Geological Survey of Denmark and Greenland Bulletin 23, 2011, 9–12 9 The Cambrian to Lower Silurian succession in Denmark is mostly composed of organic-rich black shales that were deposited in an epicontinental sea during a period of high global sea level (Haq & Schutter 2008). The mid-Cambrian to early Ordovician Alum Shale was intensively studied in the 1980s for its source-rock properties (e.g. Buchardt et al. 1986). Recent attention has focused on its potential as an unconventional shale gas source (Energistyrelsen 2010). On southern Bornholm, many wells have been drilled through the Lower Palaeozoic succession because of its importance for groundwater exploitation. In western Denmark, only the deep exploration wells Slagelse-1 and Terne-1 have pene- trated the Alum Shale, and knowledge of the unit west of Bornholm is thus very limited (Fig. 1). The project ‘Shale Gas in Europe (GASH)’ was launched in 2009 to address the European shale gas potential (Hors- field et al. 2008) and is organised by the German Research Centre of Geosciences and sponsored by oil and energy com- panies. It deals with basic research of key aspects of gas shale from regional to reservoir scales, and focuses on four ‘natural laboratories’, namely the Lower Palaeozoic Alum Shale, the Carboniferous (Namurian) shales, the Lower Jurassic Pose- donia Shale and as a reference the North American Barnet Shale. As part of GASH, a European black shale database is under construction in order to facilitate exploration and exploitation of gas shales in Europe. The Geological Survey of Denmark and Greenland (GEUS) has taken part in both the gas shale research and in the national data compilation for the European shale database. This paper presents the results of drilling on Bornholm in August 2010 by GEUS with the aim of obtaining fresh core material relevant to shale gas studies within the GASH project (the Skelbro-2 core) and providing new stratigraphic and geochemical information on the Lower Palaeozoic (the Billegrav-2 core). In particular, logging of the Silurian was needed to improve the log-stratigraphical template of Pe- dersen & Klitten (1990) which will enable the correlation of geophysical logs from non-cored water wells. Drilling and logging project on Bornholm The southern part of Bornholm is characterised by a mosaic of fault blocks (Graversen 2010). Outcrops, old core data and logs from water wells were used to find the best drill- ing locations for the two new wells. Both wells were fully cored and subsequently subjected to an extensive logging programme by GEUS in order to characterise the litho- logy as well as the water composition and the flow capacity of the fracture systems (Fig. 2). Spectral gamma and density scanning of the cores was subsequently carried out at GEUS. Skelbro-2 well The Skelbro-2 well was drilled 275 m east of Skelbro-1 (DGU 246.749; Pedersen 1989). The mid-Ordovician Komstad Limestone is 4 m thick at this locality. The top of the Alum Shale was encountered at 8.5 m below surface, and a total of 33.5 m of Alum Shale was drilled. The well was terminated at 42.9 m in the Lower Cambrian Rispebjerg Member (Læså Formation). The cored succession is virtually identical to the one described by Pedersen (1989) from the Skelbro-1 core. Billegrav-2 100 km Skelbro-2 Caledonian front Slagelse-1 Terne-1 Bornholm Norwegian–Danish Basin Ringkøbing–Fyn High Fig. 1. Map of Denmark showing the distribution of Lower Palaeozoic stra- ta (modified from Buchardt et al. 1997) and the location of the Slagelse-1 and Terne-1 exploration wells, and the Skelbro-2 (DGU 246.817) and Bil- legrav-2 (DGU 248.61) wells. Shale gas investigations in Denmark: Lower Palaeozoic shales on Bornholm Niels Hemmingsen Schovsbo, Arne Thorshøj Nielsen, Kurt Klitten, Anders Mathiesen and Per Rasmussen © GEUS, 2011. Geological Survey of Denmark and Greenland Bulletin 23, 9–12. Open Access: www.geus.dk/publications/bull 1010 L .C . M .C . U p p er O rd o vi ci an L o w er S ilu ri an Fu ro n gi an L .O . Q Grey shaleBlack shale SandstoneLimestone Siltstone (no log below 90 m)(no log below 90 m) K Lithology DepthPeriod (m) Natural gamma (API)10 Formation resistivity (ohm-m)10 Fluid conductivity (mScm–1)600 Flow log (%)0 10 20 30 40 50 60 70 80 90 100 110 120 1600800 100 Strat. R .M . A lu m S h al e Fm D ic el lo gr ap tu s Sh . L in d eg år d D1 D2 D3 A B1 B2 B3 B4 F3 F5 F4 E1 E2 E3 F1 F2 Log unit R as tr it es S h al e (API)10 2000 250 Sonic velocity (km/s) 62 Fig. 2. Stratigraphy, lithology, natural gamma ray, formation resistivity, sonic velocity, fluid conductivity and flow logs for Billegrav-2. Log units A–F according to Pedersen & Klitten (1990). Dashed lines: uncertain biostratigraphical boundaries. Green arrows: major inflow zones; black arrows: minor inflow zones. Q: Quaternary. K: Komstad Limestone. L.O.: Lower Ordovician. L.C.: Lower Cambrian. M.C.: Middle Cambrian. R.M.: Rispebjerg Member. API: American Petroleum Institute (a standard unit for gamma-ray measurements). Note change in scale for natural gamma at 90 m. 11 Billegrav-2 well The Billegrav-2 well was drilled close to locality 14b of Bjer- reskov (1975) and 800 m south of the Billegrav-1 well (DGU 247.560; Pedersen 1989). The Silurian Rastrites Shale was cored from 4.5 m below surface down to 60.5 m. The Ras- trites Shale comprises light to dark mudstone except for a distinct grey mud- to siltstone unit containing carbonate- cemented sandy beds at a depth between 31.2 and 46.0 m (Fig. 2). The Upper Ordovician includes the Lindegård For- mation (previously referred to as the Tretaspis Shale or Tom- marp and Jerrestad Mudstones), comprising grey mud- and siltstone and the dark organic-rich Dicellograptus Shale (Fig. 2). The base of the Dicellograptus Shale is located at a depth of 95 m. In its lowermost part the shale contains numerous bentonite beds including a 1 m thick K-bentonite bed that represents an important regional marker bed (Bergström & Nilsson 1974). The Komstad Limestone is 0.1 m thick and only repre- sented by its basal conglomerate. A thin bentonite rests di- rectly on the Komstad Limestone conglomerate; there is no conglomerate at the base of the overlying Dicellograptus Shale. The Alum Shale Formation is 27 m thick and includes the Middle Cambrian Andrarum and Exsulans Limestone beds that are important regional marker beds (Nielsen & Schovsbo 2006). The base of the Alum Shale was reached at a depth of 122 m, and the well was terminated at 125.9 m in the Rispebjerg Member (Fig. 2). Log-stratigraphy of the Billegrav-2 well Correlation of water wells based on gamma variation has served as an effective mean of correlation between wells (Pedersen & Klitten 1990). All gamma-ray, log-defined units identified in nearby water wells can also be recognised in the Billegrav-2 well (Fig. 2). The resistivity and sonic logs provide important additional information (Fig. 2). In the Rastrites Shale, the resistivity is particularly powerful in resolving the lithological variation, since the carbonate-cemented sandy beds in the middle part (the F3 unit) stand out as high-resis- tivity beds (Fig. 2). Water-flow zones in the Billegrav-2 well The flow log from this well clearly indicates that the water flow is related to three major and three minor influx zones (Fig. 2). The majority of the water flow takes place in the lowermost part of the Rastrites Shale at around a depth of 56–50 m. Relatively high water influx is also seen in the Ras- trites Shale at about a depth of 20 and 30 m, whereas there is very little water flow from log unit F3 (Fig. 2). Water influx is seen in the Dicellograptus Shale at 77 and 90 m as well as in the uppermost part of the Alum Shale at 96 m. No flow is observed deeper in the well. Interestingly, the conductiv- ity data suggest that the pore water in the Alum Shale has a much higher conductivity than in the shales above, suggest- ing that it is stagnant. This indicates that the Alum Shale acts as a hydraulic barrier between the silt- and sandstone aquifers below and the shale aquifers above this unit. Potential shale gas units onshore Denmark Shale gas units of potential economic interest have to be (1) matured to at least the gas generative stage, (2) organic rich (total organic carbon (TOC) >2 wt%), (3) volumetrically im- portant (thickness >20 m and regionally distributed) and (4) preferentially located away from structurally complex areas. The Lower Palaeozoic succession on Bornholm contains up to 10 wt% TOC in the Alum Shale, up to 5 wt% in the Dicel- lograptus shale and up to 2 wt% in the Rastrites Shale (Bu- chardt et al. 1986). Also Mesozoic organic-rich units occur in Denmark, but those onshore are all thermally immature to marginally mature (Petersen et al. 2008) and thus have no potential for shale gas. Thermal maturity and burial of the Lower Palaeozoic succession The thermal maturity of Lower Palaeozoic shales in Den- mark is only known from a few wells that all have vitrinite reflectance values >2.5%, indicative of a post-mature rank with regard to oil generation (Fig. 3). The shales are thus the- oretically favourable for shale gas. The high maturity reflects deep burial and a high geothermal gradient in Late Silurian – Early Devonian time (Buchardt et al. 1997). The present burial depth of the Lower Palaeozoic can be evaluated from the pre-Zechstein depth map (Fig. 4) that rep- resents the deepest level that can be mapped with some con- Fig. 3. Maturity of the Lower Palaeozoic sequence based on reflectance of vitrinite-like particles. The thermal maturity increases towards the Cale- donian front, reflecting deep burial in Late Silurian to Early Devonian time. Modified from Buchardt et al. (1997). ? ? ? ? ? ? Immature Mature Over-mature Anchi-metamorphic Caledonian front 1212 fidence on a regional basis (Vejbæk 1997). Lower Palaeozoic shales are buried to very deep levels in the central parts of the Norwegian–Danish Basin and are probably not within reach of shale gas exploration. Lower Palaeozoic shales buried to moderate depths of 2–4 km occur in a broad belt around the margin of the Norwegian–Danish Basin. Shale gas investi- gations have focused on this region (Energistyrelsen 2010). Conclusions Shale gas research and exploration in Denmark is currently focused on Palaeozoic shales. The prospective units are poor- ly known in Denmark outside Bornholm. The play involves deeply buried, post-mature shale in which many basic rock properties are still unknown. Key questions that remain to be addressed include the gas storage capacity of the shales, their mineralogy and how they respond to fracturing. The hydrogeology of water wells on Bornholm may provide a test case to assist in unravelling how fracture systems and flow are distributed in the Lower Palaeozoic shales. Acknowledgements The GASH project financed the Skelbro-2 drilling. Funding for the Bil- legrav-2 drilling was provided by GEUS, the Natural History Museum of Denmark, the University of Southern Denmark and Bornholm’s Region- skommune. Peter and Kristian Turner from Faxe Kalk A/S assisted with the drilling. The land owners Andres Ipsen, Jørn Erik Koefoed and Jesper Koefoed kindly permitted us to drill on their properties. References Bergström, S.M. & Nilsson, R. 1974: Age and correlation of the Middle Ordovician bentonites on Bornholm. Bulletin of the Geological Society of Denmark 23, 27–48. Bjerreskov, M. 1975: Llandoverian and Wenlockian graptolites from Bornholm. Fossils and Strata 8, 1–94. Buchardt, B., Clausen, J. & Thomsen, E. 1986: Carbon isotope composi- tion of Lower Palaeozoic kerogen: effects of maturation. Organic geo- chemistry 10, 127–134. Buchardt, B., Nielsen, A.T. & Schovsbo, N.H. 1997: Alun Skiferen i Skandinavien. Geologisk Tidsskrift 3, 1–30. Energistyrelsen 2010: Denmark’s oil and gas production 2009, 156 pp. Copenhagen: The Danish Energy Agency. Graversen, O. 2010: Structural analysis of superposed fault systems of the Bornholm horst block, Tornquist Zone, Denmark. Bulletin of the Geo- logical Society of Denmark 57, 25–49. Haq, B.U. & Schutter, S.R. 2008: A chronology of Paleozoic sea-level changes. Science 322, 64–68. Horsfield, B., Schulz H.-M. & GASH Team 2008: GASH: a shale gas initiative for Europe. EGU general assembly. Geophysical Research Ab- stracts 10, EGU 2008-A-01508. Nielsen, A.T. & Schovsbo, N.H. 2006: Cambrian to basal Ordovician lithostratigraphy in southern Scandinavia. Bulletin of the Geological Society of Denmark 53, 47–92. Pedersen, G.K. 1989: The sedimentology of Lower Palaeozoic black shales from the shallow wells Skelbro 1 and Billegrav 1, Bornholm, Denmark. Bulletin of the Geological Society of Denmark 37, 151–173. Pedersen, G.K. & Klitten, K. 1990: Anvendelse af gamma-logs ved kor- relation af marine skifre i vandforsyningsboringer på Bornholm. Dan- marks Geologisk Forening Årskrift 1987–89, 21–35. Petersen, H.I., Nielsen, L.H., Bojesen-Koefoed, J.A., Mathiesen, A., Kristensen, L. & Dalhoff, F. 2008: Evaluation of the quality, thermal maturity and distribution of potential source rocks in the Danish part of the Norwegian–Danish Basin. Geological Survey of Denmark and Greenland Bulletin 16, 66 pp. Vejbæk, O.V. 1997: Dybe strukturer i danske sedimentære bassiner. Geolo- gisk Tidsskrift 4, 31 pp. Fig. 4. Depth to the Top pre-Zechstein surface. Note the depocentre in the Norwegian–Danish Basin (>5000 m) and the shallow Ringkøbing–Fyn High (<1000 m). Areas where the Top pre-Zechstein surface coincides with basement are shown in grey. Modified from Vejbæk (1997). Faults at Top pre-Zechstein Top pre-Zechstein on basement 5000 1000 Depth to Top pre-Zechstein (m) 100 km Authors’ addresses N.H.S., K.K., A.M. & P.R., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: nsc@geus.dk A.T.N., Natural History Museum of Denmark, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark.