The Lower Jurassic Johansen Formation, northern North Sea – Depositional model and reservoir characterization for CO2 storage

Marine and Petroleum Geology 77 (2016) 1376e1401

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Research paper

The Lower Jurassic Johansen Formation, northern North Sea e Depositional model and reservoir characterization for CO2 storage Anja Sundal a, *, Johan Petter Nystuen a, Kari-Lise Rørvik b, Henning Dypvik a, Per Aagaard a a b

University of Oslo, Department of Geosciences, Sem Sælands vei 1, 0371, Oslo, Norway Ross Offshore AS/Gassnova, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 October 2014 Received in revised form 7 September 2015 Accepted 19 January 2016 Available online 2 February 2016

The Lower Jurassic, Johansen Formation sandstone, located in the Northern North Sea, has been proposed as a reservoir candidate for CO2 storage by Norwegian authorities. The objective of this study is to evaluate the reservoir quality of the Johansen Formation, as function of depositional history and architecture. We propose a depositional model comprising an early phase delta progradation, with clinothems building into deep waters, associated with delta front and pro-delta turbidites sourced from river mouths or/and upper delta front collapse. During a subsequent, aggradational stage, thick spit bar deposits developed in the southern, down-current part, sheltering a brackish lagoon, before rapid transgression caused back-stepping and preservation of sandy deposits encased in mud. Considering the depositional model presented, the inferred high porosity spit bar deposits would provide a suitable injection site and reservoir for CO2. Climatic controlling factors, rather than structural, are interpreted to have exerted the major force on the asymmetric sand distributions observed in the Johansen Formation, an architectural style which is repeated in later Jurassic successions on the Horda Platform. On a local scale, accommodation was created by differential compaction above rotated, Permian fault blocks, in addition to regional, postthermal subsidence and rising sea-level. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Reservoir characterization Depositional model North Sea Jurassic Sandstone CO2 storage Sedimentology Geological heterogeneity Trapping mechanisms

1. Introduction The objective of this study is to develop an integrated depositional model and characterize the reservoir quality of the Johansen Formation (Fm. hereafter) located in the northern North Sea, on the Horda Platform (Fig. 1), with respect to its potential for CO2 storage. In order to evaluate the reservoir properties of a sandstone formation for potential CO2 storage, a geo-conceptual model of the unit has to be developed, similarly to models needed for evaluation and characterisation of hydrocarbon reservoirs (Finley and Tyler, 1986; Weber, 1986; Howell et al., 2008). The depositional model comprises the stratigraphy, dimensions, geometries and heterogeneities of sandstone bodies. The Norwegian continental shelf is covered by a series of seismic surveys and penetrated by thousands of wells since the late 1960's in search for hydrocarbons or for production development. Data scarcity is a common problem with regards to evaluating the storage potential of formations that have not previously been * Corresponding author. E-mail addresses: [email protected] (K.-L. Rørvik).

(A.

Sundal),

[email protected]

considered to hold enough economical potential (i.e. hydrocarbon bearing) to justify extensive data collection. In this study it has been a challenging task to make a depositional model of a complex delta system from a restricted data base of rather few, widely spaced wells and a limited coverage of seismic data. The interpretation of sedimentary facies has been made by integrating all types of available data (i.e. seismic, well log, core, side wall cores and well bore cuttings) into a generic depositional model that explains all observations within the current knowledge of how deltas may evolve. This study has benefittd from existing depositional models of the Johansen Fm. (Marjanac, 1995; Marjanac and Steel, 1997). However, new information, particularly from the 3D seismic survey GN1001 (Gassnova, 2012), combined with re-interpretations of the geological setting, petrography, well log stratigraphy and sequence stratigraphic framework, are the basis for the depositional model of the Johansen Fm. presented herin. This work is an independent study, but has been conducted in cooperation with Gassnova, providing access to recently acquired seismic 3D data, attribute analyses, geological and petrophysical reservoir evaluations (Gassnova, 2012).

http://dx.doi.org/10.1016/j.marpetgeo.2016.01.021 0264-8172/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Fig. 1. Structural elements of the study area in the northern North Sea. The map is based on the Rhyazanian level, Lower Creataceous, modified from Rattey and Hayward (1993) and Kyrkjebø et al. (2001). Main NeS oriented faults; Svarta, Tusse, Vette and Øygarden are delineating the study area as well as the gas accumulations in the Troll Field, and are marked with framed letters; S, T, V and Ø, respectively. The central part of the study area is located mainly on Horda Platform (HP) and Uer Terrace (UT) within blocks 35 and 31. The coloured depth map is showing top Johansen Fm. at burial depths ranging from 2 to 3.5 km (Gassnova, 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2. Geological setting The Johansen Fm. belongs to the Lower Jurassic Dunlin Gp., which also comprises the Amundsen, Burton, Cook and Drake for
(1984) (Fig. 2). The Dunlin mations, as defined by Vollset and Dore deposits extend across vast areas of the North Sea, and are considered to correlate with the Fjerritslev Fm. on the Danish shelf. The Johansen Fm. sandstone units are geographically restricted to the Horda Platform in the eastern part of the northern North Sea
, 1984) (Fig. 1). (Vollseth and Dore

2.1. Basin evolution Following the Caledonian orogenic collapse during Devonian time (Fossen and Hurich, 2005; Gabrielsen et al., 2010), a rift system started to develop in North-Western Europe during the

Carboniferous-Permian (Ziegler, 1990; Ziegler and Cloetingh, 2004). The extension has been interpreted as of Late Permian-Early Triassic age in the northern North Sea area (Gabrielsen et al., 2010). The Permo-Triassic extension event was succeeded by a thermal relaxation phase from Middle Triassic, during which a broader post-rift basin developed (Badley et al., 1988; Steel and Ryseth, 1990; Gabrielsen et al., 2010). The Dunlin Gp. was deposited during Late Sinemurian e Early Pliensbachian time (Partington et al., 1993). Some minor tectonic events of moderate local to regional uplifts and subsidence may have taken place and influenced thickness and facies variation within the Dunlin Gp. (Husmo et al., 2003). A regional uplift has been suggested to have taken place in Late Toarcian-Aalenian (Ziegler, 1982). Before this, and during a period of tectonic quiescence, differential subsidence in different parts of the basin may have been a major controlling factor in the distribution and input of clastic material during the early Jurassic post-

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rift phase (Steel and Ryseth, 1990; Færseth, 1996). On a local scale, basin lows and highs inherited from the Late Permian-Early Triassic rifting in company with differential subsidence brought about by variation in depositional thickness and mechanical compaction (e.g. Bertram and Milton, 1988; Thorne and Watts, 1989) may have affected generation of accommodation. Subsidence also occurred on a regional, basin scale as a result of post-rift, thermal relaxation (Ziegler and Cloetingh, 2004). A renewed rifting event was onset in Late Jurassic when EWextension resumed and subsidence rates rose. Initiation of rifting events was not synchronous throughout the basin, e.g. in the northern North Sea rifting was onset during Bajocian (Johannessen et al., 1995; Hesthammer et al., 1999). The rifting peaked at the JurassiceCretaceous transition and was succeeded by subsidence and sediment loading throughout Cretaceous (Gabrielsen et al., 2010). Some faults were reactivated faults from the PermoTriassic rift phase, whereas the major faults delimiting the Troll Field have been interpreted to have originated during the Jurassic extension (Færseth, 1996). During the Cenozoic several significant uplift events were followed by tectonic subsidence (Jordt et al., 2000; Jarsve et al., 2014). Burial depths of the Johansen Fm. vary from ~2 km on the northern Horda Platform, with gradually increasing burial depths southwards into the Stord Basin (~3 km). On the Uer Terrace the top of Johansen Fm. is located at depths down to ~4 km (Fig. 1). 2.2. Stratigraphy The understanding of the stratigraphic framework for the Dunlin Gp. has developed throughout the last few decades (e.g.
et al., 1984; Partington Bowen, 1975; Deegan and Scull, 1977; Dore et al., 1993; Steel, 1993; Parkinson and Hines, 1995; Husmo et al., 2003). Various depositional models have been suggested (e.g. Marjanac and Steel, 1997; Charnock et al., 2001). There are also inconsistencies in the use of lithologic and/or stratigraphic criteria with regards to the definition of formation boundaries (e.g. in the Norwegian Petroleum Directorate's database). In this study, the formations of the Dunlin Gp. are referred to within a sequence stratigraphic framework, based on well log and seismic interpretations of bounding surfaces (sensu Van Wagoner et al., 1988, 1990). Major boundaries are more or less in accordance with the interpretation by Steel (1993) of three mega-sequences: Johansen, Cook and Drake, in which the Johansen Fm. occurs in megasequence number 4 (MS4 as shown in Fig. 2). These megasequences form the main regressive-transgressive depositional units of the Dunlin Gp. The mega-sequences can be correlated by the application of maximum transgressive surfaces (or maximum flooding surfaces, MFS); chronostratigraphically dated and defined by Partington et al. (1993). The Johansen mega-sequence is defined between time-lines J12 and J14 (Fig. 2), and referred to as the Johansen sequence in the following, for simplicity. The basal unit of the Dunlin Gp.; the Amundsen Fm., comprises laterally extensive, marine mudstones and siltstones deposited in Sinemurian during a regionally extensive transgression, draping the underlying Latest Triassic e Early Jurassic deposits of the
et al., 1984; Nystuen et al., 1989; Steel and Statfjord Gp. (Dore Ryseth, 1990; Nystuen and F€ alt, 1995; Ryseth, 2001; Husmo et al., 2003). This time interval marked a climate change from arid and semi-arid conditions in late Triassic towards a humid climate prevailing throughout the Jurassic, coinciding with northwards tectonic drift and a sub-tropical paleo-latitude (Torsvik et al., 2002; Nystuen et al., 2014). The J12 MFS time line marks the base of the Johansen sequence, as well as the onset of an upward shallowing and regressive succession into the increasingly more sandy deposits of the Johansen

Fm. The Amundsen and Johansen formations are in this way also time equivalents, with the distal, open shelf mud of the Amundsen Fm. deposited in front of prograding sand deposits of the Johansen Fm. The maximum areal extent of the Johansen Fm. deltaic sand facies are draped by marine mud deposits of the upper part of the Amundsen Fm., before flooding culmination is marked by the MFS J14. In the more proximal depositional setting on the eastern part of the Horda Platform, the top of the Johansen Fm. is an erosional unconformity, forming an SU/TS surface overlain by the upper Amundsen Fm. In some areas there is a direct, erosional contact with the overlying Cook Fm., which comprises sandy, progradational parts of succeeding mega-sequence 5 (MS5 sensu Steel, 1993, Fig. 2). 3. Data, methodology and terminology 3.1. Lithology and petrophysics The well data base of this study comprises 41 wells located in the northern North Sea (Fig. 3A), on and adjacent to the Horda Platform (Fig. 1), all of which penetrate the Dunlin Gp. into the underlying Statfjord Gp. (Appendix 1). In recognizing stratigraphic surfaces, performing seismic well ties and studying facies developments, wire line logs were used. Gamma (API), bulk density (g/cm3), resistivity (ohm/m), neutron porosity (%) and velocity (uS/ feet) logs are publicly available from the Norwegian Petroleum Directorate's database for most wells (NPD, 2014). High mica and potassium feldspar content in the sandstones of the Johansen Fm. is a challenge in interpreting the gamma log response, as the potassium from these minerals increases total natural radioactivity. Spectral gamma log is available only in few wells. Mica may also increase the bulk density, which was taken into account in calculating density derived porosities. There is only one cored interval from the Johansen Fm. in the study area; between levels 2116 m and 2134 m in well 31/2-3. Porosity and horizontal permeability were measured in 54 core plugs by the operator in the cored interval (Appendix 2). There are no measurements of vertical permeability from the Johansen Fm. 3.2. Petrology From the cored interval (2116e2134 m) in well 31/2-3, 9 thin sections were prepared for this study and examined by optical- and electron-microscopy (SEM). The scanning electron microscope used is JEOL JSM 6460LV at the University of Oslo. Volume percent of the mineral distribution and the porosity was estimated based on point counting (300 points) (Appendix 4). Cathodoluminence and microsonde (Cameca SX100 at the University of Oslo) were used in detailed studies of grain overgrowths and mineral compositions. Another 10 thin sections from the same interval were borrowed from Statoil, as well as 5 thin sections from side wall cores in well 31/5-2. Due to relatively poor quality (rough surfaces) these thin sections were only used for facies description purposes. Well bore cuttings have been inspected for estimation of grain size and depositional facies. The majority of penetrating wells are old (>30 years) and additional sidewall cores that were made at the time are no longer available. Quantitative XRD data were derived from the 9 core samples in well 31/2-3, and from 46 cuttings samples from wells 31/2-3 (below the cored interval), 31/5-2, 31/2-1, 31/2-2, 31/3-1 and 31/2-4, using the Rietveld refinement technique (software TOPAS 4.2). The estimated bulk mineralogical compositions are given in weight percent (wt%) (Appendix 3). For the purpose of internal reference in the quantification procedure, 10 wt% ZnO was added to each sample. Cuttings samples are “polluted” by drilling mud to various degrees (e.g. bentonite), and quantification is thus not reliable with respect

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Fig. 2. Bio-, sequence- and lithostratigraphy of the Dunlin Group, based on major boundaries as defined by Partington et al. (1993) and Steel (1993). The Amundsen and Johansen formations are comprised within mega-sequence 4 (MS4) between maximum transgressive surfaces J12 and J14. The sandstone wedge of the Johansen Formation is outlined in red. Modified from Husmo et al. (2003). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

to clay mineral content. Barite, gypsum, syngenite and sylvite are present in some samples and interpreted to have precipitated from intruded drilling mud. These fractions are subtracted in the normalized XRD data (Appendix 3). It should be noted that salt precipitation may have influenced to some extent the porosity and permeability measurements. Mineral weight fractions were normalized without calcite in order to compare the clastic constituents between different facies. Calcite content is listed separately (Appendix 3). Samples were analysed using a Bruker D8 Advance instrument located at the University of Oslo. 3.3. Geophysical data The seismic database available for this study comprises 3D cubes GN1001, NPD-TW-08-4D-TROLLCO2, NH0301, SG9202, TNE01, GN1101 and SG9603 (Fig. 3B). Additionally, 2D data were used for extrapolation of facies distributions and locating pinch outs, with key lines shown in Fig. 3B. A consistent seismic database for interpretation, attribute analysis and inversion was created by

merging GN1001, NPD-TW-08-4D-TROLLCO2 and NH0301 into one volume; GN10M1 (Fig. 3B) (Gassnova, 2012). In a seismic inversion study (Gassnova, 2012) elastic properties, Vp/Vs ratios, acoustic impedance and density were quantified for GN10M1 using ISIS simultaneous AVO inversion technology. Density- and acoustic impedance volumes were generated from the GN10M1 cube. A corresponding porosity volume was made by defining lithology classes (sand, shale) related to measured porosities in wells and the acoustic impedance response (Gassnova, 2012). Root mean square (RMS) of seismic amplitude was calculated as a measure of reflectivity in GN10M1. Within the same volume, sweetness attribute was mapped, representing a combination of envelope and instantaneous frequency attributes, which was used for identification of seismic features visible due to change in energy signatures. Maps were generated using the software SVI Pro 8.1 to select frequency classes and highlight the amplitude response in a colour scheme of red, green and blue (RGB blending) (Gassnova, 2012). In the study of depth converted sections and generation of

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Fig. 3. (A) Well data base comprising 41 wells penetrating the Johansen and/or Amundsen formations. Well 31/2-1 is the type well for the Johansen Fm. and is used here as a tiewell in well correlation panels and for reference in map figures. Well correlation panel 1: from well 31/3-3 (E) to 30/3-3 (W), well correlation panel 2: from 31/2-19S (NNW) to 31/62R (SSE), well correlation panel 3: from 35/11-4 (SW) to 35/10-2 (NE), well correlation panel 4: from 35/11-1 (E) to 35/11-6 (W). (B) Seismic database. GN10M1 is a merged 3D volume combing the surveys GN1001, NPD-TW-08-4D-TROLLCO2 and NH0301, forming the basis for detailed studies and attribute analysis. SG9603, TNE01, SG9202 and GN1101 are 3D seismic data sets that have been interpreted with respect to main reflectors (Statfjord, Dunlin, Brent and Viking groups) and seismic facies. Key in-lines, composite and 2D lines are marked in yellow and red. The orange polygon outlines the velocity model used in this study (Gassnova, 2012) for depth and thickness mapping (Fig. 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

thickness maps, a velocity model hiQbeR was used (Gassnova, 2012). The uncertainty within the area covered by high quality 3D data (GN10M1) is in the order of ± 1%. It is expected, however, that the uncertainties are larger in areas without well control, and where there is a large structural dip. Depth and thickness maps were available for central parts of the study area; within a polygon (Fig. 3B) comprising 3D surveys GN10M1, SG9202, EO0801 and TNE01, with additional data supplied from 2D surveys (HRTRE00, MN88, MN9103, NSR, SG8043R91, SG9206, SH8001R92, SHP91, ST8408R91, ST8518, TE93 and TW08).

3.4. Delta terminology Due to lack of well core material, interpretation and modelling of the Johansen Fm. deltaic deposits are mainly based upon cuttings, well log data and seismic sections. By this reason the depositional environment and processes of the Johansen Fm. are referred to as four principal morphological elements: delta plain, delta front, prodelta and open-shelf, each of which may include

various sub-environments (cf. Reading and Collinson, 1996; Bhattacharya, 2010). We apply the term delta complex for the entire delta system, including sub-environments and related depositional processes from delta plain to prodelta settings.

4. Results 4.1. Well data interpretation 4.1.1. Core description The logged 18 m core interval of well 31/2-3 displays a general upward coarsening trend (Fig. 4). The lower section (2128.5e2134 m) consists of fairly to poorly consolidated, thin bedded, light grey, micaceous, feldspathic arenite. The sandstone is fine-to medium grained. Elongated mica flakes and coalified plant fragments make up draping layers (Fig. 4C). Individual beds display upwards fining trends, occasionally with Skolithos and Ophiomorpha burrows. The sorting is moderate to poor, and the grains are sub-angular to sub-rounded. Average measured porosity from core

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Fig. 4. Sedimentary log from well 31/2-3 through cored interval 2116e2134 m (measured depth e MD) showing sedimentary structures, average grain size and sample points (thin sections). The lithology colour scheme is in accordance with facies assosiations from well logs (Fig. 6). (A) Photomicrograph of thin section sample from level 2120.7 m (cross polarized light, 5
). Note porefilling, poikilotopic calcite cement (CC) and moldic porosity (%). (B) Photomicrograph of thin section sample from level 2125.4 m (plane light, 5
). Open porespace (%) is filled with blue epoxy. (C) Photomicrograph of thin section sample from level 2129.5 m (plane light, 5
). Note layered, elongated grains of biotite (B), muscovite (M) and carbonaceous fragments (C). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

plugs (15 points) is 26.5 %, with horizontal permeabilities varying across a large range; 228-5959mD (Appendix 2). In the upper part (2116e2128.5 m) the grain size is medium to coarse, and beds are thicker and generally structureless. Thin pebble lag conglomerates occur at the base of some beds. In this interval the average porosity is 29.9 % (34 points, Appendix 2). Measured horizontal permeabilities display large variance, in the order of 218-4594 mD (Appendix 2). An example of typical high porosity texture of this section is displayed in photomicrograph from 2125.4 m (Fig. 4B). The cored section contains some completely carbonate cemented

layers (Fig. 4A), occurring associated with bed boundaries, or within beds independent of any sedimentary boundaries. T volume fraction measured in two thin sections (Appendix 4) is 31.3 % and 30.3 %, and the porosity 5.5 % and 5.3 %, respectively. Interpretation: The relatively coarse grain size and the high amount of organic material indicate a near-shore environment. The sandstone facies represented in the cored section is interpreted to represent an upper delta front environment. The massive beds and cross stratified finer intervals may have formed as proximal mouth bars during repeated river floods. Normal graded, erosionally based

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beds with pebble lags are likely deposited in associated distributary channels. Slight decreases in the energy regime would allow deposition of inter-granular, micro-micaceous clay and the settling of plant debris and mica particles as draping layers over small sand dunes. Carbonate cementation is mainly by calcite and a postdepositional, diagenetic feature. A likely carbonate source in a high energy delta front depositional setting, could be mollusks present in situ, as reworked storm beds or transgressive lags (Bjørkum and Walderhaug, 1990). 4.1.2. Petrography and diagenesis The sandstone lithology of the Johansen Fm. is micaceous, feldspatic arenite sensu Dott (1964). The average quartz content in core samples is in the order of 45 % by volume (Appendix 4). Some sutured grain contacts with contact dissolution were observed, as well as euhedral, autigenic overgrowths on quartz grains, occasionally enclosing kaolinite (Fig. 5A). Quartz cementation and accompanying dissolution at points of contact between quartz grains are mainly controlled by temperature and silica concentration (Walderhaug, 1996; Bjørlykke, 2014) and is onset at approximately 70  C (Bjørlykke and Egeberg, 1993). The estimated current temperature for the cored interval in well 31/2-3 is 65  C at 2100 m. The current burial depth on the Horda Platform is not likely equivalent to maximum burial, due to several post depositional uplift events (i.e. during Late Jurassic, early Paleocene and early Oligocene (Sømme et al., 2013). The average volumetric feldspar content is in the order of 13 %, with potassium feldspar being more abundant than plagioclase (Appendix 4). Potassium feldspar grains display high K-content and low to negligible Na-content in SEM. In plagioclase, both well preserved and more altered or dissolved grains mostly display Narich chemistry and low or negligible Ca-contents, indicating that albite is the most common phase. The microcline grains are noticeably less corroded, dissolved or altered to various degrees to kaolinite and sericite, compared to plagioclase grains. Examples of authigenic microcline overgrowths, as well as partly altered, or dissolved, feldspar grains with internal secondary porosity are common. Feldspar is the most likely precursor for many of the clay mineral pseudomorphs, consisting in large parts of sericite. Diagenetic, porefilling kaolinite (Fig. 5A) sometimes displaying typical vermicular morphologies, is probably in large parts an alteration product from weathering (Keller, 1978). Meteoric flushing during early burial is interpreted to have been the most important mechanism providing high porewater fluxes and excess ion removal (Bjørlykke, 1994). Muscovite is relatively abundant, constituting 1e6 vol% and 2e6 wt% (Appendix 3, 4). Individual grains display high degree of orientation and are commonly quite long (up to 3 cm) (Fig. 4C). All grades from fresh to completely diagenetically transformed muscovite are present. Biotite is usually less abundant than muscovite and also appears to be almost completely diagenetically altered into kaolinite and chlorite (<1 wt% and not included in XRD quantification). The dissolution of biotite and resultant Ti-oxide precipitation is likely an early diagenetic process operating more or less parallel with kaolinitization (cf. Pe-Piper et al., 2011) during meteoric flushing. Pyrite (<1 wt%) occurs mostly as framboidal, small grains associated with degraded biotite (Fe-source) and carbonaceous fragments, indicating reducing conditions due to bacterial, anaerobic decomposition of organic matter shortly after deposition. The lithic fragments comprise grains of metamorphic origin; quartz rich, gneissic fragments and mica schists, as well as some igneous, granitic rock fragments. Zircon, apatite and amphibole (hornblende) are common detrital heavy minerals, constituting less than 2 vol%. This is consistent with the inferred eastern hinterland geology, with sediments sourced from granitic and

Fig. 5. Photomicrographs from thin section (TS) samples from the cored section in well 31/2-3 (2116e2131 m), using scanning electron microsopy. Sample locations are marked in Fig. 4.(A) TS 2129.3 m. Autigenic quartz (Q) overgrowth, enclosing porefilling kaolinite (K). Thick chlorite (Chl.) coating in embayment. (B) TS 2125.4 m. Calcite cement (CC) enclosing kaolinite (K) and filling inside chlorite ghost rims (Chl.) where the original, detrital coated grain is missing.

metamorphic rocks both from Caledonian thrust sheets and Precambrian gneissic basement rocks (Nystuen and F€ alt, 1995; Knudsen, 2001). Calcite occurs as pore-filling, poikilotopic cement (Figs. 4A and 5B). Pore-filling kaolinite is commonly enclosed in, and chlorite ghost rims are filled with calcite cement (Fig. 5B). Euhedral, rhombic crystals are also observed, as well as patches of microcrystalline cement and isolated carbonate growth between mica lamellae. SEM and electron microprobe reveals an overall Ca-rich chemistry (52e55 %) in all the observed crystal habits, with some varying contents of Mg, Fe and Mn in the range of 0e1.5 % (Appendix 5). The Sr-content in the calcite cement was very low in all sample points (Appendix 5), in the range of 0e120 ppm (close to detection limit), which is indicative of later diagenetic precipitation, possibly as a result of re-precipitation (Bjørlykke and Brendsdal, 1986). In-situ porewater would be rich in Sr at the time of deposition, and subsequently diluted during meteoric flushing. Cementation post-dates early diagenetic processes such as feldspar leaching, precipitation of kaolinite, swelling and fanning of mica grains, and chloritization of mica and grain-coating clay, but is likely to have occurred before the onset of quartz cementation, as quartz overgrowths are rare in cemented samples. With the summed volume fraction of calcite cemented pore-space plus porosity being higher in calcite cemented samples (>35 %) compared to the porosity observed in the associated uncemented facies environments (<30 %), it may be inferred that initial cementation occurred during early burial, increasing the rock rigidity and resisting some later, porosity reducing, mechanical compaction. Chlorite is relatively abundant (2e9 wt %) (Appendix 3), occurring in different habits as grain-coating and ooids, as replacing other Fe-

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Mg minerals, pore-filling and pore-lining, and in chaotic mud clasts. The chlorites display an Fe-rich chemistry close to chamosite (low to negligible Mg-contents). The grain coats are patchy and of varying thickness, sometimes thicker in embayments (Fig. 5). Deltaic environments are common depositional settings for chlorite-precursor clay grain coats in sandstone. Quartz cementation is inhibited on chlorite coated grains, effectively preventing chemical compaction and preserving porosity, as observed down to 4 km burial depth in the Cook Fm. (Ehrenberg, 1993). 4.1.3. Facies associations from logs Facies associations obtained from wire line logs in all studied wells (Fig. 3A), abbreviated to log facies associations, or log FA (Fig. 6), have been defined by interpretation of patterns and relative magnitudes of gamma ray (GR), specific potential (SP), velocity (DT), resistivity (R) density (RHOB) and neutron porosity (NPHI) wire line logs. Lithological interpretations from logs have been adjusted manually by comparison with data from drilling reports, cuttings samples, side wall cores (SWC) and biostratigraphy. Facies interpretations were made by a combined evaluation of grain size distribution patterns and mineralogy. Nine distinct facies associations have been identified, within the principal depositional systems of delta plain, delta front, pro-delta and open shelf environments. Log FA 1. Log intervals representative of this facies association are recognised by low gamma-log readings (in the order of 30e40 gAPI) and display a slightly serrated pattern of stacked,

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sometimes individually fining (GR increase) upwards (1e5 m thick), often sharp based units. Low resistivity and density readings, intermediate neutron porosity (RHOB e NPHI crossover plot indicative of clean sand) are additional log characteristics. Velocities are in the order of 80 us/ft. Porosities inferred from logs are in the order of 30 %. Associated cuttings and side wall cores are described as containing medium-to very coarse-grained, poorly consolidated sandstone or friable sand The degree of sorting is mostly moderate, with angular to sub-angular grains, and occasional presence of carbonaceous material, as well as kaolinite and argillaceous matrix. The average mineral content measured by XRD based on 12 cuttings samples from 4 wells is 69 % quartz, 18 % feldspar, 2 % mica and 10 % clay (chlorite and kaolinite). Additional calcite content varied from 2 to 6 % (Appendix 3). Interpretation. Log FA 1 is interpreted as thick, amalgamated and sometimes fining upwards sandstone beds, often with erosional bases. These log trend units occur in the uppermost part of progradational clinothem sets, and log FA 1 is inferred to represent point bars or channel fill deposits of fluvial distributary rivers on a sub-aerial delta plain. According to recorded log interval thickness, individual channel thicknesses vary in the range of 1e5 m. Log FA 2. This facies association is recognised by slightly elevated gamma-log readings (in the order of 45e55 gAPI) compared to the low values typical of log FA 1, higher density and neutron porosity readings. This well log pattern is closely associated with gamma readings interpreted as log FA 1. These well-log records and trends are not very frequent in this data set (only

Fig. 6. Well correlation panels (locations are shown in Fig. 3A), showing interpreted correlations of the bounding lower and upper maximum flooding surfaces (MFS) J12 and J14 (Fig. 2) defining the Johansen sequence, as well as the maximum regressive surface (MRS). Log facies associations (log FA 1e9) are colour coded and interpreted for each well, shown as infill with gamma log pattern. All panels are displayed in meters sub-sea true vertical depth (SSTVD) and flattened on MFS J12. (A) Well correlation panel 1: from well 31/3-3 (E) to 30/3-3 (W). (B) Well correlation panel 2: from 31/2-19S (NNW) to 31/6-2R (SSE) (C) Well correlation panel 3: from 35/11-4 (SW) to 35/10-2 (NE). (D) Well correlation panel 4: from 35/11-1 (E) to 35/11-6 (W). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. (Continued).

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Fig. 6. (Continued).

noted in wells 31/3-3, 31/3-1 and 31/2-2). Interpretation. Log FA 2 is interpreted to reflect beds and bed sets of siltstone or mudstone between fluvial channel deposits of log FA 1 in a delta plain setting. Units of this facies association are thus assumed to be floodplain deposits between channel infill successions, formed due to avulsion or associated with flooding events. Rare occurrences of this facies association within the data set are interpreted to be due to high degree of channel amalgamation and low preservation potential. Log FA 3. Well-log intervals of this facies association display blocky to serrated gamma ray patterns, with magnitudes in the order of 30e40 gAPI. Velocities are around 80 us/ft, resistivity and density readings are relatively low, with intermediate neutron porosities (RHOB e NPHI crossover plot indicative of clean sand). Parts of the observed core from well 31/2-3 are representative of this facies association (Fig. 4), to which the descriptions of cuttings material and side wall cores were compared. Estimated porosities are in the order of 30 %. The average mineral content as measured by XRD in 8 cuttings samples from 3 wells (Appendix 3) is 67 % quartz, 18 % feldspar, 3 % mica and 12 % clay (chlorite and kaolinite). Additional calcite content varies in the range of 3e7 %. Interpretation. Log facies FA 3 is interpreted to represent massive, amalgamated and highly porous sandstone beds. The sand may have been deposited as mouth bars, distributary channel deposits, upper shoreface and beach deposits. Log FA 3 deposits form the top of the prograding delta complex in the study area. Log FA 4. These log units are characterized by slightly elevated gamma log responses, 50e60 gAPI and higher density compared to log FA 1 and FA 3. This may be assigned to higher contents of mica, concurrent with observations from thin sections and sediment descriptions from cuttings and sidewall samples. Carbonaceous

material and pyrite were also noted in descriptions of cuttings material. The log signature is serrated and occurring in aggradational blocks, as part of progradational and retrogradational successions. Estimated porosities are in the order of 26 %, and the average mineral content as measured by XRD in 16 cuttings samples from 5 wells (Appendix 3) is 61 % quartz, 20 % feldspar, 4 % mica and 16 % clay (chlorite and kaolinite). Additionally, the calcite content varied up to 17 %. Interpretation. The sediments of log facies FA 4 are interpreted to be porous, heterogeneous fine-to medium grained sandstone beds. The beds are inferred to have been deposited in a moderate energy environment at the lower delta front, allowing the settling of mica flakes and light, carbonaceous material. Log FA 4 appears to represent a more distal setting compared to log FA 3 and may reflect distal mouth bar or/and lower shoreface environments. Log FA 5. The log signature of this facies association is characterised by gamma readings in the order of 60e90 gAPI. Cuttings from these well log intervals are described as siltstones and silty- and clayey fine to very fine sandstones. The varying relative contents of mica and clay within these fine-grained sediments affect the log signatures with highly fluctuating gamma ray responses, variable compaction and estimated porosities according to net gross. The XRD mineral content in 7 cuttings samples from 5 wells (Appendix 3) is 57 % quartz, 22 % feldspar, 4 % mica and 17 % clay (chlorite and kaolinite), with an additional calcite fraction up to 11 %. Interpretation. This log facies association is interpreted as the distal pinch out deposits of progradational delta front successions. The log facies association comprises siltstones, as well as silty- and clayey fine to very fine sandstones, deposited in a low energy, prodelta environment. Log FA 6. These well log intervals display serrated, aggradational

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gamma ray patterns of high magnitude (>80 gAPI), relatively low density and high neutron porosity, with the degree of RHOB-NPHI cross over curve separation depending on clay content and degree of compaction. Descriptions of cuttings samples confirm that these intervals comprise mudstone, claystone, shale and siltstone. Spikes of very high gamma values (>150 gAPI) are also comprised in this log facies association. Interpretation. The deposits of log FA 6 are interpreted as clayand silt-rich sediments deposited mainly as suspension fallout in a low energy pro-delta to open shelf setting. Particularly high-value gamma spikes are interpreted as condensed intervals of shale with high organic content and radioactivity, associated with transgressive events. Log FA 7. This type of well-log pattern is recognised as sharp based, stacked, metre-scale upwards fining or graded units in the gamma-log, with low readings (30e40 gAPI). Velocities are around 80 us/ft, resistivity and density readings are relatively low, with intermediate neutron porosities (RHOB e NPHI crossover plot indicative of clean sand). Cuttings material and side wall cores contain medium to coarse-grained clean sandstone and very fine to fine sandstone. The log FA 7 intervals occur with abrupt changes in log response above intervals of log FA 6, and is only recognised in wells in quadrants 35/10 and 35/11. Individual thin (<1 m) upwards fining or graded units may be organised in thicker (several meters) upward coarsening or upward fining log intervals, resting top of log FA 5 and FA 6 units. Interpretation. Log FA 7 is interpreted as erosionally based sandstones deposited in a distal setting by turbidity currents, supplied more and less directly from river mouths and/or by delta front slope failure. The depositional environment is interpreted as delta front (log FA 5) and pro-delta (log FA 6), with sandy sediments introduced as gravity flow deposits. Individual upward fining (or graded units) may represent turbidite beds, whereas larger upward fining and upward coarsening units may be turbidite channels and delta front/pro-delta turbidite fans or lobes, respectively. Log FA 8. These intervals display heterogeneous log patterns, with relatively high gamma ray values; in the range of 50e100 gAPI. The identification of this facies association is based mainly on comparison with observations from well 31/6-6, from which a more comprehensive biostratigraphy report is available (Bell et al., 1984). The sediments are described as heterolithic, with beds of fine to very coarse-grained, micaceous, pyritic and glauconitic sandstone, and some beds of claystone and siltstone. In well 31/6-6 wood fragments have been recorded, with rare to common occurrences of inertinite, terrestrial palynomorphs and membranous material. Marine biostratigraphic indicators are sparse, with Micrhystridium (acritarchs) only in the lower interval of the Johansen Fm. in well 31/6-6 (below 2205 m) being indicative of some marine influence. Occurrences of rare cysts and Botryococcus (fresh water algae) in the upper part of Johansen Fm. in well 31/6-6 are in favour of brackish depositional conditions according to Bell et al. (1984). Log FA 8 is recognised only in the wells 31/6-6, 31/6-2, and 31/6-3. Interpretation. Log FA 8 is interpreted to represent heterolithic deposits of sandstones and siltstones, rich in clay and glauconite (causing an increase in gamma log response) deposited in a near shore, protected lagoonal to back basin environment, with occasional tidal or flood events causing marine influence and brackish water conditions. Log FA 9. This log facies association is characterised by distinct log spikes, with a characteristic drop in GR, SP and NPHI readings and corresponding increase in RHOB, DT and R. This type of log response is observed within all other log facies associations. Carbonate cemented sandstone and mudstone is observed in cuttings and side wall cores, as well as in the cored section of well 31/23 (Fig. 4A). The minimum detectable carbonate cemented layer

thickness in well logs is estimated to be in the order of 0.5 m, whereas from the core it is evident that there are also thinner layers present. This log facies association is only picked where several definite log indicators are observed. Interpretation. Log FA 9 is interpreted to represent low porosity, hard carbonate cemented zones in sandstones and mudstones formed from dissolution of calcareous material (supplied for example from mollusc fragments in proximal parts and pelagic material in distal areas) and subsequent precipitation during burial. Log FA 9 is thus recognized within all previously described log facies associations. 4.1.4. Well correlation All wells penetrating central, sandy parts of the Johansen sequence display a distinctive, crescent shaped log pattern; with a lower progradational part, a middle aggradational interval and a retrogradational upper part, interpreted as corresponding to one regressive-transgressive cycle, or R/T-sequence. The progradational to retrogradational succession varies in thickness and log facies associations according to the depositional and structural setting within the basin. Wire line log correlations between four well panels are shown in Fig. 6, with the respective locations noted in Fig. 3A. In well correlation panels (Fig. 6), the Johansen Fm. is interpreted to form a clastic wedge building out towards the west, between the lower and upper maximum flooding surfaces J12 and J14 (Fig. 2). In some areas the lower contact between the Johansen and Amundsen formations appears to be an erosional unconformity, and the TS and MFS correlate geometrically with this sub-aerial unconformity (SU), thus the lower contact in these areas are forming a combined SU/TS/ MFS surface. Such surfaces represent abrupt increase in water depth and accommodation space (cf. Ahokas et al., 2014a, b; Jarsve et al., 2015). The R/T-Johansen sequence is locally interrupted by erosional surfaces (ES) and flooding surfaces (FS), some of which correlate between wells. The numerous calcite cemented layers (log FA 9) seem to be local features in some areas, but in other cases they represent correlatable abandonment surfaces, or erosional transgressive surfaces (ravinemental, calcareous lag deposits). The Johansen sequence contains several parasequences (e.g. Marjanac (1995) subdivided the Johansen sequence in 17 parasequences.) which may be observed in the correlation panels (Fig. 6). We consider the stratigraphic resolution to be too low and the uncertainty too high for definition and correlation of individual parasequences. Thus only overall stacking patterns (parasequence sets) are described. In central parts of the study area the general stacking pattern of the Johansen sequence is well defined as initially progradational from the J12 MFS, then aggradational up to the MRS, and finally retrogradational or backstepping towards J14 MFS. Well correlation panel 1 (Fig. 6A) displays a roughly depositional dip parallel cross section, from the most proximal setting in the east towards more distal facies in the west, with a gradual thinning of sandy facies. Delta plain facies associations (log FA 1, 2), with aggradational, amalgamated sandy channel deposits are recognised as far west as in well 31/2-5 (Fig. 6A). There are multiple significant erosional surfaces. Abrupt deepening with respect to facies associations across a distance of 8.3 km is observed between the wells 31/2-5 and 31/1-1. There is also a large structural down-throw (in the order of 500 m) between these wells. In the north-south oriented well correlation panel 2 (Fig. 6B), delta plain facies associations (log FA 1, 2) are recognised in the central, proximal wells 31/2-1, 31/5-2 and 31/6-1. Towards the north the panel section displays a transition from delta plain into shallow marine facies of interchanging upper and lower delta front (log FA 3, 4) in wells 31/2-3 and 31/2-4R. The core in well 31/2-3 is located within delta front facies (log FA 3, 4) in the aggradational, sandy part of the Johansen sequence and comprises the MRS (depending on the accuracy of measured depths). There appears to be a high degree of

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amalgamation of sandstone units in proximal parts, and numerous calcite cemented layers (log FA 9) within each parasequence, but less frequent preservation of fine-grained facies associated with flooding events (log FA 5, 6). The Johansen Fm. is erosionally based in 31/2-19S, interpreted as a proximal, turbidite feeder channel (log FA 7). Towards the southern end of well correlation panel 2 (Fig. 6B), heterolithic sediments and brackish water indicators in well 31/6-2R (log FA 8) is interpreted to represent a transition from delta plain (log FA 1, 2) in well 31/6-1 to a lagoonal environment. The top of the Johansen Fm. appears as an unconformity, likely a subaerially exposed surface, in most proximal wells (Fig. 6A, B), sometimes correlating with the MRS (e.g. well 31/3-3, Fig. 6A). In well 31/6-1 (Fig. 6B), thick, aggradational delta plain and uppermost delta front deposits (log FA 1 and FA 3) are overlain by an erosively based Cook Fm.; the upper MFS (J14) as well as the retrograding upper units of the Johansen sequence are absent. Otherwise, the rise in relative sea level up to the J14 MFS appears to be a more gradual trend. In well panel 3 (Fig. 6C) stacked, prograding delta front units (log FA 3, 4) make up the south-eastern, proximal part (wells 31/11-7 and 35/11-4). Sharp based, gravity flow deposits (log FA 7) are recognised towards the outer part of the basin in the north-west, together with a general thickening of the Johansen Fm. in well correlation panel 4 (Fig. 6D), a markedly thicker succession makes

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up the Johansen Fm. in well 35/11-1. The overall log signature in this well reveals no distinct progradational to retrogradational pattern. The lower boundary of the Johansen Fm. appears sharply erosional in the wells 35/11-1, 35/11-2 and 35/11-6 (Fig. 6D), with coarse-grained sand (log FA 7) directly overlying distal, very finegrained marine sediments (log FA 5, 6). The Johansen Fm. is relatively thin in the wells 35/11-6 and 35/11-2 (Fig. 6C). 4.2. Seismic interpretation The Johansen Fm. is characterized by 3e4 seismic reflectors, which make a recognizable pattern in central parts of the study area within the merged 3D volume GN10M1 (Fig. 3B), at burial depths in the range of 2e3 km. The distribution of the Johansen Fm. is displayed in two seismic composite lines in Fig. 7 (the location is shown in Fig. 3B). The top Johansen reflector is interpreted at the onset of an S-crossing (between a seismic trough and peak) and is present within all the interpreted 3D volumes (Fig. 3B). The underlying Statfjord Gp. produces a strong, consistent reflector. The lower Amundsen Fm. is recognised throughout the seismic 3D volumes as a high-amplitude through, although distorted in some areas by slumping. The upper Amundsen Fm. gives rise to a slightly weaker reflector, and is picked as a through; however, it is not always unambiguous. The upper Amundsen Fm. appears to be very

Fig. 7. Seismic composite lines 1 and 2 from 3D volume GN10M1 (Fig. 3B). Intra-Dunlin reflectors are shown, representing the Johansen, Cook and Drake formations. The top of the Brent Group and the Sognefjord Formation are indicated for reference, as well as flat spots associated with the gas accumulations in Troll East and Troll West. Note 5
vertical exaggeration. (A) NW-SE striking composite line 1 intersecting wells 31/2-4, 31/2-3 and 31/3-1, parallel with the interpreted depositional dip. The line intersects the major, NSstriking regional faults Svarta and Tusse, which display large throws and cause segmentation of the Johansen Formation. Insert showing down-lapping reflections and clinoform geometries in the Johansen Fm. (B) Composite line 2 through tie well 31/2-1, southwards through 31/5-2 and south-westward to the edge of seismic volume GN10M1. A group of NW-SE striking faults east of the regional N-S striking Svarta Fault display relatively small throws, not compartmentalizing the Johansen Fm. Insert showing high amplitude seismic reflections in the Johansen Fm.

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Fig. 8. West-east seismic cross section between wells 32/4-1 and 32/2-1 along in-line 1026 in seismic volume GN1101 (Fig. 3B). The line shows eastward thinning and erosional subcrop of the Dunlin Group, while the underlying Statfjord Group reflector is present throughout. The throw across the Vette Fault is large (>50 s TWT), juxtaposing the Johansen Formation towards Basement in the east and the Sognefjorden Formation in the west. The Jurassic is not interpreted east of the Øygarden Fault. Note 5
vertical exaggeration.

Fig. 9. The interpretation of the north-south oriented seismic 2D line HRTRE-00212 (Fig. 3B) shows coinciding stratigraphic pinch outs of the deltaic systems in the Johansen and Sognefjord/Krossfjord formations (Gassnova, 2012). Note 10
vertical exaggeration.

Fig. 10. Seismic facies with examples from in-lines in seismic 3D volume GN10M1 (Fig. 3B). (A) Seismic FA 1. Planar parallel reflections. (B) Seismic FA 2. High amplitude reflections. (C) Seismic FA 3. Chaotic reflections. (D) Seismic FA 4. Down-lapping reflections displaying clinoform geometries.

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thin, or missing, in the south-western part of the GN10M1 volume, where there is an apparent erosional contact between the Johansen and Cook formations (confirmed in well 31/6-1, Fig. 6D). The Cook Fm. is thinning towards the east (Fig. 7A) and is not present in wells east of 31/3-1. The Drake Fm. is present throughout the study area and is recognised as a weak, but consistent reflector. The central parts of the study area are underlying the Troll gas reservoir, with the hydrocarbon accumulations clearly visible as flat spots within the Late Jurassic Sognefjord Fm. (Fig. 7). In the SG9603 survey area (Fig. 3B), the Johansen Fm. is recognised at burial depths down to 3e4 km. The sedimentary succession is heavily faulted, resulting in lower resolution and distorted reflectors, but the Johansen Fm. appears to be present throughout the volume. The areal extent, with the Johansen Fm. sandstones pinching out towards the west, is in agreement with previous studies, as summarized in Husmo et al. (2003). In the east, there is an NeS running (semi-parallel to the Øygarden Fault Zone) erosional sub-crop of the Lower Jurassic strata towards the overlying Brent Gp. The Dunlin Gp. is missing in well 32/2-1. The unconformity is observed within the GN1101 volume (Fig. 8). A well-defined southward, stratigraphic pinch out into the Stord Basin of the thick sand accumulations associated with the Johansen Fm. is displayed in the seismic 2D line HRTRE00-212 (Fig. 9). The location of the pinch-out coincides with the position of the pinchout of the overlying Sognefjord Fm. In the central parts of the study area the Johansen Fm. displays depth and thickness variations as shown in Fig. 1 (as estimated within the outline of the velocity model, polygon in Fig. 3B). The formation depth increases towards the north and south along the large, regional faults delineating the Horda Platform (Fig. 1). There is a steepening structural dip into the Stord Basin, and increasing burial depths from 2.2 km in central parts down to 3.2 km in the south. The formation thickness increases westwards in the range of 80e180 m. 4.2.1. Seismic facies Within the GN10M1 merged volume, as well as in TNE01, SG9202 and SG9603 (Fig. 3B), high resolution data provide for evaluation of seismic facies distribution. Four main categories in seismic occurrence were recognised (Fig. 10). Seismic facies are ambiguous with respect to their depositional facies equivalents; the acoustic impedance is also strongly related to fluid content. The interpretations given are constrained to plausible occurrences (i.e. siliciclastic sediment, saline pore water). No hydrocarbons (gas or oil) have been encountered within the Johansen Fm., but thick gas accumulations in overlying strata (i.e. Troll Field, Sognefjord Fm.) may to some degree weaken the seismic signal. The amplitudes show great variance, but the frequencies are less variable, due to the formation thickness in relation to seismic resolution. The varying degree of heterogeneity with respect to the number and thicknesses of internal flooding surfaces and calcite cemented layers in different parts of the formation (i.e. log FA 6, 9) provide the largest uncertainty with respect to interpreting seismic facies. Seismic FA 1. Planar parallel facies. This seismic facies consists of horizontal, roughly parallel reflectors of medium to low, fairly constant amplitudes (Fig. 10A). These reflectors are commonly laterally continuous. Interpretation. The planar parallel facies is interpreted to represent water-filled sandstone and/or shale lithologies with sheet-like geometries. The beds have likely been deposited in a medium to low energy environment. Seismic FA 2. High amplitude facies. The facies is defined by relatively parallel, sometimes discontinuous, high amplitude reflectors (Fig. 10B). Interpretation. The facies of high amplitude reflectors is inferred

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to be highly porous, fairly homogenous, water filled sandstone yielding large acoustic impedance contrast due to over- and underlying more silty- and shaly lithologies. The inferred amalgamated sandstone beds of this seismic facies are interpreted to represent a high to medium energy system. Seismic FA 3. Chaotic facies. This facies is recognised as discontinuous, medium-to low amplitude reflectors with no apparent systematic or directional trend (Fig. 10C). Interpretation. The facies is interpreted to reflect heterolithic sediments, displaying frequent property changes both laterally and vertically, due to in situ depositional environment with soft sediment deformation or slumped/faulted depositional surfaces. The energy system has been variable. Seismic FA 4. Clinoform facies. In some areas down-lapping, sometimes sigmoid, medium-to low amplitude reflectors with a systematic, directional trend are found (Fig. 10D). Clinothem thicknesses are in the order of 50 ms TWT (i.e. in the range of 100e140 m minimum thickness, not considering post-depositional compaction), generally displaying a steep dip and a total lap distance around 500 m. Interpretation. The facies may represent alternating lithologies, sandstone lobes and siltstone/shale drapes with overall clinoform geometries, formed as progradational clastic wedges. 4.2.2. Seismic attributes The high resolution GN10M1 seismic cube provided the basis for attribute analysis. In the RMS (Root Mean Square) amplitude map (Fig. 11A) an elongate body of high reflectivity is conspicuous at a level of 25 ms below the top Johansen reflector. This amplitude anomaly correlates with high amplitude seismic facies (seismic FA 2, Fig. 10B) distribution as shown in the correlating seismic crosssection along in-line 1757, where there is a development from chaotic seismic FA 3 in the east, via high amplitude seismic FA 2, and an abrupt change to chaotic seismic FA 3 in the west. The acoustic impedance data, representing average values in the uppermost part of the Johansen Fm., display generally low values in the central parts of the study area, indicative of porous sandstone, and anomalously low values in a restricted, NNW e SSE oriented, elongate shape (Fig. 11B). This geometry is consistent with a coast parallel spit bar type of deposit. The area of these impedance anomalies corresponds with the lateral extent of the amplitude anomaly observed in the RMS map. Acoustic impedance is the main input for property modelling, thus a similar spatial pattern may be recognised in all attribute maps. The density map (Fig. 11C), representing average values within the Johansen Fm., mainly shows low densities within the study area, indicative of porous sandstone, only with some increased values (red colour), indicative of compacted mudstone, in the south-eastern part of the GN10M1 volume. Some selected map details from the SVI-Pro attribute analysis are shown in Fig 12. The blended Red-Blue-Green (RGB) map (Fig. 12A) shows an interpreted horizon parallel slice at 56 ms below top Johansen Fm. Each colour corresponds to an amplitude response at a given frequency. Blending three frequency classified volumes, property contrasts within the data set become evident. A NNW-SSE oriented, high amplitude, elongate shape is visible in green in the southern half of the data-set, corresponding to observations shown in Fig. 11. This feature is present more or less throughout the Johansen Fm., as interpreted from stacked RGB images (time slices). Also, a distinct fan-shaped, high reflectivity feature protruding towards the east is visible in the southern part of the map in Fig. 12A. This may be interpreted as a flood tidal delta geometry. This feature is becoming vaguer and disappearing towards the top of the Johansen Fm. Semi-circular and sinusoidal grey/white features in Fig. 12B are

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Fig. 12. Details from frequency analysis within seismic volume GN10M1 (Fig. 3B), modified from Gassnova (2012). (A) Blended Red-Green-Blue (RGB) colour map representing a horizon parallel slice at 56 ms (TWT) below the interpreted top Johansen Formation. Each colour corresponds to a given frequency channel with a distinct amplitude response. A strong response from all frequencies produces white or light grey areas (high). A weak response from all frequency channels produces black or dark colours (low). A NNW-SSE oriented, high amplitude, elongate shape is visible in green in the southern half of the data-set, with a distinct fan-shaped feature protruding towards the east, marked by arrows. (B) Marked with arrows and displayed as a small-scale detail from (A) close to tie well 31/2-1 are sinusoidal (channel-like) grey/white features (high). Fault lines or fractures are seen as dark lines (low). (C) Sweetness attribute map representing a time slice at 2127 ms in the Johansen Formation close to tie-well 31/2-1. The colour scheme shows where there is a change in energy signatures (high, low), due to the combination of envelope and instantaneous frequency attributes. Sinusoidal, channel-like features are indicated with arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

interpreted as channel geometries, and they are also recognised in the sweetness attribute maps (Fig. 12C). A strong response from all frequencies produces white or grey colour. Faults are visible as dark (Fig. 12A, B) or grey (Fig. 12C) lines. In the modelled porosity cube (Fig. 13) the two stratigraphically lowermost slices (Fig. 13AeB) are located within the Amundsen Fm. and display relatively low porosities (5e15 %) throughout. There is a trend of porosity increase upwards and towards the west (from blue and purple colours to turquoise and green). The middle section of the Johansen Fm. (Fig. 13CeE) displays high porosities in the range of 20e30 % (correlating with log FA 1, 2, 3 and 4 in proximal wells, Fig. 6A, B). An area of low porosities (5e10 %) is shown in the eastern part of the section (correlating with log FA 8 in well 31/62R, Fig. 6B). Towards the formation top (Fig. 13EeH) an area of low porosity in the south-east is increasing in size. In the upper part of the Johansen Fm. (Fig. 13GeH) there is a marked distribution of higher porosities in the central study area, around tie-well 31/2-1. High average porosities (~30 %) in south-western parts of the

GN10M1 volume, where burial depths are in the order of 3 km (Fig. 1), compared to average porosities ~20 % in central parts around well 31/2-1 (Fig. 6A, B), where burial depths are in the order of 2 km, contradicts the general trend of porosity decreasing with depth, as recorded in well data. The observed increase in porosity with depth in some areas is due to lateral, basinward change in sedimentary facies. The high porosity NNW-SSE elongated feature (Fig. 13) is interpreted as sandy spit bar deposits. 4.2.3. Faults Troll East and West reservoirs are separated and held in rotated fault blocks, delimited along the NeS striking major faults Tusse and Svarta, formed during the late Jurassic rift phase (Fig. 1). These faults display large throws and juxtapose the Johansen Fm. towards the Cook and Drake formations, as well as the underlying Statfjord Gp (Fig. 7A). Between the Svarta and Tusse faults there is a series of NWeSE striking faults, with limited extent and small throws that do not cause segmentation of the Johansen Fm. (Fig. 7B).

Fig. 11. Seismic attribute maps generated from the merged seismic volume GN10M1 (modified from Gassnova, 2012). The data location is given in Fig. 3B, as the outlined polygon of GN10M1 for the maps in A, B and C and in-line 1757 for the corresponding cross sections A0 , B0 and C0 . (A) RMS (Root Mean Square) amplitude map, displaying property distribution on a surface 25 m below the top Johansen Formation. Note an elongately shaped, NNW-SSE oriented high amplitude anomaly. The associated cross section in (A0 ) shows the variation in seismic signal and reflectivity from east to west. The vertical exaggeration is 5
. (B) Acoustic impedance map, representing average values throughout the Johansen Formation volume. The south-eastern part of the volume displays higher values (warm colour schemes). In the cross section (B0 ) the corresponding EeW variation is displayed. (C) Density map, representing average values within the Johansen Formation volume. Higher values are observed in the SeE corner of the GN10M1 volume. In the associated cross section (C0 ) the EeW variation is displayed.

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Fig. 13. Porosity trends intra Amundsen and Johansen formations are displayed as slices through the GN10M1 volume (Fig. 3B) calculated from acoustic impedance data and basic lithology classes (sand, shale). The layers are drawn relative to the interpreted formation tops and represent the development of the Johansen sequence through geologic time (PS 16). Modified from Gassnova (2012). (A) Middle Lower Amundsen Formation (B) Top Lower Amundsen Formation (C) Lower Johansen Formation (PS1) (D) Lower Johansen Formation (PS2) (E) Middle Johansen Formation (PS4) (F) Middle Johansen Formation (PS5) (G) Upper Johansen Formation (PS6) (H) Top Johansen Formation.

Within the GN10M1 seismic cube the westward dipping Svarta and Tusse faults are observed to die out towards the south (Fig. 1). The Svarta Fault appears to be segmented towards the south (Fig. 1). The NS trending Vette Fault, more or less delineating the TNE01 and SG9202 datasets towards the east, dies out towards the south at around 61.5oN. These main faults are sub-parallel with the Øygarden Fault Zone. Major movements took place after the deposition of the Sognefjord Fm. (Late Jurassic) (e.g. Badley et al., 1988; Færseth, 1996; Sømme et al., 2013). A group of faults with similar strikes, NWeSE, dipping both north- and southwards with relatively small throws compared to the main faults Svarta, Tusse and Vette, form horst and graben structures. They terminate below the base of Cretaceous and seem to be associated with Late Jurassic rifting, having initiated post deposition of the Dunlin Gp. These are oriented oblique to the Svarta and Tusse faults (Figs. 1, 7B). Within the Johansen Fm., and parallel to depositional dip, some small scale syn-sedimentary slumping with displacements on single reflector scale, are evident.

There is no definite indication of fault movement during the deposition of the Johansen Fm. In Fig. 14A, the same in-line as used to illustrate seismic attributes (Fig. 11), has been exaggerated on the vertical scale by 20
, and flattened from the overlying top Brent Gp. reflector (Fig. 14B) in order to examine the thickness variation within the Johansen Fm. (and within the Dunlin Gp.) across faults. In this area, the Johansen Fm. is down-faulted towards the west by the major NeS striking Svarta and Tusse faults, and two minor, semi-parallel segments forming a horst. There is a gradual increase in thickness of the Johansen Fm. across all four faults, from east to west. The high amplitude seismic reflection (i.e. the NWeSE oriented elongated area of seismic high reflectivity facies FA 2) discussed in Chapter 4.2.2, is cross cut in this section. It is evident that the abrupt change from seismic facies FA 2 to chaotic seismic facies FA 3 from east to west happened independent of the faults, and that these were activated later (in connection with the Late Jurassic rifting). Repeated concave, cuspate reflection signatures are observed in deeper parts in both

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Fig. 14. (A) Faults and structural setting shown across an EeW oriented seismic cross section from GN10M1 along in-line 1757 (Fig. 3B). Note 20
vertical exaggeration (corresponding for comparison to the less exaggerated cross sections given in Fig. 11). The Johansen Formation is down-faulted towards the west by major NeS striking faults; Svarta and Tusse, and two minor segments forming a horst. There is a change in seismic facies from chaotic (seismic FA 3) in the east, via high amplitude (seismic FA 2), to chaotic (seismic FA 3). There is a general thickening trend in the Johansen Formation towards the west. (B) In-line 1757 flattened from the top Brent Group reflector. Note the gradual increase in thickness of the Johansen Formation across all four faults, from east to west. There are repeated concave, cuspate reflector signatures in deeper parts, above some underlying rotated fault block geometries, as indicated with dotted lines.

cross sections (Fig. 14AeB) above underlying rotated fault block geometries. These observations are consistent throughout the southern half of the GN10M1. In the central part of the study-area (around tie-well 31/2-1), underlying geometries are slightly more complicated. 5. Discussion It has previously been suggested that the Johansen Fm. was formed as a wave dominated delta prograding mainly north-

westwards from mainland Norway (Marjanac, 1995) during a fall in relative sea level, with constricted deposition in a broad, incised valley (Marjanac and Steel, 1997). From the seismic data set applied in the present study (Fig. 3B) it is evident that delta progradation has taken place towards NW, W, SW and S in central parts of the study area with an overall westerly direction of advancement of the delta complex. However, no indication of any major incised valley beneath the Johansen Fm. has been recorded in the present data set. These observations, as well as the recognition of a distinct NWeSE oriented land-detached, elongate sandstone body and the

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present interpretations of depositional facies with their lateral variation justify a revision of the sedimentary architecture and depositional model of the Johansen Fm. 5.1. Depositional architecture and sequence stratigraphy 5.1.1. The Statfjord Gp. e Dunlin Gp. boundary The lower boundary of the Amundsen Fm. is a marine flooding surface (FS) and also a regional transgressive surface (TS) (Charnock et al., 2001; Ryseth, 2001). In areas where the top Statfjord Gp. is represented by a sub-aerially exposed unconformity (SU) (where the marine Nansen Fm. is absent, e.g. Nystuen et al., 1989), the boundary between the Statfjord and Dunlin groups is thus a sub-aerial unconformity superseded by a transgressive surface, a combined SU/ TS-surface (cf. Ahokas et al., 2014a, b). The unconformity above the Statfjord Gp. and beneath the marine Amundsen Fm. (Marjanac and Steel, 1997) may have been formed by forced regression and incision or by normal regression with sediments that have filled the accommodation space, before subsequent sea level rise and deposition of the Amundsen Fm. An erosional surface formed by forced regression may have acted as the floor of an incised valley, or a regional low-relief erosional unconformity, sub-aerial or/and shallow-marine (Zaitlin et al., 1994). The SU/TS surface thus forms a sequence boundary between the Statfjord and Dunlin groups. 5.1.2. The J12 maximum flooding surface (MFS) The MFS J12 lies within the lower Amundsen Fm. (Fig. 2). It marks the peak of transgression, and defines the base of the Johansen sequence. Upwards from MFS J12 a shallowing and regressive succession into the increasingly more sandy deposits of the Johansen Fm. is onset. The Amundsen and Johansen formations are in this way also time equivalents, with the distal, open shelf mud of the Amundsen Fm. deposited in front of prograding sand deposits of the Johansen Fm. as observed in the proximal to distal well correlation panel 1 (Fig. 6A). The lower MFS (J12) is located very close to the Statfjord Gp. e Dunlin Gp. sequence boundary in most wells (Fig. 6). This indicates that accommodation for the sandy Johansen delta complex was created very fast after the initial flooding of the Statfjord Gp. at the base of the Amundsen Fm. 5.1.3. The Johansen Fm. e Amundsen Fm. boundary The Johansen and Amundsen formations are in part time equivalents, and the boundary between the formations is lithos
, tratigraphic and defined by facies transitions (Vollset and Dore 1984). The nature of this boundary is observed in this study as either conformable or erosional contacts between the distal, muddy open shelf or pro delta facies of the Amundsen Fm. (log FA 5, 6) and the more proximal, sandy delta front or turbidite facies of the Johansen Fm. (log FA 3, 7) (Fig. 6). The most common conformable contact relations between the Amundsen and the Johansen formations are 1) thin-bedded sandstones of lower delta front facies overlying pro-delta or open shelf mudstone of the Amundsen Fm. at the base of an upward coarsening prograding succession (e.g. well 31/3-1, Fig. 6A), and 2) thin turbidite sandstone beds interfingering with Amundsen mudstone (e.g. well 35/11-5, Fig. 6C). These two contact types are in this study interpreted as formed by progradation of the Johansen delta complex into a basin dominated by mud sedimentation, represented by the Amundsen Fm. The third contact type is erosional and recognised as 3) thick turbidite or other types of gravitational sandstone units with sharp contact on top of Amundsen mudstone. Such unconformable, sharp lithological contacts between thick sandstone units and underlying Amundsen mudstone (e.g. wells 35/10-2, 35/11-6, 35/11-2 and 35/ 11-1, Fig. 6C, D), was by Marjanac (1995) inferred as formed

subsequent to fall in sea-level and incision into the Amundsen Fm. and locally also into the underlying Statfjord Fm. The thick sandstone units overlying the unconformity were interpreted as delta plain deposits of the Johansen Fm., filling in the inferred deep incised valley. Charnock et al. (2001) interpreted the sandstone units above the Amundsen shales to be tidally influenced, estuarine sandstones on top of shelf deposits. The regional unconformity with the alleged incised valley was also reconstructed on palaeotopographic maps (Marjanac and Steel, 1997). There is no indication in the seismic surveys of any deep incision in the Amundsen and Statfjord formations. An incision in the order of about 200 m followed by a corresponding rise in sea level during the short time of formation of the Johansen sequence must have been forced by block faulting. No tectonic activity in this scale has been proposed for this area in Early Jurassic. In our model, the Johansen Fm. is interpreted as formed by ordinary deltaic deposition succeeding a rise in relative sea level after the transgression of the Statfjord Gp. Thus, the sharp boundary between open-shelf Amundsen mudstone and overlying sandstone units in the north-western, distal position of the Johansen delta complex (Fig. 6BeD) represents, according to our interpretation, the contact between marine mud and turbidite fan, lobe and channel sand deposited in front of the prograding delta e not the infill of an incised valley by delta plain or estuarine sediments. 5.1.4. The maximum regressive surface (MRS) The MRS is located within or at the top of the Johansen Fm. and represents maximum basinward progradation of the Johansen Fm. marking a shift in the regressive-transgressive Johansen sequence. The surface is interpreted as conform in most distal parts (towards west and north-west in well correlation panels 1e4, Fig. 6), but diachronous where sub-areal exposure, erosion and/or sediment bypass occurred in the proximal delta plain area (e.g. well 31/3-3, Fig. 6A). The paleogeographical map representing the Pliensbachian depositional environment and maximum extent of the Johansen Fm. outlined by Husmo et al. (2003) (Fig. 15A) is in accordance with the work of Steel (1993). The facies distributions interpreted in this study differ from previous models, and are shown in Fig. 15B. Fig. 15 forms the basis for further discussions on facies distributions and comparisons with previous work. The wells on the facies distribution map (Fig. 15A) are colour coded according to the interpreted facies at the intersecting the MRS line (log FA 1e9, Fig. 6), and with additional observations of seismic facies distributions and observations of pinch outs (Figs. 7, 9 and 11), facies transitions were outlined. Some genetic elements included here differ from previous models and maps; turbidites (log FA 7, Fig. 6) and spit bar deposits (attribute analysis, Figs. 11 and 13). Facies development in space and time is explained as part of the depositional model (Chapter 5.2). 5.1.5. Flooding surfaces (FS) and parasequences The Johansen Fm. displays a progradational to retrogradational signature from east to west, with down-lapping clinothems thinning basinwards (Fig. 6A) as also recognised by Steel (1993). The thickness of the clinothem set (up to ~140 m not considering compaction, e.g. Fig. 7A) indicates the minimum paleo-water depth and hence the space of accommodation at the onset of the delta progradation. Parasequences are defined by marine flooding surfaces representing increased water depth (Van Wagoner et al., 1988). Correlations of bounding surfaces and parasequences in the stratigraphic interval from MFS J12 to J14 (Fig. 2) varies between previous studies (e.g. Steel (1993), Marjanac (1995), Marjanac and Steel (1997), Charnock et al. (2001) and Husmo et al. (2003)). Although not correlated between wells in this study, several flooding surfaces (FS) are recognised within the Johansen sequence. Within the Johansen Fm., some calcite

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cemented layers in the upper delta front to delta plain facies (log FA 9 on top of log FA 1, 2, 3 or 4) are interpreted to reflect calcareous transgressive lag deposits (e.g. well 31/2-1, Fig. 6A). In more distal parts (e.g. well 31/1-1, Fig. 6A), flooding events are marked by mud draping (log FA 6 on top of log FA 4 or 5) associated with deepening, and/or marly deposits (log FA 9 on top of log FA 4 or 5) associated with clastic sediment starvation. The number and distribution of calcite cemented layers between wells seem uneven, possibly reflecting different origins and processes; the combination of primary patchy separate shell banks, frequent delta lobe shifts, subsequent erosion and reworking and secondary distribution patterns of calcite due to variation in dissolution and cementation during burial diagenesis. The layers are thus likely to display varying geometries and lateral extents (i.e. from meters to kilometre scale) (Bjørkum and Walderhaug, 1990). The clinoforms of the prograding delta appear to be quite steep (e.g. between well 31/2-3 and 31/2-4, Fig. 7A), implying that the regional extent of individual flooding surfaces may have been restricted and that they do not necessarily correlate between wells (>6 km). Cemented transgressive lags may also be confined to the base of channels at the delta plain, where they are not laterally extensive. Calcrete in soil profiles may also give rise to carbonate cemented layers in delta plain deposits, for example in fluvial channel sand€lt, 1995). However, calcrete nodstone bodies (e.g. Nystuen and Fa ules disappear from the Upper Triassic to the Lower Jurassic in the northern North Sea area in response to a change from semiarid to humid climate (Nystuen et al., 2014). 5.1.6. The J14 maximum flooding surface (MFS) The MFS J14 marks the upper boundary of the Johansen sequence and the peak of transgression, and occurs within open shelf deposits (log FA 6) of the upper Amundsen Fm. (Figs. 2 and 6). Clear pinch-outs of the Johansen Fm. are observed in the seismic lines (Fig. 9), which in combination with the interpreted spit deposits indicate south-ward progradation of the Johansen delta complex, in addition to the northwestwards component described in previous studies (e.g. Marjanac and Steel, 1997). Generally, the relative thickness between the MRS and MFS J14 decreases landward (Fig. 6A), reflecting the direction of flooding. In some eastern parts the upper Amundsen Fm. is not present, and the upper boundary of the Johansen sequence is an unconformity (e.g. well 31/6-1, Fig. 6B). The spatial depositional architectural elements and sequence stratigraphy discussed above form the basis of the model of the development of the Johansen delta complex in time and space. 5.2. Depositional model 5.2.1. Progradational phase: advancing delta plain The lower part of the Johansen sequence between the MFS J12 and the maximum regression surface (MRS) (Fig. 6AeD), represents a prograding deltaic system, in which the sediment input exceeded the generation of accommodation and caused shoreline regression and a basinward shift in facies belts. As a result, facies associations of increasingly proximal character were stacked on top of each other as the depositional system advanced seaward. The upward coarsening successions are interpreted to represent clinothems bounded by flooding surfaces. A high degree of amalgamation of sandstone units indicates limited accommodation or/and a high rate of sediment supply, whereas thin, fine-grained deposits in the deeper parts of the basin in front of the advancing delta front reflects sediment starvation. During deposition of the lower prograding part of the Johansen Fm the palaeoshoreline was located somewhere slightly west of well 31/2-2, as seen from the distribution of delta plain channel deposits (log FA 1), and abrupt facies change to deeper water

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settings towards the west (Fig. 6A). The initial phase of delta progradation brought large volumes of sandy deposits into central parts of the study area. Flood plain facies (log FA 2) is missing in this interval (Fig. 6). Fluvial sand may have been reworked into amalgamated beach ridges. However, the sand-dominated facies may also be attributed to highly mobile distributary braided streams; just leaving a rather uniform sand blanket on the delta plain. All well sections (Fig. 6) indicate abrupt deepening towards west and north-west. The gravitational deposits (Figs. 6C, D and 16A) are interpreted to be sourced from shallower, more proximal settings, due to sediment bypass from fluvial to submarine turbidity current transport, or by collapse of delta front sediments, including mouth bar and shore face deposits (Fig. 16A). There is large variance in accumulated sediment thicknesses in this area (Fig. 6C, D), which may be due to inherent basin topography, e.g. larger accommodation above deep lying basement than above shallow lying basement blocks (Fig. 14). 5.2.2. Aggradational phase: delta plain, spit and lagoon development The middle part of the Johansen sequence comprises the MRS and represents the farthest basinward location of the shoreline (Figs. 15B and 16B). Delta plain deposits (log FA1) are recognised as far west as in well 31/2-5 (Fig. 6A). Aggradational stacking patterns are characteristic for this depositional phase (e.g. well 31/3-3, Fig. 6A). The thick channel features identified in the seismic data (Fig. 12) are interpreted to represent anastomosing or meandering river systems on the delta plain, being stationary over long periods of time. Lack of coal beds may indicate that the delta plain was too dynamic, for example due to frequent floods, for swamps to establish and accumulate plant debris. However, the presence of coalified plant fragments (Fig. 4) indicate that swamps may have developed in a coastal plain setting not preserved within the study area. Stationary channels and aggradational stacking patterns imply that sediment supply was close or equal to the generation of accommodation. Sandy beds often appear amalgamated in the most proximal wells, with only occasional floodplain deposits (log FA 2) and/or flooding surfaces (often calcareous lag deposits) preserved. This phase of delta plain aggradation correlates with delta front deposits (log FA 4 and 5) in the more distal wells, and with continued deposition of delta front to pro-delta turbidites (log FA 7) in distal northern wells (Figs. 15B and 16B). A contrast in depositional environments within lower delta front to prodelta regimes from north to south are revealed by the wells 30/3-3 at 60 460 1100 N (Fig. 6A) and 35/10-2 at 61020 2300 N (Fig. 6B), about 30 km apart (Fig. 3A). Both wells are located at  about 3 E, approximately along depositional strike of the westerly prograding delta complex (Fig. 15B). Well 30/3-3 is characterized by prodelta and open shelf mudstone between the J12 and J14 MFS's and well 35/10-2 by turbidite fans, lobes and channels, interfingering with mudstone in the same interval. This model for the Johansen delta complex (Figs. 15B and 16B), including turbidite facies (log FA 7, Fig. 6C, D), differs from previous models which interpreted sandy deposits at 3 E as subaerial delta plain deposits (Fig. 15A) (e.g. coastal-alluvial plain (Husmo et al., 2003), estuarine infill (Charnock et al., 2001)). In comparison the total advancement of the palaeo-coastline is less in this model (Fig. 15B). The NWeSE elongated sand body identified within the seismic volume GN10M1 (Figs. 11 and 13) is interpreted as spit bar deposits (Fig. 16B). Steep, downlapping clinoform geometries are observed basinwards (west) in northern parts, whereas the depositional dip along the NWeSW long axis appears to have been very low. The overall morphology and clinoform geometry is consistent with spit deposition and indicates a balanced or slight surplus in sediment supply relative to generation of accommodation in comparison

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Fig. 15. Facies distribution maps for the northern North Sea as interpreted for the Pliensbachian (190-183 Ma). (A) Facies distribution map for the Johansen Fm. modified from Husmo et al. (2003). (B) Facies distribution map for the Johansen Fm. at the stage of maximum regression, as interpreted in this study. The wells (Fig. 3A) are coloured based on the interpretation of the log facies association (FA) at maximum regression (MRS), and as displayed in Fig. 6. The polygon delineating the velocity model is included for reference (Fig. 3B). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

with models by Nielsen and Johannessen (2008). Based on the seismic signature, aggradational nature and its large total thickness (Figs. 7A, 11e13) it seems likely that the sand spit bar formed by means of southward progradation from a relatively stationary anchor point at a river mouth. If the spit was formed solely by “beach detachment” processes (e.g. Hoyt, 1967; Swift, 1975; Martin and Dominguez, 1994) during late transgression, as many modern sand spits, prograding beach ridges would expectedly be present below thin lagoon deposits; however, this is not consistent with our observations. The anchor point must have been established sometime after the main stage of the delta progradation, when the shoreline reached its furthest westerly location (Figs. 15B and 16B). Strong NeS alongshore currents must have been dominant in order to transport large volumes of sand from the river mouths (Fig. 16B). This signifies a marked increase in sediment supply and maintenance of the delta plain relative to sea level, with sediment bypass to the shoreline and the spit system and the filling of an increasing accommodation towards the south. Differential compaction of Triassic sediments above rotated Permian fault blocks (Fig. 14) may have contributed to local accommodation (Fig. 16B). Brackish water conditions in part of the Johansen sequence, as

suggested by Steel (1993) and Bell et al. (1984) in well 31/6-6, are interpreted as part of lagoonal log FA 8 (recognized in wells 31/6-1, 62 and 6-6, Fig. 15B) and corresponding to chaotic seismic facies FA 3 (Fig. 10). A partially protected lagoonal bay is thus included in the depositional model (Fig. 16B). A spit bar or a barrier island is a premise for a coastal lagoon (Martin and Dominguez, 1994). In this environment clay-rich sediments from suspension fallout would be mixed with fluvial sand filling the lagoon from the north, and from washover-processes east to west. There is some indication (e.g. Fig. 12A) of a tidal inlet intersecting the spit in an east-west direction, with accumulation of an early stage flood tidal delta, which would be plausible in a lagoonal environment (Fig. 16B). The coastal belt of sand and progradational patterns in the Johansen Fm. is indicative of a main sediment distributing locus in the EeNE (Fig. 15). It is likely that the fluvial Sognefjorden drainage system was already established at this time (Holgate et al., 2013) and provided an important sediment source to the deltaic system of the Johansen Fm. The southward pinch-out of the Johansen Fm. delta complex corresponds with the southwards pinch-out of the Oxfordian, deltaic Sognefjord Fm. (Fig. 9), which was controlled by sediment supply, and possibly subsidence and deepening towards the Stord Basin. The present hinterland bedrock geology of the

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Sognefjord drainage system is in accordance with the known mineralogy and petrography in the Johansen Fm. and is also interpreted as the main provenance in succeeding Jurassic deltaic successions on the Horda Platform (e.g. Ramm, 2000; Dreyer et al., 2005). It is likely that there were several sediment sources and coeval, local deltaic systems along the coast, as indicated in the area of well 35/9-2 (Figs. 3A and 15B). North-south oriented spit and/or barrier deposits associated with delta progradation have been suggested also to be present elsewhere in the northern North Sea area; in the southern part of the Viking Graben during the Bathonian e Lower Oxfordian (Varadi et al., 1998); and on the Horda Platform during the Oxfordian; i.e. in the Krossfjord Fm. (Holgate et al., 2013) and in the Sognefjord Fm. (Dreyer et al., 2005). Westward progradation and structural setting vary between the locations. These examples indicate that the sandy sediment distribution on the Horda Platform sourced from the Sognefjord drainage area during the Jurassic may have been controlled mainly by marine currents acting upon prograding deltas. In turn, the currents must have been controlled by size, geometry and bathymetry of the marine basin besides prevailing wind systems and other climatic mechanisms acting at this time in northern Europe. 5.2.3. Retrogradational phase: rapid transgression The uppermost part of the Johansen sequence, above the MRS, displays a retrogradational stacking pattern and is bounded on top by MFS J14. During these stages of deposition the shoreline was retreating landward (Fig. 16C). The retrogradational sediment succession in the central parts of the study area (Fig. 6A, B) is relatively thin compared to the aggradational package, thus implying rapid transgression or/and decreased rate of sediment influx after a long period of a relative stable shoreline and balance between rates in creation of accommodation versus rate in sediment supply during the aggradational phase. During rising sea level and transgression, increasing amounts of sandy sediments may have been deposited into the lagoonal bay by wash-over processes during storm events and through tidal inlets, steadily supplied by sand along the shoreface of the spit by coast parallel reworking processes. A thick spit bar has a high preservation potential if it is quickly submerged during transgression and engulfed in mudstone deposits (Johannessen and Nielsen, 2006). This is a likely scenario in the case of the later transgressive stages in deposition of the Johansen Fm. (Fig. 16C). Indication of thick, muddy fill in the lagoonal setting are confirmed in seismic data (e.g. Figs. 11C and 13EeH). The retrogradational stage was terminated by deposition of open shelf mud referred that is referred lithostratigraphically to the upper part of the Amundsen Fm. The following Cook Fm. represents a new phase of sand influx to the Horda Platform area. 5.3. Reservoir characterization The depositional model forms the basis for reservoir characterization of the Johansen Fm. as a potential sequestration site for CO2. If the Johansen Fm. is selected as a future storage reservoir for CO2, a suitable injection point must be selected. This would be a decision for the dedicated operator and the authorities, but considering the geological model proposed here, and given a premise of zero interference with ongoing hydrocarbon production at the nearby Troll Field, a suitable choice would be to place an injection well in the elongated, high porosity sandstone body far south from the Troll reservoir (Figs. 7, 11), interpreted here to have formed as a spit bar. The Johansen Fm. sandstones north of Troll are in parts buried too deep for economic feasibility (~4 km), or situated in the vicinity of the Fram Field. This region is densely faulted, providing high risk with respect to CO2 storage.

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Based on well data, the reservoir quality in central, proximal parts appears to be good, with thick (20e100 m), amalgamated and aerially extensive sandstone bodies (Figs. 6AeB, 15, 16) displaying porosities and horizontal permeabilities up to 32 % and 5000 mD, respectively (Appendix 2). The main challenge in selecting the spit bar deposits as the locus for injection and immediate reservoir is the lack of well data. In evaluation of the reservoir quality in this area, which at this point relies mainly on geophysical attributes, it is necessary to interpolate the reservoir characteristics according to inferred lateral facies changes as well as an increased burial depth. Slight differences in reservoir properties between facies are mainly related to grain size and shape. In fine grained sandstone layers higher contents of clay and oriented, elongated flakes of mica and coal fragments are likely to reduce the vertical permeability (e.g. Fig. 4C). Extensively reworked sand deposits in the high energy environment of a spit bar complex are, however, expected to be fairly well sorted and mature, with low preservation potential for aerially extensive mudstones (Martin and Dominguez,1994; Dreyer et al., 2005; Nielsen and Johannessen, 2009; Eide et al., 2014). Lithologic layers of low permeability may form intra-reservoir flow barriers to buoyant CO2. Measurements show that the permeability in calcite cemented layers in the Johansen Fm. is close to zero (Appendix 2). If present and laterally continuous, such layers may affect CO2 injectivity and distribution patterns in the reservoir, as shown in fluid flow models for the Johansen Fm. (Sundal et al., 2015). The temperature and pressure conditions in central parts of the study area, at 2.1 km depth on the Horda Platform, is in the order of 65  C and 210 bar, respectively. Assuming hydrostatic pressure and an average geothermal gradient of 28.8  C, the in situ temperature and pressure in the potential storage reservoir in the sandstone spit bar, at approximately 3 km burial depth, would be in the order of 86.5  C and 300 bar, respectively, not considering differential uplift. With bulk rock quartz contents in the order of 60 %, we estimate a porosity reduction of maximum 8 % due to quartz cementation at 3 km burial (Walderhaug, 1996; Marcusssen et al., 2010), relative to core observations at 2.1 km (in well 31/2-3, Fig. 5A, Appendix 3, 4). With widespread chlorite coatings preventing overgrowth, however, the chemical compaction is probably much less. Illitization of kaolinite in the presence of potassium feldspar is known to cause severe reduction in macro porosity and permeability, but is unlikely to be significant at a reservoir depth of 3 km (Ehrenberg and Nadeau, 1989; Bjørlykke et al., 1992; Ramm, 2000). Sub-seismic faults are likely to be present within the immediate reservoir sandstone. Deformation bands are known to occur in poorly consolidated, highly permeable sandstones, and may form flow baffles, especially if the timing of deformation and reservoir conditions (>90  C) allows for preceding cementation (Fossen et al., 2007). If quartz cementation is prevented by chlorite coating, as is likely in the Johansen Fm., preferred quartz cementation may have occurred mostly along deformation bands where fresh, reactive mineral surfaces are exposed due to grain crushing (Fossen et al., 2007). 5.3.1. Effect of trapping mechanisms for CO2 In evaluating the reservoir quality for CO2 storage, the effect of trapping mechanisms (i.e. structural/stratigraphical, residual, dissolution and mineralization) for CO2 must be considered (IPCC, 2005). The storage security will increase with time according to the amount of CO2 immobilised as residual bubbles, dissolved in formation water or precipitated as carbonates (Fig. 17A). Under the inferred reservoir conditions at 3 km depth CO2 would be in a supercritical state. With available volumes for pressure migration and aquifer compression much larger than the immediate reservoir,

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Fig. 17. (A) General representation of the relative effect of trapping mechanisms over time, as presented by the International Panel for Climate Change (IPCC, 2005). Storage security increases with increasing amount of CO2 in progressively more immobile phases, from initial free gas phase, to capillary trapped, residual bubbles in swept reservoir volumes, and CO2 dissolved in formation water, and finally completely immobilised as mineralized carbon. (B) A modified representation of the relative effect of trapping mechanisms as would be expected in a heterogeneous, dynamic reservoir such as the Johansen Formation, in which the reservoir sweep is expected to be large; enhancing residual and dissolution trapping (e.g. Sundal et al., 2015). Geochemical studies show that mineralization is likely to be onset rather early (already at 10 years) at the prevailing reservoir conditions and yield significant contribution to the trapped fraction in the Johansen Formation (Sundal et al., 2014).

the potential for storage is quite high (e.g. 150 Mt, Halland et al., 2011). In fluid flow simulations by Eigestad et al. (2009) storage capacity for injecting at least 3.5 Mt/CO2 for 50 years was found feasible. Multi-phase flow experiments show that in a clean sandstone with 25% porosity, the capacity for residual trapping could amount to 8% of the reservoir volume swept by CO2 (Lu et al., 2013). Within the Johansen Fm. it is probable that the capacity for residual trapping is high. Residually trapped CO2 will gradually dissolve in formation water. With regards to formation water chemistry, there is currently no data available from the Johansen Fm. In pore water samples from different formations (i.e. Statfjord, Fensfjord), at similar depths in nearby fields (i.e. Oseberg, Brage), total dissolved solids (TDS) are in the order of 20e70 g/l (Egeberg and Aagaard, 1989). This is relatively low and thereby provides higher capacity for dissolution of CO2. Assuming 35 g/l TDS at reservoir conditions, CO2 would have a solubility in the order of 1.18 mol/l or 10.4 kg/m3 rock. Geological heterogeneities as observed in the Johansen Fm. may be advantageous, in that they contribute to spreading the CO2 plume and thereby increase the effect of trapping mechanisms along the migration path, thus providing better storage security (Hovorka et al., 2004). The potential for immobilizing CO2 by dissolution and residual trapping in the Johansen Fm. may be as high as 50% of injected fluid within relatively short time frames (150 years) (Sundal et al., 2015). Some bulk estimates for the mineral trapping potential in the Johansen Fm. show that chlorite and albite are the most important contributors of cations in precipitated carbonates. The CO2 trapped in solid phase has been estimated to 1.7 mol/l after 1 year and up to 5 mol/l after 1000 years, corresponding to 50 kg CO2 stored per m3 rock at 20% porosity (Sundal et al., 2014). Note that this capacity is high in comparison to solubility estimates, and that the relatively early onset of carbonate precipitation would drive further dissolution.

Based on the above considerations, providing that pressure conditions are manageable and top seal secure, the reservoir quality of the Johansen Formation seems quite suitable for CO2 storage. The reservoir may be characterized as a dynamic reservoir, in which migration is a predicted mechanism for increasing the storage security (Fig. 17B).

6. Conclusions  A revised depositional model for the Johansen sequence is proposed based on interpretation of new seismic and mineralogical data. In this study the Johansen Fm. is characterised as a suitable reservoir with respect to CO2 storage.  The Johansen sequence, comprising the lower/upper Amundsen and Johansen formations, displays a combined SU/FS/MFS sequence stratigraphical lower boundary with the underlying Statfjord Gp.  The sandy Johansen Fm. was deposited as a delta system, prograding EeW and filling in the accommodation created after a rather abrupt rise in relative sea level, overlying and interfingering with overall transgressive, deep marine mudstones of the Amundsen Fm.  During the middle, aggradational phase of deposition, strong NeS oriented, alongshore currents caused sediment bypass and reworking on the delta front, with resulting sand deposition in a NNW-SSE oriented spit bar. A protected lagoon environment was established east of the spit, with brackish water conditions and deposition of heterolithics. Turbidites sourced from collapse of the delta front were deposited pro delta and on the lower delta front towards the north and west.  At the latest depositional stage of the Johansen Fm., rapid transgression and retrogradation prevented significant erosion, thus engulfing and preserving the spit deposits.

Fig. 16. Depositional model for the development of the stages for Johansen Formation progradation and retrogradation during Sinemurian-Pliensbachian (A) The initial, progradational phase. A delta builds into and fills increasing accomodation towards the west. Steep clinothems and a strong current regime at the delta-front cause sediment bypass and turbidite deposition in deeper settings. (B) The middle, aggradational phase. The delta plain is interpreted to have reached its maximum, basinward position, and balance between sediment input and accomodation is reached, with sand being transported from the river mouths and accumulating down-current, building into an elongate spit bar. Behind the spit a protected lagoon is formed, with sand introduced from washover processes in the west and from the deltaeplain in the north. (C) The final, retrogradational phase of deposition, during which rapid transgression causes back-stepping of depositional environments, submerging underlying sand-deposits and subsequent deep-water deposition engulfing the Johansen Fm. in marine mud of the overlying Amundsen Fm.

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 On a regional scale accommodation was created by post-rift cooling, subsidence and rising sea level. Locally, accommodation was created partly by inherent basin geometries, with differential compaction of different sediment thicknesses overlying rotated Permian fault blocks.  Repetitive pattern in spatial facies distributions on the Horda Platform (i.e. asymmetric, wave and fluvially dominated deltas with attached, down-current spit bar deposits) throughout the Jurassic is indicative of climate-controlled sediment distributions.  The Johansen Fm. would provide a suitable reservoir for CO2, with inferred high porosity and permeability in the spit bar deposits, ideal P, T conditions and high potential for trapping and immobilization of CO2. Acknowledgements The authors greatly appreciate the cooperation with Gassnova AS, which provided access to seismic data and reports. Also, we would like to thank Jan Inge Faleide, Trude Ravn, and Ivar Midtkandal for fruitful discussions, and Statoil AS for granting access to core material. Technical assistance, lab- and software related, was kindly provided by Berit Løken Berg, Michel Heeremas, Mufak Naoroz, Muriel Erambert and Maarten Aerts. This work has been partly funded by the University of Oslo and the SUCCESS centre for CO2 storage under grant 193825/S60 from the Research Council of Norway. SUCCESS e SUbsurface CO2 storage e Critical Elements and Superior Strategy is a consortium with partners from industry and science, hosted by Christian Michelsen Research, and is one of the centres for Environmentfriendly Energy Research (FME) assigned by the Norwegian Research Council. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.marpetgeo.2016.01.021. References Ahokas, J.M., Nystuen, J.P., Martinius, A.W., 2014a. In: Depositional Dynamics and Sequence Development of the Paralic Early Jurassic Neill Klinter Group, Jameson Land Basin, East Greenland: Comparison with the Halten Terrace, Mid Norwegian Continental Shelf. Ahokas, J.M., Nystuen, J.P., Martinius, A.W., 2014b. Stratigraphic signatures of punctuated rise in relative sea-level in an estuary-dominated heterolithic succession: incised valley fills of the Toarcian Ostreaelv Formation, Neill Klinter group (Jameson Land, East Greenland). Mar. Petrol. Geol. 50, 103e129. Badley, M.E., Price, J.D., Dahl, C.R., Agdestein, T., 1988. The structural evolution of the Northern Viking Graben and its bearing upon extensional modes of basin formation. J. Geol. Soc. 145, 455e472. Bhattacharya, J.P., 2010. 10. Deltas. In: James, N.P., Dalrymple, R.W. (Eds.), Facies Models 4, Geotext 6. Geological Assocation of Canada, pp. 233e264. Bell, D.G., Bjærke, T., Skarbø, O., 1984. Biostratigraphy Kerogen Analysis. Stratlab for Statoil (Report). Bertram, G.T., Milton, N.J., 1988. Reconstructing basin evolution from sedimentary thickness; the importance of palaeobathymetric control, with reference to the North Sea. Basin Res. 1, 247e257. Bjørkum, P.A., Walderhaug, O., 1990. Lateral extent of calcite-cemented zones in shallow marine sandstones. In: Buller, A.T., Berg, E., Hjelmeland, O., Kleppe, J., Trosæter, O., Aasen, J.O. (Eds.), North Sea Oil and Gas Reservoirs II, Proceedings of the 2nd North Sea Oil and Gas Reservoirs Conference. The Norwegian Institute of Technology, Graham and Trotman, Trondheim, Norway, pp. 331e336. Bjørlykke, K., 1994. Fluid flow processes and diagenesis in sedimentary basins. Geol. Soc. Lond. Spec. Publ. 78, 127e140. Bjørlykke, K., 2014. Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic process in sedimentary basins. Sediment. Geol. 301, 1e14. Bjørlykke, K., Brendsdal, A., 1986. Diagenesis of the Brent sandstone in the Statfjord field, North Sea. In: Gautier, D.L. (Ed.), Roles of Organic Matter in Sediment Diagenesis. Society of Economic Paleontologists and Mineralogists, Tulsa, USA,

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