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    Jan Andsbjerg

    A sequence stratigraphic framework has been erected for the Upper Jurassic of the Danish Central Graben. The framework consists of sequence boundaries, maximum flooding surfaces and transgressive surfaces at different hierarchial levels.... more
    A sequence stratigraphic framework has been erected for the Upper Jurassic of the Danish Central Graben. The framework consists of sequence boundaries, maximum flooding surfaces and transgressive surfaces at different hierarchial levels. The framework is primarily based on correlation of petrophysical logs and detailed palynology, supported by sedimentological interpretation of cores in sandy interwals and by interpreted regional seismic lines in areas with poor well coverage.
    The Middle Jurassic Bryne and Lulu Formations of the Søgne Basin (northern part of the Danish Central Graben) consist of fluvially-dominated coastal plain deposits, overlain by interfingering shoreface and back-barrier deposits. Laterally... more
    The Middle Jurassic Bryne and Lulu Formations of the Søgne Basin (northern part of the Danish Central Graben) consist of fluvially-dominated coastal plain deposits, overlain by interfingering shoreface and back-barrier deposits. Laterally continuous, mainly fining-upwards fluvial channel sandstones that locally show evidence for tidal influence dominate the alluvial/coastal plain deposits of the lower Bryne Formation. The sandstones are separated by units of fine-grained floodplain sediments that show a fining-upwards - coarsening-upwards pattern and locally grade into lacustrine mudstones. A regional unconformity that separates the lower Bryne Formation from the mainly estuarine upper Bryne Formation is defined by the strongly erosional base of a succession of stacked channel sandstones, interpreted as the fill of a system of incised valleys. Most of the stacked channel sandstones show abundant mud laminae and flasers, and rare herringbone structures, suggesting that they were depo...
    A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used... more
    A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isochore maps. In the Lower Jurassic (Hettangian-Pliensbachian) succession, five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the uppermost Aalenian/lowermost Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped. On the basis of the sequence stratigraphic correlation and the construction of isochore maps for individual sequences, the Jurassic basin history of the Danish Central Graben can be subdivided into seven discrete phases: (1) Shallow marine and offshore sediments deposited in a prerift basin extending from the Nor...
    Schiøler, P., Andsbjerg, J., Clausen, O.R., Dam, G., Dybkjær, K., Hamberg, L., Heilmann-Clausen, C., Johannessen, E.P., Kristensen, L.E., Prince, I. & Rasmussen, J.A. 2007: Lithostratigraphy of the Palaeogene – Lower Neogene succession of... more
    Schiøler, P., Andsbjerg, J., Clausen, O.R., Dam, G., Dybkjær, K., Hamberg, L., Heilmann-Clausen, C., Johannessen, E.P., Kristensen, L.E., Prince, I. & Rasmussen, J.A. 2007: Lithostratigraphy of the Palaeogene – Lower Neogene succession of the Danish North Sea. Geological Survey of Denmark and Greenland Bulletin 12, 77 pp. + 5 plates. As a result of a lithological, sedimentological and biostratigraphic study of well sections from the Danish sector of the North Sea, including some recently drilled exploration wells on the Ringkøbing–Fyn High, the lithostratigraphic framework for the siliciclastic Palaeogene to Lower Neogene sediments of the Danish sector of the North Sea is revised. The sediment package from the top of the Chalk Group to the base of the Nordland Group is subdivided into seven formations containing eleven new members. The existing Våle, Lista, Sele, Fur, Balder, Horda and Lark Formations of previously published lithostratigraphic schemes are adequate for a subdivision ...
    During the Mid- to Late Jurassic, long-term eustatic sea-level rise and half-graben subsidence in the Danish Central Trough resulted in a major transgression. Deposition of widespread alluvial plain sediments in the Sogne Basin and Tail... more
    During the Mid- to Late Jurassic, long-term eustatic sea-level rise and half-graben subsidence in the Danish Central Trough resulted in a major transgression. Deposition of widespread alluvial plain sediments in the Sogne Basin and Tail End Graben was terminated in the Late Bathonian due to bypass and erosion following a short-term relative sea-level fall. The sea-level fall was followed by a relative sea-level rise during the Callovian which caused the development of a transgressive succession of coastal plain and shallow marine deposits. The transgressive succession consists of several prograding sediment wedges commonly terminated by a coal bed and a flooding surface. As transgression continued during the Late Jurassic, the shoreline shifted westwards. Transgressive Oxfordian shoreface to back-barrier sandstones were deposited. Another short-term relative sea-level fall occurred during the Kimmeridgian during which regressive shoreface to beach sandstones were deposited. During the Late Kimmeridgian and Volgian, the long-term relative sea-level rise resulted in deposition of thick Volgian offshore claystones in the central part of the Central Trough (Heno Plateau, Gertrud Graben and Feda Graben). During the Volgian and Early Cretaceous, the transgression continued towards the west and shallow marine sandstones may have been deposited along the western margin of the Central Trough fringing the Mid North Sea High (Grensen Nose Basin and Outer Rough Basin). Along the Ringkobing Fyn High in the easternmost part of the Central Trough, Volgian and Early Cretaceous fan-delta sandstones and basin floor turbidite sandstones are interstratified with offshore claystones.
    A complete updated and revised lithostratigraphic scheme for the Jurassic succession of the onshore and offshore Danish areas is presented together with an overview of the geological evolution. The lithostratigraphies of Bornholm, the... more
    A complete updated and revised lithostratigraphic scheme for the Jurassic succession of the onshore and offshore Danish areas is presented together with an overview of the geological evolution. The lithostratigraphies of Bornholm, the Danish Basin and the Danish Central Graben are described in ascending order, and a number of new units are defined. On Bornholm, the Lower–Middle Jurassic coal-bearing clays and sands that overlie the Lower Pliensbachian Hasle Formation are referred to the new Sorthat Formation (Lower Jurassic) and the revised Bagå Formation (Middle Jurassic). In the southern Danish Central Graben, the Middle Jurassic succession formerly referred to the Lower Graben Sand Formation is now included in the revised Bryne Formation. The Lulu Formation is erected to include the uppermost part of the Middle Jurassic succession, previously referred to the Bryne Formation in the northern Danish Central Graben. The Upper Jurassic Heno Formation is subdivided into two new members...
    Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding of the siliciclastic Palaeogene succession in the Danish North Sea sector (Fig. 1). Many of the new wells were drilled... more
    Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding of the siliciclastic Palaeogene succession in the Danish North Sea sector (Fig. 1). Many of the new wells were drilled in the search for oil reservoirs in sand bodies of Paleocene–Eocene age. The existing lithostratigraphy was based on data from a generation of wells that were drilled with deeper stratigraphic targets, with little or no interest in the overlying Palaeogene sediments, and thus did not adequately consider the significance of the Palaeogene sandstone units in the Danish sector. In order to improve the understanding of the distribution, morphology and age of the Palaeogene sediments, in particular the economically important sandstone bodies, a detailed study of this succession in the Danish North Sea has recently been undertaken. An important aim of the project was to update the lithostratigraphic framework on the basis of the new data.The project was ca...
    Complex fabric created by multiple small-scale sand intrusions (light) into dark mudstones – such enigmatic fabrics are commonly associated with the sand-rich units of the Rogaland Group in the Siri Canyon area, offshore Denmark. The... more
    Complex fabric created by multiple small-scale sand intrusions (light) into dark mudstones – such enigmatic fabrics are commonly associated with the sand-rich units of the Rogaland Group in the Siri Canyon area, offshore Denmark. The illustrated section of core is about 10 cm across
    of particular interest in relation to hydrocarbon explo-ration, as they include units of reservoir sandstones as well as oil- and gas-producing source rocks (Fig. 1). Hydrocarbon discoveries have been made in both Middle and Upper... more
    of particular interest in relation to hydrocarbon explo-ration, as they include units of reservoir sandstones as well as oil- and gas-producing source rocks (Fig. 1). Hydrocarbon discoveries have been made in both Middle and Upper Jurassic sandstones, and the Middle Jurassic Harald Field is under production. The distribution of both reservoirs and source rocks reflects the complex
    ABSTRACT Middle Jurassic strata from the Harald gas field in the Danish Central Graben include five coal seams. The precursor mires were situated in coastal plain environments, and peat formation occurred during water table rise related... more
    ABSTRACT Middle Jurassic strata from the Harald gas field in the Danish Central Graben include five coal seams. The precursor mires were situated in coastal plain environments, and peat formation occurred during water table rise related to relative sea level rise. The coals that represent peat accumulation during rapid relative sea level rise are characterized by a higher average hydrogen index, extractability, hydrocarbon yield, and thermally extracted and generated bitumen content (Sâ+Sâ) compared to the coals that represent peat formation during the initial stages of relative sea level rise (slow rate of water table rise). In the latter coals, Pr/Ph ratios are higher, and the content of Cââ steranes is higher and the content of Cââ is lower. With regard to thermal maturity, the coals are in the oil window and may possess the potential to generate and release liquid hydrocarbons. This is supported by the composition of oil samples derived from sandstones in the Harald field. Multivariate data analysis shows a positive correlation between Sâ+Sâ, which is taken to indicate the generative potential of the coals, and in particular the macerals telinite, telocollinite, and cutinite, the microlithotypes vitrite and clarite, and total organic carbon. Such a petrographic composition is favored in precursor mires characterized by continuously waterlogged, anoxic conditions. These conditions are best met in mires situated on the lower coastal plain during rapid relative sea level rise. The knowledge obtained from multivariate modeling of the data, and sedimentological and sequence stratigraphic interpretations of the coal-bearing strata, may thus make it possible to locate the coals with the highest generative potential.
    Cover Complex fabric created by multiple small-scale sand intrusions (light) into dark mudstones – such enigmatic fabrics are commonly associated with the sand-rich units of the Rogaland Group in the Siri Canyon area, offshore Denmark.... more
    Cover Complex fabric created by multiple small-scale sand intrusions (light) into dark mudstones – such enigmatic fabrics are commonly associated with the sand-rich units of the Rogaland Group in the Siri Canyon area, offshore Denmark. The illustrated section of core is about 10 cm across and is from the lower Tyr Member (Lista Formation) in the Cecilie-1B well (2346.8 m). Photograph: Jakob Lautrup.
    A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used... more
    A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the con- struction of isochore maps. In the Lower Jurassic (Hettangian-Pliensbachian) succession, five sequences have been