource appraisal methodology. In: Spencer, A.M. et al. (eds): Arctic petroleum geology. Geological Society Memoirs (London) 35, 663–672. Hamann, N.E., Whittaker, R.C. & Stemmerik, L. 2005: Geological development of the Northeast Greenland shelf. In: Doré, A.G. & Vining, B.A. (eds): Petroleum geology: North-West Europe and global perspectives. Proceedings of the 6th Petroleum Geology Conference, 887–902. London: Geological Society. Hopper, J.R., Dahl-Jensen, T., Holbrook, W.S., Larsen, H.C., Lizarralde, D., Korenaga, J., Kent, G.M. & Kelemen, P.B. 2003: Structure of the SE Greenland margin from seismic reflection and refraction data: Implications for nascent spreading center subsidence and asymmetric crustal accretion during North Atlantic opening. Journal of Geophysical research 108, 2269, http://dx.doi.org/10.1029/2002JB001996 Larsen, H.C. & Saunders, A.D. 1998: Tectonism and volcanism at the southeast Greenland rifted margin: a record of plume impact and later continental rupture. In: Saunders, A.D., Larsen, H.C. & Wise, S.W. (eds): Proceedings of the Ocean Drilling Program Scientific results 152, 503–533. Mjelde, R., Raum, T., Breivik, A.J. & Faleide, J.I. 2008: Crustal transect across the North Atlantic. Marine Geophysical Research 29, 73–87. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: mbwf@geus.dk 64
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 intrusion 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 licences 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 Greenland (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 examination. 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 compare 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 positive 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 intrusion are much more difficult to recognise than on Nuussuaq, 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 available for analysis in the present investigation. The material, which is a waxy paraffinic hydrocarbon of C 28 H 56 , was located in vugs of pegmatite veins and labelled as an evenkitelike mineral by Konnerup-Madsen et al. (1979). Otherwise hydrocarbons in the Ilímaussaq intrusion only exist in fluid inclusions, mainly as C 1 –C 5, and as dispersed bitumen invisible to the naked eye. The stable carbon isotopic ratio of methane (δ 13 C = –7‰) released from the inclusions by crushing (Petersilie & Sørensen 1970) differed from that of associated methane in most oil and gas reservoirs, which has δ 13 C values of –30 to –50‰. The ratio was closer to the isotopic ratio of primodial carbon of the Earth’s mantle, which has δ 13 C values around –5‰. The paraffinic hydrocarbons of ‘evenkite’ on the other hand had a δ 13 C value of –29‰, which is within the expected range for hydrocarbons generated by thermal maturation of organic matter (Konnerup-Madsen et al. 1988). 66°N Greenland 60°N Nuussuaq Marraat 100 km Ilímaussaq intrusion 48°W 66°N Fig. 1. Map of south-western Greenland showing the distribution of Mesozoic/Cenozoic rift basins offshore (green). Modified from Chalmers & Pulvertaft (2001). Oil seeps occur on Nuussuaq. © 2012 GEUS. Geological Survey of Denmark and Greenland Bulletin 26, 65–68. Open access: www.geus.dk/publications/bull 65
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Geological Survey of Denmark and Greenland Bulletin 26 ... - Geus

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