Evaluation of the Quality, Thermal Maturity and Distribution of Potential Source Rocks in the Danish Part of the Norwegian–Danish Basin

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GEOLOGICAL SURVEY OF DENMARK AND GREENLAND BULLETIN 16 · 2008 Evaluation of the quality, thermal maturity and distribution of potential source rocks in the Danish part of the Norwegian–Danish Basin

Henrik I. Petersen, Lars H. Nielsen, Jørgen A. Bojesen-Koefoed, Anders Mathiesen, Lars Kristensen and Finn Dalhoff

GEOLOGICAL SURVEY OF DENMARK AND GREENLAND MINISTRY OF CLIMATE AND ENERGY Geological Survey of Denmark and Greenland Bulletin 16

Keywords Norwegian–Danish Basin, source rocks, generation potential, maturation

Cover Photomicrograph of alginite-rich organic matter from lacustrine sediments of the Frederikshavn Formation in the Terne-1 well (200–210 m depth; see also Fig. 39); the photograph is taken in fluorescence-inducing blue light. The sub-circular structure (c. 200 µm across) is an algal body that resembles the freshwater green alga Botryococcus.

Chief editor of this series: Adam A. Garde Editorial board of this series: John A. Korstgård, Geological Institute, University of Aarhus; Minik Rosing, Geological Museum, University of Copenhagen; Finn Surlyk, Department of Geography and Geology, University of Copenhagen Scientific editor of this volume: Jon R. Ineson Editorial secretaries: Jane Holst and Esben W. Glendal Referees: Anthony M. Spencer (Norway) and an anonymous referee Illustrations: Stefan Sølberg, with contributions from Jette Halskov Digital photographic work: Benny M. Schark Layout and graphic production: Annabeth Andersen Printers: Schultz Grafisk, Albertslund, Denmark Manuscript received: 7 December 2007 Final version approved: 25 August 2008 Printed: 28 November 2008

ISSN 1604-8156 ISBN 978-87-7871-232-5

Geological Survey of Denmark and Greenland Bulletin The series Geological Survey of Denmark and Greenland Bulletin replaces Geology of Denmark Survey Bulletin and Geology of Greenland Survey Bulletin.

Citation of the name of this series It is recommended that the name of this series is cited in full, viz. Geological Survey of Denmark and Greenland Bulletin. If abbreviation of this volume is necessary, the following form is suggested: Geol. Surv. Den. Green. Bull. 16, 66 pp.

Available from Geological Survey of Denmark and Greenland (GEUS) Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Phone: +45 38 14 20 00, fax: +45 38 14 20 50, e-mail: [email protected] or at www.geus.dk/publications/bull

Abstract ...... 5 Introduction ...... 7 Aims of this study ...... 7 Tectonic setting ...... 10 Depositional development and stratigraphy ...... 14 Zechstein evaporites ...... 14 Early–Middle Triassic clastic deposition ...... 14 Late Triassic clastic and evaporitic deposition and marine flooding ...... 16 Hettangian – Early Pliensbachian transgression and basin expansion ...... 16 Late Pliensbachian – Early Aalenian sea-level fluctuations ...... 22 Late Early – Middle Jurassic uplift and erosion ...... 22 Late Middle – Late Jurassic basin expansion ...... 24 Cretaceous continued basin expansion and chalk deposition ...... 26 Cenozoic clastic deposition and Neogene exhumation ...... 27 Regional coalification curves corrected for net exhumation ...... 28 Shale velocity-based curve ...... 29 Chalk velocity-based curve ...... 33 Depth to the top of the oil window ...... 33 Distribution and thermal maturity of potential source rocks ...... 36 Pre-Permian units ...... 36 Permian units ...... 37 Triassic units ...... 37 Lower Jurassic units ...... 38 F-I member ...... 39 F-II member ...... 39 F-III and F-IV members ...... 40 Middle Jurassic units ...... 49 Upper Jurassic – Lower Cretaceous units ...... 52 Lower Cretaceous units ...... 56 Potential reservoirs ...... 56 Gassum reservoir ...... 56 Haldager Sand reservoir ...... 57 Additional reservoirs ...... 57 Discussion ...... 58 Source-rock quality and distribution ...... 58 Source-rock maturity ...... 59 Reservoirs and migration ...... 61 Active Mesozoic petroleum system? ...... 62 Conclusions ...... 62 Acknowledgements ...... 62 References ...... 63

3 4 Abstract

Petersen, H.I., Nielsen, L.H., Bojesen-Koefoed, J.A., Mathiesen, A., Kristensen, L. & Dalhoff, F.* 2008: Evaluation of the quality, thermal maturity and distribution of potential source rocks in the Danish part of the Norwegian–Danish Basin. Geological Survey of Denmark and Greenland Bulletin 16, 66 pp.

The quality, thermal maturity and distribution of potential source rocks within the Palaeozoic–Mesozoic succession of the Danish part of the Norwegian–Danish Basin have been evaluated on the basis of screening data from over 4000 samples from the pre-Upper Cretaceous succession in 33 wells. The Lower Palaeozoic in the basin is overmature and the Upper Cretaceous – Cenozoic strata have no petroleum generation potential, but the Toarcian marine shales of the Lower Jurassic Fjerritslev Formation (F-III, F-IV members) and the uppermost Jurassic – lowermost Cretaceous shales of the Frederikshavn Formation may qualify as potential source rocks in parts of the basin. Neither of these potential source rocks has a basinwide distribution; the present occurrence of the Lower Jurassic shales was primarily determined by regional early Middle Jurassic uplift and erosion. The generation potential of these source rocks is highly variable. The F-III and F-IV members show significant lateral changes in gene - ration capacity, the best-developed source rocks occurring in the basin centre. The combined F-III and F-IV members in the Haldager-1, Kvols-1 and Rønde-1 wells contain ‘net source-rock’ thicknesses (cumulative thickness of intervals with Hydrogen Index (HI) >200 mg HC/g TOC) of 40 m, 83 m, and 92 m, respectively, displaying average HI values of 294, 369 and 404 mg HC/g TOC. The Mors-1 well contains 123 m of ‘net source rock’ with an average HI of 221 mg HC/g TOC. Parts of the Frederikshavn Formation possess a petroleum generation potential in the Hyllebjerg-1, Skagen-2, Voldum-1 and Terne-1 wells, the latter well containing a c.160 m thick highly oil-prone interval with an average HI of 478 mg HC/g TOC and maximum HI values >500 mg HC/g TOC. The source-rock evaluation suggests that a Mesozoic petroleum system is the most likely in the study area. Two primary plays are possible: (1) the Upper Triassic – lowermost Jurassic Gassum play, and (2) the Middle Jurassic Haldager Sand play. Potential trap structures are widely distributed in the basin, most commonly associated with the flanks of salt diapirs. The plays rely on charge from the Lower Jurassic (Toarcian) or uppermost Jurassic – lowermost Cretaceous shales. Both plays have been tested with negative results, however, and failure is typically attributed to insufficient maturation (burial depth) of the source rocks. This maturation question has been investigated by analysis of vitrinite reflectance data from the study area, corrected for post-Early Cretaceous uplift. A likely depth to the top of the oil window (vitrinite reflectance = 0.6%Ro) is c. 3050–3100 m based on regional coalification curves. The Frederikshavn Formation had not been buried to this depth prior to post-Early Cretaceous exhumation, and the potential source rocks of the formation are thermally immature in terms of hydrocarbon generation. The potential source rocks of the Fjerritslev Formation are generally immature to very early mature. Mature source rocks in the Danish part of the Norwegian–Danish Basin are thus dependent on local, deeper burial to reach the required thermal maturity for oil generation. Such potential kitchen areas with mature Fjerritslev Formation source rocks may occur in the central part of the study area (central–northern Jylland), and a few places offshore. These inferred petroleum kitchens are areally restricted, mainly associated with salt structures and local grabens (such as the Fjerritslev Trough and the Himmerland Graben).

Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: [email protected] *Present address: Vattenfall A/S, Støberigade 14, DK-2450 Copenhagen SV, Denmark.

5 ),

TTZ Hi. Gr. Baltic Sea High Fault Wells Geosections National border 100 km Skåne Øresund-5,-7 Sweden Margretheholm-1 Lavø-1 Fig. 5B Fig. Hans-1 Sjælland ( Graben and the Himmerland Trough luding the Fjerritslev Kattegat Terne-1 ons of geosections displayed in Figs 4–6 are also shown. Named wells are wells Named also shown. 4–6 are in Figs ons of geosections displayed Slagelse-1 wells e.g. Fig. 15). Assessment of the source-rock potential is based on data from 15). Assessment of the source-rock e.g. Fig. Zone Stenlille

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Jylland weg F Jelling-1 Vedsted-1 r m lt Kvols-1 o u Fault u Farsø-1 l g Haldager Fa N r v Hyllebjerg-1 Hi. Gr. Hi. ø Skive-1 Skagerrak le B s t

i Germany r r Fjerritslev-2

je 5A Fig. J-1 Mejrup-1 Vinding-1 Mors-1 FjerritslevTrough F Rødding-1 Grindsted-1 Fig. 6B Fig. Nøvling-1

13/2-U-2 y Vemb-1 –F g Felicia-1, 1A Felicia-1, in b C-1 ø k g in

K-1 R

Norway Fig. 6A Fig. Graben Horn Inez-1 F-1 R-1 R-1 11/9-1

Farsund Basin 11/10-1 6ºE 8ºE 10ºE 12ºE 14ºE , Teisseyre–Tornquist Zone. , Teisseyre–Tornquist Ibenholt-1 TTZ Sea High East North 10/8-1 D-1 D-1 Block Complex Lista Fault Platform Stavanger L-1 10/7-1

EgersundBasin 58ºN 57ºN 56ºN 55ºN Danish T- 1 Graben Central 4ºE Fig. 1. Map showing well positions and the structural outline of the Norwegian–Danish Basin, the Sorgenfrei–Tornquist Zone, inc Zone, the Sorgenfrei–Tornquist Basin, positions and the structural well outline of the Norwegian–Danish showing 1. Map Fig. and the Skagerrak–Kattegat Platform. The major fault systems indicated are those active in the Mesozoic (cf. Figs 2, 3). Positi Figs (cf. in the Mesozoic those active The major fault systems indicated are Platform. and the Skagerrak–Kattegat discussed in the text or used in log-panels; unnamed wells are indicated to illustrate the deep wells used in map compilation ( indicated to illustrate the deep wells are discussed in the text or used log-panels; unnamed wells indicated in italics. wells

6 Introduction

This study focuses on an evaluation of source rocks in the north of the study area, has recently been drilled, and the Danish part of the Norwegian–Danish Basin, which is large- well encountered Jurassic sandstones and Lower Permian ly equivalent to the Danish Embayment and the Danish – Carboniferous rocks without indications of hydrocar- Subbasin of earlier workers (Figs 1, 2; Sorgenfrei & Buch bons. However, oil probably charged from Lower Jurassic 1964; Larsen 1966; Michelsen 1975, 1978, 1989a, b; (Fjerritslev Formation) and Upper Jurassic (Tau Formation) Bertelsen 1978). The Danish part of the Norwegian–Danish source rocks is under production from Middle–Upper Basin (referred to hereafter as ‘the study area’) covers onshore Jurassic sandstones (Sandnes reservoir) in the Yme Field, Denmark and Danish offshore territory extending as far west situated in the Egersund Basin some 75–100 km west of as the eastern margin of the Danish Central Graben in the the study area in the north-western part of the Nor - North Sea. Several sub-basins occur in the area, including the wegian–Danish Basin (Fig. 1; Husmo et al. 2003). Himmerland Graben and the Fjerritslev Trough (Fig. 2). Since 1935, more than 60 deep wells have been drilled throughout the study area (see Fig. 2), yielding valuable in - formation related to petroleum prospectivity (Fig. 1). Some Aims of this study of the wells were drilled for geothermal energy or gas stor- The primary aim of this bulletin is to provide a compre- age, but the majority of the wells in the basin were hydro- hensive assessment of the quality, thermal maturity and carbon exploration wells, the main target being the Mesozoic distribution of potential source rocks within the succession (Sorgenfrei & Buch 1964; Nielsen & Japsen Palaeozoic–Mesozoic succession of the central area of the 1991). The Mesozoic petroleum system that relies on Lower Danish part of the Norwegian–Danish Basin (Fig. 2). Jurassic source rocks of the Fjerritslev Formation is con- Source-rock quality has been assessed by evaluation of sidered to be the principal petroleum system in the screening data (total organic carbon (TOC) contents, S2 Norwegian–Danish Basin. In the study area, the dominant pyrolysis yields, Hydrogen Index (HI) values) from more play models have involved sandstones of the Upper Triassic than 4000 samples from the pre-Upper Cretaceous suc- – lowermost Jurassic Gassum Formation or the Middle cession in 33 wells (Fig. 1). Tmax values and vitrinite reflec - Jurassic Haldager Sand and Bryne Formations charged tance (VR) data have been used to determine the thermal from source rocks within the Lower Jurassic (Fjerritslev maturity of potential source rocks. Formation) or the uppermost Jurassic – lowermost Creta- In order to assess the maturity of the various source- ceous (Børglum, Frederikshavn, Tau and Mandal Forma - rock levels in the study area, an understanding of Neogene tions). These plays have been tested by more than 30 wells exhumation is essential. A previously published coalifica- placed on structures or stratigraphic pinch-outs. The results tion curve that was corrected for post-Early Cretaceous of wildcat drilling in the study area have so far been dis- exhumation on the basis of sonic velocity data suggested appointing as only poor indications of hydrocarbons have that the Lower Jurassic source-rock interval is immature to been encountered in the wells. The primary source rocks, very early mature and the uppermost Jurassic – lowermost the F-III and F-IV members of the Fjerritslev Formation, Cretaceous interval is immature in the Danish wells (Peter - have previously been determined to be immature to very sen et al. 2003a). In this study, new regional coalification early mature in the wells drilled. The wells have mainly (maturation) curves are presented based on a large num- been drilled on positive structures, but regional seismic ber of VR measurements (a total of 560 VR values from data also support a relatively shallow burial depth for the 26 wells). The present-day depths of the samples have been Lower Jurassic source-rock intervals over much of the study corrected for (1) post-Early Cretaceous net-exhumation area. Sufficient burial of the source rocks prior to Late magnitudes derived from chalk velocities, and (2) post- Cretaceous – Palaeogene inversion events and Neogene Early Cretaceous net-exhumation magnitudes derived exhumation may have occurred in localised depressions, from re-evaluation of the shale sonic ve locity data. however, such as in rim synclines developed adjacent to The secondary aim of this bulletin is to review the poten- salt diapirs. The Mesozoic succession has also been tested tial Mesozoic reservoirs in the study area, another critical in the Norwegian part of the basin and the border zone by element in a viable petroleum system. c. 10 wells, also with poor results in terms of hydrocarbons. The Norwegian Farsund Basin (Fig. 1), immediately

7 Fig_2 Boundary Study area 1–2 km 2–3 km 3–4 km 4–5 km 5–6 km 6–7 km 7–8 km 8–9 km >9 km Fault zone n n n e e e n b nn n a abe abe ø r r Rø R G G

e n o cov- the extent of study area Basin; ta in the Norwegian–Danish

Z (1997). Vejbæk from ). Modified

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r F f h h g ––Fy . . . .

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r e a Sea Hi Sea Hi S East No E o

N o d n unu n s i r s e a g BasiB E 4ºE 100 km 58ºN 57ºN 56ºN 55ºN Fig. 2. Depth structure map of the top pre-Zechstein surface, seismic da the deepest surface conventional that can be mapped by map of the top pre-Zechstein structure 2. Depth Fig. ered by this bulletin is also shown. Note the large thicknesses in the Fjerritslev Trough and the Himmerland Graben ( Graben and the Himmerland Trough the large thicknesses in Fjerritslev Note this bulletin is also shown. by ered

8 Basement below TPZ Basement below TPZ Palaeozoic below Zechstein salt pillow Zechstein salt diapir front Caledonian deformation Limit of Zechstein deposits Fault National border of known salt structures. The blue line indicates the limit of Zechstein salt structures. of known 10ºE 12ºE 14ºE 16ºE 6ºE 8ºE , Top pre-Zechstein. , Top TPZ 4ºE 100 km 58ºN 57ºN 55ºN Fig. 3. Map showing the strata subcropping the top pre-Zechstein surface (and lateral correlative surface) and the distribution surface (and lateral correlative the top pre-Zechstein the strata subcropping showing 3. Map Fig. deposits (Vejbæk 1997). deposits (Vejbæk

9 SW T-1 F-1 K-1 0.0

1.0

2.0

3.0

TWT sec. 4.0

5.0 25 km 6.0 Danish Norwegian–Danish Basin Central Graben

Tectonic setting Fig. 4

The Danish part of the Norwegian–Danish Basin is a Lower Palaeozoic strata unconformably overlain by Permian WNW–ESE-trending intracratonic basin, containing rocks (Figs 4–6; Liboriussen et al. 1987; Vejbæk 1989, Permian–Cenozoic strata, that to the south is bounded by 1997; Michelsen & Nielsen 1991, 1993; Jensen & Schmidt elevated Precambrian basement of the Ringkøbing–Fyn 1993; Christensen & Korstgård 1994; Vejbæk & Britze High and to the north-east and east by the Fennoscandian 1994). The crests of the fault blocks are deeply truncated Border Zone. The Fennoscandian Border Zone consists of and this top pre-Zechstein surface is the deepest regional the Sorgenfrei–Tornquist Zone and the Skagerrak–Kattegat surface that can be mapped on reflection seismic data in Platform that westwards passes into the Stavanger Platform the Norwegian–Danish Basin and the Fennoscandian north-east of the Egersund Basin; the zone marks the tran- Border Zone (Fig. 3). The surface is relatively flat and sition to the stable Precambrian Baltic Shield (Fig 1; smooth, indicating pronounced erosion prior to the Sorgenfrei & Buch 1964; Bergström 1984; EUGENO-S Zechstein transgression. The unconformity is penetrated Working Group 1988; Michelsen & Nielsen 1991, 1993; by wells that testify to the presence of Precambrian crys- Vejbæk 1997; Nielsen 2003). The Ringkøbing–Fyn High talline rocks on the Ringkøbing–Fyn High (Glams bjerg-1, separates the Norwegian–Danish Basin from the North Grindsted-1, Ibenholt-1, Jelling-1) and the Skager - German Basin and was probably formed contemporane- rak–Kattegat Plat form (Frederikshavn-1) and Lower Pa - ously with the Norwegian–Danish Basin as an area of less laeozoic sedimentary rocks in the Norwegian–Danish Basin crustal stretching (Figs 1–3). On the Skagerrak–Kattegat and Fennoscandian Border Zone (Figs 1, 3; Nøvling-1, Platform, the Mesozoic section is relatively undisturbed; it Rønde-1, Slagelse-1, Terne-1; Sorgenfrei & Buch 1964; onlaps tilted fault blocks comprising Precambrian crys- Poulsen 1969, 1974; Larsen 1971, 1972; Christensen 1971, talline basement, Lower Palaeozoic and Lower Permian 1973; Nielsen & Japsen 1991; Michelsen & Nielsen 1991, strata and wedges out towards the north-east (Figs 4, 5A). 1993). The shallow IKU/Sintef borehole 13/2-U-2 encoun- The Sorgenfrei–Tornquist Zone is a highly block-faulted tered Silurian strata subcropping the Quaternary north- 30–50 km wide zone that runs SE–NW from the Rønne east of the Farsund Basin (Fig. 1). Graben in the Baltic Sea across southern Sweden through The tilted fault-block crests are deeply truncated by the the Kattegat and northern Jylland to the Skagerrak, where mid-Permian unconformity showing that regional post- it turns westwards across the Norwegian shelf (Figs 1–3, rift thermal subsidence was somewhat delayed (Vejbæk 5B). The zone includes the deep Fjerritslev Trough with a 1997). The unconformity that defines the base of the post- Zechstein–Mesozoic succession that locally is more than 9 rift sequence is overlain by a relatively complete succession km thick, and the shallower Farsund Basin where the of Zechstein salts and carbonates, Triassic clastics, carbon- Zechstein–Mesozoic section locally attains a thickness of ates and salts, Jurassic – Lower Cretaceous clastics, Upper slightly more than 6 km (Figs 2, 6). Cretaceous chalks and Cenozoic clastics that attain a thick- The principal rifting phase of the Norwegian–Danish ness of 5–6.5 km along the basin axis. A similar thickness Basin and the Sorgenfrei–Tornquist Zone is defined by the of the post-rift succession is found in the Farsund Basin, occurrence of tilted fault blocks with basement rocks and where the succession is more than 3 sec. (TWT) thick in

10 Felicia-1/1A Sæby-1 (projected) J-1 (projected) NE 0.0

1.0

2.0

3.0 Cenozoic L.–M. Jurassic

U. Cretaceous Triassic 4.0 TWT sec. L. Cretaceous Zechstein 5.0 U. Jurassic Rotliegend 6.0 Norwegian–Danish Basin Sorgenfrei–Tornquist Zone Skagerrak–Kattegat Platform

Fig. 4. Regional geosection from the Central Graben in the south-west to the Skagerrak–Kattegat Platform in the north-east showing pinch-out of the Mesozoic strata on the Skagerrak–Kattegat Platform. For location of section, see Fig. 1. TWT, Two-Way Travel Time.

places, corresponding to slightly more than 6 km (Jensen Sorgenfrei–Tornquist Zone, as shown by well sections in & Schmidt 1993; Vejbæk & Britze 1994), whereas the suc- the Øresund region (Øresund-5, -7), in the Kattegat area cession is more than 9 km thick locally in the Fjerritslev (Terne-1, Anholt-4), and in the Fjerritslev Trough (Børg - Trough and the Himmerland Graben (Fig. 2). Isochore lum-1, Fjerritslev-2, Flyvbjerg-1, J-1, Haldager-1, Vedsted- maps of the Triassic and Jurassic – Lower Cretaceous suc- 1), but at a much lower rate than in Triassic – Early Jurassic cessions show a relatively uniform regional thickness over times (Fig. 7). Danish well sections on the Skagerrak most of the basin, except for areas influenced by local halo- –Kattegat Platform (Frederikshavn-1, -2, -3, Skagen-2, kinetic movements, indicating relatively uniform thermal Sæby-1) indicate that only limited erosion occurred north subsidence (Vejbæk 1989, 1997; Britze & Japsen 1991; of the Fjerritslev Trough. Japsen & Langtofte 1991). Although the thick Upper Regional subsidence gradually resumed during late Permian – Triassic succession indicates rapid subsidence Middle – Late Jurassic times and tectonic tranquillity gen- that exceeds rates normally associated with post-rift ther- erally prevailed, except for local salt movements, until inver- mal contraction (Fig. 7), a prolonged or new rifting phase sion occurred in the Sorgenfrei–Tornquist Zone. A Late is precluded by the general lack of pronounced extensional Cretaceous – Palaeogene age is generally accepted for the faulting in the Mesozoic succession; phase transformations inversion, which is interpreted to have been caused by com- in the deep crust have been proposed to explain the rapid, pression related to a change in the regional stress field from early post-rift subsidence (Vejbæk 1989, 1997). Deposition extensional to compressional, linked to Alpine deformation and preservation of great thicknesses of Mesozoic sedi- and opening of the North Atlantic (Liboriussen et al. 1987; ments in the Himmerland Graben and the Fjerritslev Trough Ziegler 1990; Michelsen & Nielsen 1991, 1993; Mogensen were facilitated by transtensional strike-slip move ments in & Korstgård 2003). The inversion may have occurred in the Sorgenfrei–Tornquist Zone or large-scale salt move- several phases, however, and it is assumed to have begun ments (Pegrum 1984; Vejbæk 1989; Christensen & in Turonian times or earlier in the south-east, with 1.5–3 Korstgård 1994; Mogensen 1994, 1996). Growth of salt km of uplift in the Skåne–Bornholm area, and propagated structures influenced Mesozoic deposition locally. north-westwards with decreasing intensity of deformation The Ringkøbing–Fyn High was uplifted significantly (Berthelsen 1992; Mogensen & Jensen 1994; Michelsen in early Middle Jurassic times and the Norwegian–Danish 1997; Petersen et al. 2003a; Japsen et al. 2007). After ces- Basin became tilted to the north-east (Michelsen 1978; sation of the inversion, a new quiescent tectonic phase Koch 1983; Andsbjerg et al. 2001; Nielsen 2003). The began with regional subsidence of the greater North Sea uplift and tilting caused progressively deeper erosive trun- Basin. Contemporaneously with the regional down-warp- cation of the Lower Jurassic and Triassic across the basin ing of southern Scandinavia, significant uplift and erosion towards the Ringkøbing–Fyn High, where erosion removed began to influence parts of the Norwegian–Danish Basin the entire Lower Jurassic and much of the Triassic on the and the Ringkøbing–Fyn High in Neogene times (Japsen most elevated parts of the high (Fig. 8). In contrast, sub- 1993; Jensen & Schmidt 1993; Japsen et al. 2002a, b, sidence continued during Middle Jurassic times in the 2007).

11 A NE Haldager-1 Flyvbjerg-1 Frederikshavn-1 0

2 TWT (sec.) 3 10 km 4 Fjerritslev Trough Skagerrak–Kattegat Platform Sorgenfrei–Tornquist Zone

SW Nøvling-1 0

2 TWT (sec.) 3

4 Ringkøbing–Fyn High Norwegian–Danish Basin

Cenozoic Lower Cretaceous/ Upper Triassic Upper Jurassic Zechstein Palaeozoic Upper Cretaceous Middle–Lower Jurassic Lower–Middle Rotliegend Basement Triassic

B Sorgenfrei–Tornquist Zone SW Hans-1 Børglum Fault NE 0 L. Cretaceous? ? Jurassic ? Jurassic Jurassic Triassic Triassic Zechstein 1 Triassic Z R

.) Rotliegende U.Car. Z U.Car. Lower Palaeozoic R U. Carboni- ferous 2

TWT (sec Ordovician-Silurian Ordovician-Silurian Cambrian

L. Cretaceous? Zechstein Ordovician–SilurianCambrian 3 Basement Jurassic Rotliegend Cambrian Triassic U. Carboniferous Basement 5 km

Fig. 5. Geosections modified from Michelsen & Nielsen (1991); for locations, see Fig. 1. A: Regional geosection across the Danish part of the Norwegian–Danish Basin showing thickening of the Mesozoic section in the Fjerritslev Trough / Sorgenfrei–Tornquist Zone. B: Geosection from the Kattegat through the Hans-1 well; note Palaeozoic fault blocks.

12 A S K-1 N 0 Seabed

TWT (sec.) 3

Farsund Basin

5 B S J-1 N 0

Seabed 1

TWT (sec.) 4

Fjerritslev 6 Fault Fjerritslev Trough

20 km 7 Quaternary Lower Cretaceous Lower Jurassic Zechstein salt

Cenozoic Upper Jurassic Triassic Palaeozoic Upper Cretaceous & Middle Jurassic Triassic Salt Basement Danian (Haldager Sand Fm) (Oddesund Fm) Internal reflectors Reflector truncation

Fig. 6. Geosections modified from Jensen & Schmidt (1993); for locations, see Fig 1. A: Geosection from the Norwegian–Danish Basin across the Fjerritslev Fault and the Farsund Basin. The Middle Jurassic Haldager Sand reservoir overlying the Lower Jurassic potential source rocks is marked in red. Note Palaeozoic fault blocks overlain by Zechstein salt. B: Geosection from the Fjerritslev Trough.

13 Late Triassic Early Jurassic Middle Jurassic Late Jurassic E.Cret. Fig. 7. Subsidence curves for the Late Triassic – 0 Early Cretaceous constructed for different structural elements/sub-basins within the Norwegian–Danish Basin, Skagerrak – Ringkøbing–Fyn High Kattegat Platform and Ringkøbing–Fyn High Skagerrak–Kattegat Plat based on a sequence stratigraphic break-down form (Nielsen 2003). 500

Basin cent re Depth (m) 1000 Himmerland Graben

Fjerritslev Trough

1500

220 210 200 190 180 170 160 150 140 Time (Ma)

Depositional development and stratigraphy

A review of the Permian–Cenozoic depositional evolution influenced Mesozoic deposition in places. Towards the of the basin is presented here to provide a framework for east, the evaporites are replaced by thin clastic Zechstein discussion of potential components of a petroleum system deposits as shown by the Terne-1 and Hans-1 wells in the in the Danish portion of the Norwegian–Danish Basin. Sorgenfrei–Tornquist Zone (Michelsen & Nielsen 1991, For detailed, comprehensive accounts of the basin evolu- 1993). Farther north-west in the Sorgenfrei–Tornquist tion and stratigraphy, the reader is referred to Michelsen Zone, however, evaporites are present in the Fjerritslev et al. (2003) and Nielsen (2003). Trough and the Farsund Basin (e.g. Liboriussen et al. 1987; Jensen & Schmidt 1993; Christensen & Korstgård 1994; Vejbæk 1997). Zechstein evaporites Deposition of the post-rift succession was initiated in Late Permian times with the accumulation of thick Zechstein Early– Middle Triassic clastic deposition evaporites in most of the Norwegian–Danish Basin. Mar - In Early Triassic times, the depositional environment ginal facies were developed along parts of the Ring- changed to more continental conditions. Similarity in depo- købing–Fyn High in Late Permian times, and as the Lower sitional facies in the North German and Norwegian–Danish Triassic Bunter Shale Formation seems to rest on deeply Basins indicates that the two basins were connected, at weathered basement in the Grindsted-1 well, it is likely least periodically (Bertelsen 1978; Michelsen & Clausen that the high formed a barrier between the Southern and 2002). The facies encountered in the Jelling-1 and Northern Zechstein basins (Ziegler 1982; Stemmerik et al. Grindsted-1 wells located on the northern flank of the 1987; Vejbæk 1997). Later mobilisation of Zechstein salt Ringkøbing–Fyn High belong to the Bunter Shale, Bunter led to the formation of salt structures such as pillows and Sandstone, Ørslev, Falster and Tønder Formations; these for- diapirs (Fig. 3), and the continued growth of salt structures mations may be traced farther northwards to the Mors-1

14 SW NE Chronostratigraphy Sequ- Key Tectonics Lithostrati- ence surfaces Norwegian–Danish Basin graphy SB MFS RKF STZ SKP Ma 27 Frederiks- Ryazanian Fr 3 havn Fm 26 142.0 Fr 2 25 25 Renewed Fr 1 24 regional subsidence Volgian 24 including Ringkøbing– Bø 1 Fyn High Børglum Fm 150.7 23 Condensed Himmerland Fjerritslev Kimmeridgian section Graben Trough 154.1 23 22 Flyvbjerg Fm Oxfordian Basin Fl 1 expansion 159.4 22 Callovian Fault-con- 164.4 trolled sub- Ha 3 sidence of Bathonian Sorgenfrei– Haldager 21 Tornquist 169.2 Sand 20 Zone Fm Ha 2 Bajocian Uplift of Ringkøbing– 20 Fyn High, 176.5 Ha 1 19 NE-tilting Base Middle Jurassic unconformity Local Aalenian of basin 19 ‘Mid-Cimmerian Unconformity’ incision 180.1 Fj 10 18 18 Fj 9 17 F-IV mb 17 Fj 8 16 Toarcian 16 Erosion over salt structures Fj 7 15 F-III mb 189.6 Fj 6 15 14 14 Pliensbachian Fj 5 13 F-II mb 13 Thermal subsidence Fm Fjerritslev 195.3 Fj 4 12 local fault- 12 ing and F-Ib Sinemurian Fj 3 11 halokinesis F-I mb

201.9 Fj 2 11 10 F-Ia Hettangian Fj 1 9 205.7 9 7 Ga 1 Gassum Rhaetian 5 Fm 209.6

Vi 1 1 Vinding Norian Fm Sk Od

Shoreface sandstones/siltstones Dolomitic limestones Alluvial conglomerates, sandstones Source rocks

Estuarine/lagoonal sandstones, Fluvial sandstones Lacustrine mudstones heteroliths, mudstones, coal beds Offshore mudstones; occasionally Sabkhas and lacustrine calcareous, Hiatus sandy or silty evaporitic mudstones

Fig. 8. Time-stratigraphic scheme of the Upper Triassic – Lower Cretaceous trending SW–NE across the Norwegian–Danish Basin. Sequence stratigraphic key surfaces are included, numbered sequence boundaries are shown in red (SB) and maximum flooding surfaces shown in blue (MFS). The SBs bound the named sequences (e.g. Vi 1, Ga 1). The main tectonic events are indicated. Note the significant base Middle Jurassic unconformity that cuts deep into older strata to the south-west and is onlapped as the area of subsidence expanded during Late Jurassic times. Od, Oddesund Formation; RKF, Ringkøbing–Fyn High; Sk, Skagerrak Formation; SKP, Skagerrak–Kattegat Platform; STZ, Sorgenfrei–Tornquist Zone. Modified from Nielsen (2003); time-scale from Gradstein et al. (1994).

15 well, in which the succession reflects a transition to the and resulted in the formation of a large epicontinental sea. contemporaneous Skagerrak Formation that dominates The transgression led to deposition of oolitic limestones suc- along the northern and eastern basin margin (Bertelsen ceeded by marlstones and fossiliferous claystones of the 1980; Nielsen & Japsen 1991). The Bunter Shale and Vinding Formation, which is typically 40–100 m thick Bunter Sandstone Formations are present in Felicia-1A, over most of the basin (Figs 8–10; Bertelsen 1978, 1980; whereas the Ørslev, Falster and Tønder Formations are Nielsen 2003). At its maximum extent in Late Norian replaced by the Skagerrak Formation. The Bunter Sandstone times, the shallow sea covered most of the central basin Formation may also be present in the Terne-1 well where and the Ringkøbing–Fyn High, whereas deposition of flu- the lower 155 m of the Triassic succession consists mainly vial arkosic sands and lacustrine muds of the Skagerrak of fine-grained, well-sorted sandstones. The Bunter Formation continued in the Sorgenfrei–Tornquist Zone Sandstone Formation mainly consists of red-brown and and on the Skagerrak–Kattegat Platform. Maximum trans- yellow-brown, medium- to fine-grained, well-sorted sand- gression was followed by phased regression, and shoreface stones with intraformational claystone clasts and thin mud- and fluvial sands of the lower Gassum Formation were stone beds, largely recording deposition in ephemeral, deposited in stepwise, more basinward positions; the sands braided fluvial channels in an arid desert environment are interbedded with clays of the upper Vinding Formation (Bertelsen 1980; Pedersen & Andersen 1980). Eolian dune in the basin centre. Deposition of fluvial sand and lacus- sand, and mud deposited in ephemeral lakes, may consti- trine mud of the Skagerrak Formation continued in the tute minor proportions of the formation. Up-section and Fjerritslev Trough (Figs 8, 10). Regression culminated in towards the northern and north-eastern basin margin, the the early Rhaetian with the formation of an extensive, flu- Bunter Sandstone Formation passes into the Skagerrak vially incised sequence boundary (SB 5 of Nielsen 2003; Formation. On the Skagerrak–Kattegat Platform, Lower Figs 8, 10). The following, widespread marine flooding –Middle Triassic strata are all referred to the Skagerrak was initiated with deposition of fluvial–estuarine sediments, Formation, which here seems to include large parts of the up to 30 m thick in the basin centre, above the sequence Upper Triassic as well (Frederikshavn-1, -2, -3, Sæby-1; boundary. The transgression was punctuated by two short- Nielsen & Japsen 1991; Figs 8, 9). The formation consists term, forced regressions that led to deposition of wide- of a heterogenous succession of interbedded conglomerates, spread shoreface sand sheets encased in offshore mud sandstones, siltstones and claystones that were mainly (Hamberg & Nielsen 2000; Nielsen 2003). The trans- deposited as alluvial fans along the basin margins. gression reached its maximum in the latest Rhaetian, when the entire study area and the Ringkøbing–Fyn High were covered by the sea, and marine mudstones were deposited widely (MFS 7; Figs 8, 10–11). Late Triassic clastic and evaporitic deposition and marine flooding In Late Triassic times, the arid or semi-arid climate continued and deposition of variegated red-brown or brown, Hettangian – Early Pliensbachian calcareous, anhydritic and pyritic mudstones and siltstones transgression and basin expansion with thin beds of dolomitic limestone and marl commenced During latest Rhaetian times, the climate changed to the in sabkhas and ephemeral lakes. In the central, deep parts subtropical to warm-temperate and humid conditions that of the basin, more permanent lakes were established. The characterised the Jurassic period, when large quantities of deposits are included in the Carnian – Lower Norian Od- clay were supplied to the basin due to weathering of Pa - desund Formation, which passes into the Skagerrak laeozoic shales and granitic basement of the Baltic Shield. Formation towards the basin margins to the north and The Jurassic transgression was interrupted by two phases north-east (Figs 8, 9; Bertelsen 1980). In places, the Od- of coastal progradation that caused deposition of two thin, desund Formation includes two halite units up to 90 m thick, regressive, shoreface sand sheets, which constitute the upper- which in some areas have contributed to the formation of most part of the Gassum Formation over much of the study salt domes together with the Zechstein salts (Liboriussen area. The regression culminated in coastal progradation far et al. 1987; Jensen & Schmidt 1993; Christensen & into the basin accompanied by fluvial erosion and incision Korstgård 1994). in the Himmerland Graben, the Fjerritslev Trough and the A gradual change to more humid conditions took place Skagerrak–Kattegat Platform (SB 9; Figs 10–11). Sub - in Late Triassic times, associated with an Early Norian sequently, the regional transgression continued (TS 9; early marine transgression that probably came from the south Hettangian Planorbis Zone), so that fully marine mud-

16 System Marine mudstones and siltstones Danish Central Norwegian– Stage Graben Danish Basin Series SW NE Offshore organic-rich marine shales U Åsgard Formation Submarine fan sandstones and siltstones Valanginian Vedsted Marine calcareous claystones, L Leek Member Formation Vyl carbonates Fm Lower U Shallow marine sandstones and Ryazanian siltstones, offshore mudstones Cretaceous L Bo Member Paralic and non-marine sandstones, Frederikshavn siltstones, mudstones and coals U Formation Poul Alluvial sandstones and Volgian M Farsund Formation Fm lacustrine mudstones L

r Hiatus e U Børglum p Kimmeridgian Heno Fm p Formation Source rocks U L Unconformity U Lola Formation Flyvbjerg Oxfordian M Formation L Middle Graben U Formation Lulu Callovian M Formation L U Haldager Sand Bathonian M Formation L Bryne Formation Middle

Jurassic U Bajocian L

Aalenian U L F-IV U mb Toarcian M L F-III mb U Pliensbachian F-II mb L

F-Ib Fjerritslev Formation Lower U Sinemurian L F-I mb Fjerritslev Formation F-Ia Hettangian

Gassum Rhaetian Formation Fig. 9. Lithostratigraphy of the Upper Triassic – Lower Cretaceous in the Norwegian–Danish Basin. The stratigraphy of the Danish Central Upper Winterton Vinding Triassic Skagerrak Norian Formation Formation Formation Graben is included for reference. Modified from Oddesund Fm Michelsen et al. (2003).

stones belonging to the F-Ia unit of the F-I member of the 9, 10), interrupted by a short-term regression in the mid- Fjerritslev Formation overlie the sandy Gassum Formation dle Hettangian (SB 10; Figs 8, 10–12). Deposition of trans- over most of the study area (Figs 8–14). The mudstones gressive paralic deposits along the basin margin was have a high content of land-derived organic matter. The interrupted briefly by a fall in sea level soon after the transgression peaked in the early and late Hettangian (MFS Hettangian–Sinemurian boundary. This resulted in fluvial

17 , self-

SP ., Ringkøbing–Fyn High Lowstand Transgressive Highstand Sequence boundary marine Maximum surface Transgressive Sequence Shoreface Offshore Hi. Gr Fluvial Estuarine Lacustrine Depositional environments LST flooding surface flooding Systems tracts TST HST Bounding surfaces SB MFS TS 1 Vi , gamma ray; GR NE MFS 9 TS 9 SB 9 MFS 8 SB 8 MFS 7 TS 7 SB 7 SB 6 MFS 5 TS 5 SB 5 MFS 4 TS 4 SB 4 SB 3 MFS 6 MFS 3 SP Vedsted-1 Himmerland Graben) showing the Norian–Hettangian showing Graben) Himmerland Skagerrak, Vinding, of the depositional environments that Note Fjerritslev Formations. and lowermost Gassum sequence stratigraphic surfacesthe prominent (MFS1, 1, Ga Vi MFS7, MFS9, SB5, SB9) define major sequences subdivided into minor 1, and Fj 1; these sequences are SB3 etc. sequences bounded by (2003). Nielsen from potential. Modified Fig. 10. Well-log panel trending SW–NE across the SW–NE across panel trending Well-log 10. Fig. (see inset map; Basin Norwegian–Danish Skagerrak Fm Skagerrak Sorgenfrei–Tornquist Zone Sorgenfrei–Tornquist 41 km 50 m LST Vinding Fm GR Oddesund Formation Formation Oddesund Hyllebjerg-1 5 km Himmerland Graben Gassum Fm Fjerritslev Fm Fjerritslev GR Farsø-1 37 km TS 7

e SP Skagerrak–Kattegat

Rødding-1 Platform SB 2 an–Danish Basin

Sorgenfrei–Tornquist Zon Hyllebjerg-1 Farsø-1

Hi. Norwegi Gr. 25 km Vedsted-1 Fj 1 Vi 1 Ga1 Mejrup-1 Fjerritslev LST TST TST TST HST HST HST Trough Oddesund Formation Formation Oddesund Rødding-1 50 km GR Mejrup-1 SB 6 SB 9 SB 5 SB 3 SB 7 SB 8 SB 4 TS 4 TS 9 TS 1 TS 5 MFS 9 MFS 4 MFS 6 MFS 5 MFS 3 MFS 8 MFS 7 MFS 1 SW

18 MFS 12 TS 12 SB 12 MFS 11 TS 11 SB 11 MFS 9 TS 9 SB 9 SB 8 MFS 7 TS 7 NE ., Himmerland Graben) ., Himmerland 100 m Hi. Gr SP Skagerrak Fm Skagerrak Frederikshavn-2 Fig. 11. Well-log panel trending SW–NE panel trending Well-log 11. Fig. (see Basin the Norwegian–Danish across inset map; the Rhaetian–Sinemurian showing of the upper depositional environments Fjerritslev Formations. and lower Gassum (2003). Nielsen from Modified Platform LST Fault LST Børglum 42 km Skagerrak–Kattegat Sequence Ringkøbing–Fyn High SP

Børglum-1 Fj 1

LST 15 km LST TST TST TST HST HST Fjerritslev Fm Fjerritslev Lowstand Transgressive Highstand regressive Forced Sequence boundary marine flooding surface Maximum surface Transgressive Gassum Fm Fjerritslev Trough Fjerritslev

SB TS LST TST MFS HST FRST Systems tracts Bounding surfaces Flyvbjerg-1 FRST FRST 30 km SP Vedsted-1 Fluvial Estuarine Lacustrine Lagoonal Shoreface Offshore Depositional environments LST LST TST TST TST TST 41 km HST HST HST/FRST

Skagerrak– Zone Kattegat in

LST Platform Frederikshavn-2 GR Graben Flyvbjerg-1 HST Hyllebjerg-1 rgenfrei–Tornquist Himmerland

Fj 4 Fj 3 Fj 2 Fj 1 So Børglum-1 Hyllebjerg-1

Hi. Norwegian–Danish Bas Gr. Vedsted-1 Fjerritslev Fm Fjerritslev 65 km Ga 1

Fjerritslev Trough Mejrup-1 50 km TS 7 GR Mejrup-1 SW SB 9 SB 8 TS 9 SB 12 SB 11 SB 10 TS 11 TS 10 MFS 9 MFS 8 MFS 7 MFS 12 MFS 11 MFS 10

19 ns. SB 23 SB 19–22 SB 16 MFS 15 MFS 22 MFS 16 NE

Skagerrak–Kattegat Platform Frederikshavn-2 Sæby-1 SP Skagen-2 Flyvbjerg-1 Skagen-2

Sorgenfrei–Tornquist Zone 35 km SB 11 SB 5 SB 13 SB 9 SB 15 MFS 12 SB 12 MFS 7 MFS 9 MFS 11 Børglum-1 Hi. Gr. Norwegian–Danish Basin Vedsted-1 ., Himmerland Graben) showing the showing Graben) ., Himmerland

Fjerritslev Trough SP Hi. Gr Frederikshavn-2 50 km

SB 17 Flyvbjerg Fm Flyvbjerg 10 km Skagerrak–Kattegat Platform Skagerrak–Kattegat GR Sæby-1 Fault Børglum Ringkøbing–Fyn High Sequence Ga 1 34 km SB Sequence boundaryBounding surfaces Sequence SB surface MFS Transgressive TS marine flooding surface Maximum Fj 9 Børglum-1

SB 17 MFS 17 MFS Depositional environments Fluvial Estuarine Lacustrine Lagoonal Shoreface Offshore 15 km MFS 1 Børglum Fm Norian–Oxfordian depositional environments of the Skagerrak, Gassum, Fjerritslev, Haldager Sand, Flyvbjerg and Børglum Formatio Flyvbjerg Sand, Haldager Fjerritslev, Gassum, of the Skagerrak, depositional environments Norian–Oxfordian (2003). Nielsen from SB 19–22. Modified truncation erosional at the amalgamated sequence boundary, the pronounced Note Fig. 12. Well-log panel trending SW–NE across the Norwegian–Danish Basin (see inset map; Basin the Norwegian–Danish SW–NE across panel trending Well-log 12. Fig. Flyvbjerg-1 Fl 1 Sorgenfrei–Tornquist Zone Sorgenfrei–Tornquist Ha 3 Fjerritslev Fm Fjerritslev 100 m 30 km Fm Fm Gassum Skagerrak Haldager Sand Fm SP SP SP Vedsted-1 Fj 3 Fj 2 Fj 1 Fj 7 Fj 6 Fj 5 Fj 4 Fj 8 Vi 1 Ga 1 Ha 2 Ha 1 Himmerland Graben SB 9 SB 5 TS 19 SB 16 SB 15 SB 13 SB 23 SB 19 SB 14 SB 12 SB 22 SB 21 SB 20 SB 11 SB 10 MFS 9 MFS 7 MFS 14 MFS 13 MFS 16 MFS 15 MFS 12 MFS 11 MFS 10 MFS 22 SW

Fm Fjerritslev

Skagerrak– Zone Børglum Fm Fm Kattegat F-ll F-lll F-lb F-lV (Jurassic) (Triassic) Platform Vedsted Fm Vedsted Gassum Fm Flyvbjerg Fm Flyvbjerg Haldager Sand Fm Skagerrak Fm Skagerrak Frederikshavn Frederikshavn-2 Flyvbjerg-1 Res Sorgenfrei–Tornquist n–Danish Basin High Børglum-1 Hyllebjerg-1 SP

Hi. Norwegia Gr. Frederikshavn-2 Vedsted-1 Platform Trough Fjerritslev Skagerrak–Kattegat Mejrup-1 Rødding-1

Ringkøbing–Fyn 50 km SP Res Flyvbjerg-1 F-ll F-lb F-lll F-lb F-lV Res SP Børglum-1 Sorgenfrei–Tornquist Zone Sorgenfrei–Tornquist ., SP Res Vedsted-1 Hi. Gr F-ll F-la F-lll F-lb F-lV F-la Sonic GR Hyllebjerg-1 . (2003). et al Norwegian–Danish Basin Norwegian–Danish Sonic Mejrup-1 GR F-ll F-lll F-la F-lb Fm Marine mudstones marine sandstones and siltstones Shallow Paralic and non-marine sandstones, and coals mudstones siltstones, (Triassic) Børglum Fm Fm Vedsted Fm Vedsted Gassum Fm Frederikshavn 100 m Flyvb.–Hald. Fjerritslev Fig. 13. Well-log panel trending SW–NE across the basin (see inset map; SW–NE across panel trending Well-log 13. Fig. Himmerland Graben) showing the lithostratigraphy of the Upper Triassic – Lower Triassic the lithostratigraphy of Upper showing Graben) Himmerland Michelsen from Modified Cretaceous.

21 incision on the Skagerrak–Kattegat Platform, while regres- of F-IIb and F-IIc beds, F-II member; Michelsen 1989a, sive shoreface sand was deposited in the Fjerritslev Trough b). The ensuing sea-level rise reached a peak in the late (SB 11; Figs 8, 11–12). Farther basinwards, heteroliths and Late Pliens bachian (early Spinatum Zone). Marine silty silty mudstones were deposited above the conformable part mud accumulated in the basin, while muddy marine sand of the sequence boundary. with bivalves was deposited in the Fjerritslev Trough and A rapid sea-level rise followed in the earliest Sinemurian presumably also in the Farsund Basin (lower part of the (upper part of the Bucklandi Zone), and transgressive F-III member, Fjerritslev Formation; Figs 8, 9, 13). Strata marine muds of the F-Ib unit of the F-I member of the from this period are absent on the Skagerrak–Kattegat Fjerritslev Formation finally overstepped fluvial and marine Platform due to bypass or later erosion (Fig. 8). sands of the Gassum Formation in the Sorgenfrei–Tornquist A subsequent sea-level fall caused the formation of a Zone and on the Skagerrak–Kattegat Platform (Figs 8, 9, widespread marine regressive surface of erosion and depo- 13, 14). The early Sinemurian transgression led to deposition of 5–10 m of lowstand shoreface sandstones in the sition of up to 150 m of uniform mudstones in the basin, central parts of the Fjerritslev Trough (SB 15; Figs 8, 12); showing a marked thinning towards the northern and Lower Pliensbachian strata were eroded on the Ska - north-eastern basin margin (Sequence Fj 3 in Figs 8, 11, gerrak–Kattegat Platform. 12). Following a minor sea-level fall in the Late Sinemurian, The subsequent sea-level rise caused marine flooding the overall Early Jurassic sea-level rise continued and reached over the entire study area, and the transgression reached its a maximum in the latest Sinemurian (Fig. 14). In the cen- maximum in the Early Toarcian Falciferum Zone (MFS 15; tre of the basin, the diversity and abundance of the ostra- Figs 8, 12, 14). Due to oxygen-poor conditions, the ostra- cod fauna decreased and infaunal bivalves and some of the cod fauna disappeared and an increasing amount of algal- epifaunal bivalves disappeared due to reduced oxygen con- derived marine organic matter was preserved, resulting in ditions (Pedersen 1986; Michelsen 1989a). the accumulation of organic-rich, oil-prone mudstones in A gradual decrease in the rate of sea-level rise in the parts of the basin (F-III and F-IV members, see below; Figs Early Pliensbachian Jamesoni Zone caused a distinct ba - 8, 9, 13). During the remainder of the Early Jurassic and sinward progradation of shoreface clinoform sandstones in the Early Aalenian Opalinum Zone (early Middle on the Skagerrak–Kattegat Platform. The regression cul- Jurassic), a succession of up to 150 m of marine mudstones minated in the middle Early Pliensbachian (early Ibex was deposited in the Sorgenfrei–Tornquist Zone (F-III and Zone) and deposition changed from fine-grained mud F-IV members, Fjerritslev Formation; Figs 8, 9, 12–14). (F-Ib unit, F-I member) to silty and sandy heteroliths Interbedded with the mudstones are three shoreface sand- (F-IIa unit, F-II member, SB 13; Figs 8, 9, 12–14; Michelsen stones, 5–15 m thick, overlying regressive marine erosion 1989a; Nielsen 2003). When the sea level started to rise surfaces. These sandstones were deposited during sea-level again, deposition of fine-grained mud resumed in the ba - falls and could act as carrier beds for any petroleum gen- sin (lower part of F-IIb unit, F-II member, Fjerritslev erated and expelled from thermally mature Toarcian source Formation), while backstepping parasequences of marine rocks. sand were succeeded by transgressive mud on the Ska - gerrak–Kattegat Platform. Peak transgression was reached in the late Early Pliensbachian Davoei Zone. Thereafter, the rate of sea-level rise decreased and a coarsening-upward Late Early – Middle Jurassic succession of mud and fine-grained heteroliths was deposited uplift and erosion in the study area (middle part of F-IIb unit, F-II member). The Ringkøbing–Fyn High and most of the Nor we - gian–Danish Basin were uplifted in late Early Jurassic – early Middle Jurassic times, and the Triassic – Lower Jurassic successions were eroded on the highest parts of the Late Pliensbachian – Early Aalenian Ringkøbing–Fyn High (Figs 8, 14, 15). The Lower Jurassic sea-level fluctuations succession, including the potential source-rock intervals of Significant erosion took place on the Skagerrak–Kattegat the Fjerritslev Formation, was deeply eroded in the up - Platform during a sea-level fall in the early Late Pliensbachian lifted area north of the high. Outside the study area, to- Margaritatus Zone (SB 14; Figs 8, 12). Basinwards, depo- wards the north-west, Norwegian wells (10/7-1, 10/8-1, sition changed to silty and sandy mud and fine-grained 11/9-1, 11/10-1; Fig. 1) confirm the deep regional ero- sand, showing pronounced thinning over some salt struc- sion of the Lower Jurassic succession, although the large tures possibly reflecting shallow-water depths (upper part hiatuses recorded in these wells may not be representative

22 4°E 8°E 12°E 4°E 8°E 12°E ABNorway Hettangian Norway Sinemurian – 58°N 100 km 58°N Early Pliensbachian 100 km 16°E 16°E

Sweden Sweden

Denmark Denmark 56°N 56°N

The Germany The Germany Netherlands Netherlands

4°E 8°E 12°E 4°E 8°E 12°E CDNorway Toarcian – Norway Late Aalenian – 58°N Early Aalenian 58°N Bathonian 100 km 100 km

16°E 16°E

Sweden Sweden

Denmark Denmark 56°N 56°N

The Germany The Germany Netherlands Netherlands

4°E 8°E 12°E 4°E 8°E 12°E E Norway Callovian F Norway Kimmeridgian 58°N 100 km 58°N 100 km

16°E 16°E

Sweden Sweden

Denmark Denmark 56°N 56°N

Germany Germany The The Netherlands Netherlands

4°E 8°E 12°E Offshore marine (mud-dominated) G Norway Volgian – 58°N Early Ryazanian Deep marine (sands, gravels) 100 km Shallow marine sandstones and siltstones 16°E Paralic and non-marine sandstones, siltstones, mudstones and coals Sweden Non-deposition/erosion Denmark 56°N

Fig. 14. Paleogeographic maps showing changes in the general Germany The depositional environments through Jurassic times. Modified from Netherlands Michelsen et al. (2003). since they intersect positive Jurassic structures that were els closer to the Sorgenfrei–Tornquist Zone (Fig. 15). exposed to deep erosion during regional uplift. In the Within the fault-bounded Sorgenfrei–Tornquist Zone itself, Egersund Basin, located on-strike farther to the north-west where subsidence still occurred (but at a much lower rate (Fig. 1), the Fjerritslev Formation is also deeply truncated than before), the dramatic changes in regional basin con- and is locally absent. Erosion did not reach such deep lev- figuration are marked by a shift from offshore mudstones

23 6°E Norway 10°E 58°N Intra-Aalenian basinward shift in facies Middle–Upper Jurassic absent Subcrop to the base Mid- dle Jurassic unconformity U. Toarcian Subcrop to the base Cretaceous unconformity Fault Well Distribution of F-III/F-IV mbs U. Pliensbach. SinemurianL. Pliensbach.L. Toarcian of the Fjerritslev Fm Hettangian U. Triassic 100 km

Sweden

56°N

Hettangian ? Hettangianurian urian L. Sinem The Netherlands Germany U. Sinem ?

Fig. 15. The subcrop below the base Middle Jurassic unconformity and the distribution of the stratigraphic interval with the Lower Jurassic potential source rocks (F-III and F-IV members). The coarse-dotted area indicates the area within which the shift from marine mudstones to shallow marine or fluvial sandstones occurred in the Aalenian without major erosion. Modified from Nielsen (2003).

of the F-IV member (Fjerritslev Formation) to shallow marine sandstones (Haldager Sand Formation; Figs 8, Late Middle – Late Jurassic 12–16). During the rest of the Aalenian, the Bajocian and basin expansion the early Bathonian, deposition was more or less confined Regional subsidence resumed during late Middle – Late to the narrow zone bounded by the Fjerritslev and Børglum Jurassic times and the shallow marine clastic depocentre grad- Faults and their south-eastward continuation in the Kattegat, ually expanded. The Upper Jurassic thus onlaps onto the Øresund and southern Sweden (Figs 8, 14). It is assumed base Middle Jurassic unconformity, showing significant that the westward continuation of the faults defining the younging of the onlap towards the south-west and north- southern margin (the Fjerritslev Fault) and the northern east, on both sides of the Sorgenfrei–Tornquist Zone (Figs margin of the Farsund Basin also delineate an area of con- 14, 17). Deposition overstepped the bounding faults of tinued subsidence in Middle Jurassic times, thus protect- the Sorgenfrei–Tornquist Zone, with deposition of Bath - ing the potential source rocks in the upper Fjerritslev onian(?) braided fluvial sands on the Skagerrak–Kattegat Formation from erosion in that area. The Fjerritslev Trough Platform and in the Himmerland Graben. Deposition also and presumably also the Farsund Basin received detritus from resumed outside the study area where Bathonian(?) sand- the uplifted areas to the west and south-west, and from the stones of the Haldager Sand and Bryne Formations are Baltic Shield to the north, resulting in deposition of the preserved in the 10/7-1 and 10/8-1 wells. Haldager Sand Formation, which is 29–155 m thick in A marine transgression close to the Callovian–Oxfordian the well sections in the Fjerritslev Trough (Figs 8, 9, 16). boundary influenced most of the basin, and accommoda- The formation is dominated by shoreface and fluvial sand- tion space was also created in the former by-pass zone of stones interbedded with thin marine and lacustrine mud- the southern part of the basin and on the Skagerrak–Kattegat stones, and thin coaly beds in places. Platform, where fluvial sands were deposited. During the

24 – ak rr Terne-1

Skageattegat K rm o atf Pl 100 m one

n ., Himmerland

Frederikshavn-2 i

Skagen-2 –Tornquist Z Flyvbjerg-1 sh Bas i n

Da Hi. Gr Haldager-1 TS 22 SB 22 MFS 16 SB 16 TS 23 SB 23 MFS 22 Sorgenfrei Hi. Gr

Norwegian– NE ugh Fjerritslevo Tr 50 km SP Skagen-2 35 km

SP Sequence LST Lowstand Ha 1 Frederikshavn-2 TST Transgressive TST Highstand HST

Systems tracts SB Sequence boundaryBounding surfaces Sequence SB marine surface Maximum MFS surface flooding Transgressive TS 32 km Skagerrak–Kattegat Platform Skagerrak–Kattegat Fault Børglum TST LST SP Depositional environments Fluvial Estuarine Lagoonal Shoreface Offshore Flyvbjerg-1 Graben) showing the Toarcian–Oxfordian depositional environments of the upper Fjerritslev, Haldager Sand, Flyvbjerg Sand, Haldager of the upper Fjerritslev, depositional environments Toarcian–Oxfordian the showing Graben) depending Jurassic truncation the varying of erosional at the base of Middle severity Note and Børglum Formations. succession. Modified Jurassic the truncation by on structuralof the sequences defined in Lower position, as reflected (2003). Nielsen from Fig. 16. Well-log panel trending SW–NE across the Norwegian–Danish Basin (see inset map; Basin the Norwegian–Danish SW–NE across panel trending Well-log 16. Fig. 148 km LST GR Terne-1 TST TST HST HST HST Børglum Fm

8 Fm Fjerritslev Flyvbjerg Fm Flyvbjerg Haldager Sand Fm 145 km MFS 19

MFS 1 TS 22 TS Fl 1 Fj 9 Fj 8 Fj 7 Bø 1 Ha 3 Ha 2 Ha 1 Fj 10 LST LST LST TST LST TST LST SP Sorgenfrei–Tornquist Zone Sorgenfrei–Tornquist Haldager-1 TST TST TST HST HST SW SB 23 SB 22 SB 21 SB 20 SB 19 SB 18 SB 17 SB 16 TS 23 TS 21 TS 20 TS 19 MFS 22 MFS 20 MFS 17 MFS 16

25 Oxfordian, the sedimentation area was further enlarged, and a north-eastwards thickening wedge of transgressive, fos- Cretaceous continued basin siliferous marine sand and mud was deposited above lagoonal expansion and chalk deposition deposits on the Skagerrak–Kattegat Platform (lower Flyv - In Early Cretaceous times, the area of marine deposition bjerg Formation; Figs 8, 9, 13, 16). The transgression expanded further with coastal progradation from the north peaked in the mid-Oxfordian, with deposition of marine and north-east. Deposition of marine mud prevailed over mudstones over most of the northern part of the study area most of the study area. The Lower Cretaceous mudstones including the Fjerritslev Trough and the Skagerrak–Kattegat with sandy intercalations (most common towards the north- Platform. A sea-level fall in the latest Oxfordian resulted east) are included in the Vedsted Formation (Fig. 9), which in coastal progradation on the Skagerrak–Kattegat Platform consists of four depositional units (Michelsen & Nielsen and in the Fjerritslev Trough; fluvial and shallow marine 1991). sands were deposited, and a south-west prograding wedge In Late Cretaceous – Danian times, a high sea level dom- was formed (upper Flyvbjerg Formation; Figs 8, 9, 16). inated and the study area was covered by an epicontinen- Extensive marine flooding occurred in Kimmeridgian tal sea. The dry climate and low relief of the hinterland times, and sedimentation of marine mud (Børglum For - reduced clastic input, and biogenic, pelagic chalk deposi- mation) characterised the whole area, although marked tion (with coccolith plates being the dominant constituent) thinning towards the south-west of the study area empha- occurred over the entire study area. In the easternmost part sises the reduced accommodation space there (Figs 8, 12–14, of the basin and within the Fennoscandian Border Zone, 16). During Volgian–Ryazanian times, the depositional deposition of marine greensands ceased in late Cenomanian environment was dominantly a shallow shelf, with three times and in the Early Turonian intermittent deposition of to four major phases of coastal progradation (sequences marls and mudstones in parts of the basin also came to an Fr 1–3 of the Frederikshavn Formation; Figs 8, 14). Marine end (Stenestad 1972; Surlyk 1980). In central parts of the muds were deposited over much of the study area. study area, in the deepest parts of the Cretaceous basin, coc-

6°E Norway 10°E 58°N Intra-Aalenian basinward shift in facies Oxfordian Middle–Upper Jurassic absent Onlap limits Onlap limits uncertain Bajocian–Bathonian Fault Well 100 km

14°E Oxfordian

Kimmeridgian–Volgian

56°N Sweden

The Netherlands Germany ?

Fig. 17. Map showing the Upper Jurassic – Lower Cretaceous onlap onto the base Middle Jurassic unconformity, indicating the gradual expansion of the depositional area. Modified from Nielsen (2003).

26 colith-dominated chalks accumulated in water depths that may have been up to 500–600 m (Surlyk & Lykke-Andersen Cenozoic clastic deposition 2007). Closer to the basin margins and over structural and Neogene exhumation highs, chalks rich in bryozoans and other benthic fossils were After cessation of carbonate deposition in the Paleocene, deposited at mid to inner shelf depths (?100–200 m). In deep marine sedimentation of fine-grained hemipelagic more shallow water areas in the Fennoscandian Border deposits took over in the study area. The northern and Zone, benthos-rich chalks pass into bryozoan wackestones eastern limits of these fine-grained sediments are unknown and packstones that locally developed as mound complexes, due to later erosion. In the Oligocene, major clastic wedges while skeletal grainstones and oyster bank carbonates formed began to build out from the Baltic Shield, while prodeltaic closer to the shoreline (Surlyk 1997). glauconite-rich clayey sediments were deposited farther Centrally in the study area, south-west of the Sorgen - basinwards. Coarse-grained sediments reached the south- frei–Tornquist Zone, 1.5–2 km of chalk was deposited, ern part of the basin and the Ringkøbing–Fyn High in while 500–750 m accumulated over the Ringkøbing–Fyn Neogene times (Larsen & Dinesen 1959; Friis et al. 1998; High. In the Sorgenfrei–Tornquist Zone, the original thick- Rasmussen 2004). Deposition in the greater North Sea ness of the chalk succession is masked by Late Cretaceous basin continued during the Pliocene, and up to 500 m of – Palaeogene inversion and erosion (Liboriussen et al. 1987; sediments were deposited in the Norwegian–Danish Basin Nielsen & Japsen 1991; Jensen & Schmidt 1993; Michelsen during the Late Miocene and Pliocene (Overeem et al. & Nielsen 1993; Erlström & Sivhed 2001). The inversion 2001). During the post-Late Cretaceous period, parts of began in Coniacian times and accelerated rapidly during the Norwegian–Danish Basin and the Fennoscandian Border Santonian–Campanian times (the sub-Hercynian phase; Zone were uplifted and eroded. This major tilting contin- Ziegler 1990); quiescence during Maastrichtian–Danian ued into the Quaternary (Japsen 1993; Jensen & Schmidt times was followed by pronounced inversion again in the 1993; Japsen et al. 2002a, b; Rasmussen et al. 2005). Late Paleocene (the Laramide phase; Liboriussen et al. 1987; Ziegler 1990).

27 Regional coalification curves corrected for net exhumation

Due to Late Cretaceous – Early Cenozoic inversion of fault In the present study, new regional coalification curves blocks in the Sorgenfrei–Tornquist Zone and Neo - for the Norwegian–Danish Basin have been constructed in gene–Pleistocene regional uplift of the Norwegian–Danish order to evaluate the depth to the oil window. The depths Basin (e.g. Michelsen & Nielsen 1991, 1993; Jensen & of the samples have been corrected for net exhumation, Schmidt 1993; Japsen 1993, 1998; Petersen et al. 2003a), the magnitudes of which have been derived from both shale present-day burial depths must be corrected for post-Early and chalk velocities. A total of 560 vitrinite reflectance Cretaceous net exhumation (Fig. 18). Petersen et al. (2003a) (VR) measurements from 26 wells in the Norwegian–Danish presented a regional coalification curve for the Norwe- Basin were available for construction of the coalification gian–Danish Basin based on 249 measurements from 15 curves (Fig. 18). The VR values are from Thomsen (1980, wells (onshore wells and the Hans-1 well in the Kattegat). 1983), Schmidt (1985, 1988, 1989) and GEUS unpub- That study did not include offshore wells from the Skagerrak lished data. All VR values are random measurements per- area. The well-sections were corrected for net-exhumation formed on core samples, sidewall cores or cuttings, and as values obtained from the analysis of sonic velocities of many particles as possible were measured in each sample. shales by Japsen (1993). The accuracy of this method is Untreated rock samples (whole rock) were used, as such sam- dependent on a uniform shale unit covering the entire ples – compared to kerogen concentrates – have the advan- study area and a valid sonic velocity reference curve. Net tage that it is easier to identify the primary vitrinite particles exhumations for wells not included in Japsen’s (1993) study and thus avoid oxidised and bituminous organic matter were estimated by comparison to nearby wells and inter- (e.g. Barker 1996). Identification of the primary, i.e. indige- polation. nous or autochthonous, vitrinite is essential for obtaining reliable VR values as this vitrinite reflects the actual ther-

Table 1. Net exhumation magnitudes* Well Shale† Chalk† Mean§ VR Comments Anholt-1 1400 1400 - - Comparison to Hans-1, Terne-1 and Frederikshavn-1 (very uncertain) Års-1 442 461 - - Børglum-1 1185 600 - - C-1 300 306 - - Comparison to Inez-1 and Vemb-1 (264 m) D-1 50 50 - - Comparison to L-1 F-1 481 357 419 1200 Farsø-1 377 530 - - Felicia-1/1A 1020 712 866 800 Fjerritslev-2 1443 802 - - Frederikshavn-1 1000 702 - - Comparison to Sæby-1 (1051 m) Gassum-1 1090 579 - - Comparison to Voldum-1 (845 m) + 250 m deeper chalk truncation Haldager-1 1400 486 - - Comparison to Børglum-1 and Fjerritslev-2 Hans-1 1735 - - - Hobro-1 450 543 - - Comparison to Års-1 and Kvols-1 Hyllebjerg-1 470 552 - - Inez-1 344 445 395 850 K-1 624 414 519 1300 Kvols-1 361 420 - - L-1 0 0 - - Mors-1 690 602 - - R-1 150 176 - - Comparison to C-1, L-1, S-1 (0 m) and Vemb-1 (238 m) Rønde-1 394 447 - - Skagen-2 1000 1000 - - Comparison to Børglum-1, Frederikshavn-1 and Sæby-1 (1051 m) Terne-1 1373 - - - Vedsted-1 1300 500 - - Comparison to Børglum-1, Fjerritslev-2 and Haldager-1 Vinding-1 175 250 - - Comparison to Mejrup-1 (sh: 231 m; ch: 253 m) and Vemb-1 (sh: 264 m; ch: 238 m)

* Net exhumation magnitudes based on shale and chalk sonic velocity data (Japsen et al. 2007) † Magnitudes in italics are estimated; see comments column § Mean of shale (sh) and chalk (ch)

28 A Vitrinite reflectance (%Ro) 0.20.4 0.6 0.8 1.0 1.2 1.4 1.6 0

2000 (m) present day present

n = 560 Depth Anholt-1 Hobro-1 Års-1 Hyllebjerg-1 4000 Børglum-1 Inez-1 C-1 K-1 D-1 Kvols-1 F-1 L-1 Farsø-1 Mors-1 Felicia-1/1A R-1 Fjerritslev-2 Rønde-1 Frederikshavn-1 Skagen-2 Gassum-1 Terne-1 Haldager-1 Vedsted-1 Hans-1 Vinding-1 6000 Fig. 18. A: Vitrinite reflectance values plotted against present-day depths for 26 onshore and offshore wells.

mal maturity of the organic matter at the sampled depth. Higher reflecting vitrinite particles may represent recycled Shale velocity-based curve organic matter that attained higher maturity from the tem- The shale sonic velocity reference curve of Japsen (1993) perature history of its former host rock (e.g. Bostick 1979; has recently been refined and new, modified net-exhuma- Hunt 1996; Taylor et al. 1998). In contrast, vitrinite that tion values based on shale velocities have been proposed yields anomalously low reflectance values may be suppressed (Table 1; Japsen et al. 2007). The revised magnitudes of net (Buiskool Toxopeus 1983; Carr 2000a, b). A more detailed exhumation are reduced by about 100 m compared to the description of the prerequisites for construction of a regional values used by Petersen et al. (2003a). The new net-exhuma- coalification curve is presented in Petersen et al. (2003a). tion values have been used to revise the coalification curve

29 B Vitrinite reflectance (%Ro) 0.2 0.30.4 0.5 0.6 0.7 0.8 0

1000 (m)

present day present 2000 Depth

3000

4000 n = 545

Fig. 18. B: As in 18A, but excluding the Felicia-1/1A deep well; for legend, see Fig. 18A.

for the study area. In Fig. 19, the present depths of sam- (2003a), the Fjerritslev-2, Haldager-1 and Vedsted-1 wells ples from 25 wells have been corrected for the magnitudes from the Fjerritslev Trough are not included as they define of net exhumation obtained from shale velocities. For some an atypically steep maturation gradient. This was explained wells, net-exhumation values based on shale velocities are by a probable overestimation of shale velocities in this area not available, and the amount of net exhumation for these due to an increased coarse-grained component within the wells has been estimated by comparison to nearby wells lowermost Fjerritslev Formation. The Anholt-1 and Ter- and stratigraphic evaluation (Table 1). Despite the VR data ne-1 wells show unusually low VR values and are thus not showing considerable scatter, 13 of the wells define a well- included. Compared to the curve in Petersen et al. (2003a), constrained VR trend (Fig. 20A). Following Petersen et al. the Horsens-1, Lavø-1 and Ullerslev-1 wells have been

30 Vitrinite reflectance (%Ro) 0.20.3 0.4 0.5 0.6 0.7 0.8 0

1000 (m)

2000 corrected (shale) corrected

Depth n = 545

Anholt-1 Hyllebjerg-1 Års-1 Inez-1 Børglum-1 K-1 C-1 Kvols-1 3000 D-1 L-1 F-1 Mors-1 Farsø-1 R-1 Fjerritslev-2 Rønde-1 Frederikshavn-1 Skagen-2 Gassum-1 Terne-1 Haldager-1 Vedsted-1 Hans-1 Vinding-1 Hobro-1 4000 Fig. 19. Vitrinite reflectance values from 25 wells plotted against depths corrected for post-Early Cretaceous uplift on the basis of shale sonic velo - city data.

omitted, as only one VR measurement is available from each a VR of c. 0.6%Ro, the ‘shale curve’ suggests that the top well. The VR data from the Børglum-1 well are, however, of the oil window occurs at a burial depth of c. 3050 m. included in the new coalification profile. Accurately deter- VR data from offshore wells in the Skagerrak are not mined VR values will define a straight line in a semi-log included in the coalification curve (Fig. 20A). Generally, plot (Dow 1977), and the established VR profile yields a the VR values from these wells lie above the maturity curve correlation coefficient of 0.89 (Fig. 20A). The linear regres- (Fig. 19), and data from four of the wells (D-1, F-1, Inez-1 sion line intercepts the surface at a VR of c. 0.26%Ro which and K-1) provide a relatively well-defined VR trend (Fig. is at the upper end of the reflectance values recorded for 20B). The regression line yields a correlation coefficient of peaty organic matter at the surface (c. 0.10–0.25%Ro; 0.80 and intercepts the surface at a VR of c. 0.28%Ro. This Cohen et al. 1987). If the start of the oil window is set at alternative maturity gradient may be applied for the off-

31 A Vitrinite reflectance (%Ro) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.26 0 Fig. 20. A: Regional coalification curve for the Norwegian–Danish Basin based on 12 onshore wells and the Hans-1 well, selected from the wells in Fig. 19 (see text). A total of 263 vitrinite reflectance 1000 values have been used. The regres - sion line has a correlation coefficient of r2 = 0.89 and the line

intercepts the surface at 0.26%Ro. (m) The depth to the top of the oil

r2 = 0.89 window at 0.6%Ro is c. 3050 m. 2000 n = 263 B: Coalification curve based on four wells from the Skagerrak area. Års-1 corrected (shale) corrected Børglum-1 The regression line yields a corre - Farsø-1 lation coefficient of 0.80 and the Depth Frederikshavn-1 line intercepts the surface at a VR

Gassum-1 of c. 0.28%Ro. The top of the oil Hans-1 window (VR c. 0.6%R ) occurs at a 3000 Hobro-1 o burial depth of c. 2600 m according Hyllebjerg-1 Kvols-1 to this curve. Mors-1 Rønde-1 Skagen-2 Vinding-1 4000

B Vitrinite reflectance (%Ro) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.28 0

1000 (m) corrected (shale) corrected

2000 2 Depth r = 0.80 n = 166 D-1 F-1 Inez-1 K-1 3000

32 shore part of the study area. According to this curve, how- c. 0.28%Ro, which is at the upper end of the VR values of ever, the top of the oil window (VR c. 0.6%Ro) occurs at peaty organic matter (Fig. 22A). Compared to the ‘shale a burial depth of c. 2600 m which, given the absence of curve’, the slightly lower gradient of the ‘chalk curve’ yields petroleum discoveries in this part of the basin, is probably a burial depth of c. 3100 m for the top of the oil window too shallow. Underestimation of the magnitude of exhuma- using a VR of 0.6%Ro. tion in the area may provide an explanation for the appar- As with the shale velocity-corrected samples, the chalk ently erroneous curve. velocity-corrected samples from offshore wells in the Skagerrak yield VR values that generally lie above the established maturity profile (Fig. 21). Three of these wells (F-1, Inez-1, K-1) provide a well-constrained VR gradient (cor- Chalk velocity-based curve relation coefficient of 0.90) that intercepts the surface at Chalk velocities have – like shale velocities – been used to c. 0.26%Ro (Fig. 22B). According to this curve, the top of estimate the magnitude of net exhumation by Japsen (1998). the oil window is located at about 2450 m depth, which is Considerable uncertainty in the estimated exhumation may considered an unrealistically shallow burial depth probably be introduced if the chalk section is <300 m thick. Net- due to underestimation of the magnitude of exhumation. exhumation magnitudes based on new chalk velocities (Japsen et al. 2007) have been used to correct the present- day sample depths in this study (Table 1). Compared to the net-exhumation magnitudes in Japsen (1998), the Depth to the top of the oil window adjusted values presented here are reduced by about 200 If the top of the oil window is set at a maturity level cor- m. In Fig. 21, the present depths of samples from 25 wells responding to c. 0.6%Ro, the two depth-corrected matu- have been corrected for the magnitudes of net exhumation rity gradients, derived from the onshore wells, yield similar obtained from chalk velocities. The VR data show a rela- depths to the top of the oil window (c. 3050–3100 m). tively large scatter, although the data seem to group into According to the relationship between VR and burial peak two elongate populations with the upper population prin- temperature of Barker & Pawlewicz (1994), c. 0.6%Ro cor- cipally formed by offshore wells in the Skagerrak. The lin- responds to c. 95°C. This fits reasonably well with the pres- ear regression line for a regional VR curve based on 15 ent-day thermal gradient of 31.4°C/km in Felicia-1/1A, as onshore wells and the Hans-1 well has a correlation coef- estimated from corrected bottom-hole temperature mea- ficient of 0.87 (Fig. 22A), which is slightly poorer than the surements. In contrast, depth-corrected maturity gradients ‘shale curve’. The linear regression line is, however, based based on offshore wells from the Skagerrak yield unrealis- on more data (n = 282) as VR data from the Fjerritslev-2 tically shallow burial depths to the top of the oil window. and Vedsted-1 wells are included in the coalification pro- Burial depths of only 2450–2600 m would imply the pres- file. Like the ‘shale curve’, the Haldager-1 well yields an ence of mature source rocks in the area, which is inconsis- abnormally steep maturity gradient, while the Anholt-1 tent with the lack of direct evidence of generated petroleum. and Terne-1 wells display much too low VR values compared to the overall trend defined by the majority of wells (Fig. 21). The curve intercepts the surface at a VR of

33 Vitrinite reflectance (%Ro) 0.2 0.30.4 0.5 0.6 0.7 0.8 0

1000 (m)

2000 corrected (chalk) corrected

n = 545 Depth Anholt-1 Hyllebjerg-1 Års-1 Inez-1 Børglum-1 K-1 C-1 Kvols-1

3000 D-1 L-1 F-1 Mors-1 Farsø-1 R-1 Fjerritslev-2 Rønde-1 Frederikshavn-1 Skagen-2 Gassum-1 Terne-1 Haldager-1 Vedsted-1 Hans-1 Vinding-1 Hobro-1 4000

Fig. 21. Vitrinite reflectance values from 25 wells plotted against depths corrected for post-Early Cretaceous uplift on the basis of chalk sonic velo - city data.

Fig. 21

34 A Vitrinite reflectance (%Ro) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.28 Fig. 22. A: Regional coalification 0 curve for the Norwegian–Danish Basin based on 14 onshore wells and the Hans-1 well, selected from the wells in Fig. 21 (see text). A total of 282 vitrinite reflectance values have been used. The regression line has a 1000 correlation coefficient of r2 = 0.87 and the line intercepts the surface at 0.28%R . The depth to the top of the o (m) r2 = 0.87 oil window at 0.6%Ro is c. 3100 m. n = 282 B: Coalification curve based on three offshore wells from the Skagerrak 2000 Års-1 Børglum-1 area. The VR data form a very well- Farsø-1 corrected (chalk) corrected constrained VR gradient (correlation Fjerritslev-2 coefficient of 0.90) that intercepts the Frederikshavn-1 Depth surface at c. 0.26%Ro. According to Gassum-1 this curve, the top of the oil window Hans-1 is located at only c. 2450 m depth. Hobro-1 3000 Hyllebjerg-1 Kvols-1 Mors-1 Rønde-1 Skagen-2 Vedsted-1 Vinding-1 4000

B Vitrinite reflectance (%Ro) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.26 0

1000 (m) corrected (chalk) corrected

2000 Depth r2 = 0.90 n = 157 F-1 Inez-1 K-1 3000

35 Distribution and thermal maturity of potential source rocks

The regional petroleum generation potential and thermal petroleum was generated from Lower Palaeozoic source maturity of the pre-Upper Cretaceous succession in the rocks. Lower Palaeozoic source rocks and oils are known study area have been assessed by the evaluation of more than from the Baltic area, where Lower Silurian shales consti- 4000 data points (S1 and S2 yields, Hydrogen Index (HI), tute the principal source (Zdanaviciuté & Bojesen-Koefoed Tmax, total organic carbon (TOC)). These are derived from 1997). Moreover, oil has been generated from Palaeozoic Rock-Eval pyrolysis and TOC determination of principally units in Sweden (Sivhed et al. 2004 and references therein). cuttings samples from 33 wells situated onshore Denmark The up to 90 m thick Middle Cambrian – Lower and offshore in the Skagerrak and Kattegat areas (Fig. 1). Ordovician Alum Shale, which is exposed in Skåne, on Most of the wells drilled the Lower Cretaceous – Triassic Bornholm and in southern Norway, possesses source-rock succession, but pre-Permian and Permian strata were also properties. The black marine shales are highly organic-rich encountered in some wells. with an organic matter content locally up to 30% (Thomsen Unreliable Tmax or HI values derived from low S2 yields et al. 1987; Bharati et al. 1992). In central Sweden, the Alum (i.e. approximately <0.2 mg HC/g rock) or TOC contents Shale is immature to marginally mature and has HI values have been omitted from the data set. The following thresh- up to above 600 mg HC/g TOC, but in the Fennoscandian olds for HI have been applied to evaluate the petroleum Border Zone the shales are overmature (Buchardt et al. generation capacity of the predominantly Type II kerogen 1986; Buchardt & Lewan 1990; Bharati et al. 1992). The dominated source rocks (Peters & Cassa 1994): Alum Shale was encountered in the Slagelse-1 and Terne-1 HI <200 mg HC/g TOC: gas-prone wells. In the Slagelse-1 well, TOC contents are below the HI = 200–300 mg HC/g TOC: mixed oil/gas-prone reliable detection limit of the carbon analyser, but in the HI >300 mg HC/g TOC: oil-prone Terne-1 well, TOC contents range from 5.39–10.08 wt%. Determination of the thermal maturity is based on vitri- The Alum Shale in the Terne-1 and Slagelse-1 wells is, nite reflectance (VR) and Tmax values, and the potential however, overmature, with HI values <8 mg HC/g TOC source rock is considered immature for VR values <0.6%Ro in the Terne-1 well. or Tmax values <435°C. The Tmax range 435–460°C defines High TOC contents and HI values were recorded in the the oil window (Bordenave et al. 1993; Peters & Cassa Upper Silurian Nøvling Formation in the Rønde-1 well, 1994). Tmax is, however, also influenced by kerogen type where the formation consists of interbedded basalts, grey and the mineral matrix, and single Tmax values may there- and red-brown claystones and sandstones with some car- fore be less reliable as a maturity parameter (Peters 1986). bonates (Christensen 1971, 1973). The samples are, how- Average Tmax values for specific formations or members ever, contaminated by various drilling mud additives, such with source rocks are therefore used. as diesel, Black Magic®, and starch-based mud, which ren- der the data unreliable. Upper Carboniferous strata were encountered by the Hans-1 well, but no source-rock data are available. The Pre-Permian units drilled redbed section consists of interbedded sandstones, Seven wells have encountered pre-Permian strata in the siltstones and claystones overlain by volcanic rocks and Danish part of the Norwegian–Danish Basin and claystones, which are regarded to have no source-rock Fennoscandian Border Zone. Source-rock data are available potential based on the lithology (Michelsen & Nielsen from five of them: Frederikshavn-1, Nøvling-1, Rønde-1, 1991, 1993). Upper Carboniferous coals are, however, sig- Slagelse-1 and Terne-1 (Fig. 1). Bitumen and oil stains have nificant gas source rocks regionally in Northwest Europe been reported from Lower Palaeozoic rocks onshore Norway and the southern North Sea (Lokhorst 1998; Gautier 2003). and Sweden. In the Oslo Graben, bitumen has been found West of the study area, the Carboniferous has been drilled in fractures in Ordovician limestones and corals, and in by nine released wells in the Danish Central Graben and southern Norway, close to the Oslo Graben, oil and gas are the southern part of the Norwegian and UK Central Graben; trapped in Permian volcanic intrusive rocks (Pedersen et al. these strata are of Early Carboniferous age (Bruce & 2005, 2007). In southern Sweden, at Österplana, oil has Stemmerik 2003). Thin Lower Carboniferous coals were been found in Upper Ordovician carbonate rocks (Pedersen encountered in the Gert-2 well, and these coals possess a et al. 2007). Pedersen et al. (2005, 2007) suggested that the gas/condensate generation potential inherited from the

36 original vegetation, which is a general aspect of Car - in agreement with the high organic maturity. Tmax values boniferous coals (Petersen 2006; Petersen & Nytoft 2006, of 480–494ºC correspond roughly to a vitrinite reflectance 2007a, b). Reworked Carboniferous palynomorphs are range of 1.6–1.9%Ro, and at this maturity level the fluo- common in the Jurassic of the Norwegian–Danish Basin rescence behaviour of all types of organic matter has dis- suggesting that Carboniferous strata, probably coal-bear- appeared. On the basis of presently available data, therefore, ing, were originally more widespread (Nielsen & Koppelhus the Permian succession does not exhibit petroleum gener- 1990) and may be preserved below the regional Permian ation potential, although it is acknowledged that data are unconformity in local deep grabens. These strata may thus few and the Permian potential cannot be categorically dis- potentially constitute a deep-seated source for gas/con - counted. Thin black, bituminous shales possibly equiva- densate. lent to the Kupferschiefer in the North German Basin were In summary, the well data described here do not demon- encountered in the Rønde-1 well (Jacobsen 1971) and thin strate the occurrence of viable source rocks in the pre-Per - shales appear to be locally present in the Zechstein succession. mian succession. It should be acknowledged, however, that The distribution of these facies in the Norwegian–Danish the Silurian is only known from the Rønde-1, Nøvling-1 and Basin is poorly known. Terne-1 wells where less than 450 m of the up to 2600 m thick succession have been investigated (Christensen 1971, 1973; Michelsen & Nielsen 1991, 1993). Similarly, the distribution and nature of Carboniferous strata are very Triassic units poorly known and it can be speculated that gas/condensate- Triassic strata are dominated by continental to marginal prone Carboniferous strata may be preserved in local grabens marine sandstones, mudstones, marls, carbonates and evap- and half-grabens, as recorded from the Central Graben orites, and good quality source rocks are not common in (Bruce & Stemmerik 2003). the Triassic. Petroleum generation potential has been detected in Upper Triassic strata in the Hans-1, Mejrup-1, Rønde-1 and Skagen-2 wells. A few HI values up to above 500 mg HC/g TOC have been recorded from mudstones of the Permian units marine Oddesund Formation in the Rønde-1 well, and in Strata of Zechstein and Rotliegend age are present in nine the Mejrup-1 well a c. 8 m thick interval in the brackish wells, and results of source-rock analyses are available from Vinding Formation has HI values up to nearly 700 mg five of them (C-1, Nøvling-1, Rønde-1, Slagelse-1, Sæby-1; HC/g TOC. Fig. 1). No petroleum generation has been observed in the In the Hans-1 well, where parts of the Gassum Formation analysed well sections. High TOC contents and HI values consist of aggrading parasequences of coastal plain deposits in the Rotliegend Group in the Rønde-1 well, where the with coal beds, a c. 10 m thick interval possesses gas gen- formation is mainly composed of reddish brown sandstones eration potential. In the Skagen-2 well, the uppermost part (Jacobsen 1971; Nielsen & Japsen 1991), are artefacts from of the Gassum Formation and the lowermost part of the drilling mud additives. F-I member of the Fjerritslev Formation comprise lagoonal Cuttings samples, showing a resemblance to black shale, sediments with a restricted capacity to generate liquid petro- from the Rotliegend section (5092–5260 m) in the Feli - leum (see below). In the Mejrup-1 well, the Gassum cia-1A well, yield extraordinarily high HI values (exceed- Formation is more fine-grained than normal, and certain ing 1000 mg HC/g TOC) and low Tmax from 425–440ºC mudstone intervals have HI values up to 534 mg HC/g (Petersen et al. 2003b). However, these data are flawed as TOC; the average HI value for a c. 73 m thick mudstone- the samples are contaminated by oil-based drilling mud dominated section is 216 mg HC/g TOC. This unusual (Shellsol D-70), which was used below a depth of about development of the formation in Mejrup-1 is probably 2000 m. Core samples collected from thin black, shaly related to the position of the well in the secondary rim syn- intervals yield HI values <32 mg HC/g TOC and Tmax val- cline of the Vejrum salt dome, in the centre of the basin. ues from 480–494ºC. This indicates overmaturity of the Movement of salt influenced depositional patterns at many organic matter and no source potential. Visual inspection locations in the basin, as shown by seismic data, but only of the dispersed organic matter (DOM) by reflected light few wells have drilled rim synclines and none show a devel- microscopy (white and fluorescing-inducing blue light) opment similar to that of the Gassum Formation in the reveals lack of fluorescence of the DOM that is classified Mejrup-1 well. However, detailed seismic mapping of the as vitrinite (Type III kerogen) and inertinite (Type IV kero- Upper Triassic may determine if similar depositional situ- gen). Lack of fluorescence and visible liptinitic material is ations and sufficient burial of Upper Triassic potential

37 700 Års-1 Mejrup-1 Type II F-1 Mors-1 Farsø-1 Nøvling-1 Felicia-1 Rødding-1 600 Fjerritslev-2 Rønde-1 Hans-1 Skagen-2 Hobro-1 Skive-1 Hyllebjerg-1 Slagelse-1 Inez-1 Sæby-1 500 J-1 Terne-1 K-1 Vedsted-1 Kvols-1 Vinding-1 Lavø-1 Voldum-1 400

Hydrogen Index (mg HC/g TOC) Index (mg HC/g Hydrogen 300

Type III

200

100 Fig. 23. Hydrogen Index versus Tmax plot of 497 samples from the F-I member of the Lower Jurassic Fjerritslev Formation. The F-I member is a poor source rock, generally with HI values

0 <100 mg HC/g TOC. 400 450 500 Tmax (°C)

source rocks occur adjacent to other salt structures. have been encountered in all the investigated wells. All four In summary, apart from a few local occurrences of Upper members of the Fjerritslev Formation (F-I – F-IV) are, Triassic units with a limited potential, the Triassic does not however, not present in all wells due to Middle Jurassic possess a petroleum generation potential. uplift and erosion over much of the basin (Andsbjerg et al. 2001; Nielsen 2003). As a result of the uplift and associated erosion, the F-I member at the base of the formation is regionally the most widespread, being present in all but Lower Jurassic units one well, whereas the F-IV member in the uppermost part With the sole exception of the C-1 well, the Lower Jurassic of the formation is only present in 20 of the 33 wells. The offshore marine mudstones of the Fjerritslev Formation F-II and F-III members are present in 25 of the wells.

38 700 Års-1 K-1 Type II F-1 Mejrup-1 Farsø-1 Mors-1 Felicia-1 Nøvling-1 600 Fjerritslev-2 Rødding-1 Flyvbjerg-1 Rønde-1 Frederikshavn-1 Skive-1 Frederikshavn-2 Sæby-1

500 Hans-1 Terne-1 Hobro-1 Vedsted-1 Hyllebjerg-1 Voldum-1 J-1

400

Hydrogen Index (mg HC/g TOC) Index (mg HC/g Hydrogen 300

Type III

200

100 Fig. 24. Hydrogen Index versus Tmax plot of 176 samples from the F-II member of the Lower Jurassic Fjerritslev Formation. The F-II member is a poor source rock, generally with HI values

0 <150 mg HC/g TOC. 400 450 500 Tmax (°C)

F-I member F-II member Only in the Skagen-2 well does the F-I member possess a A marginal petroleum generation potential has been recorded potential for petroleum generation (Fig. 23). As noted in the F-II member in the Farsø-1 and Mors-1 wells, whereas above (Triassic units), a 10 m thick interval spanning the the member in the Hobro-1 well shows a somewhat bet- uppermost part of the Gassum Formation and the lower- ter potential (Fig. 24), albeit over a very narrow interval. most part of the F-I member shows HI values ranging from Within a 6 m thick interval in Hobro-1, the HI values 255–273 mg HC/g TOC. This interval is interpreted to range from 203–340 mg HC/g TOC. The topmost 10 m consist of stacked lagoonal deposits that are only locally devel- of the F-II member in Mors-1 has an average HI value of oped (Sequence Fj 9 of Nielsen 2003). 183 mg HC/g TOC, whereas the average HI value of the member in Farsø-1 is 156 mg HC/g TOC. The F-II mem-

39 ber in the latter two wells can thus principally be regarded interval with an average HI value of 243 mg HC/g TOC. as gas-prone. In the other wells, the mudstones of the F-II In Rønde-1, the topmost c. 18 m of the F-III member are member possess no source-rock potential. Generally, the highly oil-prone, with HI values ranging from 263–428 mg source-rock potential of the F-II member can thus be HC/g TOC, averaging 355 mg HC/g TOC (Fig. 29B). regarded as limited and primarily gas-prone. Similarly, the F-III member in the Haldager-1 well contains a c. 25 m thick oil-prone interval, with HI values reaching 425 mg HC/g TOC and averaging 323 mg HC/g TOC (Fig. 30A). F-III and F-IV members In a number of other wells, the F-III member also posses - The distribution of the F-III and F-IV members is controlled ses a variable petroleum generation potential. The Farsø-1 by post-depositional erosion related to the regional early and J-1 wells show maximum HI values around 300 mg Middle Jurassic uplift that influenced most of the HC/g TOC and average HI values of 214 mg HC/g TOC Norwegian–Danish Basin and the Fennoscandian Border and 202 mg HC/g TOC, respectively, indicating a limited Zone. The map in Fig. 15 displays the area within which liquid petroleum generation potential (Fig. 30B). The sediments of F-III or F-IV are preserved; in some wells the Mors-1 well has an average HI value of the same order, but combined thickness of the F-III and F-IV members amounts the maximum HI value only reaches 266 mg HC/g TOC to nearly 400 m (Fig. 25A; see also Table 2). The isopach (Fig. 31A). In the Hobro-1 well, a c. 12 m thick interval map suggests that the largest combined thicknesses occur of the F-III member has an average HI of 233 mg HC/g in the Himmerland Graben (Fig. 25B). The entire strati- TOC (Fig. 31B). The F-III member in Fjerritslev-2, graphic interval is only preserved in the Sorgenfrei–Tornquist Hyllebjerg-1 (Fig. 32A) and Voldum-1 (Fig. 32B) contains Zone. The analyses of the 33 well sections show that the intervals with average HI values around 190 mg HC/g average quality of the F-III and F-IV members, in terms TOC, whereas in the Års-1 well, the average value is only of potential source rocks for oil generation, is highly vari- 156 mg HC/g TOC. The F-III member of these latter four able both stratigraphically and geographically (Figs 26, 27). wells is considered to be principally gas-prone. In the Kvols-1 and Rønde-1 wells, part of the F-III In the Rønde-1 well, the F-IV member (55 m thick) is member constitutes an excellent potential oil source rock an excellent, organic-rich (average TOC of 3.58 wt%), oil- (Fig. 26). The uppermost alginite-bearing 40 m of the prone source rock with an average HI value of 435 mg member in Kvols-1 has an average TOC content of 2.95 HC/g TOC and a maximum HI value of 543 mg HC/g wt%, and the interval displays HI values consistently >300 TOC (Figs 27, 29B). The organic material in this highly mg HC/g TOC, with a maximum value of 529 mg HC/g oil-prone interval consists of abundant fluorescing AOM, TOC and an average value of 429 mg HC/g TOC (Fig. detrital liptinite and Leiosphaeridia type alginite (Fig. 33). 28, 29A). The marine mudstones in this interval contain Both the AOM and detrital liptinite are probably com- abundant algal-derived organic material composed of flu- posed of degraded algal material. The occurrence of fram- orescing, amorphous organic matter (AOM) and alginite boidal pyrite is indicative of oxygen-deficient conditions of the Tasmanites and Leiosphaeridia types (Fig. 28). during deposition of the organic-rich shales. A nearly 50 Associated framboidal pyrite testifies to the oxygen-defi- m thick section of the F-IV member in the Hyllebjerg-1 cient, organic-rich conditions in the sediment during depo- well has an average HI value of 210 mg HC/g TOC, with sition. This highly oil-prone section overlies a c. 25 m thick a maximum HI value of 371 mg HC/g TOC (Fig. 32A).

Table 2. Net source-rock (SR) thicknesses, F-III and F-IV members of the Fjerritslev Formation Well F-III Net SR F-III Gross F-III F-IV Net SR F-IV Gross F-IV Sum Net SR Sum Gross Sum Average HI*, Net SR interval (m) (m) Net/gross (m) (m) Net/gross (m) (m) Net/gross (number of samples) Haldager-1 31 117 0.26 9 127 0.07 40 244 0.16 294 (7) Hobro-1 19 154 0.12 7 32 0.22 26 186 0.14 229 (4) Hyllebjerg-1 35 228 0.15 21 55 0.38 56 283 0.20 246 (14) J-1 19 98 0.19 0 43 0 19 141 0.13 267 (2) Kvols-1 79 201 0.39 4 18 0.22 83 219 0.38 369 (28) Mors-1 118 147 0.80 5 25 0.20 123 172 0.72 221 (17) Rønde-1 48 145 0.33 44 55 0.80 92 200 0.46 404 (12) Skagen-2 7 59 0.12 0 29 0 7 88 0.08 203 (1) Sæby-1 0 53 0 4 38 0.11 4 91 0.04 261 (1) Voldum-1 14 111 0.13 0 0 0 14 111 0.13 215 (2) * Average HI in Net SR intervals based on HI values ≥200 mg HC/g TOC

40 A 400 m F-IV 200 m F-III 0 m

High Fault National border

50 km

Jylland

B <150 m 150–200 m 200–250 m 250–300 m 300–400 m >400 m Limit of F-III and F-IV members High Fault National border 50 km

Jylland

Fig. 25. A: Thickness of the Lower Jurassic lithostratigraphic F-III and F-IV members in the Fjerritslev Formation in the studied wells; the base of each schematic stratigraphic column is located at the well site. B: Isopach map of the combined thickness of the F-III and F-IV members. The largest thicknesses occur in the Himmerland Graben.

41 700 Års-1 J-1 Type II Børglum-1 K-1 F-1 Kvols-1 Farsø-1 Mejrup-1 600 Felicia-1 Mors-1 Fjerritslev-2 Rødding-1 Flyvbjerg-1 Rønde-1 Frederikshavn-1 Skive-1

500 Frederikshavn-2 Sæby-1 Haldager-1 Terne-1 Hobro-1 Vedsted-1 Hyllebjerg-1 Voldum-1

400

300 Hydrogen Index (mg HC/g TOC) Index (mg HC/g Hydrogen

Type III

200

Fig. 26. Hydrogen Index versus Tmax plot of 364 samples from the Toarcian 100 F-III member of the Fjerritslev Formation. The F-III member is principally a good, mixed gas/oil-prone source rock, but with some wells containing highly oil-prone intervals

0 with HI values >350 mg HC/g TOC. 400 450 500 Tmax (°C)

A marginal gas generation potential is shown by the mem- HI values >200 mg HC/g TOC (Fig. 35A; Table 2) and ber in the J-1 well (Fig. 30B), where the average HI value the net/gross ratio (cumulative thickness/total thickness of is only 130 mg HC/g TOC and the maximum HI value is F-III + F-IV members). The map thus displays the occur- 178 mg HC/g TOC. rence and thickness of source rocks with a mixed gas/oil or The average HI values in the F-III and F-IV members, oil generation potential and suggests that the thickest cumu- calculated from the entire thickness of the members, clas- lative source-rock section with HI >200 mg HC/g TOC sify the majority of well-sections as gas-prone and a few as occurs in the basin centre (Kvols-1, Mors-1 and Rønde-1 gas-/oil-prone (Fig. 34). The source-rock quality of the wells), where the best source-rock quality also occurs. The F-III and F-IV members, as shown above, varies consider- Kvols-1 and Rønde-1 wells contain about 80–90 m net ably between wells and within the members in individual oil-prone source rock with average HI values of 369 and wells; average values thus mask specific oil-prone intervals. 404 mg HC/g TOC, respectively. It is also notable that To illustrate further the evaluation of the source rocks in the Haldager-1 well in the Sorgenfrei–Tornquist Zone con- the F-III and F-IV members, a ‘net quality map’ was contains c. 40 m net source rock with an average HI of 294 structed showing the cumulative thickness of intervals with mg HC/g TOC. The thinner net source-rock thicknesses

42 700 Års-1 J-1 Type II Børglum-1 K-1 Farsø-1 Kvols-1 Fjerritslev-2 Mors-1 600 Flyvbjerg-1 Rønde-1 Frederikshavn-2 Skagen-2 Gassum-1 Sæby-1 Haldager-1 Terne-1 500 Hobro-1 Vedsted-1 Hyllebjerg-1

400

Hydrogen Index (mg HC/g TOC) Index (mg HC/g Hydrogen 300

Type III

200

Fig. 27. Hydrogen Index versus Tmax plot of 142 samples from the Toarcian F-IV member of the Fjerritslev Formation. The F-IV member is 100 principally a good, mixed gas/oil-prone source rock, but with the Hyllebjerg-1 and Rønde-1 wells containing considerably richer intervals with HI values reaching c. 450–550 mg HC/g TOC in

0 Rønde-1. 400 450 500

Tmax (°C) in the Hobro-1, Hyllebjerg-1 and Voldum-1 wells, which mally immature in the investigated wells, also demonstrated are also situated centrally in the basin, may partly be by the Tmax values (Fig. 35B). The largest discrepancy explained by erosion of the upper part of the F-IV mem- between Tmax and VR values is observed in the Terne-1 ber during the Middle Jurassic uplift event. In other cen- well, which compared to the other wells also yields unusu- trally placed wells, such as Mejrup-1, Rødding-1 and ally low VR values (Figs 18, 19, 21). The notable lack of Skive-1, the entire F-IV member and most of the F-III hydrocarbon shows in the wells drilled in the study area, member have been eroded, resulting in potential source including the central deep part, supports the suggestion that rocks being thin or absent in these areas. the F-III and F-IV source rocks had not, prior to post- The limiting factor for the petroleum potential of the Early Cretaceous uplift, been buried to the necessary depth study area is the thermal maturity of the richest source-rock for petroleum generation to occur. The Mors-1 and Års-1 units, the F-III and F-IV members. It has been shown that wells represent locations where the Fjerritslev Formation the depth to the top of the oil window, corresponding to has been buried below the depth of the top of the oil win- a VR of 0.6%Ro, is about 3050–3100 m (Figs 20A, 22A). dow, but in both wells the petroleum generation potential Using this threshold, the F-III and F-IV members are ther- of the sediments is relatively poor to non-existent.

43 P AOM P

L T

a b

AOM P

I P

c I d

AOM

P P L L

e f

Fig. 28. Paired photomicrographs (reflected light, oil immersion; scale bar is c. 30 µm) of the organic material in the oil-prone F-III member in the Kvols-1 well; left (a, c, e): white light; right (b, d, f): fluorescence-inducing blue light. a-d: Fluorescing, amorphous organic matter (AOM; probably algal-derived) and alginite with Tasmanites (T) and Leiosphaeridia (L) morphology in cuttings from c. 1996 m. Inertinite (I) and framboidal pyrite (P) are also present. TOC = 2.56 wt%, HI = 416 mg HC/g TOC. e, f: Fluorescing AOM (probably algal-derived) and several alginites with Leiosphaeridia (L) morphology in cuttings from c. 2012 m. Abundant framboidal pyrite (P) present. TOC = 3.57 wt%, HI = 487 mg HC/g TOC.

44 Kvols-1 A Gamma ray (API) DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100

1931 oil Oil Fair Gas Gas/ Poor Good Bø. Fm 1950 Excellent Very good 1959 H. S. Fm 1974 Fj. Fm, F-IV 1992 2000 2006 MFS-15

2050 2062 TS-15 2068 SB-15

2094

Depth (m) MFS-14 2100 Fj. Fm, F-III

2150

2183 TS-14 2192 SB-14 2200

Fj. Fm, F-II

Rønde-1 B Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 400 300 200100 40 0 200300 400 600 0.1 1 10 100 oil Oil Fair Gas Gas/

2100 2100 Poor Good Excellent

Very good Bø. Fm

2138 2150 Fj. Fm, F-IV

2193 2200 2205 MFS-15

2240 TS-15

Depth (m) 2250 2257 SB-15

2285 MFS-14

2300 Fj. Fm, F-III

2338 TS-14 2342 SB-14 2350

Fig. 29. Plots showing well logs, Hydrogen Index (HI) values, S2 yields, sequence stratigraphic key surfaces and formations in the Kvols-1 (A) and Rønde-1 (B) wells. Note the oil-prone upper part of the F-III member in Kvols-1 and the oil-prone F-IV member in Rønde-1. DT, sonic velocity; MFS, maximum marine flooding surface; SB, sequence boundary; TS, transgressive surface. Bø., Børglum; Fj., Fjerritslev; H.S., Haldager Sand.

45 Haldager-1 A Gamma ray (API) SP (mV) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120-100 -50 0 50 100 150 0 200300 400 600 0.1 1 10 100 oil Oil Fair Gas Gas/ Poor 1250 Good Excellent Very good

1280 1284 MFS-18 1295 SB-18 1300

1324 MFS-17 Fj. Fm, F-IV 1350 1350 SB-17

1372 MFS-16 Depth (m) Depth 1400 1403 SB-16 1407

1450 Fj. Fm, F-III 1471 MFS-15 1485 TS-15 1495 SB-15 1500

1524 TD

J-1 B Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100 oil Oil Fair Gas Gas/ Poor Good Excellent Very good

1092 1100 H. S. Fm 1111

1132 SB-16 Fj. Fm, F-IV

1150 1154

1190 Depth (m) MFS-15 1200 Fj. Fm, F-III 1208 TS-15

1222 SB-15

1237 MFS-14

1250 1252

1264 SB-14

Fj. Fm, F-II

1300

Fig. 30. Plots showing well logs, Hydrogen Index (HI) values, S2 yields, sequence stratigraphic key surfaces and formations in the Haldager-1 (A) and J-1 (B) wells. Note the oil-prone interval in the F-III member in Haldager-1. DT, sonic velocity; SP, self-potential; TD, total depth. For further abbreviations, see Fig. 29.

46 Mors-1 A Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100 oil Oil Fair Gas Gas/ Poor Good Excellent Very good

2150 2155 Fj. Fm, F-IV

2179 SB-16

2200

2213 MFS-15

2240 TS-15

Depth (m) 2250 2252 SB-15

2275 MFS-14 Fj. Fm, F-III

2300

2327 TS-14 2335 SB-14

2350

Hobro-1 B Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100 oil Oil Fair 1884 Gas Gas/ Poor Good Excellent 1900 Very good H. S. Fm

1923 1932 MFS-16 Fj. Fm, F-IV 1950 1955

1972 MFS-15

2000 2008 TS-15 2013 SB-15

Depth (m) Fj. Fm, F-III 2039 MFS-14 2050

2100 2109

2122 SB-14 Fj. Fm, F-II

2140 2150 Fj. Fm, F-I

Fig. 31. Plots showing well logs, Hydrogen Index (HI) values, S2 yields, sequence stratigraphic key surfaces and formations in the Mors-1 (A) and Hobro-1 (B) wells. Note the c. 125 m thick interval in the F-III member in Mors-1 in which HI values exceed 200 mg HC/gfig34 TOC. DT, sonic velocity. For further abbreviations, see Fig. 29.

47 Hyllebjerg-1 A Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100 oil Oil Fair Gas Gas/ Poor 1894 Good 1900 Excellent

Very good Fl. Fm 1913 1922 H. S. Fm 1927 MFS-16

1950 1947 SB-16 Fj. Fm, F-IV

1977

2000

2050 2060 MFS-15 Depth (m) 2093 TS-15 Fj. Fm, F-III 2100 2114 SB-15

2145 2150 MFS-14

2200 2205 2213 SB-14 Fj. Fm, F-II 2250

Voldum-1 B Gamma ray (API)DT (µsec./ft) HI (mg HC/g TOC) S2 (mg HC/g rock) fig35 04080120 160 120 80 40 0 200300 400 600 0.1 1 10 100 oil Oil

1350 Fair Gas Gas/ Poor Good Excellent Very good

1379 Bø. Fm 1393 1400 H. S. Fm

1423

1450 Depth (m) Fj. Fm, F-III

1500

1533

1550 Fj. Fm, F-II

1562 Fj. Fm, F-I

Fig. 32. Plots showing well logs, Hydrogen Index (HI) values, S2 yields, sequence stratigraphic key surfaces and formations in the Hyllebjerg-1 (A) and Voldum-1 (B) wells. In Hyllebjerg-1, note the increased HI values in the upper part of the F-III member and in the middle part of the F-IV member. DT, sonic velocity. For further abbreviations, see Fig. 29. Fig_36

48 L AOM P

a b

AOM

P L T

c d

Fig. 33. Paired photomicrographs (reflected light, oil immersion; scale bar is c. 30 µm) of the organic material in cuttings sample (c. 2152 m) from the oil-prone F-IV member in the Rønde-1 well; left (a, c): white light; right (b, d): fluorescence-inducing blue light. The cuttings contain an abundance of fluorescing, amorphous organic matter (AOM) and detrital liptinite (probably algal-derived) together with Leiosphaeridia (L) and Tasmanites (T) alginite. P, framboidal pyrite. TOC = 5.81 wt%, HI = 543 mg HC/g TOC.

Middle Jurassic units (Petersen et al. 1998, 2000; Petersen & Brekke 2001). In the Yme Field in the Egersund Basin, where the Bryne Coals formed in the coastal reaches of the mires are more Formation (equivalent to the Haldager Sand Formation) oil-prone than their more landward equivalents. is thickly developed, the formation contains shales that In the study area, the Middle Jurassic Haldager Sand show a good to excellent oil generation potential with TOC Formation is dominated by fluvial, estuarine and shallow values of 2–13 wt% and HI values of 100–480 mg HC/g marine sandstones interbedded with marine and lacustrine TOC. Coal seams within the Middle Jurassic – lower Upper mudstones and thin coal seams (Nielsen 2003). The gen- Jurassic of this field are also oil-prone in places, with HI eration potential of the formation is low, with only a few values exceeding 400 mg HC/g TOC. Similarly, Middle exceptions. In the Haldager-1 well, two samples from a Jurassic coals and carbonaceous lacustrine–brackish shales 4–5 m thick marine mudstone have high HI values (342 of the Lulu Formation are considered to have sourced the and 637 mg HC/g TOC), and the Terne-1 well shows HI oil and gas/condensate accumulations in the Lulita and values of 260 mg HC/g TOC. Both wells are situated in Harald Fields in the Søgne Basin of the North Sea. In this the Sorgenfrei–Tornquist Zone where the Middle Jurassic area, the type of generated petroleum is considered to have is thickest. been controlled by lateral coal facies variations related to There may thus be a relationship between the gross the proximity of peat formation to the coeval coastline thickness of the Middle Jurassic succession, the palaeo-

49 2.48 A 0.75 S2 (mg HC/g rock) 1.7 0.75 TOC (wt%) Oil: HI = 300–600 mg HC/g TOC Gas/oil: HI = 200–300 mg HC/g TOC 2.68 Gas: HI <200 mg HC/g TOC 1.38 0.15 0.75 High 0.90 0.18 0.69 Fault 1.05 0.31 1.01 0.57 1.95 National border 1.28 0.44 1.69 0.92 1.14 50 km 1.18 0.65 0.77

1.27 2.54 2.63 2.74 1.29 1.30 1.68 1.43 4.42 1.92 0.61 0.92 1.17 0.62 1.02 1.04 0.15 5.2 0.59 0.97 1.68 0.75 0.72 1.32 3.38 0.07 0.89 1.10 1.66 0.25 Jylland

0.34 S2 (mg HC/g rock) B 1.49 2.16 0.34 TOC (wt%) Oil: HI = 300–600 mg HC/g TOC

Gas/oil: HI = 200–300 mg HC/g TOC Gas: HI <200 mg HC/g TOC 1.38 1.03 High 0.93 3.1 3.53 4.07 Fault 0.35 3.12 0.93 National border 50 km 1.02 1.1 0.16 0.17 1.47 1.42 0.48 0.29

2.03 2.47 1.68 1.2 1.19 1.4 1.04 1.38 1.28 1.37

1.22 1.04 0.5 0.6 16.09 3.58 Jylland

Fig. 34. The average source-rock quality of the Lower Jurassic F-III member (A) and F-IV member (B) of the Fjerritslev Formation.

50 A Skagen-2 203 (1) Average HI of samples with HI >200 203 (1) 7 (no. of samples) 0.08 7 Net thickness in metres 0.08 Net/gross ratio Oil: HI = 300–600 mg HC/g TOC J-1 267 (2) Sæby-1 Gas/oil: HI = 200–300 mg HC/g TOC 19 261 (1) Gas: HI <200 mg HC/g TOC 0.13 4 0.04 High Fault

Haldager-1 National border 40 294 (7) 0.16 50 km

123 Mors-1 221 (17) Hyldebjerg-1 0.72 246 (14) 56 0.20

26 Hobro-1 0.14 229 (4) Kvols-1 369 (28) 83 0.38 Voldum-1 215 (2) 14 0.13 Rønde-1 92 404 (12) Jylland 0.46

Fig. 40 B 0.40 0.34 Vitrinite reflectance (%Ro)

Immature: <435°C Tmax Early oil window: 435–445°C

High Fault 0.46 National border

0.45 0.45 50 km 0.49

0.56 0.52 0.58 0.51

0.51 0.50

0.53 0.34

0.54 Jylland

Fig. 35. A: Cumulative net source-rock thickness of the intervals in the F-III and F-IV members with HI values exceeding 200 mg HC/g TOC, i.e. Fig. 41 a source rock with a mixed gas/oil or oil generation potential. B: Thermal maturity of the F-III and F-IV members of the Fjerritslev Formation.

Vitrinite reflectance and Tmax values indicate immaturity with regard to petroleum generation.

51 0.34 S2 (mg HC/g rock) 2.2 0.34 TOC (wt%) 1.01 Oil: HI = 300–600 mg HC/g TOC Gas/oil: HI = 200–300 mg HC/g TOC 0.32 Gas: HI <200 mg HC/g TOC 0.52 0.49 0.71 0.73 High 0.11 0.94 0.45 Fault 0.24 0.52 National border 0.17 50 km 0.21 0.20 0.56 0.91 0.90 0.26 1.18 0.45 0.72 0.86 0.61 0.88 0.98 0.95

0.26 0.26 0.53 0.20 0.53 1.74 0.47 1.09 0.23 0.55 0.27 1.07 23.78 2.08 4.39 0.50 0.54 0.76 Jylland 0.07 0.37

Fig. 36. Average source-rock quality of the entire uppermost Jurassic – lowermost Cretaceous Frederikshavn Formation. geographic position and the occurrence of shales and coals ing 580 mg HC/g TOC, averaging 478 mg HC/g TOC with an oil generation potential. Oil-prone coals in the (Fig. 38A; Table 3). Microscopical kerogen analyses show Middle Jurassic may be best developed in areas with most an abundance of amorphous algal organic matter in the form pronounced subsidence and hence relatively large rates of of filamentous lamalginite and alginite with morphology formation of accommodation space during deposition. similar to the extant fresh to brackish water Botryococcus algae (Type I kerogen; Fig. 39). Brackish, oxygen-deficient conditions during deposition are suggested by the abundance of framboidal pyrite associated with the organic matter Upper Jurassic – Lower Cretaceous units (Fig. 39). The probable lacustrine origin of these deposits In most wells, the uppermost Jurassic – lowermost Creta- in Terne-1 suggests a local development, as the formation ceous Frederikshavn Formation is dominated by shallow is typically of shallow marine to offshore origin (Michelsen marine and paralic siltstones and sandstones (Michelsen et et al. 2003). al. 2003) with no petroleum generation potential. The The Skagen-2 well contains a c. 78 m thick net source- average HI of the entire Frederikshavn Formation indi- rock interval with an average HI value of 241 mg HC/g cates a gas-prone source potential (Fig. 36), but the formation contains intervals with good to excellent oil generation po - tential, as demonstrated by the Gassum-1, Hyllebjerg-1, Table 3. Net source-rock (SR) thicknesses, Frederikshavn Fm Skagen-2, Sæby-1, Terne-1 and Voldum-1 wells (Figs 37, Well Net SR Gross Net/ Average HI*, Net SR interval 38A; Table 3). The section in Terne-1 is particularly note- (m) (m) gross (number of samples) worthy, with HI values >1100 mg HC/g TOC, although Gassum-1 17 101 0.17 320 (3) these values in part reflect contamination by gel mud and Hyllebjerg-1 29 146 0.20 243 (4) cement applied during drilling. Nevertheless, solvent- Skagen-2 78 176 0.44 241 (7) Sæby-1 10 105 0.10 329 (1) extracted samples are still encouraging, with a cumulative Terne-1 150 258 0.58 478 (10) net source-rock unit of c. 150 m containing on average 5.7 Voldum-1 26 66 0.39 257 (3) wt% TOC and HI values of the extracted samples reach- * Average HI in Net SR intervals based on HI values ≥200 mg HC/g TOC

52 1000 Gassum-1 (n = 15)

Hyllebjerg-1 (n = 17)

900 Type I Skagen-2 (n = 15) Terne-1 (n = 14, extracted)

Voldum-1 (n = 7) 800

700 Type II

600

500

Hydrogen Index (mg HC/g TOC) Index (mg HC/g Hydrogen 400

300

Type III

200 Fig. 37. Hydrogen Index versus Tmax plot of 68 samples from the uppermost Jurassic – lowermost Cretaceous Frederikshavn Formation. In the Terne-1 100 well, in particular, the Frederikshavn Formation constitutes an excellent oil- prone source rock with HI values 0 exceeding 500 mg HC/g TOC. 400 450 500 550 Tmax (°C)

TOC and a maximum value close to 350 (Fig. 38A). In the Formation is known as the principal source for oil, and in Gassum-1 well, most HI values are low but a few samples the North Sea the uppermost Jurassic – lowermost Creta - yield HI values >300 mg HC/g TOC (Fig. 38A); these ceous marine shales of the Farsund Formation and equiv- high values are abnormal compared to the general trend and alents (Kimmeridge Clay, Mandal and Draupne formations) may reflect thin layers with higher quality kerogen in an are well known as the primary oil source rocks (e.g. Ineson otherwise sand-dominated succession. The Hyllebjerg-1 et al. 2003). As for the Lower Jurassic Fjerritslev Formation, well has a c. 29 m thick net source-rock section with an aver- the major problem in the Danish area is to find areas where age HI value of 243 mg HC/g TOC, whereas the average the Frederikshavn succession is sufficiently buried to be HI value of c. 26 m net source rock in the Voldum-1 well thermally mature with regard to petroleum generation. VR is 257 mg HC/g TOC (Fig. 38A; Table 3). values from 0.36–0.53% Ro and the majority of Tmax val- The Frederikshavn Formation thus locally contains good ues <430–435°C indicate that the potential source rocks to excellent oil source rocks in the study area. Towards the are thermally immature (Fig. 38B). west in the Egersund Basin, the broadly time-equivalent Tau

53 A Skagen-2 78 241 (7) Average HI of samples with HI >200 241 (7) 0.4 (no. of samples) 78 Net thickness in metres 0.4 Net/gross ratio Oil: HI = 300–600 mg HC/g TOC Gas/oil: HI = 200–300 mg HC/g TOC Sæby-1 10 Gas: HI <200 mg HC/g TOC 329 (1) 0.1 High Fault National border 50 km

29 0.2 Hyldebjerg-1 243 (3)

Gassum-1 17 320 (3) 0.17

150 Voldum-1 Terne-1 257 (3) 0.58 26 478 (10) 0.39 Jylland

Fig . 44 B 0.34 Vitrinite reflectance (%Ro) 0.34 Immature:: <435°C Tmax Early oil window: 435–445°C

High 0.41 Fault 0.44 National border

0.44 50 km 0.45

0.53

0.51 0.48

0.46 0.44

0.46

0.51 Jylland

Fig. 38. A: Cumulative net source-rock thickness of the intervals in the Frederikshavn Formation with HI values exceeding 200 mg HC/g TOC, i.e. a source rock with a mixed gas/oil or oil generation potential. B: Thermal maturity of the Frederikshavn Formation. Vitrinite reflectance values indicate immaturity, whereas Tmax values may suggest early oil window maturity in four wells located in the central part of the basin or in the Sorgenfrei–Tornquist Zone.

54 P

FL B

a b

B FL

P B B P

c d

B L B

B P

e f

Fig. 39. Paired photomicrographs (reflected light, oil immersion; scale bar is c. 30 µm) of the organic material in the oil-prone lacustrine Frederikshavn Formation in the Terne-1 well; left (a, c, e) white light; right (b, d, f): fluorescence-inducing blue light. a-f: Abundance of fluorescing, filamentous lamalginite (FL) and Botryococcus-type alginites (B) of varying size in cuttings from c. 200–210 m. Abundant framboidal pyrite (P) has been formed within the large Botryococcus-type alginite bodies. TOC = 7.30 wt%, HIextracted = 498 mg HC/g TOC.

55 Lower Cretaceous units 388 mg HC/g TOC; the interval is thus an excellent poten- In general, the Lower Cretaceous succession of the study tial oil and gas source rock. Similarly, a c. 20 m thick inter- area contains few and relatively thin potential oil source val in the Års-1 well possesses a mixed oil/gas generation rocks. Three wells, the Lavø-1, Sæby-1 and Års-1, contain potential, although the potential is poorer than in Sæby-1 intervals in the Lower Cretaceous (Vedsted Formation or as the HI values range from 210–411 mg HC/g TOC, undifferentiated Lower Cretaceous) with a petroleum gen- averaging 294 mg HC/g TOC. In the Lavø-1 well, a c.18 eration potential. In the Sæby-1 well, a c. 20 m thick m thick potential source-rock interval with HI values from organic-rich interval has TOC contents up to 5.28 wt% 175–242 mg HC/g TOC is present. The Lower Cretaceous and HI values from 320–472 mg HC/g TOC, averaging is, however, thermally immature in all well sections.

Potential reservoirs

The principal sedimentary units of interest with respect to stones are predominantly well to moderately sorted, fine- potential reservoirs in the Norwegian–Danish Basin are to medium-grained, locally coarse-grained and slightly the sandstones of the Upper Triassic – lowermost Jurassic pebbly. The shoreface sandstones occur as widespread Gassum Formation and the Middle Jurassic Haldager Sand sheets, 4–30 m thick, separated by marine transgressive Formation. The growth of salt pillows caused local topo- mudstones and lagoonal heteroliths. Thick fluvial–estuarine graphic relief that influenced the deposition of these two sandstones mostly overlie the major SB 5 sequence bound- reservoir units, as well as the intervening Fjerritslev For - ary (Figs 10, 12; Nielsen 2003). mation. The units thicken into rim synclines as indicated, The formation is 50–150 m thick in central parts of the for example, by the Felicia-1/1A section, and may thin Norwegian–Danish Basin, its thickness being influenced considerably over salt pillows. These principal potential by proximity to salt structures and faults. The formation reservoirs are reviewed below with respect to their gross dis- thickens to 170–200 m in the fault-bounded Himmerland tribution, thickness development and properties. Graben and the northern part of the Sorgenfrei–Tornquist A number of secondary potential reservoir units are also Zone. It thickens further to more than 300 m in the south- known from the Norwegian–Danish Basin and the ern part of the fault zone where deposition of sand con- Fennoscandian Border Zone, including the Lower Triassic tinued from the Triassic until the Early Sinemurian (Nielsen Bunter Sandstone, the Lower–Upper Triassic Skagerrak 2003). The thickness ranges from 69–205 m in the F-1, Formation, the Lower Jurassic F-II member of the Fjerritslev K-1, Felicia-1/1A and J-1 wells (Fig. 1). The large thick- Formation, the Upper Jurassic Flyvbjerg Formation and the ness (205 m) of the formation in Felicia-1/1A, with thick uppermost Jurassic – lowermost Cretaceous Frederikshavn mudstones in the middle part of the formation, probably Formation. These secondary reservoir units are briefly reflects an excess of accommodation space in the rim syn- described after the principal reservoirs. cline associated with the nearby large salt pillow. The thickness decreases to 10–80 m on the Skagerrak–Kattegat Platform. It is generally assumed that the siliciclastic material was Gassum reservoir mainly supplied from the Baltic Shield to the north and Shoreface and fluvial–estuarine sandstones interbedded east. However, the dominance of mineralogically mature with marine mudstones, lagoonal heteroliths and mud- and better sorted sandstones in the Stenlille area and in the stones, lacustrine mudstones and thin coal seams occur in Ullerslev-1 well suggests that sand may have been supplied the Gassum Formation (Nielsen 2003). In the Himmerland from the erosion of older sediments, such as the Triassic Graben, the Sorgenfrei–Tornquist Zone and the Skagerrak Bunter Sandstone on the Ringkøbing–Fyn High (Larsen –Kattegat Platform, sandstones are commonly the domi- 1966; Nielsen 2003). nant lithology, and petrophysical log evaluations typically The Gassum Formation is utilised in geothermal energy show net-to-gross ratios of 0.3–0.7 and porosities of installations onshore Denmark at a depth of c. 1200 m 15–25%. The formation is more sand-poor in the central (Thisted, northern Jylland) and is used for storage of nat- part of the basin with net-to-gross ratios of 0.1–0.2 (e.g. ural gas in a structure at c. 1550 m depth in the eastern Mejrup-1, Nøvling-1, Vemb-1, Vinding-1). The sand- part of the basin (Stenlille area; Fig. 1).

56 Haldager Sand reservoir Additional reservoirs The Haldager Sand Formation consists primarily of sand- The Lower Triassic Bunter Sandstone and the Lower–Upper stones interbedded with thin mudstones. The sandstones Triassic Skagerrak Formations constitute additional poten- are medium- to coarse-grained, slightly pebbly, commonly tial reservoir units in the study area. The Bunter Sandstone well to moderately sorted but locally poorly sorted. Formation consists of orange, red-brown and yellow-brown, Sandstones are the dominant lithology, and petrophysical medium- to fine-grained, moderately to well-sorted, log evaluations typically show net-to-gross ratios of 0.4–0.8 cemented sandstones deposited mainly in braided ephemeral and porosities of 15–30%. In the Sorgenfrei–Tornquist rivers and by eolian dunes. The Skagerrak Formation con- Zone, where subsidence continued despite regional uplift, sists of interbedded sandstones, siltstones, claystones and the formation consists of four thick fluvial–estuarine to anhydrites. The sandstones are arkosic, grey, red, orange- shallow marine sandstone units separated by marine and brown, fine- to coarse-grained, poorly sorted, angu- lagoonal–lacustrine mudstones (Nielsen 2003). Beyond lar–subangular and partly cemented, and were deposited the fault-bounded graben, in areas that experienced uplift on alluvial fans or braided river plains. Both formations are in the early part of the Middle Jurassic, sandstones were dominated by sandstones and potential internal barriers mainly deposited by braided rivers, and the sand bodies are or seals are rare. expected to be laterally coherent without significant pri- Another potential reservoir unit is represented by a mary hydraulic barriers. Anomalies with respect to facies muddy sandstone unit, 20–30 m thick, in the upper part and thickness occur locally in rim synclines associated with of the Lower Jurassic F-II member (Fjerritslev Formation) salt structures. on the Skagerrak–Kattegat Platform; the muddy sandstones The distribution and thickness of the Haldager Sand were deposited by coastal progradation and ensuing trans- reservoir are strongly influenced by regional syndeposi- gression (sequences Fj 4, Fj 5; Figs 8, 12; Nielsen 2003). tional tectonism, local faulting and salt structures. Sediments The sandstones are only well developed north-east of the were supplied from the north and east, but deep erosion Børglum Fault, where they show good porosity. of Triassic and older strata on the Ringkøbing–Fyn High In the south-eastern part of the study area, a series of and Lower Jurassic mudstones along the northern flank of Lower Jurassic (Sinemurian–Pliensbachian) shoreface sand- the high added a substantial amount of material. As a result stones were encountered in the Lavø-1 and Margrethe - of the uplift of the Ringkøbing–Fyn High, high-energy holm-1 wells, where they interfinger with marine mudstones braided rivers shed erosion products into the Sorgen- of the Fjerritslev Formation. The sandstone unit is 30–70 m frei–Tornquist Zone, which experienced slow fault-con- thick with porosities of 10–25%. trolled subsidence. Between the Fjerritslev and Børglum The shoreface sandstones present in the lower and upper Faults, the formation attains a thickness of 30–175 m. parts of the Upper Jurassic Flyvbjerg Formation are also con- Outside the Sorgenfrei–Tornquist Zone, the thickness and sidered potential reservoir rocks; these sandstones show a number of sandstone units decreases. On the Skagerrak general thinning from north to south and from east to west –Kattegat Platform, the formation is 15–50 m thick and (Figs 8, 13). in the central part of the basin it is 25–50 m thick. In the On the Skagerrak–Kattegat Platform, the uppermost southern and south-western part of the study area, the for- Jurassic – lowermost Cretaceous Frederikshavn Formation mation is thin and has a patchy distribution with thicknesses includes shallow marine and fluvial sandstones that pos- below 10 m. sess reservoir properties, although the shale content increases rapidly towards the basin.

57 Discussion

The evaluation presented here of potential source rocks in whereas the F-III and F-IV members were deposited under the Danish portion of the Norwegian–Danish Basin, more reducing bottom conditions with a higher contribu- together with a review of potential reservoirs, suggests that tion of oil-prone Type II kerogen (Figs 28, 33). a Mesozoic petroleum system may be present. Two pri- The generation potential of the Toarcian part of the mary plays are possible: the Upper Triassic – lowermost F-III and F-IV members shows significant lateral changes, Jurassic Gassum play and the Middle Jurassic Haldager with the best-developed source-rock units occurring in the Sand play, both relying on charge from Lower Jurassic basin centre (Fig. 35A; Table 2). The source quality of the (Toarcian) or uppermost Jurassic – lowermost Cretaceous F-III member seems to be particularly well developed in source rocks. Both plays have, however, been tested with the Mors-1, Kvols-1, Hyllebjerg-1 and Farsø-1 wells, where negative results in a number of wells. It is generally pro- the average HI values indicate a gas/oil generation poten- posed that the main reason for the failure of these plays so tial (Fig. 34A). The Mors-1 well is located close to a major far has been the insufficient maturation (burial depth) of salt diapir (Fig. 3), which may suggest that the good source- the potential source rocks. In the light of this study, then, rock quality is related to deposition in the deeper rim syn- it is useful to revisit the important elements of the Mesozoic cline of the diapir. It is likely that development of rim petroleum system. synclines adjacent to salt structures influenced source-rock formation, and it may thus be possible to infer the presence of source-rock intervals based on the analysis of lateral changes in seismic attributes in the rim synclines. Source-rock quality and distribution Indeed, Thomsen et al. (1987) suggested that the increased The regional petroleum generation potential and thermal HI values and TOC contents in Kvols-1 were the result of maturity of the pre-Upper Cretaceous succession in the anoxic depositional conditions in a rim syncline; note, study area have been assessed by evaluating the stratigraphic however, that this well was drilled adjacent to a minor salt units drilled by 33 wells both onshore and offshore in the pillow rather than a diapir. Michelsen (1989b) proposed Skagerrak and Kattegat areas (Fig. 1). It is generally accepted that the organic-rich section in the Kvols-1 well resulted that the Upper Cretaceous – Cenozoic strata have no source- from reduced siliciclastic influx and constant organic depo- rock potential in the study area. Within the Lower Palaeozoic sition (i.e. a condensed section) as reflected in the rela- – Lower Cretaceous succession, only the Lower Jurassic tively thin succession between TS 15 and the base of the (Toarcian) F-III and F-IV members of the Fjerritslev F-IV member compared to other wells; this interpretation Formation and the uppermost Jurassic – lowermost Creta - is compatible with the fact that the highest HI values seem ceous Frederikshavn Formation contain intervals that qual- to be related to MFS 15 (Fig. 29A). The Hyllebjerg-1 and ify as potential oil-prone source rocks in the successions Farsø-1 wells were not drilled close to salt structures, but drilled to date (Figs 25–27, 34–38). None of these poten- both wells are located in the deepest part of the Himmerland tial source rocks have a basinwide distribution. It is further Graben (Figs 2, 3). In Hyllebjerg-1, the highest HI values emphasised that only parts of the lithostratigraphic units actually occur in the basal part of the F-IV member imme- have a good to excellent petroleum generation potential and diately above SB 16, which may suggest a relationship to the potential source-rock units have highly variable gener- a transgressive surface (Fig. 32A). The Rønde-1 well is ation potentials depending on the interaction of a number located on the eastern flank of the Voldum structure, but of geological processes during their formation. An overall the F-III member is not developed as a good source rock upwards increasing petroleum generation potential is in this well. In contrast, the well-developed source rocks observed from the F-I member to the upper F-III and occur in the F-IV member and they do not seem to be F-IV members of the Lower Jurassic Fjerritslev Formation associated with either maximum flooding or transgressive (Figs 23, 24, 26, 27). This difference in generation poten- surfaces (Fig. 29B). It is commonly assumed that the for- tial is partly attributed to a change in depositional condi- mation of marine black mudstones with high values of HI tions through Early Jurassic times (Thomsen et al. 1987; is associated with initial flooding of lowstand systems cre- Michelsen 1989b; Nielsen 2003). The F-I and F-II mem- ating sediment-starved environments with sufficient nutri- bers were deposited in more oxic and shallow marine envi- ents for high organic production (e.g. Wignall & Maynard ronments with a higher contribution of Type III kerogen, 1993). During continued transgression, organic-rich black

58 shales may be preserved. The formation of oil-prone shales of marine organic matter in the basin was a complex inter- is also considered to occur at the time of maximum marine play between a number of factors probably including bot- flooding, and they are often best developed in the upper tom topography and depth, variation in primary organic part of the transgressive systems tract close below the max- productivity, sedimentation rate, bottom-water oxygena- imum flooding surface (Bohacs 1993; Robison & Engel tion and distance to fluvial sources and coastlines; the rel- 1993; Pasley et al. 1993). It is clear, however, from Figs 29–32 ative importance of these different factors in influencing that no simple relationship exists in this case between the source-rock formation in the area is poorly understood in sequence stratigraphic key surfaces and high HI values. detail. The data from the wells analysed in this study thus indi- In contrast to the uncertainty related to regional pre- cate that the potential oil source rocks occur in different diction of source-rock quality, the present-day geographi- intervals within the Toarcian part of the F-III and F-IV mem- cal occurrence of the stratigraphic interval spanning the bers. These intervals may be locally developed, and 2–4 F-III and F-IV members is well understood (Fig. 15). Their stacked intervals occur in some areas. The combined F-III occurrence is primarily dependent on the regional early and F-IV members in the Rønde-1, Kvols-1 and Haldager-1 Middle Jurassic uplift event that caused deep widespread wells, for example, possess oil-prone net source-rock inter- erosion, with truncation of the source-rock interval over vals (i.e. HI >200 mg HC/g TOC) with average HI values large parts of the study area. Indeed, even within the cen- ranging from 294–404 mg HC/g TOC, whereas the net tral area where the F-III and F-IV members are typically source-rock interval in other wells is less oil-prone (Fig. preserved (Figs 15, 25B), this interval may be absent over 35A). The large variation in net/gross ratio shows that sed- local structural highs such as salt structures. To assess the imentation rate was not the sole controlling parameter (Fig. distribution of this interval, it is necessary to map seismi- 35A), but rather that optimum conditions required the cally both the base Middle Jurassic unconformity (base right combination between sedimentation rate and organic Haldager Sand Formation or Bryne Formation) and the seis- productivity. Local depressions in the basin may further have mic reflector corresponding to top F-II member. The lat- favoured preservation of the organic matter by promoting ter coincides with a significant Upper Pliensbachian flooding stratification of the bottom waters. A detailed analysis of surface typically situated 100–200 m below the best source- outcrops and cores from the Lower Toarcian Posidonia rock intervals. If possible, reflectors between these two hori- Shale in south-west Germany has shown that the forma- zons should also be mapped out in order to further constrain tion of this rich source rock was governed by a complex inter- the position of the potential source rocks. play of factors, important amongst which was water column Parts of the uppermost Jurassic – lowermost Cretaceous stratification controlled by sea-level changes (Röhl et al. Frederikshavn Formation, which is dominated by siltstones 2001). Stratigraphic subdivision of the Danish well sections and sandstones in most wells, possess a petroleum gener- (Michelsen 1989a; Nielsen 2003) indicates that the two ation potential in the Hyllebjerg-1, Skagen-2, Terne-1 and lower intervals of organic-rich mudstones in the F-III mem- Voldum-1 wells, with the Terne-1 well having a particu- ber are similar in age to the organic-rich shales in the larly rich c. 160 m thick oil-prone interval with an average Falciferum and Bifrons Zones of the Posidonia Shale. This HI of 478 mg HC/g TOC (Fig. 38A; Table 3). The Type suggests that, in addition to local factors, external factors I kerogen composition of this Terne-1 interval (Fig. 39) indi- such as regional/global anoxic events and sea-level changes cates, however, freshwater to slightly brackish lacustrine may have played an important role in the formation of depositional conditions in contrast to the marine and par- organic-rich shales at this time in the Norwegian–Danish alic conditions that characterised the regional depositional Basin and the Fennoscandian Border Zone. The regional environment of the formation; the unit may thus be only or global anoxia in the Early Toarcian that seems to have a local development in the Terne-1 area in the Kattegat. favoured the formation of organic-rich mudstones at some stratigraphic levels in parts of the basin, was apparently not a significant factor in other areas, possibly owing to high clastic input or shallow water depth that prevented the Source-rock maturity development of oxygen-deficient conditions and thus Regional maturation profiles constructed from VR mea- masked the event. It is intriguing, however, that the anoxic surements corrected for post-Early Cretaceous exhuma- event appears to have influenced deposition along the basin tion yield a likely depth to the top of the oil window (VR margin in shallow marine/lagoonal areas on Bornholm of 0.6%Ro) of c. 3050–3100 m, based on the regional coal- (Koppelhus & Nielsen 1994; Hesselbo et al. 2000). The ification curves principally derived from onshore wells (Figs implication is, therefore, that the deposition and preservation 20A, 22A). Accepting this depth, the potential source rocks

59 Vitrinite reflectance (%Ro) Fig. 40. The regional VR curve based on 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 chalk velocity corrected depth (see Fig. 0 22A). The depth to the top of the oil

window at 0.6%Ro is indicated together with the range of depths to the base of the F-III and F-IV members before post- Early Cretaceous exhumation in the Range of depths to the studied wells. In the Mors-1 well, the 1000 base of the F-IV member, based on 14 studied wells base of the F-III member has been buried slightly deeper, but still above the top of the oil window. Only in the Års-1 well (m)

has the F-III member been within the top part of the oil window before Range of depths to the exhumation. The thickness of the F-III 2000 base of the F-III member, based on 15 studied wells member ranges from 30–279 m in the corrected (chalk) corrected 15 studied wells, whereas the F-IV member ranges from 9–127 m in 13 of Depth the 14 studied wells. In Gassum-1, the F-IV member is considerably thicker, Mors-1, base F-III mb 3000 Top of the oil window namely 320 m. Års-1, base F-IV mb Års-1, base F-III mb

4000

need a burial depth of c. 1.75–2 sec. TWT to reach the oil graphic unit with the most promising source rocks (F-III window if velocity data from the Års-1 well, placed cen- and F-IV members) in the Fjerritslev Formation. If the trally in the basin, are used as guidelines (Nielsen & Japsen present-day depths are corrected for post-Early Cretaceous 1991). The shales of the Frederikshavn Formation are thus uplift (Table 1; Japsen 1998) and information of the com- regionally thermally immature in the study area. The bined thickness of the F-III and F-IV members (Fig. 25) Toarcian shales of the F-III and F-IV members of the is included, it is possible to locate potential areas with Fjerritslev Formation constitute the most obvious poten- mature source rocks (Fig. 41). These potential, locally devel- tial source rocks, but over most of the study area they have oped petroleum kitchens are mainly located in the central not been buried sufficiently to have entered the oil win- part of the study area (central–northern Jylland), where dow (Fig. 40). Of the investigated wells, only in the Års-1 they are associated with rim synclines of salt structures. well, on the flank of the Himmerland Graben immedi- Offshore, in the Skagerrak, a minor kitchen may be pre- ately west of a major salt structure, has the F-III member sent in the Fjerritslev Trough close to the Fjerritslev Fault. been within the uppermost part of the oil window prior to Farther to the west, a kitchen may occur between two salt post-Early Cretaceous exhumation. The source-rock qual- structures (Fig. 41) although in this area the F-III mem- ity of the F-III member in this well, however, is poor with ber is probably very thin and the F-IV member is absent a maximum HI value of 186 mg HC/g TOC and an aver- (Fig. 25). The onshore petroleum kitchens indicated on Fig. age HI of only 113 mg HC/g TOC. Hence, occurrence of 41 correspond to the possible kitchens in the Harboør mature source rocks in the study area requires local burial –Uglev, Mors and Tostrup rim synclines mapped by anomalies, such as local grabens or rim synclines adjacent Thomsen et al. (1987); apart from the kitchen area in the to salt diapirs, to reach thermal maturity for oil generation. Mors rim syncline, however, the kitchens identified in the A map showing the depth to the base of the Middle Juras - present study are areally smaller. The potential kitchen in sic (i.e. the base Middle Jurassic unconformity) in the the Vejrum rim syncline suggested by Thomsen et al. (1987) Danish part of the Norwegian–Danish Basin was con- cannot be confirmed in the present study, principally due structed by Bidstrup et al. (2002). This surface corresponds to very thin source-rock units (e.g. Mejrup-1 well; Fig. 25A) to the top of the F-IV member, i.e. the top of the strati- or the absence of the F-III and F-IV members (Fig. 15).

60 Fig. 41. Map showing the location of 6ºE 8ºE 10ºE 12ºE potential petroleum kitchen areas with Potential mature source rocks of the F-III and kitchen area Zechstein salt F-IV members of the Lower Jurassic diapir or pillow SoS Fjerritslev Formation. The kitchens are Farsund Basin o Fje rgr g locally developed and are principally rr e itslev T n f associated with salt structures. The most r r 57ºN oug ei–Toe h i westerly kitchen in the Skagerrak is highly – T o rrnqu uncertain due to the absence of the F-IV N o Hi. n o q member and possibly only a very thin r w e Gr. u e isti s Zo (or absent?) F-III member. Note that only g i a t i a Z n o salt structures associated with potential – D a n D e kitchen areas are shown (cf. Fig. 3). a n i s h B a s i 56ºN s Hi. Gr., Himmerland Graben. h B a s East North i RiR n Sea High i ngn g købik ø bib i ngn g –Fy– F y n Hi H i g h 100 km

The Mors-1 well is located adjacent to the northern flank Thin fine-grained shoreface sandstones formed during of the largest potential kitchen area. The main targets of short-lived regressive events occur within the Lower Jurassic this well were the Zechstein and Rotliegend, but the well marine mudstones in the Fjerritslev Trough and Skagerrak reached TD in Lower Triassic sandstones at a depth of 5303 –Kattegat Platform. These sandstones can be traced rela- m. The well intersected 123 m of net source rock with gas tively far into the basin as thin silty sandstone intercala- and oil generation potential (average HI = 221 mg HC/g tions in the mudstones and may function as conduits for TOC) and the source rocks are close to being early mature hydrocarbon migration. (Fig. 35). The well encountered only small traces of asphalt The Gassum Formation reservoir is overlain by the lat- in the Haldager Sand Formation, and the poor hydrocar- erally consistent, thick marine mudstone succession of the bon indications may be explained by the lack of structural Fjerritslev Formation. Sandstone or siltstone units up to 5–10 closure (Thomsen et al. 1987; GEUS, unpublished data). m thick are present in the basal Fjerritslev Formation in places, but their influence on seal integrity over the Gassum Formation reservoir is generally expected to be limited. Close to the basin margin, however, for example on the Reservoirs and migration Skagerrak–Kattegat Platform in the eastern part of Kattegat Both reservoirs, the Gassum and Haldager Sand Formations, and Sjælland, Hettangian–Pliensbachian sandstones are are proved to be present regionally and sealed by Lower common and constitute an additional potential reservoir, Jurassic and Upper Jurassic mudstones, respectively. The overlain by marine mudstones of the upper Fjerritslev Haldager Sand reservoir conformably overlies the Lower Formation. The Haldager Sand reservoir is overlain by Jurassic potential source rocks in the relatively deep Fjerritslev marine mudstones of the Flyvbjerg and Børglum For - Trough and presumably also in the Farsund Basin (Fig. 2). mations. Sandstones are present in the lower and upper In contrast, in the remaining parts of the study area, where part of the Flyvbjerg Formation in places, and their thick- the F-III and F-IV members and the Haldager Sand ness and grain size are expected to increase towards the Formation are present, the Haldager Sand reservoir uncon- northern and eastern basin margin, where they may form formably overlies the succession with the potential source an additional reservoir section. The middle part of the rocks. Migration of hydrocarbons to the Haldager Sand reser- Flyvbjerg Formation is dominated by marine mudstones voir is thus simple. Migration to the Gassum reservoir with some seal capacity, and the Flyvbjerg Formation itself requires stratigraphic downward migration of the hydro- is overlain by the thick, regionally continuous, marine carbons and may thus require structural components, i.e. mudstone succession of the Børglum Formation. faulting or salt domes.

61 Active Mesozoic petroleum system? nous immature organic matter (GEUS, unpublished data). In agreement with the general immaturity of the potential Thus, reports of thermally generated petroleum in this well source rocks in the study area, very few oil shows have been cannot be confirmed. The well was also drilled in an area reported. One exception is the K-1 well (Fig. 1), in which in which the presence of mature source rocks is deemed weak shows in sandstone and sandstone stringers were unlikely (Fig. 41). The presence of generated petroleum in noted at several depths. The well was, however, drilled with the Danish part of the Norwegian–Danish Basin thus still diesel that was added several times during drilling opera- has to be documented, although it cannot be excluded that tions; geochemical data of cuttings samples from the K-1 petroleum has been generated in localised potential kitchen well show the presence of a low-boiling distillation cut such areas (Fig. 41). as diesel, with a minor contribution representing indige-

Conclusions

Two primary plays are possible in the study area: the Upper 3. Based on interpretation of regional coalification Triassic – lowermost Jurassic Gassum play and the Middle curves, the top of the oil window (vitrinite reflec - Jurassic Haldager Sand play, both relying on charge from tance = 0.6%Ro) is located at c. 3050–3100 m depth. Lower Jurassic (Toarcian) or uppermost Jurassic – lower- The uppermost Jurassic – lowermost Cretaceous most Cretaceous source rocks. Both plays have, however, Frederikshavn Formation had not been buried to been tested with negative results. This study shows that two this depth prior to post-Early Cretaceous exhuma- main uncertainties are present in the Danish part of the tion, and the potential source rocks of the formation Norwegian–Danish Basin and the Fennoscandian Border are thermally immature in terms of hydrocarbon Zone: (1) the patchy distribution of well-developed, oil- generation. Similarly, the potential source rocks of prone, potential source rocks, and (2) the thermal matu- the Lower Jurassic F-III and F-IV members of the rity of the potential source rocks. The latter factor is Fjerritslev Formation are generally immature to very considered here to be the most significant uncertainty in early mature. However, potential kitchen areas with proving the integrity of this Mesozoic petroleum system. mature source rocks of the F-III and F-IV members The evaluation of source-rock quality, thermal maturity may occur in the central part of the study area and distribution allows the following principal conclusions (central–northern Jylland) and a few places offshore. to be drawn: These potential petroleum kitchens are considered to be of local development, mainly associated with 1. Lower Palaeozoic rocks are overmature in the study salt structures and grabens (Fjerritslev Trough and area and Upper Cretaceous – Cenozoic strata possess Himmerland Graben). no petroleum generation potential.

2. Toarcian marine shales of the Lower Jurassic F-III and F-IV members of the Fjerritslev Formation and the uppermost Jurassic – lowermost Cretaceous shales Acknowledgements of the Frederikshavn Formation constitute oil-prone The study was in part supported financially by the Danish potential source rocks in parts of the basin. The Energy Authority and DONG Energy. A.M. Spencer and generation potential of these potential source rocks an anonymous referee are thanked for constructive reviews is highly variable geographically, and the F-III and which greatly improved the manuscript. F-IV members in the centre of the basin possess the best-developed source potential. The highly oil-prone lacustrine mudstone interval of the Frederikshavn Formation in the Terne-1 well is probably only a local development.

62 References

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