Geological Survey of Denmark and Greenland Bulletin 41, 2018, 29-32 29 In recent years there has been an increased interest in Neo- gene hydrocarbon accumulations in the North Sea. The production of gas from Pliocene–Quaternary deposits in the Dutch sector, the discovery of oil-bearing Miocene sands in the Lille John area and oil accumulation in middle Miocene deposits in the T-1 well in the northern part of the Danish Central Graben area, have documented Neogene hydrocar- bon accumulations. Some of these deposits are of economic interest. This study presents an oil-bearing, middle Miocene diatom ooze in the Valhall Field (well 2/8-G-2), within the Norwegian sector (Fig. 1). The Valhall Field is located just north of the Danish–Norwegian sector boundary. Geological setting During the Miocene, the North Sea formed a silled-bound- ed basin with connection to the Atlantic Ocean via a strait between Norway and Shetland (Rasmussen et al. 2008; Fig. 1). The main sediment source areas were the Shetland Plat- form, which supplied sediments to the northern North Sea and the southern Scandes from which sediments were routed southwards into the south-eastern North Sea (Fig. 1). Dur- ing the early Miocene relatively large delta complexes formed from these areas and resulted in eastward progradation off the Shetland Platform (Skade Formation; Eidvin et al. 2014) and south-westward progradation south of the southern Scandes (Ribe Group; Rasmussen et al. 2010). During the Miocene oil-bearing diatom ooze from the North Sea Emma Sheldon, Erik S. Rasmussen, Karen Dybkjær, Tor Eidvin, Fridtjof Riis and Rikke Weibel 0°E 10°E 20°E 50°N 60°N Shetland Platform So ut he rn Sc an de s 2/8-G-2 Mittelgebirge Highs Fig. 1. Palaeogeographical reconstruction of the early Miocene North Sea. Note that the main sediment influx from the Shetland Platform (yellow arrows) filled the northern North Sea and that sediment supply to the eastern North Sea had its source in southern Scandes. Based on Rasmussen et al. (2008). B C A 10 µm 20 µm 2 mm Fig. 2. Cored diatom ooze from the Valhall Field, Norwegian sector of the North Sea. A: Optical microscope image of chip of diatom ooze, note the diatom in the upper part (black arrow). B: ?Denticulopsis kanayae. C: ?Den- ticulopsis nicobarica. © 2018 GEUS. Geological Survey of Denmark and Greenland Bulletin 41, 29–32. Open access: www.geus.dk/bulletin http://www.geus.dk/bulletin 3030 middle Miocene these delta complexes where flooded due to reorganisation of the tectonic regime in North-West Europe which resulted in accelerated subsidence of the basin mar- gins. Consequently, much of the North Sea was starved of sediment during the middle and early late Miocene. The North Sea area was located in the western wind belt with seasonal storms. Therefore, the coast was strongly influ- enced by wave action. In the deeper basin, which was up to c. 1000 m deep, hemipelagic deposition predominated. A coun- ter-clockwise current system redistributed and reshaped mud- dy sediments along the delta and shelf slopes within the basin (e.g. Hansen et al. 2004). During the early Miocene a humid, warm temperate climate predominated, similar to present day western Florida (USA). A change to a cooler climate com- menced in the middle Miocene which probably also resulted in the enhanced influence of cold-water current systems from the Atlantic Ocean. Under these cooler climatic conditions diatoms bloomed and resulted in deposition of diatom ooze. Diatom ooze The diatom ooze is fine-grained and grey to brown, since it contains oil (Fig. 2). Diatom valves and radiolarians could be seen in optical and scanning electron microscopes (Figs 3, 4). The diatom ooze consists of a mixture of abundant diatom valves, radiolarians and clay (Fig. 4). The porosity is 50–60 %. Porosity is mainly associated with diatom valves and ei- ther occurs inside the valves or in pockets next to the valves or other fossils. Reduced porosity is observed in some sam- ples, attributed to clay intruding into the diatom valves. The permeability is assumed to be low, due to the small size of the pores and tortuous connectivity between the largest pores. The measured porosity of the diatom ooze would correspond to a permeability of 0.006–0.02 mD in diatom ooze from various localities in the Pacific Ocean (Gamage et al. 2011). Nannofossils and microfossils Core sample (1802.7 m from the well 2/8-G-2) was analysed for nannofossil, microfossil and the presence of diatoms. Diatom valves and debris were found to be common. Dia- toms include ?Thalassiosira spp., ?Denticulopsis kanayae and ?Denticulopsis niobarica. D. kanayae and D. nicobarica range from the early to middle Miocene (Barron 1985; Fig. 2). The sample was barren with respect to nannofossils and the mi- 2 µm 10 µm 10 µm50 µm Fig. 3. Four scanning electron microscope images of the diatom ooze. Note the valve of the centric diatom Thalassiosira spp. in the upper right image. 31 crofossil fraction yielded one radiolarian (Cenodiscus spp.) and no foraminifera. Palynology Six core samples were analysed for palynology (1797.0 m, 1802.7 m, 1803.4 m, 1813.0 m, 1819.0 m and 1827.0 m). In all samples, the assemblages of organic particles are charac- terised by a dominance of marine dinoflagellate cysts (di- nocysts). Bisaccate and non-saccate pollen and wood parti- cles occur very sporadically while no freshwater algae were recorded. The dinocyst assemblage is rich and diverse and the consistent presence of Nematosphaeropsis spp. and Im- pagidinium spp. indicates an outer neritic to oceanic set- ting (Brinkhuis 1994). An increase in cold-water tolerant dinocyst taxa (mainly Habibacysta tectata) was found (Fig. 5), ranging from no recordings in the lowermost sample, to sporadic occurrences in the next samples and common oc- currences in the two uppermost samples. The occurrences of the dinocyst species Unipontodinium aquaductum in all six samples (Fig. 5) strongly indicate that the cored inter- val should be referred to the Unipontodinium aquaductum Zone of Dybkjær and Piasecki (2010). The age of this zone is mid-Langhian to early Serravallian (middle Miocene). The A B C D 1 2 3 4 5 1) Stephanopyxis turris (diatom) 2) cross section of a diatom valve 3) ? Peridinium longispinum (Radiolarian) 4) ? Stephanopyxis turris 5) ????? 30 µm 30 µm 30 µm 30 µm Fig. 4. Optical microscope images of the diatom ooze. The ooze comprises siliceous microfossils (e.g. diatoms and radiolarians) and clay. Porosity inside microfossils is recognised by the blue stain- ing of the epoxy impregnating the ooze. A and B are identical; B with crossed nicols. 1: Stephano- pyxis turris (diatom). 2: cross section of a diatom valve. 3: ?Peridinium longispinum (radiolarian), 4: ?Stephanopyxis turris. 5: diatom frustule. Alexander Mitlehner, UK, kindly helped with the identification of the diatoms. 20 µm 20 µm A B Fig. 5. Dinoflagelate cysts from the oil-bearing cores. A: Unipotodinium aquaductum. B: Habibacysta tectata. Increased abundance of Habibacysta tectata, a cold-water tolorant species, is probably associated with the climatic deterioration in the middle Miocene (Serravalian). 3232 Unipontodinium aquaductum Zone occurs in the upper part of the Hodde Formation defined onshore Denmark which correlates with the lowermost part of the Nordland Group. Depositional environment Based on palynology and seismic stratigraphic studies (e.g. Rasmussen et al. 2005), the depositional setting was outer neritic to oceanic, with a water depth just below 1000 m. Late early Miocene biosiliceous, organic-rich sediments of the upper Lark Formation in the Central Graben area have been described by Sulsbrück & Toft (2018). This part of the upper Lark Formation was deposited at the termination of shoreline progradation from the southern Scandes (Ribe Group). The studied section represents slightly younger de- posits than those laid down during the transgression of the lower Miocene Ribe Group. Consequently, the depositional environment was sediment starved and fully marine. The oc- currence of cold water dinocysts in the studied cores, prob- ably reflects the beginning of the middle Miocene (Serraval- ian) global climatic deterioration (Zachos et al. 2001). Petroleum system The oil-bearing Miocene diatom ooze from the Norwegian sector of the North Sea described here documents oil migra- tion into younger deposits, which are normally considered to be non-prospective. In the Danish North Sea area, a number of wells have penetrated hydrocarbon-bearing strata of Mio- cene and Pliocene ages as described above. The oil-bearing deposits are found in the western and central parts of the Danish and Norwegian Central Graben. The oil probably has a source in the Jurassic shale deep in the Central Graben. Migration into Cenozoic deposits probably occurred along salt structures. Due to early Quaternary tilting of the North Sea Basin (Rasmussen et al. 2005), up-dip migration into stratigraphic and structural traps located in the eastern part of the Central Graben area and the Ringkøbing–Fyn High may have occurred. This calls for a total re-evaluation of the petroleum system of the Cenozoic succession in the North Sea area. References Barron, J.A. 1985: Miocene to Holocene planktic diatoms. In: Bolli, H.M., Saunders J.B. & Perch-Nielsen, K. (eds): Plankton stratigraphy, 763–809. Cambridge: Cambridge University Press. Brinkhuis, H. 1994: Late Eocene to Early Oligocene dinoflagellate cysts from the Priabonian type-area (Northeast Italy): biostratigraphy and paleoenvironmental interpretation. Palaeogeography, Palaeoclimatology, Palaeoecology 107, 121–163. Dybkjær, K. & Piasecki, S. 2010: Neogene dinocyst zonation in the eastern North Sea Basin, Denmark. Review of Palaeobotany and Palynology 161, 1–29. Eidvin, T., Riis, F. & Rasmussen E.S. 2014: Oligocene to Lower Pliocene deposits of the Norwegian continental shelf, with correlation to the Norwegian Sea, Greenland, Svalbard, Denmark and their relation to the uplift of Fennoscandia. Marine and Petroleum Geology 56, 184– 221. Gamage, K., Screaton, E., Bekins, B. & Aiella, I. 2011: Permeability-poros- ity relationships of subduction zone sediments. Marine Geology 279, 19–36. Hansen, J.P.V., Clausen, O.R. & Huuse, M. 2004: 3D seismic analysis reveals the origin of ambiguous erosional features at a major sequence boundary in the eastern North Sea: near top Oligocene. Geological So- ciety Memoirs (London) 29, 83–90. Rasmussen, E.S., Vejbæk, O.V., Bidstrup, T., Piasecki, S & Dybkjær, K. 2005: Late Cenozoic depositional history of the Danish North Sea Basin: implications for the petroleum systems in the Kraka, Halfdan, Siri and Nini fields. In: Dore, A.G. & Vinding, B.A. (eds): Petroleum geology: North-West Europe and global perspectives. Proceedings of the 6th petroleum geology conference, 1347–1358. London: Geologi- cal Society. Rasmussen, E.S., Heilmann-Clausen, C., Waagstein, R. & Eidvin, T. 2008: Tertiary of Norden. Episodes 31, 66–72. Rasmussen, E.S., Dybkjær, K, & Piasecki, S. 2010: Lithostratigraphy of the Upper Oligocene – Miocene succession of Denmark. Bulletin of the Geological Survey of Denmark and Greenland 22, 92 pp. Sulsbrück, H. & Toft, J. 2018: A new observation of a biosiliceous opal bearing sequence in the Miocene Lark Formation in the Danish North Sea. 33rd Nordic Geological Winter meeting, Lyngby. Ab- stract http://2dgf.dk/foreningen/33rd-nordic-geological-winter-meeting/ ngwm-2018-abstracts/3-sedimentary-rocks-and-processes/ Zachos, J.C., Pagani, M., Sloan, L., Thomas, E. & Billups, K. 2001: Trends, rhythms, and aberrations in global climate 65 Ma to Present. Science 292, 686–693. Authors’ addresses E.S., E.S.R., K.D. & R.W., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: es@geus.dk. T.E. & F.R. Norwegian Petroleum Directorate (NPD), P. O. Box 600, N-4003 Stavanger, Norway. http://2dgf.dk/foreningen/33rd-nordic-geological-winter-meeting/ngwm-2018-abstracts/3-sedimentary-rocks-and-processes/ http://2dgf.dk/foreningen/33rd-nordic-geological-winter-meeting/ngwm-2018-abstracts/3-sedimentary-rocks-and-processes/ mailto:es@geus.dk