Sedimentary environments offshore Norway--- Palaeozoic to Recent : an introduct ion

Ole J. Martinsen and Tom Dreyer

Sedimentary environments offshore Norway have evolved through time as a response to changing climatic conditions, basin

physiography and tectonic setting. This volume includes examples from various time periods and offshore basins, including some

adjacent onshore and offshore analogues from Greenland, Denmark and Great Britain. These examples illustrate characteristics of the sedimentary environments of various time periods, from the Devonian of the onshore Hornelen Basin to the Holocene of the Mid-Norway

area, including continental, shallow-water and deep-water depositional settings. Cases range from detailed facies analysis of highly prolific, hydrocarbon-bearing Jurassic reservoir rocks, through to Recent, giant submarine slides, showing the changes in processes and setting that the Norwegian offshore areas have experienced. The examples from analogues in East Greenland are particularly important

both to understand the downdip evolution of environments in the MOre and VOring Basins, but also because the onshore areas on the Norwegian mainland largely lack a post-Devonian sedimentary record. This volume presents an account of the sedimentary development

of the Norwegian basins offshore Mid-Norway and in the North Sea region, and thus complements earlier volumes dealing with the Arctic areas.

Introduction

From May 3-5, 1999, more than 200 geologists assembled in Grieghallen, Bergen to discuss the sed- imentary development of the Norwegian offshore areas and their analogues. The conference was organ- ised by the Norwegian Petroleum Society. The partic- ipants came from universities, government agencies and oil and consulting companies in Norway, Den- mark, Great Britain, Holland, France, Spain, Canada and USA. The presentations included 25 talks, 10 core examples and 37 posters. Particular emphasis was on the posters and the core examples, recognising that it was within these presentations that most of the data were presented. Three keynote addresses were given: by Robert Dalrymple on non- and marginal- marine environments, by H. Edward Clifton on shal- low-marine environments, and by Arnold Bouma on deep-water environments. For various reasons, papers from these keynote addresses were not included in the volume, but their abstracts are included in the abstracts volume (Martinsen and Dreyer, 1999).

Sedimentary environments in Norway through time

This conference volume includes 23 papers, from both the poster and the oral presentations. They range from detailed analysis of single stratigraphic units to

overview articles. Previous volumes treating the Nor- wegian offshore geology have dealt with correlation methodology (Collinson, 1988), sequence stratigra- phy (Steel et al., 1995; Gradstein et al., 1998) or concentrated on Arctic geology (Vorren et al., 1993). Since the present volume has papers mainly from the Mid-Norway and North Sea Basins, it comple- ments the volume edited by Vorren et al. (1993) on the Arctic areas. Together, these two volumes give a comprehensive account of how both the northern and southern basins evolved through time. Both publica- tions are required reading for geologists working in the offshore areas. In the following, we give a brief review of the papers contained in the present volume and put the information into a time-stratigraphic con- text and provide some comparison with onshore data from Norway.

Palaeozoic

As is the case in the Arctic and in the Barents Sea region (cf. Vorren et al., 1993), Palaeozoic sedi- mentary successions are poorly known from offshore Norway, although they most likely exist in consider- able thickness and may link up with the onshore suc- cessions (e.g. Fa~rseth et al., 1995). Onshore, Precam- brian strata in Finnmark (e.g. Siedlecka, 1975) and Cambrian-Carboniferous sedimentary successions in the Oslo region (e.g. Olaussen et al., 1994) are well

Sedimentary Environments Offshore Norway - Palaeozoic to Recent edited by O.J. Martinsen and T. Dreyer. NPF Special Publication 10, pp. 1-5, Published by Elsevier Science B.V., Amsterdam. �9 Norwegian Petroleum Society (NPF), 2001.

2 O.J. Martinsen and T. Dreyer

known. Furthermore, The onshore Devonian sedi- mentary record from western Norway (Andersen, 1998; Folkestad and Steel, 2001) is a widely known and most impressive record of basin formation and sedimentation by anyone's standard. However, the ex- istence of pre-Devonian sedimentary record in west- ern Norway is less well known (e.g. Brekke and Sol- berg, 1987; RavnSs and Fumes, 1995 and references therein). Although the Devonian and older rocks are unlikely to form reservoir rocks offshore Norway, their onshore existence and character are valuable for implications of the offshore development.

The overview given by Brekke et al. (2001) on pre-Mesozoic offshore sedimentation is valuable al- though the database is sparse. Carboniferous reservoir successions are well known from the British offshore areas (e.g. Collinson et al., 1993). The Carboniferous succession offshore Norway is perhaps a speculative reservoir target because of large burial depths in most basins, but as of the present day it is untested and still a possibility where burial depth is not excessive.

Permian sedimentary rocks are well known in the Barents Sea (Vorren et al., 1993), but further south, there is little information from offshore areas. In the Oslo Graben, there is a Permian sequence (Olaussen et al., 1994). In fact, the Permian may be the least well-known time period in from Norwegian offshore and onshore areas. Therefore, the data and interpreta- tions given by Kreiner-Mr and Stemmerik (2001) from well-exposed Permian deep-water deposits in East Greenland are valuable, because they provide important ideas for the sedimentary evolution of this time period in adjacent areas offshore Norway.

Mesozoic

The Mesozoic Era is obviously the most impor- tant time period offshore Norway for hydrocarbon resources and implicitly for the occurrence of reser- voir and source rocks. Especially, there was a major change of sedimentary environments from continen- tal in the Triassic through mainly coastal plain and nearshore marine to deep water in the Cretaceous. Brekke et al.'s (2001) overview paper covers several aspects of these changing sedimentary environments.

The Triassic period was dominated by arid, conti- nental deposition (e.g. Steel and Ryseth, 1990; Steel, 1993), while the latest Triassic to earliest Jurassic saw a progressive change into a wetter climate with the initiation of shallow-marine sedimentation. Ry- seth (2001) documents the important changes that took place during the deposition of the Statfjord For- mation, the major reservoir-bearing sandstone forma- tion that is important both on the eastern and western margin of the Viking Graben in major fields such as

Snorre, Statfjord and Oseberg. In Mid-Norway, equiv- alent Lower Jurassic sandstones are also highly im- portant as hydrocarbon reservoirs and Svela (2001) describe the Are Formation in the Heidrun Field. An- other important stratigraphic unit in the Mid-Norway area is the Tilje Formation, which forms both a primary and secondary reservoir unit in several Mid-Norway fields. The tidally dominated Tilje Formation marks the change from mainly fluvial deposition in the ,~re Formation to shallow-marine, tidally dominated sedi- mentation. Martinius et al. (2001) document the facies and sedimentary patterns within the Tilje.

Early Jurassic sedimentation in the North Sea Basin was dominated by shallow marine sedimenta- tion recorded by the Dunlin Group. The Dunlin Group has played a subordinate role as a reservoir unit, but Charnock et al. (2001) document the regional deposi- tional patterns and sequence stratigraphy. The Dunlin Group may have an unexplored reservoir potential, and the hydrocarbon resources could be underesti- mated.

The Brent Group, the most prolific oil reservoir unit on the Norwegian shelf, lies on top of the Dunlin Group. It marks a change from margin fed shallow marine systems in Dunlin Group (see Charnock et al., 2001), to an axial system which prograded and ret- rograded largely along a N-S axis. The Brent Group has been amply documented in earlier publications (see for instance the papers in Steel et al., 1995).

While the Early and Middle Jurassic periods were dominated by relative tectonic quiescence, tectonism becomes very important in the Late Jurassic. Ex- tensional faulting controls depositional patterns, and sedimentary environments vary considerably on a lo- cal scale. This situation is important both offshore Mid-Norway, in the North Sea and in adjacent areas, and Corfield et al. (2001), Andsbjerg et al. (2001) and Smelror et al. (2001) show examples of the relation- ships between tectonism and sedimentation during this period.

One particularly important part of the Jurassic reservoirs is the reservoir behaviour and characteri- zation. Analogue work is important for this aspect and Yoshida et al. (2001) show an example from tidal sandstones in southern England that compares with the tidally dominated Tilje Formation. Other field analogues for the Jurassic and the lower Creta- ceous stratigraphy offshore Norway are found in East Greenland, and both Larsen et al. (2001) and Surlyk and Noe-Nygaard (2001) show important examples.

Tectonism and rifting in the Late Jurassic and earli- est Cretaceous mark the change from shallow-marine sedimentation to deep-water sedimentation. The ex- ample by Larsen et al. (2001) from Greenland (see above) shows this change. In the Norwegian offshore

Sedimentary environments offshore N o r w a y - Palaeozoic to Recent: an introduction 3

areas, Cretaceous reservoirs have been difficult to lo- cate as the passage from the Jurassic to Cretaceous time also records a change from a sand-rich to a mud- rich depositional setting. Bugge et al. (2001) describe the depositional history in the northern North Sea, an area with some Cretaceous discoveries, such as in Agat, but where the volumes of hydrocarbons so far have been too small for commercial exploitation.

Understanding the basin development through the Cretaceous-Cenozoic period is challenging, and se- vere effort has been put into solving this important question for making predictive models. The three papers by KyrkjebO et al. (2001), Kjennerud et al. (2001) and Gillmore et al. (2001) show different ap- proaches of restoring palaeobathymetry. Such mod- elling is important because they provide ideas on how basins filled and thus where source and reservoir rocks may be located.

Sand-rich deep-water turbidite systems are present in the western Voring Basin (Brekke et al., 2001). These systems relate to supply from East Greenland, showing the importance of the western basin margin of the offshore Mid-Norway area for supply of sands (Larsen et al., 2001; Surlyk and Noe-Nygaard, 2001). The western margin of the VOring and MOre Basins still has a high potential for hydrocarbon exploration.

Cenozoic

The Palaeocene of the North Sea and the Mid- Norway area has proven to be a prolific, oil- and gas-bearing reservoir succession. Sedimentation took place in deep-water fans and related depositional sys- tems (Badescu, 2001; Cecchi et al., 2001; Gjelberg et al., 2001). The change from a relatively mud-rich Cretaceous period to more sandy Palaeocene systems relate to basin margin uplift and tectonism in concert with incipient rifting in the North Atlantic (Martinsen et al., 1999; Brekke et al., 2001).

This volume lacks papers dealing with the Eocene-Pliocene periods, and thus we give a short review to complete this introduction. The Eocene- Miocene period saw a change from deep-water to shallow-marine conditions in the North Sea (Dalland et al., 1988; Isaksen and Tonstad, 1989; Martin- sen et al., 1999). This period is in general poorly documented on the Norwegian shelf in terms of sedimentary history. Many wells, drilled for deeper targets, have no data from much of this stratigraphic succession. The petroleum potential of the Eocene- Miocene is highly questionable because of shallow burial depths. Nevertheless, the depositional history has more than academic interest, because the accu- mulation of these stratigraphic successions, and the overlying Pliocene, caused underlying packages to

reach burial depths where hydrocarbons could form, migrate and accumulate in reservoir-bearing succes- sions of older age.

The Pliocene is also relatively poorly documented in general, despite its great importance for burial of older units, and the fact that it records the highest sedimentation rate on the Norwegian shelf (e.g. Jordt et al., 1995; Rokoengen et al., 1995; Henriksen and Weimer, 1996). The thick Pliocene wedges probably relate to glaciation, high erosional rates and conse- quent high sedimentation rates (cf. Rokoengen et al., 1995 and references therein).

The Pleistocene and Holocene periods are domi- nated by glacial sedimentation and erosion in the off- shore area. Ottesen et al. (2001) show how glacial ero- sion has created large-scale erosional features and de- positional products on the Mid-Norwegian continental shelf. In a time-stratigraphic sense, erosion is proba- bly more important than sedimentation in many areas (e.g. Sejrup et al., 1996; Ottesen et al., 2001). A ma- jor component of the Pleistocene and Holocene his- tory is the occurrence of giant submarine slides. Vor- ren and Laberg (2001) describe several examples and their occurrence. Bugge et al. (1987) and Haflidason et al. (1999) described the Storegga slide, the largest known submarine slide (see also the illustration on the front cover of the book). The sedimentary environ- ments and patterns during this recent period have ma- jor implications for installation of hydrocarbon-pro- ducing equipment on or above the sea floor, and thus the importance of understanding the Quaternary devel- opment cannot be underestimated. In addition, stud- ies of the Recent Skagen Odde complex in northern Denmark show a valuable modern analogue for Juras- sic, fault block-related shoreface sands (Nielsen and Johannessen, 2001), which shows the importance of studying the present to understand the ancient.

Conclusions

This volume covers many important aspects of sed- imentation and sedimentary environments offshore Norway from the Palaeozoic to the Recent. The sed- imentary geological setting has changed significantly over this time period as a response to changing tec- tonic setting, basin physiography and morphology. Further knowledge will be attained through an in- creasing database with new wells, and in particular a growing seismic database where 3-D data play a vital role. Our prediction is that the most important progress in understanding the development of sedi- mentary environments through time offshore Norway will be made from studying seismic data. There is a general knowledge on how sedimentary environments changed from core data, and naturally these data have

4 O.J. Martinsen and T. Dreyer

to be supplemented by new core data. However, a fully three-dimensional understanding of plan view morphology and cross-sectional architecture can only be attained from high-resolution seismic data used with a sedimentologist's eye.

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O.J. MARTINSEN

T. DREYER Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway