A mid-Cretaceous prograding sedimentary complex in the Sisimiut Basin, offshore West Greenland—stratigraphy and hydrocarbon potential

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Marine and Petroleum Geology 24 (2007) 15–28 www.elsevier.com/locate/marpetgeo

A mid-Cretaceous prograding sedimentary complex in the Sisimiut Basin, offshore West Greenland—stratigraphy and hydrocarbon potential Ulrik Gregersen, Nina Skaarup Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Received 24 February 2006; received in revised form 14 September 2006; accepted 21 October 2006

Abstract A prograding and aggrading depositional system has been mapped from seismic data in the Sisimiut Basin, offshore West Greenland. Seismic stratigraphic analysis resulted in mapping of three seismic sequences and internal boundaries. Mounds at the base of the prograding sequences may represent massflows, which could be prospective. The sealing deposits are Cretaceous claystone, indicated by correlation to the Ikermiut-1 well. Local closures of mounds, regional seal and hydrocarbon traces in the region indicate a potential for stratigraphic hydrocarbon leads. Amplitude interpretation indicates possible mass-flow deposits within the sequences as well as halfgraben syn-rift deposits below, both of which may be prospective. r 2006 Elsevier Ltd. All rights reserved. Keywords: Cretaceous sequences; Sisimiut Basin; West Greenland

1. Introduction During the 1970s, the first regional seismic surveys were acquired and five exploration wells were drilled offshore West Greenland. Two of the wells (Ikermiut-1 and Kangaˆmiut-1) are located at the westernmost edge of the study area. In the late 1980s and early 1990s, seismic stratigraphic mapping and structural analysis were applied to the 1970s seismic data. Acquisition of new seismic data and discovery of oil seeps onshore facilitated a new exploration phase, including further data acquisition and exploration drilling (Qulleq-1) in 2000. Promising results from regional mapping of leads and analyses of stratigraphy and seeping oils, mainly by the Geological Survey of Denmark and Greenland (GEUS), lead to licensing rounds in 2002 and 2004, and two licences were awarded to EnCana Corporation in 2002 and 2005 south of the study area. Exploration is still active offshore West Greenland with announcement of a licence round in 2006. Some of the early research in the area, e.g. by Henderson et al. (1981), Corresponding author. Tel.: +45 38 14 20 00; fax: +45 38 14 20 50.

E-mail address: [email protected] (U. Gregersen). 0264-8172/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2006.10.005

Ottesen (1991) and later work by GEUS, discovered supposed sedimentary features with geometries suggesting progradation towards northwest from the Nukik Platform (Fig. 1) in the study area. The present study was performed to map the prograding complex in more detail using more recent seismic data. The study area covers approximately 70  90 km, mainly blocks 6655/1–6. Other work on the prospectivity and the regional geology of Cretaceous and Palaeogene deposits offshore West Greenland has previously been performed on seismic data in the region (e.g. Chalmers et al., 1993; Chalmers et al., 1995; Dalhoff et al., 2003; Skaarup et al., 2000; Skaarup, 2002). 2. Seismic data and methods Seismic data from four surveys (GGU/90, GGU/92, BUR/BG and NU/9801), covering approximately 6300 km2, were used in the seismic stratigraphic analysis and interpretation (Figs. 1 and 2). The seismic grid is irregular and line separations vary from 2 to 10 km. The studied progradational complex contains some clear boundaries and special features. The boundaries, reflection configuration, geometrical shapes, and amplitude

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Fig. 1. Gravity map offshore West Greenland with the study area indicated. The map shows 150 km high-pass filtered Bouger Gravity data. The study area is marked by a yellow rectangle and is located on the West Greenland continental shelf, in the northwest part of the Nukik Platform and southernmost Sisimiut Basin. Hydrocarbon shows from wells and samples, and satellite slick-clusters are indicated. The gravity data are kindly provided from the Danish National Space Center and prepared/filtered by GEUS. The insert map with the studied region marked (Bathymetric Chart of the Arctic Ocean, Jakobsson et al., 2000).

variations were evaluated using seismic stratigraphic methods. Seismic sequence boundaries (sb) were traced, mapped, and depth converted. The mapped horizons were chosen because they represent stratigraphic breaks and boundaries useful for the depth conversion and may mark hydrocarbon leads. Internal elements of the seismic sequences were analysed, such as tops of basinal mounds. Finally, isopach maps were constructed.

3. Regional geology 3.1. Structural framework The area studied is located on the West Greenland continental shelf, in the northwest part of the Nukik Platform and southernmost Sisimiut Basin (Fig. 1), which are structures that are revealed on 150-km high-pass filtered Bouger Gravity data.

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The West Greenland shelf with Mesozoic–Cenozoic basins extends from a structural trend of linked structural highs and basins (the Ungava Fault Zone) and the Davis Strait High and east to the coast of Greenland (Fig. 1). The basins contain mainly Cretaceous and Cenozoic sediments deposited on continental crust (Chalmers et al., 1993). However, an older succession may exist, as Jurassic spores and Ordovician deposits have been found by seabed sampling south of the study area (Fig. 1). Pre- and early syn-rifts deposition (Deep Sequence and Kitssisut Sequence, Fig. 3) occurred in the Davis Strait region during Early Cretaceous time (Chalmers and Pulvertaft, 2001). The offshore margin of West Greenland was mainly formed by extensional opening of the Labrador Sea in late Mesozoic to early Cenozoic time resulting in complex fault blocks and rift basins in the Labrador Sea and the Davis Strait area (southern part and central parts of Fig. 1, respectively) (Chalmers et al., 1993; Chalmers and Pulvertaft, 2001). The Cretaceous rift phase with E–W extension culminated in late Early Cretaceous time and formed N–S trending normal faults and rotated fault blocks, with syn-rift deposits being a part of the Appat 57°00W

67°00´N

55°00W Fig.6 Fig.5 Fig.7,12 Fig.7,12

N

Fig.8 Fig.9

Ikermiut-1 Fig.10

Fig.4

Fig.11

Fig.5

66°40´N 10 km

W-E W-Elimits limitsof ofthe themapped mappedarea areaininFigs Figs13-19 13-19

Fig. 2. Map with the seismic lines used and figure locations indicated. The Ikermiut-1 well, which is about 30 km west of the studied sequences, is also shown. The study area is marked by a yellow rectangle in Fig. 1 and is located on the West Greenland continental shelf.

Period

Age

Santonian Coniacian

Sisimiut Basin (Rolle, 1985)

(Balkwill, 1987)

Ikermiut Fm.

U. Markland Fm.

? Kangeq Sequence

Late Albian

? Appat Sequence

Early Albian L. Cretaceous

Aptian

Kitsissut Sequence

(This paper)

? L. Markland Fm. Freydis Mb.

Turonian Cenomanian

Labrador Basin Sisimiut Basin

Sisimiut Basin

Campanian U. Cretaceous

sequence (Fig. 3), found in major parts offshore West Greenland (Chalmers et al., 1993; Chalmers and Pulvertaft, 2001). During mid-Cretacous (mainly Santonian) time sandy deposits have been found widespread in the West Greenland region, from marine sands in the Qulleq-1 well (Christiansen et al., 2001) in the south, and in the Kangaˆmiut-1 well, and to the northeast in the Nuussuaq Basin with deltaic sands (Sønderholm and Dam, 1998), indicating possibly tectonic activity. In Late Cretaceous thermal subsidence formed wide basin areas, which in major parts generally were filled with fine-grained post-rift deposits of the Kangeq Sequence (Fig. 3), drilled in Ikermiut-1 and Qulleq-1 wells, but more sandy sediments may have been deposited locally in the basins and locally at structures and basin margins (Chalmers and Pulvertaft, 2001). In all wells, the uppermost Cretaceous (Maastrictian to Campanian) and Lower Paleocene deposits are absent (Lower Paleocene basalt was drilled in Nukik-2) and a base Cenozoic unconformity truncating structures at this level on seismic data, indicate regional erosion, though deposits may have been preserved locally (Chalmers and Pulvertaft, 2001; Dalhoff et al., 2003). In the Palaeogene, NNE-trending strike-slip movements took place along the Ungava Fault Zone, forming flower structures and N and NE-trending compressional ridges, e.g. the Ikermiut Fault Zone, where the Ikermiut-1 was drilled (Chalmers et al., 1993; Chalmers and Pulvertaft, 2001). Tectonic activity during late Paleocene to Eocene time may also be indicated by the basalts found on Disko, and in the Hellefisk-1 and the Nukik-2 wells (Dalhoff et al., 2003). In the Davis Strait region, Late Paleocene to midEocene fine-grained deposits with local mass-flow sands deposited in the basin areas, but from north to southwest in the Sisimiut basin a major system of progradational sedimentary sequences developed with thick basin-marginal sands, drilled in Hellefisk-1, and may possibly relate to new tectonic activity (Dalhoff et al., 2003). Through main parts of the Neogene, basinal parts subsided with deposition of clastics, but towards the Greenland coast, phases of

(Chalmers et.al. 1993)

Maastrichtian

17

Narssariut

?

Studied prograded Sequences ?

?

U. Bjarni Fm. L. Bjarni Fm.

Barremian Fig. 3. Stratigraphic scheme with selected Cretaceous part of the stratigraphy comparing the studied interval with other work. The studied seismic sequences may be the time- as well as depositional equivalent of the Freydis Member of the Lower Markland Formation of the Labrador Basin.

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uplift in Miocene and Pliocene occurred, e.g. exposing Cretaceous deposits and Paleocene sections with seeping oil in the Nuussuaq region (Chalmers, 2000; Japsen et al., 2006). 3.2. Stratigraphy The stratigraphy of the sedimentary basins offshore southern West Greenland (Fig. 3) is partly known from five exploration wells (Fig. 1) drilled in the 1970s (Rolle, 1985; Nøhr-Hansen, 1998, 2003; Dalhoff, et al., 2003; Rasmussen and Sheldon, 2003; Sheldon, 2003) and the Qulleq-1 (6354/4-1) well drilled in 2000 (Nøhr-Hansen et al., 2000; Christiansen et al., 2001; Pegrum et al., 2001) and by the regional seismic correlation (Chalmers et al., 1993, 1995; Chalmers & Pulvertaft, 2001; Dalhoff et al., 2003). The Nukik-1 and Kangaˆmiut-1 wells terminated in Precambrian basement, while Nukik-2 and Hellefisk-1 terminated in Paleocene basalts. Only the Ikermiut-1 and the Qulleq-1 wells drilled significant sections of pre-Cenozoic sediments, with TD in Campanian claystone of the Kangeq sequence (Fig. 3) and Santonian sandstone, respectively (Chalmers et al., 1995; Sønderholm et al., 2003). Rolle (1985) used the 1970s wells to divide the lithostratigraphy of the area into 7 sedimentary formations ranging in age from Cretaceous (Fig. 3) to Neogene (not shown), and Upper Cretaceous sediments (Ikermiut-1) were the oldest drilled sediments at that time of Rolles’ (1985) compilation. Four pre-Cenozoic seismic sequences, the Kangeq, the Appat, the Kitsissut and the Deep Sequence (Fig. 3), have been recognised (Ottesen, 1991; Chalmers et al., 1993; Chalmers et al., 1995). The Kangeq seismic sequence is recognised throughout the southern West Greenland Basin area, and it overlies the seismic sequences studied in this project and is often seismically transparent. More proximal sediments of the same age are exposed onshore Disko and in adjacent areas farther north (Pedersen and Pulvertaft, 1992). The seismic character of the prograding succession is similar to the character of the Freydis Member of the Markland Formation on the Labrador Shelf (Balkwill, 1987), and the sequences are possibly time-equivalent

Ikermiut-1 well

(Fig. 3). The facies of the Freydis Member was described by Balkwill (1987) as locally thick, coarse-grained wedges, deposited contemporaneously with rifting, and other places deltaic sandstones of the syn-rift tectonic mega-sequence occur, and the deposits are partly marine, grading eastward into marine shales. The Appat seismic sequence is interpreted as having been deposited during the main phase of Cretaceous rifting, with wedge-shaped units reaching thicknesses of up to 2000 m, possibly locally with graben fill sands (Chalmers et al., 1993, 1995). The Kitsissut seismic sequence is recognised between the basement or the Deep sequence, and the Appat seismic sequence, and are referred to be of Early to mid-Cretaceous age based on a seismic character similar to the Lower and Upper Bjarni Formation on the Labrador Shelf (Balkwill, 1987; Balkwill et al., 1990). Equivalent age sediments are exposed on the Nuussuaq peninsula and on islands to the north (Rosenkrantz and Pulvertaft, 1969). 4. Seismic sequences The studied interval of seismic sequences forms a wedge between the Appat seismic sequence and the overlying Kangeq seismic sequence, and the studied sequences wedges out ca. 30 km east of the Ikermiut-1 well (Fig. 4). Further south, the studied sequences correlate to the top Santonian sandstone, penetrated by the Qulleq-1 well. Initially, this system was interpreted as prograding Paleocene hyaloclastites, mainly because of moderate to high seismic velocities and analogy with onshore Nuussuaq hyaloclastites (Henderson et al., 1981). Ottesen (1991) also proposed that the prograding clinoforms (the uppermost part of his Unit I) could be lava flows, but suggested, alternatively, that the system is prograding sediments, derived from doming of the transparent high in block 6655/ 4. Interpretation of the prograding feature as a sedimentary system (‘‘fan’’) and its possible prospectivity was also described by Chalmers et al. (1995) and the present study supports interpretation of the seismic sequences as clastic sediments. In some places interval velocities in the seismic sequences are ca. 3300–4400 m/s, though lower values

S.P. 2500

3000

3500

W

4000 sb4

sb3

E

Top Kangeq

sb1

3000

Base Kangeq

Twt (msec)

sb2

3500 Top seismic basement

4km

Fig. 4. The W–E seismic section GGU92-05, with the Ikermiut-1 well, with interpreted horizons and normal faults. The studied seismic sequences to the east, are succeeded by the Kangeq Sequence, which is tied to the Ikermiut-1 well and show drilled Campanian mudstone. Location of the line is shown in Fig. 2.

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4.1. Seismic basement

S.P. 1000

500

W

E

sb4 sb3

3000

sb2

Twt (msec)

2000

Top seismic basement

sb1 4km

Fig. 5. E–W seismic section GGU92-04, with interpreted horizons and normal faults. The seismic sequences prograded generally towards the NW from the shallow basement of the Nukik Platform. Location of the line is shown in Fig. 2.

S.P. 1500

1000

sb4

2000

sb3 Top seismic basement basemen sb1

Twt (msec)

S

N

sb2

19

3000 4km 4k

Fig. 6. The S–N seismic section BURBG-25N, with interpreted horizons and normal faults. Note the flower structure at SP 1150. The major basement fault at SP 800 divides the area with mapped seismic sequences from the shallow Nukik Platform area to the SE, where the seismic sequences are absent. The seismic sequences prograded generally towards the NW from the shallow basement of the Nukik Platform. Location of the line is shown in Fig. 2.

possibly is most real. Similar velocities for clastic deposits are seen elsewhere in the region, with e.g. nearly 3300 m/s at the base of the Ikermiut-1 well and around 4400 m/s at the base of the Kangaˆmiut-1 well. The reason for high velocities is not known, but could be connected to preservation of compaction during later uplift, or may be other reasons such as diagenesis. Three seismic sequences were interpreted that prograde towards the west and northwest. These seismic sequences are termed seismic sequences 1–3 in what follows. The interpreted horizons are shown on seismic sections in Figs. 4–11 (locations are shown in Fig. 2), and a schematic outline of the seismic sequences is shown in Fig. 12, based on seismic stratigraphy/facies. Interpretation of the horizons was terminated where the seismic sequences wedge out. To the west they wedge out below the Kangeq Sequence ca. 30 km from the Ikermiut-1 well. The well drilled the upper part of the Kangeq Sequence of Santonian–Campanian age, so the seismic sequences 1–3 below, may possibly be of mid-Cretaceous age (Fig. 4).

The basement interpreted here is the acoustic basement (Figs. 4–6), interpreted from the seismic character. Resolution at this level is very poor, especially below 2.5 s two-way time. The seismic character below the horizon is diffuse and transparent, and crystalline basement is mainly expected there, but also old sedimentary rocks may be present in fault blocks, where reflections are observed locally below sequences (e.g. in the northernmost faultblock in Fig. 6). It is possible that these deeper reflections may represent older compact Mesozoic deposits (Appat and Kitssisut sequences) or even deposits of Palaeozoic age (Chalmers et al., 1995), e.g. Ordovician Limestone, which have been found in sea-bed samples in 2003–04 south of the study area (Larsen and Dalhoff, 2006). In other places, no distinct reflections other than multiples are observed below the acoustic basement horizon. The area is dominated by a shallow platform dipping towards W and NW, transected along its margins by mainly N- and NE-trending fault systems (Fig. 13). Only a few of these normal faults affect the seismic sequences. The faults appear to have been produced by an episode of E–W extension during the Early Cretaceous, as are the major normal faults limiting the seismic sequences to the south. In the central part of the study area, a fault complex with a flower structure (Fig. 6-SP1200) and thrust faults affect the seismic sequences, but not to great extent their thickness, and may have been formed after seismic sequences 1–3 were deposited. The faults seem to have been formed by strike-slip movements, and may have been caused by regional movements associated with the Ungava Fault Zone (Fig. 1) in the Davis Strait area (Chalmers et al., 1993; Chalmers and Pulvertaft, 2001) during Paleocene–Eocene times. 4.2. Seismic sequences Seismic sequences 1–3 (Figs. 7A, 14–16) were deposited across the northwestern part of the Nukik Platform and have a combined total thickness of up to ca. 2000 m (ca. 5–700 ms twt) in their northwestern part (Figs. 5, 6). The depocentres parallel the rim of the Nukik Platform. The depositional area is restricted to the south by mainly a major E-trending normal fault and partly to the west by a N-trending normal fault. 4.2.1. Seismic sequence 1 Seismic sequence 1 is bounded below by horizon sb1 and above by horizon sb2 (Fig. 7A). The horizon mound surface 1 (ms1) is the upper boundary of seismically defined minor mounds, occurring in the basal part of Seismic sequence 1 (Fig. 7A, B). Mounds have been observed in the seismic sequence with thicknesses of more than 150 m and with an average lateral extent of approximately 5  8 km

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S.P. 5000

5400

N

S

Seq.3 sb4

2000

sb3 sb4

Twt (msec)

Seq.2 Seq. 1 sb2

ms3 sb1 ms2 Top seismic basement

ms1

2km

3000

S.P. 5400

5000

S

N

2000 Twt (msec)

sb4

sb3 ms3

sb2 sb1

ms2 Top seismic basement ms1 2km

3000

Fig. 7. (A) The S–N seismic section GGU92-03 with interpreted seismic sequence boundaries and (B) with amplitudes. Details of internal downlap, onlap and truncation can be observed. The amplitudes in Fig. 7B elucidate dipping, internal-stacked high amplitudes at the lower slopes and mounded features at the base of the sequences. The interpretation of the seismic sequences is shown in Fig. 12. Location of the line is shown in Fig. 2.

(Fig. 17). Due to the spacing of the seismic grid, the limits of the mounds are uncertain and it is possible that additional mounds exist between the seismic profiles. The sb1 horizon is defined by onlap (Fig. 7B; Sp.5200) from the succeeding deposits and by truncation of reflections below (Fig. 7B, Sp. 5410). Seismic sequence 1 has locally been displaced by normal faults (Fig. 5). The depocentres (locally more than 800 m) of Seismic sequence 1 occur to the east and north in the study area (Fig. 14). The depocentres of the succeeding seismic sequences were deposited successively to the west (compare Figs. 14–16).

4.2.2. Seismic sequence 2 Horizon sb2 is the lower boundary for the Seismic sequence 2, and horizon sb3 is the upper limit of the sequence (Fig. 7A). Sb2 is defined by onlap (Fig. 6, Sp. 1250), downlap and truncation (Figs. 7B, 11). The dip of reflections becomes steeper from sequence 1 to above the sb2 boundary, in sequence 2 (Fig. 7B). The depocentre of Seismic sequence 2 (Fig. 15), locally more than 1200 m, is located slightly west of the Seismic sequence 1 depocentre (Fig. 14). The ms2 horizon is the upper boundary of seismically defined minor mounds in the basal part of Seismic

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S.P. 5100

5000

5200

N

S

Seq.3

2200 sb3

Seq.2 sb2 ms2

Toplap

Downlap

Truncation

2400

500 m

ms3 Onlap

Twt (msec)

sb4

Offlap break

Fig. 8. Enlarged part of Fig. 7 (line GGU92-03), showing details of amplitude variations and seismic boundary relations by lapouts and truncation of sequences 2 and 3. Downlap onto mounds (ms2 and ms3) occur lowermost in the sequences, onlap occurs especially onto sequence boundaries (sb3 and sb4), toplap occurs in upper part of sequences at the level of the offlap break, and truncation appears to occur below upper parts of the sequence boundaries. Sequences 2 and 3 show progradation in front of the offlap break, and an element of aggradation, by a moderate rise of the trajectory. The interpretation of the seismic sequences is shown in Fig. 12. Location of the line is shown in Fig. 2.

S.P. 11100

11200

N

sb3 sb4

S

sb2

Mounds

2200

ms3 2400

Twt (msec)

11000

sb1

ms2

1000 m Fig. 9. N–S seismic section (NU9801-407), showing details at the clinoform toes of sequences 1–3. At the clinoforms toes, lowermost in the sequences, mounded features occur. Some of them are stacked at the same location (Sp. 11160). Location of the line is shown in Fig. 2.

1100

S.P. 1125

1150

N

S

Erosional trough

1800

sb3

Twt (msec)

1075

2000

Onlap

Downlap

sb2 Truncation

500 m

Fig. 10. N–S seismic section (NU9801-409), showing a ca. 850 m wide and ca. 70 m deep trough (Sp. 1125) on sequence boundary 3 (sb3). Note that reflectors below seem to have been truncated at the base of the trough. The trough could represent erosion at the base of an incised valley. Succeeding reflectors of sequence 3 downlap the sequence boundary. Location of the line is shown in Fig. 2.

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S.P. 5450

5400 N

5500 S

Erosion

1600

sb4 sb3

1800 sb2 Onlap

Downlap

Twt (msec)

22

500 m

Truncation

Fig. 11. N–S seismic section (GGU92-03), showing sequences 2–3 with onlap, downlap and truncation at sequence boundaries. In the northern part of the section a c. 150 m wide and ca. 30 m deep trough (Sp. 5390) appears on sequence boundary 4 (sb 4) and truncation of reflectors below the sequence boundary indicate erosion. Location of the line is shown in Fig. 2.

Seq.3 Seq.2 sb4

Seq.1

sb3 sb2

sb1

ms3 ms2

200 ms

2km

ms1

Legend Sequence boundary (conform/unconform)

Possible shallower marine to coastal deposits

Mound top surface (ms) Seismic reflection (bedding surface)

Possible facies transition at the offlap break

Onlap

Possible marine mud- and sandstones

Toplap

Mounds, possible fans / massflows

Downlap

Possible turbidites

Truncation Offlap break

Fig. 12. Interpreted seismic sequences and facies. The interpretation is mainly based on the seismic section GGU92-03 (Figs. 7A, B). Seismic sequence boundaries have been defined by mainly onlap, downlap or truncation (e.g. Fig. 8). Internal reflection configurations and seismic amplitudes were used to subdivide elements of each seismic sequence such as mounds and to suggest possible facies by comparison to other analysis of similar systems and predictive models for sand content and morphology of prograding to aggrading depositional systems (e.g. Johannessen and Steel, 2005; Orton and Reading, 1993).

sequence 2 (Figs. 7A, 9). The minor mounds at the base of the seismic sequence show downlap onto horizon sb2, and downlap onto their tops (ms2) are observed (Fig. 9). Several mounds have been observed in the seismic sequence with thicknesses between 100 and 200 m and most have lateral extensions of about 3  3 km, but one, however, is more than 25 km long (Fig. 18).

4.2.3. Seismic sequence 3 Seismic sequence 3 is bounded below by horizon sb3 and at the top by horizon sb4 (Fig. 7A). Internal reflection configurations and seismic facies were used to subdivide elements of the seismic sequence. Local downlap surfaces from minor shingled (heights of ca. 100–150 m) reflections (Fig. 7B—Sp. 5400), may be local deltas, have been

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55°00´W

56°00´W

45 00

0 00 6006

Contour interval: 500 m 10 km

N

0

550 0 4000

67°00´N 40000 3500 35 30000

50000

500

2000 20 00

25000

40000

1500 1500

1000 0

25000

00 450

66°40´N

20 00

2500 2500

Fig. 13. Depth-structure map to top of the seismic basement with contour intervals of 500 m. The lower right area (SE) is the Nukik Platform. Note that the rim of the platform to the left (W) and upper parts are affected by N and NE trending normal faults. (Ikermiut-1 well is located west of the map limit in this and in subsequent maps—see Fig. 2).

55°00´W

56°00´W

56°00´W

67°00´N N

0

55°00´W

67°00´N N

0 400

400

400

400

0 00 80 8

66°40´N

66°40´N

Contour interval: 200m Contour interval: 200m

10 km

10 km

Fig. 14. Isopach map of seismic sequence 1 with contour intervals of 200 m. The depocentre is located along the rim of the Nukik Platform, especially to the north and east, where it reaches more than 800 m. The seismic sequence is affected by minor faults and central parts have probably been removed by erosion. The seismic sequence is limited to the south and partly to the west by a major normal fault.

observed above and landward (southeast) of the offlap break, which is the point on the seismic sequence boundary where the dip changes significantly (Fig. 7A, at Sp. 5220 and 2.1 section twt). In sb3 troughs are seen (e.g. Fig. 10) to truncate reflections below probably caused by erosion, and some of the larger may be incised valleys. One of the troughs seems to run towards the NNE (becoming wider), perpendicular to the progradation direction (Fig. 7B—SP. 5400). Mounds are observed lowermost in the seismic sequence, at the clinoform toes. The ms3 horizon is the upper boundary of the seismically defined minor mounds, lowermost in seismic sequence 3 (Figs. 7A and 19). The mounds show seismic downlap on the sequence boundary

Fig. 15. Isopach map of seismic sequence 2 with contour intervals of 200 m. The depocentre is located along the rim of the Nukik Platform, especially to the west, where it reaches more than 1400 m. The seismic sequence is slightly affected by minor faults. The seismic sequence is limited to the south and partly to the west by a major normal fault.

below and are identified at their top by downlap from clinoforms above (Figs. 8, 9). Reflection amplitudes in the mounds are usually moderate, but are locally moderate to strong (Figs. 7B, 8). Mounds have been observed in the seismic sequence with thicknesses up to nearly 200 m (50–100 m on average) generally with lateral extent of ca. 5  5 km, though a few are up to 10 km long (Fig. 19). The depocentre (locally up to more than 1200 m) of seismic sequence 3 (Fig. 16) occurs west of the older depocentre of seismic sequence 2, which again occurs southwest of the seismic sequence 1 depocentre (compare Figs. 14–16). The shift of depocentres towards the southwest and west may indicate the shifting position of accommodation space and the dominant progradation direction, and a small area of truncation of seismic sequence 3 occurs in its centre. Since time-equivalent

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56°00´W

55°00´W 0

67°00´N

N 400

800

800

0

66°40´N

Contour interval: 200m

10 km

Fig. 16. Isopach map of seismic sequence 3 with contour intervals of 200 m. The depocentre is located along the rim of the Nukik Platform, slightly west and southwest of the prior seismic sequences, where it reaches more than 1200 m. The seismic sequence is missing in the central part, where it probably has been eroded. The seismic sequence is limited to the south and partly to the west by a major normal fault.

56°00´W

55°00´W

67°00´N

N

sedimentary progradation also occurs much further southwest on the east Canadian shelf (Freydis member), regional causes may explain control on sediment supply, such as tectonic pulses and/or eustatic sea-level oscillations controlling accommodation space and wave erosion areas. Sequence 1 and partly sequence 2 seem to have been locally affected by faulting (Figs. 5, 6) and the origin of progradation of the sequences seems to have been centred at the edge of a structural high, a corner of the Nukik Platform, indicating the possibility of tectonism and related erosion and sediment supply from the platform. 4.2.4. The upper sequence boundary Horizon sb4 is the upper boundary of the studied seismic sequences, with onlap from succeeding deposits (Fig. 7A— Sp. 5000, Sp. 5420; Fig. 8) onto the boundary and by truncation of reflections below, which is observed below small troughs in the horizon (Figs. 8, 11). In wells further west and south (Ikermiut-1 and Kangaˆmiut-1), the Upper Cretaceous interval (above sb4) consists of mudstones and thus seems to form a thick regional seal (Chalmers et al., 1995). 4.3. Mounds and high-amplitude sheets at the lower slope

Contour interval: 100m

66°40´N 56°00´W

55°00´W

67°00´N

N

66°40´N

Contour interval: 100m

56°00´W

10 km

55°00´W

67°00´N

66°40´N

10 km

N

Contour interval: 100m

10 km

Figs. 17–19. Isopach maps of the mounds in seismic sequences 1–3: ms1–3, respectively, with contour interval of 100 m. All mounds are located N–W of the Nukik Platform, at the clinoform toe-sets, lowermost in the seismic sequences. They are mostly approximately 50–150 m thick and 5  5 km, but locally more than 25 km long. These mounds are possibly mass flows, e.g. mounded lobes, which could form stratigraphic leads.

The mounds may be interpreted as larger mass-flow deposits, and it is difficult with only the 2D seismic data to go into details whether it could be e.g. sets of mounded lobes, fans, or chute deposits or other deposits. Also secondary features could have affected or caused some of the mounds, such as stress or slides. However, a typical 2D seismic pattern of e.g. mounded lobes in slope to base of slope settings is a low-relief mounded feature with nested lobe geometry, with thicknesses of 10–100 m and lateral extension of 1–100 km2 (Galloway, 1998). Similar physical extensions and characteristics are observed (see e.g. the section Seismic sequence 3) for the mounds in the study area and might be one option. If the mounds represent mass flows, a sandy content may be possible, but though clayey content is less likely due to a possible sand rich system of high angle clinoforms progradation, it can not be ruled out. More knowledge is needed e.g. from seismic inversion using nearby wells to relate seismic signals to lithology in the area. However, mounded silts and sands of mass-flows related to similar seismic characters as studied have been reported in the North Sea (e.g. Shanmugam et al., 1995; Gregersen, 1997, 1998; Gregersen et al., 1998). Prediction of deepwater sands at slopes and base of shelf-margin clinoforms from offshore Ireland and Spitsbergen have been proposed (Johannessen and Steel, 2005) for similar shelf-slope-basin complexes as studied here, and as the trajectories (offlap break migration direction) gradually decreased in steepness, from rising (sequences 1 and 2) to very moderate rising/nearly flat (sequences 3 and 4), it may indicate an increased sand content (Johannessen and Steel, 2005).

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The angle of the clinoform slopes (not decompacted) of the seismic sequences is in average nearly ca. 6–81 for sbs1 and 2, increasing up to more than 101 for sbs3 and 4. The high angles (up to 100–250 m/km) may indicate a sand rich system, as high sand content (medium to coarser fractions) have been found in submarine clinoforms with similar angles e.g. with 50–500 m/km dip of front (Orton and Reading, 1993). Clay/silt dominated systems rarely build clinoforms slope angles of more than 20 m/km and would also often show shallow rotational slides developed on the slope (Orton and Reading, 1993), and this is not suggested to occur in the study area. Also other factors than grain size may affect slopes and their steepness, such as processes related to rate of deposition, bypassing and resedimentation. The dominant prograding clinoform succession occurs mainly basinward of the offlap break at Sp. 5220 (Figs. 7A, 8), but approximately from the level of the offlap break and landward (southeast) the reflections of the sequence becomes mainly semi-horizontal with high amplitudes (Figs. 7B, 8). The semi-horizontal reflections may represent a facies transition from shallow-marine silt or sandstone (high amplitudes) above to mainly marine clay stone below, as may predicted from the position at the top of prograding clinoforms, and as clay content may increase towards the basinal area in a system with rising trajectories (Johannessen and Steel, 2005). For all seismic sequences the height (not decompacted) of the clinoforms is approximately 2–400 ms (3–600 m), and as marine clinoforms height may give some idea of the order of size of the relative sea level, the mounds at the seismic sequence base were probably deposited in a relatively deep water. Within the distal, basinal parts of the clinoforms in the sequences, series of inclined high-amplitudes occur (Figs. 7B—at e.g. 2.3–2.5 section twt of Sp. 5100 and 5300). These features may, by analogy to similar features, with high amplitudes at slopes be sandy deposits (e.g. Shanmugam et al., 1993; Galloway, 1998; Gregersen and Rasmussen, 2000; Johannessen and Steel, 2005), e.g. deposited by processes such as mass-flows, contourites, turbidites or other processes at the slope. Sheet turbidites have typical a 2D seismic pattern with continuous highamplitudes in an irregular sheet geometry in areas of more than 100 km2 (Galloway, 1998), and as these high amplitude features at the lower slope of the sequences studied are also seen, turbidite deposition may be an option. With the given seismic grid size it has not been possible to link the features in map displays, or to link/correlate troughs with mounds to further clarify the actual slope processes. However, both the mounded possible mass flows and the high amplitudes sheets at the slope front, possibly are locally sand-rich and are likely play types. 5. Hydrocarbon potential Some stratigraphic leads have been identified, which indicate that the area may have a potential to trap

25

hydrocarbons. The potential depends on many factors, e.g. such as the presence of reservoir quality, closures, source rock presence, burial history/hydrocarbon generation, migration, etc. Recent work (see below) indicates the possible presence of marine source rocks in the area, and the possible petroleum potential in the region has also been discussed previously (e.g. McMillan, 1980; Henderson et al., 1981; Rolle, 1985; Bojesen-Koefoed et al., 1999; Chalmers et al., 1993, 1995; Chalmers & Pulvertaft, 2001). 5.1. Stratigraphic plays Some semi-horizontal horizons at the level of the offlap breaks (Figs. 8, 12) may be facies boundaries between marine mudstones below (basinward) and possibly more proximal deposits with coarser grain sizes above and could be of interest as possible reservoirs if a seal is present above. Since horizons, and especially the base (sb4) of the expected sealing clays, generally dip upwards and becomes shallow towards the south and southeast (Figs. 4, 6A), there is a risk that hydrocarbons, if generated, could have leaked in this direction. The mounds that have been identified in the lower parts of the seismic sequences, could be e.g. mounded lobes or fans (ms1–3), that could contain sands and therefore may represent stratigraphic leads. Obviously, since these or equivalent deposits have not been drilled, neither their lithology is known, nor the quality, porosities or permeabilities of the deposits. The identified mounds (Figs. 9, 17–19) seem to be possible stratigraphic leads in the area: they form minor local closures probably with sealing marine clays above. The mounds of the sequences are up to approximately 200 m thick (50–100 m thick on average) and have average extensions of approximately 5  5 km, but are locally more than 25 km long. A comparison of the three maps (Figs. 17–19) with the mounds and seismic sections show that the locations of the mounds several places are located near the top of each other (Fig. 9) and thus may be stacked targets. In the basinal parts of the seismic sequences, at the clinoform toes, a series of short high-amplitude reflections (Figs. 7B, 12) may, by analogy (e.g. Shanmugam et al., 1993; Galloway, 1998; Gregersen and Rasmussen, 2000; Johannessen and Steel, 2005), indicate the presence of turbidites, due to their location at the toes of the clinoforms and their local moderate to high amplitudes. These amplitudes may be caused by sandy facies or/and porefluid contacts and might form minor reservoirs, but this is obviously uncertain, especially as no well has penetrated these in the area. The suggested turbidites may thus have a play potential. Other suggested sand-rich intervals may be the upper, more landward parts of the seismic sequences, above the offlap break, e.g. shallow marine sands may be expected there. The difference in reflection character between the possible mudstones in the seismic sequences above and the half-graben fill, sometimes with high amplitudes below,

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probably indicates different sediments. Thus syn-rift sand and coarser deposits, filling the half-grabens in the basement may locally be expected. However, this is unsure and not considered further. 5.2. Seal Mudstones of the Kangeq seismic sequence (in the Ikermiut-1 well) overlie the seismic sequences and may constitute a regional seal. Alternatively, possible mudstones within the seismic sequences, draping the mounds and possible turbidites may be expected to act as seals. The base of the regional seal forms closures towards the major faults, which are assumed to seal. Minor faults have been observed in the seal above the seismic sequences and may pose risks for leakage pathways for possible hydrocarbons. Moderate overpressure in the area cannot be excluded because similar seismic discontinuous reflection pattern locally found in parts of the studied deposits also have been found in overpressured lower Cenozoic deposits in the Kangaˆmuit-1 well (Bate, 1997). 5.3. Source rocks In the two wells Ikermiut-1 and Kangaˆmiut-1 at the western part of the study area (Fig. 1), mudstones of late Cretaceous and Palaeogene age with gas-prone material have been found. Fluid inclusions have been reported from the Ikermiut-1 well (Fluid Inclusion Technologies, 2001; GHEXIS, 2002), and the Kangaˆmiut-1 well experienced a gas kick and wet gas recorded during the control operations. Together these observations may indicate the presence of oil or condensate in a reservoir (Bate, 1997; Chalmers and Pulvertaft, 2001). An evaluation of the thermal maturity data from offshore wells (BojesenKoefoed et al., 2000) and basin modelling for the region (Mathiesen, 2000) suggest active kitchen in many depocentres. Cretaceous marine mudstones with source rocks may be present below seismic sequences 1–3 and be buried by more than 2 km of deposits, between the sb1 and the Basement horizon (Fig. 7A), where the interval have the same seismic characters as the interval with mudstones at TD in the Ikermuit-1 well. The discovery of extensive oil seeps in Cretaceous sediments and Palaeogene basalts onshore central West Greenland (Christiansen, 1993; Christiansen et al., 1994, 1995, 1997) shows that the West Greenland area contains oil-prone source rocks. Analysis of the onshore oils (Bojesen-Koefoed et al., 1999, 2004) in the area of Disko (Fig. 1) and further north, shows that they come from a number of source rocks of Mesozoic and earliest Palaeogene age. The presence offshore of Cenomanian–Turonian, and older marine mudstones as source rock for some of the seepage is possible and, although this succession has not been penetrated by any well yet, the presumed most important source rock in the region may be Cenomanian–Turonian mudstones (Bojesen-Koefoed et al., 2004). The source rocks may

extend from offshore West Greenland into deep basinal parts along the Ungava Fault Zone and into the study area just east of this fault zone. Oil from a possibly Paleocene source rock has been encountered on the western Nuussuaq (Bojesen-Koefoed et al., 1999), and Dalhoff et al. (2003) has shown that such a source rock may be mature in the Sisimiut Basin. Ordovician source rocks have been found south of the study area by seabed sampling (Fig. 1.) organised by GEUS in 2003. Furthermore, satellite slick studies issued by Nunaoil A/S and EnCana Corporation indicate possible seeps at structural highs in the region (Fig. 1). Thus, the various indications suggest a potential for source rocks in the study area region. 6. Conclusion Three seismic sequences of mid-Cretaceous age have been interpreted on the southern margin of the Sisimiut Basin, offshore southern West Greenland. The system prograded generally towards the west and northwest, but wedged out before reaching the nearest well, the Ikermiut-1 well. The system was deposited across the northwestern margin of the Nukik Platform and the total thickness of the seismic sequences is more than 2000 m. The successive depocentres of the three mapped seismic sequences were deposited gradually to the west and south—at the margin of the Nukik Platform. Analyses of internal elements of the seismic sequences indicate the possible presence of mounded mass-flows with thicknesses of up to 100–200 m and dimensions mainly of 5  5 km and may together with possible turbidites at the slope toes constitute potential plays. High-angle prograding clinoforms indicate a sand rich system and sandy stratigraphic leads may be present within the seismic sequences and possibly as graben sands below. The petroleum prospectivity in the region seems promising. If a source rock is present below the seismic sequences or within a short migration distance, potential stratigraphic traps, sealed by thick mudstones, may be present in the area. Acknowledgment The careful review and constructive comments by Flemming Getreuer Christiansen and James A. Chalmers and by the referees, and the figure drawings by Claus B. Jakobsen and Eva Melskens, are acknowledged. The permission from Danpec and Fugro-Geoteam to publish the seismic section BUR/BG25N, and the permission from NUNAOIL A/S to publish the NU9801-407, -409 seismic lines are acknowledged. The Bouger Gravity data are kindly provided from the Danish National Space Center. The work was funded by the Mineral Resources Administration for Greenland (MRA), the Bureau of Minerals and Petroleum (BMP) under the Greenland Home Rule and by the Danish Energy Agency (DEA). The work and seismic sections (GGU lines) were published with

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permission from the Geological Survey of Denmark and Greenland (GEUS).

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