NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 87

A depositional and sequence stratigraphic model for the Rannoch and Etive formations, Oseberg Field, northern North Sea

Tore M. Løseth & Alf Ryseth

Løseth, T.M. & Ryseth, A.: A depositional and sequence stratigraphic model for the Rannoch and Etive formations, Oseberg Field, northern North Sea. Norwegian Journal of Geology,Vol. 83, pp. 87-106. Bergen 2003. ISSN 029-196X.

The Rannoch and Etive formations, a shallow-marine sandstone succession of the economically important Middle Jurassic Brent Group of the nor- thern North Sea, has been re-examined in the Oseberg area, using twenty-six cored wells. An updated sedimentologic and sequence stratigraphic model for the two formations, and the lowest part of the overlying Ness Formation, is presented. The Rannoch and Etive formations were largely deposited within a wave-dominated, shore-zone setting. The orientation of the shoreline shifted gradually from a transverse (N-S) to an axial (E-W) orientation during a regression, under stable to slightly falling relative sea level. After a subse- quent period of oblique aggradation involving a stable to slightly rising relative sea level, a more pronounced fall of relative sea level caused the development of a regional subaerial unconformity and an incised-channel complex in the Oseberg Gamma area.

Norsk Hydro Research Centre, Sandsliveien 90, N-5020 Bergen, Norway ([email protected], [email protected])

Introduction Richards 1992, Ravnås et al. 1997, Løseth et al. 2001). In spite of the large number of studies on the Brent The Middle Jurassic Brent Group is the main hydrocar- Group, there are still disputes regarding several key bon reservoir unit of the northern North Sea (Fig. 1). issues, one of which concerns the nature of the strati- The group was initially related to deposition in a major graphic relationships between the Rannoch and Etive delta system (e.g. Bowen 1975, Eynon 1981), and subse- formations. Whereas the Rannoch Formation is gene- quent studies have in general supported this view (e.g. rally believed to have been deposited in a wave-domi- Graue et al. 1987, Rønning & Steel 1987, Richards et al. nated lower - middle shoreface or lower delta-front 1988, Helland-Hansen et al. 1989, Ryseth 1989, Fält & environment, there are disagreements regarding both Steel 1990, Helland-Hansen et al. 1992, Fjellanger et al. the sedimentological and sequence stratigraphic inter- 1996, Ravnås et al. 1997, Færseth & Ravnås 1998, pretations of the Etive Formation and its relationship Ryseth et al. 1998, Davies et al. 2000). Deposition com- with the underlying Rannoch Formation. Some argue menced with a phase of lateral (E to W) fan-delta depo- that the Etive Formation represents mainly fluvial dis- sition (Broom and Oseberg formations; Graue et al. tributaries and mouth-bar deposits (Brown & Richards 1987), possibly triggered by Late Aalenian thermal 1989, Johannessen et al. 1995), whereas others propose doming and hinterland uplift (Steel 1993, Underhill & either tidal channel and inlet deposits (Daws & Prosser Partington 1993). Subsequent flooding of the lateral 1992, Scott 1992), or wave-dominated shoreface and systems initiated widespread northwards progradation strand plain settings (Cannon et al. 1992, Mitchener et of a wave-dominated delta system during the Early al. 1992, Jennette & Riley 1996). Furthermore, some Bajocian (Fig. 2). The sandy Rannoch and Etive forma- authors argue for an ascending (or "normal") regres- tions record shallowing of the basin in response to sion, with coeval deposition of the Rannoch, Etive and northward progradation of the delta, and are overlain Ness formations (e.g. Brown et al. 1987, Graue et al. by coal-bearing delta-plain deposits of the Ness Forma- 1987, Brown & Richards 1989, Fält et al. 1989, Helland- tion (e.g. Livera 1989, Ryseth 1989, Scott 1992, Ryseth Hansen et al. 1992), whereas other more recent studies et al. 1998). The Tarbert Formation formed during argue for descending (or "forced") regression in certain landward retreat of the Brent delta, and comprises shal- parts of the basin and at particular periods during low marine sandstones and associated coal-bearing deposition of the Rannoch-Etive sedimentary succes- intervals deposited in a series of back-stepping progra- sion (Van Wagoner et al. 1993, Olsen & Steel 1995, dational prisms of successive transgressive/regressive Reynolds 1995, Jennette & Riley 1996). There are also events (e.g. Rønning & Steel 1987, Fält et al. 1989, disagreements as to whether the depositional system 88 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

Fig. 1. Map illustrating the geographical and structural setting of the study area. The area is located about 125 km off Norway’s west coast bet- ween the relatively unfaulted Horda Platform to the east and the Viking Graben proper to the west. Structurally the area is dominated by a series of tilted half-grabens, formed in response to Permo-Triassic and Late Jurassic extension. Locations of the studied cored wells are indicated. (Map to the right is modified from Færseth 1996 and the cross section is modified from Nipen 1987.) was dominated by axial (south to north) or transverse ii) Investigate the thickness distribution of the under- (east to west) drainage patterns (e.g. see Richards et al. lying Oseberg Formation compared to the Rannoch 1988, Fält & Steel 1990). and Etive formations. One key question is whether depositional lobes within the Oseberg Formation This study updates the geological model of the Ran- control subtle thickness variations within the Ran- noch and Etive formations in the Oseberg Field (inclu- noch and Etive formations. ding the Alpha North, Alpha, Alpha South and Gamma iii) Investigate the truncation of Rannoch and Etive for- areas; Fig.1), and aims to gain a more detailed under- mations by some Ness Formation channel comple- standing of the facies distribution, stratigraphy, erosio- xes in parts of the study area. What is the respon- nal events, thus shedding light on some of the issues sible process for this truncation and how extensive mentioned above. More specifically, the aims are to: is it? i) Review, undertake and update core descriptions, iv)Understand the nature of the shoreline trajectory facies, depositional model, sand-body architecture (migration pattern of the shoreline through time) and sequence stratigraphy of the Rannoch and Etive of the Rannoch and Etive depositional system. formations in the Oseberg Field. NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 89

Fig. 2. Schematic south to north stratigraphic section of the Brent and Vestland groups showing formations and timelines within the overall regressive-to-transgressive megasequence (modified from Helland-Hansen et al. 1992).

Geological setting faults show substantial thickening (up to 100%) of Lower and Middle Jurassic strata in their hangingwalls, Rifting and crustal extension in the Viking Graben implying that they were established prior to the Late commenced in Late Permian/Early Triassic time (Gilt- Jurassic rift phase (Færseth 1996, Fristad et al. 1997, ner 1987, Badley et al. 1988, Steel & Ryseth 1990, Fær- Ryseth 2000). However, relatively constant thicknesses seth 1996) and was followed by post-rift thermal subsi- in the Rannoch and Etive formations adjacent to these dence during the Triassic to Middle Jurassic. A second faults indicate that these units were not affected by phase of rifting occurred in Late Jurassic and earliest fault-related differential subsidence in the study area. Cretaceous, forming the present-day, fault-block struc- Thus, during deposition of the Rannoch and Etive for- ture, and was followed by renewed crustal contraction mations, faulting was subtle, with only subtle facies and thermal subsidence throughout Cretaceous and changes across the main faults (Graue et al. 1987, Fjell- Tertiary time (e.g. Badley et al. 1988, Ziegler 1990, Yiel- anger et al. 1996). This indicates that during prograda- ding et al. 1992, Færseth 1996). The stretching events tion, the rate and amount of sediment supply to the led to crustal thinning of up to 17 km in the deepest shoreline balanced the accommodation space created parts of the Viking Graben (e.g. Klemperer 1988). by the faulting and thermal subsidence.

The study area (Fig.1) is located approximately 125 km The lower Brent Group in the Oseberg area is domina- offshore western Norway. The east-dipping Oseberg ted by the fan-delta sandstones of the Aalenian Oseberg Fault Block is located on the Bergen High (Badley et al. Formation (Steel 1993, Muto & Steel 1997a), which 1984), between the moderately faulted Horda Platform were supplied to the area from the east (lateral fill). The area to the east and a down-faulted terrace area adja- overlying, composite Rannoch/Etive unit is generally cent to the deeply subsided Viking Graben proper to related to the axial (northwards) prograding system, the west (Fig. 1, see also Færseth & Ravnås 1998). Most and forms a relatively thin (5 - 15m) upward coarse- faults in the study area strike N-S and NNW-SSE, sub- ning sandstone unit of shallow marine origin. The Ness parallel to the Viking Graben (Fig.1). The majority of Formation (Early Bajocian) in the Oseberg area, in con- 90 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

A

B

Fig. 3. (A) Selected core photographs of sedimentary facies identified in the Rannoch Formation. (1) Transition from Oseberg Formation to Rannoch Formation (marked by arrow, stratigraphic way-up is from lower right to upper left), (2) Detail of the flooding surface (marked by arrow) separating Oseberg Formation (below) and the Rannoch Formation (above), (3) Lower part of Rannoch Formation consisting of very fine-grained sandstones interbedded with mudstones and coarse-grained sandstones and conglomerates, (4) Typical expression of the Rannoch Formation; very fine-grained, very well sorted sandstone with mm-scale laminations, HCS and wave ripples, (5) Detail of typical sedimentary structures and bioturbation seen in the Rannoch Formation, (6) Detail of sedimentary structures typically found in the central Alpha area. In this location HCS is rare, but wave ripples and mud drapes are relatively common. Scale: all core slabs are 10 cm wide, full core length (e.g. pho- tographs 1 and 3) is one metre.

(B) Selected core photographs detailing sedimentary facies of the Etive Formation. In all photographs stratigraphic way-up is from lower right to upper left. (1) In some locations the transition from Rannoch Formation to the Etive Formation may be gradational and quite subtle, especi- ally in the central Alpha area, (2) More commonly, however, the transition from Rannoch Formation (right) to Etive Formation (left) is very sharp (marked by arrow), (3) Typical expression of mouth-bar facies composed of fining upward sets of conglomerates (inversely, inversely-to- normally and normally graded) to fine-grained sandstones (high angle cross-stratified and wave-ripple laminated), (4) Typical expression of a prograding strand-plain succession. Sedimentary structures pass upward from dominantly wave ripples (w) and high- and low-angle cross- stratification (c), to plane-parallel lamination (p), to massive sand (m) with root structures (r) at the top, (5) Photograph detailing the transi- tion from foreshore/backshore sandstones of the Etive Formation (right) to lacustrine coals of the Ness Formation (left). Scale: all core slabs are 10 cm wide. NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 91 trast to areas further north and north west, does not Sedimentological descriptions include any lower delta plain sediments but consists of fluvio-lacustrine sandstones, mudrocks and coal beds Rannoch Formation deposited solely within an upper delta plain environ- The Rannoch Formation is usually between 3-6 m ment (Ryseth 1989, Ryseth & Fjellbirkeland 1995, thick. It can be separated into two distinctly different Ryseth et al. 1998). Locally, particularly on the Gamma facies associations each of which are found at specific fault block, basal fluvial-channel systems of the delta geographical locations. plain incise the Oseberg Formation (see Ryseth et al. 1998). These incisions have implications for the under- The first association is found in the northern standing of possible forced regression in the Ran- Alpha/Alpha North area and in the southernmost noch/Etive system, and will be discussed below. Alpha/Alpha South/Gamma area (Fig. 3A - #4 and 5, Fig. 4 – logs 1 and 3, Fig. 6A-C – indicated by yellow colour). It is characterized by pale to medium grey and light brown (partly oil stained), well- to very-well sor- Dataset and methodology ted, very fine- to fine-grained, micaceous sandstones This study is based upon core data from 26 wells and with spectacular mm-scale lamination. The lamination additional log data from 7 wells located within the Ose- is controlled by varying concentrations of mica and berg Alpha North, Alpha and Gamma areas (i.e. within mud. Sedimentary structures range from flat lamina- Blocks 30/6 and 30/9 of the Norwegian North Sea). tion to very low-angle (swaley?) and hummocky cross- Environmental interpretations are based upon sedi- stratification (SCS and HCS), the latter having upward mentological description and interpretation of about and downward curvature of laminations, and low-angle 330m of slabbed core from the 26 wells, fully or parti- scour surfaces. The thickness of individual sets varies ally covering the stratigraphic level of the upper Ose- from 10 to 70 cm. Wave ripple lamination is also pre- berg, Rannoch, Etive and lower Ness formations. sent, usually at the top of hummocky cross-stratified sets. Carbonaceous material, including rare coaly frag- From the cores, key stratal surfaces with potential for ments, muddy streaks, scattered and discrete horizons local or semi-regional correlation were identified. Sur- of pebbles and rare pyrite mini-concretions (<1 cm) faces identified include candidate Regressive Surfaces of are subsidiary components. Small-scale soft sedimen- Marine Erosion (RSME) (Plint 1988) and Subaerial tary deformation structures and synerisis cracks have Unconformities (SU), as well as candidate Flooding also been observed. Surfaces (FS) (Van Wagoner et al. 1988). Key stratal surfaces are indicated on graphical logs and correlation This facies association of the Rannoch Formation usu- panels. Surfaces of regression or relative sea-level fall ally has a slight upward increase in grain size, although are marked in red (RSME and SU), and surfaces associ- usually within the very fine-grained category (‘very- ated with transgression or relative sea-level rise in blue fine lower’ to ‘very-fine upper’). There is also an (FS). upward change from single HCS and parallel-lamina- ted sandstone sets interbedded with black mudstone The macro-biological content and degree of bioturba- bands, to amalgamated sandstone beds with low-angle tion were also noted during sedimentary logging of the to hummocky cross-stratification. This upward change cores following the methodology of Taylor & Gaw- is sometimes punctuated by mudstone layers between thorpe (1993). The burrowing intensity (or Bioturba- amalgamated sandstone intervals. tion Index - BI) scale runs from 0 to 6; 0 being virtually non-bioturbated, and 6 being completely bioturbated. The second facies association of the Rannoch Forma- Highly bioturbated intervals often correspond to peri- tion is found in the central Alpha area (wells 30/6-13, ods of low sedimentation or marine flooding, i.e. 30/9-B50H, 30/9-B49H, 30/9-B23, 30/9-B20 and 30/6- Omission or Flooding surfaces (e.g. Taylor & Gaw- 4) and in the Gamma area (wells 30/9-B26 and 30/9-2) thorpe 1993). Gradual changes in the degree of biotur- (see Fig. 3A - #6, Fig. 4 – log 4, and Fig. 6A-C – lilac bation also often reflect facies changes or changes in colour). Here the sedimentary structures are somewhat stacking patterns (i.e. progradation to retrogradation). different compared to the rest of the study area. Instead of being dominated by HCS and plane-parallel lamina- Architectural and sequence stratigraphical analyses are ted to very low-angle cross-stratification, the sandsto- based upon vertical stacking patterns of identified nes are bioturbated and have muddy to silty inter-beds facies, as well as the identification and correlation of that contain abundant mica and organic-rich material. key stratal surfaces. Correlation panels oriented both The sandstones, which are sometimes coarser (fine- to parallel and normal to the local structure, have been medium grained) and less sorted than in other areas, constructed to investigate the spatial and temporal dis- are frequently wave-ripple laminated and draped by a tribution of sedimentary facies and environments. pair of thin, lenticular dark mudstones. 92 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth andplain, shoreline). and (4) embayed r ised channel complex/valley, (3) st ig. 4. Sedimentological logs core representing depositional the different environments (i.e. mouth bar, (1) wave-dominated (2) inc F NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 93

Fig. 5. Stratigraphic thickness maps measured in meters normal to structure (i.e. true vertical thickness). (A) Oseberg Formation, (B) Rannoch Formation, (C) Etive Formation and (D) Rannoch and Etive formations combined. Thickness (in meters TVT) is indicated individually for each map by colour legend. 94 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

For both associations, burrowing intensity (BI) varies well-sorted, fine- to medium-grained sandstone that is from 0 to 5 and generally decreases upwards through often overlain by a 20-50 cm thick unit of heavily roo- the formation. The most common trace fossils are Ber- ted silt- and mudstones. gaueria, Palaeophycus, Planolites, Rosselia/Cylindrich- nus/ Conichnus, Skolithos and bivalve escape structures. Bioturbation is rare or absent (BI 0-2, Fig. 4) with scat- Asterosoma, Gyrochorte, Teichicnus and rare Ophiomor- tered Asterosoma, Bergaueria, Conichnus, Planolites and pha and Macaronichnus are also found. Typically, Tei- Skolithos.Locally, units of siltstone to fine sandstone chicnus is found only in the lower part of the formation with coal and wood clasts, mud drapes, and structures whereas Bergaueria is found in the upper part. such as soft sedimentary deformation, ripple lamina- tion and cross stratification, can be significantly biotur- The lower part of the formation usually contains 3-130 bated (BI 4), dominantly by Diplocraterion and cm thick beds (except for the 30/6-1 and –2 wells where Rosselia-like traces. they are up to several meters thick) with erosive bases and sharp or reworked tops, consisting of medium to The Rannoch-Etive boundary can either be sharp or very coarse-grained, moderately sorted sandstones and gradual, or more rarely interfingering (Figs. 3B and 4). conglomerates with commonly rounded quartz grains that are 1-5 cm in diameter (Figs. 3A (#3) and 4). The sandstones often display a general fining-upward grain- size distribution, and are either massive, parallel- and Thickness trends wave-ripple laminated, or cross stratified. The conglo- merates are clast-supported and are typically normally From core and well data it is evident that the Rannoch- graded, although inverse grading occurs. Etive succession is significantly thicker in the northern Alpha/Alpha North area (10-15 m) than in the central- southern Alpha/Alpha South/Gamma area (5-8 m) Etive Formation (Figs. 5D and 6). This may be due to a gentle deepening of the basin to the north, similar to that observed in the The Etive Formation is dominated by grey to yellowish- Tarbert Formation in this area (Løseth et al. 2001). This brown and dark brown (oil stained), moderately- to is also indicated by a generally more diverse marine poorly-sorted, medium- to coarse-grained sandstones. trace fossil assemblage in the Alpha North and nor- Both mica and mud are less abundant than in the thernmost Alpha area than in the rest of the study area. underlying Rannoch Formation. Sedimentary structu- The Rannoch Formation is relatively thin in the Alpha res include wave and current ripple-lamination, high- North area, and relatively thick in the central Alpha and low-angle cross-stratification and plane parallel area (Figs. 5B and 6). A gross thickness distribution stratification (Fig. 3B, #4). Some sands appear structu- map of the underlying Oseberg Formation shows that reless. Pebbles, 2-40 mm in diameter, are common and there is an inverse relationship between the thickness of occur at bases or along the set boundaries of low-angle, Oseberg and Rannoch formations (Fig. 5A, B). This cross-stratified sandstones. Conglomerates are most suggests that the geometry of the Oseberg Formation frequent in the Alpha North area (wells 30/6-21, -C5, created subtle basin topography, which then controlled -C8 and to a small extent in well 30/6-C10), but are also the thickness distribution of the overlying Rannoch observed in the southern Alpha area (well 30/9-1). Formation. For example, an unusually thick Rannoch The conglomerates may be ungraded, inverse graded, succession is developed in wells 30/6-1 and -2 near the inverse-to-normal graded or normal graded. Most are westward pinchout of the Oseberg Formation (Fig. 5B). clast supported, but some are supported by a medium- The Etive Formation, however, is relatively thin in the to coarse-grained sand matrix. central Alpha area, but thickens markedly towards the Alpha/Alpha North areas in the north (Figs. 5C and 6). Fining-, and more rarely, coarsening upward units (cm to few metres thick) with sharp and/or erosive bases, are present within the formation (Figs. 3B (#3) and 4). These units often show an upward change in sedimen- Interpretations tary structures from low- or high-angle cross-stratifica- tion near the base to massive in the middle portions to Rannoch Formation wave-ripple lamination towards the top (i.e. reworked Grain size, sorting, sedimentary structures (particularly by wave ripples). SCS and HCS) and types of biogenic traces suggest that facies association one of the Rannoch Formation was The Etive Formation commonly displays an overall deposited in a wave/storm-dominated lower-middle coarsening-upward grain-size distribution (e.g. Fig. 4, shoreface or delta-front environment (cf. Leckie 1986, log 3), except towards the top, which is characterised by Duke et al. 1991, Cheel & Leckie 1993). The lack of such a 0.5-3.5 m unit of plane-parallel laminated to massive, wave/storm-generated sedimentary structures in the NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 95

6A

6B NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 96

Fig. 6. (A) North-south-trending correlation panel across the Alpha block. Depositional environments range from wave-domi- nated delta with mouth bars in the south, to an embayed protec- ted shoreline in the north. (B) Southeast-northwesttrending cor- relation panel across the southern Alpha and Gamma blocks. Note the prominent fluvial incision on the Gamma block. 97 NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field

6C

central Alpha area suggests that this area was probably Etive Formation more protected from waves and storms than other parts Grain-size distribution, grain-size trends, sedimentary of the study area. Furthermore, two markedly different fabric and sedimentary structures suggest that the Etive energy regimes are suggested by the presence of mud Formation represents deposition within a wave-domi- drapes and muddy to silty inter-beds, indicative of a nated delta front to strandplain, with coast-parallel, tidally-influenced deposition environment (cf. Ter- subaqueous sheet sands (spits and bars). The sheet windt 1981, Nio & 1991). We therefore interpret the sands were locally cut by distributary channels, which central Alpha area to have been an area where tidal deposited subaqueous distributary mouth bars in a energies dominated over storm/wave energies. wave/storm-dominated basin. Upper shoreface to fore- shore successions are characterized by a coarsening- The Oseberg-type coarse-grained beds within the lower upward grain-size distribution. The distributary mouth part of the Rannoch Formation are either formed by bars and channels are either dominated by fining- wave reworking during strong storms or by flood upward packages that reflects fluvial input to the wave- events in a still active Oseberg distributary system. This dominated basin, or by coarse-grained sandstones and does not suggest, however, interfingering between Ose- conglomerates, deposited mainly by subaqueous cohe- berg and Rannoch formations, but rather that the sionless debris flows (e.g. Nemec & Steel 1984) genera- boundary between the two depositional systems is ted by oversteepening of the mouth bars. The coarse genetically defined by the maximum flooding surface material was probably supplied to the basin from the lying slightly above the first flooding surface that defi- Horda Platform via transverse (westward flowing) nes the top of the Oseberg Formation. rivers. Upper shoreface deposits, with abundant peb- bles strewn along depositional sets, probably represent deposition down-current of the distributary mouth 98 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

Fig. 6. (C) North-south-trending correlation panel across the northern Alpha and Alpha North blocks. The Rannoch and Etive formations are thicker here than in other parts of the study area and the trace fossil assem- blage is more diverse (indicating more open marine con- ditions).

bars, with coarse clasts being transported from nearby The less storm wave-dominated, more heterogeneous mouth bars by longshore currents. and somewhat more tidally-influenced Rannoch-Etive deposits of the central Alpha area (Fig. 4, log 4) are The Rannoch-Etive depositional system situated between mouth bar deposits in the Alpha The dominance of storm wave-generated structures North and locally in the southern Alpha/Alpha South and gradual coarsening upward grain-size trends, and areas (e.g. Fig. 4, log 1). This suggests that there was a the rarity of fining-upward units and interbedded coastal embayment in the central Alpha area fringed by mudstones all favour an interpretation of a prograding, lateral shoreline protuberances related to the distrib- wave-dominated shoreline for the Rannoch-Etive utary systems. In an outer mouth-bar setting (i.e. 30/6- depositional system. However, local fluvial input is C15 and partly 30/6-C10 within the northern Alpha indicated by the presence of distributary area), the protective effect of the mouth bars is only channels/mouth bars in Alpha North and southern recorded in the upper part of the Rannoch Formation, Alpha/Alpha South areas. Furthermore, the overlying as indicated by the overall finer grain size and lack of Ness Formation is believed to have been deposited wave-generated structures. More exposed conditions mainly within an upper delta plain environment are recorded in the lower part by the presence of wave- (Ryseth 1989, Ryseth & Fjellbirkeland 1995, Ryseth et dominated structures (i.e. facies association one of the al. 1998). The Rannoch-Etive depositional system Rannoch Formation is overlain by facies association represents, therefore, a wave-dominated deltaic to two). This indicates that the protective bar or spit was strandplain depositional system, where wave domina- not yet formed during deposition of the lower unit in ted deltas developed close to fluvial input points and this area. strandplain systems occurred between the input points. NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 99

6D

Fig. 6. (D) Southeast-northwest-trending log correlation panel for the Gamma area (modified from Ryseth et al. 1998). The major subaerial unconformity at the base of the incised Ness channel complex in the 30/6-9, 30/9-B21 and 30/9-B27 wells rises stratigraphically southeast- wards, to lie just above the lowermost coal beds of the Ness Formation.

Scott (1992) interpreted fine-grained, organic-rich land-Hansen & Gjelberg 1994, Helland-Hansen 1995, deposits within the Rannoch-Etive depositional system Helland-Hansen & Martinsen 1996). The preferred as having been deposited within troughs commonly sequence stratigraphic model for the Rannoch, Etive found along dissipating shorelines. This interpretation and lower Ness formations is shown conceptually in does not account for the lateral changes described above. Figure 7 and is based on log and core interpretations (see e.g. Fig. 6). The predominance of a simple (not vertically interfing- ering) transition from marine sediments (Etive) to con- tinental (Ness) environments in most wells suggests Oseberg-Rannoch boundary that the two formations might be slightly disconnected. The definition of the boundary between Oseberg and This is discussed further below. Rannoch formations is based on lithostratigraphic principles and is marked by a regional flooding surface (e.g. Graue et al. 1987). The flooding surface is easily recognizable on logs and in cores (Figs. 3, 4 and 6). Sequence stratigraphy Genetically however, the boundary between the Ose- Following sequence stratigraphic principles, cyclic suc- berg and Rannoch-Etive depositional systems belongs to cessions generated by shoreline regressions and trans- a maximum transgressive surface (MTS) that lies above gressions can be divided into genetic units of chrono- but close to the formation-bounding flooding surface. stratigraphic significance (Posamentier et al. 1988, We interpret the coarse-grained beds of Oseberg-like Posamentier & Vail 1988, Galloway 1989, Van Wagoner facies within the basal part of the Rannoch Formation et al. 1990, Helland-Hansen & Gjelberg 1994, Helland- to represent distal flood-related pulses of the retreating Hansen & Martinsen 1996). In the present study, subtle Oseberg depositional system. As a consequence, we subaerial erosion surfaces and flooding surfaces have place the MTS above these coarse-grained beds, and been used to correlate facies and depositional units bet- below the main coarsening upward (prograding) com- ween the cored wells. Subaerial erosion surfaces are pound Rannoch-Etive succession. usually referred to as sequence boundaries in the litera- ture (e.g. Posamentier & Vail 1988, Van Wagoner et al. 1988, Van Wagoner et al. 1990, Posamentier & Weimer Rannoch-Etive boundary 1993). The behaviour of the shoreline in terms of its In the southern and northern parts of the study area landward or seaward migration pattern, with superim- (Alpha North/northern Alpha and southern Alpha- posed upward or downward components, is critical Alpha South areas), the Rannoch-Etive boundary is because it controls stratal architectural patterns and the sharp and well defined (see Fig 3B) and there is no formation of stratigraphic significant surfaces (Hel- interfingering between the two formations. This sug- 100 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

Fig. 7. Conceptual southeast to northwest section through the Rannoch, Etive and lower Ness formations, showing the favoured sequence-strati- graphic interpretation of the depositional system. Note that the cross-section is extended far beyond the Oseberg study area. gests that the surface separating them is an erosive sur- Ness Formation. Ramm (2000) also suggested that the face. Based on the interpretation that the Rannoch- Rannoch and Etive formations have a different source Etive depositional system is a regressive shoreline sys- area than the Ness Formation in the area. Ryseth (1989) tem, where the Rannoch Formation represents lower argued that the lower Ness Formation in the study area shoreface deposits and the Etive Formation represents represents an upper delta plain succession and that a upper shoreface deposits, this surface is interpreted as a lower delta plain succession is almost always missing. regressive surface of marine erosion (RSME). In the This is evident from the common direct change from central study area (central Alpha area), the boundary is marine deposits of the Etive Formation to lacustrine more gradational (see Fig. 3B) and possibly even inter- coals of the Ness Formation. This relationship requires fingering. This indicates that the boundary between a rapid progradation and migration of the shoreline Rannoch and Etive formations changes character late- away from the area, with creation of a broad beach- rally from being erosive in the north and south, to ridge plain, before the lowermost Ness coal started to being conformable in the central area. This can be accumulate beyond the influence of marine waters. The explained by lower basinal energies in the interpreted Okefenokee Swamp is probably a good analogue for the marine embayment situated in the central study area, Etive-Ness transition. Here, the transition from a marine relative to the storm wave-dominated areas developed substratum to peat-forming swamps represents a subtle in the south and north. hiatus of some thousands of years (Cohen 1984).

Etive-Ness boundary Shoreline trajectory and relative sea-level behaviour There are some indications that the Etive-Ness transi- A regressive surface of marine erosion is commonly tion represents an unconformity. Hurst & Morton formed during a period of relative sea-level fall (e.g. (1988) and Morton (1992) conclude, based on garnet Helland-Hansen & Gjelberg 1994, Plint & Nummedal associations, that the Rannoch and Etive formations in 2000). The rapid progradation and detachment bet- the Oseberg Field have a different provenance from the ween Etive and Ness formations explained above is the- NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 101

Fig. 8. Inferred palaeogeographic evolution of the Rannoch, Etive and lower Ness formations. (A) Maximum transgression (base Rannoch For- mation times), (B) early stage of progradation of the Rannoch-Etive shoreline system, (C) later stage of progradation of the Rannoch-Etive sho- reline system, indicating the initation of incision in the Gamma area, (D) Short period of slight rise to stillstand of relative sea level in between period of slight fall (B-C), and (E) more pronounced fall of relative sea level that is characterised by fluvial incision, (F) deposition of Ness For- mation during rise of relative sea level after incision. 102 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth oretically associated with a descending regressive shore- ted in the Oseberg area (e.g. Ramm 2000). For example, line trajectory that is less steep than the gradient of the is it unlikely that coarse material within the Etive For- upper delta plain (cf. Larue & Martinez 1989, Cant mation in the Alpha North area, as observed in well 1991). We therefore suggest that the Rannoch and Etive 30/9-1, could have been transported axially from a dis- formations in the Oseberg area were deposited during a tant southern source area. Although the top of the Ose- slow and small fall of relative sea level. berg Formation is marked by a transgressive event, transgressive deposits associated with this event are A higher amplitude fall in relative sea level in the area is very thin in the study area. This suggests that the mor- indicated by incision in the Gamma area (e.g. Figs. 6B, phology during progradation of the Oseberg system 6D and 7, unit 3), where both Rannoch and Etive for- was not extensively modified despite the preceding mations, as well as some of the Oseberg Formation, are marine flooding event. Rivers that fed the Etive shore- eroded. This fall of relative sea level, however, occurred line thus probably followed the same pathways as some after deposition of the Rannoch and Etive formations of the rivers of the Oseberg system (although the exact in the area, when the shoreline was basinward of the source area was slightly different). Strong longshore study area. This is indicated by the presence of local flo- currents, coupled with the established transverse drai- odplain, coal and small channel deposits of the Ness nage pattern, caused quick filling of the shallow areas Formation below the surface that correlates with the above the Oseberg Formation. The shoreline of the base of the incised channel complex in the Gamma area Oseberg depositional system had an overall N-S orien- (Ryseth et al. 1998, correlation of this surface shown in tation and, as a consequence, so had initially the Etive Figs. 5-7 is, where no core coverage is available, taken coastline (see Fig. 8B). from panels constructed by Alf Ryseth). The deposits of the Ness Formation observed below the main subaerial The coastline also had local protuberances in relation unconformity (unit 2 in Fig. 7) were probably deposi- to the mouth bars developed in the Alpha North and ted during a slight rise or stillstand of relative sea level Alpha South areas (Fig. 8B). In between these areas in between the period of gentle fall during when Ran- (central Alpha area) the coastline took the form of a noch and Etive formations were deposited and the pro- broad embayment. In this sheltered setting it is sugge- minent fall during which the incision in the Gamma sted that tide-related currents dominated over wave area took place. and storm currents. This interpretation is supported by the core data in the central Alpha area (see Fig. 3 and Eventually, the regressive shoreline trajectory must have discussion of facies in this area). The Oseberg Forma- shifted from being descending to highly ascending, to tion is relatively thin in the central Alpha area compa- account for the accumulation of a thick unit of Ness red to the Alpha North and Alpha South areas (Fig 5A), Formation deposits. The younger part of the Ness For- where thick lobes developed This indicates that some of mation is probably deposited during an overall trans- the rivers of the Rannoch-Etive depositional system gression (not shown in Fig. 7) when the whole deltaic may initially have followed along the same routes as system retreated. parts of the Oseberg drainage system (compare Figs. 8B and 5A).

Due to a general northward progradation of the whole Palaeogeography Brent deltaic complex, the shoreline gradually shifted from a dominant north-south trend to an east-west ori- The transgression of the transverse (east to west) fan- entation (Fig. 8C). deltaic Oseberg depositional system culminated in the latest Aalenian/earliest Bajocian times (Helland-Han- During continued northwards progradation of the sen et al. 1992). Due to a westerly pinchout of the Brent delta, transverse fluvial systems located south and underlying Oseberg Formation, the basin deepened southeast of the shoreline were gradually incorporated from east to west in the area during onset of prograda- into the larger axial fluvial systems (schematically tion of Brent delta proper (Fig. 8A). shown in Fig. 8C). Axial fluvial systems eventually pro- graded across the Oseberg area and incised down into Although the whole Brent deltaic complex prograded the abandoned Etive strandplain system in response to from south to north (Graue et al. 1987, Helland-Han- a fall in relative sea level (Fig. 8D). Thus, this period sen et al. 1989, Fält & Steel 1990, Helland-Hansen et al. was characterised by both fluvial incision and contem- 1992), there are several indications that the Etive coast- poraneous peat accumulation in lakes behind stranded line, at least initially, had a strong north-south compo- Etive beach ridges (Fig. 8C, cf. Cohen 1984). nent in the Oseberg area. (cf. Helland-Hansen et al. 1992, their Fig. 6). The presence of transverse (i.e. east During the subsequent relative sea-level rise, significant to west or south-east to north-west) sediment input accommodation space for upper delta-plain sediments points for the Rannoch-Etive system is well documen- of the Ness Formation was created (Fig 8F). Accommo- NORWEGIAN JOURNAL OF GEOLOGY Depositional model, Oseberg Field 103 dation space developed due to incision in the north- fall when the shoreline was basinward of the study area. western Gamma area was in-filled during this time. The This is consistent with physical and numerical model- shoreline at the time was most likely located north of ling concepts that show it to be unlikely that deltas can the study area, and the main rivers flowing across the prograde as far as the Brent Delta did, with even a study area were therefore mostly axially oriented and modestly continuous rise of the relative sea level (Muto flowed towards the north. This explains why the source & Steel 1997b, Muto 2001). area of the Rannoch and Etive is different from the source area of the Ness Formation in the Oseberg The conflict between the different models is related to region (cf. Hurst & Morton 1988, Ramm 2000). scale of investigation, temporal changes of the shore- line trajectory and possibly bathymetric variations wit- hin the basin. On a long time-scale there was probably an overall (but non-continuous) rise of relative sea Discussion level during deposition of the whole Brent deltaic com- plex (Graue et al. 1987, Helland-Hansen et al. 1992). Two end-member sequence stratigraphic models for Superimposed on this, there were periods of falling the Rannoch-Etive shoreline depositional system are relative sea level. The lack of mature, oxidised paleosols described in the literature, 1) an ascending regressive in the Ness Formation (e.g. Ryseth et al. 1998) indicates model and 2) a descending regressive model. An ascen- that the relative sea-level falls probably occurred over ding regressive model assumes that the overall progra- short time periods (cf. Jennette & Riley 1996, Ryseth et dation of the Rannoch and Etive Formations took place al. 1998, Olsen & Steel 2000). These falls were especially during a gradually rising relative sea level. This model important along the flanks of the basin where subsi- appears to be supported by: (1) a local, gradual dence rates were low. Thus, on a longer time-scale, both upwards facies change from the Rannoch to the Etive descending and ascending regressive shorelines are formations (e.g. Graue et al. 1987, Helland-Hansen et expected to develop during deposition of the Rannoch al. 1992, Scott 1992, Johannesen et al. 1995, Olsen & and Etive formations. The variation in style of regres- Steel 2000, our observation in the central Alpha area), sion gives rise to marked variability of the Rannoch- and (2) time lines that locally pass through the Ness, Etive boundary, both spatially and temporally. Etive and Rannoch Formations as indicated by palyno- logical studies (Helland-Hansen et al. 1992, Johannesen The different interpretations regarding physical proces- et al. 1995). These data appear to suggest a close genetic ses acting during deposition of the Rannoch and Etive relationship between the Rannoch, Etive and Ness for- formations (see introduction) can be explained by tem- mations (Ryseth 1989, Helland-Hansen et al. 1992, poral as well as spatial changes in the relative impor- Johannesen et al. 1995, Olsen & Steel 1995, Fjellanger et tance of wave, fluvial and tidal energies, as is common al. 1996, Ryseth et al. 1998, Olsen & Steel 2000). Howe- along shorelines today. ver, this model contrasts with the development of local deeply incised channels (Ryseth 1989, 2000, Ryseth & Fjellbirkeland 1995, Jennette & Riley 1996, Ryseth et al. 1998, Olsen & Steel 2000, core data from three Gamma Conclusions wells in this study), which strongly suggest a descen- ding regressive model, whereby deposition took place Core data from the Oseberg Field indicate the presence during a fall of the relative sea level. Models of both of the following sedimentary facies and depositional long-duration/large-magnitude and short-term, perio- environments in the Rannoch and Etive formations: dic fall of the relative sea level have been proposed for the Rannoch-Etive depositional system. A long-dura- •A prograding wave-dominated delta with distributa- tion/large magnitude relative sea-level fall results in a ries and mouth bars. We interpret two main distrib- widespread valley-fill cut down into the Rannoch For- utary mouth bars; one located in the Northern mation, and the development of a regional unconfor- Alpha/Alpha North area, and the other in the sout- mable Rannoch-Etive boundary. In this case deposits of hern Alpha/Alpha South/Gamma area. Deposition of the Etive Formation are interpreted as being braided the two mouth bar units is interpreted to be related to fluvial deposits (Elliott 1989, Van Wagoner et al. 1993, the transverse drainage systems. The rivers that fed Reynolds 1995). Models involving small and short them are believed to have been influenced by the term,periodic fall of relative sea level include develop- topography created by the earlier Oseberg Formation ment of more subtle erosion surfaces (Jennette & Riley drainage system. 1996, Olsen & Steel 2000, the model presented here). •A progradational strand plain, which developed late- The model presented here argues for a stable to slightly rally of the two major mouth bars. Longshore drift is falling relative sea level during deposition of the Ran- believed to be the main process of sediment distribu- noch and Etive formations, and a subsequent more tion. A protected embayment was created between pronounced, but short-lived, period of relative sea-level the mouth-bar complexes. 104 NORWEGIAN JOURNAL OF GEOLOGY T. M. Løseth & A. Ryseth

Parts of the Rannoch-Etive shoreline system developed References in the Oseberg area were deposited during a stable to slightly falling relative sea level. At an early stage of pro- Badley, M.E., Egeberg, T. & Nipen, O. 1984: Development of rift basins gradation, the shoreline probably had a strong south- illustrated by the structural evolution of the Oseberg feature, Block north orientation with transverse drainage stemming 30/6, offshore Norway. Journal of the Geological Society 141, 639-649. from the Norwegian mainland. As the major Brent del- Badley, M.E., Price, J.D., Rambech Dahl, C. & Agdestein, T. 1988: The structural evolution of the Northern Viking Graben, and its bea- taic complex prograded northwards, the shoreline gra- ring upon extensional models of basin formation. Journal of the dually shifted towards an east-west trend causing incor- Geological Society 145, 455-472. poration of the transversely flowing fluvial systems into Bowen, J.M. 1975: The Brent Oilfield. 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