Geological Setting/Structural Description
A reduced capacity Smeaheia prospect:
Structural risks associated with an Alpha-only storage scenario
This publication has been produced with support from the NCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the
following partners for their contributions: Aker Solutions, ANSALDO Energia, CoorsTek Membrane Sciences, EMGS, Equinor, Gassco, KROHNE, Larvik Shipping, Norcem, Norwegian Oil and Gas, Quad Geometrics,
Shell, TOTAL, and the Research Council of Norway (257579/E20).
Mark Joseph Mulrooney1, Johnathon Lee Osmond1, Elias H. Leon1, Elin Skurtveit1,2 and Alvar Braathen1
1. Department of Geosciences, University of Oslo (UiO), PO Box 1047, Blindern, 0316 Oslo, Norway
2. Norwegian Geotechnical Institute (NGI), PO Box 3930, Ullevaal Stadion, 0806 Oslo, Norway
Smeaheia (Fig.1), a potential subsurface storage unit situated in the northern North Sea, east of
Troll East was previously considered as the most suitable location for storage of CO2for a
proposed full-chain CCS project in Norway. Originally, two closures, the Alpha (to the west) and
Beta (to the east) where defined. CO2injected into the Alpha structure was envisaged to fill the
structure (approx. 100 Mt) and spill over to the Beta structure (an additional 100 Mt capacity;
Fig. 2). Juxtaposition of the potential storage unit (the Jurassic Sognefjord-Fensfjord-Krossfjord
formations), however, against Caledonian basement adjacent to the Beta closure has been
considered too high risk for the Beta structure to be considered for the full-chain CCS project.
Moreover, the Alpha structure alone is not considered to have sufficient capacity, and has lead
to an alternative site being chosen (Aurora). The Alpha structure, however, prevails as a
potential “add-on” site to expand future storage capacity in the North Sea. Key to the success
of Alpha is the nature of cross-fault juxtaposition of the Vette Fault Zone where the storage unit
in Alpha is primarily self-separated across the fault zone, and juxtaposed with mixed
siliciclastic-carbonate successions of the Cretaceous overburden. A complex relay zone of
several Vette Fault Zone segments to the south primarily exhibit cross-fault self-juxtaposition of
the storage unit. This relay zone likely provides pressure communication between Smeaheia
and the Troll field suggesting Smeahiea is depleted. Subsidiary faults that intersect the storage
unit in Alpha pose potential baffles to CO2migration and require consideration when
positioning injector wells. A small population of tectonic faults, including the Vette 01 fault,
continue up section where they intersect the caprock and overburden. These faults provide
potential caprock-bypass risk, and as such the likeliness of their reactivation under various
pressure regimes is considered. Moreover, a network of low-displacement polygonal faults that
intersect the overburden have been mapped, and in places are hard-linked to tectonic faults.
Finally, the distribution of pock-marks both on the seafloor and on intra-Quaternary horizons
are considered in relation to underlying faults to discern whether these paleo-seepage events
are from significant depths or from shallow methanogenic gas.
Figure 1. A) Primary structural elements map of the North Sea (faults, basins, and highs) emphasising the trilete rift system and dominantly Permo-
Triassic and Jurassic depocentres. Sector boundaries shown with red stippled line. Compiled from Roberts et al. (1995), Færseth et al. (1995), (1996)
and Domínguez (2007).Inset: Simplified outline of Western Europe showing geographical location of the North Sea rift system. B) Structural elements
and oil/gas accumulations of the northern Horda Platform in the Norwegian sector of the northern North Sea. Redrafted from the Norwegian
Petroleum Directorate Fact Maps (http://npd.no/en/Maps/Fact-maps). Location of the GN1101 Survey subject to this study shown in blue.
Figure 2. Interpreted WSW–ENE trending seismic transect of the GN1101 3D survey. Location shown in Figure 1. The transect intersects both historical exploration wells 31/4-1 and 32/2-1 that targeted the Alpha and Beta prospects, respectively.
The seismic interpretation shows the Smeaheia target formation and caprock unit are bounded by two basement-involved faults, the Vette Fault Zone and the Øygarden Fault Complex. Prospective CO2–water contacts are shown for an original fill-
to-spill scenario. Migration of CO2from a filled-to-spill Alpha prospect is depicted by blue arrows. Removing the Beta prospect, significantly reduces Alpha capacity to mitigate against spill towards Beta. The target formation in the Alpha prospect is
not intersected by intra-block subsidiary faults in this section, whereas the Beta prospect is intensely faulted. Note, a large population of closely spaced, low displacement normal faults affect the Upper Cretaceous to Lower Cenozoic but do not
occur above the base Quaternary unconformity. A narrow graben with possible Jurassic infill (no well constraints) is interpreted in the footwall of the Øygarden Fault Complex. At dept, high amplitude basement reflectors likely representing
Devonian and Caledonian structures and metasediment are highlighted.
Figure 3. Fault heave maps for key stratigraphic surfaces with fault dip
direction indicated by colour, A) Top Draupne Formation, B) Top
Sognefjord Formation and C) Top Brent Group. A rose diagram showing
fault strike for the Top Sognejford Formation is also shown in (B). The
Smeaheia fault block bounding Vette Fault Zone and the Øygarden Fault
Complex trend north–south and dip towards the west. Subsidiary faults
strike northwest–southeast and are predominantly synthetic to the
fault block bounding faults, although a small population of antithetic
(east-dipping) faults also exist.
Figure 4. A) Variance attribute map of the Vette Fault Zone (left) and
interpretation (right) for the Top Sognefjord Formation. Red faults dip
towards the west whereas blue faults dip towards the east. Note the fault
zone continues both north and south beyond the coverage of the GN1101
3D seismic survey. Three primary segments of the fault zone that form a
complex relay zone are labelled (Vette 01,02 and 03). B) A 3D perspective
view of the relay zone as expressed at the Top Sognefjord Formation (top)
with interpretation (bottom) of the primary fault segments and smaller
splays. C) Simplified juxtaposition diagram of faults in the vicinity of the relay
zone. See figures 5 and 6 for more fault attributes of the Vette 01 and 02
faults and for the juxtaposition colour legend.
Figure 5. Perspective images of an approx. 15 km long section of the
main segment of the Vette Fault Zone segment (Vette 01) showing A)
fault –horizon intersection (cut-offs), B) simplified lithological
juxtaposition across fault and C) fault throw distribution. The lightest
blue colour in (B) represents where the reservoir in Alpha is
juxtaposed against the Cretaceous overburden. This is the dominant
Figure 6. Perspective images of an approx. 3 km long section of the Vette 02
that intersects the alpha prospect showing A) fault –horizon intersections
(cut-offs), B) simplified lithological juxtaposition across fault and C) fault
throw distribution. For the most part, the reservoir interval is self-
juxtaposed across the Vette relay zone, and as such a pathway for reservoir
depletion (due to Troll production) is envisaged.
Figure 8. Thickness variation map of the Draupne Formation from the GN1101 3D
seismic survey. Note the formation is drastically thinner in the immediate footwall
of the Vette Fault Zone. Outline of the Alpha and Beta prospect’s CO2–water
contacts in fill-to-spill scenarios also shown.
Figure 11. Mohr Circles showing effective in situ stress (normal and shear) for the proposed reservoir interval at A),
hydrostatic pressure, 12.6 MPa at the midpoint, B) depletion scenario 1, 9.6 MPa at the midpoint, and C) depletion
scenario 2, i.e. 7.6 MPa at the midpoint. D) Mohr circle plot for the caprock interval for a hydrostatic pressure
regime, 10.3 MPa at the midpoint. Coloured lines represents the Mohr-Columb failure envelope, i.e. the shear stress
at which the faults in Smeaheia are envisaged to fail (τ=σtan (Φ) + C). The blue and black lines show the Mohr
circle and the Mohr-Columb failure envelope for results of Slip Stability (σn -τ/Φ) for a classic fault (friction angle =
31o) and reservoir derived rock properties (friction angle = 15o), respectively. whereas the red line shows results for
Fracture Stability (σn + [C2-τ2]/2C). Note all Fracture Stability scenarios, which considers faults to have similar
cohesion as the undeformed country rock, show no portions of the fault planes are at risk of failure. For Slip Stability
with classic fault properties, again no scenario predicts fault failure. Conversely, for Slip Stability, where faults are
modelled with host rock friction angle, but zero cohesion, all scenarios show intersection of the Mohr Circle with
the Mohr-Columb failure envelope. Increasing reservoir depletion (B and C) reduce the likelihood of fault failure.
Figure 12. Horizontal projections of faults intersecting the potential storage formation with stress analysis results
expressed as coloured fault attributes. A) Slip Tendency, B) Dilation Tendency, C) Slip Stability (classic fault), D)
Fracture Stability (reservoir rock properties).
Figure 9. A) Expanded view of the WSW–ENE trending seismic transect shown if Figure 2 highlighting overburden-confined, low displacement,
closely spaced faulting. A small number of faults affecting the overburden can be seen continuing down-section and intersecting the potential CO2
reservoir (Jurassic Sognefjord-Fensfjord-Krossfjord formations). B) Perspective view of faults effecting the Smeaheia overburden. The top of the
Draupne Formation underlying the low displacement faults are shown in grey. The green line represents the northern extent of the GN1101
survey, while the white line delineates the alpha prospect’s potential CO2–water contacts in a fill-to-spill scenario. Vertical exaggeration (V.E.) is
denoted in (A) and (B). C) Elevation depth maps showing fault–horizon intersection trace lines for the top of the Shetland and Kromer Knoll
groups. Tectonic faults that extend down-section into the Sognefjord Formation are shown in pink, while the Alpha prospect is outlined in purple.
Figure 10. Variance attribute map of the seafloor, showing random clustering of pockmarks. Such features have been identified on several
intra-Quaternary horizons alongside glacial striations. Pockmarks are interpreted to results from degassing events sourced by shallow
methanogenic gases during deglaciation. Fluids are envisaged to have migrated up dip along subcropping shallow reflectors. No evidence of
leakage of deeper fluids from the reservoir interval in Smeaheia has been seen. B) Blue outlines represent the Alpha (left) and Beta (right)
closures in fill-to-spill scenarios.
Two primary closures (Sognefjord-Fensfjord-Krossfjord formations) are defined, Alpha, a 3-way closure in the footwall of the Vette Fault Zone (VFZ),
and Beta, a 3 way closure in the hanging wall of the Øygarden Fault complex (ØGC). The structures formed as a footwall rebound anticline, and a
hanging wall rollover anticline, respectively.
Considering an Alpha-only prospect significantly reduces the original estimated storage volume of Smeaheia. Smeaheia, however, can still be used as
an “add-on” structure at a later stage in development of North Sea infrastructure for CCS.
A satellite storage unit, the Johansen-Cook Formation has a similar closure geometry in Alpha to the originally assessed Songnefjord-Fensfjord-
Krossfjord storage unit. Potential deeper Triassic storage units show no Alpha closure, and would migrate immediately to the high-risk Beta closure.
The VFZ and ØFZ are north–south striking basement involved faults which were active both during the Permo-Triassic and the Late Jurassic –Early
Cretaceous phases of rifting. For the latter, most displacement accrued in the Early Cretaceous, later than recorded on fault blocks further west.
The Alpha closure is primarily juxtaposed against the Cromer Knoll Group across the VFZ. Modest Vshale estimates indicate the fault is sealing.
The primary caprock is significantly eroded (Northern North Sea Unconformity Complex), and possibly absent in parts of the Alpha closure. A highly
argillaceous overburden, however, probably provides a secondary seal, analogous to Troll East.
Linkage of a major relay zone in the VFZ took place in the second rift phase. The reservoir interval is primarily self-juxtaposed across these faults and
provides a pressure communication pathway with Troll East; depletion is probable.
NW–SE striking intra-block subsidiary faults developed during the second rift phase. The reservoir unit is primarily self-juxtaposed across these faults,
however, they may support small CO2columns and subsequent pressure buildups during injection; dip direction dependant.
A population of low displacement polygonal faults are mapped within the Cromer Knoll, Rogaland and Hordaland groups and developed during the
Eocene to middle Miocene. They are primarily strata-bound but occasionally hard linked with tectonic faults at depth.
A dense population of pockmarks were mapped on the seafloor and within Quaternary sediments and indicate post-glacial dissociation and leakage
of shallow methanogenic gas hydrates. The features are not consistent with leakage of hydrocarbons from deeper seated reservoirs.
There is a low to moderate risk (especially on north–south striking faults) of reactivation on faults given a hydrostatic reservoir pressure. Depleted
scenarios (probable), and considering conservative cohesion on fault surfaces significantly reduce reactivation risk.
Figure 7. Simplified juxtaposition of the Vette Fault Zone as depicted in figures 5 and 6. The Alpha closure is primarily juxtaposed against the Cretaceous
overburden across the Vette 01 fault. The Vette relay zone, however, primarily shows self-juxtapostiion of the reservoir. A calibration study (Osmond et al.
2019) is currently underway, and attempts to relate quantifiable fault seal of the neighbouring Tusse fault (Troll field) to that of the Vette Fault Zone.