Significance of fault seal in assessing CO2 storage capacity and containment risks – an example from the Horda Platform, northern North Sea

Long Wu, Rune Thorsen, Signe Ottesen, Renata Meneguolo, Kristin Hartvedt, Philip Ringrose and Bamshad Nazarian
Petroleum Geoscience, 27, petgeo2020-102, 30 March 2021, https://doi.org/10.1144/petgeo2020-102

Abstract

An understanding of fault seal is crucial for assessing the storage capacity and containment risks of CO2 storage sites, as it can significantly affect the projects on across-fault and along-fault migration/leakage risk, as well as reservoir pressure predictions. We present a study from the Smeaheia area in the northern Horda Platform offshore Norway, focusing on two fault-bounded structural closures, namely the Alpha and Beta structures. We aim to use this study to improve the geological understanding of the northern Horda Platform for CO2 storage scale-up potentials and illustrate the importance of fault seal analysis in containment risk assessment and storage capacity evaluation of a CO2 storage project. Our containment risk assessment shows that the Alpha structure has low fault-related containment risks; thus, it has a potential value to be an additional storage target. The Beta structure shows larger fault-related containment risks due to juxtaposition of the prospective storage aquifer with the basement across the Øygarden Fault System. The storage capacity of Smeaheia will be determined by the long-term dynamic interplay between pressure depletion and recharging. Our study shows that across-fault pressure communication between Smeaheia and the depleting Troll reservoir is likely to be through several relay ramps of the Vette Fault System. However, Smeaheia also shows pressure-recharging potentials, such as through the subcropping areas at the Base Nordland Unconformity. The depletion observed in the newly drilled well 32/4-3S gives a good validation point for our fault seal predictions and provides valuable insights for future dynamic simulations.

Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

The geoscientists in a CO2 storage project must answer two key questions: (1) How much CO2 can be stored? – the storage capacity; (2) Is it safe to store and how can this be demonstrated? – the containment risks (discussed previously by Bachu et al. 2007; Eiken et al. 2011; DECC 2012; Ringrose et al. 2013; Tucker et al. 2013; Pawar et al. 2015). The importance of faults in assessing these two questions for CO2 storage in the subsurface has been widely recognized (e.g. Dockrill and Shipton 2010; Bretan et al. 2011; Yielding et al. 2011; and reviews in IEAGHG 2015, 2016) since they can significantly impact the project in the following ways: (1) across-fault CO2 migration/leakage risk; (2) along-fault CO2 migration/leakage risk; and (3) across-fault reservoir pressure predictions (Fig. 1a).

Fig. 1.
Fig. 1.

(a) A conceptual model of CO2 injection in a storage reservoir with faults. Faults have important roles in controlling the across-fault flows and pressure communication between different fault blocks, as well as along-fault flows to overburden sections or to the surface. (b) A phase diagram of CO2 showing the pressure–temperature conditions for CO2 transportation and subsurface storage examples from offshore Norway (modified from Ringrose 2020). *Note that 800 m is the critical depth for CO2 to maintain its dense form (CO2CRC 2008).

CO2 storage projects have two distinctive features compared to typical projects in the oil and gas industry. One is that CO2 has unique physical properties and flow behaviour, which vary significantly with pressure and temperature conditions (Fig. 1b) (Eiken et al. 2011; Ringrose 2020). CO2 is in the gas phase at normal surface conditions. While for transportation (e.g. by ship) and subsurface storage (e.g. at the Sleipner and Snøhvit fields: Eiken et al. 2011), the CO2 needs to exceed a critical pressure and temperature threshold to maintain its dense form, either as a liquid phase or a supercritical phase (Fig. 1). In this case, to meet the requirements of pressure–temperature conditions, the critical depth for a subsurface storage reservoir is around 800 m (assuming a standard surface condition: 1.013 bara and 15°C). At shallower depths, CO2 will gasify and the volume can expand more than 300 times (CO2CRC 2008). The other distinctive feature is that CO2 storage projects typically need to deal with a c. 1000 year timescale into the future. In contrast, the timescale for hydrocarbon development–production projects is typically c. 20–50 years, and the fluid behaviour can be monitored and validated during the project lifespan. Hydrocarbon exploration projects also work with basin history over a geological timescale, which is millions of years in the past, for which the long-term trap integrity can be directly validated by drilling (e.g. discovery, dry holes, residual hydrocarbons). However, these validation methods (e.g. monitoring or drilling) may not be entirely relevant for CO2 storage projects, considering that the storage timescale is c. 1000 years into the future. Thus, this c. 1000 year forecasting timescale is a new challenge for CO2 storage projects and needs to be considered when evaluating the storage capacity and containment risks.

In support of efforts to reduce CO2 emissions and achieve the global climate goals as defined in the Paris Agreement, the Northern Lights project (https://northernlightsccs.com/en) is under development as Norway's first full-scale carbon capture and storage (CCS) project integrating onshore industrial CO2 capture with integrated solutions for transportation, injection and subsurface storage of CO2. The Sleipner and Snøhvit CCS projects, which involve the removal of CO2 from natural gas, provide useful experience of industrial-scale CCS but are not integrated with the decarbonization of European industry. To fully explore the greater Horda Platform area's storage potential, two candidates have been evaluated for offshore subsurface storage – namely Smeaheia and Aurora. They are both located in tilted fault blocks near the Troll Field area on the Horda Platform, offshore Norway (Fig. 2). The candidates must meet both storage capacity and containment risk criteria to be considered as workable storage sites. As we will show in this study, Smeaheia was found to have unacceptably large uncertainties for use as the initial storage site for the Northern Lights project. Thus, Aurora was selected and is currently under development, and is proceeding as the primary storage target. Nevertheless, Smeaheia remains important as a site for additional volumes because of its scale-up potential and location between the onshore CO2 terminal in western Norway and the Aurora storage site.

Fig. 2.
Fig. 2.

Structural map of the Horda Platform area, showing the main fault systems, intrablock faults, Troll Field (red shaded), the Aurora license boundary and two structural prospects within Smeaheia analysed in this study (i.e. Alpha and Beta: shaded in cyan). Note that the fault traces are mapped on the top Sognefjord Formation level. The dark-grey colour in the index figure (after Færseth 1996) represents the Permo-Triassic rift axis, while the light-grey colour is the Jurassic rift axis.

In this paper, we present our analysis of the structural geology and fault seal of CO2 storage prospects in the Smeaheia area. We also include pressure measurements and a triangle plot from the newly drilled (October 2019) Gladsheim exploration well (32/4-3S in Fig. 2), which provides supporting validation for our pre-drill fault seal prediction and provides valuable insights for future dynamic simulations. We aim to use this study to: (1) help to improve the geological understanding of the northern Horda Platform area for CO2 storage scale-up potentials; and (2) demonstrate the significance of fault seal analysis in the containment risk assessment, as well as in controlling reservoir pressure profile and CO2 storage capacity.

Geological setting

Tectonostratigraphic evolution

The Horda Platform is a north–south-trending structural high along the eastern margin of the northern North Sea, with the North Viking Graben to its west (Fig. 2). The regional setting and tectonic evolution of the Horda Platform and the northern North Sea have been well documented and widely discussed in the published literature and references (e.g. Ziegler 1990; Færseth 1996; Færseth and Ravnas 1998; Odinsen et al. 2000; Bell et al. 2014; Whipp et al. 2014; Duffy et al. 2015; Deng et al. 2017; Fossen et al. 2017a; Phillips et al. 2019).

There are two major extensional events observed on the Horda Platform: the Permo-Triassic rifting (Synrift 1) and the Late Jurassic–Early Cretaceous rifting (Synrift 2: Fig. 3). The Permo-Triassic rifting, as a result of the break-up of Pangaea, affected the entire northern North Sea Basin, with the rift axis beneath the Horda Platform (e.g. Færseth 1996; Odinsen et al. 2000; Bell et al. 2014; Phillips et al. 2019). This rifting event resulted in the formation of a series of easterly-tilted, pre-Jurassic half-graben bound by several north–south-trending, large-displacement normal fault systems (Færseth 1996; Bell et al. 2014; Whipp et al. 2014; Phillips et al. 2019). The north–south-trending strikes of the Permo-Triassic faults offshore and the Permian dykes observed onshore western Norway (Torsvik et al. 1997; Fossen et al. 2017a) suggest an east–west extension orientation during this rifting event.

Fig. 3.
Fig. 3.

Tectonostratigraphic chart of the Troll–Smeaheia area. Well 31/6-6 (see Fig. 2 for the location) is used to show the well–seismic correlation and the seismic stratigraphic framework. *The term ‘synrift’ is used in this study to mark the faulting activities in the Troll–Smeaheia area on the Horda Platform, although Synrift 2 is diachronous across the northern North Sea area (see more details in Bell et al. 2014). Unc., unconformity.

The Jurassic rifting event in the northern North Sea area was diachronous (Bell et al. 2014). To the west of the Horda Platform, the evidence of extensional episodes during the mid-Jurassic has been observed near the central segment of the northern North Sea, especially around the Viking Graben and Sogn Graben areas (e.g. Færseth and Ravnas 1998; Bell et al. 2014; Deng et al. 2017). On the Horda Platform's northern margin, several faults were active in the Uer Terrace area (see Fig. 2) in the mid-Jurassic (from the Bajocian: see Bell et al. 2014). On the western margin of Horda Platform, the Brent Group sequences on the hanging-wall side of the Troll Fault System also show minor fault-controlled thickness changes, suggesting Late Bajocian–Middle Callovian aged faulting (Whipp et al. 2014). However, observations of active faults in the mid-Jurassic have not been reported within the Horda Platform. In this region, from Late Triassic to Late Jurassic times, the Horda Platform was stable and experienced a phase of tectonic quiescence (marked as Post-rift 1 in Fig. 3). Several fluvio-deltaic to shallow-marine systems were deposited during this phase, including the Statfjord Group, the Dunlin Group, the Brent Group and the Viking Group (Fig. 3). In seismic sections, these sedimentary packages are nearly tabular across the Horda Platform (Færseth and Ravnas 1998; Bell et al. 2014; Whipp et al. 2014) (also see Fig. 4).

Fig. 4.
Fig. 4.

(a) Uninterpreted seismic profile from Troll West Oil Province, Troll West Gas Province, Troll East to Smeaheia. The location of this profile is in Figure 2. This seismic line is a composite line from both the CGG and GN1101 seismic surveys. (b) Interpreted profile with well controls and seismic stratigraphic framework shown in Figure 3. The spill-point level of the Alpha structure is marked. TWT, two-way travel time.

During the Late Jurassic–Early Cretaceous rifting (Synrift 2 in Fig. 3), pre-existing north–south-trending major faults were reactivated and formed several half-graben depocentres on the Horda Platform (Bell et al. 2014; Whipp et al. 2014; Duffy et al. 2015) (Fig. 4). The Draupne Formation, which is composed of deep-marine mudstones, was deposited during the early phase of this rifting event, and has shown minor rotated onlaps and thickness changes toward the Tusse Fault (Whipp et al. 2014) (see Fig. 2). The Cromer Knoll Group shows clear wedge shapes and rotated onlaps in the half-graben, which mark the main phase of faulting on Horda Platform (Fig. 4) (Whipp et al. 2014). It is worth noting that the top Cromer Knoll Group used in previous studies (Bell et al. 2014; Whipp et al. 2014) is now reclassified as the top Svarte Formation in this study, based on new well data interpretation and correlation (Fig. 3). Despite the change in stratigraphic allocation, this synrift package mainly comprises shale, marl and some limestones. Numerous NW–SE-trending faults formed within each fault block during this time (Whipp et al. 2014).

After the rifting, the Horda Platform evolved into a post-rift, thermal subsidence phase (Fig. 3). Thick strata were deposited in this area, including the deep-water clastics and carbonates of the Shetland Group, and siliciclastic-dominated Rogaland and Hordaland groups (Figs 3 and 4). The Horda Platform, along with the whole western continental margin, was uplifted and eroded during the Neogene time (e.g. Fossen et al. 1997; Japsen and Chalmers 2000; Baig et al. 2019), and then unconformably overlain by the Nordland Group, which is mainly composed of Quaternary post-glacial deposits in our study area (Figs 3–5).

Fig. 5.
Fig. 5.

A well correlation section with simplified lithology from the Viking Group level to the shale-dominated overburden. It shows the faulting of the Vette Fault, as well as the thickness change in the growth strata. The top Svarte Formation is used here as the datum for wells around the Vette Fault, while it is truncated by the Base Nordland Unconformity towards well 32/2-1.

Troll and Smeaheia

The Troll Field is situated within three main tilted fault blocks on the Horda Platform, and can be divided into Troll East, Troll West Gas Province and Troll West Oil Province (Figs 2 and 3). The main reservoir of the Troll Field is the Viking Group sandstones, including the Sognefjord, Fensfjord and Krossfjord formations (Figs 3 and 4). The Troll Field was discovered in 1979 and has been producing since 19 September 1995. The ‘flat spot’ (the reflection of the hydrocarbon–water contact) can still be observed on the CGG BroadSeis™ seismic data, which was acquired during 2014–2016 (see Fig. 4). The presence of the Troll Field itself demonstrates a sealed system on the geological timescale, from both top seal and fault seal perspectives, although the charging history includes episodes of tilting and hydrocarbon re-migration during the Neogene (Horstad and Larter 1997).

Smeaheia is a fault block immediately east of the Troll Field, bounded between the Vette Fault System (VFS) and the Øygarden Fault System (ØFS: see Fig. 2). Smeaheia shows similar stratigraphy and faulting styles to the Troll fault blocks, while it experienced slightly more uplift and erosion of the overburden than the Troll area (Figs 3 and 5). The main storage aquifers in Smeaheia that we have focused on in this study are the Viking Group sandstones, which contain three shallow-marine sandstone packages – namely the Sognefjord, Fensfjord and Krossfjord formations – interfingering with equivalent shelfal deposits called the Heather Formation (Figs 3 and 4). Other potential storage units in Smeaheia include the deeper sandstones within the Brent, Dunlin, Statfjord and Hegre groups.

We investigated two structural closures at Smeaheia in this study: Alpha and Beta (Fig. 2). These two structures were drilled by the exploration well 32/4-1T2 (in 1996) and well 32/2-1 (in 2008), and no hydrocarbons were discovered. The Viking Group sandstones in well 32/4-1T2 showed hydrostatic pressure. Well 32/2-1 has no pressure measurements from the Viking Group sandstones, with only one pore-pressure measurement from the Ness Formation (Brent Group) showing a hydrostatic pressure. Thus, the sealing potential of these structures cannot be directly proven or disproven by these drilling results. A third important structural closure, the Gamma structure (Fig. 2), was drilled in 2019 (the Gladsheim well 32/4-3S) and was mainly used as a validation point for this study.

Methodology and data

Seismic and well data

In this study, the seismic interpretation was performed using 3D seismic data from both the GN1101 and CGG (BroadSeis™) Northern Viking Graben surveys (e.g. Fig. 4). The CGG survey covers most of the Smeaheia fault block and the whole Troll Field area to its west. The GN1101 survey only covers a limited portion of the Smeaheia area but has more coverage of the basement area to the east of the ØFS than the CGG survey. We used data from several exploration wells, including four wells from the Troll East (i.e. 31/3-3, 31/6-6, 31/6-2 and 31/6-3) and three wells from Smeaheia (i.e. 32/4-1T2, 32/2-1 and 32/4-3S). See the well locations in Figure 2. Well 32/4-3S (namely the Gladsheim well) was drilled in October 2019. The main types of well data used in this study include well logs (e.g. gamma ray), stratigraphic picks and pressure measurements.

Mapping and structural analysis

Seismic-well ties have been performed on all wells in this study. Seismic interpretations and fault mapping were carried out on seismic data in the time domain (e.g. Fig. 4). Multiple horizons have been mapped from the seismic data, while in this study we focus on 10 key horizons that have adequate well control (Fig. 3). We used stratigraphic picks from well logs (e.g. Fig. 5) for correlating and dating the key seismic horizons (e.g. Figs 3 and 6).

Fig. 6.
Fig. 6.

(a) Oblique view of the Vette Fault System, with two fault segments selected for juxtaposition analysis (i.e. VF #1 and VF #2). (b) Map view of the two selected Vette fault segments and locations of three cross-sections. (c) Juxtaposition diagrams of two Vette fault segments, with the footwall side of the Viking Group sandstones (VKS) coloured by different juxtapositions with itself, the Draupne Formation (Drp.), and the Cromer Knoll Group–Svarte Formation (CK  +  Sv); the locations of three cross-sections are also marked. (d)–(f) Three east–west cross-sections through two key wells on Smeaheia and the fault overlap zone; the locations are shown in (b). VF, Vette Fault; IBF, intrablock fault.

The interval velocity information from these wells was used for time–depth conversion of mapped horizons and faults. After the time to depth conversion, we constructed a 3D geomodel in the depth domain in order to perform the fault-throw analysis and statistical analysis of fault data, including fault-throw measurements in the depth domain, fault-throw frequency, maximum displacement–length (D–L) relationships and fault strike statistics (Fig. 7).

Fig. 7.
Fig. 7.

(a) Time–structure map of the Beta structure at the base Draupne Formation level, with faults and locations of seismic sections and time slices. (b) An east–west section through well 32/2-1 shows the rollover structure with the crestal normal faults. The Viking Group sandstones juxtapose with the basement across the Øygarden Fault. The Øygarden Fault System shows clear fault kinks/bends in this area. Two main growth packages and unconformity surfaces can be observed in the overburden. The boundary (marked by reverse triangles) between the two growth packages is around the top Åsgard Formation (lowest Cromer Knoll Group) level. (c) A north–south section through the highest point of Beta structure, showing a small (c. 67 ms TWT) separation between the Viking Group sandstones and the Jurassic (?) strata on the footwall. (d) A section from the southern side of the Beta structure, showing normal faults in the strata intersecting the top of the basement. (e) A time slice from 720 ms with the seismic coherence attributes showing normal faults in the Jurassic strata on top of the basement. The location of the time slice is shown in (a) and (d). Q, Quaternary.

Fault seal analysis

Fault juxtaposition and fault-rock seal analysis

We investigated the fault seal for the Smeaheia storage prospects mainly from two key aspects. The first aspect is fault juxtaposition: how are different host-rock lithologies juxtaposed against each other across faults (Allan 1989)? The second aspect is fault-rock seal: what might the sealing capacity of the ‘membrane’ materials (i.e. the fault rock) be between faulted blocks (Yielding et al. 1997; Yielding 2002; see the review in Yielding et al. 2010)?

Following seismic interpretation of horizons and faults, the 3D geomodel, which captures the fault juxtaposition information across all faults, is used for implementing grid-based fault seal calculations. The implementation of fault seal calculations into the grid requires calculations on fault surfaces in 3D, with considerations of lateral facies variations (thus shale content) along the fault planes (e.g. Knai and Knipe 1998; Ottesen Ellevset et al. 1998). However, the 3D grid-based fault seal calculations are too extensive to be included in this study. Nevertheless, to help illustrate the main findings, we generated Allan diagrams for key faults to show the across-fault juxtapositions between different stratigraphic intervals (e.g. Fig. 6) and used triangle plots (see the following subsection) to illustrate the fault-rock seal evaluation.

In siliciclastic rocks, the sealing capacity of a fault is mostly determined by the fault-rock permeability and, in areas where extensive cementation is absent (as is the case for this study), the fault-rock permeability depends mostly on the amount of fine-grained materials in the fault rock. There are two main processes that determine the fine-grained material content in the fault rock: (1) mechanical damage processes; and (2) kinematic mixing. The first includes mechanical compaction, shearing, grinding, breakage of mineral grains in the host rocks and formation of fault rock, including disaggregation bands and cataclasites (e.g. Fisher and Knipe 1998, 2001; see the review in Fossen et al. 2007). The second process concerns introducing pre-existing fine-grained materials (e.g. clay/phyllosilicates) into the fault rock by the mixing of different host-rock lithologies during fault slip, with typical fault-rock types including phyllosilicate-framework fault rock (PFFR) and clay smear (e.g. Fisher and Knipe 1998, 2001; Færseth 2006). Thus, the clay/phyllosilicate content of the fault rock depends on both fault slip and the host-rock lithologies.

For assessing the fault rocks from the mechanical perspective, we investigate the fault timing and estimate the burial depth at the time of faulting based on the growth strata and cross-cutting relationships of faults seen in seismic data. The results allow us to estimate the general fault-rock types in the clean sandstones (e.g. disaggregation bands v. cataclasites; clay content <20%) and their potential permeability reductions. We also use host-rock lithology and fault throws to investigate the potential fault-rock clay content resulting from the kinematic mixing. Since there are no fault-rock permeability data measured in the Smeaheia area, we used published fault-rock permeabilities for the Troll reservoir (Gabrielsen and Koestler 1987) as key data, along with our internal fault-rock database as a guideline to estimate fault-rock permeability reduction ranges.

Triangle plots

We use the T7/TrapTester software to make 1D triangle plots for the wells in the study area using shale volume (Vsh) curves. The triangle plot method uses data from single wells and can be populated with different shale/clay content algorithms, which is ideal for providing a first-pass look at across-fault juxtaposition and potential sealing capacities (see more details in Knipe 1997). The Vsh values represent the volume percentage of shale/phyllosilicates in the host rock, and are derived from the gamma-ray (GR) curves. A Vsh value of 0 represents pure sand with no shale component, while a value of 1 means 100% shale.

Fault-rock seal algorithms

We use the shale gouge ratio (SGR: Yielding et al. 1997) to predict the clay/phyllosilicate content in the fault rock and also calculate the shale smear factor (SSF: Lindsay et al. 1993) to evaluate the potential clay smear effect. The SGR algorithm calculates the amount of clay/phyllosilicate in the host-rock strata that has moved past a point on the fault (see details in Yielding et al. 1997). We used values of 0.15, 0.2, 0.3 and 0.4 as the SGR criteria for colour classification: for example, green indicates a low shale/clay content in the fault rock (i.e. 15%), while red represents a high shale/clay content in the fault rock (i.e. >40%).

The SSF algorithm helps to estimate how far a continuous clay/shale bed can extend from the source layer by calculating the ratio of fault throw to source-layer thickness (see details in Lindsay et al. 1993). We used 0.5 as the shale cutoff value, which means that intervals with over 50% shale content are considered as shale beds that can smear. In this study, SSF values <4 are considered as continuous shale smear; SSF>7 is considered no longer smear; and values between 4 and 7 are considered to have potential discontinuities in the shale. It is worth noting that the SSF analysis is focused only on the sealing potential provided by the shale beds, while SGR uses the whole shale/clay content in the strata to evaluate the membrane sealing of the fault zone.

Structural analysis and fault seal analysis

Structural configuration and fault juxtaposition of the Alpha structure

The Alpha structure is an upthrown three-way closure on the footwall crest of the VFS (Figs 2, 4 and 6). The present-day depth of the Alpha structure is c. 1.3 s in two-way travel time (TWT) (Fig. 4), which is c. 1.2 km in true vertical depth subsea (TVDSS) (also see Fig. 5 for the well picks of 32/4-1T2 in depth). The closure height above the spill-point level is c. 50 ms (see Fig. 4), around 70 m in depth. It formed as the footwall of the VFS during the Synrift 2 phase, and then it experienced minor fault reactivation during the Post-rift 2 phase (Figs 4 and 5). The thickness variation of the Synrift 2 growth package can also be observed from the well correlation across the VFS (Fig. 5). Vette fault segment 1 (VF #1 in Fig. 6) and a NW–SE-trending fault bounding the Alpha structure (i.e. IBF1 in Fig. 6) have reached the Base Nordland Unconformity (Figs 4 and 6) (also see Mulrooney et al. 2020).

The VFS has several segments linked by relay ramps (some breached) that control the across-fault juxtaposition of strata between Smeaheia and Troll East (Figs 2 and 6). Across two Vette fault segments adjacent to the Alpha structure and the Gamma structure (i.e. VF #1 and VF #2: Fig. 6), the Viking Group sandstones in the footwalls are juxtaposed mainly with the Draupne Formation and the Cromer Knoll Group–Svarte Formation in the hanging wall (Fig. 6c, d and f). For both fault segments, the throws decrease towards their northern and southern ends, where the Viking Group sandstones are self-juxtaposed (Fig. 6c). The overlapping zone between these two segments is characterized by a (upper ramp) breached relay structure, where the Viking Group sandstones are self-juxtaposed across both fault segments (Fig. 6b, c and e). The geometry, linkage and timing of the relay ramp to the south of the Alpha structure have also been described in detail in Mulrooney et al. (2020).

Structural configuration and fault juxtaposition of the Beta structure

The Beta structure is a hanging-wall three-way closure in which the strata are juxtaposed with the basement via the ØFS, with the highest point (c. 900 ms TWT; c. 800 m TVDSS) near its northern end (Fig. 7a–c). Beta is a north–south-trending fold structure parallel to the Øygardern Fault, with a series of normal faults near the crest striking parallel to the overall folding trend (Figs 2 and 7b, d). The Øygarden Fault beneath the Beta structure clearly shows fault kinks/bends in the dip direction (see Fig. 7b). Fault branches in the basement are possible (Fig. 7b) but cannot be confirmed due to the lack of constraints from seismic reflections. The observed growth strata on top of Beta can be roughly divided into two packages (Fig. 7b–d), with an unconformity as the boundary around the top Åsgard Formation level (i.e. lowest Cromer Knoll Group: Fig. 3). We identified two folding events based on the truncations and onlaps. Growth package 1 and its top unconformity mark the first event. The second folding event is manifested by the folding of growth package 2 (Fig. 7b).

We interpret the Beta structure to have been formed as an extensional fault-bend fold (e.g. Xiao and Suppe 1992; Shaw et al. 1997; Serck and Braathen 2019) controlled by the geometry of the Øygarden Fault during the Late Jurassic–Early Cretaceous rifting. This interpretation is consistent with the normal faults observed in both the Beta structure itself and in other neighbouring areas during that time (see also the following subsection) (Bell et al. 2014). The second folding event imposed on the Beta area slightly folded the structure, especially around well 32/2-1 (Fig. 7b); this folding is less clear on the southern side of the structure (Fig. 7d). The exact timing and mechanism of the second folding event are uncertain due to the absence of the overburden record. We believe this can result from an additional phase of dip slip along the kinks/bends on the Øygarden Fault (Fig. 7b). Still, we cannot completely rule out the possibility of a minor local inversion during the Neogene uplift of the western Norway area. Nevertheless, no faulting or folding activities have been identified above the Base Nordland Unconformity on top of the Beta structure.

The Viking Group sandstones of the Beta structure are juxtaposed with the basement across the ØFS (Fig. 7b–d). Although there are no well penetrations of the basement in the study area, the basement can be correlated to the basement outcrops exposed in the west Bergen area, which is mainly composed of pre-Cambrian granitic gneiss and migmatites (e.g. Fossen et al. 1997, 2017a). Near the southern part of the Beta structure, a series of normal faults can be identified intersecting the top of the basement reflector and the overlying Jurassic strata (age constrained from 3D seismic correlation: Fig. 7d). A seismic coherency time slice through the Jurassic strata shows that those normal faults are parallel to sub-parallel to the ØFS (Fig. 7e). Although identifying faults in the basement is difficult, the basement in this area is likely to be faulted and fractured. This is also supported by observations from the onshore Bergen area where the basement rocks are highly faulted and fractured, having undergone multiple tectonic and faulting events since the Devonian (e.g. Fossen et al. 1997, 2017a; Ksienzyk et al. 2016).

Fault throw and fault population

The fault throws are measured in depth after the structural mapping and time–depth conversion (Fig. 8a). The fault map is coloured based on the top Brent Group level (Fig. 3), which can represent typical fault throws in the Viking Group sandstones in this area. The results show that most intrablock faults have fault-throw values of less than 50 m (green-coloured in Fig. 8a), while the VFS and ØFS have throws of more than 300 m (red-coloured in Fig. 8a). The sampled Vette fault segment (i.e. VF #1 in Fig. 6) bounding the Alpha structure has throws of around 400–500 m, and its fault throw decreases towards both ends (<50 m; see Fig. 8b). The cumulative frequency of throw (Fig. 8c) shows that around 70% of fault population in Smeaheia have throws of less than 50 m, while more than 90% faults have fault throws of less than 300 m; A linear power-law relationship in the population can be observed between 20 and 300 m throw. The maximum displacement–length relationship chart of the fault population (Fig. 8d) shows that the Dmax/L values are mainly around 10−2, with a scattered range largely between 10−3 and 10−1, which is a typical range for reported normal fault populations (see the review in Kim and Sanderson 2005). The rose diagram of the fault strikes (Fig. 8e) shows the fault strike ranges from north–south to NW–SE: the near north–south-trending cluster is mainly from the main faults and crestal faults in the Beta structure, while the NW–SE-trending clusters can be attributed to the intrablock faults.

Fig. 8.
Fig. 8.

(a) Fault-throw map on the top Brent Group level. Most intrablock faults show fault throws of less than 50 m; a sampling area/line is used to plot the fault throw of the Vette fault segment bounding the Alpha structure. (b) Fault-throw profile of VF #1 along the sampling direction; coloured by value ranges. (c) Cumulative frequency of fault throws, showing that more than 70% of faults have throws of less than 50 m, and 90% of faults have throws of less than 300 m; a linear power-law relationship can be observed from 20 to 300 m. (d) Maximum displacement–length chart of the fault population showing that the Dmax/L values are mainly around 10−2, with the range between 10−3 and 10−1. (e) Rose diagram of fault strikes.

Fault timing and fault-rock type prediction

Our fault timing analysis was based on identifying synrift/synkinematic growth strata and cross-cutting relationships in seismic (e.g. Figs 4, 6 and 7). Here we divided the fault timing results on Smeaheia into three main phases: (1) Synrift 1 faulting in the Permo-Triassic; (2) Synrift 2 faulting in the Late Jurassic–Early Cretaceous; and (3) Post-rift 2 faulting and reactivation.

During the first phase, the VFS and ØFS were active during the east–west extension and largely controlled the development of half-graben (Fig. 9a; see also Fig. 4) (Bell et al. 2014; Whipp et al. 2014). Most faults intersecting the Viking Group sandstones formed during the Synrift 2 phase, which includes both (a) the reactivation of pre-existing north–south-striking Permo–Triassic faults and (b) newly formed intrablock faults (Fig. 9b; see Synrift 2 faults in Figs 4, 6 and 7). The reactivated VFS and ØFS show much larger fault throws, accompanied by clear fault-growth strata (e.g. see Figs 4, 6 and 7). The newly formed intrablock faults strike mainly NW–SE, indicating a NE–SW extension direction in the Smeaheia area. During the Post-rift 2 phase, some faults reactivated (e.g. Post-rift 2 reactivation in Figs 4 and 6d), and an array of NW–SE-trending normal faults formed to the south of the Alpha structure (Fig. 9c) (see also Mulrooney et al. 2020). These newly formed faults (e.g. Post-rift 2 fault in Fig. 4) show a thin-skinned feature, with larger fault displacement in the overburden section (v. Viking Group sandstone level) and a fault timing in the Paleocene–Eocene (see Mulrooney et al. 2020).

Fig. 9.
Fig. 9.

Schematic fault timing maps of the Smeaheia area in three tectonic phases based on growth strata identification and cross-cutting relationships with horizons. Some faulting activities on the Øygarden Fault System are uncertain due to strata erosion, and are thus labelled with question marks. Note that fault traces used here are based on the base Draupne Formation level, so the first map only represents the general position of the Synrift 1 faults.

Since the second phase of faulting started during the deposition of the Draupne Formation and the early Cromer Knoll Group, the burial depth should be quite shallow at the time of faulting, most probably between c. 0.1 and 0.5 km. Thus, given a normal faulting regime, the fault-rock type in the clean sandstones (clay content <20%) should be dominated by disaggregation bands, perhaps with very minor cataclasis. In this case, the permeability reduction in the fault rock is likely to be only c. 0–1 orders of magnitude lower than the host rock, which is consistent with the published fault-rock data (Gabrielsen and Koestler 1987). We plotted these published Troll fault-rock data in Figure 10, together with our expected permeability reduction ranges of different fault-rock types. For those faults that were active during the third phase (i.e. Fig. 9c), the depth at the time of faulting is generally deeper than 500 m when considering the thickness of overburden strata below the Base Nordland Unconformity (see Fig. 5). The burial depth can go up to 2 km when considering the potential burial compaction and the potential Neogene uplift (e.g. c. 1 km uplift in Baig et al. 2019). We thus predict a bit more cataclastic fault rock in these faults, in which the permeability reduction can be up to 2 orders of magnitude in the high-quality sandstones (Fig. 10). We also expect more PFFR to occur in the low-quality sandstones with a higher clay/phyllosilicate content (e.g. 15–40%) in this area (Fig. 10). Clay smear is also expected for shaly strata, like the Draupne Formation and the Cromer Knoll Group above the Viking Group sandstones (Fig. 5).

Fig. 10.
Fig. 10.

Fault-rock–host-rock permeability chart showing potential fault-rock types and permeability reductions of the Viking Group sandstones in this area. High- and low-quality sandstone ranges show typical permeabilities observed in the reservoir sandstones of Troll Field. Troll data plotted in the chart are from Gabrielsen and Koestler (1987). PFFR, phyllosilicate-framework fault rock.

Triangle plots

We calculated the SGR and SSF on both the storage unit and the overburden level (e.g. Figs 11 and 12). Our results show that when fault throw is less than about 50 m, the Viking Group sandstones are self-juxtaposed at the formation scale and usually have low SGR windows with values of less than 15–20% (i.e. the green and yellow colours in Fig. 11). This low-SGR window is quite prominent in two Smeaheia wells (32/4-1T2 and 32/2-1: Fig. 11a and b) and the Troll well 31/6-6 (Fig. 11d) at the Sognefjord Formation level. The other three wells have more heterogeneity in the Viking Group sandstones but still show low-SGR windows when fault throw is around 20–50 m (Fig. 11c, e and f). Generally, when the fault throw is larger than 150 m, the SGR values from all of these wells are around 20–40%; the SGR values around the shaly Draupne Formation and the upper Dunlin Group interval (namely the Drake Formation in Fig. 3) are greater than 40% (Fig. 11).

Fig. 11.
Fig. 11.

Triangle plots with SGR values from six key wells in our study area. See the well locations in Figure 2. SGR, shale gouge ratio; SSF, shale smear factor; B, Brent; CK, Cromer Knoll; D/Drp, Draupne; H, Heather; K, Krossfjord; N, Nordland.

Fig. 12.
Fig. 12.

Shale gouge ratio (SGR) and shale smear factor (SSF) triangle plots from the overburden section of the 32/4-1T2 well in our study area. B, Brent; CK, Cromer Knoll; D, Draupne; H, Heather; K, Krossfjord; N, Nordland.

Based on the calibration studies described in Yielding et al. (2010), SGR values of less than 15–20% at burial depths of less than 3000 m are likely to indicate good across-fault communication (on the geological timescale). In this study, given that the fault rocks with low SGR values (<15–20%) are likely to be dominated by the disaggregation bands (see the previous subsection), they should have permeabilities quite close to the host rock based on our prediction in Figure 10 (i.e. 0–1 orders of magnitude reduction). This implies that these low-SGR fault rocks (SGR <15–20%) between sand-rich intervals should have a good across-fault flow potential, although in this study we do not have SGR calibrations from the production or storage timescales.

The overburden section in the well 32/4-1T2 is dominated by the shaly and marly strata (Figs 3 and 5); thus, the SGR values are generally higher than 30%, and the SGR values associated with lower Rogaland Group and Draupne Formation are over 40% (Fig. 12a). In the SSF plot, the Draupne shale has a SSF value of less than 4 when the fault throw is less than 85 m, and the lower Rogaland Group shale has a SSF value of less than 4 when the fault throw is less than 190 m (Fig. 12b) – suggesting that these shale intervals are most likely to be continuous across faults within the corresponding throw ranges. With larger fault throws, the SSF values fall between 4 and 7 (i.e. yellow coloured area in Fig. 12b), and suggest that the clay smear becomes semi-continuous in this fault-throw range. The upper-bound fault-throw values for these two shale intervals are 148 and 334 m, respectively. Beyond these two values, the fault throws are probably too large to allow the shale intervals to provide a clay smear effect on faults (i.e. SSF>7: Fig. 12b).

Containment risk assessment

The containment risk assessment for the Smeaheia storage prospects integrates both geology-related and well-related containment risks. For the geology-related risks, we evaluated fault-related risks and cap-rock-related risks by further breaking down the questions into geological, geomechanical and geochemical perspectives. After identifying potential containment risks, we gathered the evidence (for or against) and ranked them based on the quality and types (e.g. hard quantitative data, analogue reasoning or expert judgement). Then the Bow-tie method (e.g. Tucker et al. 2013; see the review in Pawar et al. 2015) was used to allocate and visualize the causes of unwanted events, consequences, preventive controls and mitigation measures. The method provides a simple way to visualize and demonstrate that risks are understood and controlled at different levels. The complete assessment is too voluminous to be included in this paper, so we only focus on the critical evidence used in the assessment of containment risk.

Alpha structure

The trap integrity of the Alpha structure is determined by both the top seal and fault seal. The Draupne Formation, mainly composed of deep-marine shale in the study area (Fig. 5), provides a high-quality top seal for the Alpha and Beta structures. The thickness of Draupne shale penetrated in well 32/4-1T2 is over 100 m (Figs 4 and 5). Although some footwall erosion close to the VFS is expected, the Alpha structure still has sufficient thickness of the Draupne Formation at the crest based on seismic mapping (see Figs 4 and 6c, d).

Regarding fault seal, the Viking Group sandstones in the Alpha structure juxtapose with the Cromer Knoll Group across the VFS (Fig. 4). The Cromer Knoll Group was deposited in a neritic environment during the Synrift 2 phase. Well penetrations in our study area (Fig. 5) show that it is generally marly and shaly in this region, with some interbedded limestones but no permeable beds were discovered in this interval. Since there is no well control for the thickest part of growth strata in the half-graben, potential lateral lithological variations were also considered during our investigation of this growth package. However, no seismic anomalies were observed in that sequence to suggest the existence of permeable beds (e.g. sandbodies). As to the fault-rock seal (i.e. the ‘membrane’ seal), we would expect a very high shale content (>40%) in the Vette fault rock for this growth section. In addition, clay smear, sourced from both the Draupne Formation and the Cromer Knoll Group, can also provide a very good across-fault sealing mechanism for the Alpha structure (e.g. Fig. 12b: the Draupne can cover c. 85 m fault throw with SSF<4).

The along-fault leakage risk of the Alpha structure is also considered very low, even though the VFS experienced minor reactivation in the Post-rift 2 phase and reached the Base Nordland Unconformity level (e.g. Figs 4 and 6d). The reasons for a low along-fault leakage risk here are: (a) the fault rock of the VFS in the overburden is expected to be very shaly due to the clay-rich strata (Fig. 5; see also SGR in Fig. 12a), and the shale beds are more likely to be continuous across the VFS in the overburden (e.g. the Rogaland Group: Fig. 12b) due to small fault throws; (b) the VFS is truncated by the Base Nordland Unconformity and overlain by intact Quaternary sediments (Figs 4 and 6d), demonstrating that the VFS has been inactive for the last few million years; thus, the reactivation risk of VFS due to natural processes (e.g. earthquakes) in the next c. 1000 years is considered to be very unlikely; and (c) the updated high-quality in situ stress data (i.e. from extended leak-off tests) acquired in the last decades suggest that the stress regime in the sedimentary packages in this area are in a normal faulting regime, and faults are typically not critically stressed (Andrews et al. 2016; De Lesquen et al. 2020). Thus, we consider that the risk of fault reactivation due to injection to be very low if injection is managed properly (e.g. injector position, injection rates, and associated pressure and thermal effects).

Moreover, the existence of the Troll Field is direct evidence that the faults, as well as the top seals, can provide an effective sealing capacity on the geological timescale (i.e. for millions of years, since the Neogene tilting event: Horstad and Larter 1997). The Alpha structure and the neighbouring fault blocks in Troll experienced nearly identical geohistories, and share numerous geological similarities regarding faulting activity and juxtaposition (e.g. see Fig. 4). Thus, the Troll Field is arguably the best analogue for the Alpha structure, thus suggesting very low geology-related storage containment risks for the Alpha structure. The gas column in Troll is more than 210 m (up to 250 m in Troll East), while the Alpha structure's height from the top to the spill point is only c. 70 m in depth (c. 50 ms in TWT: Fig. 4). This means that Alpha can seal effectively on the geological timescale with only one-third of Troll's proven (i.e. minimum) sealing capacity, assuming the same fluid and conditions.

There are uncertainties regarding the CO2 sealing capacity of clay-rich fault rock due to the preferred CO2 wettability of some clay minerals (see Miocic et al. 2019; Karolytė et al. 2020), so that the clay-rich fault rock might not be able to hold the same column of CO2 on the geological timescale as hydrocarbons. However, for the containment risk assessment of a CO2 storage project, the consequence (thus the risk) of having CO2 flow across a shale/clay-rich (e.g. >40% clay content) fault zone is most likely to be negligible, given the very slow flow rate (controlled by the very low permeability of the fault rock) and the c. 1000 year storage timescale.

The geochemical reaction between CO2 and minerals in the cap rock and fault rocks are also included in our containment risk assessment but are not considered a major factor. This is mainly because: (a) the order of magnitude for the effect of the CO2–brine–rock interaction is considered to be of secondary importance (IPCC 2005); (b) the consequences are variable since CO2 can either corrode minerals or precipitate carbonates to reduce permeability, and the interaction is dynamic so that the porosity/permeability modifications can change with time (Kampman et al. 2014).

Beta structure

Similar to the Alpha structure, the cap-rock-related containment risk on the Beta structure is considered very low. The Draupne Formation above the structure, as well as the Cromer Knoll Group, can provide very good top seals (Fig. 5). However, the Beta structure has higher across-fault and along-fault flow risks for CO2 storage. The main reasons for this conclusion are:

  • Beta is a three-way closure with its highest point towards the northern end of the fold (Fig. 7a and c). The crestal faults on Beta are mostly parallel to the fold structure (Fig. 7a), and they have small fault throws (<50 m; Fig. 8a) and weak across-fault permeability reduction potentials (Figs 10 and 11b). Thus, there are no adequate fault compartments on the Beta structure, and the faults on the Beta structure are less likely to be effective barriers to retard CO2 flow towards the northern end (Fig. 7c).

  • The Viking Group sandstones are directly juxtaposed with the basement, which is most likely to be a faulted, fractured and potentially weathered Precambrian granitic basement (Fig. 7; see the earlier subsection ‘Structural configuration and fault juxtaposition of the Beta structure’). The fault/fracture permeability and connectivity in the basement, as well as the ØFS fault-rock permeability, are highly uncertain.

  • Around the structural high of the Beta structure (see Fig. 7c), the small separation (c. 65 ms TWT; <100 m in depth) between the Viking Group sandstones in the Beta structure and the Jurassic sediments on the basement implies that the ØFS in this section might have multiple slip surfaces, which can form sand–sand juxtaposition (Wibberley et al. 2008) and/or fault lenses (Childs et al. 2009) below the resolution of the seismic. Thus, CO2 can potentially migrate to the Jurassic sediments on the footwall side along the Øygarden Fault.

  • The present-day depth of the structure is c. 0.8 km TVDSS, close to the critical depth, and thus not ideal for CO2 to maintain its liquid or supercritical phase (see Fig. 1).

  • The ØFS in the Uer Terrace–Måløy Slope area (e.g. see fig. 4 in Bell et al. 2014), which is c. 30 km to the north of the Beta structure, has branches that reach to the seabed, although the lateral fault linkage of the ØFS in the crystalline basement is uncertain.

  • Recorded earthquakes indicate ongoing strain occurring around the ØFS and the Øygarden Complex (i.e. basement) areas, although the magnitudes are generally low (magnitude <4.0) based on records from the Norwegian National Seismic Network database (http://nnsn.geo.uib.no/nnsn/).

Based on this set of observations and arguments, we do not consider the Beta structure to be suitable for large-scale CO2 storage due to the potential containment risks. However, it could still be considered a buffer volume (secondary storage zone) in the case where CO2 spills out from the Alpha structure in the long term. Such secondary storage concepts may be important for assessing storage risks over a longer timescale (c. 1000 years). We suggest that future work should focus on the critical volume for CO2 spill from Alpha to Beta, with considerations of CO2 plume migration speed and other CO2-trapping mechanisms in the long term (e.g. residual CO2 trapping and solubility trapping). Future studies on analogous basement-contact fault zones in this region would also help to de-risk the ØFS (e.g. fault-zone architecture and permeabilities).

Storage capacity

The storage capacity for large quantities of CO2 is mainly determined by the container's size (i.e. gross rock volume (GRV)) and the pressure–temperature conditions in the reservoir. At a given thermal condition in the reservoirs, the reservoir pressure evolution becomes a significant factor for estimating the storage capacity. The future fluid-pressure profile in Smeaheia area largely depends on two factors: Troll depletion and reservoir pressure recharging (Fig. 13).

Fig. 13.
Fig. 13.

The pressure setting of the Viking Group sandstones in Smeaheia, showing a dynamic interplay between pressure depletion from Troll East and several pressure-recharging potentials. ØC, Øygarden Complex (i.e. the basement).

Troll depletion

The Troll Field has been under production by depletion for the last 25 years, mainly from the Sognefjord Formation level. The pressure drawdown in the Viking Group sandstones therefore becomes a major issue for estimating the storage capacity of Smeaheia, especially for the Alpha structure (e.g. Lauritsen et al. 2018; Nazarian et al. 2018).

Our fault seal analysis suggests that the pressure communication between the Troll Field and Smeaheia are likely for the following reasons:

  • The reservoir sandstones are self-juxtaposed across those VFS relay ramps and the intrablock NW–SE-trending faults (see Figs 6 and 14), where the fault throws are commonly less than 50 m (Fig. 8).

  • The SGR values for these self-juxtaposed windows are typically less than 15% (Fig. 11).

  • The fault-rock type is expected to be dominated by disaggregation bands in the high-quality sandstones, thus fault-rock permeability reduction is only c. 0–1 order of magnitude (Fig. 10; see the earlier subsection ‘Fault timing and fault-rock type prediction).

Fig. 14.
Fig. 14.

(a) A seismic section through the (breached) relay ramp D3 of the Vette Fault System (see Fig. 13), showing the self-juxtaposition of the Viking Group sandstones from Smeaheia to Troll East. Two intrablock faults (IBF 1 and IBF 2: see Fig. 6) formed during Post-rift 2 phase (see Fig. 9). (b) A seismic section showing the Viking Group sandstones from the Alpha structure to the subcropping area to the north of the Øygarden Fault. (c) A detailed image showing that the Viking Group sandstones are truncated at the Base Nordland Unconformity. The Quaternary sediments covering the subcropping area include a seismically transparent/chaotic package (light blue) and then the layered package (pink) above. These two sand-rich packages are encountered in well 32/2-1 and are expected to be highly permeable (Darcy level). Locations of the lines are shown in Figure 13.

We were recently able to validate our models of the Smeaheia–Troll pressure communication using pressure data obtained in the Gladheim well 32/4-3S (see details in the section ‘Well 32/4-3S – a new validation point’ later in this paper).

Pressure recharging

Despite the depletion from the Troll gas production, Smeaheia also has many potential pressure-recharging mechanisms (Fig. 13). The first mechanism is via recharge from the aquifers of the structural downslope areas north and south of Smeaheia (i.e. A1 and A2 in Fig. 13). Furthermore, our seismic mapping results also show that the Viking Group sandstones in Smeaheia are subcropped and truncated at the Base Nordland Unconformity level near the northern and southern section of ØFS (i.e. R1 and R2 in Fig. 13; see also the map in Fossen et al. 2017b). Thus, the Viking Group sandstones are in direct contact with the Quaternary sediments (Fig. 14b and c) with permeability likely to be in the Darcy-level range, indicating good pressure-recharging potential from the seabed. Recharging across the ØFS (‘Rf’ in Fig. 13) is also possible, but the fault-rock permeability of the ØFS and the basement fault/fracture permeability and connectivity are uncertain.

Pressure profile predictions

The Smeaheia storage prospects have a dynamic reservoir pressure setting. The long-term pressure profile is determined by the interplay between depletion and recharging, which are further controlled by how well Smeaheia communicates (across-fault) with the Troll Field and recharging areas (Fig. 13). We illustrate the concept of this interplay in Figure 15. Three conceptual cases with different depletion/recharging ratios show how fast the reservoir pressure can reach the lowest point and rebound back to the initial (e.g. hydrostatic) pressure level. At a given GRV, a structure's storage capacity is determined by the lowest average reservoir pressure, which could result in an unwanted movement of the CO2 plume. Examples include CO2 spilling out from the trap and moving into neighbouring licences or areas with uncertain containment quality and/or pressure–temperature conditions. The lowest pressure point should also be higher than the critical pressure level, which is either: (1) the required minimum pressure for CO2 to remain its dense form (a technical limit); or (2) an artificial level to meet certain capacity requirements for a structure (e.g. an economic limit).

Fig. 15.
Fig. 15.

Conceptual long-term average reservoir pressure diagram of Smeaheia, showing three cases with different depletion–recharging scenarios determined by the level of (across-fault) communication of Smeaheia with Troll and recharging areas. The depletion–recharging ratio influences the lowest (average) pressure point, which can affect the storage capacity of a given structure (to avoid spill, for instance). The timing of injection is also a factor: injecting before the lowest point requires good predictions of the pressure evolution (e.g. the Smeaheia case); if injecting in an already-depleted reservoir, the lowest pressure point can be directly measured.

Moreover, the timing of the injection period on the pressure evolution timeline is also an important factor to consider if:

  • injecting into an already-depleted reservoir (i.e. on the right-hand side of the curves: Fig. 15), then the start point is the lowest pressure that can be directly measured, so that the capacity is easier to estimate;

  • or

  • injecting into a currently under-depleted system, as in the Smeaheia case, where the prediction of the lowest point is crucial.

In this manner, we can show how dynamic simulations, with fault seal analyses implemented, will be needed using the regional Troll–Smeaheia model(s) to capture the long-term pressure uncertainties for storage in the Viking Group sandstones in the Smeaheia area.

Well 32/4-3S – a new validation point

The 32/4-3S Gladsheim well was drilled recently (October 2019) on the southern side of the Smeaheia fault block (see the location in Figs 2 and 13) and provided a valuable constraint for our predictions of the Smeaheia–Troll pressure communication. Pressure measurements were performed in several sandstone intervals stretching from the Sognefjord Formation in the Viking Group to the Lunde Formation in the Hegre Group (Fig. 16). As expected (e.g. Wu et al. 2019), depletion is observed from the Viking Group sandstones in Smeaheia (Fig. 16a). However, the pressure measurements in this well also show differential degrees of depletion with depth (Fig. 16a), which can be divided into three main intervals:

  • Interval 1: the upper Viking Group sandstones, including the Sognefjord and upper Fensfjord formations, have the largest depletion (c. 14 bars);

  • Interval 2: the lower Fensfjord Formation–Brent Group has c. 10 bar depletion;

  • Interval 3: the Johansen Formation–Hegre Group interval is close to hydrostatic pressure.

Fig. 16.
Fig. 16.

(a) Reservoir pressure measurements from the Gladsheim well 32/4-3S (see the location in Fig. 2) showing depletions in the Smeaheia area. There are three main intervals with different pressure gradients. Two potential vertical pressure baffles/barriers can be correlated with high-Vsh intervals from the intra-Fensfjord Formation and the Drake Formation, respectively. The shorter grey bars with question marks are potential minor vertical baffles/barriers. (b) 1D triangle diagram (SGR) of the Gladsheim well with shale bed juxtaposition. There are five across-fault communication paths between these three pressure intervals. Five juxtaposition windows for the five paths are marked on the triangle diagram accordingly. Two potential vertical communications are marked as V1 and V2. A, Amundsen Formation; J&C, Johansen and Cook formations; S, Statfjord Group.

We calculated the 1D triangle plot with SGR and shale bed juxtaposition for this well (Fig. 16b), and stratigraphically correlated it with the pressure plot. Based on these results, two potential vertical pressure baffles/barriers (Fig. 16a) can be drawn between three pressure domains/intervals, which correlate well with the high Vsh intervals from the intra-Fensfjord Formation and the Drake Formation (Fig. 16b). Of course, there are also some minor pressure-gradient differences (c. 1–2 bar) within the intervals (Fig. 16a), which can be correlated with some high-Vsh intervals in the base Fensfjord Formation and the shaly Amundsen Formation (lower Dunlin Group: Fig. 16b). To simplify the discussions, we only focus on the three main intervals here.

The understanding of the different levels of depletion observed in the Gladsheim well should be mainly governed by two aspects: (1) across-fault communication; and (2) vertical communication (i.e. across-bedding). The current Troll gas production is mainly from the Sognefjord Formation in Troll East. Thus, the depletion in the first interval (Sognefjord–upper Fensfjord formations) in the Gladsheim well is most likely to be through those relay-ramp areas along the VFS (Figs 6 and 13): for example, the intact relay-ramp area D4 and breached relay-ramp area D3 (see Figs 13 and 14a). The triangle plots from both the Gladsheim well (Path 1 in Fig. 16b) and 31/6-3 (Fig. 11f) show some low-SGR windows when the fault throw is small (i.e. <20–66 m).

The c. 10 bar depletion in Interval 2 could be from three possible routes:

  • the juxtaposition of the Interval 2 in Smeaheia with Interval 1 in Troll East (i.e. Path 2 across VFS: Fig. 16b);

  • vertical communication between Interval 1 and Interval 2 in Smeaheia (V1 in Fig. 16b);

  • or

  • depletion in the Interval 2 in Troll first (V2 in Fig. 16b), then moving across the VFS (i.e. Path 3 in Fig. 16b).

However, it is uncertain which route or which mechanism (vertical v. across-fault flow) plays a bigger role, since this is largely controlled by the sand/shale (thus permeability) continuity and connectivity (e.g. lateral facies variations; see wells 32/4-3S and 31/6-3) in the Brent and Viking groups.

The near-hydrostatic pressure from Interval 3 is probably caused by two main factors: (1) the thick shaly Drake Formation (Figs 11f and 16b) acting as an effective vertical pressure barrier; and (2) very limited across-fault communication, as shown in the triangle plots where the SGR value in the juxtaposition windows is quite high (e.g. SGR >30%: Figs 11f and 16b). The Drake Formation can also provide clay smear on the fault surfaces.

It is important to note that this well only shows how the depletion pressure has been propagated after 25 years of Troll production. Thus, the long-term depletion profile, fault seal effects and vertical baffles/barriers should be analysed in future dynamic simulations. Nevertheless, the Gladsheim well is a valuable validation point for our (pre-drill) fault seal analyses and provides new insights into the Smeaheia pressure depletion. It is also a useful constraint on the long-term pressure profiles (as illustrated in Fig. 15) for future dynamic modelling of the Smeaheia storage aquifers.

Conclusions and future work

We have used the Smeaheia storage prospects as a case study to illustrate how fault seal analysis plays an important role in assessing the storage capacity and containment risks for subsurface CO2 storage projects. We conducted detailed structural analysis and fault seal analysis in the study area, and were able to draw the following conclusions:

  • During the Late Jurassic–Early Cretaceous rifting, the Viking Group sandstones’ burial depth was less than 500 m. The fault-rock type in the clean sandstones is likely to have been dominated by disaggregation bands with minor cataclasites, with general permeability reduction around 0–1 orders of magnitude (up to 2). Most of the intrablock faults in Smeaheia have fault throws of less than 50 m. Triangle plots show low SGR values when fault throws are small (<50 m), suggesting good communication across most intrablock faults.

  • The Alpha structure has very low across-fault leakage risks due to the fault juxtaposition to the synrift Cromer Knoll Group; and the fault rock in the Vette Fault close to the top Alpha structure is expected to have high clay content. Clay smear from the Draupne Formation and the Cromer Knoll Group can also be an effective sealing mechanism. The along-fault leakage risk on Alpha is considered low because of the high clay content in the overburden successions, fault inactivity for the last few million years and the present-day in situ stresses. The Troll Field is a sealed system on a geological timescale, providing arguably the best analogue to the Alpha structure sealing potential.

  • The Beta structure analysis reveals much larger fault-related containment risks. The crestal faults cannot provide adequate sealing to prevent CO2 from migrating to the Beta structure's highest point, where the Viking Group sandstones are juxtaposed with the faulted/fractured Precambrian basement across the ØFS. There are also large uncertainties in across-fault and along-fault rock permeabilities of the ØFS.

  • The storage capacity uncertainty of the Viking Group sandstones in Smeaheia is mainly determined by the reservoir pressure, which is a dynamic interplay between two mechanisms: depletion and recharging. Our fault seal study shows that the pressure communication between the Troll Field and Smeaheia is likely to be through several relay-ramp areas along the VFS. The Viking Group sandstones in Smeaheia also have considerable pressure-recharging potential from the subcropping areas at the Base Nordland Unconformity around the ØFS.

  • The depletion observed in the newly drilled (October 2019) Gladsheim well 32/4-3S provides a good validation for our fault seal prediction. Differential depletions from three different intervals in the well can be attributed to both across-fault communication and vertical communication. These data and analyses provide valuable insights for future dynamic simulations of CO2 storage site performance.

In order to maximize the CO2 storage scale-up potential of the Smeaheia area, we suggest that future work should focus on:

  1. Implementation of static fault seal analysis into dynamic simulations, including history matching of the depletion observed in the Gladsheim well and the evaluation of long-term pressure effect (c. 1000 year scale).

  2. Evaluation of the value and risks of using ØFS-associated structures (e.g. the Beta structure) as buffer storage sites.

  3. Further evaluation of storage potentials in other prospective storage aquifers with near-hydrostatic pressures (e.g. the Johansen and Cook formations) in Smeaheia.

Acknowledgements

The authors wish to thank Equinor, Gassnova and CGG for permission to publish seismic and well data used in this work. Some seismic data used is proprietary to CGG but available to many research and academic organizations. We would also like to thank Equinor, DNO Norge, Lundin and Petoro for their permission to use the Gladsheim well data in this paper. Inputs and discussions from many colleagues in the Northern Lights subsurface team (Equinor, Shell and Total) are acknowledged. Peter Bretan and an anonymous reviewer are thanked for constructive reviews.

Author contributions

LW: conceptualization (lead), investigation (lead), methodology (lead), software (lead), visualization (lead), writing – original draft (lead); RT: conceptualization (equal), data curation (equal), investigation (equal), methodology (supporting), project administration (lead), software (equal), writing – review & editing (equal); SO: conceptualization (supporting), investigation (supporting), methodology (supporting), software (supporting), writing – review & editing (equal); RM: conceptualization (supporting), investigation (supporting), methodology (supporting), writing – review & editing (equal); KH: conceptualization (supporting), data curation (equal), software (supporting), writing – review & editing (equal); PR: conceptualization (supporting), methodology (supporting), project administration (equal), writing – review & editing (equal); BN: conceptualization (supporting), data curation (supporting), visualization (supporting), writing – review & editing (supporting).

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data availability

The data that support the findings of this study are available from Equinor but restrictions apply to the availability of these data, which were used under licence for the current study, and so are not publicly available. Data are, however, available from the authors upon reasonable request and with permission of Equinor.

References

  1. ↵
    1. Allan, U.S
    . 1989. Model for hydrocarbon migration and entrapment within faulted structures. AAPG Bulletin, 73, 803–811.
    OpenUrlAbstract
  2. ↵
    1. Andrews, J.S.,
    2. Fintland, T.G.,
    3. Helstrup, O.A.,
    4. Horsrud, P. and
    5. Raaen, A.M.
    2016. Use of unique database of good quality stress data to investigate theories of fracture initiation, fracture propagation and the stress state in the subsurface. Paper ARMA-2016-887 presented at the 50th US Rock Mechanics/Geomechanics Symposium, 26–29 June 2016, Houston, Texas, USA.
  3. ↵
    1. Bachu, S.,
    2. Bonijoly, D.,
    3. Bradshaw, J.,
    4. Burruss, R.,
    5. Holloway, S.,
    6. Christensen, N.P. and
    7. Mathiassen, O.M
    . 2007. CO2 storage capacity estimation: Methodology and gaps. International Journal of Greenhouse Gas Control, 1, 430–443, https://doi.org/10.1016/S1750-5836(07)00086-2
    OpenUrlCrossRef
  4. ↵
    1. Baig, I.,
    2. Faleide, J.I.,
    3. Mondol, N.H. and
    4. Jahren, J
    . 2019. Burial and exhumation history controls on shale compaction and thermal maturity along the Norwegian North Sea basin margin areas. Marine and Petroleum Geology, 104, 61–85, https://doi.org/10.1016/j.marpetgeo.2019.03.010
    OpenUrl
  5. ↵
    1. Bell, R.E.,
    2. Jackson, C.A.-L.,
    3. Whipp, P.S. and
    4. Clements, B
    . 2014. Strain migration during multiphase extension: observations from the northern North Sea. Tectonics, 33, 1936–1963, https://doi.org/10.1002/2014TC003551
    OpenUrlCrossRef
  6. ↵
    1. Bretan, P.,
    2. Yielding, G.,
    3. Mathiassen, O.M. and
    4. Thorsnes, T
    . 2011. Fault-seal analysis for CO2 storage: an example from the Troll area, Norwegian Continental Shelf. Petroleum Geoscience, 17, 181–192, https://doi.org/10.1144/1354-079310-025
    OpenUrlAbstract/FREE Full Text
  7. ↵
    1. CO2CRC
    2008. Storage Capacity Estimation, Site Selection and Characterisation for CO2 Storage Projects. CO2CRC Report RPT08–1001. Cooperative Research Centre for Greenhouse Gas Technology, Canberra, Australia.
  8. ↵
    1. Childs, C.,
    2. Manzocchi, T.,
    3. Walsh, J.J.,
    4. Bonson, C.G.,
    5. Nicol, A. and
    6. Schöpfer, M.P
    . 2009. A geometric model of fault zone and fault rock thickness variations. Journal of Structural Geology, 31, 117–127, https://doi.org/10.1016/j.jsg.2008.08.009
    OpenUrlCrossRefWeb of Science
  9. ↵
    1. DECC
    2012. CO2 Storage Liabilities in the North Sea – An Assessment of Risks and Financial Consequences – Summary Report for DECC May 2012. Department of Energy and Climate Change (DECC), London.
  10. ↵
    1. De Lesquen, C.,
    2. Andrews, J.S. and
    3. Rasheva, S.
    2020. Good quality LWD Images in horizontal wells invalidate the strike-slip theory in the Northern North Sea. Paper ISRM-EUROCK-2020-103 presented at the ISRM International Symposium – EUROCK 2020, 14–19 June 2020 (physical event not held).
  11. ↵
    1. Deng, C.,
    2. Fossen, H.,
    3. Gawthorpe, R.L.,
    4. Rotevatn, A.,
    5. Jackson, C.A.-L. and
    6. Fazlikhani, H.
    2017. Influence of fault reactivation during multiphase rifting: the Oseberg area, northern North Sea rift. Marine and Petroleum Geology, 86, 1252–1272, https://doi.org/10.1016/j.marpetgeo.2017.07.025
    OpenUrl
  12. ↵
    1. Dockrill, B. and
    2. Shipton, Z.K
    . 2010. Structural controls on leakage from a natural CO2 geologic storage site: Central Utah, USA. Journal of Structural Geology, 32, 1768–1782, https://doi.org/10.1016/j.jsg.2010.01.007
    OpenUrlCrossRefWeb of Science
  13. ↵
    1. Duffy, O.B.,
    2. Bell, R.E.,
    3. Jackson, C.A.L.,
    4. Gawthorpe, R.L. and
    5. Whipp, P.S
    . 2015. Fault growth and interactions in a multiphase rift fault network: Horda Platform, Norwegian North Sea. Journal of Structural Geology, 80, 99–119, https://doi.org/10.1016/j.jsg.2015.08.015
    OpenUrl
  14. ↵
    1. Eiken, O.,
    2. Ringrose, P.,
    3. Hermanrud, C.,
    4. Nazarian, B.,
    5. Torp, T.A. and
    6. Høier, L
    . 2011. Lessons learned from 14 years of CCS operations: Sleipner, In Salah and Snøhvit. Energy Procedia, 4, 5541–5548, https://doi.org/10.1016/j.egypro.2011.02.541
    OpenUrlCrossRefWeb of Science
  15. ↵
    1. Færseth, R.B
    . 1996. Interaction of Permo-Triassic and Jurassic extensional fault-blocks during the development of the northern North Sea. Journal of the Geological Society, London, 153, 931–944, https://doi.org/10.1144/gsjgs.153.6.0931
    OpenUrlAbstract/FREE Full Text
  16. ↵
    1. Færseth, R.B
    . 2006. Shale smear along large faults: continuity of smear and the fault seal capacity. Journal of the Geological Society, London, 163, 741–751, https://doi.org/10.1144/0016-76492005-162
    OpenUrlAbstract/FREE Full Text
  17. ↵
    1. Færseth, R.B. and
    2. Ravnas, R
    . 1998. Evolution of the Oseberg fault-block in context of the northern North Sea structural framework. Marine and Petroleum Geology, 15, 467–490, https://doi.org/10.1016/S0264-8172(97)00046-9
    OpenUrlCrossRefWeb of Science
  18. ↵
    1. Fisher, Q.J. and
    2. Knipe, R.J.
    1998. Fault sealing processes in siliciclastic sediments. Geological Society, London, Special Publications, 147, 117–134, https://doi.org/10.1144/GSL.SP.1998.147.01.08
  19. ↵
    1. Fisher, Q.J. and
    2. Knipe, R.J
    . 2001. The permeability of faults within siliciclastic petroleum reservoirs of the North Sea and Norwegian Continental Shelf. Marine and Petroleum Geology, 18, 1063–1081, https://doi.org/10.1016/S0264-8172(01)00042-3
    OpenUrlCrossRefWeb of Science
  20. ↵
    1. Fossen, H.,
    2. Mangerud, G.,
    3. Hesthammer, J.,
    4. Bugge, T. and
    5. Gabrielsen, R.H
    . 1997. The Bjorøy Formation: a newly discovered occurrence of Jurassic sediments in the Bergen Arc System. Norsk Geologisk Tidsskrift, 77, 269–287.
    OpenUrlWeb of Science
  21. ↵
    1. Fossen, H.,
    2. Schultz, R.A.,
    3. Shipton, Z.K. and
    4. Mair, K
    . 2007. Deformation bands in sandstone: a review. Journal of the Geological Society, London, 164, 755–769, https://doi.org/10.1144/0016-76492006-036
    OpenUrlAbstract/FREE Full Text
  22. ↵
    1. Fossen, H.,
    2. Khani, H.F.,
    3. Faleide, J.I.,
    4. Ksienzyk, A.K. and
    5. Dunlap, W.J
    . 2017a. Post-Caledonian extension in the West Norway–northern North Sea region: the role of structural inheritance. Geological Society, London, Special Publications, 439, 465–486, https://doi.org/10.1144/SP439.6
  23. ↵
    1. Fossen, H.,
    2. Ksienzyk, A.K. and
    3. Jacobs, J
    . 2017b. Correspondence: Challenges with dating weathering products to unravel ancient landscapes. Nature Communications, 8, 1502, https://doi.org/10.1038/s41467-017-01457-9
    OpenUrl
  24. ↵
    1. Gabrielsen, R.H. and
    2. Koestler, A.G
    . 1987. Description and structural implications of fractures in late Jurassic sandstones of the Troll Field, northern North Sea. Norsk Geologisk Tidsskrift, 67, 371–381.
    OpenUrlWeb of Science
  25. ↵
    1. Horstad, I. and
    2. Larter, S.R
    . 1997. Petroleum migration, alteration, and remigration within Troll field, Norwegian North Sea. AAPG Bulletin, 81, 222–248.
    OpenUrlAbstract
  26. ↵
    1. IEAGHG
    2015. Criteria of Fault Geomechanical Stability During A Pressure Build-Up. IEAGHG Report 2015/04. IEA Greenhouse Gas R&D Programme (IEAGHG), Cheltenham, UK.
  27. ↵
    1. IEAGHG
    2016. Fault Permeability. IEAGHG Report 2016/3. IEA Greenhouse Gas R&D Programme (IEAGHG), Cheltenham, UK.
  28. ↵
    1. IPCC
    2005. Carbon Dioxide Capture and Storage: Special Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.
  29. ↵
    1. Japsen, P. and
    2. Chalmers, J.A
    . 2000. Neogene uplift and tectonics around the North Atlantic: overview. Global and Planetary Change, 24, 165–173, https://doi.org/10.1016/S0921-8181(00)00006-0
    OpenUrlCrossRefWeb of Science
  30. ↵
    1. Kampman, N.,
    2. Bickle, M.,
    3. Wigley, M. and
    4. Dubacq, B
    . 2014. Fluid flow and CO2–fluid–mineral interactions during CO2-storage in sedimentary basins. Chemical Geology, 369, 22–50, https://doi.org/10.1016/j.chemgeo.2013.11.012
    OpenUrlCrossRef
  31. ↵
    1. Karolytė, R.,
    2. Johnson, G.,
    3. Yielding, G. and
    4. Gilfillan, S.M
    . 2020. Fault seal modelling – the influence of fluid properties on fault sealing capacity in hydrocarbon and CO2 systems. Petroleum Geoscience, 26, 481–497, https://doi.org/10.1144/petgeo2019-126
    OpenUrlAbstract/FREE Full Text
  32. ↵
    1. Kim, Y.S. and
    2. Sanderson, D.J
    . 2005. The relationship between displacement and length of faults: a review. Earth-Science Reviews, 68, 317–334, https://doi.org/10.1016/j.earscirev.2004.06.003
    OpenUrl
  33. ↵
    1. Knai, T.A. and
    2. Knipe, R.J
    . 1998. The impact of faults on fluid flow in the Heidrun Field. Geological Society, London, Special Publications, 147, 269–282, https://doi.org/10.1144/GSL.SP.1998.147.01.18
  34. ↵
    1. Knipe, R
    . 1997. Juxtaposition and seal diagrams to help analyze fault seals in hydrocarbon reservoirs. AAPG Bulletin, 81, 187–195.
    OpenUrlAbstract
  35. ↵
    1. Ksienzyk, A.K.,
    2. Wemmer, K. et al.
    2016. Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating. Norwegian Journal of Geology, 96, 275–299, https://doi.org/10.17850/njg96-3-06
    OpenUrl
  36. ↵
    1. Lauritsen, H.,
    2. Kassold, S.,
    3. Meneguolo, R. and
    4. Furre, A.
    2018. Assessing potential influence of nearby hydrocarbon production on CO2 storage at Smeaheia. In: Conference Proceedings of the Fifth CO2 Geological Storage Workshop, November 2018. European Association of Geoscientists and Engineers (EAGE), Houten, The Netherlands, https://doi.org/10.3997/2214-4609.201802970
  37. ↵
    1. Lindsay, N.G.,
    2. Murphy, F.C.,
    3. Walsh, J.J. and
    4. Watterson, J
    . 1993. Outcrop studies of shale smear on fault surfaces. International Association of Sedimentologists Special Publications, 15, 113–123.
  38. ↵
    1. Miocic, J.,
    2. Johnson, G. and
    3. Bond, C.E
    . 2019. Uncertainty in fault seal parameters: implications for CO2 column height retention and storage capacity in geological CO2 storage projects. Solid Earth, 10, 951–967, https://doi.org/10.5194/se-10-951-2019
    OpenUrl
  39. ↵
    1. Mulrooney, M.J.,
    2. Osmond, J.L.,
    3. Skurtveit, E.,
    4. Faleide, J.I. and
    5. Braathen, A
    . 2020. Structural analysis of the Smeaheia fault block, a potential CO2 storage site, northern Horda Platform, North Sea. Marine and Petroleum Geology, 121, 104598, https://doi.org/10.1016/j.marpetgeo.2020.104598
    OpenUrl
  40. ↵
    1. Nazarian, B.,
    2. Thorsen, R. and
    3. Ringrose, P.
    2018. Storing CO2 in a reservoir under continuous pressure depletion; A simulation study. Paper presented at the 14th Greenhouse Gas Control Technologies Conference (GHGT-14), 21–26 October 2018, Melbourne, Australia.
  41. ↵
    1. Odinsen, T.,
    2. Reemst, P.,
    3. Van Der Beek, P.,
    4. Faleide, J.I. and
    5. Gabrielsen, R.H
    . 2000. Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling. Geological Society, London, Special Publications, 167, 83–103, https://doi.org/10.1144/GSL.SP.2000.167.01.05
  42. ↵
    1. Ottesen Ellevset, S.,
    2. Knipe, R.J.,
    3. Svava Olsen, T.,
    4. Fisher, Q.J. and
    5. Jones, G.
    1998. Fault controlled communication in the Sleipner Vest Field, Norwegian Continental Shelf; detailed, quantitative input for reservoir simulation and well planning. Geological Society, London, Special Publications, 147, 283–297, https://doi.org/10.1144/GSL.SP.1998.147.01.19
  43. ↵
    1. Pawar, R.J.,
    2. Bromhal, G.S. et al.
    2015. Recent advances in risk assessment and risk management of geologic CO2 storage. International Journal of Greenhouse Gas Control, 40, 292–311, https://doi.org/10.1016/j.ijggc.2015.06.014
    OpenUrl
  44. ↵
    1. Phillips, T.B.,
    2. Fazlikhani, H. et al.
    2019. The influence of structural inheritance and multiphase extension on rift development, the northern North Sea. Tectonics, 38, 4099–4126, https://doi.org/10.1029/2019TC005756
    OpenUrl
  45. ↵
    1. Ringrose, P.
    2020. How to Store CO2 Underground: Insights From Early-Mover CCS Projects. Springer Nature, Cham, Switzerland.
  46. ↵
    1. Ringrose, P.S.,
    2. Mathieson, A.S. et al.
    2013. The In Salah CO2 storage project: lessons learned and knowledge transfer. Energy Procedia, 37, 6226–6236, https://doi.org/10.1016/j.egypro.2013.06.551
    OpenUrlCrossRef
  47. ↵
    1. Serck, C.S. and
    2. Braathen, A
    . 2019. Extensional fault and fold growth: impact on accommodation evolution and sedimentary infill. Basin Research, 31, 967–990, https://doi.org/10.1111/bre.12353
    OpenUrl
  48. ↵
    1. Shaw, J.H.,
    2. Hook, S.C. and
    3. Sitohang, E.P
    . 1997. Extensional fault-bend folding and synrift deposition: An example from the Central Sumatra Basin, Indonesia. AAPG Bulletin, 81, 367–379.
    OpenUrlAbstract
  49. ↵
    1. Torsvik, T.H.,
    2. Andersen, T.B.,
    3. Eide, E.A. and
    4. Walderhaug, H.J
    . 1997. The age and tectonic significance of dolerite dykes in western Norway. Journal of the Geological Society, London, 154, 961–973, https://doi.org/10.1144/gsjgs.154.6.0961
    OpenUrlAbstract/FREE Full Text
  50. ↵
    1. Tucker, O.,
    2. Holley, M.,
    3. Metcalfe, R. and
    4. Hurst, S
    . 2013. Containment risk management for CO2 storage in a depleted gas field, UK North Sea. Energy Procedia, 37, 4804–4817, https://doi.org/10.1016/j.egypro.2013.06.390
    OpenUrl
  51. ↵
    1. Whipp, P.S.,
    2. Jackson, C.L.,
    3. Gawthorpe, R.L.,
    4. Dreyer, T. and
    5. Quinn, D
    . 2014. Normal fault array evolution above a reactivated rift fabric; a subsurface example from the northern Horda Platform, Norwegian North Sea. Basin Research, 26, 523–549, https://doi.org/10.1111/bre.12050
    OpenUrl
  52. ↵
    1. Wibberley, C.A.,
    2. Yielding, G. and
    3. Di Toro, G
    . 2008. Recent advances in the understanding of fault zone internal structure: a review. Geological Society, London, Special Publications, 299, 5–33, https://doi.org/10.1144/SP299.2
  53. ↵
    1. Wu, L.,
    2. Thorsen, R.,
    3. Ringrose, P.,
    4. Ottesen, S. and
    5. Hartvedt, K.
    2019. Significance of fault seal in assessing CO2 storage capacity and leakage risks – an example from offshore Norway. In: Conference Proceedings of the Fifth International Conference on Fault and Top Seals, September 2019. European Association of Geoscientists and Engineers (EAGE), Houten, The Netherlands, https://doi.org/10.3997/2214-4609.201902319
  54. ↵
    1. Xiao, H. and
    2. Suppe, J
    . 1992. Origin of Rollover (1). AAPG Bulletin, 76, 509–529.
    OpenUrlAbstract
  55. ↵
    1. Yielding, G.
    2002. Shale gouge ratio – Calibration by geohistory. Norwegian Petroleum Society Special Publications, 11, 1–15.
  56. ↵
    1. Yielding, G.,
    2. Freeman, B. and
    3. Needham, D.T
    . 1997 Quantitative fault seal prediction. AAPG Bulletin, 81, 897–917.
    OpenUrlAbstract
  57. ↵
    1. Yielding, G.,
    2. Bretan, P. and
    3. Freeman, B
    . 2010. Fault seal calibration: a brief review. Geological Society, London, Special Publications, 347, 243–255, https://doi.org/10.1144/SP347.14
  58. ↵
    1. Yielding, G.,
    2. Lykakis, N. and
    3. Underhill, J.R
    . 2011. The role of stratigraphic juxtaposition for seal integrity in proven CO2 fault-bound traps of the Southern North Sea. Petroleum Geoscience, 17, 193–203, https://doi.org/10.1144/1354-0793/10-026
    OpenUrlAbstract/FREE Full Text
  59. ↵
    1. Ziegler, P.A.
    1990. Tectonic and palaeogeographic development of the North Sea rift system. In: Blundell, D.J. and Gibbs, A. (eds) Tectonic Evolution of North Sea Rifts. Clarendon Press, Oxford, UK, 1–36.
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Petroleum Geoscience: 27 (3)
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