DGF bulletin volume 71 is a thematic volume dedicated to the results from the LOCRETA project (2017–2021) on aspects of some Lower Cretaceous chalk formations in the North Sea Basin. As for the ordinary DGF bulletin volumes, the articles will be published successively immediately after acceptance and technical production.
Guest editor: Bodil Wesenberg Lauridsen
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Contents
Fabricius, I.L. & Lauridsen, B.W. 2025. LOwer CRETAceous project, summary of results.
Lauridsen, B.W., Lode, S., Sheldon, E., Frykman, P., Anderskouv, K. & Ineson, J. 2022. Lower Cretaceous (Hauterivian–Aptian) pelagic carbonates in the Danish Basin: new data from the Vinding-1 well, central Jylland, Denmark.
Lorentzen, M. C. L., Bredesen, K., Smit, F. W. H., Hansen, T. H., Nielsen, L. and Mosegaard, K. 2022. Mapping Cretaceous faults using a convolutional neural network – A field example from the Danish North Sea.
Jelby, M.E., Ineson, J.R., Thibault, N., Bodin, S., Blok, C.N., Edvardsen, N., Clemmensen, T.S., Buls, T. & Anderskouv, K. 2022. Facies and depositional processes of Lower Cretaceous carbonates, Danish Central Graben.
Ineson, J.R., Petersen, H.I., Andersen, C., Bjerager, M., Jakobsen, F.C., Kristensen, L., Mørk, F. & Sheldon, E. 2022. Early Cretaceous stratigraphic and basinal evolution of the Danish Central Graben: a review.
Blok, C.N., Adatte, T., Ineson, J.R., Sheldon, E., Jelby, M.E., Smit, F.W.H., Lauridsen, B.W., Anderskouv, K. & Bodin, S. 2023. Clay mineral assemblages as a tool in source-to-sink studies: an example from the Lower Cretaceous of the North Sea Basin.
Fabricius, I.L. & Meireles, L.T.P. 2025. Assessment of pore collapse in North Sea Lower Cretaceous carbonates using downhole elastic strain.
Fabricius, I.L., Hjuler, M.L., Jelby, M.E. & Anderskouv, K. 2025. Microscopic character of the Lower Cretaceous Tuxen and Sola formations in the Boje-2C well, Danish Central Graben.
LOwer CRETAceous project, summary of results.
Fabricius, I.L. & Lauridsen, B.W. 2025. LOwer CRETAceous project, summary of results.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 1–6.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2025-71-01
Abstract: We present summary of results from the LOCRETA project, grouped according to scientific approach: 1) Sedimentology and stratigraphy, 2) Structural geology, 3) Petrography and Petrophysics, and 4) Rock mechanics.
Keywords: Chalk, North Sea Basin, sedimentology, rock mechanics, geophysics.
Address: Ida L. Fabricius [ilfa [at] dtu [dot] dk], Department of Environmental and Resource Engineering, Technical University of Denmark, Denmark. Bodil W. Lauridsen [bwl [at] geus [dot] dk]. Geological Survey of Denmark and Greenland, Denmark.
Download PDF (3,15 Mb)Lower Cretaceous (Hauterivian–Aptian) pelagic carbonates in the Danish Basin: new data from the Vinding-1 well, central Jylland, Denmark.
Lauridsen, B.W., Lode, S., Sheldon, E., Frykman, P., Anderskouv, K. & Ineson, J. 2022. Lower Cretaceous (Hauterivian–Aptian) pelagic carbonates in the Danish Basin: new data from the Vinding-1 well, central Jylland, Denmark.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 7–29.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2022-71-02
Abstract: Understanding of the shallow shelf system in the Danish Basin during the Early Cretaceous has benefitted significantly from studying the previously overlooked Hauterivian–Aptian section of the Vedsted Formation of the Vinding-1 drill core. The presence of chalks in this section demonstrates that carbonate-rich pelagic sediment accumulated locally in the siliciclastic-dominated Danish Basin and that benthic carbonate production was insignificant. The area was not a carbonate platform in the Early Cretaceous and does not indicate any reworked carbonate supply from platform environments in the vicinity. The scarcity of benthic macrofossils in the cored section is due to the lack of a specialised boreal chalk fauna at that time, and the adjacent nearshore environment apparently did not support any substantial benthic carbonate production. A revised biostratigraphy of the cored section is presented based primarily on calcareous nannofossils, supported by foraminifera, ostracods, and belemnites. Four lithofacies describe the spectrum from marlstone to slightly marly chalk, and the facies succession characterises four depositional units recording two discrete transgressive–regressive cycles. The study provides a depositional record that permits sequence stratigraphic correlation to the Valdemar and Adda Fields in the Central Graben.
Keywords: Danish Basin, Lower Cretaceous, chalk and marl, old wells, biostratigraphy.
Address: Bodil W. Lauridsen [bwl [at] geus [dot] dk], Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Stefanie Lode [stefanie [dot] lode [at] ntnu [dot] no], Department of Geoscience and Petroleum, Norwegian University of Science and Technology, Postbox 8900, NO-7491 Trondheim, Torgarden, Norway. Emma Sheldon, [es [at] geus [dot] dk], GEUS. Peter Frykman [pfr [at] geus [dot] dk], GEUS. Kresten Anderskouv [ka [at] ign [dot] ku [dot] dk], Department of Geosciences and Natural Resource Management, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark and Jon Ineson [ji [at] geus [dot] dk], GEUS.
Download PDF (27,99 Mb)Mapping Cretaceous faults using a convolutional neural network – A field example from the Danish North Sea.
Lorentzen, M. C. L., Bredesen, K., Smit, F. W. H., Hansen, T. H., Nielsen, L. and Mosegaard, K. 2022. Mapping Cretaceous faults using a convolutional neural network – A field example from the Danish North Sea.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 31–50.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2022-71-03
Abstract: The mapping of faults provides essential information on many aspects of seismic exploration, characterisation of reservoirs for compartmentalisation and cap-rock integrity. However, manual interpretation of faults from seismic data is time-consuming and challenging due to limited resolution and seismic noise. In this study, we apply a convolutional neural network trained on synthetic seismic data with planar fault shapes to improve fault mapping in the Lower and Upper Cretaceous sections of the Valdemar Field in the Danish North Sea. Our objective is to evaluate the performance of the neural network model on post-stack seismic data from the Valdemar Field. Comparison with variance and ant-tracking attributes and a manual fault interpretation shows that the neural network predicts faults with more details that may improve the overall geological and tectonic understanding of the study area and add information on potential compartmentalisation that was previously overlooked. However, the neural network is sensitive to seismic noise, which can distort the fault predictions. Therefore, the proposed model should be treated as an additional fault interpretation tool. Nonetheless, the method represents a state-of-the-art fault mapping tool that can be useful for hydrocarbon exploration and CO2 storage site evaluations.
Keywords: Machine learning, fault detection, cap-rock integrity, reservoir modelling, Cretaceous, Danish North Sea.
Address: 1Department of Geophysics and Sedimentary Basins, Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark. 2Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, Aarhus C, Denmark. 3Depart-ment of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark. 4Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark. *Correspondence: mlo [at] geus [dot] dk.
Download PDF (11,60 Mb)Facies and depositional processes of Lower Cretaceous carbonates, Danish Central Graben.
Jelby, M.E., Ineson, J.R., Thibault, N., Bodin, S., Blok, C.N., Edvardsen, N., Clemmensen, T.S., Buls, T. & Anderskouv, K. 2022. Facies and depositional processes of Lower Cretaceous carbonates, Danish Central Graben.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 51–74.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2022-71-04
Abstract: The Lower Cretaceous Tuxen (lower Hauterivian – upper Barremian) and Sola (upper Barremian – Albian) Formations in the Danish Central Graben (North Sea) constitute one of the oldest chalk successions recorded globally, but have received little attention with regards to sedimentary facies and depositional processes. This study presents the first comprehensive carbonate facies analysis of the succession, retrieved from seven drill cores from the Valdemar and Adda Fields. A total of 50 facies are identified, based on a continuum of six lithologies ranging from chalk to marlstone and tuffaceous siltstone to sandstone that display eight different sedimentary structures or fabrics, and two redox-associated lithological color variations (green and red) in the Adda Field. The eight sedimentary structures record: (i) comprehensive bioturbation of homogeneous sediment during fully oxygenated benthic conditions and low sedimentation rates; (ii) a similar bioturbation process but in heterogeneous sediment with lithological contrasts permitting visible burrows to form, perhaps due to rhythmic alternation between pelagic (clay-poor) and hemipelagic (clay-rich) sedimentation; (iii) pelagic to hemipelagic suspension settling in dysoxic to anoxic bottom-water conditions; (iv) patchy cementation of the shallow sea bed during incipient hardground formation; (v) reworking of bioclasts and chalk intraclasts by bottom or wave-induced currents and cohesive debris flows; (vi) pressure solution during late burial diagenesis; (vii) shear deformation by intense plastic deformation of unlithified sediment from limited lateral displacement; and (viii) silicification during burial diagenesis. The facies distribution indicates that active tectonism took place prior to the onset of anoxia that resulted in deposition of the Munk Marl Bed, which in the Valdemar Field was followed by tectonic waning and repeated anoxia. The Valdemar Field constituted a basinal depocenter and was flanked to the east by an early inversion high in the Adda Field characterized by condensation and bypass. The Fischschiefer Member represents a return to prevailing anoxia, consistent with global records of the early Aptian Oceanic Anoxic Event 1a (OAE-1a).
Keywords: Aptian, Barremian, chalk, marlstone, Sola Formation, Tuxen Formation.
Address: Mads E. Jelby [madsjelby [at] gmail [dot] com], Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Jon R. Ineson [ji [at] geus [dot] dk], Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Nicolas Thibault [nt@ ign.ku.dk], Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Stéphane Bodin [stephane [dot] bodin [at] geo [dot] au [dot] dk] and Carlette N. Blok [carletteblok [at] geo [dot] au [dot] dk], Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark. Niklas Edvardsen [niklas [dot] edvardsen [at] gmail [dot] com], Tatjana S. Clemmensen [tatjanasc [at] gmail [dot] com], Toms Buls [toms [dot] buls [at] gmail [dot] com] and Kresten Anderskouv [ka [at] ign [dot] ku [dot] dk], Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Download PDF (15,60 Mb)Early Cretaceous stratigraphic and basinal evolution of the Danish Central Graben: a review.
Ineson*, J.R., Petersen, H.I., Andersen, C., Bjerager, M., Jakobsen, F.C., Kristensen, L., Mørk, F. & Sheldon, E. 2022. Early Cretaceous stratigraphic and basinal evolution of the Danish Central Graben: a review.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 75–98.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2022-71-05
Abstract: An integrated seismic–stratigraphic study of the Lower Cretaceous Cromer Knoll Group was undertaken as part of a recent comprehensive analysis of the Upper Jurassic – lowermost Cretaceous petroleum system in the Danish Central Graben. This study of the basal group of the post-rift package yielded an updated regional assessment of the distribution of the Valhall, Tuxen, Sola and Rødby Formations. This is documented by four high resolution isochore maps (presented here) that record temporal shifts in subsidence patterns from the latest Ryazanian to the earliest Cenomanian. The distribution and thickness variation of the mud-dominated Valhall Formation (latest Ryazanian – early Hauterivian) at the base of the group attests to the progressive fill of inherited syn-rift morphology. The dominant depositional theme is thus ponding in, and onlap from, the main inherited depocentres, although growth faults and incipient inversion locally controlled stratigraphic architecture, and new depocentres were initiated in the east of the graben (Ål and Outer Rough Basins). The isochores for the succeeding, increasingly chalk-rich Tuxen, Sola and Rødby Formations (Hauterivian – earliest Cenomanian) document the regional weakening of syn-rift patterns but emphasize the shift in sedimentation patterns controlled by accelerating inversion activity in the east (Adda–Tyra area, Søgne Basin) and increased local subsidence. The latter sometimes coincided with syn-rift depocentres, such as the Roar Basin and the Arne-Elin Graben, but was also significant in the new Early Cretaceous depocentres in the west of the graben, particularly the Outer Rough Basin. The evolution of the Early Cretaceous basin recorded by this dataset reveals significant shifts in the subsidence pattern in the late Valanginian – early Hauterivian, in large part due to early inversion in the east, and during the late Aptian – early Albian when subsidence was focussed in central and western sub-basins, probably locally due to salt withdrawal. These events, in combination with sea-level change, had implications for the accumulation and preservation of Barremian and early Aptian reservoir chalks.
*Corresponding author; email: ji [at] geus [dot] dk
Keywords: Danish Central Graben, North Sea, Cromer Knoll Group, Lower Creta-ceous, updated seismic mapping, lithostratigraphy, subsidence history.
Address: Jon R. Ineson [ji [at] geus [dot] dk], Henrik, I. Petersen [hip [at] geus [dot] dk], Claus Andersen [ca@geus. dk], Morten Bjerager [mbj [at] geus [dot] dk], Finn C. Jakobsen [fj [at] geus [dot] dk], Lars Kristensen [lk@ geus.dk], Finn Mørk [fm [at] geus [dot] dk], Emma Sheldon [es [at] geus [dot] dk], Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Download PDF (14,65 Mb) Download supplementary file A (4,46 Mb) Download supplementary file B (11,06 Mb)Clay mineral assemblages as a tool in source-to-sink studies: an example from the Lower Cretaceous of the North Sea Basin.
Blok, C.N., Adatte, T., Ineson, J.R., Sheldon, E., Jelby, M.E., Smit, F.W.H., Lauridsen, B.W., Anderskouv, K. & Bodin, S. 2023. Clay mineral assemblages as a tool in source-to-sink studies: an example from the Lower Cretaceous of the North Sea Basin.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 99–113.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2023-71-06
Abstract: The alternating marlstone and chalk of the Lower Cretaceous succession in the Danish Central Graben (DCG) are important for the understanding of the evolution of the larger North Sea Basin. This study focusses on the clay mineral assemblages of the upper Hauterivian – lower Aptian in the DCG and Danish Basin (DB) and their implications. Clay mineral assemblages are predominantly used to assess palaeoclimate. In this study, however, they were additionally used in a source-to-sink context. Kaolinite was found to form a dominant component of the clay mineral assemblage in the sampled wells of the DCG and in the DB, suggesting that a feldspar- or kaolinite-rich source was present and actively eroded in the region during the Early Cretaceous. Moreover, a decreasing gradient west to east of average kaolinite content is observed in the three studied wells for the early Hauterivian to late Barremian (BC9-BC17), with the highest content observed in the North Jens-1 well (av. 74%), followed by the Boje-2C well (av. 49%) and lastly in the Vinding-1 well (av. 39%). Due to the relatively rapid settling of kaolinite in marine environments compared to other clay minerals, this gradient suggests that the main clay mineral source was located in the south-western part of the DCG. Isochore maps, a new palaeogeographic map of the DCG and the western part of the German sector of the North Sea illustrates where Lower Cretaceous rocks are absent in this region, due to either erosion or non-deposition. Potential subaerially exposed highs included the distant Baltic Shield to the north, the Ringkøbing–Fyn High to the east and the Heno Plateau within the DCG, with the latter being located closest to the North Jens-1 well and containing feldspar-rich sandstones of the Heno Formation (upper Kimmeridgian – lowermost Volgian/Tithonian). During the Early Cretaceous, part of the Heno Formation was potentially subaerially exposed or subject to wave reworking/erosion in parts of the Danish and German sectors. The sandstones could weather into kaolinite and this structural high is therefore suggested to have been the main source area for this part of the DCG, with minor sediment influxes from the Ringkøbing–Fyn High and Baltic Shield. In addition, the overall decrease in kaolinite in the DCG from the late Hauterivian to the late Barremian indicates a climatic change towards drier conditions, with some minor, slightly more humid periods.
Keywords: Barremian, Danish Central Graben, Heno Plateau, kaolinite, source-to-sink, Tuxen Formation.
Address: Carlette N. Blok [carletteb [at] unis [dot] no], Department of Arctic Geology, The University Centre in Svalbard, P.O. Box 156, N-9171 Longyearbyen, Norway. Thierry Adatte [thierry.adatte@ unil.ch], Institute of Earth Sciences (ISTE), University of Lausanne, 1015 Lausanne, Switzerland. Jon R. Ineson [ji [at] geus [dot] dk], Emma Sheldon [es [at] geus [dot] dk], Florian W.H. Smit [fs@ geus.dk], Bodil W. Lauridsen [bwl [at] geus [dot] dk], Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Mads E. Jelby [mads.e.jelby@uib. no], Department of Earth Science, University of Bergen, Allégaten 41, P.O. Box 7803, N-5020 Bergen, Norway. Kresten Anderskouv [ka [at] ign [dot] ku [dot] dk], Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Stéphane Bodin [stephane [dot] bodin [at] geo [dot] au [dot] dk], Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark.
Download PDF (1,37 Mb) Download supplementary file (21,78 Kb)Assessment of pore collapse in North Sea Lower Cretaceous carbonates using downhole elastic strain.
Fabricius, I.L. & Meireles, L.T.P. 2025. Assessment of pore collapse in North Sea Lower Cretaceous carbonates using downhole elastic strain.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 115–125.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2025-71-07
Abstract: The potential for pore collapse is a concern when producing hydrocarbons from fragile sedimentary rocks, like chalks. The reason is that although pore collapse can be a valuable drive mechanism, it can also cause large scale compaction and surface subsidence. This risk is as a rule evaluated from results of rock mechanical testing of core samples, but because a given reservoir typically comprises layers of different porosity, the stress required for fracturing or pore-collapse is then generalized as a function of porosity. Due to different diagenetic history, different reservoirs can have different degree of cementation (induration) for a given porosity, so each reservoir requires a separate stressporosity relationship. Recent studies of chalk indicate that pore collapse happens at a critical elastic strain irrespective of porosity, pore fluid, and induration. This should not be surprising, bearing in mind that the strain of the rock represents an equal strain of the mineral crystals constituting the rock frame, so that onset of pore collapse in chalk is governed by the in situ elastic strain of the calcite. Chalk can be rich in minerals like clay or quartz, but when calcite constitutes the frame, it is calcite that is elastically strained. The simple point is that elastic strain of the calcite crystals is the same as the elastic strain of the rock. A complication with rock mechanical data is that the tested material has been brought to the surface by coring, a process that typically causes tested rocks to contain microcracks (small unloading fractures), so that the tested rocks do not truly represent the rock in situ. This issue points to using downhole deformation data, but these are sparse. A possibility is then to use information from geophysical well logs, because in situ elastic strain can be calculated from well log data, so that the results from rock mechanical testing can be generalized in a simple way. The method for strain calculation is here applied to data from the Valdemar field, which produces hydrocarbons from the Tuxen and Sola formations, a clay-rich Lower Cretaceous chalk interval in the Central North Sea.
Keywords: Aptian, Barremian, Cretaceous, chalk, marlstone, rock physics, pore collapse, small strain.
Address: Ida L. Fabricius [ilfa [at] dtu [dot] dk] and Leonardo T. P. Meireles*, Department of Environmental and Resource Engineering, Technical University of Denmark, Denmark. *present address: [LETE@ equinor.com].
Download PDF (8,89 Mb)Microscopic character of the Lower Cretaceous Tuxen and Sola formations in the Boje-2C well, Danish Central Graben.
Fabricius, I.L., Hjuler, M.L., Jelby, M.E. & Anderskouv, K. 2025. Microscopic character of the Lower Cretaceous Tuxen and Sola formations in the Boje-2C well, Danish Central Graben.
Bulletin of the Geological Society of Denmark, Vol. 71, pp. 127–162.
ISSN 2245-7070. https://doi.org/10.37570/bgsd-2025-71-08
Abstract: The Lower Cretaceous Tuxen and Sola formations in the Danish Central Graben (North Sea) constitute pelagic chalks and marlstones, which locally are hydrocarbon-bearing. Based on previous identification of 21 sedimentary facies in the cored Boje-2C well of the Boje Field, 66 samples were analysed through back scattered electron microscopy to delineate how the microscopic character reflects the macroscopic sedimentary facies and to identify indications of diagenesis. For most samples, microscopic characters can clearly be related to the previously identified sedimentary facies. Bioturbation and shear deformation, however, were not consistently reflected in the microscopic character. In all samples, the microtexture is characterised by massive or laminated calcareous mudstone with varying content of mainly kaolinitic clay and only sparse remains of microfossils or shell fragments, whereas particles of quartz silt and kaolinitic claystone silt are common. Porosity is mainly interparticular, typically reaching 25% in the Sola and upper Tuxen formations, and declining with depth to ca. 17% in the lower Tuxen and upper Valhall formations. Diagenesis is reflected in the occurrence of calcite cementation, pyrite and calcite-rimmed dolomite, and some pores may represent moulds after silicious microfossil fragments, which probably sourced the locally observed silicification of the sediment. Further, in clay-poor intervals below the oil zone, a significant diagenetic feature constitutes biogenic calcite mud particles that become increasingly recrystallized and euhedral with depth. It is therefore possible that burial-related crystal ripening was delayed in the oil zone represented by the upper part of the core. In the underlying transition zone, which in the Boje-2C well comprises most of the Tuxen Formation, the degree of recrystallization increases with depth as water saturation increases. Pressure dissolution features were noted in clay-rich intervals, but proper stylolites were not encountered, reflecting the generally high clay content of these marly chalks.
Keywords: Aptian, Barremian, chalk, diagenesis, marlstone, SEM.
Address: Ida L. Fabricius [ilfa [at] dtu [dot] dk] and Morten L. Hjuler, Department of Environmental and Resource Engineering, Technical University of Denmark, Denmark. Mads E. Jelby [jelby [at] geo [dot] au [dot] dk], Department of Geoscience, Aarhus University, Aarhus, Denmark. Kresten Anderskouv [ka [at] ign [dot] ku [dot] dk], Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark.
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