Geological Survey of Denmark and Greenland Bulletin 33, 2015, 9-12 9 The Lower Palaeozoic now fully cored and logged on Bornholm, Denmark Niels H. Schovsbo, Arne T. Nielsen and Kurt Klitten A 558 m long, complete section of the Lower Palaeozoic succession preserved onshore southern Bornholm has been compiled from fi ve fully cored scientifi c wells, carried out between 2005 and 2012. Th e scientifi c programme included coring and geophysical logging of the fi ve scientifi c wells that yielded a total of c. 750 m of partially overlapping cores as well as re-logging of water wells and acquisition of shal- low seismic data. Th e last well drilled, the Sommerodde-1, cored the youngest preserved Silurian strata on Bornholm including strata not exposed in outcrops. Th e well penetrat- ed 168.1  m of Silurian shales, 42.7 m of Upper Ordovician shales and 27.9 m of Alum Shale before it terminated at a depth of 250.3 m in the Lower Cambrian Norretorp Mem- ber of the Læså Formation. Th e Sommerodde-1 well docu- ments that the Lower Silurian Cyrtograptus shale is at least 91.7 m thick and that the Rastrites shale is 76.4 m thick. Th e complete Lower Cambrian succession has previously been covered by the 316.0 m deep Borggård-1 well that terminated in basement rocks (Nielsen et al. 2006). The Lower Palaeozoic in Denmark Th e Lower Palaeozoic is very thick in Denmark, locally more than 4 km thick, and the deposition marks the transition from passive margin to a rapidly subsiding foreland basin de- veloped in front of the Caledonian orogen towards the south and west (Lassen & Th ybo 2012). Th e Palaeozoic strata on- shore Bornholm have been described in papers published over the last 150 years and constitute a classical topic in Danish geology. However, previous studies have primarily focused on biostratigraphical aspects. Contemporaneous strata through- out most of central and western Denmark are deeply buried and hence, have only been reached by a few deep exploration wells (Fig. 1), and thus the succession is rather poorly known. Only in Slagelse-1 and Terne-1 in Kattegat have the Palaeo- zoic shales been fully penetrated whereas only the topmost parts of the Silurian shales were reached by the Rønde-1, Nøv- ling-1, Pernille-1 and Stina-1 wells (Fig.  1). A new shale gas exploration well, Vendsyssel-1, is planned to be drilled in 2015 and is expected to penetrate the entire Lower Palaeozoic shale sequence in northern Jylland (Fig. 1). The Sommerodde-1 well Th e well was drilled as close as technically possible to the Sommerodde beach locality described by Bjerreskov & Jør- gensen (1983) where the youngest Silurian strata onshore Bornholm are exposed at low tide. Prior to drilling, a seismic section was recorded in April 2012 and relevant water well Sweden Lower Palaeozoic strata Caledonian front Borehole Ringkøbing–Fyn High Pernille-1 Kattegat Terne-1 Vendsyssel-1 Rønde-1 Nøvling-1 Slagelse-1 Stina-1 Denmark Norwegian–Danish Basin 50 km Germany Bornholm Fig. 2 Jylland Fig. 2. Geological map of southern Bornholm showing the positions of sci- entific wells drilled 2005–2012. Borggård-1 (DGU 247.627), Skelbro-2 (DGU 246.817), Billegrav-2 (DGU 248.61), Hjulmagergård-1 (DGU 246.838) and Sommerodde-1 (DGU 248.62) are shown as red dots, and the re-logged water wells Sømarken-3 (DGU 248.39), Sømarken-4 (DGU 247.312) and Golf banen-1 (DGU 248.54) are shown as blue dots. Areas where 2D seismic data were acquired are shown as black rectangles. The geological map is from a compilation by Graversen (2009). Nexø Sommerodde-1 Billegrav-2 Golfbanen Sømarken-4 Sømarken-3 Borggård-1 Precambrian Mesozoic Palaeozoic Fault 2 km 15°5´E 54°59.2´N Hjulmagergård-1 Skelbro-2 Fig. 1. Distribution of Lower Palaeozoic strata and deep wells reaching at least top Silurian in Denmark north of the Caledonian Front. The Vendsyssel-1 well is planned to be drilled in 2015. © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 9–12 . Open access: www.geus.dk/publications/bull 1010 0 20 40 60 80 100 120 140 160 180 200 220 240 Flow % Fluid conductivity (mS/m) 0 40 80 120 160 3 4 510 100 1000 0 0 500 100 0 400 800 1200 R.M. A lu m S h al e D ic e ll o g r. L in d e g. Log Unit L.C. M .C . U p p e r O rd o v ic ia n L o w e r S il u ri an F u ro n . Q R as tr it e s sh al e P-wave velocity (km/s)Formation resistivity (ohm m)Gamma ray (cps)OPTVLithologyPeriod Strati- graphy L.O. C y rt o gr ap tu s sh al e Casing (scale change) D1 D2 D3 A B2 B3 B4 G1 G5 G4 G2 E3 E1 E2 F1 F5 F4 F3 F2 G3 N.M. Depth (m) Grey shale LimestoneSandstoneBlack shale Dark grey shale Light grey shale Fig. 3. Selected logs measured in the Sommerodde-1 well. The arrows indicate significant water inf low zones. The optic televiewer (OPTV) picture of the well is a 360° view of the borehole wall. Flow %: water f low scaled to the total f low of 1.4 m3/h. Q: Quaternary. LO: Lower Ordovician. Furon.: Furongian. MC: Middle Cambrian. LC: Lower Cambrian. Lindeg.: Lindegård Formation. Dicellogr.: Dicellograptus shales. K: Komstad Limestone. R M: Rispebjerg Member. NM: Norretorp Member of the Læså Formation. cps: counts per second. 11 reports were examined to locate potential faults in the area (Fig. 2). Th e well was drilled in November 2012, well logging was done by both the Geological Survey of Denmark and Greenland and Rambøll and the hole was plugged and per- manently abandoned in November 2013. A total of ten log runs were made including a full wave sonic log and an optic televiewer recording that provided a high resolution image of the borehole wall (Fig. 3). Th e Sommerodde-1 well cored Silurian shales between 6.5 and 174.6 m, Ordovician shale above the Alum Shale Forma- tion between 174.6 and 217.3 m, a thin Komstad Limestone (Middle Ordovician) between 217.3 and 217.9 m, the Cam- bro-Ordovician Alum Shale Formation between 217.9 and 245.8 m and Lower Cambrian sand- and siltstone between 245.8 and 250.3 m (Fig. 3). A total of 168.1 m Silurian shales were thus cored. Th e lower Silurian Rastrites shale was com- pletely cored and is 76.4 m thick in the well. Th e Alum Shale Formation is 27.9 m thick and includes the Middle Cambri- an Andrarum and Exsulans Limestone beds. Th e well was terminated at 250.3 m in the Norretorp Member of the Læså Formation aft er having penetrated the Rispebjerg Member (4.5 m thick). Log stratigraphy in the Sommerodde-1 well Pedersen & Klitten (1990) established a detailed log strati- graphical scheme (labelled units A–G on Figs 3, 4) based on the gamma-ray variation in the Lower Palaeozoic shales on Bornholm. Th e log stratigraphy permitted correlation of un-cored water wells with fully cored scientifi c wells. Th e log stratigraphy was later emended to also include the resistivity log, from which more detailed lithological information can be gained (Schovsbo et al. 2011). In the Sommerodde-1 all log-based units defi ned by Pedersen & Klitten (1990) were identifi ed and the previously un-divided G unit was divided into four new log units, labelled G1–G4 (Fig. 3). Th e units A–F were described by Schovsbo et al. (2011) and, hence, only the new log units are briefl y described here. Th e G unit is correlated here with the Cyrtograptus shale as was originally done by Pedersen & Klitten (1990). Th e base of the G unit has not previously been cored nor recognised in logs from water wells on Bornholm. It is here placed at 98.2 m; above this level the resistivity and sonic velocity log read- ings increase (Figs 3, 4). Th e G1 subunit is 24 m thick and consists of light green to dark grey shales. Th e unit is char- acterised by a steady increase in resistivity readings. Subunit G2 is 18.5 m thick and consists of light grey to dark green shale. Subunit G3 is 16 m thick and consists of light grey to dark green shale. Th e top of the G3 subunit is defi ned where an increase occurs in the resistivity log, refl ecting a change to darker lithologies. Numerous bentonite beds character- ised by low resistivity and high gamma ray readings occur in G2 and G3. Subunit G4 is a 10 m thick, dark-coloured interval. Th e subunit is readily identifi ed in the core and on the optic televiewer log and by a slightly lower gamma-ray log response and distinctly higher resistivity readings compared to the subunits above and below. Th e top of the G4 subunit is defi ned at a point of decrease in the resistivity log. Subunit G5 is 23.5 m thick and comprises light grey to green shale characterised by low and stable gamma-ray values. Th e subu- nit includes numerous silty to sandy beds similar to those ex- posed on the beach just south of the well location (Bjerreskov & Jørgensen 1983). Gamma ray (cps) 40 80 120 160 Log units Gamma ray (cps)Gamma ray (cps) Sommerodde-1 Resistivity (Ohm m) 100 1000 Resistivity (Ohm m) 10 100 1000 Billegrav-2Billegrav-2 D ep th (m ) 0 20 40 60 80 100 120 140 160 180 0 40 80 120 160 50 100 150 200 20 40 60 D ep th (m ) D ep th (m ) 0 20 40 60 Golfbanen-1 G2 G1 G5 G4 G3 F2 F1 F3 F4 F5 E3 Fig. 4. Correlation of the Silurian sections in the Sommerodde-1, Bille- grav-2 and Golf banen-1 wells based on the gamma-ray (black line) and resistivity log (red line) responses. Grey intervals show intervals where the log response was measured through the steel casing. 1212 Correlation of Silurian shales on southern Bornholm Th e G unit was originally defi ned based on the gamma- ray signature in the un-cored water well Golf banen-1 that penetrated a section of the Cyrtograptus shale (Pedersen & Klitten 1990). As part of the present study the Golf banen-1 well was re-logged in order to expand the log signature and to enable correlation with the Sommerodde-1 well (Fig. 4). Based on comparison with the Sommerodde-1 well, the gamma ray and resistivity logs show that the Golf banen-1 well penetrated a sequence from the G3 to the uppermost F5 unit. Th e Sommerodde-1 well is also correlated with the fully cored Billegrav-2 well that penetrated the succession from the lower part of the F5 unit and downwards, thereby clos- ing the correlation gap between the Billegrav-1 well and the exposures in the Øle Å water course as presented by Pedersen & Klitten (1990). Several of the Rastrites shale log units are slightly thicker in the Sommerodde-1 well than in the Bil- legrav-2 well (Fig. 4). In the latter several fault zones occur in this interval and it appears that parts of the section are miss- ing due to faulting. In the Sommerodde-1 well no fault zones are identifi ed and it is believed that the recorded thicknesses refl ect the true stratigraphical thicknesses. Water-flow properties Th e water infl ow in Sommerodde-1 was evaluated from an impeller fl ow log during discharge of 1.4 m3/h supplemented by the fl uid conductivity log measured during the same dis- charge from the well. Together the two logs show that most of the infl ow took place within the uppermost 75 m of the well (Fig. 3). Th e infl ow rates decrease rapidly with depth and the section below 80 m contributes with less than 20% of the total infl ow into the well. Th e water conductivity is highest in the Alum Shale Formation and in the lower part of the Ras- trites shale, 350–400 mS/m, but from a depth of 125 m and upwards several small infl ow sources with lower conductivity stepwise reduce the conductivity of the upwards fl owing wa- ter until it has a conductivity of 50 mS/m (Fig. 3). In spite of the many infl ow points the specifi c yield is quite low, less than 0.1 m3/h/m, suggesting that only a few of the fractures in the shales are open and that these fractures occur within the up- permost 75 m. Similar conditions are seen in many wells pen- etrating Danian limestone and Cretaceous chalk found near the surface, where fracture-based hydraulic conductivity is mainly found in the upper part, less than 70 m below the pre- Quaternary surface. Th is is probably because the fractures were formed due to pressure release in connection with the last deglaciation. In contrast, the deep, water-bearing open fractures on Bornholm, as for instance observed in the Nexø Formation in the Borggård-1 well at a depth of c. 300 m, were probably formed due to tectonic uplift . Conclusions New knowledge on the Palaeozoic geology of Bornholm has been gained since 2005 from fi ve fully cored and geophysi- cally logged scientifi c wells, re-logging of some water wells and acquisition of seismic data. A complete section of the Lower Palaeozoic has been pieced together by correlation of overlapping scientifi c well sections. Th e research on Bornholm has provided detailed insight into the Palaeozoic stratigraphy and established a litho- and log-stratigraphical frame that is applicable in a regional con- text. Combined with ongoing drilling activities in northern Jylland this will provide a much better understanding of the evolution of the Palaeozoic in Denmark. Acknowledgements We are thankful to landowner Jette Staberg, Peter Turner of Faxe Kalk A/S, Kurt Nielsen of Rambøll and Klaus Bauer of GFZ. Th e greater part of the funding was received from Geocenter Denmark and Total E&P Denmark B.V. References Bjerreskov, M. & Jørgensen, K.Å. 1983: Late Wenlock graptolite-bearing tuff aceous sandstone from Bornholm, Denmark. Bulletin of the Geo- logical Society of Denmark 31, 129–149. Graversen, O. 2009: 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. Lassen, A. & Th ybo, H. 2012: Neoproterozoic and Palaeozoic evolution of SW Scandinavia based on integrated seismic interpretation. Precam- brian Research 204–205, 75–104. Nielsen, A.T., Klitten, K. & Hansen, H.P.B. 2006: Borggård-1: en ny stra- tigrafi sk kerneboring på Bornholm. Geologisk Nyt 6, 4–10. Pedersen, G.K. & Klitten, K. 1990: Anvendelse af gamma-logs ved kor- relation af marine skifre i vandforsyningsboringer på Bornholm. Dansk Geologisk Forening Årsskrift 1987–89, 21–35. Schovsbo, N.H., Nielsen, A.T., Klitten, K., Mathiesen, A. & Rasmussen, P. 2011: Shale gas investigations in Denmark: Lower Palaeozoic shales on Bornholm. Geological Survey of Denmark and Greenland Bulletin 23, 9–13. Authors’ addresses N.H.S. & K.K., 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.