Post-Eocene strata of the southern Viking Graben, northern North Sea

60 Pages·2008·10.53 MB·English

NORWEGIAN JOURNAL OF GEOLOGY Post-Eocene strata of the southern Viking Graben, northern North Sea 391
Post-Eocene strata of the southern Viking Graben,
northern North Sea; integrated biostratigraphic,
strontium isotopic and lithostratigraphic study
Tor Eidvin1 & Yngve Rundberg2
Tor Eidvin & Yngve Rundberg: Post-Eocene strata of the southern Viking Graben, northern North Sea; integrated biostratigraphic, strontium
isotopic and lithostratigraphic study. Norwegian Journal of Geology, vol. 87, pp. 391-450.Trondheim 2007. ISSN 029-196X.
Based on an extensive study of biostratigraphic and strontium isotopic data from wells in the southern Viking Graben and eastern flank of the
Utsira High we present an improved chronology of the post-Eocene section of the northern North Sea. Emphasis has been placed on the sandy
Utsira and Skade formations. Detailed analyses of foraminiferal and Bolboforma fossil assemblages supported by strontium isotopic data from six
exploration and two production wells suggest that the Skade sands were deposited mainly during the Early Miocene whereas the Utsira sands were
deposited during the Late Miocene and Early Pliocene. All biostratigraphic data are presented in range charts and have been integrated with wire-
line log and seismic data. Strontium isotope stratigraphy has been used as an additional dating tool and has proved powerful in the sandy sections.
This work also demonstrates a need for an update or modification of the lithostratigraphic nomenclature of the post-Eocene succession in the
Norwegian North Sea, and a proposal for a revision is presented.
1 Norwegian Petroleum Directorate, P.O. Box 600, N-4003 Stavanger, Norway
2 Svenska Petroleum Exploration AS, Lilleakerveien 2-E3, P.O. Box 153, N-0216 Oslo, Norway
Introduction This paper forms the third part of our study of the upper
Cainozoic succession in the central and northern North
In this paper we present an improved chronology of the Sea. A stratigraphical investigation of the upper Caino-
post-Eocene section in blocks 15/9, 16/1, 24/12 and 25/10 zoic succession in the Tampen area was presented in Eid-
of the Norwegian North Sea (Fig. 1). vin & Rundberg (2001) and a synthesis of the Oligocene
Our investigation includes data from six exploration to Miocene depositional history of the northern and
wells (15/9-13, 15/12-3, 24/12-1, 16/1-2, 16/1-4 and central North Sea was presented in Rundberg & Eidvin
25/10-2) and two production wells (15/9-A-11 and 15/9- (2005).
A-23) located in the southern Viking Graben and on the All absolute ages referred to in the present study are based
western flank of the Utsira High. It is a combined bio- on Berggren et al. (1995), and all depths are expressed as
stratigraphically, seismostratigraphical, lithostratigraphi- meters below the rig floor (m RKB).
cal and geochemical study. Well 15/12-3 has previously
been investigated by Eidvin et al. (1999), but has been re-
Previous works
analysed and reinterpreted in this study.
The main purpose has been to improve the chronology
Biostratigraphy
of the upper Cainozoic sequences and to constrain better
the age of the main sequence boundaries. Emphasis has Although extensively explored by a number of explora-
been placed on the upper part of the Hordaland Group tion and production wells, the biostratigraphy of the
including the Skade Formation and the lower part of the post-Eocene section of the southern Viking Graben has
Nordland Group including the Utsira Formation. The not previously been treated in detail. In most of these
Upper Pliocene has been studied in two wells, and the wells, contracted consultants have carried out routine
lower part of the Upper Pliocene has been investigated in biostratigraphic dating. Traditionally, the Upper Caino-
four wells. The lower part of Pleistocene is considered in zoic has been given low priority, and the datings are con-
two wells. The error in the definition of Utsira and Skade sequently often insufficient. Correlation of wells based
type wells (Fig. 2), as described by Rundberg & Eidvin on a variety of consultant-generated data can often be
(2005), has evidently caused much confusion and many problematic, because of different taxonomic nomencla-
ture and interpretations. Unpublished consultant reports
problems for stratigraphic workers in the northern North
are often particularly inadequate with regard to the use
Sea. These problems have also demonstrated a need for
of planktonic foraminifera and Bolboforma (calcareous
updating the existing lithostratigraphic nomenclature of
microfossils of uncertain origin). One reason for this is
the Upper Cainozoic (Isaksen & Tonstad 1989), and we
that planktonic/benthic ratios are very low in some sec-
present a proposal for its revision.
tions. In this study we overcame this problem by pre-
392 T. Eidvin & Y. Rundberg NORWEGIAN JOURNAL OF GEOLOGY
1° 2°
59°
Figure 1. Study area showing
STUDY AREA
major structural elements, loca-
tion of studied wells (red) and
wells (black) that are shown on
seismic sections and log correla-
tions. Blue lines show location of
58°
seismic lines.
Figure 1.
Fig.2 Correlation conflict
Figure 2. Correlation conflict
between the Utsira and Skade
formations, in which the lower
part of the Utsira Formation in
type well 16/1-1 correlates with
the Skade Formation in its type
well 24/12-1 (from Rundberg &
Eidvin 2005).
Figure 2.
NORWEGIAN JOURNAL OF GEOLOGY Post-Eocene strata of the southern Viking Graben, northern North Sea 393
paring and analysing large sediment samples. Another cession in an exploration well in the Danish sector based
reason is that the tests of Bolboforma are very small and on the analysis of benthic foraminifera in ditch cutting
some species are difficult to identify without the use of samples. Eidvin et al. (1999) and Eidvin & Rundberg
the electron microscope. (2001) have investigated the Oligocene to Pleistocene
succession in a number of exploration wells based on
Based on the analysis mainly of ditch cutting samples but analysis of benthic and planktonic foraminifera, Bolbo-
including also samples from sidewall cores and conven- forma and strontium isotopes in mainly ditch cutting
tional cores from a large number of exploration wells, samples, but including sidewall cores and conventional
King (1983, 1989) published a detailed foraminiferal cores. Piasecki et al. (2002) investigated the Lower Plio-
zonation for the Cainozoic of the entire North Sea. Grad- cene succession (upper part of the Utsira Formation) on
stein & Bäckström (1996) established a similar forami- the Sleipner field based on the analysis of dinoflagellate
niferal zonation for the North Sea and the Haltenbanken cysts in a cored sample. Head et al. (2004) have investi-
areas based on the analysis of the same kind of material gated the base Upper Pliocene section on the same oil
in a number of wells. field based on analysis of dinoflagellate cysts, benthic and
planktonic foraminifera in three cored samples.
In recent years, several papers have been published deal-
ing with the chronology of upper Cainozoic deposits in Lithostratigraphy
exploration wells from the central and northern North
The first stratigraphic subdivision of the Cainozoic was
Sea areas. Rundberg & Smalley (1989) performed age
presented by Deegan & Scull (1977). They subdivided
determinations in a number of exploration wells based
Eocene to Recent strata into the Hordaland and Nordland
on strontium isotope analysis of mollusc fragments in
groups. The only formation defined by these authors
ditch cutting samples. Knudsen & Asbjörnsdóttir (1991)
within the post-Eocene was the sandy Utsira Forma-
have investigated the Upper Pliocene and Pleistocene in
tion at the base of the Nordland Group. Isaksen & Ton-
an exploration well in the English sector based on the
stad (1989) adopted this nomenclature, and also intro-
analysis of benthic foraminifera in ditch cutting samples.
duced two sandy formations in the Oligocene part of the
Seidenkrantz (1992) studied the same succession in sev-
Hordaland Group, which they termed Skade and Vade
eral exploration wells on the Gullfaks field based on the
formations, present in the Viking Graben and central
analysis of the same kind of fossils and material. Grad-
North Sea, respectively.
stein et al. (1992, 1994) published data from the entire
Cainozoic succession in a number of exploration wells
In the UK part of the basin, Knox & Holloway (1992)
based on the analysis of dinoflagellate cysts, benthic and
established the Westray Group as a new lithostratigraphic
planktonic foraminifera in mainly ditch cutting samples
unit. It formed the uppermost of two groups which they
but including sidewall cores and conventional cores.
had introduced to replace the Hordaland Group. They
Konradi (1996) has analysed the post mid-Miocene suc-
introduced the Lark Formation for the distal, mudstone-
Figure 3. Oligocene-Miocene lithostratigraphy of the Norwegian northern North Sea (modified from Rundberg & Eidvin 2005).
Figure 3
394 T. Eidvin & Y. Rundberg NORWEGIAN JOURNAL OF GEOLOGY
dominated facies of the Westray Group and used the vin (2005). The most detailed works have focussed on
Skade Formation for glauconitic sandstones and siltstones the Utsira Formation, first by Gregersen et al. (1997) and
of shelf-facies. Fyfe et al. (2002) published an updated later by Galloway (2002). The latter represents a mile-
lithostratigraphy of the central and North Sea, based on stone in the understanding of this unique sandy system,
Isaksen and Tonstad (1989), Knox & Holloway (1992), with a thorough discussion of the depositional pattern.
Eidvin et al. (1999) and Eidvin & Rundberg (2001).
Recently, Rundberg & Eidvin (2005) presented a revised Post-Eocene structural and depositional
lithostratigraphic and chronostratigraphic subdivision
history of the southern Viking Graben
of the Oligocene-Miocene of the Norwegian northern
North Sea (Fig. 3). They assigned the Skade Formation to
The main structural elements of the study area are shown
the Early Miocene, and not to the Oligocene, as suggested
in Fig. 1. The structural setting is depicted by the two
by Isaksen & Tonstad (1989) and a more precise age
regional transects shown in Figs. 4 and 5. These figures
assignment was given to the Utsira Formation. Further-
illustrate the Palaeocene-Eocene sedimentary and struc-
more, Rundberg & Eidvin (2005) demonstrated an error,
tural pattern, characterized by Palaeocene gravity-flow
or inconsistency, in the definition of the Skade and Utsira
deposition in the southern Viking Graben (SVG) and on
formations. As presented in Fig. 2, there is an overlap in
the eastern flank of the Utsira High, and Eocene subsid-
definitions of the Utsira and Skade formations in which
ence of the SVG and relative uplift of the Utsira High.
the Skade Formation, as defined in type well 24/12-1,
The Grane and Balder fields represent two important
correlates with the lower part of the Utsira Formation as
hydrocarbon traps that developed as a response to the
defined in type well 16/1-1.
structural activity. This history is well-known and will
not be outlined further here.
Sequence stratigraphy
The post-Eocene sedimentary and structural history,
Regional sequence stratigraphic studies on the post- however, is less well described. Important accounts have
Eocene of the northern North Sea have been presented been presented by Rundberg (1989), Galloway et al.
by Rokoengen & Rønningsland (1983), Rundberg (1993), Galloway (2002), Jordt et al. (1995), Gregersen et
(1989), Jordt et al. (1995), Gregersen et al. (1997, 1998), al. (1997), Faleide et al. (2002) and Rundberg & Eidvin
Martinsen et al. (1999), Galloway et al. (1993), Galloway (2005).
(2002), Eidvin & Rundberg (2001) and Rundberg & Eid- Recently, Rundberg & Eidvin (2005) presented a tec-
W CNST-97-06 E
UK | N
(a)
UK16/17-15 15/6-315/6-5 15/6-7 16/4-1 16/5-1
(b)
Utsira Fm sands
Middle Miocene Unconformityat topHordaland
Lower Miocene
Figure 4.
Figure 4. (a) Regional seismic line across the southern Viking Graben and Utsira High close to wells 15/6-3 and 15/6-5, (GR-logs displayed); (b)
detail from line shown above with projections of wells that are used in log correlation (Fig.18). Location of line shown in Fig. 1.
NORWEGIAN JOURNAL OF GEOLOGY Post-Eocene strata of the southern Viking Graben, northern North Sea 395
Fig.5
UpperPliocene
Utsira Fm
Middle Miocene
Skade Fm
LowerMiocene
Oligocene
Eocene
UpperPliocene
Utsira Fm
Middle Miocene
Skade Fm
LowerMiocene
Oligocene
Paleocene
Eocene
FigurFei g5ur.e 5. (a) Regional seismic line across the southern Viking Graben and Utsira High; (b) interpretation of line above. Location of line shown
in Fig. 1.
tonostratigraphic framework for the Oligocene-Miocene of also much of the Utsira sands are restricted to the central
the northern North Sea. They suggested that the struc- part of the basin, denoting a dramatic relative sea-level
tural and stratigraphic changes seen in the northern fall with infill of sediments within the southern Viking
North Sea were closely linked to the larger scale struc- Graben.
tural evolution of the NW European passive margin.
Structural doming on the mid-Norwegian margin, for Rundberg & Eidvin (2005) suggested that the post-
example, coincides fairly well with uplift of the northern Eocene of the northern North Sea was affected by three
North Sea Basin. As a response to this uplift, a distinct phases of coarse clastic input. The first phase of sandy
unconformity was created (referred to as the mid-Miocene deposition took place during Early Oligocene and was
unconformity) in the northern North Sea Basin. In Fig. concentrated to areas north of 60°N (unnamed Oligocene
3, this is illustrated by a northwards increasing strati- sands in Fig. 3). Gravity-flow sands with gross thicknesses
graphic break. In regional transects across the southern in excess of 400 m were sourced from the East Shetland
Viking Graben and Utsira High, this unconformity is Platform and deposited in a fairly deep-water environ-
particularly conspicuous, as exemplified in Figs. 4a and ment. The second phase of coarse clastic input took place
5a. Below the mid-Miocene unconformity, Oligocene and during Early Miocene and was concentrated mainly to
Lower Miocene strata are present across the entire basin, the southern Viking Graben. This deposition comprises
suggesting relatively deep marine conditions. Above the gravity-flow sands belonging to the Skade Formation,
mid-Miocene unconformity, Middle Miocene strata and which reaches a gross thickness of about 400 m in the
396 T. Eidvin & Y. Rundberg NORWEGIAN JOURNAL OF GEOLOGY
Fig. 6a
WELL 16/1-4 BENTHIC FORAMINIFERA FPOLRAANMKINTOIFNEIRCA BOLBO-FORMA OTHERFOSSILS
DEPTH (mRKB) GAMMA RAY LOGUNIT: gAPI LITHOLOGY LITHOSTRATIGRAPHIC GROUP SERIES/SUBSERIES BENTHIC FOSSILASSEMBLAGES PLANKTONIC FOSSILASSEMBLAGES PALEOBATHYMETRY SIDEWALL CORES SAMPLES (meters) DITCH CUTTINGS SAMPLES (meters) CIBICIDES LOBATULUSCASSIDULINA RENIFORMEBULIMINA MARGINATAANGULOGERINA FLUENSBUCCELLA TENERRIMAELPHIDIUM EXCAVATUMHAYNESINA ORBICULARECASSIDULINA TERETISCIBICIDES SP.ELPHIDIUM USTULATUMELPHIDIUM GROENLANDICUMVIRGULINA LOEBLICHINONION AFFINEQUINQUELOCULINA SEMINULUMCIBICIDES GROSSUSELPHIDIUM ALBIUMBILICATUMCIBICIDES SCALDISIENSISCIBICIDES PACHYDERMASIGMOILOPSIS SCHLUMBERGERIEPONIDES PYGMEUSLOXOSTOMOIDES LAMMERSIBOLIVINA SKAGERRAKENSISCIBICIDOIDES PACHYDERMAELPHIDIUM SUBARCTICUM HETEROHELIX SP.NEOGLOBOQUADRINA PACHYDERMA (SIN.)NEOGLOBOQUADRINA PACHYDERMA (DEX.)GLOBOROTALIA INFLATATURBOROTALIA QUINQUELOBAGLOBIGERINA BULLOIDESGLOBIGERINITA GLUTINATANEOGLOBOQUADRINA ATLANTICA (DEX.)HEDBERGELLA SP. BOLBOFORMA CLODIUSI GEODIA SP.RADIOLARIADIATOMS PYRITIZED
357,5
M-S 375,5
UTI
440500 PLEISTOCENE LPHIDIUM EXCAVATCASSIDULINA TEREASSEMBLAGE 445024,,55
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700 NEOGLOBOQUAD-RINA ATLANTICA(DEXTRAL)ASSEMBLAGE OUTER TO MIDNERITIC 666668878983517,,,,,55555 667790810000
04080120 OD 0306022
Figure 6a. Range chart of the most important index fossils, fossil assemblages, gamma ray log, lithology, lithostratigraphic unit, series/subseries,
paleobathymetry, and samples investigated in the upper part of well 16/1-4. For abbreviations and key to symbols, see Fig. 6c.
study area (Fig. 5). The third phase of coarse clastic input is based on material from cored sections in basal part
relates to the sands of the Utsira Formation, which were of Upper Pliocene and Lower Pliocene deposits, respec-
deposited during Late Miocene and Early Pliocene (in tively. The Pleistocene succession is analysed in well
some wells the deposition started during latest Middle 15/12-3 and 16/1-4, but the uppermost approximately 90
Miocene). m in well 15/12-3 and approximately 220 m in well 16/1-
4 are not sampled. Ditch cuttings are usually sampled at
10 m intervals in upper Cainozoic sections, and we have
Material and methods analysed all the available samples. However, some of the
stored samples contain so little material that they could
Samples. In most of the studied wells, the biostrati- not be released for analyses. This is especially the case for
graphic analyses were performed largely on ditch cut- some sections of of wells 16/1-4 and 24/12-1, but these
ting samples. Sidewall cores were available in well 16/1-4 sections are analysed in other wells.
(48 cores). In wells 15/9-A-11 and 15/9-A-23, the work
NORWEGIAN JOURNAL OF GEOLOGY Post-Eocene strata of the southern Viking Graben, northern North Sea 397
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398 T. Eidvin & Y. Rundberg NORWEGIAN JOURNAL OF GEOLOGY
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Figure 6c. Range chart of the most important index fossils, fossil assemblages, gamma ray log, lithology, lithostratigraphic units, series/
subseries, paleobathymetry, strontium isotope ages and samples investigated in the lower part of well 16/1-4.
NORWEGIAN JOURNAL OF GEOLOGY Post-Eocene strata of the southern Viking Graben, northern North Sea 399
Altogether 365 ditch cutting samples, 48 sidewall cores marily with the biozonation of King (1983, 1989), which
and four samples from conventional cores were analysed, outlines a micropalaeontological zonation for Cainozoic
primarily for planktonic and benthic foraminifera and sediments in the North Sea. Gradstein & Bäckström’s
Bolboforma. Pyritized diatoms were used to establish the (1996) faunal zonation from the North Sea and Halten-
stratigraphy in Lower Miocene and Oligocene deposits. banken is also extensively used. In addition, a number
of articles describing benthic foraminifera from onshore
Between 50 g and 100 g of material were used to analyse basins in the area surrounding the central and south-
conventional core samples and ditch cuttings. Sidewall ern North Sea are utilized. The zonations of planktonic
cores contain less sample material, and thus sometimes foraminifera (Weaver 1987, Weaver & Clement 1986 and
produce incomplete, non-representative faunal assem- 1987, Spiegler & Jansen 1989) and Bolboforma (Spie-
blages. Sidewall core and conventional core analyses do, gler & Müller 1992, Müller & Spiegler 1993) from ODP
however, provide useful in situ assemblages, because the and DSDP drillings in the Norwegian Sea and the North
material is generally not contaminated by cavings. Atlantic are also very important for dating the sediments.
Correlation with these zones yields the most accurate
Fossil identifications were performed in the 106-500 age determinations, because the zones are calibrated
μm fraction. In some cases, the fraction larger than 500 with both nannoplankton and palaeomagnetic data. The
μm and the fraction less than 106 μm were also studied. zonations of King (1983, 1989) and Gradstein & Bäck-
If possible, 300 individual fossils were selected for each ström (1996) are based on the last appearance datums
sample. In order to optimise the identification of the (LADs) of various taxa. The planktonic foraminifera and
foraminiferal assemblages, most samples rich in terri- Bolboforma zonations from the ODP and DSDP drillings
genous grains were gravity-separated in heavy liquid. In are based on first appearance datums (FADs).
such cases, 1000-1500 individuals were analysed in fossil-
rich samples. Fossil assemblages. In the eight wells examined in this
study, a system of 18 assemblages based on benthic fora-
The lithologic analyses are based on visual examina- minifera and 18 assemblages based on planktonic fossils
tion of the samples prior to treatment, and also of the is devised. Three intervals with very poor planktonic fau-
dissolved and fractionated material after preparation. nas (undefined intervals P1, P2 and P3) and one interval
Owing to problems caused by caved material, only a with very poor benthic foraminiferal fauna (undefined
very generalised description was deemed appropriate for interval F1) are also described. These units are presented
most sections. However, the sidewall cores in well 16/1-4 in the Appendix. The total results are also summarized
and the short conventional cores in wells 15/9-A-11 and in stratigraphic range charts (Figs. 6 through 13). Cor-
15/9-A-23 allowed more accurate lithologic descriptions relation of fossil assemblages between the studied wells
to be performed. is shown in Fig. 14. Correlation of fossil assemblages
between wells 15/12-3, 16/1-4 and 16/1-2 to the fossil
Strontium isotope analyses. In total, 215 samples were zonation of King (1989) and the planktonic fossil zona-
analysed for Sr isotopic composition. The analyses were tion in the ODP sites on the Vøring Plateau is shown
mainly conducted on tests of calcareous foraminifera in Fig. 15. Correlation of planktonic fossil assemblages
and mollusc fragments. In some samples, Bolboforma between wells 24/12-1 and 25/10-2 with the planktonic
were also used. The analytical work was conducted by fossil zonation in the ODP sites on the Vøring Plateau is
the Mass Spectrometry Laboratory at the University of shown in Fig. 16.
Bergen, Norway. All Sr isotopic ratios were normalized
to 86Sr/88Sr = 0.1194 and to NIST 987 = 0.710248. Sr val-
ues were converted to age estimates using the SIS Look- Log correlations, log markers and
up table of Howard & McArthur (1997). The within-run
seismic illustrations
precision for single analyses was commonly of the order
of ±7 to 9 x 10-6. The precision of the analyses, however, Four log correlations profile (profiles 1-4) of Oligocene
is considered to be of the order of ±20 x 10-6, which is the - Miocene strata are shown in Figs. 17 through 20. The
long-term precision of the mass spectrometer using the correlations are calibrated with seismic and biostrati-
standard reference material. See also Chapter 7 for more graphic data in key wells of the study area.
details about Sr isotope stratigraphy. In the southern Viking Graben (Fig. 18, profile 2),
seven gamma ray maxima or peaks (GR1-GR7) were
Biostratigraphic zonation identified within the mudstone-dominated Oligocene-
The standard Cainozoic biostratigraphic zonation is Miocene section. The stratigraphic sequences bounded
based on planktonic foraminiferal and calcareous nan- by these peaks have been mapped in a number of wells
noplankton distributions established in tropical and sub- using the GR-markers as a guiding tool (see below for
tropical areas. In middle and high latitudes, the assem- further details). Between wells that are relatively closely
blages become progressively less diverse and many key spaced, the correlations can be carried out with good
species are lacking (King, 1983). precision and a high degree of confidence. Jump corre-
In this study, the fossil assemblages are correlated pri-
400 T. Eidvin & Y. Rundberg NORWEGIAN JOURNAL OF GEOLOGY
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Fig. 7a WELL 24/12- GO )LB KY YAR GIRmPO A(A LHgMO T:TMHPIANTEIGUDL 500 550 600 650 700 750 800 18402000 Sea floor = 138,5 meters bDC = Ditch cuttings gAPI = American Petroleu = Abundant molluscs or G = Abundant glauconite
Figure 7a. Range chart of the most important index fossils, fossil assemblages, gamma ray log, lithology, lithostratigraphic units, series/subseries,
paleobathymetry, strontium isotope ages and samples investigated in the upper part of well 24/12-1.