The Lower Palaeozoic now fully cored and logged on Bornholm, Denmark

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 shallow 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 penetrated 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 Member of the Læså Formation. Th e Sommerodde-1 well documents 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 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 shallow 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 penetrated 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 Member of the Laeså Formation. Th e Sommerodde-1 well documents 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 onshore 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 throughout 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 Palaeozoic shales been fully penetrated whereas only the topmost parts of the Silurian shales were reached by the Rønde-1, Nøvling-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 & Jørgensen (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  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 permanently 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 Formation between 174.6 and 217.3 m, a thin Komstad Limestone (Middle Ordovician) between 217.3 and 217.9 m, the Cambro-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) Log stratigraphy in the Sommerodde-1 well Pedersen & Klitten (1990) established a detailed log stratigraphical 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 readings 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 characterised 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 characterised 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 subunit includes numerous silty to sandy beds similar to those exposed on the beach just south of the well location (Bjerreskov & Jørgensen 1983).

Correlation of Silurian shales on southern Bornholm
Th e G unit was originally defi ned based on the gammaray signature in the un-cored water well Golfbanen-1 that penetrated a section of the Cyrtograptus shale (Pedersen & Klitten 1990). As part of the present study the Golfbanen-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 Golfbanen-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 closing 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 Billegrav-2 well (Fig. 4). In the latter several fault zones occur in this interval and it appears that parts of the section are missing 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 m 3 /h supplemented by the fl uid conductivity log measured during the same discharge 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 Rastrites 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 water 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 m 3 /h/m, suggesting that only a few of the fractures in the shales are open and that these fractures occur within the uppermost 75 m. Similar conditions are seen in many wells penetrating 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 geophysically 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 context. Combined with ongoing drilling activities in northern Jylland this will provide a much better understanding of the evolution of the Palaeozoic in Denmark.