Pliocene paleoenvironment evolution as interpreted

ARTICLE IN PRESS

Marine and Petroleum Geology 25 (2008) 173–189 www.elsevier.com/locate/marpetgeo

Pliocene paleoenvironment evolution as interpreted from 3D-seismic data in the southern North Sea, Dutch offshore sector Gesa Kuhlmanna,, Theo E. Wonga,b a

Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands TNO-NITG, National Geological Survey of the Netherlands, Princetonlaan 6, 3584 CB Utrecht, The Netherlands

b

Received 25 February 2006; received in revised form 19 April 2007; accepted 3 May 2007

Abstract A high-resolution 3D-seismic survey from the Dutch offshore sector has been interpreted and subsequently correlated with existing regional seismo-stratigraphic concepts derived from conventional 2D-seismic data sets. The interpreted 13 seismic units have been related to a newly established chrono-stratigraphic framework [Kuhlmann et al., 2006a, b. Chronostratigraphy of Late Neogene sediments in the southern North Sea Basin and paleoenvironmental interpretations. Palaeogeography, Palaeoclimatology, Palaeoecology, 239, 426–455; Integrated chronostratigraphy of the Pliocene–Pleistocene interval and its relation to the regional stratigraphical stages in the southern North Sea region. Netherlands Journal of Geosciences-Geologie en Mijnbouw, 85(1), 19–35] resulting in up-dated age control for the seismic units. The generation of amplitude maps, time slices and isopach maps from the 3D-seismic data enabled detailed spatial and temporal reconstruction regarding the paleoenvironmental and climatological development as depicted by Kuhlmann et al. [2006a. Chronostratigraphy of Late Neogene sediments in the southern North Sea Basin and paleoenvironmental interpretations. Palaeogeography, Palaeoclimatology, Palaeoecology, 239, 426–455]. The lowermost seismic units S1–S4 comprise condensed Middle Miocene to Piacencian sediments, deposited under warm open marine conditions. These sediments show a uniform seismic facies of lowamplitude reflectors. The boundary of seismic unit S4–S5 (around 2.6 Ma) delineates a shift towards generally colder climate conditions that are connected to the onset of Northern Hemisphere Glaciation. Seismic unit S5 includes alternations of warmer and colder periods. During warmer periods, bottom currents generated elongated structures (2.5–4 km long, 300–500 m wide) on the horizon display. These layers show as well shallow gas accumulations with a more regional extent and are related to coarser-grained sediments sealed by clayey sediments of the cold phases. A homogenous seismic facies is characteristic for the colder periods. Within seismic units S6 and S7, internal fore set structures display the regressional trend related to the cooling of this interval. Seismic units S8–S11 display a pattern of unoriented lineaments which was probably caused by icebergs drifting into the North Sea and souring the sea floor. This finding is in accordance with the shallow marine, arctic conditions of this interval. Within these units a bright spot of 3 by 7 km in size, is interpreted to result from shallow gas accumulation related to a grounded iceberg. This feature represents another, more local area of gas accumulation that is closely related to iceberg activity within the study area. The uppermost seismic units S12 and S13 belong to a fluvial, paralic paleo-environment, that is characterised by aggradational seismic reflectors. Pleistocene glacial valleys were recognised in the upper 200 m below sea floor. r 2007 Elsevier Ltd. All rights reserved. Keywords: 3D seismic; Paleoenvironment; Pliocene; North Sea

1. Introduction

Corresponding author. Present address: GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. Tel.: +49 331 288 1348; fax: +49 331 288 1436. E-mail address: [email protected] (G. Kuhlmann).

0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.05.009

Late Cenozoic sedimentation in the North Sea Basin was dominated by a large clastic depositional delta system. Progradation developed mainly from the north and east and subsequently from the south (Michelsen et al., 1998; Huuse et al., 2001; Overeem et al., 2001; Kuhlmann et al., 2004). Huge amounts of sediment were deposited in the

ARTICLE IN PRESS G. Kuhlmann, T.E. Wong / Marine and Petroleum Geology 25 (2008) 173–189

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has been investigated (Schroot and Schu¨ttenhelm, 2003) within the North Sea. Other studies show the potential of 3D seismics in recognising different sedimentary facies (Reymond and Stampfli, 1994; Gregersen, 1997a, b; Gregersen, 1998; Steeghs et al., 2000). This study aims to resolve climate-controlled changes in sedimentary facies of the depositional sequences in an 3D-seismic data set. For this purpose, well-log data are integrated in the high-resolution seismic data set from the northern part of the Dutch offshore sector (block A15, Fig. 1). In combination with a recently established chronology, and together with paleoenvironment and—climate interpretation by Kuhlmann et al. (2006a), the geological development as expressed in the 3D seismic is described with high temporal and spatial resolution. This detailed data set has been combined with existing regional seismic studies (Sørensen et al., 1997; Overeem et al., 2001), covering a broader lateral extent to discuss some contradicting conclusions of these studies.

North Sea Basin during the Neogene, reaching 1500 m in thickness in the area of the Central Graben (Ziegler, 1990). The main controlling mechanisms for long-term processes, lasting over million of years, have been recognised in the interplay of tectonic factors, such as uplift of Scandinavia and subsidence of the North Sea Basin (Cloetingh et al., 1992; Rijs, 1992, 1996; Stuevold and Eldholm, 1996; Japsen and Chalmers, 2000). For short-term processes, hundreds to tens of thousand of years, climate processes are proposed to play another important role (Sørensen et al., 1997; Huuse et al., 2001; Overeem et al., 2001). Within the late Neogene this includes the shift from warm Miocene conditions towards the cold Pliocene climate related to the onset of Northern Hemisphere Glaciation with first glacier build-up and retreat in the surrounding areas of the North Sea and Scandinavia (Mangerud et al., 1995). Initial studies, using 2D-seismic interpretation, provided a regional seismo-stratigraphic framework describing the main depositional units (or sequences) for the Neogene North Sea (Cameron et al., 1993; Michelsen, 1994; Sørensen and Michelsen, 1995; Sørensen et al., 1997; Michelsen et al., 1998; Overeem et al., 2001). Though 2D-seismic interpretation covers a larger area, 3D-seismic data allow studying the sedimentary setting in more detail because of the higher lateral and vertical resolution of the data. Until now, 3D-seismic data have mostly been used by the oil industry for exploration targets. Exploration targets, however, focussed mainly on deeper (older) successions or on structural traps, and less attention has been paid on shallower (younger) sediments. Only recently, gas occurrences related to stratigraphic traps in shallow successions 2°

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Seismic interpretation was carried out on conventional 2D-seismic lines of the NOPEC surveys SNSTI-NL-83 and SNSTI-NL-87 covering the Dutch offshore sector (Fig. 1). These are post-stack migrated surveys that form a grid with an average distance of about 10 km between individual seismic lines. Within this framework, a 3D-seismic survey was used for interpretation. This survey was located in the Dutch offshore block A15 (Figs. 1 and 2) and was acquired

(fig.

56°

2. Database and methods

Fig. 1. Location of the study area in the northern part of the Dutch off-shore sector, southern North Sea. Indicated are the area of the 3D-seismic survey as well as the 2D-seismic lines of the NOPEC SNSTI-NL surveys from 1983 and 1987 together with the location of the boreholes used for the chronostratigraphy established by Kuhlmann et al. (2006a). The red lines give the profiles used for correlation purposes and shown in Figs. 3 and 4. Superimposed are the salt structures as taken from the gas-atlas of TNO-NITG.


with the upper 1500 m as target interval by Wintershall Noordzee B.V. in 2000. The survey covers an area of 25 by 25 km in size and consists of 1910 inlines and 2067 crosslines with 12.5 m horizontal and 2 ms vertical sample spacing. The data were pre-processed by CGG, London, 4° E 20 83- 4) . (fig

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55°N Fig. 2. Detailed map of the seismic lines used for connecting the regional 2D-seismic framework (bold lines, see also Figs. 3 and 4) to the 3D-seismic survey of the A15 block. Inline 3270 (small dashed line) is taken to connect the reference well A15-3 to the seismic data as shown in Fig. 6. The locations of the boreholes (squares) used in chronostratigraphy are shown. The large dashed line represents the well correlation of Fig. 13.

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and subsequently loaded onto a standard Unix workstation. Seismic interpretation was performed with the software package Geoframe (Version 3.6) of Schlumberger. Initially, the 2D-seismic lines were interpreted using the seismostratigraphic concept of Mitchum and Vail (1977) in which a seismic boundary is defined by reflector terminations of seismic bodies, i.e., by onlap, downlap, toplap or erosional truncation. The interpreted seismic units were connected via seismic line SNSTI-NL-83-9 and via seismic line SNSTI-NL-87-6 (Fig. 1) to the regional seismostratigraphic context of Sørensen et al. (1997) and Overeem et al. (2001), who used these lines in their studies. Two of the interpreted 2D-seismic lines, the north-south trending SNSTI-NL-83-20 and the northeast-southwest trending line SNSTI-NL-87-3 (Figs. 2–4), were traced through the 3D-seismic survey and the seismic sequences were subsequently interpreted within this survey. From the interpreted horizons and seismic units various maps were generated, i.e. horizon-based depth and amplitude maps, volume-based isopach maps as well as time slices. On basis of paleomagnetic and biostratigraphic data from eight wells located in the vicinity of the investigated 3D-seismic survey Kuhlmann et al. (2006a, b) developed a detailed age model. This age model was linked to the boreholes by their gamma-ray (GR) curves (Fig. 5). Since the GR logs are recorded in the depth domain and seismic data is measured in the time domain the well-log data and seismic data has to be tied together by time-depth conversion. Clausen and Huuse (1999), Japsen (1999) and Van der Molen (2004) found a linear relation between the velocity and the depth for the Tertiary sedimentary

area of 3D seismic survey

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Fig. 3. East-west trending seismic profile with interpretation of the main regional seismic units (S1–S13). The lower boundary of the interpreted interval is the Mid-Miocene-Unconformity (MMU). The areal extent of the 3D-seismic survey is marked by a large rectangle and two of the boreholes (A15-3 and B13-3), closest to the seismic line are indicated.


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area of 3D survey A 12-3 A15-3 A15-4 17000 16500 19511750 1500 1250 SNST93-20 500 500

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Fig. 4. North-south trending seismic profile with interpretation of the main regional seismic units (S1–S13). The areal extent of the 3D-seismic survey is indicated by the large rectangle and four of the boreholes (A12-3, A15-3, A15-4, and B16-1), closest to the seismic line are indicated.

Fig. 5. Gamma-ray curve of the reference well (A15-3) and associated log units (Kuhlmann, 2004) with the correlative seismic units from this study and the studies of Overeem et al. (2001) and Sørensen et al. (1997). The chronology (left side of the panel) and paleoenvironmental intervals (right side of the panel) are shown as given by Kuhlmann (2004). The sea-surface temperaturerecord derived from dinoflagellate cyst (SSTDino) and the grain-size distribution (Kuhlmann, 2004) are displayed. The grey bars indicate times of cold climate. Note that the time scale is not linear but that the vertical scale is linear depth.

succession for the eastern and central North Sea with an interval velocity of approximately 2000 m/s. This relation implies that the seismic two-way travel time (TWT) in milliseconds (ms) corresponds to the depth in metres, i.e.

1000 ms TWT corresponds to 1000 m. This relation was used to integrate the GR curves and age model by Kuhlmann et al. (2006a) with the seismic data of this study (Fig. 6).


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Fig. 6. Inline 3270 showing the interpreted 13 seismic horizons within the 3D survey together with the reference well A15-3. The scale is given as two-way travel time (TWT) in ms and depth in m. The arrows point to the horizons and depth levels where time slices and amplitude maps are made with the associated figures (Figs. 13–18). The five paleoenvironmental intervals, as interpreted by Kuhlmann (2004), are circled and marked with numbers. The circles A and B mark levels of high-amplitude reflectors indicating shallow gas occurrences. At the top (o250 ms) of the seismic profile Pleistocene channels are pointed out. The infill of these channels disturbs the seismic signal below.

3. Results 3.1. Seismo-stratigraphic interpretation Based on 2D-seismic interpretation, 13 seismic units (S1–S13) were identified (Figs. 3, 4 and 6). These seismic units are presented in two seismic profiles from the northern part of the Dutch offshore sector. One sections is perpendicular to the prograding units (east–west oriented line SNSTI-NL-87-3, Fig. 3), and the other is along strike (north-south oriented seismic line SNSTI-NL-83-20, Fig. 4). These lines are also running through the area of the 3D-seismic survey and were used to trace the seismic units through the data set of the 3D survey (inline 3270, Fig. 2). Inline 3270 crosses the location of well A15-3 (indicated with its GR curve in Fig. 6) which has been used as reference well for age control. The correlation of these units to the regional seismo-stratigraphic framework of Sørensen et al. (1997) and Overeem et al. (2001) is shown in Fig. 5. The base of the interpreted succession is formed by a high-amplitude reflector on which the above lying seismic units onlap (Fig. 3). This surface has been described as the Mid Miocene unconformity (MMU), one of the major sequence boundaries within North Sea Basin (Cameron

et al., 1993; Huuse and Clausen, 2001). The MMU has been interpreted to be a transgressive surface that appears on seismic profiles as an onlap surface towards the northeastern part of the North Sea Basin, as a downlap surface within the southern North Sea region and as a conformable surface in the Central Trough area (Cameron et al., 1993; Huuse and Clausen, 2001). In our study area, the MMU reaches down to 1600 m towards the north-east and delineates a north-south trending basin (Fig. 7a). The 3D survey is located in the deeper part of the basin reaching down to 1500 m (Fig. 7b) displaying the location in the depocentre during the Late Neogene. The subsequent infill of the basin is characterised by huge prograding wedges (Fig. 3) that have been mapped in different studies for the North Sea region (Cameron et al., 1984; Sørensen et al., 1997; Overeem et al., 2001) and have also been recognised along the continental slope off Norway and Great Britain (Henriksen and Vorren, 1996; Evans et al., 2000; Stoker et al., 2002). In the area of the 3D-survey isopach maps of the interpreted seismic units indicate the main transport direction of the sedimentary wedges. Sediments of seismic unit S1 fill the deeper parts of the area, seen in thickest sediment infill in the through structure of the area. The isopach maps of seismic units S2 until S5 indicate sediment


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Fig. 7. Two-way-travel time (TWT) map of the Mid-Miocene Unconformity (MMU) for the study area of 2D-seismic lines (a) and for the 3D-seismic survey; the scale is in ms.

supply from the northeast while seismic units S7 and S8 show distinct progradation from east to west. From seismic unit S9 onward, the prograding wedges have passed the studied area and differences in thickness have been levelled out. Progradation, however, continues further to the west of the studied area as can be seen in Fig. 3. These sediment supply directions are in accordance with the depocentre shifts within the regional development (Sørensen et al., 1997; Michelsen et al., 1998; Clausen et al., 1999; Huuse et al., 2001). 3.2. Age assignment of the seismic units and paleoenvironmental intervals Seismic interpretation can be used to unravel the geological development of an area. The succession of seismic units places the order of geological events relative in time. However, for coupling of the seismic units to the conditions under which sedimentation took place, such as climate changes or tectonic phases, the seismic sequences must be related to absolute ages. Since the chronology established by Kuhlmann et al. (2006a, b) is coupled to the GR signal of reference well A15-3 (Fig. 5), and the GR log is tied to the seismic through time depth conversion, the derived ages could directly assigned to the seismic units as shown in Fig. 6. From this it results that seismic unit S1 is equivalent to log unit 1 (Fig. 5) and comprises condensed Middle to Late Miocene sediments corresponding to the

distal parts of the sedimentary system that started deposition onto the MMU. The onset of sedimentation onto the MMU is estimated at 12 Ma (Middle Miocene) in the working area (Kuhlmann et al., 2006a). The boundary of seismic units S2 and S3 was assigned to the Zanclean– Piancenzian boundary at 3.6 Ma corresponding to log units 2 and 3 (Kuhlmann et al., 2006a). The Gauss–Matuyama boundary at around 2.6 Ma between log units 4 and 5 (Kuhlmann et al., 2006a) coincides with the boundary of seismic units S4 and S5 (Fig. 6). The base of the Olduvai subchron (at 1.9 Ma) matches the top of log unit 16 (Kuhlmann et al., 2006a) and the seismic unit S11. These three absolute ages were taken as fix points for age control for the seismic units. The less certain ages for the top of the Olduvai as well as the Reunion subchron and the X-event (Kuhlmann et al., 2006a) were not taken into account in this study. From this age assignment it appears that the main part of seismic units in the study area belong to the Late Pliocene time interval, i.e. from the Zanclean to Gelasian stage. Through the GR log responses the seismic units have been correlated to the sedimentological parameters and paleoenvironment interpretations as elaborated by Kuhlmann et al. (2006a). They distinguished five paleoenvironmental intervals for the Pliocene section of the study area on basis of dinoflagellate cysts, pollen and foraminifers (Fig. 5). Subsequently, the 3D-seismic data set has been interpreted with regard to the prevailing environmental


conditions of these intervals and specific features have been visualised by time slices and amplitudes maps. The depth of the presented time slices and amplitude maps is shown in Fig. 6. Seismic units S1–S4 were deposited during warm climate conditions as is revealed from the sea-surface temperature (SSTDino) curve and is described as paleoenvironment interval 1 (open marine and temperate) by Kuhlmann et al. (2006a) (Fig. 5). Fig. 8 shows a time slice for these units, with the seismic character of the MMU and the infill of seismic units 1 and 2. Towards the base of seismic unit S5 cooling took place and seismic units S5–S7 are characterised by alternating warm and cold phases, coinciding with paleoenvironment intervals 2 and 3 (transitional and restricted marine) (Fig. 5). The amplitude map shown in Fig. 9 displays the seismic expression within seismic unit S5 with its high amplitudes during warm conditions. Whereas the cold interval within seismic unit S5 shows a homogenous character (Fig. 10). From seismic unit S8 onward the SSTDino decreases further and arctic conditions prevailed (paleoenvironmental interval 4, Fig. 5). The seismic time slices for this interval (Fig. 11) show unoriented lineaments resulting from icebergs scouring the sea floor. The uppermost seismic units S12 and S13 belong to a fluvial, paralic environment with still arctic conditions (Fig. 6) that is represented in Fig. 12.

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4. Discussion 4.1. Correlation to regional 2D-seismo-stratigraphic studies In the following, the new chronology, as outlined for the seismic units in this study, will be compared with those published by Sørensen et al. (1997) and Overeem et al. (2001). Generally, the ages derived for the seismo-stratigraphic units (S2–3.6, S4–2.6 and S11–1.9 Ma) show significant differences (in the order of 0.6–2 Myr) with the ages of the seismostratigraphic units given by Sørensen et al. (1997) and Overeem et al. (2001) (Figs. 13 and 14). The linearly interpolated ages between the biostratigraphic zones of Sørensen et al. (1997) seem generally to be too old for the lower part of the sequences and too young for the upper part. Overeem et al. (2001) calculated a best-fit line through age points derived from literature, and the resulting ages for their seismic units seem too old. As a result of their different approaches and the uncertainties in the age assessment of these two studies, opposite conclusions on climate-related sediment supply were proposed by the authors, even though the same data set (2D-standard seismic lines, NOPEC SNSTI-87) was used. While Sørensen et al. (1997) stated that during periods of increasing and high-temperatures sediment accumulation rates were

Fig. 8. Time slice of 1250 ms; the arrows point to the cauliflower fault pattern of the MMU and the infill of seismic units S1 and S2 in the trough structure of the eastern part of the 3D-seismic survey.

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Fig. 9. Amplitude map of the high-amplitude reflector within seismic unit S5 (40 ms offset from the upper boundary).

Fig. 10. Amplitude map of the low-amplitude reflector within seismic unit S5 (10 ms offset from the upper boundary).


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Fig. 11. Time slice at 660 ms showing unoriented lineaments of iceberg plough marks.

high, Overeem et al. (2001) stressed that sediment supply was low under warm, temperate conditions and vice versa. To deal with this contradiction, we placed their units within the chronostratigraphic framework of this study and calculated new accumulation rates. In an environment of non-linear sedimentation, as it is given in a progradational setting, it is not sufficient to calculate (linear) sedimentation rates between age points since this would only describe the situation at the given position, and will not give the true sedimentation rate of the entire prograding unit. For the calculation of accumulation rates, the volume of a unit has to be known. Because there are no such volumes for the study by Sørensen et al. (1997) we took the volumes of the seismic units from Overeem et al. (2001) (Fig. 15a). Sediment accumulation rates were then calculated by dividing the volume by the duration of a seismic unit. The duration of a unit was inferred from the gradient of the line between the age points as shown in Fig. 14. We are aware that the duration per unit is assumed to be linear between the age points, but the closer the points, the better a linear line approximates the ‘true’ duration. In Fig. 15b the derived accumulation rates for the corresponding seismic units are plotted against time. From these calculations it is obvious that seismic units S2–S4, that were deposited under warm climate conditions, show relatively

low accumulation rates (less then 6 km3/Ma). During deposition of seismic units S5–S7 sediment accumulation increased substantially (up to 30 km3/Ma). This increase coincides with the enhanced cooling during this time and it seems reasonable that the commencing glacial activity produced high amounts of erosional material through their seasonal waxing and waning. A global increase in sedimentation rates recorded from several regions and geological settings could be explained by synchronous climate changes around 4–2 Ma (Peizhen et al., 2001) but they argue as well that fluctuations of the erosional processes play an important role for the resulting sedimentation rates. Sørensen et al. (1997) showed that these units (their units 20–22) have a broad lateral extent and larger volumes (Fig. 15a) within the North Sea Basin. From seismic unit S8 onward sediment accumulation rates decrease again. As seen in Fig. 3, these units show thick prograding wedges but decreasing sediment accumulation rates could indicate a more local extent of these seismic units with a smaller volume (Fig. 15a). Although glacial climate conditions prevailed and cooling proceeded as indicated by the SSTDino curve (Fig. 5), the proposed mechanism for glacially derived material contributing to higher accumulation rates must have been changed for units S8 and onward. One explanation might be that the



Fig. 12. Pleistocene glacial valleys as seen on a time slice at 200 ms.

existing glaciers remained more stationary due to enhanced cooling and had consequently shorter seasonal melting periods, in which they generally produce highest rates of erosional detritus. At the time of deposition of seismic unit 12 and onwards, the basin was entirely filled up and a fluvial to paralic environment developed. The aggradational character of these seismic units resulted from periods when sea level was high enough to allow basin wide sedimentation. 4.2. Paleoenvironment expression in 3D seismics The 3D-seismic data set of this study has been interpreted with regard to the prevailing environmental conditions and specific features have been visualised by time slices and amplitudes maps. In Fig. 6 the corresponding depths of the environmental intervals and discussed seismic horizons are shown. Furthermore, internal climate variations as depicted from the SSTDino curve and from sedimentological parameters (Fig. 5, Kuhlmann, 2004; Kuhlmann et al., 2004, 2006a) are discussed with regard to the seismic facies observed in the 3D-data set. From the paleoclimatic constraints from Kuhlmann et al. (2006a) and the detailed SSTDino curve (Fig. 5) it is apparent that after the general shift from warm to cold climate conditions at the boundary from seismic unit

S4–S5, climate fluctuations of warmer and colder intervals occur in a generally cold period. The general cooling and the resulting regression are displayed by the overall progradational character of the seismic units. Moreover, the expression of short-term climate fluctuations can be observed within a seismic unit as discussed below. 4.2.1. Seismic units S1–S4: open marine, temperate The base of the investigated succession, the MMU, is a high-amplitude reflector where the fault distribution resembles a ‘cauliflower’ pattern in time slice view (Fig. 8). Such structures might be related to dewatering processes of generally clayey sediment. Cartwright (1994a, b) and Dewhurst et al. (1999) describe similar features in lower tertiary sediments from the North Sea in relation to polygonal fault systems. The overlying sediment infill is highly condensed and these sediments show a uniform seismic character with low-amplitude reflectors (Fig. 8). The temperate to warm climate with open marine conditions and respectively higher sea level favoured such homogenous seismic character. The main sediment input of these units was from the northeast as indicated by the isopach map. These seismic units represent the distal part of the prograding units with their depocentre more towards the east.


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Fig. 13. Correlation panel of reference well A15-3 and borehole A18-1 used in the study of Sørensen et al. (1997). The superimposed line indicates the associated ages for the seismic units (Sørensen et al., 1997; Kuhlmann et al., 2006a, b; Cande and Kent, 1995; Berggren et al., 1995).

= max. ages = min. ages

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= age model used by Overeem et al. (2001) = age points taken from Kuhlmann et al. (2006a)

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10 8 6 4 S2

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Seismo-stratigraphic units of Overeem et al. (2001) Fig. 14. Graph of the age model of Overeem et al. (2001) showing the used data points from literature and the best-fit line of their age model. The ages for seismic units of our study (closed triangles; S2–3.6, S4–2.6 and S11–1.9 Ma) are plotted within this graph.

4.2.2. Seismic unit S5: transitional in temperature and oceanic influence This interval is characterised by seismic reflectors of extremely high amplitudes alternating with low-amplitude

reflectors. These amplitude variations coincide with strong fluctuations in the GR signal (Fig. 6). The levels with the extremely high amplitudes are equivalent to low GR values and those with lower amplitudes to high GR values. This interval is assigned to the first cold inceptions of the onset of Northern Hemisphere glaciation (Kuhlmann et al., 2006a). The GR highs reflect periods of predominantly cold climate and the GR lows indicate warmer climate. One conspicuous feature of these alternations is, that during cold periods grain size was very fine (clayey to clayey silt) while during warm periods grain size was coarser (silty). This has been explained by the interplay of glacial activity during cold periods and enhanced precipitation during warm periods (Kuhlmann, 2004). The high-amplitude reflectors are discontinuous and show distinct 15 m deep incisions within the reflectors (Fig. 6). When seen in the horizontal plane of an amplitude map (Fig. 9) these incisions form NNW–SSE elongated elliptical structures that are approximately 2.5–4 km long and 300–500 m width. These features are believed to result from strong bottom currents that were active during warm periods. Together with the paleo-environmental information by Kuhlmann et al. (2006a), an estuarine circulation model is delineated for the warmer stages in the North Sea


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0 1.7

15

S10/11

7

2.7

25

14

0.5

3.7

30

10

3

12

volume (103 km3)

9

35 S5 to S7

3.5

3.7

S2

S3

5

sediment accumulation rate (103 km3)

184

S4

0 2.7

1.7

Age (Ma) Age (Ma) Fig. 15. (a) Volumes of the seismic units versus age; (b) sediment accumulation rates of the seismic units versus age. The numbers of the seismic units of Overeem et al. (2001) are indicated within the dots, the corresponding seismic units of our study are written beside the dots. The ages are given by this study.

sea-level high stand

bottom water inflow

surface water outflow

ts e ren cur g slop n alo

N

run-off through high precipitation

Shorezone and deltaic facies Shelf facies

sea-level low stand suface mixing glacially derived run-off

N Shorezone and deltaic facies Shelf facies

N Shorezone and deltaic facies Shelf facies

Fig. 16. (a) Depositional model of the Pliocene North Sea Basin during warm intervals. (b) Depositional model of the Pliocene North Sea Basin during cold intervals. (c) Model of iceberg activity within the Pliocene North Sea Basin.


(Fig. 16a). This depositional model implies a low-saline surface water outflow and vigorous bottom currents that are also reflected in the deposition of coarser-grained sediments (Kuhlmann et al., 2004). Cartwright (1995) reported ridge trough sedimentary structures within the same Pliocene delta system from the British North Sea sector. Although these structures are larger, they resemble those presented here. Cartwright (1995) discussed two possible depositional models to explain their origin; one is associated with bottom currents and the other with currents downslope of the prograding wedge. Our data support the first model since the observed structures are elongated parallel to the slope of the prograding wedges and not downslope as it is the case in the second model. During the cold stages that are associated with high GR values in the presented data set, the reflectors have low amplitude (Fig. 6). In the horizontal view of the amplitude map (Fig. 10) a homogenous surface can be observed. In these periods only fine-grained sediment were deposited in the basin (Kuhlmann et al., 2004) which can be related to relatively calm hydrodynamic conditions or no significant input of coarse-grained sediments into the basin. The surface circulation in the North Sea was generally weak and a mixing occurred only by convection through cooling (Kuhlmann et al., 2006b). Fig. 16 displays the proposed depositional model with only surface mixing present during cold stages. 4.2.3. Seismic units S6 and S7: restricted marine enhanced cooling This paleoenvironmental interval constitutes of two seismic units, S6 and S7. Each of these units can internally be divided into cold and warm periods as shown in Fig. 5. Seismic unit S6 includes one thick sedimentary section with a transparent seismic facies that coincides with a broad GR peak. The sediments at the base of this unit were deposited in a warmer climate while those at the GR peak under colder conditions. Towards the unit boundary the climate became warm again. During deposition of the lower part of seismic unit S7 cold conditions prevailed and at the top a warmer climate dominated again (Fig. 17). Within the cold intervals, internal fore set structures can be observed that are the result from regression induced by the cooling.

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Towards the landward (eastern) direction east-west striking lineations probably indicate erosion on the amplitude map. Toplap reflectors as shown in Fig. 17 are a sign for erosion. Isopach maps show that progradation of seismic unit S6 and S7 occurred from east towards west. In terms of the sequence stratigraphic concept that has been applied by Michelsen (1994) and Sørensen et al. (1997) the GR peaks and the associated fine-grained sediments were interpreted as ‘maximum flooding surfaces’. However, in the case of the Pliocene North Sea, this concept cannot be applied straight away. As shown in Fig. 17 the GR peaks comprise periods of cold climate with associated eustatic low sea level, while GR lows coincide with warm climate conditions during sea-level high stands. Consequently, maximum flooding should be assigned to the GR lows in this peculiar sedimentary system. This is not only the case in the restricted marine environment under which seismic units S6 and S7 were deposited but also in the open-marine setting of the underlying unit S5. 4.2.4. Seismic units S8–S12: shallow marine, arctic conditions From seismic unit S9 onward the seismic section shows aggradational reflectors in the 3D-seismic survey. From the top of seismic unit 7 to the top of the investigated succession (as marked in Fig. 6) a distinct pattern of unoriented lineaments has been observed. These structures resemble scratches on the sea floor and are shown on the time slice at 660 ms in Fig. 11. During this interval relatively cold sea-surface temperature (sub-polar to polar) prevailed together with a shallow-marine environment (Fig. 5, Kuhlmann et al., 2006a). In the context of arctic climate conditions, these scratches represent most probably iceberg plough marks on the sea floor of the North Sea Basin. Such a buried ice-scoured surface has been described for the first time from boomer profiles for the Central North Sea (Stoker and Long, 1984) and has later been found in several 3D-seismic surveys (Long and Praeg, 1997; Schroot and Schu¨ttenhelm, 2003). Within this paleoenvironmental interval other reflectors with high amplitude are observed (Fig. 6, circled area B). A time slice near the top of seismic unit S9 (Fig. 17) shows that the high amplitudes cover an distinct area with an A15-3 GR

600

4

800

transgressive regressive 3 transgressive regressive

900

transgressive

1100

warm, sea-level high

5 km

warm cold S7 cold S6 warm

cold,sea-level low S5

transition restricted to open marine

1000

warm

restricted marine

TWT (ms)

700

Fig. 17. Schematic diagram showing the internal structures of seismic units S5–S7. The cold-warm periods and implied transgressive–regressive trends are indicated according to the climate information given in Fig. 5 with reference to Kuhlmann et al. (2006a).



extension of ca. 7 by 3 km in diameter in the middle of the investigated area. The amplitude map of horizon S9 (Fig. 18) displays scratches originating from the area of the bright spot and directing towards the south. These scratches support the idea that an iceberg that became stationary at this position featured this bright spot. The scratches might thus have resulted from the break-off of smaller icebergs that were transported towards the south. Here, the seismic data provide evidence that icebergs have reached the North Sea Basin around 2 Ma ago. In the North Atlantic ice-rafted detritus is found since 3.1 Ma (Kleiven et al., 2002; Jansen et al., 2000) and icebergs could thus, potentially have been transported further to the south from this time onward.

At the top of the seismic section, from 100 to 300 ms, incised valley structures are observed (Fig. 12). These Pleistocene glacial valleys have been first described for the North Sea by Stoker and Long (1984) and later recorded by Praeg (1997), Praeg and Long (1997) and Gregersen (1997a, b); a distribution map of these valleys for the North Sea Basin has been compiled by Huuse and LykkeAndersen (2000). For our study it is noteworthy to state that the infill of these structures disturbs the underlying seismic signal. A pull-up effect is visible in the seismic section resulting in an artefact seen in time slices that should be taken into account when interpreting underlying reflectors (e.g. the expression of a fluvial channel in Fig. 11).

4.2.5. Seismic units S12 and S13: fluvial, paralic and pleistocene glacial valleys The reflectors of the upper most seismic units, S12 and S13, have an aggradational character and the units do not vary in thickness within the 3D-seismic survey. Also, in the greater areal extent of the 2D-seismic survey (Fig. 3), the horizontal reflectors imply that the basin was filled entirely. During this interval a fluvial to paralic environment persisted (Kuhlmann et al., 2006a) and sedimentation occurred probably only at sea level high stands with sufficient accommodation space.

4.3. Shallow gas accumulations The study area of the 3D-seismic survey is located in the northern Dutch offshore block A15 that constitutes of the deeper part of the Neogene North Sea Basin at the western flank of the Mesozoic Step Graben structure, being part of the Central Graben. The distribution of salt domes and pillows are lined along these Mesozoic structures as shown in Fig. 1. Three of the wells, in the vicinity of the seismic survey (A12-3 in the north, B16-1 in the south east and B13-3 in the east; Fig. 2) are located above such salt

Fig. 18. Amplitude map of a bright spot indicating shallow gas accumulation at the location of a possibly stranded iceberg. The scratches directing towards the south are interpreted as iceberg plough marks.


structures, as can be seen on the seismic profiles of Figs. 3 and 4. In a seismic section ‘blanking’ of the seismic signal and a ‘pull-down-effect’ is often related to gas occurring in the sediments (Schroot and Schu¨ttenhelm, 2003). From Figs. 3 and 4 it appears that the gas escapes from deeper lying source rocks along these salt dome structures towards the shallow unconsolidated Neogene sediments. Other indications for gas occurrences are bright spots caused by high amplitudes of the reflectors. Such bright spots can be seen at distinct levels within the studied Neogene sediments. In our study, one level with high-amplitude reflectors is found approximately between 850 and 1100 m depth (as marked by a circled area A in Fig. 6), including seismic sequences S5 and the lower part of S6. A second level is located between 450 and 750 m as marked by the circled area B in Fig. 6. South-east of our study area, Schroot and Schu¨ttenhelm (2003) recorded comparable shallow gas accumulations in sediment layers between 400 and 600 ms (F03 block, Dutch offshore sector). In the following, reasons for gas accumulation at these levels will be discussed with regard to the existing climate conditions at the time of sediment deposition. The lower gas occurrence is confined to sediments deposited in the warm periods of paleoenvironmental interval 2 (transitional in temperate and oceanic influence, Kuhlmann et al., 2006a). In the amplitude map view of Fig. 9, the highest amplitudes occur in a north-south trending area of 17–18 km width over the whole length (25 km) of the survey. The intermittent elongated structures of low amplitudes were interpreted to be the result of strong bottom currents. Probably these elongated structures represent stronger compacted sediments while the sediments around these structures have a larger porosity and are capable to store gas. The alternation of coarser-grained (silty) sediments, having a higher porosity and finer-grained (clayey) sediment, acting as seal apparently provides the possibility for gas storage at this level. The other level of gas accumulation is associated with the bright spot shown in Fig. 18. This feature is more regional (3 by 7 km) in its extent and is related to paleoenvironmental interval 4 (shallow marine, arctic conditions Kuhlmann et al., 2006a) comprising the seismic units with signs of ice-scratches. This gas occurrence seems closely related to the ice-rafting active during this interval. Probably, the partial release of (coarser-grained) material from melting periods of the icebergs provided the porosity necessary for gas accumulation while the intermitting layers acting as seal, were compacted by the weight of the iceberg. Comparable small scale shallow gas occurrences can be seen in the same depth interval at the location of boreholes A12-3 and B16-1 just outside of the 3D-seismic study area. However, more detailed 3D-seismic studies are needed to confirm the proposed hypotheses.

5. Conclusions

Thirteen seismic units have been interpreted on conventional regional 2D-seismic profiles in the northern

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Dutch offshore sector and have subsequently been traced within a high-resolution 3D-seismic survey of block A15. Age assessment revealed absolute ages for the seismic units through integration of formerly dated wells (Kuhlmann et al., 2006a, b). Seismic unit S1 comprises condensed Middle to Late Miocene sediments; the boundary of seismic units S2 and S3 has been assigned to the Zanclean–Piacenzian boundary at 3.6 Ma. The Gauss–Matuyama boundary at around 2.6 Ma coincides with the boundary of seismic unit S4 and S5 and the base of the Olduvai subchron (at 1.9 Ma) is coupled to the top of seismic unit S11. Comparison of these new age assignments to those of existing seismic studies showed age differences in the order between 0.6 and 2 Myr for individual seismic units. Five paleoenvironmental intervals, as depicted by Kuhlmann et al. (2006a), been tied to the seismic units and different seismic facies were visualised: seismic units S1–S4 (paleoenvironmental interval 1; open marine, temperate) have a uniform seismic facies with lowamplitude reflectors. Seismic unit S5 (paleoenvironmental interval 2; transitional in temperature and oceanic influence with alternations of warmer and colder intervals) show elongated structures (2.5–4 km long, 300–500 m width) caused by bottom currents during warmer periods, and a homogenous seismic facies during the colder periods. Seismic units S6 and S7 (paleoenvironmental interval 3; restricted marine, enhanced cooling) are characterised by internal fore set structures related to regression caused by cooling. At seismic units S8–S12 (paleoenvironmental interval 4; shallow marine, arctic conditions) plough marks imply iceberg transport into the North Sea. The aggradational seismic reflectors of seismic units S12 and S13 (paleoenvironmental interval 4; fluvial, paralic) provide evidence that the basin was entirely filled. From then on sedimentation only took place at times when the basin was sporadically flooded. At the topmost 200 ms of the seismic data Pleistocene glacial valleys were observed. In this study climate data (SSTDino) and the age model were obtained from the same boreholes that were in turn integrated with the seismic data. Based on this firm data set, low sediment accumulation rates were proposed for warmer periods of seismic units S1–S4. The following interval (seismic units S5–S7), dominated by cold climate conditions has enhanced sediment supply rates when seasonal glacial activity has been high. This is explained by high erosion rates during frequent waxing and waning of (initial) glacier activity. The preceding cold temperatures led the glaciers to grow and stay more stationary resulting in a lowering of sediment accumulation rates for seismic units S8–S11 although cold conditions prevailed. Shallow gas occurrences are related to gas escapement routes along salt structures and to subsequent accumulation in stratigraphical traps in the unconsolidated


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Pliocene sediments. Two levels of gas accumulation have been described from the 3D-seismic survey. These accumulations have been interpreted with respect to climate conditions: one level occurs within the warm periods of paleoenvironmental interval 2 when alternations of coarser- and finer-grained sediments provide higher porosity layers and seal layers. The other gas accumulation has been recognised in the interval when icebergs were transported into the North Sea and ablation processes created the conditions for gas accumulation. An understanding of the sedimentological and climatological processes has proven to be useful in recognising the extent of gas occurrences in stratigraphical traps of shallow marine sediments.

Acknowledgements We gratefully acknowledge the funding for the Ph.D. project of G.K. and data supply by Wintershall Noordzee B.V. and its partners Marathon Oil Company, Dana Petroleum and Energie Beheer Nederland (EBN). We thank Wintershall Noordzee B.V. and Nederlandse Aardolie Maatschappij (NAM) for the permission to publish the results. This work has been carried out at Utrecht University and the Geological Survey of the Netherlands (TNO-NITG).

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