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Rogaland Group

Harald Brunstad1, Felix Gradstein2, Luis Vergara3, Jan Erik Lie4 and Øyvind Hammer2

Note: Postal addresses to be completed

  1. Lundin Petroleum AS Norway, Lysaker, Norway
  2. Museum of Natural History, N-0318 Oslo, Norway
  3. RWE-Dea, Hamburg, Germany
  4. RWE-Dea, Oslo, Norway.

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The whole document, including formation and members, is available as a Word file (27 MB).

Abstract

A revision was undertaken of the lithostratigraphy of the Rogaland Group for the Norwegian North Sea. An abundance of recent well and seismic data sheds new light on lithology, biostratigraphy, provenance and geographic distribution of all Rogaland units.

Whilst finer siliciclastic units largely remain as previously defined, sandstone/siltstone formations and one (reworked) chalky unit are now included as members. With the new definitions and redefinitions the Rogaland Group now consists of four formations and seventeen members (Table 1), which span the stratigraphic interval from lower Paleocene to lower Eocene. The revisions concerning the sandstone bodies are of four different types:

Table 1. Overview of the Formations and Members of the Rogaland Group, Paleocene - Lower Eocene, North Sea, Norway.

1. GEOLOGICAL SETTING FOR THE ROGALAND GROUP, NORWEGIAN NORTH SEA.

1.1 General setting

Fig. 1. Structural elements of the Norwegian North Sea at BCU level, with the first Paleogene hydrocarbon discoveries in Norway (Balder, 1967), UK (Arbroath, 1969) and Denmark (Siri, 1996) outlined.

Paleocene to lowermost Eocene strata are extensively distributed throughout the whole of the North Sea Basin, and provide one of the most prolific hydrocarbon plays in the Norwegian North Sea. Hydrocarbons were first discovered in strata belonging to this age in 1967 at the Balder Field (Fig. 1) in the Norwegian sector and the first discovery in the UK, the Arbroath Field was in 1969. In the Danish sector, the Siri Field discovery was made in 1996 (Ahmadi et al., 2003).

Paleocene to lowermost Eocene sediments in the central and northern North Sea range in depths close to sea bed to more than 3000m, and reach a thickness of approximately 1000 m in the Outer Moray Firth. Fig. 2 shows a time depth map and areas of shallow and deep burial, whereas the time thickness map in Fig. 3 shows distribution of thin and thick development of the Rogaland group.

Fig. 2. Seismic time map of the Base Rogaland Group (Top Shetland Group) seismic horizon. Orientation of regional seismic lines I-III are shown as blue lines.

Fig. 3. Seismic time thickness map (TWT) of the Rogaland Group (Top Balder Formation to Top Shetland Group). Outlined regional seismic lines I-III are shown in Figs. 16-18.

Improvements in seismic acquisition and processing quality and the application of high resolution biostratigraphy have caused a sustained interest in this stratigraphic interval. Although this play model has existed for a long time, discoveries are still being made, exemplified by the 16/4-4 and the Storskrymten discoveries made in 2007.

The Rogaland Group comprises the most sandprone interval of the Paleogene (Ahmadi et al, 2003; Liu & Galloway, 1997). The preserved strata in the North Sea consist of siliciclastic sediments with volumetrically minor amounts of coal, tuff, volcaniclastic rocks, marls and reworked carbonate sediments. The Scotland-Shetland hinterland was the primary source for the great volumes of siliciclastic sediment (Morton et al., 1993; Ahmadi et al., 2003), but also Fennoscandia acted as a provenance.

During the Early Paleogene, basin architecture evolved from basin-centered to basin margin deposition as Atlantic and European tectonic events, oceanographic trends, eustacy, differential tilting and subsidence, climate and variable sediment supply all combined to influence the depositional systems (Milton et al., 1990; Ziegler, 1990).

Lithofacies distribution within individual depositional systems were controlled by the combined effects of evolving basin physiography, underlying Mesozoic structures, syndepositional tectonics and halokinesis (Ahmadi et al., 2003).

Tectonic activity at this time was related to the development of the Iceland Plume, which caused regional uplift and gradual enclosing and isolation of the North Sea basin from the Atlantic Ocean (Fig. 4) . Adjacent to the North Sea, the Scottish Highlands and the East Shetland Platform were uplifted, together with a somewhat less magnitude of uplift of Western Fennoscandia. Further stresses along the line of the future north-east North Atlantic Ocean led to major volcanic activity in that region, which is represented in the sedimentary strata as tuffs. The seismic time thickness map in Fig. 3 clearly demonstrates the development of thick Paleocene-Earliest Eocene deposits adjacent to these areas.

Fig. 4. Global reconstruction of North West Europe in the Late Thanetian, the period of maximum basin restriction of the Early Paleogene. Modified from Ziegler (1988), Torsvik et al. (2002) and Coward et al. (2003).

1.2 Basin development in North Sea - Proto Norwegian Sea Basin, a semi-enclosed to enclosed basin system

A short summary of the basin evolution is given below and in Fig. 6 . For reference in the following text we include the 2004 Geological time scale (Fig. 5).

Fig. 5. Geological Time Scale 2004 (Gradstein et al., 2004, modified by Simmons et al, 2007) highlighting stages with Global Stratotype Selection Points (GSSP), which is a location specific bedding plane where the base of each stage is defined. This definition is tied to an event in the rock record useful for correlation. As can be seen one GSSP has been defined within the time period when the Rogaland Group was deposited, the Thanetian/Ypresian transition 55.8Ma, which is internally in the Sele Formation, close to the boundary between the Upper and Lower Sele Formation.

Fig. 6. Simplified basin development in the North Sea Basin through the Early Paleogene.

Early Danian

The warm, well oxygenated marine environment that existed in the North Sea during the Late Cretaceous continued into the Danian. Southern and Central parts of the North Sea were depositional sites remote to siliciclastic input, and sediments were dominated by calcareous, coccolithic mudstones. In the northern North Sea however, shorter distance to terrestrial provenance areas caused more siliciclastic sedimentation in that area than further south.

Late Danian-Early Selandian, increased siliciclastic input (Våle Fm)

The first pulse of the Alpine orogeny (Cretaceous to Early Paleogene, e.g. Ebner, 2002) affected the southern and central parts of the North Sea with inversion tectonics and local movements along some of the deeper fault lineaments inherited from pre-Cretaceous times. This caused tectonic uplift and sea level drop with exposure of more terrestrial areas closer to the North Sea Basin. There was also a change to a more temperate and more humid environment. With increased siliciclastic input and less favourable temperature the coccolithic carbonate systems of the Chalk Group were gradually switched off, and deposition of the Rogaland Group started.

Late Selandian-Early Thanethian, decreased connection to the Atlantic ocean (Lista Fm)

In this period the calcareous input from microorganisms, and reworking from exposed chalk of the Shetland Group had practically come to an end, and the fines deposited in the basin were dominated by siliciclastic minerals. In general the trace fossils became smaller in size and the sediments became darker, reflecting less oxygenation of the basin in this period. Coarser sediments were reworked and redeposited during three major episodes of sea level drop in this period.

Global warming, sea level drop and basin isolation in the Late Thanetian (Sele Fm)

Tectonics seem to have caused basin restriction due to establishment of threshold barriers between the North Sea and the Atlantic Ocean. The thresholds were established by inversion tectonics in the London - Brabant Platform area in the southwest, rise of the Wyville Thompson Ridge west of Shetland, and early plate collisions in the south and east, isolating the North Sea from the Tethys Ocean and the Arctic Oceans. As a consequence the North Sea basin became dysoxic to anoxic for a period that lasted for 3 million years, and thus well into the Early Ypresian. Coarser sediments were introduced into the basin during two major episodes of sea level drop.

Early Ypresian (Balder Fm)

The basin isolation and restriction during the Late Thanetian continued into the Early Ypresian. During this period the basin became strongly influenced by tuffaceous ash falls related to magmatism and extrusive activity associated with the breaking up of the North Atlantic. Sandy sediments were introduced into the North Sea Basin during a sea level drop in this period.

Middle to Late Ypresian

In the Middle to Late Ypresian the North Sea Basin and the connection to the Atlantic and possibly the Tethys oceans were temporarily reestablished, and the basin became better circulated. Benthic fauna fossils were reestablished and light green grey to red colored, heavily burrowed mudstones were deposited within the lowermost part of the Hordaland Group.

1.3. Some stratigraphic markers of regional importance

Intra Danian unconformity (Top Shetland/Base Rogaland Group seismic marker)

The boundary between the Shetland Group and the Rogaland Group is regionally the most important unconformity affecting the Rogaland Group. It is extensively developed and is usually a very good lithostratigraphical marker, making the distinction between the Rogaland and Shetland Groups easy in most cases (Fig. 7) . However, the unconformity is diachronous in many areas and can hence not be used as a high resolution chronostratigraphic marker.

Fig. 7. Showing the unconformity at the base of the Rogaland Group, with the Våle formation resting on the chalk of the Shetland Group. Example is taken from Well 25/11-17 drilled by Norsk Hydro. Photo from NPD faktasider at http://www.npd.no.

Late Thanetian unconformity - correlative conformity (Base Sele/Top Lista Formation regional seismic marker)

Within the stratigraphic interval spanning the Sele Formation there are two important unconformities in the southern North Sea and the English Channel - London Basin area (Fig. 8) . Of these two the most important and easiest to recognize is the one represented by the Lista-Sele facies transition (Latest Thanetian-Sparnacian) which reflects a dramatic sea level fall, as seen throughout the UK shelf and onshore, and a simultaneous basin restriction with anoxia in central and deeper parts of the North Sea basin.

Fig. 8. Chronostratigraphic chart, modified after Knox (1994) for the Paleogene of the North Sea-Paris Basin region, showing that the type areas of the Paris Basin, the Ypresian and Thanetian stages, are bounded by unconformities. To capture the missing time represented by these unconformities, the stages could be expanded downward to incorporate the time represented by the basinal sediments of the Central and Northern North Sea Basin where there is continuous deposition. Modified from Simmons et al. (2007).

The overall fall in sea level for Southern England is inferred as being at least 100 m. In Bradwell (Knox et al. 1994), the results of this sea level drop can be seen as an interval of fine grained marine mudstones of the Rhabdamina biofacies (Thanet Formation, Lista equivalent), overlain by the pedogenically altered continental to lagoonal Reading Formation, Sele Fm equivalent. At the Isle of Wight, offshore South England (Fig. 9) , the pedogenically influenced strata of the Reading Formation are seen to lie unconformably between marine sediments of the Shetland Group and the London Clay Formation (time equivalent to the Balder Formation). Knox (1996) and Neal (1996) argue that a sea level fall of a magnitude and rapidity to have caused this effect must be related to tectonical uplift. Ziegler (1988) and Ahmadi et al. (2003) infer that inversion tectonics has influenced the Southern and Central parts of the North Sea Basin, whereas e.g. Nilsen et al. (2005) also demonstrate substantial inversion along the Tornquist line and in the Norwegian Danish Basin during the Paleocene. From France and Belgium there is documented inversion of the Brabant Massif and the Artois Axis (Vandenberghe et al. 1998). For reference, see Fig. 4.

Fig. 9. Exposure of the Reading Formattion at Whitecliff Bay, Isle of Wight. As can be seen, the terrestrial sediments of the Reading Formation can lie inconformably between marine sediments, and must represent a dramatic sea level drop relative to the adjacent sediments (Photo by H. Brunstad).

According to Simmons et al. (2007) the Sele-Lista Formation interboundary is present as an unconformity as far as the southern part of the Central North Sea (Fig. 8) , and Dreyer et al. (2004) report shallow marine indicators within the overall deep marine sandstones of the Latest Thanetian Forties Member in the Pierce and the Josephine High areas. These shallow marine indicators could be a result of local tectonics/salt tectonics, but could also as well be explained as a result of more regional causes, reflecting the large scale inversion related sea level drop as mentioned above.

In the sediments from the Central and Northern North Sea can be seen an abrupt transition from bioturbated to practically undisturbed dark shales. This transition is due to the above mentioned anoxia, lasting through deposition of the entire Sele and Balder Formations, ending with the deposition of the green and red coloured bioturbated shales of the Hordaland Group. Both top and base boundaries of this anoxic shale zone are sharp and considered to be very good litho- and chronostratigraphic markers. In the Central to Northern North Sea the basal part of this zone represents the correlative conformity to the subregional unconformity seen at the base of the Reading Formation in the London Basin and at the Isle of Wight, and in the South to Central North Sea.

The Sele-Lista Formation interboundary thus seems to represent an unconformity in the Southern North Sea and along the east and west margins of the basin further north, and a correlative conformity in the basinal areas from the Central North Sea and northwards. This interboundary is one of the best regional stratigraphic markers, easily detectable from seismic, lithology and biostratigraphy.

Paleocene - Eocene Thermal Maximum, PETM, global biostratigraphic marker

Superimposed on the effects of this period of basin isolation there was also a global warming pulse (Fig. 10) , the Paleocene-Eocene Thermal maximum (PETM) with global extinction of marine benthos outside the North Sea Basin. This pulse raised the global oceanic temperature by 6-7°C for a period of ~60.000 years (~54.98-54.92Ma) by the beginning of the Ypresian (Bowen et al. 2004). This short term effect is difficult to distinguish from other parts of the Sele Formation, since the basin was already anoxic. Sea surface temperatures rose between 5 and 8°C over a period of a few thousand years. Although the extreme PETM warming lasted only for a short period, global sea surface temperatures of the Paleocene-Early Eocene period before and after this event were still much warmer than today. The Arctic and Antarctic seas were rather warm, and ice free (Bowen et al., 2004 and Moryan et al. 2006).

Fig. 10. Marine and Terrestrial records of the PETM, correlated to an age model for ODP site 690 (Antarctic Ocean). a) Marine d13C records derived from the surface dwelling genus Acarania at ODP sites 690 (Antarctic, blue circles) and 1290 (subtropical Pacific Ocean, red squares). b) Paleosol carbonates d13C from northern Spain (Blue circles), Hunan, Chona (red squares) and Wyoming, USA (green diamonds). c) Temperature anomalies for sites 690 and 1209 calculated from monospecific d18O and Mg/Ca records, respectively (symbols in a.). d) normalized composite carbon isotope curves for paleosol carbonates (green) and planktonic foraminiferal carbonate (dark blue). Interval of terrestrial CIE amplification (PETM temperature increase) is shown in grey. From Bowen et al., 2004.

With its short time duration, the PETM is almost a perfect chronostratigraphic marker. However, the definition of this stratigraphic event is not possible macroscopically, and must rely on high resolution biostratigraphic analysis with dense sampling.

Volcanic extrusives regional seismic marker

The igneous activity in the North Atlantic (Fig. 4) shows a wide age range, but peaks between 55 and 50 Ma (Torsvik et al, 2002), spanning syn rift and a continental break-up phase. During the late rift phase, and especially during deposition of the Balder Formation (~53-54 Ma), large amounts of tuffaceous ash material were introduced into the atmosphere, and distributed over vast areas of North Europe. The lower and upper boundary of the tuff rich zone (Fig. 11) is a significant litho- and chronostratigraphic marker that makes recognition of the base of the Balder Formation rather easy in most cases. Some minor tuff stringers are also sometimes seen in the Lista and Sele Formations, but are not easy to use for correlation.

Fig.11 Series of bright to medium dark greenish grey tuff layers of variable thickness, interlaminated with black anoxic shales. Example is taken from the Balder Formation Well 25/11-20 drilled by Norsk Hydro. Picture from NPD Faktaside at http://www.npd.no.

2. LITHOSTRATIGRAPHIC NOMENCLATURE

2.1 General

This study deals with a revision and update of the lithostratigraphic nomenclature and classification of the Rogaland Group of Paleogene age in the Norwegian sector of the North Sea below 62°N.

In broad terms, the Rogaland Group of this study corresponds with the formal definition by Hardt et al. in Isaksen & Tonstad (1989), which authors joined Deegan & Scull's (1977) Montrose and Moray Groups into the single Rogaland Group.

In this study, the finer siliciclastic formation units remain as previously defined, but the sandstone formations of Hardt et al. (1989) are changed into members. This procedure follows much of the same principle as in Mudge & Copestake (1992) and Knox & Holloway (1992) for the UK sector of the North Sea.

Since Hardt et al.'s (1989) work on the Norwegian sector, an abundance of new stratigraphic information from wells and 3D seismic has become available. The data have shown a more complex situation concerning sand provenance and sand distribution. The geographic distribution of the sand units has been refined. The stratigraphic resolution using microfossils has also become more detailed.

As a result, several new member names have been introduced in the period after Hardt et al.'s (1989) revision (Knox & Holloway, 1992; Ahmadi et al., 2003; Schiøler et al., 2007) and are used here. In addition, new members are proposed in this study, and several type and reference wells originally defined by Hardt et al. (1989) have been amended. Several additional wells are also added when new stratigraphic units are established. This study also provides comprehensive geological figures of the new members.

2. 2. Note on Members (see also www.stratigraphy.org - International Stratigraphic Guide)

Member is a formal lithostratigraphic unit, next in rank below formation, possessing lithologic properties distinguishing it from adjacent parts of the formation. No fixed standard is required for the extent and thickness of a member. A formation need not be divided into members unless a useful purpose is served. Specially shaped forms of members (or of formations) are lenses and tongues. A member can transect in more than one formation.

In the current revision of the Rogaland Group, it is intended to use existing naming as far as practical for the sandstone members. However, some major additions have been made in the North Eastern North Sea and in the Siri Canyon of the southern parts of the Norwegian Danish Basin since the former official nomenclature (Hardt et al., 1989) did not describe any sandstones there. Since the 90s several wells have given more information about sandstones in these areas and they are correspondingly included and defined here.

The fifteen sandstone members of the Rogaland Group have been defined according to their provenance area, sedimentary distribution and structural boundaries. This practice has been used by: Hardt et al. (1989) to separate their easterly derived 'Fiskebank Formation' from the westerly sourced 'Forties Formation'; Schiøler et al. (2007) to separate easterly sourced sandstones of the Siri Canyon and Tail End Graben/Søgne Graben from westerly sourced sandstones of the Central Graben; and Hardt et al. (1989) and Knox and Holloway (1992), distinguishing northwestern from southwestern sandstones in the Central Graben and the Viking Graben.

Roughly, sandstone members within each shale formation are bounded by the two crossing Member separation lines shown in Figs. 12 and 13, which give four sub areas each with their separate sandstone member:

  1. North-South separation line.
  2. West-East separation line.

Fig. 12. Sketch showing rough distribution of the sandstone members belonging to the four shale formations of the Rogaland Group, relative to the Member separation lines.

Fig. 13. Sketch demonstrating subareas and separation lines used to limit the various sandstone members.

2.3 Subjects covered in Lithostratigraphic nomenclature

In this revision each of the four formations and the fifteen members are described in the same successive steps: The amount of text dedicated to each formation or member is dependent on the data coverage and importance.

In general the shale formations are described in more detail concerning biostratigraphic events and stratigraphic criterias, and in the descriptions of the sandstones members it is often referred back to the shale formation concerning this issue.

A large amount of composite well diagrams have been designed, and are shown under the various stratigraphic units. The lithological codes used are shown in Fig. 14.

Fig. 14. Lithological codes used in well composites in Chapter 4-7.

3. Rogaland Group, general description

Unit Definition of the Rogaland Group

The Rogaland Group of the Norwegian sector of the North Sea corresponds to the combined Montrose and Moray Groups of the UK sector (Knox & Holloway, 1992). It is divided into four shale/mudstone formations. From old to young these are the Våle, Lista, Sele and Balder Formations, each containing their respective sandstone members (Table 1 and Fig. 15 ).

The formations in this lithostratigraphic revision are defined geographically to include the main shale units of the Paleocene and Early Eocene of the North Sea up to 62°N.

An overview of the Rogaland Group with its four shale formations and seventeen sandstone members is shown in Fig. 15.

Fig. 15. Lithostratigraphic summary chart of the Rogaland Group, Northern and Central North Sea.

Name

The Rogaland Group was given name by Deegan and Scull (1977) to Paleocene to Early Eocene (Late Danian-Early Ypresian) siliciclastic sediments of the Central and Northern North Nea.

Derivatio Nominis

The name Rogaland is after the county of Rogaland in southwest Norway.

Lower Boundary

In the central North Sea, the Norwegian-Danish Basin and in the southern Viking Graben the base of the Rogaland Group is picked at the change from marly mudstones with local sandstones and thin interbeds of limestone of the lower parts of the Våle Formation into the chalks and marls of the Shetland Group. In the northern North Sea the basal boundary is not so cleary defined as further south, since there is less distinct lithological difference between the Våle Formation and a less calcareous underlying Shetland Group in that area compared to further south.

Upper boundary

The top of the Rogaland Group is taken at the transition between the dark grey, laminated and partly tuffaceous shales of the Balder Formation into the commonly pale green grey to red colored basal parts of the overlying Hordaland Group.

General lithological characterization

The Rogaland Group is characterised by a siliciclastic succession of sediments following the more carbonate rich Cretaceous Shetland Group. The sediments are composed of basin-wide mudstones and shales, with intercalations of sandstones at several levels that vary widely in geographical distribution.

Wireline log characterization

From wire line logs (Table 2) the Rogaland Group is defined by a significant upwards increase in gamma readings and a decrease in acoustic velocity when compared to the chalky or calcareous mudstones of the Shetland Group. The top of the Rogaland Group is often taken at or close to the top of the bell shape seen on wireline logs for the Balder Formation.

Table 2. High resolution shale stratigraphy of an idealized desanded log of the Rogaland Group in the Viking Graben.

Thickness

The thickness of the Rogaland Group varies significantly within the North Sea Basin. In the Norwegian sector a thickness of 918m was found in well 24/9-3, 714 m in 25/4-3, and 647 m in well 35/9-2. Very minor thicknesses of the Rogaland Group are seen on top of salt diapirs in the Central Trough (eg. 26 m in well 1/6-5), and on the Utsira High (88 m in well 16/6-1), and in the easternmost parts of the Norwegian Danish Basin (55 m in well 9/2-2).

Isochore map ROGALAND-SHETLAND from well data

Seismic characterization

A tectonic map of the Central and Northern North Sea with the main structural elements is shown in Fig. 1 and regional seismic maps from the Rogaland Group in Figs. 2 and 3. These show an axial deep close to the UK/Norwegian border, with its deeper parts in the south. Greatest thicknesses are seen in the southern Viking Graben Area and along the eastern margin of the Sogn Graben.

The top of the Rogaland Group is often distinguishable as the top Balder seismic reflector. However, in some areas the top of the Balder tuffaceous zone or the base of the Balder Formation are better defined than the formation top, and is often easier to map regionally.

Regional Cross Sections

From regional seismic cross sections (Figs. 16-18) , prominent progradational wedges with clinoforms can be seen west of the main depositional troughs of the Central Graben and Viking Graben. Along the eastern margin of the Stord Basin and the Sogn Graben similar wedges can be seen to dip from the East. The general impression is that clinoforms along the western flank have a steeper dip than those from the east. This may be interpreted as the result of different sand to mud ratio in the sediments, and to a more active tectonic regime of the Shetland Platform than in southern Fennoscandia. During the Paleocene there were periods of active rifting along the western margin of the Shetland Platform which uplifted the platform and generated a more immature topography exposing erodable sediments, whereas southern Fennoscandia to the east remained a tectonically stable platform with a rather mature topographic profile in a landscape dominated by basement rocks.

Fig. 16. Regional seismic line I, northern North Sea. Note signs of dipping reflections/shelf buildout in the East Shetland basin, and dipping reflections representing slope wedges in the Horda Platform area. In the basinal area between the wedges there are thin deposits, reflecting a relatively sparse input of gravity flow material in that area compared to the South Viking Graben and the Central Graben.

Fig. 17. Regional seismic line II, from the Shetland Platform to the Stord Basin, North Sea. Dipping reflections east of the Stord Basin are attributed to slope progradation from the East. The Viking Graben formed a deep basin filled with a mixture of gravity flow sediments sourced from the East Shetland Platform and background hemipelagic siliciclastics. Utsira High is characterized by a thinning of the sequence, and is believed to have formed a submarine basinal high during deposition.

Fig. 18. Regional seismic line III, through the Central Graben and the Norwegian Danish Basin, North Sea. Thickening in the Central Graben is mainly attributed to input of siliciclastics from the East Shetland Platform and the Moray Firth area in the north, but input from Southern Fennoscandia in the east and possibly the Midland Valley area may also have contributed. The Central Graben area is commonly interpreted as a deep marine environment, whereas the Norwegian Danish Basin is considered as a shallow to deep marine transition.

Seismic character on a local scale

Sandstones are present in all of the fine-clastic formations. Top and base of sand packages are often mappable due to contrast between blocky sandstones and adjacent shales. In other cases the tops of the sandstones have a more gradual transition to the shales above and there is not a good seismic contrast. In these cases presence of sand is often seen to correspond to a mounded seismic character with discontinuous internal reflectors. An example of seismic character of sandy facies is shown in Fig. 19. Mounds are often interpreted as containing submarine channels/channel complexes or elongated submarine fan systems. In many instances seismic attribute analysis can delineate the areal extension of these systems.

Fig. 19. Example of seismic character of sandy facies of the Rogaland Group, seen from a WE section through southern parts of block 25/10.

Sediment composition and processes

Sandstone facies

In the Norwegian North Sea the Rogaland Group contains a wide variety of sandstone facies, ranging from deep marine and gravity flow to shallow marine and deltaic sandstones. Deep marine sandstones are dominating, but shallow marine deposits have also been recorded. In general shallow marine facies are found in the easternmost parts of the Norwegian North Sea, but at some stages possibly also in parts of the southern Central Graben sucession (Dreyer et al, 2004).

The most pronounced deep marine sandstones are found along the trends of the Central Trough - Viking Graben and the Sogn Graben. On the East Shetland platform (UK), sandstones intercalated with shales and well developed coals or lignitic beds witness deltaic/coastal plain setting. Distinct coal beds are not found in wells penetrating the Rogaland Group of the Norwegian North Sea.

Most wells in the Norwegian sector have been drilled in a paleo-slope to basin floor setting and sandstones here are in general interpreted as gravity flow/turbidites of submarine fans (e.g. Hardt et al., 1989, Knox & Holloway, 1992). Possible exceptions include some occurrences of the intra Sele Formation sandstones of the Forties (Dreyer et al., 2004) and Fiskebank Members. This is discussed in more detail in the subchapters on these members. A nice seismic scale example of submarine fan deposits is shown in the seismic amplitude map in Fig. 20. Examples of turbidites in cores from the Rogaland Group are shown in Figs. 21-24.

Fig. 20. Seismic amplitude maps of an intra Sele Formation seismic marker, displaying submarine fan deposits of the Hermod Member in north eastern parts of Q25.

Fig. 21. Turbidite Example 1. Clean, massive sandstones in Heimdal Member well 25/8-6, interpreted as high density turbidites. Picture from NPD fact pages at http://www.npd.no.

Fig. 22. Turbidite Example 2. Dish structured massive sandstone in Heimdal Member well 25/10-7, interpreted as high density turbidites influenced by water escape after deposition. Picture by H. Brunstad.

Fig. 23. Turbidite Example 3. Normal graded beds, with sharp base and massive structureless sandstone in lower parts of beds, and faintly cross laminated sandstones in the upper parts. Example from Heimdal Member, well 16/1-1. Deposits are interpreted as low-density turbidite deposited in a distal fan or overbank position. Picture by H. Brunstad.

Fig. 24. Turbidite Example 4. Beds of clean, massive, structure-less sandstone at top, and thin-bedded, ripple cross laminated, heterolithic sandstones below. Massive sandstone is interpreted as high density turbidite deposited in a proximal/axial fan position, whereas heterolithics are interpreted as low density turbidites deposited in a fan fringe position. Example from Heimdal member, well 25/8-6. Picture from NPD fact pages at http://www.npd.no.

There is a great variability in the composition of the turbidites. Some consist of thick beds of massive sandstones with frequent fluid escape structures and only very sparse primary structures (Figs. 21 and 22) . In other cases turbidites consist of interbedded shales and sandstone layers with primary structures as horizontal lamination and ripple cross bedding (Figs. 23 and 24) .

In general, the sandstones have very fine to medium sized grains and are moderate to well sorted. Mineralogy is dominated by quartz and feldspar, although in some cases the sandstones may contain a large proportion of glauconite or mica grains. Rip-up clast material of clay and drifted wood fragments may be common in the sandstones in some areas, especially in lower parts of sandstone intervals.

Sandy debris flow deposits may also make up an important part of the primary deposited sandstones in some areas, consisting of massive, rather structureless sandstones, with variable content of fine grained clayey matrix and larger angular mudstone clasts. In the sandstones of the Maureen and Mey Members of the Central Trough, and the Ty Member of the Viking Graben, chalk clasts and occasional chert clasts are also common within the sands. An example of sandy debris flow deposits is shown in Fig. 25.

Fig. 25. Example of sandy/gravelly debris flow deposits from the intra Lista Formation Mey Member. Cores are from well UK 30/14-1, Flyndre discovery, close to the UK/NOR border. The clast material consists of reworked chalk, mudstone, chert and sandstone. The sediments are matrix supported to grain supported. Photo by H. Brunstad.

Injectites

One of the most prominent post depositional features of sandstones in the Rogaland Group is the frequently occurring sand injection structures (e.g. De Boer et al. 2007, Huuse et al. 2004). In the Rogaland Group such features are observed on several scales from seismic (Fig. 26) to well core scale (Figs. 28 and 29) . In the cores the injectites are often seen to be of different generations with networks crosscutting and connecting each other. This indicates that the sand injection process occurred at repeated stages through burial.

Fig. 26. Example of seismic scale injectite systems in the southern parts of block 25/10.

Sand intrusions (injectites) and extrusions

A sketch showing various types of sand extrusives and intrusives is shown in Fig. 27.

Fig. 27. Sketch showing various sand intrusion to extrusion types.

Sand injectites are fluidized and mobilized sands that were injected into sediments, generating dikes and sill systems (Figs. 27, 28 and 29) , whereas sand extrusions were fluidized sands that extruded to sea bottom as sand volcanoes, sometimes developing into turbidites.

Fig. 28. Sandy dike injectites. Core example from the upper parts of the Lista Formation, well 25/10-7S. Photo by H. Brunstad.

Fig. 29. Sandy sill injectite. Example from the Balder Formation well 25/11-23. Photo from NPD Fact pages on http://www.npd.no.

Sand intrusion and extrusion is a result of pressure release and pressure reduction from overpressured sands through water escape and escape of fluidized sands from higher pressured areas to lower pressured areas. Most commonly this involved stratigraphical upwards movements of sands, but downwards and lateral movements also seem to have occurred. Simultaneously with sand injection there was severe sand deformation of parent and receiver sand bodies and clay clasts being ripped up from sand/mud interboundaries, both within the mother and the offspring sand bodies as well as in the sill/dike systems themselves.

Genesis of overpressure

Sand mobilization and injection is believed to have occurred before lithification, which in most cases is believed to have happened before several hundreds of meters of burial. The system may have lived through several phases of overpressuring with several different sources of overpressure. The most relevant sources of overpressuring are considered to be
  1. Undercompaction of clay formations containing the sands.
  2. Stabilisation of sand grain framework with tighter grain packaging and fluid escape after burial.
  3. Overpressure release/fracturing from deeper strata.
The most powerful and volumetrically most important source is believed to be 3., causing escape of high pressures from deeper basin levels created by hydrocarbon generation and silica diagenesis.

Polygonal fault patterns

Polygonal fault patterns are commonly associated with the Rogaland Group. Such patterns are especially well developed and frequently occurring in the Balder Formation, but they are also common at other levels. An example of this phenomenon is shown in Fig. 30.

Fig. 30. Polygonal fault patterns seen from seismic amplitude map of the Base Balder Fm seismic surface in the Grane Area.

According to Dewhurst et al., (1999) the structure and geometry of the fault system are controlled by the colloidal nature of the sediments, and the volumetric contraction measured on seismic scale can be accounted for by syneresis of colloidal smectitic gels during early compaction.

Syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid. This process occurs due to the domination of interparticle attractive forces in marine clays, dependent on environment, and is governed by the change of gel permeability and viscosity with progressive compaction. The process of syneresis can account for a number of structural features observed within the fault systems, such as tiers of faults, the location of maximum fault throw and growth components at upper fault tips (Dewhurst et al., 1999).

Slides and slumps

In some areas chaotic folds associated with sub horizontal shear planes through the sediments are seen from drill cores, and are evidence of slumping and sliding of sediments (Fig. 31) . Slumping is seen to be developed in association with movements on slopes, but also in connection with slide movements away from growing salt diapirs.

Fig. 31. Example of slump folded sediments. Example is taken from the Lista Formation from well UK30/14-1, Flyndre Discovery, close to the UK/NOR border. Photography by H. Brunstad.

Wireline log correlations

A series of lithostratigraphic wireline log correlations have been made at four steps from north to south in the Norwegian North Sea (Figs. 32-37) .

Fig. 32. Overview map showing orientation of Regional seismic lines I-III and well correlation lines 1-4.

Fig. 33. Regional well correlation I, northern North Sea.

Fig. 34. Regional well correlation II.

Fig. 35. Regional well correlation III

Fig. 36. Regional well correlation Alternative a. Vidar Member deposited before the Forties Member.

Fig. 37. Regional well correlation Alternative b. Vidar Member deposited after the Forties Member.

Biostratigraphy, and age

The top of the Rogaland Group (top of the Balder Formation) is characterised by impoverished shelly microfossil assemblages, dominated by pyritized diatoms containing Fenestrella antiqua of Early Ypresian age (approximately 53 Ma) (Knox & Holloway, 1992; Mudge & Bujak, 1996; Gradstein & Bäckström, 1996).

The base of the Rogaland Group coincides closely with the last occurrence of Senoniasphaera inornata (Mudge & Bujak, 1996). It is marked by a reduction in diversity of benthic assemblages towards the underlying Shetland Group (Knox & Holloway, 1992). The age of the basal Rogaland Group is Late Danian and approximately 61 Ma.

Table 3 shows diagnostic biostratigraphic events based on microfossils.

Table 3. Some important biostratigraphic events of the Rogaland Group.

Correlation and subdivision

Segmentation of the Paleocene-lowermost Eocene into stratigraphic sequences was initiated by Stewart (1987), and has been continuously refined to date. Since the work of Knox and Holloway (1992), lithostratigraphic subdivision of this stratigraphic interval follows sequence stratigraphic boundaries. This is also the case for this study. The Paleocene succession in the North Sea contains two types of stratigraphic surfaces (e.g. Mudge & Bujak, 1996 and Mudge & Jones, 2004):
  1. High gamma values mudstones representing sediment condensation associated with maximum flooding surfaces (mfs.) and maximum marine transgression.
  2. Unconformity surfaces overlain by sandstones and reworked chalk or tuff and representing submarine or sub aerial erosion and missing section.
According to Mudge & Jones (2004), biostratigraphic dating allows these surfaces to be correlated throughout the North Sea Basin, as far south as the Mid North Sea High. These authors found a series of short duration (0.1-0.3 my) unconformity-mfs couplets that may be recognized within the Danian (including the Ekofisk Formation) to lowest Ypresian interval (65-53 Ma). These uplift-subsidence cycles may have been caused by episodic plume related magmatic injection near Moho and associated fluctuations in dynamic support, related to the initiation of the Iceland Plume. Alternatively, these cycles may reflect eustatic change (Mudge & Jones, 2004).

A refined sequence stratigraphic framework was established by Mudge and Bujak (1996), who used high-gamma value mudstones in combination with biostratigraphy to subdivide the Paleocene-lowermost Eocene succession (Rogaland Group plus the Ekofisk Formation of the Chalk Group) into stratigraphic sequences, high gamma mudstones being interpreted as maximum flooding surfaces. The stratigraphic position of these high-gamma mudstones shows a consistent relationship with microfossil bioevents and biostratigraphic levels (Mudge and Bujak, 1996). These high-gamma mudstones have been used as the background for the sequence stratigraphic sub division of the shale formations in this study.

Geographic distribution

The Rogaland Group is continuously present in the sedimentary fill of the Central Graben, Viking Graben and Sogn Graben. It is also present at the Måløy terrace, in the Stord Basin and in the Norwegian Danish Basin, but is sub cropping against younger strata of various ages in the eastern parts of these areas. Paleogeographic maps for the Rogaland Group are shown in Fig. 38.

Occurrences of group tops in wells

Fig. 38. Distribution of the Formations of the Rogaland Group and its sandstone members.

Depositional environments

The Rogaland Group was deposited in a bathyal to neritic environment. The majority of the wells in the Norwegian North Sea have been drilled into deep-water deposits, with few penetrating shallow-water deposits. Since exploration has been focused in areas of basin that contain deep-water deposits, the understanding of these is better than that of the shallow-water time equivalent deposits.

It was Parker (1975) who first interpreted Paleocene sandstones of the North Sea to be submarine-fan sediments. It is now widely accepted that the dominant process for sand transport from the shelfal areas into the various basins and sub basins were of confined and unconfined gravity flows derived from either point sources or line sources (Ahmadi et al., 2003; Richards & Reading, 1994). Depositional features of confined systems involve depositional channels, sometimes erosional channels, overbank and levee deposits and channelised lobes. Features of the unconfined systems include single terminal lobes, amalgamated and compensating lobes and submarine aprons (Ahmadi et al., 2003). In both confined and unconfined systems, sand deposition and preservation have been controlled by sea-floor topography. The sedimentation process included high and low-density sediment gravity flows, slurries, slumps and debris flows.

Low-density gravity flow deposits are commonly observed in overbank deposits of levee and crevasse splays, in distal terminal lobes and distal submarine aprons. They may also be a result of flow stripping from high-density gravity flows. Slumps are common features in the overbank deposits (Ahmadi et al., 2003). High-density gravity-flow deposits often occur in confined channels, proximal terminal lobes and submarine aprons, where they exhibit massive, unstructured sandstones, massive sandstones with load and dish structures and planar-laminated sandstones. High sedimentation rates and dipping morphological gradient frequently led to post-depositional mobilization and redeposition of sediments seen as debris flow deposits, sandy injectites and slumps.

Volcanic activity and deposition of tuffs

The volcanic tuffs have already been mentioned in chapter 2. In the North Sea tuffs were mostly deposited in the Balder Formation, but some minor tuff stringers are also sometimes seen in the Lista and Sele Formations. These tuffs are deposits from airborne volcanic ash material settling on the seafloor. Commonly a bright grey colour is observed in the tuffite layers/stringers, but may change into pale greenish grey.

Classification of sand type and prediction of sand presence from Sand/Gross ratio (S/G) cross plotting

By Sand/Gross ratio is meant the relative proportions (cumulative sand thickness) of sand in a Gross stratigraphic interval, read as a fraction.

Cross plotting of S/G from wells is a technique that has been much used to predict sand/reservoir presence in Paleocene exploration targets. By cross plotting S/G data from wells drilled in a certain area, a prediction of background shale, shale cut-off, sand thickness and S/G ratio of a prospect can be made. The technique is most reliable in basinal positions, and must be limited to sub basins and sub systems since there is much variability between individual sub basins.

As a working hypothesis the following values are commonly adopted (after Richards & Reading (1994): Sand rich deep marine systems have S/G ratio 1.0-0.6, Mixed sand-mud systems 0.6-0.3 and mud rich systems <0.3. Some S/G-plots are shown in Figs. 39-41.

It must be stressed that this technique works best in basinal position with aggradation, and does normally not give a good correlation in prograding slope settings.

Fig. 39. Sand/Gross ratio for the Rogaland Group from ~300 wells in the Norwegian North Sea. From Brunstad 2002.

Fig. 40. Sand/Gross Trends for the Rogaland Group. Trend lines are based on cross plots of wells in various subareas of the North Sea. From Brunstad 2002.

Fig. 41. Sand/Gross from cross plotting of each of the formations of the Rogaland Group in southern parts of Quadrant 25. From Brunstad 2002. Crossing point with the horizontal axsis gives the treshold thickness for sand, whereas slope of line indicates N/G that can be expected for sandstone bodies in each of the Formations.

Importance of provenance areas

The major source areas for sands being shed into the North Sea Basin are the East Shetland platform and south western Fennoscandia. Sandstones vary in composition from almost pure quartzite to arenaceous sandstone to highly micaceous or glauconitic sandstone. This is a result of factors such as:

ACKNOWLEDGEMENTS

Thanks to the following for their contributions:
  1. Lundin Norway and RWE Dea Norge, for their willingness to contribute with seismic and well data and man hour.
  2. Iain Prince, Statoil, Houston, USA and Mike Charnock StatoilHydro, Bergen, Norway for their discussions and contributions with biostratigraphical data.
  3. Tom Dreyer and Vigdis Løvø, Lundin Norway AS, for reading trough the document.
  4. Heidi Lerdahl, Leigh Nesbit and Nora Marie Lothe Brunstad drafting of many of the figures and Jørgen Borchgrevink, RWE Dea Norge, and for his valuable help with accessing data.

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