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A sequence stratigraphic study of lower cretaceous deposits in the northernmost North Sea
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A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
The prospectivity of Cretaceous deposits in the northern Norwegian North Sea, particularly in the Agat area, has been evaluated using a sequence stratigraphic approach. The study integrates high-resolution biostratigraphic data, biofacies analyses, well logs, core description and seismic data, all incorporated with the tectonic history of the basin. Two supersequences, KI-1 and K1-2 are informally defined within the Lower Cretaceous succession. Supersequence KI-1 ranges from Ryazanian to late Barremian age, whereas K1-2 has an Aptian to late Albian age. Palaeoenvironmental maps for the two supersequences, using the biofacies analyses from the wells, suggest that KI-1 supersequence was deposited mainly in a shelf environment, mostly within highstand and transgressive systems tracts. Supersequence K1-2, on the other hand, was deposited largely in a bathyal environment and contains more prominent lowstand systems tracts. A more detailed sequence stratigraphic subdivision is suggested within the two main genetic units. The prospective deposits within the Lower Cretaceous interval are the Asgard Sand Unit (Hauterivian-Barremian), a highstand systems tract, and the Agat Formation (Albian), a lowstand systems tract. Within the Agat area, seismic anomalies/mounds have been recognized on an Intra-Albian unconformity. The sedimentological analyses of cores from the Agat Formation indicate, in contrast to published interpretations of depositional environments, that this unit was deposited on an upper slope environment by slump/mass flow processes, causing the sand bodies to be of laterally restricted extent. A well correlation, based primarily on the identification of maximum flooding surfaces, allowed the Agat Formation to be subdivided into at least three stacked sandy units of lowstand origin. Reservoir quality and seal capacity are the major risks in the evaluation of the proposed Agat lead.
Introduction
Exploration in the Norwegian North Sea has focused on structural traps. Recently, however, stratigraphic traps have received more attention, particularly with the advent of sequence stratigraphy as an interpretive tool for prediction of sedimentary facies and stratigraphic traps (Vail and Wornardth, 1991). The Cretaceous deposits in the northern North Sea have for many years been a minor play for exploration in Norway, with the Agat Field as the only significant Lower Cretaceous discovery in the Norwegian North Sea. The field is within the tilted fault-block terrace off M~lCy, in block 35/3 (Fig. 1). The discovery was made by Saga Petroleum in 1980 and the estimated gas-in-place is 65 • 109 S m 3. The area was later relinquished. The reservoir interval is the sandy Agat Formation, previously interpreted as submarine fan deposits of late Albian age (Gulbrandsen, 1987), and reinterpreted here in terms of upper slope sand lobes. The goal of this study was to evaluate the general prospectivity of Lower Cretaceous deposits in the Norwegian North Sea between 60 ~ and 62~
(Fig. 1). The work was done using a sequence stratigraphic approach with an integrated interpretation of high-resolution biostratigraphic data, biofacies analyses, well logs, core description and seismic data, all incorporated with the tectonic history of the basin.
Fig. 1. North Sea index map showing the study area.
Sequence Stratigraphy on the Northwest European Margin edited by R.J. Steel et al. NPF Special Publication 5, pp. 389-400, Elsevier, Amsterdam. 9 Norwegian Petroleum Society (NPF), 1995.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 2. Map showing structural nomenclature in and around the study area. N W - S E and N-S traces are Figs. 5 and 6, respectively.
Tectonic setting The study area covered the northern part of the Viking Graben and particularly the Sogn Graben (Fig. 2). These graben areas are bounded to the east by the Lomre Terrace and M~lCy Fault Blocks. The Marulk Basin lies on the northwestern margin of the study area. The Tampen Spur is centrally located with a northeast to southwest axis. The dominant fault pattern is rectilinear, with Caledonian faults trending northeast to southwest, and conjugate northwestsoutheast fault geometries. We believe many of these faults to have had an important control on Mesozoic sedimentation, and because of this structural/ topographic control they are likely to have delineated areas of differing base-level change response.
The sequence stratigraphic framework To improve our understanding of the Early Cretaceous interval, beyond the terms of the already established lithostratigraphic nomenclature (Isaksen and Tonstad, 1989), we have superimposed a sequence stratigraphy on the Ryazanian to Turonian lithostratigraphy for the study area. The sequence stratigraphy is based on in-house age datings which enabled us to assign a more exact age than previously given to the established units (Fig. 3). By combining the lithostratigraphic descriptions, palaeoecology, and the new age dates for the formations we were able to transform the existing static lithostratigraphy into a dynamic sequence stratigraphy for this interval (Fig. 4). Individual formations have been interpreted in terms of sequences and systems tracts according
Fig. 3. Cretaceous stratigraphy between 60 ~ and 62~ in the Norwegian North North Sea. The main reservoirs are the Agat Formation of Albian age and a sandstone unit here informally called the Asgard Sand Unit.
to the definitions of the latter given by Mitchum et al. (1977) and Van Wagoner et al. (1988, 1990). The figure is somewhat schematic but shows the individual sequences and systems tracts interpreted within the Lower Cretaceous and the lowermost Upper Cretaceous (Ryazanian-Turonian) section of the studied area. However, Fig. 4 is a composite, and all of these sequences are rarely found together. In some areas where sequences are present, there may well be a hiatus in the same time interval in other parts of the basin. The potential reservoirs within the study interval are the sandy Agat Formation of Albian age, and sandstones of Hauterivian to Barremian age (the Asgard Sand Unit). The entire Cretaceous interval is considered to be a first-order transgressive megasequence, K1, because of an overall deepening trend. Within this megasequence, we recognize several supersequences (second-order sequences 5-50 m.y) and depositional sequences (third-order sequences, 0.5-5 m.y (Vail and Wornarth, 1991). Two supersequences, KI-1 and K1-2, were informally defined within the Lower Cretaceous deposits. The first of these, (KI-1), ranges from Ryazanian to late Barremian in age, the second (K1-2) from Aptian to Albian. Most of the wells studied have an unconformity and hiatus between these supersequences, and this may possibly be related to a relative sea-level drop, as claimed globally for the early Aptian by Haq et al. (1987). This unconformity
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
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Fig. 4. A sequence stratigraphic interpretation of the Ryazanian to Turonian succession in the Agat region.
is seen on the seismic data by a good, continuous reflection of regional extent, and is interpreted here as a major sequence boundary. Within these supersequences a number of component depositional sequences are interpreted (Fig. 4). The Asgard Sand Unit is interpreted to represent a highstand systems tract (HST) within one of the depositional sequences in K1-1. The other proven potential reservoir level is the Agat Formation which forms several stacked, sandy units mapped in the
lower levels of sequences and therefore interpreted as lowstand systems tracts (LST) in the two lower depositional sequences within K1-2. This latter conclusion is further justified below.
Regional sequence stratigraphic interpretation and the prospectivity of the area Two regional seismic lines illustrate the regional depositional setting (Figs. 5 and 6). The regional
Fig. 5. Regional (WNW-ESE) seismic section across the Viking Graben. The position of the seismic line is shown in Fig. 2. The Lower Cretaceous deposits are coloured light green (KI-1) and orange (K1-2); BCU is the basal Cretaceous unconformity.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 6. North-south regional seismic section from the Tampen Spur area and out into the Viking Graben. The position of the seismic line is shown in Fig. 2.
seismic line in Fig. 5 is an east-west section across the Viking Graben (Fig. 2), where sequences KI-1 and K1-2 are easily defined. It should be noted that sequences deposited from the Ryazanian to late Turonian have been significantly influenced by tectonic activity, as unconformities can be identified in the wells on both sides of the Viking Graben, and can be tied to the seismic section. Sedimentation from the Coniacian to latest Maastrichtian, in contrast, reflects a more constant thermal subsidence. Palaeoenvironmental maps for KI-1 and K1-2 were made using the biofacies analyses from 45 wells. A biofacies is a group of organisms which responds to and is constrained by environment and associated chemical, physical and biological parameters. Distribution patterns of the environmentally controlled organisms provide information about the depositional environment. The biofacies from the analyzed wells imply that the supersequence KI-1 (Ryazanian to Barremian) was largely deposited in a shelf environment (Fig. 7). KI-1 consists, therefore, mostly of highstand and transgressive systems tracts. Potential reservoirs within the highstand systems tract would have included deltaic and shoreface and possibly fluvial sandstones. The potential reservoirs within the transgressive systems tract would be shoreface and estuarine sandstones. A play concept
Fig. 7. Schematic map of palaeoenvironmental reconstruction for supersequence KI-1 (Ryazanian-Barremian). Red, green and blue areas record non-deposition, shelfal and bathyal areas, respectively.
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
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Fig. 8. Schematic play concept models for supersequence KI-1 emphasizing potential transgressive and high-stand reservoir sands. A seismic line from the Troll East area supports these concepts. The black arrows indicate the potential Lower Cretaceous stratigraphic/ structural traps (the position of the seismic line is shown in Fig. 7).
for supersequence KI-1 is illustrated in Fig. 8 where sands of transgressive and high-stand origin are emphasized. The sand geometry and palaeogeographic play models are supported by a seismic section from Troll East area (Fig. 8). The trapping mechanism for hydrocarbons could be both stratigraphic and structural. Supersequence K1-2 (Aptian to Albian) was more dominated by bathyal environments (Fig. 9) than supersequence KI-1 and therefore has greater potential for lowstand systems tracts. The main elements of the play concept are illustrated by a lowstand systems tract model (Fig. 10, upper part) where the most prospective sediments are located within the basin-floor mounds, and to a lesser extent within the lowstand slope fan and prograding wedge (Vail et al., 1991). A potential basin-floor mound play is further illustrated on a seismic line from 35/6 area (Fig. 10, lower part). An integration of details from the palaeoenvironmental maps, the seismic interpretation and tectonic history have led to the identification of Lower Cretaceous play areas within the Magnus/Marulk Basin, the Lomre Terrace, the Agat Field area, regions east of the Tampen Spur, and the area east of the Troll Field. A lead within the Agat area is pursued further below.
Fig. 9. Schematic palaeoenvironmental reconstruction of supersequence K1-2 (Aptian-Albian). Red, green and blue areas record non-deposition, shelfal and bathyal environments, respectively.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 10. Model of play concepts for supersequence K1-2. The seismic line shows an example of a basin-floor mound recognized within the Agat area (the position of the seismic line is shown in Figs. 9 and 11).
Sequence stratigraphy as an aid to evaluation of a Lower Cretaceous lead (Agat area) Several seismic anomalies/mounds have been identified south of the Agat area (Fig. 11) and have been evaluated in more detail. The seismic line in Fig. 12 shows the tie from between the best sandstone interval in well 35/3-5 and two interpreted seismic anomalies/mounds. An Intra-Albian unconformity is interpreted at the base of these mounded features.
A sequence stratigraphic study was carried out to evaluate these leads. Sedimentological analysis of the cores from Agat wells and high-resolution biostratigraphic analysis of the wells allowed the prediction of depositional sequences and lithofacies distributions. A total of 215 m of core from three wells (35/3-2, 35/3-4 and 35/3-5) were examined from the Agat Formation. Contorted and massive sandstone is the most common facies (facies 2) in the described cores (Figs. 13 and 14), consisting of light grey, micaceous,
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
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Fig. 11. Mounded seismic facies south of the Agat area. The map also shows the position of the seismic line Fig. 12. Fig. 13. Short core log illustrating the main features of facies 2, the contorted and massive sandstone which is the most common facies in the Agat Formation.
Fig. 12. A NNE-SSW squashed seismic line showing the tie from the Agat well 35/3-5, south to two interpreted seismic anomalies/ mounds/yellow) lying on an Intra-Albian unconformity. Note potential reservoir sand at same level in well 35/3-5.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 14. Core photograph (Agat Formation) showing discrete units of pebbly sandstone, massive sandstone, laminated sandstone and pebbly mudstone. Note the glide plane some 10 cm above base of core (lower right). poorly sorted, fine-grained sandstone. Lamination, often at disturbed high angles (up to 60 ~) occurs and contorted sandstone units are interbedded with
undisturbed mudstone units. Mud clasts (commonly at the tops of units) and water-escape structures are present. The basal contacts of some of the units
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
397
Fig. 15. Interpreted depositional model for the Agat Formation showing distribution of sandy slumps and mass flows on an upper slope environment near the shelf edge. Note the suggested differences in sediment source areas for the 35/3-5 well (from southeast) in comparison with 35/3-2 and 35/3-4 wells (from northeast), based on petrographic and biostratigraphic evidence. show evidence of glide planes with shearing and slumping. The facies analysis strongly suggests that the Agat Formation was deposited in an upper slope environment by slump/mass flow processes (Fig. 15) and not simply as part of a submarine fan model (see also Shanmugam et al., 1993). Because upper slope mass flow deposits in such environments tend to be of restricted lateral extent, they are usually not easily correlatable between wells. As part of the sequence stratigraphic analysis, high-resolution biostratigraphic data have been analyzed and used to identify possible regional flooding surfaces and sequence boundaries upon which the well correlations can be based (Fig. 16). The peaks on the abundance and diversity curves are related to condensed intervals and are the most likely candidate horizons for the main flooding surfaces. The three main flooding surfaces or their basinward equivalents were identified (horizons 1, 2 and 3 in Fig. 16) and
used to divide the Agat Formation into three sequences. The Intra-Albian unconformity interpreted on the seismic tie profile (Fig. 12) is interpreted to be near to surface 2. These flooding surfaces appear to coincide with interpreted biostratigraphic and acme horizons which are major transgressive events in the Moray Firth/South Viking Graben area (Mobil inhouse data). Other details which should be noted in Fig. 16 are the peaks of the palynomorphs in the Hauterivian to Barremian sediments. Where the total amounts of terrestrial palynomorphs increase, the total amounts of marine palynomorphs decrease, and vice versa. This strongly suggests a prograding sequence and is one of the arguments for interpreting the Asgard Sand Unit as a highstand systems tract. Three Agat wells (35/3-2, 35/3-4 and 35/3-5) were correlated based on the maximum flooding surfaces, which in these wells are close to the sequence bound-
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A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
399
Fig. 17. Well correlation based on the interpreted maximum flooding surfaces. aries (Fig. 17). Although some sand bodies may well be in communication between adjacent wells it appears to be common that sand bodies within the same sequence cannot be correlated. Drill stem tests taken in wells 35/3-2 and 35/3-4, and interpreted to be within the same sequence, showed different pressures, strongly suggesting lack of communication and direct correlation. The petrography and biostratigraphy in well 35/3-5 appears to differ from the other Agat wells significantly. This could suggest that well 35/3-5 belongs to a separate depositional system, a difference which may have been tectonically controlled. We interpret the Albian sediments encountered in wells 35/3-2 and 35/3-4 to have been derived from the northeast whereas in well 35/3-5 these sediments were derived from the southeast. These directions (fairways) were probably structurally controlled. The potential reservoir leads identified from the basin-floor anomalies
(Figs. 12 and 13) belong clearly to the fairway containing the proven sands in well 35/3-5.
Conclusions The conclusions of this study are as follows: (1) A sequence stratigraphic approach has improved our understanding of the Lower Cretaceous deposits in the northernmost areas of the Norwegian North Sea. (2) Two supersequences have been defined within the Lower Cretaceous time interval, KI-1 and K12, of Ryazanian-Barremian and Aptian-Albian ages, respectively. (3) The main prospective levels in the Lower Cretaceous interval are the Asgard Sand Unit (Hauterivian-Barremian) which is a highstand systems tract within KI-1, and the Agat Formation (A1bian) which is a lowstand systems tract within K1-2.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
(4) Sequence stratigraphy has helped to constrain the prospect risk in an area where seal capacity and reservoir quality are the major concern.
Acknowledgements The authors thank Mobil Exploration Norway for permission to publish this work, and Nopec As. for permission to publish some of the seismic sections. The support and encouragement offered by our colleagues are gratefully acknowledged. In particular, L. Fearn and R. Dunay for the biostratigraphic support, and M. Cecci for other assistance.
References Armentrout, J.M., 1990. Patterns of forminiferal abundance and diversity; implications for sequence stratigraphic analysis. In: Soc. Econ. Paleontol. Mineral., Gulf Coast Sect., 11th Annu. Res. Conf. Sequence Stratigraphy as an Exploration Tool. Concepts and Practices in the Gulf Coast, Program and Extended and Illustrated Abstracts, pp. 53-58. Gulbrandsen, A., 1987. Agat. In: A.M. Spencer, C.J. Campbell, S.H. Hanslien, E. Holter, P.H.H. Nelson, E. Nys~ether and G. Ormaasen (Editors), Geology of the Norwegian Oil and Gas Fields. Graham and Trotman, London, pp. 363-370.
M. SKIBELI K. BARNES T. STRAUME S.E. SYVERTSEN G. SHANMuGAM
Haq, B.U., Hardenbol, J. and Vail, ER., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 11561167. Isaksen, D. and Tonstad, K., 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. Norw. Pet. Dir., Bull., 5, 24 pp. Mitchum, R.M. Jr., Vail, ER. and Thomson, S., 1977. Part Two: The depositional sequence as a basic unit for stratigraphic analysis. Seismic stratigraphic applications to hydrocarbon exploration. Am. Assoc. Pet. Geol., Mem., 26: 53-63. Shanmugam, G., Lehtonen, L.R., Hodgkinson, R.J., Straume, T., Syvertsen, S.E. and Skibeli, M., 1993. Slump/mass flow dominated slope facies, North Sea and Norwegian Sea (61~176 78th Annual AAPG-SEPM-EMD-DPA-DEG Convention (New Orleans, April 25-28, 93), Abstract. Vail, P.R. and Wornardt, W.W., 1991. Well log-seismic sequence stratigraphy. Short course notes. Am. Assoc. Pet. Geol. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, ER., Sarg, J.R., Loutit, T.S. and Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 39-46. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic sequence stratigraphy in well logs, cores and outcrops. Am. Assoc. Pet. Geol., Methods Explor. Ser., 7, 55 pp.
Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration and Production Technical Center, P.0. Box 650232, Dallas, TX 75265-0232, USA
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
The prospectivity of Cretaceous deposits in the northern Norwegian North Sea, particularly in the Agat area, has been evaluated using a sequence stratigraphic approach. The study integrates high-resolution biostratigraphic data, biofacies analyses, well logs, core description and seismic data, all incorporated with the tectonic history of the basin. Two supersequences, KI-1 and K1-2 are informally defined within the Lower Cretaceous succession. Supersequence KI-1 ranges from Ryazanian to late Barremian age, whereas K1-2 has an Aptian to late Albian age. Palaeoenvironmental maps for the two supersequences, using the biofacies analyses from the wells, suggest that KI-1 supersequence was deposited mainly in a shelf environment, mostly within highstand and transgressive systems tracts. Supersequence K1-2, on the other hand, was deposited largely in a bathyal environment and contains more prominent lowstand systems tracts. A more detailed sequence stratigraphic subdivision is suggested within the two main genetic units. The prospective deposits within the Lower Cretaceous interval are the Asgard Sand Unit (Hauterivian-Barremian), a highstand systems tract, and the Agat Formation (Albian), a lowstand systems tract. Within the Agat area, seismic anomalies/mounds have been recognized on an Intra-Albian unconformity. The sedimentological analyses of cores from the Agat Formation indicate, in contrast to published interpretations of depositional environments, that this unit was deposited on an upper slope environment by slump/mass flow processes, causing the sand bodies to be of laterally restricted extent. A well correlation, based primarily on the identification of maximum flooding surfaces, allowed the Agat Formation to be subdivided into at least three stacked sandy units of lowstand origin. Reservoir quality and seal capacity are the major risks in the evaluation of the proposed Agat lead.
Introduction
Exploration in the Norwegian North Sea has focused on structural traps. Recently, however, stratigraphic traps have received more attention, particularly with the advent of sequence stratigraphy as an interpretive tool for prediction of sedimentary facies and stratigraphic traps (Vail and Wornardth, 1991). The Cretaceous deposits in the northern North Sea have for many years been a minor play for exploration in Norway, with the Agat Field as the only significant Lower Cretaceous discovery in the Norwegian North Sea. The field is within the tilted fault-block terrace off M~lCy, in block 35/3 (Fig. 1). The discovery was made by Saga Petroleum in 1980 and the estimated gas-in-place is 65 • 109 S m 3. The area was later relinquished. The reservoir interval is the sandy Agat Formation, previously interpreted as submarine fan deposits of late Albian age (Gulbrandsen, 1987), and reinterpreted here in terms of upper slope sand lobes. The goal of this study was to evaluate the general prospectivity of Lower Cretaceous deposits in the Norwegian North Sea between 60 ~ and 62~
(Fig. 1). The work was done using a sequence stratigraphic approach with an integrated interpretation of high-resolution biostratigraphic data, biofacies analyses, well logs, core description and seismic data, all incorporated with the tectonic history of the basin.
Fig. 1. North Sea index map showing the study area.
Sequence Stratigraphy on the Northwest European Margin edited by R.J. Steel et al. NPF Special Publication 5, pp. 389-400, Elsevier, Amsterdam. 9 Norwegian Petroleum Society (NPF), 1995.
390
M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 2. Map showing structural nomenclature in and around the study area. N W - S E and N-S traces are Figs. 5 and 6, respectively.
Tectonic setting The study area covered the northern part of the Viking Graben and particularly the Sogn Graben (Fig. 2). These graben areas are bounded to the east by the Lomre Terrace and M~lCy Fault Blocks. The Marulk Basin lies on the northwestern margin of the study area. The Tampen Spur is centrally located with a northeast to southwest axis. The dominant fault pattern is rectilinear, with Caledonian faults trending northeast to southwest, and conjugate northwestsoutheast fault geometries. We believe many of these faults to have had an important control on Mesozoic sedimentation, and because of this structural/ topographic control they are likely to have delineated areas of differing base-level change response.
The sequence stratigraphic framework To improve our understanding of the Early Cretaceous interval, beyond the terms of the already established lithostratigraphic nomenclature (Isaksen and Tonstad, 1989), we have superimposed a sequence stratigraphy on the Ryazanian to Turonian lithostratigraphy for the study area. The sequence stratigraphy is based on in-house age datings which enabled us to assign a more exact age than previously given to the established units (Fig. 3). By combining the lithostratigraphic descriptions, palaeoecology, and the new age dates for the formations we were able to transform the existing static lithostratigraphy into a dynamic sequence stratigraphy for this interval (Fig. 4). Individual formations have been interpreted in terms of sequences and systems tracts according
Fig. 3. Cretaceous stratigraphy between 60 ~ and 62~ in the Norwegian North North Sea. The main reservoirs are the Agat Formation of Albian age and a sandstone unit here informally called the Asgard Sand Unit.
to the definitions of the latter given by Mitchum et al. (1977) and Van Wagoner et al. (1988, 1990). The figure is somewhat schematic but shows the individual sequences and systems tracts interpreted within the Lower Cretaceous and the lowermost Upper Cretaceous (Ryazanian-Turonian) section of the studied area. However, Fig. 4 is a composite, and all of these sequences are rarely found together. In some areas where sequences are present, there may well be a hiatus in the same time interval in other parts of the basin. The potential reservoirs within the study interval are the sandy Agat Formation of Albian age, and sandstones of Hauterivian to Barremian age (the Asgard Sand Unit). The entire Cretaceous interval is considered to be a first-order transgressive megasequence, K1, because of an overall deepening trend. Within this megasequence, we recognize several supersequences (second-order sequences 5-50 m.y) and depositional sequences (third-order sequences, 0.5-5 m.y (Vail and Wornarth, 1991). Two supersequences, KI-1 and K1-2, were informally defined within the Lower Cretaceous deposits. The first of these, (KI-1), ranges from Ryazanian to late Barremian in age, the second (K1-2) from Aptian to Albian. Most of the wells studied have an unconformity and hiatus between these supersequences, and this may possibly be related to a relative sea-level drop, as claimed globally for the early Aptian by Haq et al. (1987). This unconformity
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
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Fig. 4. A sequence stratigraphic interpretation of the Ryazanian to Turonian succession in the Agat region.
is seen on the seismic data by a good, continuous reflection of regional extent, and is interpreted here as a major sequence boundary. Within these supersequences a number of component depositional sequences are interpreted (Fig. 4). The Asgard Sand Unit is interpreted to represent a highstand systems tract (HST) within one of the depositional sequences in K1-1. The other proven potential reservoir level is the Agat Formation which forms several stacked, sandy units mapped in the
lower levels of sequences and therefore interpreted as lowstand systems tracts (LST) in the two lower depositional sequences within K1-2. This latter conclusion is further justified below.
Regional sequence stratigraphic interpretation and the prospectivity of the area Two regional seismic lines illustrate the regional depositional setting (Figs. 5 and 6). The regional
Fig. 5. Regional (WNW-ESE) seismic section across the Viking Graben. The position of the seismic line is shown in Fig. 2. The Lower Cretaceous deposits are coloured light green (KI-1) and orange (K1-2); BCU is the basal Cretaceous unconformity.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 6. North-south regional seismic section from the Tampen Spur area and out into the Viking Graben. The position of the seismic line is shown in Fig. 2.
seismic line in Fig. 5 is an east-west section across the Viking Graben (Fig. 2), where sequences KI-1 and K1-2 are easily defined. It should be noted that sequences deposited from the Ryazanian to late Turonian have been significantly influenced by tectonic activity, as unconformities can be identified in the wells on both sides of the Viking Graben, and can be tied to the seismic section. Sedimentation from the Coniacian to latest Maastrichtian, in contrast, reflects a more constant thermal subsidence. Palaeoenvironmental maps for KI-1 and K1-2 were made using the biofacies analyses from 45 wells. A biofacies is a group of organisms which responds to and is constrained by environment and associated chemical, physical and biological parameters. Distribution patterns of the environmentally controlled organisms provide information about the depositional environment. The biofacies from the analyzed wells imply that the supersequence KI-1 (Ryazanian to Barremian) was largely deposited in a shelf environment (Fig. 7). KI-1 consists, therefore, mostly of highstand and transgressive systems tracts. Potential reservoirs within the highstand systems tract would have included deltaic and shoreface and possibly fluvial sandstones. The potential reservoirs within the transgressive systems tract would be shoreface and estuarine sandstones. A play concept
Fig. 7. Schematic map of palaeoenvironmental reconstruction for supersequence KI-1 (Ryazanian-Barremian). Red, green and blue areas record non-deposition, shelfal and bathyal areas, respectively.
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
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Fig. 8. Schematic play concept models for supersequence KI-1 emphasizing potential transgressive and high-stand reservoir sands. A seismic line from the Troll East area supports these concepts. The black arrows indicate the potential Lower Cretaceous stratigraphic/ structural traps (the position of the seismic line is shown in Fig. 7).
for supersequence KI-1 is illustrated in Fig. 8 where sands of transgressive and high-stand origin are emphasized. The sand geometry and palaeogeographic play models are supported by a seismic section from Troll East area (Fig. 8). The trapping mechanism for hydrocarbons could be both stratigraphic and structural. Supersequence K1-2 (Aptian to Albian) was more dominated by bathyal environments (Fig. 9) than supersequence KI-1 and therefore has greater potential for lowstand systems tracts. The main elements of the play concept are illustrated by a lowstand systems tract model (Fig. 10, upper part) where the most prospective sediments are located within the basin-floor mounds, and to a lesser extent within the lowstand slope fan and prograding wedge (Vail et al., 1991). A potential basin-floor mound play is further illustrated on a seismic line from 35/6 area (Fig. 10, lower part). An integration of details from the palaeoenvironmental maps, the seismic interpretation and tectonic history have led to the identification of Lower Cretaceous play areas within the Magnus/Marulk Basin, the Lomre Terrace, the Agat Field area, regions east of the Tampen Spur, and the area east of the Troll Field. A lead within the Agat area is pursued further below.
Fig. 9. Schematic palaeoenvironmental reconstruction of supersequence K1-2 (Aptian-Albian). Red, green and blue areas record non-deposition, shelfal and bathyal environments, respectively.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 10. Model of play concepts for supersequence K1-2. The seismic line shows an example of a basin-floor mound recognized within the Agat area (the position of the seismic line is shown in Figs. 9 and 11).
Sequence stratigraphy as an aid to evaluation of a Lower Cretaceous lead (Agat area) Several seismic anomalies/mounds have been identified south of the Agat area (Fig. 11) and have been evaluated in more detail. The seismic line in Fig. 12 shows the tie from between the best sandstone interval in well 35/3-5 and two interpreted seismic anomalies/mounds. An Intra-Albian unconformity is interpreted at the base of these mounded features.
A sequence stratigraphic study was carried out to evaluate these leads. Sedimentological analysis of the cores from Agat wells and high-resolution biostratigraphic analysis of the wells allowed the prediction of depositional sequences and lithofacies distributions. A total of 215 m of core from three wells (35/3-2, 35/3-4 and 35/3-5) were examined from the Agat Formation. Contorted and massive sandstone is the most common facies (facies 2) in the described cores (Figs. 13 and 14), consisting of light grey, micaceous,
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Fig. 11. Mounded seismic facies south of the Agat area. The map also shows the position of the seismic line Fig. 12. Fig. 13. Short core log illustrating the main features of facies 2, the contorted and massive sandstone which is the most common facies in the Agat Formation.
Fig. 12. A NNE-SSW squashed seismic line showing the tie from the Agat well 35/3-5, south to two interpreted seismic anomalies/ mounds/yellow) lying on an Intra-Albian unconformity. Note potential reservoir sand at same level in well 35/3-5.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
Fig. 14. Core photograph (Agat Formation) showing discrete units of pebbly sandstone, massive sandstone, laminated sandstone and pebbly mudstone. Note the glide plane some 10 cm above base of core (lower right). poorly sorted, fine-grained sandstone. Lamination, often at disturbed high angles (up to 60 ~) occurs and contorted sandstone units are interbedded with
undisturbed mudstone units. Mud clasts (commonly at the tops of units) and water-escape structures are present. The basal contacts of some of the units
A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea
397
Fig. 15. Interpreted depositional model for the Agat Formation showing distribution of sandy slumps and mass flows on an upper slope environment near the shelf edge. Note the suggested differences in sediment source areas for the 35/3-5 well (from southeast) in comparison with 35/3-2 and 35/3-4 wells (from northeast), based on petrographic and biostratigraphic evidence. show evidence of glide planes with shearing and slumping. The facies analysis strongly suggests that the Agat Formation was deposited in an upper slope environment by slump/mass flow processes (Fig. 15) and not simply as part of a submarine fan model (see also Shanmugam et al., 1993). Because upper slope mass flow deposits in such environments tend to be of restricted lateral extent, they are usually not easily correlatable between wells. As part of the sequence stratigraphic analysis, high-resolution biostratigraphic data have been analyzed and used to identify possible regional flooding surfaces and sequence boundaries upon which the well correlations can be based (Fig. 16). The peaks on the abundance and diversity curves are related to condensed intervals and are the most likely candidate horizons for the main flooding surfaces. The three main flooding surfaces or their basinward equivalents were identified (horizons 1, 2 and 3 in Fig. 16) and
used to divide the Agat Formation into three sequences. The Intra-Albian unconformity interpreted on the seismic tie profile (Fig. 12) is interpreted to be near to surface 2. These flooding surfaces appear to coincide with interpreted biostratigraphic and acme horizons which are major transgressive events in the Moray Firth/South Viking Graben area (Mobil inhouse data). Other details which should be noted in Fig. 16 are the peaks of the palynomorphs in the Hauterivian to Barremian sediments. Where the total amounts of terrestrial palynomorphs increase, the total amounts of marine palynomorphs decrease, and vice versa. This strongly suggests a prograding sequence and is one of the arguments for interpreting the Asgard Sand Unit as a highstand systems tract. Three Agat wells (35/3-2, 35/3-4 and 35/3-5) were correlated based on the maximum flooding surfaces, which in these wells are close to the sequence bound-
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Fig. 17. Well correlation based on the interpreted maximum flooding surfaces. aries (Fig. 17). Although some sand bodies may well be in communication between adjacent wells it appears to be common that sand bodies within the same sequence cannot be correlated. Drill stem tests taken in wells 35/3-2 and 35/3-4, and interpreted to be within the same sequence, showed different pressures, strongly suggesting lack of communication and direct correlation. The petrography and biostratigraphy in well 35/3-5 appears to differ from the other Agat wells significantly. This could suggest that well 35/3-5 belongs to a separate depositional system, a difference which may have been tectonically controlled. We interpret the Albian sediments encountered in wells 35/3-2 and 35/3-4 to have been derived from the northeast whereas in well 35/3-5 these sediments were derived from the southeast. These directions (fairways) were probably structurally controlled. The potential reservoir leads identified from the basin-floor anomalies
(Figs. 12 and 13) belong clearly to the fairway containing the proven sands in well 35/3-5.
Conclusions The conclusions of this study are as follows: (1) A sequence stratigraphic approach has improved our understanding of the Lower Cretaceous deposits in the northernmost areas of the Norwegian North Sea. (2) Two supersequences have been defined within the Lower Cretaceous time interval, KI-1 and K12, of Ryazanian-Barremian and Aptian-Albian ages, respectively. (3) The main prospective levels in the Lower Cretaceous interval are the Asgard Sand Unit (Hauterivian-Barremian) which is a highstand systems tract within KI-1, and the Agat Formation (A1bian) which is a lowstand systems tract within K1-2.
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M. Skibeli, K. Barnes, T. Straume, S.E. Syvertsen and G. Shanmugam
(4) Sequence stratigraphy has helped to constrain the prospect risk in an area where seal capacity and reservoir quality are the major concern.
Acknowledgements The authors thank Mobil Exploration Norway for permission to publish this work, and Nopec As. for permission to publish some of the seismic sections. The support and encouragement offered by our colleagues are gratefully acknowledged. In particular, L. Fearn and R. Dunay for the biostratigraphic support, and M. Cecci for other assistance.
References Armentrout, J.M., 1990. Patterns of forminiferal abundance and diversity; implications for sequence stratigraphic analysis. In: Soc. Econ. Paleontol. Mineral., Gulf Coast Sect., 11th Annu. Res. Conf. Sequence Stratigraphy as an Exploration Tool. Concepts and Practices in the Gulf Coast, Program and Extended and Illustrated Abstracts, pp. 53-58. Gulbrandsen, A., 1987. Agat. In: A.M. Spencer, C.J. Campbell, S.H. Hanslien, E. Holter, P.H.H. Nelson, E. Nys~ether and G. Ormaasen (Editors), Geology of the Norwegian Oil and Gas Fields. Graham and Trotman, London, pp. 363-370.
M. SKIBELI K. BARNES T. STRAUME S.E. SYVERTSEN G. SHANMuGAM
Haq, B.U., Hardenbol, J. and Vail, ER., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 11561167. Isaksen, D. and Tonstad, K., 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. Norw. Pet. Dir., Bull., 5, 24 pp. Mitchum, R.M. Jr., Vail, ER. and Thomson, S., 1977. Part Two: The depositional sequence as a basic unit for stratigraphic analysis. Seismic stratigraphic applications to hydrocarbon exploration. Am. Assoc. Pet. Geol., Mem., 26: 53-63. Shanmugam, G., Lehtonen, L.R., Hodgkinson, R.J., Straume, T., Syvertsen, S.E. and Skibeli, M., 1993. Slump/mass flow dominated slope facies, North Sea and Norwegian Sea (61~176 78th Annual AAPG-SEPM-EMD-DPA-DEG Convention (New Orleans, April 25-28, 93), Abstract. Vail, P.R. and Wornardt, W.W., 1991. Well log-seismic sequence stratigraphy. Short course notes. Am. Assoc. Pet. Geol. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, ER., Sarg, J.R., Loutit, T.S. and Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 39-46. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic sequence stratigraphy in well logs, cores and outcrops. Am. Assoc. Pet. Geol., Methods Explor. Ser., 7, 55 pp.
Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration Norway Inc., P.O. Box 510, 4001 Stavanger, Norway Mobil Exploration and Production Technical Center, P.0. Box 650232, Dallas, TX 75265-0232, USA