Upper Jurassic reservoir sandstones in the Danish Central Graben: new insights on distribution and depositional environments

P. N. Johannessen, K. Dybkjær, C. Andersen, L. Kristensen, J. Hovikoski and H. Vosgerau
Geological Society, London, Petroleum Geology Conference series, 7, 127-143, 4 January 2011, https://doi.org/10.1144/0070127

Abstract

Recently available well data from the northern part of the Danish Central Graben have been analysed to further understand the basin development, biostratigraphy, depositional models and palaeogeography of Upper Jurassic reservoir sandstones, which are the primary exploration targets in this basin. Notably, the discovery of the Hejre accumulation in 2001, where oil has been encountered in Upper Jurassic good reservoir quality sandstones at a depth of more than 5000 m, triggered renewed interest in the Upper Jurassic High Temperature–High Pressure sandstone play in the area.

Overall the Danish Central Graben was transgressed from east to west during the Late Jurassic. During the Late Kimmeridgian, marginal and shallow marine sandstones assigned to the Heno Formation were deposited at the margin of the Feda Graben, and on the Gertrud and Heno Plateaus and constitute the reservoirs in the Freja and Hejre discoveries. The sandstones are analogues to the UK Fulmar and Norwegian Ula Formations encountered in several hydrocarbon fields.

During the Early Volgian, the transgression continued westwards across the Outer Rough Basin along the margin of the Mid North Sea High, where shoreface sandstones with excellent porosities and permeabilities were deposited close to similar sandstones of the Fulmar Formation in the British Fergus, Fife and Angus fields.

During this overall westward transgression, the eastern and central parts of the Danish Central Graben continued to subside and offshore mudstones accumulated, locally intercalated with gravity-flow sandstones. In the easternmost Danish Central Graben, in the Tail End Graben, Upper Kimmeridgian gravity-flow sandstones of the Svane-1 well have proved the presence of gas at c. 6 km depth.

Hydrocarbon-bearing Upper Jurassic sandstone reservoirs at significant depths (deeper than 5 km) may form the future exploration targets in the northern part of the Danish Central Graben.

Keywords

Compared with neighbouring areas in Norwegian and UK waters, where production takes place from shallow marine Ula and Fulmar Formations (Fraser et al. 2003), the Upper Jurassic sandstone play in the Danish sector has historically been less successful, due mainly to large variations in reservoir quality. A number of technical discoveries have been made and two accumulations have been declared commercial although not yet in production. These are the Freja Field on the Gert Ridge, located near the Norwegian border with reservoirs in Upper Kimmeridgian back-barrier and shoreface sandstones of the Heno Formation, and the Elly Gas Field located in the south close to the German sector hosted in Upper Kimmeridgian shoreface sandstones of the Heno Formation (Figs 1 & 2) (Hemmet 2005).

Fig. 1.
Fig. 1.

Structural map of the Danish Central Graben. Location of all wells comprising Upper Jurassic deposits is shown. Position of the geotraverse in Figure 4 is shown in yellow, while the positions of the seismic lines shown in Figures 14 and 15 are shown in red-brown and pink, respectively. The positions of the log-panels shown in Figures 6–8 are shown in red, blue and green, respectively. Inserted map shows the Central Graben, Viking Graben and Moray Firth Basin.

Fig. 2.
Fig. 2.

Time stratigraphic chart covering the Upper Jurassic and Lower Cretaceous succession in the Danish Central Graben.

In 2001, oil was encountered in sandstones of the Heno Formation in the Hejre-1 well located on a tilted fault block in the Gertrud Graben area at a depth below 5000 m. The discovery was appraised down-flank by the Hejre-2 well in 2005, where good production rates of oil and associated gas were recorded (Danish Energy Agency 2005). The Svane-1 well located in a High Temperature–High Pressure (HTHP) environment in the northern part of the Tail End Graben was drilled in 2002 to a total depth of 5865 m. Four sandstone-bearing successions of presumed gravity-flow origin of Upper Kimmeridgian to Lower Volgian age were found below 5300 m with high gas saturations. The net sand thickness is about 150 m but individual sandstones are thin (up to 15 m). The recent deep-seated Hejre and Svane discoveries have triggered renewed interest in the Upper Jurassic HTHP sandstone plays in the area.

The aim of this paper is to present an updated overview of the stratigraphy, sand distribution, depositional environments and reservoir characteristics of the Upper Jurassic potential reservoir sandstones in the northern part of the Danish Central Graben. A series of palaeogeographic maps illustrates the basin evolution during the Late Kimmeridgian to Ryazanian.

Structural setting

The northern Danish Central Graben study area forms part of the Jurassic North Sea rift complex (Fig. 1). The basin consists of a system of NNW–SSE trending half-grabens bounded by the Coffee Soil Fault to the east (western margin of the Ringkøbing–Fyn High), and by the Mid North Sea High to the west (e.g. Japsen et al. 2003; Møller & Rasmussen 2003). Rifting took place from Middle Jurassic times and persisted into the Early Cretaceous. The syn-rift sedimentary fill is mudstone-dominated, rich in organic matter at certain intervals and with subordinate amounts of sandstones (Petersen et al. 2010). The early development was characterized by fault-controlled subsidence and deposition in the eastern part, especially along north–south segments of the Coffee Soil Fault (Korstgaard et al. 1993; Bruhn & Vagle 2005). During the Kimmeridgian, the tectonic trend shifted to a dominant NW–SE trend; the depocentres shifted westwards and deposition gradually covered larger areas, reaching the flanks of the Mid North Sea High during the Early Volgian. Despite the overall extensional tectonic regime, compressional features occur, caused by oblique-slip movements along different graben segments.

The structural control on deposition is illustrated by the Upper Jurassic isochore map in Figure 3 (Britze et al. 1995) and by the simplified structural profile across the northern part of the study area (Fig. 4). The dominant Jurassic structural element is the Tail End Graben in the east containing up to 3600 m of Upper Jurassic sediments dominated by claystones. It grades into the Søgne Basin in the north with reduced sediment thicknesses. Towards the west, the Tail End Graben is separated from the Heno Plateau with westward thinning Jurassic cover by the Arne–Elin Graben (Fig. 1). The Heno Plateau passes northwards into the Gertrud Plateau/Graben, which is separated from the southwestward dipping Feda Graben by the Gert Ridge. In both graben segments, the thickness of Upper Jurassic strata locally exceeds 1500 m. The Early Cretaceous Ål Basin and the shallow Outer Rough Basin are located west of the Inge High in which Jurassic strata are thin or absent (Figs 1, 3 & 4) (Britze et al. 1995).

Fig. 3.
Fig. 3.

Upper Jurassic isochore map of the Danish Central Graben (Britze et al. 1995).

Fig. 4.
Fig. 4.

Simplified structural profile across the northern part of the Danish Central Graben. For location see Figure 1.

Methods

The chronostratigraphic framework for the studied succession is primarily based on first and last occurrences of dinoflagellate cyst (dinocyst) species (Fig. 5). Each bioevent has been given a number in order to be able to show the location of that specific bioevent in the wells included in the log panels (Figs 6⇓–8). The biostratigraphic data have been compiled from published papers (Johannessen et al. 1996; Dybkjær 1998; Andsbjerg & Dybkjær 2003; Ineson et al. 2003), unpublished data and service company reports. Spores and pollen were included in the Rita-1 and Gwen-2 wells to support the stratigraphic interpretations. In addition, new biostratigraphic and palynofacies investigations have been carried out on cuttings and core samples for the present study. In the present study we have not correlated the studied succession with the ‘genetic stratigraphic sequences’ defined by Partington et al. (1993a, b), although this subdivision is used by several companies operating in the Danish Central Graben. The biostratigraphic events used for dating the sequences (and especially the maximum flooding surfaces) by Partington et al. (1993b) was mainly based on data from the Outer Moray Firth (pers. comm. M. Partington 1995). In many cases the biostratigraphic events used by Partington et al. (1993b) are acmes of specific species and these acmes do not occur in the same stratigraphic levels and the same order in the Danish Central Graben. Furthermore, the last occurrences (‘species top’ by Partington et al. 1993b) in some occasions occur in a different order in the Danish Central Graben than are indicated by Partington et al. (1993b).

Fig. 5.
Fig. 5.

Chronostratigraphic scheme showing the correlation between the bioevents (first and last occurrences of dinocyst species) recorded in the studied wells, the boreal ammonite zonation and the chronostratigraphy. Ages after Gradstein et al. (2004). The correlation is based on Davey (1979, 1982), Cox et al. (1987), Heilmann-Clausen (1987), Riding (1987), Poulsen (1991), Poulsen & Riding (1992), Riding & Thomas (1992) and Costa & Davey (1992). Note that each bioevent has been given a number in order to be able to show the location of that specific bioevent in the wells included in the log panels (Figs 6–8).

Fig. 6.
Fig. 6.

Log correlation panel. The numbers shown refer to specific bioevents recorded in the wells, see Figure 5. Position of line is shown in Figure 1. For legend see Figure 7.

Fig. 7.
Fig. 7.

Log correlation panel. The numbers shown refer to specific bioevents recorded in the wells, see Figure 5. Position of line is shown in Figure 1.

Fig. 8.
Fig. 8.

Log correlation panel. The numbers shown refer to specific bioevents recorded in the wells, see Figure 5. Position of line is shown in Figure 1. For legend see Figure 7.

The sedimentology of the cores has been studied in detail, focussing particularly on trace fossil assemblages, because most primary sedimentary structures are not preserved due to intensive bioturbation. The interpreted depositional environments from the core studies have been extrapolated via petrophysical well logs above and below the cored sections. Utilizing the new sedimentological and biostratigraphical data, an updated time stratigraphic chart has been constructed to show the overall depositional history of the northern Danish Central Graben (Fig. 2).

Log-correlation panels have been constructed to show the interrelations and distribution of depositional environments within the basin (Figs 6–8) and schematic palaeogeographical summary maps are created (Fig. 9). Note that the term ‘shoreface’ is used in this paper in a broad sense to cover various wave-influenced and wave-dominated shallow marine environments. Porosity–permeability plots are presented from cores and compared with poroperm data from the nearby Fergus and Fife fields of the UK sector.

Fig. 9.
Fig. 9.

Series of palaeogeographic maps illustrating facies distributions and basin evolution during the Late Kimmeridgian to Ryazanian.

Upper Jurassic reservoir sandstones

Initial subsidence along the Coffee Soil Fault, the eastern boundary of the Danish Central Graben, during the Middle Jurassic resulted in deposition of paralic to shallow marine sandstones in the Søgne Basin and in the Tail End Graben (Andsbjerg 2003; Andsbjerg & Dybkjær 2003). Westward propagation of active NW–SE trending normal faults occurred during the Late Jurassic. This was coeval with a eustatic sea-level rise and resulted in an overall stepwise transgression towards the west (Fig. 9a–g) (Andsbjerg & Dybkjær 2003; Johannessen 2003).

The overall depositional environments, the distribution and the reservoir characteristics of the different Upper Jurassic sandstones in the Danish Central Graben will be presented chronologically in the following sections, based on published literature and unpublished data from new wells.

Upper Kimmeridgian paralic and shoreface sandstones

Distribution and stratigraphy

The sandstones referred to as the Gert Member of the Heno Formation (Fig. 2) (Michelsen et al. 2003) were deposited in the marginal areas of the Feda Graben and on the Gertrud and Heno Plateaus during the Late Kimmeridgian (Fig. 9a, b) (Johannessen et al. 1996; Andsbjerg & Dybkjær 2003; Johannessen 2003). The succession is represented by cores in the Gert-1, Rita-1 and 2/12-1 wells in this paper (Fig. 6).

Depositional environments

The cored succession in the Gert-1 well, situated at the margin of the Feda Graben up against the Gertrud Plateau (Fig. 1), is composed of sandstones interbedded with thin claystone and coal beds often underlain by rootlets. Spores and pollen are abundant, and numerous burrows of Ophiomorpha are observed but dinoflagellate cysts are scarce; together these features suggest deposition in a back-barrier setting (Johannessen et al. 1996; Johannessen 2003). Cross-bedded sandstones that overlie claystones and are capped by claystones or coal beds (sometimes with rootlets) were probably deposited by washover fans (Fig. 10a) (Schwartz 1982). The gamma-ray (GR) and sonic log motif is highly serrated (Fig. 11). Above the cored section, the log motif is non-serrated, suggesting that a cleaner sandstone unit abruptly overlies the back-barrier sediments, indicating deposition during high energy levels on the shoreface. The uppermost part of the sandy section shows a fining-upward, back-stepping shoreface sandstone succession which fines upwards to offshore claystones of the Lola Formation, indicating an overall transgressive succession (Fig. 6) (Johannessen 2003).

Fig. 10.
Fig. 10.

Core photos of selected facies. (a) A sediment succession consisting of cross-stratified sand (1) grading upward into root-bearing sand (2) and further into a coal-layer (3). This succession overlies organic rich mudstones and is interpreted to represent a washover fan in a back-barrier setting. Gert Member, Gert-1 well. (b) Intra- and extraformational clast (1 and 2, respectively) bearing conglomeratic bed interbedded with laminated claystones (3). Farsund Formation, Tabita-1 well. (c) A sharp-based gravel bed interbedded with massive appearing fine-grained sandstone. The sandstone facies bear common outsized clasts and mud-drapes. Ravn Member, Ophelia-1 well. (d) Bioturbated muddy sandstone. Characteristic trace fossils include abundant Asterosoma (white arrows), which is re-burrowed with Chondrites (black arrow). Dashed white line indicates the location of a central vertical tube. Ravn Member, Rita-1 well. (e) Tangentially cross-stratified sandstone. ‘Outer Rough Sand’, Saxo-1 well.

Fig. 11.
Fig. 11.

Porosity and oil saturation plot for back-barrier and shoreface sandstones of the Gert Member in the Gert-1 well. PHIE is the effective porosity (highlighted by grey colour fill) as interpreted from the well log data. SW is the water saturation estimated on the basis of the porosity evaluation, resistivity log data and a set of petrophysical parameters representing the sandstone layers. The oil saturation (green colour fill) is calculated as 1-SW. For legend see Figure 7.

It is very unusual to develop and preserve such c. 60 m thick back-barrier sediments as seen in the Gert-1 well. This thick accumulation resulted firstly because there was a sand surplus and secondly because they were accumulated at the steep margins of the Feda Graben towards the plateau areas during a transgression, forcing the back-barrier sediments and the shoreface sandstones to stack vertically (Johannessen et al. 2008, 2010). The sandstones of the Gert Member were probably derived from local sediment sources during the transgression, such as the exposed Carboniferous sediments in the Gert Ridge area and sediments on the Heno and Gertrud Plateaus (Johannessen et al. 1996).

The Norwegian 2/12-1 well, situated 2.5 km NW of the Gert-1 well, farther out in the Feda Graben, comprises an equally thick Gert Member sandstone succession but one that is dominated by shoreface sandstones (Fig. 6). Only the lowermost part was deposited in a back-barrier environment (Bergan et al. 1989; Söderström et al. 1991; Johannessen 2003).

In the Rita-1 well, situated at the western margin of the Feda Graben (Fig. 1), the cored interval of the Gert Member consists of alternating mudstones, mud-draped sandstones and conglomeratic intervals. Recurring sedimentary successions consist of 1–3 m thick units of diminutive, (pyritized) Chondrites-bearing massive organic-rich mudstones that grade upward into mud-crack-bearing heterolithic bedding, which is interpreted to reflect a gradation from an oxygen-deficient lagoonal environment to an intertidal setting. Locally, these successions are further overlain by root-bearing mudstone intervals, or are truncated by 1–2 m thick rhythmically cross-stratified heterolithic sandstone successions interpreted as tidal channel and bar deposits. The base of the channel units typically contains interbedded or mixed conglomerates and highly deformed, carbonaceous mudstone beds pointing to high-energy conditions and formation of abundant fluid mud in the depositional system. The channel units are mainly unburrowed, but toward the top of the cored interval, local occurrences of Asterosoma, Cylindrichnus and Ophiomorpha were observed, suggesting increasing marine influence, and thus transgressive setting for the upper-most part of the core. In concert, the observed lagoonal, tidal and marsh sub-environments are consistent with the overall interpretation of a back-barrier and paralic setting for the Gert Member (Fig. 6).

Reservoir characteristics

Despite reservoir depths of c. 5 km, the sandstone reservoir of the Gert Member contains producible volumes of oil in the Freja Field, herein represented by the Gert-1 and 2/12-1 wells (Fig. 11) (Hemmet 2005; Danish Energy Agency 2008). The Gert Member consists of clean sandstones with good reservoir quality and reaches a thickness up to c. 90 m in both the Gert-1 and the 2/12-1 wells. Sandstone thickness decreases in a southerly and easterly direction, and in the Hejre-1 well, for example, the net sand thickness is less than 25 m (Fig. 6). Porosities are generally in the range 15–20%. The presence of high porosity sandstones of the Hejre-2 is caused by dissolution and illite replacement of detrital feldspars and early authigenic K-feldspar cement, thus creating secondary porosity (Weibel & Keulen 2008). Oil saturations up to 80% are recorded in the Gert-1, 2/12-1 and Hejre-1 wells (Fig. 11). In spite of overall good reservoir quality, core data from the Gert-1 well do not show a well defined relationship between porosity and permeability. The permeability varies considerably at a given porosity value. Generally the sandstone permeability is less than 100 mD in Gert-1, but occasionally the permeability may exceed 100 mD provided that the porosity is fairly high (Fig. 12). In the Rita-1, Ophelia-1 and Jeppe-1 wells, the reservoir sandstones are characterized by poorer reservoir quality than seen in Gert-1, Hejre-1 and 2/12-1. The reservoir quality sandstones of the Hejre-1 and Gert-1 wells are situated in a HTHP environment with overpressures of 7020 and 6800 psi, respectively (Table 1).

Fig. 12.
Fig. 12.

Porosity–permeability relationship: shoreface to paralic sandstones of the Heno Formation (Gert and Ravn Members), cored in selected wells. The plot is based on conventional core analysis data from the wells Gert-1 (Gert Mb); Ophelia-1, Gwen-2, Jeppe-1 and Rita-1 (Ravn Mb). The trend line is from Figure 17 and plotted for reference.

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Table 1.

Formation pressures of the Svane-1, Hejre-1 and Gert-1 well

Palaeogeography

The Heno and Gertrud Plateaus formed a positive north–south trending area between the Mandal High to the NE and the Mid North Sea High to the SW (Fig. 9a–d). The back-barrier and shoreface sandstones of the Gert Member were deposited during the Late Kimmeridgian, when the transgression started in the deep part of the Feda Graben and moved towards the Heno and Gertrud Plateau margins and ultimately drowned the plateau areas (Fig. 9a, b). As the regional gradient on the plateau areas was low, the transgression was rapid, leaving only a thin section of back-barrier sediments overlain by thin shoreface sands and topped by offshore claystones (Fig. 6, Jeppe-1) (Johannessen et al. 1996; Johannessen 2003).

The upper parts of the shoreface sandstones are back-stepping and are overlain by offshore claystones of the Lola Formation in the basin areas (Figs 6 & 7). On the higher plateaus, the water depth was too shallow and energy levels too high for deposition of clay and thus sandstones were deposited here, for example, Jeppe-1 (Fig. 6).

Upper Kimmeridgian shallow marine sandstones

Distribution and stratigraphy

The sandstones referred to the Ravn Member (Fig. 2) (Michelsen et al. 2003) were deposited in the same areas as the Gert Member, later during the Late Kimmeridgian (Fig. 9c–e) (Johannessen et al. 1996; Andsbjerg & Dybkjær 2003; Johannessen 2003). The succession is represented by cores in the Rita-1, Ophelia-1, Jeppe-1 and Gwen-2 wells in this paper (Figs 6 & 7).

Depositional environment

Deposition of the regressive shoreface and deltaic sandstones of the Ravn Member followed the transgression during which the paralic and shoreface sandstones of the Gert Member were deposited (Fig. 9c–e). In general, they consist of intensely bioturbated shallow marine sandstones. The lowest part is prograding whereas the uppermost part is retrograding (Johannessen et al. 1996; Johannessen 2003). This vertical trend is also seen in the Norwegian and UK sectors in the time equivalent Ula and Fulmar Formations, respectively (Bergan et al. 1989; Howell et al. 1996; Mackertich 1996). A conglomerate bed, 0.2–2 m thick, marks the maximum regression which can be mapped over a large area (Fig. 9c). The shoreface sandstones are often thick and very widespread. The best reservoir sands are developed on the Heno and Gertrud Plateaus and shale out towards the Feda and Tail End Grabens.

The 50 m long core in the Rita-1 well, situated at the SW margin of the Feda Graben, covers much of the Ravn Member interval (Figs 6 & 13). At the base it consists of a 25 m thick coarsening upward claystone to clayey sandstone succession, which represents a distal offshore to offshore-shoreface transition from the Lola Formation to the Ravn Member. This succession is truncated by a prominent erosion surface in the middle of the core, above which the deposits grade into a succession of stacked, variably developed, coarsening upward, 1–4 m thick parasequences. Figure 13 illustrates a section of the core, which contains many of the characteristic features for these sediments: the base of a parasequence typically consists of sporadically bioturbated interlaminated mud and sand, which grades upward into bioturbated heterolithic bedding and further into clayey and pebbly sand at the top of the parasequences (Figs 10d & 13). The dominance of elements of distal and archetypal Cruziana ichnofacies (Fig. 10d) (MacEachern et al. 2008) coupled with high clay content even in high-energy facies in these successions is interpreted to reflect a downdrift offshore–lower shoreface setting.

Fig. 13.
Fig. 13.

Detailed sedimentological core logs of Ravn Member intervals in Rita-1 and Ophelia-1 wells. The Rita-1 interval demonstrates an intensively bioturbated coarsening upward parasequence. The sediments are characterized by abundant soft sedimentary deformation structures, relict sand–clay couplets, and pervasive deposit feeding trace fossils such as Asterosoma and Chondrites. The succession is interpreted to reflect a downdrift offshore–lower shoreface setting. The illustrated Ophelia-1 interval is interpreted to show, for example, common tempestite intervals interbedded with tidal and wave-influenced, heterolithic fair weather facies. The interval is interpreted as mixed-influenced delta front. See Figure 16 for symbols.

Ophelia-1, located on the Heno Plateau, has a 27 m thick core consisting typically of variably developed 1–6 m thick parasequences (Fig. 13). Some typical recurring features of these sediments are shown in Figure 13, and include the following elements: the lower part of a sequence may contain interlaminated clay and sand or heterolithic climbing combined-flow ripple cross-stratification. The heterolithic intervals display occasional double mud-drapes and subdued trace fossil diversity, which is interpreted to indicate tidal and possibly local brackish water influence. Upwards, these deposits begin to alternate with erosionally based, planar-laminated and low-angle cross-stratified, decimetre-scale sandstone intervals, which are interpreted as tempestites. On the top of the parasequence, the sediments further grade or are abruptly overlain by a 1–3 m thick succession of sporadically bioturbated, massive, graded and parallel-laminated sandstone beds, which contain locally thick mud-drapes and dispersed conglomerates. In general, Ophelia-1 sediments are interpreted to represent various storm-, wave- and tide-influenced deltaic and proximal offshore environments (Figs 10c & 13).

Reservoir characteristics

Porosities exceeding 15% are interpreted from log data acquired in the Jette-1 and Gwen-2 wells, located on the Gertrud Plateau (Fig. 1). Highly porous sandstone beds, having porosities in the range 20–30%, are interpreted in the upper part of the Ravn Member in Jette-1. The net sand thickness is c. 40 m, but despite good reservoir quality it is not hydrocarbon bearing. Similarly in Gwen-2, the presence of c. 20 m net sand with porosities exceeding 20% is verified by log and core data, and it is noteworthy that the permeability range is large (0.1–100 mD) (Fig. 12). This range is presumably related to varying clay content and the presence of cement in the sandstone matrix.

In a large number of wells situated on the Gertrud Plateau and in the Feda Graben, the Ravn Member is characterized by rather poor reservoir properties, as the sandstones may be cemented by calcite and, moreover, the pores are to some extent filled with diagenetic clay (illite). In such tight Ravn Member reservoirs, the porosity is generally below 15% and the permeability is in the range 0.1–1 mD, as observed in the cores from Ophelia-1, Jeppe-1 and Rita-1 (Fig. 12). A low-porosity Ravn Member has also been encountered in the 2/12-1, Gert-1 and Hejre-1 wells. Despite such low porosities in the Ravn Member reservoir, oil has been encountered in both the Ophelia-1 and Rita-1 wells. It appears that the back-barrier and shoreface sandstones of the Gert Member in general have a better reservoir potential than the shoreface sandstones of the Ravn Member.

Palaeogeography

Regressive to transgressive shoreface sandstones of the Ravn Member were deposited during the Late Kimmeridgian on the Gertrud and Heno Plateaus and cover a large area of the northern Central Graben (Fig. 9c–e). The plateaus were flanked by two deep, broad graben areas: the Feda Graben to the NW and the Tail End Graben to the SE. The basal regressive shoreface sandstones were probably formed due to a pause in fault activity and subsidence (Andsbjerg & Dybkjær 2003). Subsequently, the shoreface sandstones back-stepped during renewed subsidence (Fig. 9d). The shoreface sandstones of the Ravn Member are comparable to Upper Jurassic sandstones of the UK Fulmar and Norwegian Ula Formations (Fraser et al. 2003). At this time, the Mid North Sea High reached as far east as the Inge and Mads Highs and contributed much of the sand to the Heno and Gertrud Plateaus, although sand was probably also derived from the Mandal High, NE of the Gertrud Plateau (Fig. 9c–e). Farther out in the basins, claystones of the Farsund Formation were deposited.

In the latest Kimmeridgian, the previously positive Heno and Gertrud Plateaus were transgressed both from the NW from the Feda Graben and from the SE from the Tail End Graben. Later the Gertrud Plateau began to subside and was thus transformed into a graben (henceforth the Gertrud Graben) (Japsen et al. 2003; Møller & Rasmussen 2003).

Upper Kimmeridgian to Lower Volgian gravity-flow sandstones

Distribution and stratigraphy

Thin sandstone units, 0.1–5 m thick, are interbedded with the offshore claystones of the Farsund Formation in the Tail End Graben (Svane-1 and Amalie-1) and the Søgne Basin (Lulu-2, Cleo-1 and 3/7-6) (Figs 6–8 & 9c–f). The sandstones are probably of Late Kimmeridgian to Early Volgian age, but are poorly dated.

Depositional environment

In the Lulu-2 well, two sandstone beds (2 and 5 m thick respectively) are encountered. A 36 m long core shows that the sandstones are fine-grained and interbedded with parallel laminated, mostly non-bioturbated claystones. The two sandstones are mostly structureless, but cross-lamination has been observed locally. Thin siltstone and very fine-grained sandstone laminae are often slumped. Partial Bouma sequences occur. The siltstones and fine-grained sandstones were probably deposited from turbidity currents while anoxic conditions prevailed at the sea-bottom. The sediments have also undergone slump processes (Johannessen 1997).

In the Svane-1 well in the Tail End Graben, the GR and sonic logs show four sandy units, c. 40–125 m thick. The log patterns are very serrated and blocky and are similar to those in the Norwegian 3/7-6 well in the Søgne Basin (Fig. 8). A 10.5 m long core from the 3/7-6 well shows thin centimetre-scale sandstone beds within offshore claystones of the Farsund Formation (Oljedirektoratet 2009). The clay and sandstones are non-bioturbated and were probably deposited from turbidity currents. Thus it is assumed that the sandstones in the Svane-1 also were deposited from turbidity currents. The Svane-1 well is almost 6000 m deep and located on a four-way dip closed structure with seismic amplitude anomalies encountering the sandstone-bearing successions below 5300 m (Fig. 14).

Fig. 14.
Fig. 14.

Seismic section across the Tail End Graben through the Iris-1 and Svane-1 locations highlighting the location of the gravity-flow sandstones. For location see Figure 1.

In the Amalie-1 and Cleo-1 wells, lying between the Svane-1 and 3/7-6 locations, thin sandstone beds of inferred turbidite origin occur interbedded within claystones of the Farsund Formation (Fig. 8). This could imply that the turbidite sandstones of the Svane-1 may well have been derived from the north, from the Søgne Basin (Fig. 9c–f). The sandstones could alternatively have been derived from the east, from the Ringkøbing–Fyn High.

The lowermost part of the Upper Jurassic section in the Amalie-1 well encounters three gas-bearing massive sandstone beds, 5–8 m thick, with no upward coarsening or fining grain-size trends, probably deposited by gravity-flow processes (Figs 2 & 8). They are Oxfordian in age (Andsbjerg & Dybkjær 2003).

Reservoir characteristics

The Svane-1 well has proved the presence of gas in sandstones at c. 6 km depth. The net sand thickness is greater than 150 m in the Svane-1 and the 3/7-6 wells. No core data are available from the Svane-1 well, but locally relatively high porosities up to 15–20% at a depth of almost 6000 m have been interpreted from log data in certain intervals. The reservoir sandstones are gas-charged and a DST conducted in the lower part of the well flowed 2.3 MMscf of gas per day; the DST tested a net sand interval of c. 100 m. The reservoir quality sandstones of the Svane-1 well are situated in a HTHP environment with overpressures of 8600 psi (Table 1). The substantial overpressures at a depth of 5350 m signify that the pore pressure is close to the fracture pressure.

The thin gas-bearing cored sandstone interval in Lulu-2, located at a depth of 3500 m, shows porosities up to 23% and permeabilities up to 100 mD, but these data may not be representative of the turbidite sandstones in general in this area.

Palaeogeography

During the Late Kimmeridgian to Early Volgian the Tail End Graben and Søgne basin subsided and thick offshore mudstones of the Farsund Formation were locally intercalated with gravity-flow sandstones (Fig. 9c–f). Sand may have been derived from the Ringkøbing Fyn High to the east, forming marginal fans, and from the north, from the Søgne Basin and Mandal High, producing basin-axial gravity-flow systems.

Lower Volgian shoreface sandstones

Distribution and stratigraphy

Shoreface sandstones, informally termed the ‘Outer Rough Sand’, were deposited in the Early Volgian on the eastern part of the Mid North Sea High, where the Outer Rough Basin began to subside. The ‘Outer Rough Sand’ is documented by the Saxo-1 and Wessel-1 wells in the Danish sector (Figs 2 & 9f). The wells are located on separate tilted fault blocks on the flanks of the Mid North Sea High about 3 km apart (Fig. 15). The shoreface sandstones of the Saxo-1 and Wessel-1 wells have now been very precisely dated to Early Volgian (Scitulus to Pectinatus Zones), deposited within a period no longer than 1.4 million years (Fig. 5). At this time the sand source area of the Mid North Sea High was situated in the easternmost part of the UK sector (Fig. 9f).

Fig. 15.
Fig. 15.

Seismic section through the Saxo-1 and Wessel-1 locations on the flank of the Mid North Sea High. The Volgian sandstone intervals are close to seismic resolution. For location see Figure 1.

Depositional environment

The Saxo-1 core records a 65 m thick succession of moderately to well sorted, fine- to medium-grained sandstones (Fig. 16). The sandstones are organized into variably developed, 2–12 m thick, coarsening upward or aggrading parasequences that are generally interpreted to represent shoreface complexes. This interpretation is supported by abundant elements of the Skolithos ichnofacies, occurrences of Macaronichnus in the top parts of the parasequences, well-sorted sandstones and local tangentially cross-stratified intervals, which in concert are consistent with a high-energy, wave-dominated shoreface setting (Figs 10e & 16).

Fig. 16.
Fig. 16.

Sedimentological core log of ‘Outer Rough Sands’ in the Saxo-1 well. The deposits are interpreted to represent stacked shoreface complexes. Note the common occurrence of elements of the Skolithos ichnofacies (e.g. Ophiomorpha, Diplocraterion and Skolithos) and Macaronichnus, which are all consistent with high-energy wave-dominated settings.

The Wessel-1 core records a 8 m thick succession of fully bioturbated, fine- to medium-grained sandstones, which are locally intercalated with few centimetre- to decimetre-thick (extraformational) pebble rich intervals. These deposits are organized into sharp-based c. 1 m thick aggrading parasequences, which are typically burrowed with abundant Macaronichnus burrows. Elements of Skolithos ichnofacies (e.g. Diplocraterion, Palaeophycus, vertical shafts) are also present in several parasequences, but their occurrence is limited to certain intervals (brief colonization windows for suspension feeders). At the top, the parasequences are capped by prominent erosive hiatal surfaces. The hiatal surfaces are typically burrowed with spreite-bearing trace fossils and locally of passively filled vertical burrows forming Glossifungites ichnofacies demarcated surfaces. The deposits are interpreted to represent low-accommodation space shoreface successions in a tectonically complex setting.

The reservoir thickness decreases rapidly from 65 m in the Saxo-1 well to less than 10 m in Wessel-1, 3 km towards the NE (Fig. 7). Towards the west (11 km) to the UK well 39/2-2 of the Fergus Field and towards the NW (15 km) to the UK Fife Field, the shoreface sandstones increase in thickness (c. 125 m thick) and are dated to be of Late Kimmeridgian to Middle Volgian age (Figs 1 & 7) (Fraser et al. 2003). This indicates that the regional sand source area and most of the accommodation space was located to the west on the Mid North Sea High and that shoreface sands prograded towards the east into the Outer Rough Basin (Fig. 9f). Detailed studies of shoreface sandstones in the British Fife Field area shows that they prograded four times: twice towards the SW and twice towards the SE (Currie et al. 1999).

No sandstone was encountered in the remaining wells in the Danish part of the Outer Rough Basin. The offshore claystones of the Farsund Formation in the Liva-1, Tordenskjold-1 and Lilje-1 wells were deposited at the same time as the prograding shoreface sandstones in the Saxo-1 and Wessel-1 wells (Fig. 9f).

In the Ål Basin area and the area north of the Inge High and the SW margin of Feda Graben, the Kim-1, Lone-1 and Rita-1 wells show an upward coarsening succession overlain by an upward fining succession in the Lower Volgian claystones of the Farsund Formation (Fig. 6). This regressive–transgressive trend in the claystone successions may reflect the distal signal of a shoreface sand progradation and retrogradation from west, but not reaching these wells.

Parts of the Inge High must have been submerged, as Lower Volgian claystones of the Farsund Formation are recorded in the Isak-1 well (Figs 7 & 9f). As Permian Rotliegend strata are unconformably overlain by Upper Cretaceous sediments in the P-1 well, Jurassic sediments were either not deposited or later eroded in this part of the high.

Reservoir characteristics

The ‘Outer Rough Sand’ is generally characterized by good to excellent reservoir properties. The porosity range is 19–31% in the two UK oil fields, but they differ with respect to permeability; in the Fergus Field the average permeability of the sandstones is about 500 mD whereas in the Fife Field it is less than 100 mD (Fig. 17) (Shepherd et al. 2003). The sandstones of the Saxo-1 well display porosities as high as 22–33% and the average permeability is about 1000 mD (range 500–2500 mD) but it is not oil-bearing (Fig. 17). The average permeability of the reservoir sandstone in Saxo-1 is thus higher than observed in the Fergus Field. The reservoir properties of the ‘Outer Rough Sand’ deteroriate in an easterly direction.

Fig. 17.
Fig. 17.

Porosity–permeability relationship for shoreface sandstones of the ‘Outer Rough Sand’ on the eastern flank of the Mid North Sea High. The plot is based on conventional core analysis data from the Wessel-1 and Saxo-1 wells supplemented with average values applying to the Fife and Fergus Fields in the UK sector (Shephard et al. 2003). The solid trend line represents the assumed porosity–permeability relation.

Palaeogeography

During the Early Volgian, the Outer Rough Basin (eastern margin of the Mid North Sea High) probably began to subside and shoreface sandstones (‘Outer Rough Sand’) were deposited while the rest of the Danish Central Graben was subsiding and offshore claystones were deposited except the Inge and Mads Highs, which probably formed sub-aerial intra-basinal highs (Fig. 9f). The sand was derived from the Mid North Sea High and the shoreline was located farther west in the eastern part of the UK sector. The ‘Outer Rough Sand’ was probably in connection with the shoreface sandstones of the Fulmar Formation in the UK sector (Fig. 9f).

Upper Middle Volgian to Ryazanian gravity-flow sandstones

Distribution and stratigraphy

After deposition and drowning of Lower Volgian shoreface sandstones on the eastern margin of the Mid North Sea High offshore, claystones of the Farsund Formation, locally intercalated with gravity-flow sandstones, were deposited in the Danish Central Graben (Fig. 9g). Gravity-flow sandstones have been recorded in the Tail End Graben, Arne-Elin Graben, Poul Plateau, Gertrud Graben, and the southern part of the Heno Plateau NE of the Mads High (Figs 1 & 9g) (Damtoft et al. 1992; Johannessen 1997; Andsbjerg & Dybkjær 2003; Michelsen et al. 2003). No shallow marine sandstones were deposited.

Depositional environment

Cores from the Tabita-1 well in the Tail End Graben include a 20 cm thick quartz clast-supported conglomerate bed with clast sizes up to c. 1 cm and also contain several claystone clasts up to 4 cm in length (Fig. 10b). The conglomerate has a sharp boundary at the base and top to the non-bioturbated claystones of the Farsund Formation. Abundant outsized quartz clasts, up to 1.5 cm, are often seen in the claystones in different concentrations, sometimes clast-supported and sometimes matrix-supported. The relatively large amount of outsized quartz clasts indicates that coarse-grained sediments were available in or at the margins of the basin, thus it may be possible to find thick coarse-grained reservoir sandstones in the basin. The clay clast conglomerates in cores show at least four types of claystone lithologies which may have been derived from different areas and may represent different ages. The conglomerates were deposited by a range of gravity-flow processes. Amalie-1, situated c. 3 km north of the Tabita-1 well, contains thin sandstone beds probably also of gravity-flow origin and of similar age (Figs 8 & 9g).

Sandstones and siltstones deposited by gravity-flows are also represented in cores in the upper part of the Farsund Formation in the Iris-1 well located in the axis of the Tail End Graben (Figs 7, 9g & 14) (Damtoft et al. 1992; Johannessen 1997). Quartz conglomerate beds, 5 cm thick, with clast size of up to 2 cm, occur within the claystones of the Farsund Formation. In addition, large claystone clasts up to 6 cm have been observed.

The sandy succession within the organic-rich claystones of the Bo Member in the Jeppe-1 well is cored and is dominated by packets of thin non-bioturbated sandstone–mudstone turbidites interbedded with hemi-pelagic laminae and beds (Figs 6 & 9g) (Ineson et al. 2003). Medium-grained sandstone turbidites and debris flows 5–20 cm thick, and sandstone-mudstone slump sheets 10–15 cm thick are also seen. Small-scale, 1–10 cm, sand injections is the main post depositional modification of the sandstones (Ineson et al. 2003). The slump sheets and debris flow sandstones and mudstones were deposited on intra-basinal slopes in the vicinity of Jeppe-1 and were probably derived from the Gert Ridge, 2 km west of Jeppe-1. Here locally exposed, uplifted fault slices of shoreface sandstones of the Heno Formation may have been eroded and transported into the basin (Ineson et al. 2003). Sandstones, c. 10 m thick, are recorded above the claystones of the Farsund Formation and the organic-rich shales of the Bo Member in the Gwen-2 well (Figs 7 & 9g). They were probably deposited by gravity-flows (Johannessen et al. 1996).

High-amplitude seismic horizons with high acoustic impedance values have been interpreted to represent sand-rich turbidites preserved in hanging-wall blocks in the depocentres of the Gertrud Graben and are correlated to the turbidites in Jeppe-1 (Fig. 9g) (Rasmussen et al. 1999; Gregersen & Rasmussen 2000). Thus these turbidite sandstones may have derived from the Gert Ridge as the turbidite sandstones of the Jeppe-1 well.

Reservoir characteristics

In both in the Jeppe-1 and Tabita-1 cores, oil shows have been encountered in the gravity-flow sandstones.

Palaeogeography

Deposition of offshore claystones of the Farsund Formation completely dominated the Danish Central Graben in Late Middle Volgian to Ryazanian times (Fig. 9g) (Ineson et al. 2003). The wells in the Danish part of the Søgne Basin do not encounter Middle to Upper Volgian and Ryazanian sediments (Andsbjerg & Dybkjær 2003; Møller & Rasmussen 2003). This may indicate a Middle to Late Volgian inversion and erosion of the Søgne Basin and Mandal High areas, which may have given rise to gravity-flow sandstones towards south in the Tail End Graben and SW in the eastern side of the Gertrud Graben. Sand may also have been shed from the Ringkøbing–Fyn High from the east forming marginal fans. Organic-rich claystones of the Ryazanian Bo Member of the Farsund Formation were deposited over most of the Danish Central Graben with the exception of the easternmost part along the Ringkøbing Fyn High, probably because of large sediment input diluting the organic matter (Figs 2 & 9g) (Dybkjær 1998; Ineson et al. 2003).

Conclusions

New released wells and detailed sedimentological studies of new cores, together with new biostratigraphic datings, have added to our understanding of the depositional history of the Upper Jurassic succession in the northern part of the Danish Central Graben. The depositional model has been re-evaluated and summarized as a series of palaeogeographic maps covering the Late Kimmeridgian to Ryazanian interval. New wells drilled on the Heno and Gertrud Plateaus during the last 5–8 years overall confirm the earlier published interpretations of the depositional environments and distribution of paralic to shallow marine sandstones of the Upper Kimmeridgian Heno Formation (Gert and Ravn Member) and thus prove the general applicability of the depositional model. However, new core data has revealed that the shoreface sandstones of the Heno Formation are more heterogeneous than previously considered, displaying substantial variability in claystone content. This variability is interpreted to be caused by the interplay between shoreface sandstones, location of deltas and long-shore current directions. The down-drift shoreface sandstones may show particularly diminished vertical permeability comparing to their up-drift counterparts due to abundant clay-drapes characteristic for these deposits. Moreover, as much of the mudstones are derived from a continental setting in the down-drift examples, the quantity of TOC may be variable in these strata. Further work in the area should include mapping of the river mouth locations so that these reservoir quality controlling factors could be predicted. The discovery of the Freja and Hejre Fields proves that Upper Jurassic reservoir quality sandstones of the Gert Member exist at great depths (5000 m or even deeper). In particular, the discovery of the Hejre accumulation triggered renewed interest in Upper Jurassic HTHP sandstone plays in the area.

Thick, porous shoreface sandstones of the Lower Volgian ‘Outer Rough Sand’ in the Danish Outer Rough Basin at the margin of the Mid North Sea High are documented east of the equivalent sandstones of the UK Fulmar Formation. The palaeogeographic understanding of this westernmost Danish area may help to a more focused exploration of the reservoir potential in the Outer Rough Basin.

Gravity-flow sandstones intercalated in offshore claystones were mainly deposited during two periods: (1) Late Kimmeridgian to Early Volgian in the Tail End Graben and the Søgne Basin; and (2) Late Middle Volgian to Ryazanian in the central part of the Tail End Graben and along the Ringkøbing–Fyn High, in the Gertrud Graben and in the southern part of the Heno Plateau NE of the Mads High. In some cases these gravity-flow sandstones are situated within or near the organic-rich offshore shales of the upper Middle Volgian–Ryazanian Bo Member, which is an excellent source rock. Surprisingly good reservoir quality of the gravity-flow sandstones at almost 6000 m of depth has been encountered in the Svane-1 well.

The frequent occurrence of gravel- to pebble-sized quartz clasts throughout the Upper Jurassic in shoreface sandstones, gravity-flow sandstones and in the offshore mudstones all over the Danish Central Graben shows that there were coarse clastic sediment source areas nearby. The challenge is to predict where these coarse-grained sediments have been deposited in thick successions where they probably will constitute good reservoirs.

Acknowledgments

Jon R. Ineson is thanked for stimulating discussions, critical reading of an early version of the text and major improvements to the English. We also thank Lars Hamberg and two anonymous reviewers for their constructive comments.

References

  1. ↵
    1. Andsbjerg J.
    (2003) in The Jurassic of Denmark and Greenland, Sedimentology and sequence stratigraphy of the Bryne and Lulu Formations, Middle Jurassic, northern Danish Central Graben, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 301–348.
    OpenUrl
  2. ↵
    1. Andsbjerg J.,
    2. Dybkjær K.
    (2003) in The Jurassic of Denmark and Greenland, Sequence stratigraphy of the Jurassic of the Danish Central Graben, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 265–300.
    OpenUrl
  3. ↵
    1. Bergan M.,
    2. Tørudbakken B.,
    3. Wandås B.
    (1989) in Correlation in Hydrocarbon Exploration, Lithostratographic correlation of Upper Jurassic sandstones within the Norwegian Central Graben: sedimentological and tectonic implications, ed Collinson J. D. (Norwegian Petroleum Society (NPF), Graham & Trotman, London), pp 243–251.
  4. ↵
    1. Britze P.,
    2. Japsen P.,
    3. Andersen C.
    (1995) Geological map of Denmark. 1:200 000. The Danish Central Graben. ‘Base Upper Jurassic’ and the Upper Jurassic (two-way traveltime and depth, interval velocity and isochore). Danmarks Geologiske Undersøgelse Kortserie 50:7.
    OpenUrl
  5. ↵
    1. Bruhn R.,
    2. Vagle K.
    (2005) Relay ramp evolution and mass flow deposition (Upper Kimmeridgian-Lower Volgian) in the Tail End Graben, Danish North Sea. Basin Research 17:551–567.
    OpenUrlCrossRefWeb of Science
  6. ↵
    1. Costa L. I.,
    2. Davey R. J.
    (1992) in A Stratigraphic Index of Dinoflagellate Cysts, Dinoflagellate cysts of the Cretaceous System, ed Powell A. J. (British Micropalaeontological Society Publication Series, Chapman & Hall, London), pp 99–153.
  7. ↵
    1. Cox B. M.,
    2. Lott G. K.,
    3. Thomas J. E.,
    4. Wilkinson I. P.
    (1987) Upper Jurassic stratigraphy of four shallow cored boreholes in the U.K. sector of the Northern North Sea. Proceedings of the Yorkshire Geological Society 46:97–109.
    OpenUrlAbstract/FREE Full Text
  8. ↵
    1. Currie S.,
    2. Gowland S.,
    3. Taylor A.,
    4. Woodward M.
    (1999) in Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference, The reservoir development of the Fife Field, eds Fleet A. J., Boldy S. A. R. (Geological Society, London), pp 1135–1145, doi:10.1144/0051135.
    OpenUrlCrossRef
  9. ↵
    1. Damtoft K.,
    2. Nielsen L. H.,
    3. Johannessen P. N.,
    4. Thomsen E.,
    5. Andersen P. R.
    (1992) in Generation, Accumulation and Production of Europe's Hydrocarbons II, Hydrocarbon plays of the Danish Central Through, ed Spencer A. M. (Springer, Berlin) Special Publications of the European Association of Petroleum Geoscientists, 2, pp 35–58.
    OpenUrl
  10. ↵
    1. Danish Energy Agency
    (2005) Press release 01-03-05: Hejre-2 boringen fandt olie.
  11. ↵
    1. Danish Energy Agency
    (2008) Oil and Gas Production in Denmark 07. World Wide Web Adress: http://www.ens.dk/graphics/Publikationer/Olie_Gas_UK/Energistyrelsen%20UK_LOW.pdf.
  12. ↵
    1. Davey R. J.
    (1979) The stratigraphic distribution of dinocysts in the Portlandian (latest Jurassic) to Barremian (early Cretaceous) of the Northwest Europe. American Association of Stratigraphic Palynologists, Contribution Series 5B:48–81.
    OpenUrl
  13. ↵
    1. Davey R. L.
    (1982) Dinocyst Stratigraphy of the latest Jurassic to Early Cretaceous of the Haldager No. 1 borehole, Denmark. Geological Survey of Denmark B6:57.
    OpenUrl
  14. ↵
    1. Dybkjær K.
    (1998) Palynological dating of the Mandal Formation (uppermost Jurassic–lowermost Cretaceous, Norwegian Central Graben) and correlation to organic-rich shales in the Danish sector. Marine and Petroleun Geology 15:495–503.
    OpenUrlWeb of Science
  15. ↵
    1. Fraser S. I.,
    2. Robinson A. M.,
    3. Johnson H. D.,
    4. Underhill J. R.,
    5. Kadolsky D. G. A.,
    6. Connell R.,
    7. Johannessen P. N.,
    8. Ravnås R.
    (2003) in The Millennium Atlas: Petroleum Geology of the Central and Northern North Sea, Upper Jurassic, eds Evans D., Graham C., Armour A., Bathurst P. (The Geological Society, London), pp 157–189.
  16. ↵
    1. Gradstein F. M.,
    2. Ogg J. G.,
    3. Smith A. G.
    (2004) A Geological Time Scale 2004 (Cambridge University Press and International Commission on Stratigraphy, Cambridge).
  17. ↵
    1. Gregersen U.,
    2. Rasmussen E. S.
    (2000) The subtle play-potential of Upper Jurassic–Lower Cretaceous block-faulted turbidites in the Danish Central Graben, North Sea. Marine and Petroleum Geology 17:691–708.
    OpenUrlCrossRefWeb of Science
  18. ↵
    1. Heilmann-Clausen C.
    (1987) Lower Cretaceous dinoflagellate biostratigraphy in the Danish Central Trough. Geological Survey of Denmark Series A 17:87.
    OpenUrl
  19. ↵
    1. Hemmet M.
    (2005) in Petroleum Geology: North-West Europe and Global Perspectives: Proceedings of the 6th Petroleum Geology Conference, The hydrocarbon potential of the Danish Continental Shelf, Geological Society, London, eds Doré A. G., Vinning B. A. pp 85–97, doi:10.1144/0060085.
    OpenUrlAbstract/FREE Full Text
  20. ↵
    1. Howell J. A.,
    2. Flint S. S.,
    3. Hunt C.
    (1996) Sedimentological aspects of the Humber Group (Upper Jurassic) of the South Central Graben, UK North Sea. Sedimentology 43:89–114.
    OpenUrlCrossRefWeb of Science
  21. ↵
    1. Ineson J.,
    2. Bojesen-Koefod J. A.,
    3. Dybkjær K.,
    4. Nielsen L. H.
    (2003) in The Jurassic of Denmark and Greenland, Volgian-Ryazanian ‘hot shales’ of the Bo Member (Farsund Formation) in the Danish Central Graben, North Sea: stratigraphy, facies, and geochemistry, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 403–436.
    OpenUrl
  22. ↵
    1. Japsen P.,
    2. Britze P.,
    3. Andersen C.
    (2003) in The Jurassic of Denmark and Greenland, Upper Jurassic–Lower Cretaceous of the Danish Central Graben: structural framework and nomenclature, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 233–246.
    OpenUrl
  23. ↵
    1. Johannessen P. N.
    (1997) in Cores from the Northwest European Hydrocarbon Province: An Illustration of Geological Applications from Exploration to Development, Upper Jurassic back-barrier and shoreface reservoir sandstones and thin turbidite sandstones in the Danish Central Trough, North Sea, eds Oakman C. D., Martin J. H., Corbett P. W. M. (The Geological Society, London), pp 11–20.
  24. ↵
    1. Johannessen P. N.
    (2003) in The Jurassic of Denmark and Greenland, Sedimentology and sequence stratigraphy of paralic and shallow marine Upper Jurassic sandstones in the northern Danish Central Graben, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 367–402.
    OpenUrl
  25. ↵
    1. Johannessen P. N.,
    2. Dybkjær K.,
    3. Rasmussen E. S.
    (1996) Sequence stratigraphy of Upper Jurassic reservoir sandstones in the northern part of the Danish Central Trough, North Sea. Marine and Petroleum Geology 13:755–770.
    OpenUrlCrossRefWeb of Science
  26. ↵
    1. Johannessen P. N.,
    2. Nielsen L. H.,
    3. et al.
    (2008) Sedimentary facies and architecture of the Holocene to Recent barrier island in the Danish Wadden Sea, 15:49–52. Geological Survey of Denmark and Greenland Bulletin.
  27. ↵
    1. Johannessen P. N.,
    2. Nielsen L. H.,
    3. Nielsen L.,
    4. Møller I.,
    5. Pejrup M.,
    6. Andersen T. J.
    (2010) in Petroleum Geology: From Mature Basins to New Frontiers – Proceedings of the 7th Petroleum Geology Conference, Architecture of an Upper Jurassic barrier island sandstone reservoir, Danish Central Graben: implications of a Holocene–Recent analogue from the Wadden Sea, eds Vining B. A., Pickering S. C. (Geological Society, London), pp 145–155, doi:10.1144/0070145.
    OpenUrlCrossRef
  28. ↵
    1. Korstgaard J. A.,
    2. Lerche I.,
    3. Mogensen T. E.,
    4. Thomsen R. O.
    (1993) Salt and fault interactions in the northeastern Danish Central Graben: observations and inferences. Bulletin of the Geological Society of Denmark 40:197–255.
    OpenUrlWeb of Science
  29. ↵
    1. MacEachern J. A.,
    2. Bann K. L.,
    3. Pemberton S. G.,
    4. Gingras M. K.
    (2008) in Applied Ichnology: SEPM, Short Course Notes, The ichnofacies paradigm: high-resolution paleoenvironmental interpretation of the rock record, eds MacEachern J. A., Bann K. L., Gingras M. K., Pemberton S. G. (SEPM, Tulsa, Oklahoma), 52, pp 27–64.
    OpenUrl
  30. ↵
    1. Mackertich D.
    (1996) The Fife Field, UK Central North Sea. Petroleum Geoscience 2:373–380.
    OpenUrlAbstract/FREE Full Text
  31. ↵
    1. Michelsen O.,
    2. Nielsen L. H.,
    3. Johannessen P. N.,
    4. Andsbjerg J.,
    5. Surlyk F.
    (2003) in The Jurassic of Denmark and Greenland, Jurassic lithostratigraphy and stratigraphic development onshore and offshore Denmark, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 147–216.
    OpenUrl
  32. ↵
    1. Møller J. J.,
    2. Rasmussen E. S.
    (2003) in The Jurassic of Denmark and Greenland, Middle Jurassic–Early Cretaceous rifting of the Danish Central Graben, Geological Survey of Denmark and Greenland Bulletin, eds Ineson J. R., Surlyk F. 1, pp 247–264.
    OpenUrl
  33. ↵
    1. Oljedirektoratet
    (2009) Norwegian Petroleum Directorates fact page for well 3/7–6. World Wide Web Address: www.npd.no/engelsk/cwi/pbl/en/wdss_index.htm.
  34. ↵
    1. Partington M. A.,
    2. Mitchener B. C.,
    3. Milton N. J.,
    4. Fraser A. J.
    (1993a) A genetic stratigraphic sequence stratigraphy for the North Sea Jurassic and Early Cretaceous: stratigraphic distribution and prediction of Kimmeridgian–Late Ryazanian reservoirs in the Viking Graben and adjacent areas, in Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (Geological Society, London), ed Parker J. R. pp 347–370, doi:10.1144/0040347.
    OpenUrlCrossRef
  35. ↵
    1. Partington M. A.,
    2. Copestake P.,
    3. Mitchener B. C.,
    4. Underhill J. R.
    (1993b) Biostratigraphic calibration of genetic stratigraphic sequences in the Jurassic–lowermost Cretaceous (Hettangian to Ryazanian) of the North Sea and adjacent areas, in Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (Geological Society, London), ed Parker J. R. pp 371–386, doi:10.1144/0040371.
    OpenUrlCrossRef
  36. ↵
    1. Petersen H. I.,
    2. Nytoft H. P.,
    3. Vosgerau H.,
    4. Andersen C.,
    5. Bojesen-Koefoed J. A.,
    6. Mathiesen A.
    (2010) Source rock quality and maturity and oil types in the NW Danish Central Graben: implications for petroleum prospectivity evaluation in an Upper Jurassic sandstone play area, in Petroleum Geology: From Mature Basins to New Frontiers – Proceedings of the 7th Petroleum Geology Conference (Geological Society, London), eds Vining B. A., Pickering S. C. pp 95–111, doi:10.1144/0070095.
    OpenUrlAbstract/FREE Full Text
  37. ↵
    1. Poulsen N. E.
    (1991) in The Jurassic of the Southern Central Trough, Upper Jurassic dinocyst stratigraphy in the Danish Central Trough, Danmarks Geologiske Undersøgelse Serie B, eds Michelsen O., Frandsen N. 16, pp 7–15.
    OpenUrl
  38. ↵
    1. Poulsen N. E.,
    2. Riding J. B.
    (1992) A revision of the Late Jurassic dinoflagellate cysts Ambonosphaera? staffinensis (Gitmez 1970) comb. Nov. and Senoniasphaera Jurassica (Gitmez & Sarjeant 1972) Lentin & Williams 1976. Palynology 16:25–34.
    OpenUrlAbstract
  39. ↵
    1. Rasmussen E. S.,
    2. Jepsen A. M.,
    3. Maver K. G.
    (1999) Upper Jurassic basin axial turbidites within the Gertrud Graben, Danish Central Graben, in Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference (Geological Society, London), eds Fleet A. J., Boldy A. R. pp 897–906, doi:10.1144/0050897.
    OpenUrlAbstract/FREE Full Text
  40. ↵
    1. Riding J. B.
    (1987) Dinoflagellate cyst stratigraphy of the Nettelton Botton Borehole (Jurassic: Hettangian to Kimmeridgian), Lincolnshire, England. Proceedings of the Yorkshire Geological Society 46:231–266.
    OpenUrlAbstract/FREE Full Text
  41. ↵
    1. Riding J. B.,
    2. Thomas J. E.
    (1992) in A Stratigraphic Index of Dinoflagellate Cysts, Dinoflagellate cysts of the Jurassic System, British Micropalaeontological Society Publication Series, ed Powell A. J. (Chapman & Hall, London), pp 7–97.
  42. ↵
    1. Schwartz R. K.
    (1982) Bedform and stratification characteristics on some modern small-scale washover sand bodies. Sedimentology 29:835–849.
    OpenUrlCrossRefWeb of Science
  43. ↵
    1. Shepherd M.,
    2. MacGregor A.,
    3. Bush K.,
    4. Wakefield J.
    (2003) in United Kingdom Oil and Gas Fields, Commemorative Millennium Volume, The Fife and Fergus Fields, Block 31/26a, UK North Sea, Memoirs, eds Gluyas J. G., Hichens H. M. (Geological Society, London), 20, pp 537–547.
    OpenUrl
  44. ↵
    1. Söderström B.,
    2. Forsberg A.,
    3. Holtar E.,
    4. Rasmussen B. A.
    (1991) The Mjølner Field, a deep Jurassic oil field in the Central North Sea. First Break 9:156–171.
    OpenUrl
  45. ↵
    1. Weibel R.,
    2. Keulen N.
    (2008) Diagenesis influencing the porosity of Upper Jurassic reservoir sandstones, Danish North Sea. Geological Survey of Denmark and Greenland Bulletin 15:9–12.
    OpenUrl
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