Article

Structural analysis of the Smeaheia fault block, a potential CO2 storage site, northern Horda Platform, North Sea

Article

Structural analysis of the Smeaheia fault block, a potential CO2 storage site, northern Horda Platform, North Sea

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Abstract

Smeaheia, a prominent fault block located on the Horda Platform, northern North Sea is identified as a potential subsurface CO2 storage site. We utilize the GN1101 3D seismic survey to generate a high-resolution subsurface geomodel to inform the structural style and evolution of the fault block, to investigate geological controls on proposed CO2 storage and provide a geometric framework as a basis for future analyses. Two basement-involved (first-order) north-south trending fault systems, the Vette Fault Zone (VFZ) and the Øygarden Fault Complex (ØFC), bound the 15 km-wide fault block. The VFZ bifurcates down-section where it is hard-linked with two separate basement structures, a phenomenon we term as “dual rooted”. Apart from activity during the Permo-Triassic (Rift Phase 1) and the Late Jurassic–Early Cretaceous (Rift Phase 2), we present evidence that rifting in this part of the North Sea continued into the Late Cretaceous with minor reactivation in the Palaeocene –Eocene. Two segments of the VFZ interacted and linked at a relay ramp during Rift Phase 2. Second-order (thin-skinned), faults show basement affinity and developed during Rift Phase 2 in two distinct pulses. A population of polygonal faults intersects the overburden and developed during the Eocene to middle Miocene. We have revised the areal extent of two structural closures that define the Smeaheia fault block, Alpha (VFZ footwall) and Beta (ØFC hanging wall) which consist of Upper Jurassic Viking Group target formations. Simplified cross-fault juxtaposition analysis of the VFZ and second-order intra-block faults are presented and inform pressure communication pathways between the Smeaheia and Tusse fault block, as well as reservoir integrity and compartmentalisation. The geomodel further identifies important geological controls on CO2 storage in the fault block including a thinning caprock above the Alpha structure, and identification of hard-linkage between deep tectonic faults and shallow polygonal faults.

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... The uppermost formation of this interval is encountered throughout the northern Horda platform and areas to the north and has an estimated total storage capacity of nearly 18 Gt (Norwegian Petroleum Directorate, 2011). Unfortunately, only limited work has been done to advance this storage concept, with the exception of a few recent structural, seismological, and petrophysical studies (e.g., Mulrooney et al., 2020;Rahman et al., 2020;Fawad et al., 2021a, b;Osmond et al., 2021b;Wu et al., 2021). ...
... A second major phase of rifting transpired from the Late Jurassic through Early Cretaceous after cooling and deflation of the North Sea dome (e.g., Underhill and Partington, 1993;Roberts et al., 1995;Faerseth, 1996;Faerseth et al., 1997;Odinsen et al., 2000;Davies et al., 2001;Coward et al., 2003). Reactivated, thick-skinned normal faults from the Permian-Triassic rift phase sustained lower slip rates and displacement during the Late Jurassic through Early Cretaceous phase (Odinsen et al., 2000;Bell et al., 2014;Phillips et al., 2019;Fazlikhani et al., 2020), while additional northeast-southwest-and northwest-southeast-trending faults with displacements under 150 m formed oblique to the dominant north-south-trending structures ( Figure 3B) Duffy et al., 2015;Deng et al., 2017;Mulrooney et al., 2020). A fully marine depositional environment prevailed during much of the second rift phase (e.g., Nøttvedt et al., 1995;Stewart et al., 1995), resulting in the deposition of the siliciclastic Viking Group (e.g., Vollset and Dor e, 1984;Sneider et al., 1995;Stewart et al., 1995;Husmo et al., 2002), and later, the mixed siliciclastic and carbonate sedimentary successions of the Cromer Knoll and Shetland Groups (e.g., Isaksen and Tonstad, 1989;Rattey and Hayward, 1993;Bugge et al., 2001;Gradstein and Waters, 2016) (Figures 2B, 3B). ...
... In much of the northern North Sea, rifting ceased by the end of the Early Cretaceous (e.g., Faerseth, 1996;Coward et al., 2003;Bell et al., 2014;Phillips et al., 2019); however, displacement continued to accrue along many faults during the late Paleocene or possibly early Eocene (Bell et al., 2014;Whipp et al., 2014;Mulrooney et al., 2020) (Figure 3B), primarily as a result of thermal subsidence and compaction of sedimentary deposits, but possibly also due to local or stress perturbations associated with North Atlantic rifting (e.g., Ziegler, 1992;Roberts et al., 1999, Faleide et al., 2002. Marine conditions dominated during the Paleogene and Neogene history, where westward-dipping, siliciclastic sediments of the Rogaland and Hordaland Groups (e.g., Isaksen and Tonstad, 1989;Jordt et al., 2000;Eidvin and Rundberg, 2007;Brunstad et al., 2013) were deposited into a thermally subsiding basin (Faleide et al., 2002;Anell et al., 2012;Jarsve et al., 2014a) (Figures 2B, 3B). ...
... Inset: Simplified outline of the North Sea area emphasising the North Sea Triple Junction. Compiled from Faerseth (1996), Goldsmith et al. (2003), and Mulrooney et al. (2020). (b) Cross-section through the northern North Sea. ...
... About 1-3 km thick packages of Triassic strata lie within the large eastward-dipping half-grabens (Smeaheia, Tusse, and Svartalv fault blocks) bounded by westward dipping normal faults (Øygarden, Vette, Tusse, and Svartalv fault zones). The faults are spaced 15-20 km apart and exhibit throw in the range of 4-5 km (Mulrooney et al., 2020) ( Figure 1c). ...
... The transitions in seismic facies below the Base PT are reminiscent of compositional heterogeneity and varying structural fabric as seen in for example, dikes and nappestructures Lenhart et al., 2019), which are characteristic of the crystalline rocks in the onshore Caledonian massif (Boundy et al., 1992). The sigmoidal reflections are also a possible candidate for a Devonian shear zones (Fazlikhani et al., 2017;Mulrooney et al., 2020). The wedge-shaped feature in the Smeaheia Fault Block, also noted by Christiansson et al. (2000), however, is striking. ...
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The Permian-Triassic alluvial rift succession in the Horda Platform area is analysed to construct a refined depositional model. The study demonstrates how subsurface continental rift successions may be stratigraphically subdivided and correlated by integrating seismic and well-log data in concert with conceptual models. Regional unconformities mark the top and base of the Permian-Triassic succession, which is sub-divided into six seismic stratigraphic sequences, delineated by erosional- and non-depositional surfaces. The definition of seismic stratigraphic sequences is based on seismic facies trends and Gamma Ray log signatures. Time-thickness maps combined with geometries in cross-section display the depocentre development. During the Permian-Triassic, the Horda Platform experienced faulting, shaping the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, contemporaneous to a gradual climate change from arid in the Permian to humid in the latest Triassic. The basin underwent three phases of rifting during the Late Permian-Middle Triassic; (1) disconnected heterogeneous depocentres with strain concentrated in the west; (2) depocentres expanded northward; and (3) mature half graben development with widely distributed strain. Vertical lithological changes in mudstone/sandstone dominance reflect varying rates of accommodation and sediment supply (A/S). Systematic A/S variation reflects strong climatic fluctuations, which controlled facies patterns during both tectonic quiescence in the Middle-Late Triassic, and during syn-rift sedimentation. The Permian-Triassic tectono-sedimentary development in the Horda Platform area provides valuable lessons on the influence of faulting on depocentre development, how the interplay between tectonic and climatic forcing is expressed in subsurface continental deposits, and aid the characterization of reservoirs.
... The thickness variation of the Syn-rift 2 growth package can also be observed from the well correlation across the VFS ( Figure 5). The Vette fault segment (VF#1 in Figure 6) and a NW-SE-trending fault bounding the Alpha structure (i.e., IBF1 in Figure 6) have reached the Base Nordland Unconformity (Figure 4 and 6; also see Mulrooney et al. 2020). The Vette fault system has several segments linked by relay ramps (some breached) which control the across-fault juxtaposition of strata between Smeaheia and Troll East (Figure 2 and 6). ...
... 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 (Figure 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). ...
... The newly formed intra-block 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 Figure 4 and 6d), and an array of NW-SE-trending normal faults formed to the south of the Alpha structure (Figure 9c; also see Mulrooney et al., 2020). These newly formed faults (e.g., Post-rift 2 fault in Figure 4) show a thin-skin feature, with larger fault displacement in the overburden section (vs. ...
Article
Understanding of fault seal is crucial for assessing the storage capacity and containment risks of CO 2 storage sites, as it can significantly affect the projects on across-fault and along-fault migration/leakage risking, 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 Alpha and Beta structures. We aim to use this study to improve the geological understanding of the northern Horda Platform for CO 2 storage scale-up potentials and illustrate the importance of fault seal analysis in containment risk assessment and storage capacity evaluation of a CO 2 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 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 CO 2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage
... Several studies have been performed on the feasibility of the Smeaheia CO2 storage site (e.g. Sundal et al., 2014;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Wu et al., 2021). The Alpha prospect 30 identified for this site is located within a tilted fault block bound by a deep-seated basement fault: the Vette Fault Zone (VFZ) (Skurtveit et al., 2012;Mulrooney et al., 2020), and hence a high fault sealing capacity is required to retain the injected CO2. ...
... Sundal et al., 2014;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Wu et al., 2021). The Alpha prospect 30 identified for this site is located within a tilted fault block bound by a deep-seated basement fault: the Vette Fault Zone (VFZ) (Skurtveit et al., 2012;Mulrooney et al., 2020), and hence a high fault sealing capacity is required to retain the injected CO2. ...
... The Smeaheia site, see Mulrooney et al. (2020, and references therein), is located approximately 40 km Northwest of the Kollsnes processing plant, and around 20 km East of Troll East, in the Northern Horda Platform (Figure 2). The Northern Horda Platform is a 300 km by 100 km, N-S elongated structural high along the eastern margin of the northern North Sea (Faerseth, 1996;Whipp et al., 2014;Duffy et al., 2015;Mulrooney et al., 2020; Figure 2). Many deep-seated, west-dipping, basement faults occur within the Horda Platform, generating several half graben bounding fault systems with km-scale throws 165 (Badley et al., 1988;Yielding et al., 1991;Faerseth 1996;Bell et al., 2014;Whipp et al., 2014). ...
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Significant uncertainties occur through varying methodologies when interpreting faults using seismic data. These uncertainties are carried through to the interpretation of how faults may act as baffles/barriers or increase fluid flow. How fault segments are picked when interpreting structures, i.e. what seismic line spacing is specified, as well as what surface generation algorithm is used, will dictate how detailed the surface is, and hence will impact any further interpretation such as fault seal or fault growth models. We can observe that an optimum spacing for fault interpretation for this case study is set at approximately 100 m. It appears that any additional detail through interpretation with a line spacing of ≤ 50 m adds complexity associated with sensitivities by the individual interpreter. Further, the location of all fault segmentation identified on Throw-Distance plots using the finest line spacing are also observed when 100 m line spacing is used. Hence, interpreting at a finer scale may not necessarily improve the subsurface model and any related analysis, but in fact lead to the production of very rough surfaces, which impacts any further fault analysis. Interpreting on spacing greater than 100 m often leads to overly smoothed fault surfaces that miss details that could be crucial, both for fault seal as well as for fault growth models. Uncertainty in seismic interpretation methodology will follow through to fault seal analysis, specifically for analysis of whether in situ stresses combined with increased pressure through CO2 injection will act to reactivate the faults, leading to up-fault fluid flow/seep. We have shown that changing picking strategies alter the interpreted stability of the fault, where picking with an increased line spacing has shown to increase the overall fault stability. Picking strategy has shown to have minor, although potentially crucial, impact on the predicted Shale Gouge Ratio.
... The Smeaheia area is bound by two N-S striking basement-involved faults, the Vette Fault Zone to the west, and Øygarden Fault Complex to the east, both active during the Permo-Triassic and the Late Jurassic-Early Cretaceous phases of rifting [11,12,13] and continued into the Late Cretaceous [13]. ...
... The Smeaheia area is bound by two N-S striking basement-involved faults, the Vette Fault Zone to the west, and Øygarden Fault Complex to the east, both active during the Permo-Triassic and the Late Jurassic-Early Cretaceous phases of rifting [11,12,13] and continued into the Late Cretaceous [13]. ...
... With our simulator, two open relay zones along the Vette Fault will result in too low pressure in the Gladsheim well (Fig. 4b). From these simulations we see that some reduced fluid flow should be expected due to uncertainty in the sealing properties of the faults This is somewhat in contradiction to the observation by [13], that has carried out detailed studies of the juxtaposition along the northern relay zone, mapping self-juxtaposition of the storage formation, promoting pressure communication. The work of [21] reviewing relay zones in extensional regime, support that relay zones often act as conduit to fluid flow due to more faults and more deformation and that this is documented for hydrocarbon systems and for ground water relay zones. ...
Article
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A question that is often raised during discussing around underground CO2 storage in saline aquifers, is whether the stored CO2 will stay in place, and how to evaluate and quantify safe storage and ensure acceptable low seepage. In this study a workflow has been tested on a potential subsurface CO2 storage site at the Smeaheia area, offshore Norway. The effect of the pressure depletion from the giant gas Troll Field, starting production in 1995 has been simulated. As a conformance test, the simulated pressure has been compared with measured pressures from a newly drilled exploration well in the study area, with good match. Simulated pressures have used as input to simulate CO2 injection and saturation varying the reservoir heterogeneities. Different conformance tests have been discussed using bottom hole pressure to evaluate potential CO2 injection, or collection new seismic.
... Several studies have been performed on the feasibility of the Smeaheia CO 2 storage site (e.g. Sundal et al., 2014;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Wu et al., 2021). The Alpha prospect identified for this site is located within a tilted fault block bound by a deep-seated basement fault: the Vette Fault Zone (VFZ) (Skurtveit et al., 2018;Mulrooney et al., 2020), and hence a high fault-sealing capacity is required to retain the injected CO 2 . ...
... Sundal et al., 2014;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Wu et al., 2021). The Alpha prospect identified for this site is located within a tilted fault block bound by a deep-seated basement fault: the Vette Fault Zone (VFZ) (Skurtveit et al., 2018;Mulrooney et al., 2020), and hence a high fault-sealing capacity is required to retain the injected CO 2 . Further, it is necessary for the fault to have no reactivation potential. ...
... The Smeaheia site (see Mulrooney et al., 2020, and references therein) is located approximately 40 km northwest of the Kollsnes processing plant, and around 20 km east of Troll East, in the northern Horda Platform (Fig. 2). The northern Horda Platform is a 300 km by 100 km, N-S elongated structural high along the eastern margin of the northern North Sea (Faerseth, 1996;Whipp et al., 2014;Duffy et al., 2015;Mulrooney et al., 2020;Fig. ...
Article
Full-text available
p>Significant uncertainties occur through varying methodologies when interpreting faults using seismic data. These uncertainties are carried through to the interpretation of how faults may act as baffles or barriers, or increase fluid flow. How fault segments are picked when interpreting structures, i.e. which seismic line orientation, bin spacing and line spacing are specified, as well as what surface generation algorithm is used, will dictate how rugose the surface is and hence will impact any further interpretation such as fault seal or fault growth models. We can observe that an optimum spacing for fault interpretation for this case study is set at approximately 100 m, both for accuracy of analysis but also for considering time invested. It appears that any additional detail through interpretation with a line spacing of <span classCombining double low line"inline-formula">≤ 50 m adds complexity associated with sensitivities by the individual interpreter. Further, the locations of all seismic-scale fault segmentation identified on throw-distance plots using the finest line spacing are also observed when 100 m line spacing is used. Hence, interpreting at a finer scale may not necessarily improve the subsurface model and any related analysis but in fact lead to the production of very rough surfaces, which impacts any further fault analysis. Interpreting on spacing greater than 100 m often leads to overly smoothed fault surfaces that miss details that could be crucial, both for fault seal as well as for fault growth models. Uncertainty in seismic interpretation methodology will follow through to fault seal analysis, specifically for analysis of whether in situ stresses combined with increased pressure through CO<span classCombining double low line"inline-formula">2 injection will act to reactivate the faults, leading to up-fault fluid flow. We have shown that changing picking strategies alter the interpreted stability of the fault, where picking with an increased line spacing has shown to increase the overall fault stability. Picking strategy has shown to have a minor, although potentially crucial, impact on the predicted shale gouge ratio.</p
... One identified candidate for CO 2 storage is the Smeaheia site within the Northern Horda Platform, in the Norwegian North Sea (Halland et al., 2011;Statoil, 2016;Lothe et al., 2019). The prospect for this site is bound by a deep-seated basement fault, known as the Vette Fault Zone (VFZ) (Mulrooney et al., 2020;Wu et al., 2021). Fault analysis, such as assessing the likelihood for fault reactivation, is critical for all fault-bound CO 2 storage sites. ...
... Smeaheia is located approximately 40 km northwest of the Kollsnes processing plant and approximately 20 km east of Troll East in the Northern Horda Platform (Halland et al., 2011;Statoil, 2016;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Figure 1a). The Northern Horda Platform is a structural high along the eastern margin of the northern North Sea (Faerseth, 1996;Whipp et al., 2014;Duffy et al., 2015;Mulrooney et al., 2020;Wu et al., 2021;Figure 1a), containing several deep-seated west-dipping basement faults. ...
... Smeaheia is located approximately 40 km northwest of the Kollsnes processing plant and approximately 20 km east of Troll East in the Northern Horda Platform (Halland et al., 2011;Statoil, 2016;Lauritsen et al., 2018;Lothe et al., 2019;Mulrooney et al., 2020;Figure 1a). The Northern Horda Platform is a structural high along the eastern margin of the northern North Sea (Faerseth, 1996;Whipp et al., 2014;Duffy et al., 2015;Mulrooney et al., 2020;Wu et al., 2021;Figure 1a), containing several deep-seated west-dipping basement faults. These basement faults, with km-scale throws, generate several half-grabens across the Horda Platform (Badley et al., 1988;Yielding et al., 1991;Faerseth, 1996;Bell et al., 2014;Whipp et al., 2014). ...
Article
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Generating an accurate model of the subsurface for the purpose of assessing the feasibility of a CO2 storage site is crucial. In particular, how faults are interpreted is likely to influence the predicted capacity and integrity of the reservoir; whether this is through identifying high risk areas along the fault, where fluid is likely to flow across the fault, or by assessing the reactivation potential of the fault with increased pressure, causing fluid to flow up the fault. New technologies allow users to interpret faults effortlessly, and in much quicker time, utilizing methods such as Deep Learning. These Deep Learning techniques use knowledge from Neural Networks to allow end-users to compute areas where faults are likely to occur. Although these new technologies may be attractive due to reduced interpretation time, it is important to understand the inherent uncertainties in their ability to predict accurate fault geometries. Here, we compare Deep Learning fault interpretation versus manual fault interpretation, and can see distinct differences to those faults where significant ambiguity exists due to poor seismic resolution at the fault; we observe an increased irregularity when Deep Learning methods are used over conventional manual interpretation. This can result in significant differences between the resulting analyses, such as fault reactivation potential. Conversely, we observe that well-imaged faults show a close similarity between the resulting fault surfaces when both Deep Learning and manual fault interpretation methods are employed, and hence we also observe a close similarity between any attributes and fault analyses made.
... The Draupne Formation is an organic rich shale deposited under anoxic conditions during the Late Jurassic transgression [31], and a common source rock and seal formation in the North Sea. In the Horda Platform area, Draupne is the primary seal formation for potential carbon dioxide storage in the Sognefjord Formation within the Smeaheia fault block [32], offshore Norway. Its total clay content is about 50% and kaolinite is the major clay mineral [33][34][35]. ...
... Comparison between the two Draupne specimens shows that the fracture topography of DST15 was more altered during shearing ( Figure 6). This may be a consequence of the high normal stresses applied during testing of DST15 (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) ...
Article
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Fractures and faults are critical elements affecting the geomechanical integrity of CO2 storage sites. In particular, the slip of fractures and faults may affect reservoir integrity and increase potential for breach, may be monitored via the resulting seismicity. This paper presents an experimental study on shale samples from Draupne and Rurikfjellet formations from the North Sea and Svalbard, Norway, using a laboratory test procedure simulating the slip of fractures and faults under realistic stress conditions for North Sea CO2 storage sites. The motivation of the study is to investigate whether the slip along the fractures within these shales may cause detectable seismic events, based on a slip stability criterion. Using a direct shear apparatus, frictional properties of the fractures were measured during shearing, as a function of the shear velocity and applied stress normal to the fracture. We calculated the friction coefficient of the fractures during the different stages of the shear tests and analysed its dependency on shear velocity. Information on velocity-dependent friction coefficient and its evolution with increasing slip were then used to assess whether slip was stable (velocity-strengthening) or unstable (velocity-weakening). Results showed that friction coefficient for both Draupne and Rurikfjellet shales increased when the shear velocity was increased from 10 to 50 µm/s, indicating a velocity-strengthening behaviour. Such a behaviour implies that slip on fractures and faults within these formations may be less prone to producing detectable seismicity during a slip event. These results will have implications for the type of techniques to be used for monitoring reservoir and caprock integrity, for instance, for CO2 storage sites.
... The band density is calculated from the logarithmic function given by Equation (1), and only depends on the fault throw. The throw of the Vette fault obtained from the seismic imaging shows local variations [47], but for simplicity we approximate the throw using a piecewise linear profile (see Figure 11), with a maximum throw of 500 m. The width of the damage zone is calculated from the throw according to Equation (2). ...
... A full assessment of this flow, including possible leakage through the fault,would require representation of two-phase effects caused by the deformation bands, which is beyond the scope of this work. Still, to indicate the effects deformation bands can have on flow transport through a fault in a CO 2 injection scenario, we set up a simulation of a large-scale CO 2 injection into the Smeaheia formation in the North Sea, which has the Vette fault as one of its structural boundaries[47]. The goals of the simulation are first to give a proof of concept that deformation bands can be included in field-scale simulations with relatively minor modifications of industry standard simulation setups, and second to gain insight into the potential impact of deformation bands on fluid flow through the fault. ...
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Subsurface storage of CO2 is predicted to rise exponentially in response to the increasing levels of CO2 in the atmosphere. Large-scale CO2 injections into the subsurface require understanding of the potential for fluid flow through faults to mitigate risk of leakage. Here, we study how to obtain effective permeability of deformation bands in the damage zone of faults. Deformation bands are relatively small, low permeability features that can have a significant effect on flow dynamics, however, the discrepancy of scales is a challenge for field-scale simulation. A new analytical upscaling model is proposed in order to overcome some of the shortcomings of conventional upscaling approaches for heterogeneous porous media. The new model captures the fine-scale impact of deformation bands on fluid flow in the near-fault region, and can be derived from knowledge of large-scale fault properties. To test the accuracy of the model it is compared to fine-scale numerical simulations that explicitly include individual deformation bands. For a wide range of different stochastically generated deformation bands networks, the upscaling model shows improved estimate of effective permeability compared to conventional upscaling approaches. By applying the upscaling model to a full-field simulation of the Smeaheia storage site in the North Sea, we show that deformation bands with a permeability contrast higher than three orders of magnitude may act as an extra layer of protection from fluid flow through faults.
... The well is within the Alpha structure of the Smeaheia fault block ( Figure 1). The Smeaheia structure is evaluated as a potential location for CO 2 storage (Gassnova, 2016) with many recent studies focusing on the structural, reservoir, and caprock characterization of the area (e.g., Dupuy et al., 2018;Mulrooney et al., 2020;Rahman et al., 2020;Fawad et al., 2021;Wu et al., 2021). Published work also studied the interplay between CO 2 storage and pressure development in connection with the production of the nearby Troll oil and gas field and possible CO 2 migration pathways towards the eastern structural boundaries of the Smeaheia fault block (e.g., Erichsen et al., 2012;Kaufmann and Gasda, 2018;Lothe et al., 2018;Nazarin et al., 2018). ...
... Several secondary caprock units are discussed in the Cromer Knoll group (Mulrooney et al., 2020) including, in some areas, the siltstones of the Heather Formation (Lothe et al., 2018). ...
Article
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Carbon capture and storage (CCS) is an inevitable action to achieve CO2 emission reduction targets including becoming net-zero by 2050. Increased efforts are therefore required to identify suitable locations for large-scale CO2 storage. In addition to large aquifers, shut down oil and gas fields in the North Sea are logical candidates for offshore large-scale CO2 storage because of their proven storage capacity, reliable caprock integrity, established infrastructure, and public acceptance. However, in some cases, old and legacy wells are subject to high uncertainties in their integrity, and they can compromise CO2 containment in such reservoirs. On the Norwegian Continental Shelf (NCS), such wells are numerous even outside of oil and gas production areas, i.e., legacy wells affecting aquifers. Therefore, there is a clear need for reliable and cost-effective technologies for well integrity evaluation and remediation. This paper discusses a workflow for screening, monitoring, and remediation of legacy wells. In a first stage, the screening of the Horda Platform areas suggested the need for integrity investigation for the exploration well 32/4-1 T2, drilled into the Alpha structure of the Smeaheia fault block if CO2 is stored in the structure. Our initial well screening of drilling documentation indicates that the well is not suitable to be reused for CO2 injection and geophysical monitoring is recommended. In a second stage, a numerical representation of the well architecture is built including realistic geological setting. We evaluate the sensitivity of non-invasive low-frequency electromagnetic monitoring to corrosion levels in the casing. Numerical end-member simulations of assuming casing corrosion of different degrees by changing material conductivity are performed. Results comparing different corrosion scenarios with a base case (no corrosion) give an above noise signal at receiver locations enabling to separate the different cases. Comparison of the gained electrical fields at seafloor suggests that well casing corrosion monitoring should be possible. Finally, the electrochemical deposition potential of the Sognefjord Formation water is analyzed, revealing depositional potential for portlandite, which might be useful for cement remediation. We recommend such an analysis for all legacy wells penetrating candidate reservoirs for future CO2 or hydrogen storage.
... b) What is the 3D geometry of basement fault zones in extensional basins as the North Sea? c) Are all major basement fault zones present-day sources of heat and fluid in older extensional terrains of NW Europe? Northern Europe, in which the study area is located, has been experiencing a revival in exploration due to its volumes of unexploited natural gas, recognised geothermal resources, and preferential location for CO2 storage (Doornenbal et al., 2019;Peuchen et al., 2019, Mulrooney et al., 2020Wu et al., 2020;Michie et al., 2021). Many of the areas posed to produce geothermal energy in Northern Europe are associated with crustal structures that are (or were in the past) capable of focusing heat and fluid into adjacent geological reservoirs (Daniilidis and Herber, 2017;Geluk et al., 2018;Gluyas et al., 2018). ...
Article
Extraordinary 3D seismic data from the Central Offshore Platform (Southern North Sea), complemented by information from 38 boreholes, reveal a 10 km-wide basement fault zone above which fluid anomalies emanate from pre-salt reservoirs to terminate in lower Cretaceous strata. Fluid blow-out pipes, chimneys and low-amplitude trails were mostly sourced from the region where NW-striking syn-rift faults intersect the N-striking basement fault zone. As a result, 73% of the mapped fluid-flow anomalies (94 out of 129) occur within the basement fault zone of interest or follow a N-S strike along its shoulders. We postulate a strong control of the basement fault zone on past fluid and heat flow, as basin models confirm that fluid and heat were mostly produced during the Cretaceous. Bottom-hole data record temperatures of ~140oC at present, highlighting the geothermal potential of the study area. These temperatures nevertheless contrast with the relatively constant gradient of ~ 32oC/km occurring both in and outside the basement fault zone. This work is important as its shows that past fluid and heat flow over a basement fault zone does not necessarily correlate with the existence of an enhanced hydrothermal system at present. However, as bottom-hole temperatures are within the benchmark values considered across Europe, this work stresses the importance of basement fault zones as key structures to find, and assess, as potential geothermal sites.
... On the eastern side of the In areas above the Troll field where no other sealing formation is present due to erosion or non-461 deposition, sandstones and marls of the Våle Formation (where present), or more commonly 462 fine-grained mudstones of the Paleocene Lista Formation act as a tertiary seal. Gamma-ray 463 responses within this interval are fairly consistent in character (seeFigure 4), but it is also eroded 464 by the URU just east of the Vette Fault Zone (e.g.,Mulrooney, et al., 2020;Wu et al., 2021a).465 466 Thickness of the Lower Jurassic Drake Formation decreases from just under 215 m to nearly 467 zero in the northeast direction, with thicknesses around 175 m around the developing Aurora 468 CO2 storage site (Figure 8A). ...
Preprint
The maturation of geological CCS along the Norwegian Continental Shelf is ongoing in the Norwegian North Sea, however, more storage sites are needed to reach climate mitigation goals by 2050. In order to augment the Aurora site and expand CO2 storage in the northern Horda Platform, regional traps and seals must be assessed to better understand the area’s potential. Here, we leverage wellbore and seismic data to map storage aquifers, identify structural traps, and assess possible top and fault seals associated with Lower and Upper Jurassic storage complexes in four major fault blocks. With respect to trap and seal, our results maintain that both prospective intervals represent viable CO2 storage options in various locations of each fault block. Mapping, modeling, and formation pressure analyses indicate that top seals are present across the entire study area, and are sufficiently thick over the majority of structural traps. Across-fault juxtaposition seals are abundant, but dominate the Upper Jurassic storage complexes. Lower Jurassic aquifers, however, are often upthrown against Upper Jurassic aquifers, but apparent across fault pressure differentials and moderate to high shale gouge ratio values correlate, suggesting fault rock membrane seal presence. Zones of aquifer self-juxtaposition, however, are likely areas of poor seal along faults. Overall, our results provide added support that the northern Horda Platform represents a promising location for expanding CO2 storage in the North Sea, carrying the potential to become a future injection hub for CCS in northern Europe.
... Sediments within the QUD unit are interpreted as coarse glacial tills and glacimarine deposits, while silt and clay deposits of marine origin are associated with the QUC interval. More interestingly, buried pockmarks were recently interpreted by Leon (2019) along a bright, discontinuous intra-QUC (iQUC) reflector located above potential CO2 storage sites in the Smeaheia area (Mulrooney et al., 2020) on the eastern flank of the Norwegian Channel. This observation is novel for the region, and suggests that additional pockmark-forming events occurred prior to those observed on the seafloor. ...
... If supposedly closed leakage paths in the caprock, like faults, fractures, or abandoned wells, are exposed to pressures beyond their critical threshold, sequestrated CO 2 might leak out of the storage site. Therefore, many in-depth case studies have been conducted on potential storage sites to ensure secure CO 2 sequestration, e.g., Elenius et al. (2018), Mulrooney et al. (2020), and Hodneland et al. (2019). Even so, in the unlikely event that CO 2 leakage paths may develop during injection, the consequences can be severe, both in terms of impact on the nearby environment and on the public acceptance of CCS. ...
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An optimization procedure for sealing leakage paths in field-scale application of microbially induced calcite precipitation (MICP) is develop and applied to CO2 storage. The procedure utilizes a recently developed field-scale MICP mathematical model implemented in the industry-standard simulator Open Porous Media (OPM) Flow. The optimization problem is solved using the ensemble-based optimization (EnOpt) algorithm where the objective function is defined such that maximizing calcite precipitation is done in the shortest possible MICP operational time. An injection strategy is developed to efficiently produce calcite in and around the leakage paths, and to avoid clogging unwanted areas of the reservoir, e.g., the near-well area. The injection strategy consists of combined injection of growth and cementation solutions in separate well segments to initiate the MICP process after establishing a biofilm in the leakage paths with an initial injection phase. The optimization procedure is applied to three synthetic CO2 leakage scenarios. The numerical results show that the leakage paths in all scenarios are effectively sealed while keeping the total MICP operational time low. Keywords Carbon capture and storage · Ensemble-based optimization · Leakage mitigation · Microbially induced calcite precipitation · Open porous media initiative
Article
Geologic carbon storage (GCS) is a promising method for reducing anthropogenic CO2 emissions to the atmosphere. To safely deploy GCS in the field, it is necessary to assess risks and the effect of uncertainty on safe storage. The effect of uncertainty can be quantified using batches of simulations, but the high computational costs of high-resolution simulations necessitate use of reduced-order models (ROMs). Previous work involves ROMs for quantifying the risk of different potential leakage paths from storage reservoirs to shallow formations. However, previous studies on development of fault-leakage ROMs have limited numbers of uncertain parameters and do not explicitly examine impacts of CO2 solubility and thermal stresses on fault reactivation, which can generate high-permeability pathways and compromise CO2 storage. In this study, we analyze an ensemble of simulations considering CO2 leakage from a storage reservoir to a shallow aquifer through a fault while varying a number of uncertain parameters related to thermo-hydro-mechanical properties and CO2 injection. We show the effects of solubility on: free-phase CO2-leakage rates, brine-leakage rates, and poroelastic fault destabilization. We find that CO2 solubility is more important for estimating free-phase CO2-leakage rates compared to brine-leakage rates or poroelastic fault destabilization. We also find that thermal stresses and overpressures have different spatial distributions within the fault, indicating that the spatial variability of overpressures due to variation in flow parameters does not necessarily make the spatial variability of thermal stresses negligible. We suggest the use of the CO2 phase-change path as a variable in future fault-leakage ROMs.
Article
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Although geological CO2 sequestration is an essential solution for reducing anthropogenic carbon dioxide from the atmosphere, the method needs critical evaluation of injection-induced mechanical risks for safe and reliable CO2 storage. 3D field-scale geomechanical modeling is a preeminent solution for assessing mechanical risks of subsurface geological CO2 storage. However, data scarcity of seals and overburden rocks might limit building the 3D field-scale geomechanical model. This study focuses on seismic data-derived 3D field-scale geomechanical modeling of potential CO2 storage site Smeaheia, offshore Norway. The geomechanical properties inverted from seismic data are resampled in the 3D grid to consider spatial variabilities of seal and overburden rock properties. This method allows us to investigate the effect of overburden rock spatial variability imposed in seismic data on the 3D geomechanical model of Smeaheia. The model was built in Petrel-2019, while the one-way geomechanical simulation is iterated using the finite element method. Simplified constant overburden property models are also constructed to analyze the sensitivity of the overburden rock properties. The results reveal that the seismic data-driven spatially distributed overburden properties model workflow used in this study is a convenient and robust solution for 3D field-scale geomechanical modeling. The maximum vertical estimation of rock deformation is doubled in the simplified (isotropic) overburden rock property model compared to the new spatially variable (anisotropic) overburden rock property model. The Mohr-Coulomb failure envelope reveals that the new modeling approach is less prone to failure than the simplified (isotropic) model, which might influence the project decision. Moreover, our study demonstrates the importance of considering the spatial variability of overburden rock properties in building the 3D field-scale geomechanical model.
Article
In this work, we propose a local updating method to test different contact depth scenarios and assess their impact on wave propagation in the subsurface. We propose to locally modify a 2D geological model and run time-dependent elastic simulations. The input model triangulation is conforming to geological structures. The 2D meshed model is locally updated, which means that only the reservoir compartment is modified. Several model geometries are generated by inserting a new interface, in this paper a gas-water contact that is defined by a scalar field. We quantitatively evaluate the impact of the gas-water contact depth on elastic wave propagation. We run the numerical simulations with Hou10ni2D code, which is based on a Discontinuous Galerkin method. The simulation results are compared to a reference depth by computing the L2-norm at a set of seismic receivers. Results show a consistent behavior: we observe a positive correlation between the depth difference and global L2-norm for all receivers. This approach could therefore be integrated into an inversion loop to determine the position of the fluid contact and reduce uncertainties in the reservoir model from a few seismic sensors. The algorithms are available on Github and distributed under a GPL license, allowing reproducibility.
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The Utsira High (North Sea) records rift faulting that culminates with Jurassic-Cretaceous crustal-scale rollover towards the Viking Graben. This deformation is superimposed on Paleozoic fault-bounded basins on a substrate of Caledonian nappes. The latter contains multilevel crustal-scale shear systems that domed under the Utsira High during the Devonian, as indicated by mapping and interpretation of a large long-offset 2D seismic reflection dataset. Restorations show that isostatically driven doming from excision of overthickened crust caused uplift and erosion of basins and the underlying nappe stack. Doming took place above a crustal rollback system as a symmetrical metamorphic core complex formed. The dome discloses vertical flattening kinematics below bi-directional shear systems, evident by opposite shear-fabric kinematics on opposite dome flanks. A major detachment marks the upper boundary of a transition between upper and lower plate strain regimes, coinciding with low-reflective granitic units above strongly reflective Caledonian nappes. This detachment hosts transport-parallel corrugations that strike E-W to NE-SW. A series of deeply eroded half-grabens on top of the upper plate are bounded by pre-Permian faults exhibiting top-NE extensional kinematics. Faults sole out in the fundamental detachment level, locating the brittle-ductile transition, with all structures subsequently rotated during doming. A new localized detachment formed at a shallower level above the region of maximum crustal uplift, seen as upwards detachment migration driven by heating. Later faulting locates to the dome flanks, either rejuvenating rotated shear zones or cutting to deeper crustal levels while recording predominantly down-eastward transport. During Jurassic-Cretaceous rifting, deformation localized to the Viking Graben Boundary Fault, giving room for thick growth wedges in the Viking Graben and establishing the Utsira High as a crustal-scale rollover structure.
Article
Subsurface storage of CO2 is predicted to rise exponentially in response to the increasing levels of CO2 in the atmosphere. Large-scale CO2 injections into the subsurface require understanding of the potential for fluid flow through faults to mitigate risk of leakage. Here, we study how to obtain effective permeability of deformation bands in the damage zone of faults. Deformation bands are relatively small, low permeability features that can have a significant effect on flow dynamics, however, the discrepancy of scales is a challenge for field-scale simulation. A new analytical upscaling model is proposed in order to overcome some of the shortcomings of conventional upscaling approaches for heterogeneous porous media. The new model captures the fine-scale impact of deformation bands on fluid flow in the near-fault region, and can be derived from knowledge of large-scale fault properties. To test the accuracy of the model it is compared to fine-scale numerical simulations that explicitly include individual deformation bands. For a wide range of different stochastically generated deformation bands networks, the upscaling model shows improved estimate of effective permeability compared to conventional upscaling approaches. By applying the upscaling model to a full-field simulation of the Smeaheia storage site in the North Sea, we show that deformation bands with a permeability contrast higher than three orders of magnitude may act as an extra layer of protection from fluid flow through faults.
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The northern North Sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement. The geometry and evolution of syn‐rift depocenters during this multiphase evolution and the mechanisms and extent to which they were influenced by preexisting structural heterogeneities remain elusive, particularly at the regional scale. Using an extensive database of borehole‐constrained 2D seismic reflection data, we examine how the physiography of the northern North Sea rift evolved throughout late Permian‐Early Triassic (RP1) and Late Jurassic‐Early Cretaceous (RP2) rift phases, and assess the influence of basement structures related to the Caledonian orogeny and subsequent Devonian extension. During RP1, the location of major depocenters, the Stord and East Shetland basins, was controlled by favorably oriented Devonian shear zones. RP2 shows a diminished influence from structural heterogeneities, activity localizes along the Viking‐Sogn graben system and the East Shetland Basin, with negligible activity in the Stord Basin and Horda Platform. The Utsira High and the Devonian Lomre Shear Zone form the eastern barrier to rift activity during RP2. Toward the end of RP2, rift activity migrated northward as extension related to opening of the proto‐North Atlantic becomes the dominant regional stress as rift activity in the northern North Sea decreases. Through documenting the evolving syn‐rift depocenters of the northern North Sea rift, we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics, controlling the segmentation of depocenters, as well as the locations, styles, and magnitude of fault activity and reactivation during subsequent events.
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Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift-related normal faults. However, the extent to which physical and kinematic interaction between pre-existing shear zones and younger rift faults control the growth of normal faults is less-well understood. Using 3D reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-SW- and N-S-striking rift faults that define a coeval Middle Jurassic – Early Cretaceous, non-colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear zone. The faults either tip-out above or physically merge with the underlying shear zone. For faults that merges with the basement shear zone, a change from tabular to wedge-shaped geometry of the hangingwall synrift strata records a transition from non-rotational to rotational extension faulting, which we attribute to the time of rift fault's linkage with the shear zone, following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset (rather than link with) intra-basement shear zones. These observations highlight the selective influence pre-existing intra-basement shear zones may (or may not) have on evolving rift-related normal faults.
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Damage zones of different fault types are investigated in siliciclastics (Utah, USA), carbonates (Majella Mountain, Italy) and metamorphic rocks (western Norway). The study was conducted taking measurements of deformation features such as fractures and deformation bands on multiple 1D scanlines along fault walls. The resulting datasets are used to plot the frequency distribution of deformation features and to constrain the geometrical width of the damage zone for the studied faults. The damage-zone width of a single fault is constrained by identifying the changes in the slope of cumulative plots made on the frequency data. The cumulative plot further shows high deformation frequency by a steep slope (inner damage zone) and less deformation as a gentle slope (outer damage zone). Statistical distributions of displacement and damage-zone width and their relationship are improved, and show two-slope power-law distributions with a break point at c. 100 m displacement. Bleached sandstones in the studied siliciclastic rocks of Utah are associated with a higher frequency of deformation bands and a wider damage zone compared to the unbleached zone of similar lithology. Fault damage zones in the carbonate rocks of Majella are often host to open fractures (karst), demonstrating that they can also be conductive to fluid flow.
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Subsurface injection of carbon dioxide (CO2) is a technique to enhance oil recovery and so the economic value of depleting fields. It complements carbon capture and storage, which is a key technology to mitigate greenhouse gas emissions. In this work, an integrated method developed by the British Geological Survey and Cardiff University uses high-resolution 3D seismic and borehole data from the Jaeren High to analyse potential seal breaches and fluid flow paths in a frontier area of the North Sea, ultimately assessing the risk of a possible carbon capture and storage site. We integrate the spatial analysis of subsurface fluid flow features with borehole and geochemical data to model the burial and thermal history of potential storage sites, estimating the timing of fluid expulsion. On seismic data, fluid pipes connect reservoir intervals of different ages. Spatial analysis reveals clustering of fluid flow features above strata grounded onto deep reservoirs intervals. Our integrated method shows that gas matured from Dinantian coal and migrated up-dip during the Triassic-Jurassic into the lower sandstone reservoir of the Rotliegend Group. The containing seal rock was breached once sufficiently large volumes of gas generated high overpressures in the reservoir. Some of these fluid flow features may still be active conduits, as indicated by bright amplitude anomalies within the pipes. This study shows how integrated analyses may enhance our understanding of fluid-flow pathways, de-risking prospective sites for carbon capture and storage. The method proposed in this work is particularly important to assess the suitability of area with trapped gas pockets and understand tertiary migration in areas proposed for geological storage of CO2.
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Layer-bound, low-displacement normal faults, arranged into a broadly polygonal pattern, are common in many sedimentary basins. Despite having constrained their gross geometry, we have a relatively poor understanding of the processes controlling the nucleation and growth (i.e., the kinematics) of polygonal fault systems. In this study we use high-resolution 3-D seismic reflection and borehole data from the northern North Sea to undertake a detailed kinematic analysis of faults forming part of a seismically well-imaged polygonal fault system hosted within the up to 1,000 m thick, Early Palaeocene-to-Middle Miocene mudstones of the Hordaland Group. Growth strata and displacement-depth profiles indicate faulting commenced during the Eocene to early Oligocene, with reactivation possibly occurring in the late Oligocene to middle Miocene. Mapping the position of displacement maxima on 137 polygonal faults suggests that the majority (64%) nucleated in the lower 500 m of the Hordaland Group. The uniform distribution of polygonal fault strikes in the area indicates that nucleation and growth were not driven by gravity or far-field tectonic extension as has previously been suggested. Instead, fault growth was likely facilitated by low coefficients of residual friction on existing slip surfaces, and probably involved significant layer-parallel contraction (strains of 0.01-0.19) of the host strata. To summarize, our kinematic analysis provides new insights into the spatial and temporal evolution of polygonal fault systems.
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The CO2CRC is undertaking a feasibility study for a planned controlled release and monitoring experiment at a shallow fault at the CO2CRC Otway Project site in 2018. Interpretation of pre-2016 seismic data could trace the height of the fault to approximately 100 m below the ground surface, at which point the resolution of the existing seismic data was insufficient to delineate the fault any further. To better understand the shallow geology at the Otway Project site and to map the extent of the shallow fault, new geophysical surveys were acquired over the Otway site during 2016. This included a high resolution, shallow focused, 3D seismic survey to provide greater delineation of the newly identified fault in the Port Campbell Limestone on the Otway Project site, and a high resolution resistivity survey to map the vertical extent of the fault towards the ground surface. Aerial imagery and LIDAR data were also collected. The seismic survey data exhibit greatly improved vertical and lateral resolution compared to previous seismic surveys. Preliminary pre-stack time migration (PreSTM) processing of the data show that the target fault can be clearly imaged at 30 ms TWT and the fault tip can be mapped to within approximately 25 m of the surface. Approximately 5 m of throw is identified at approximately 140 m depth and the throw appears to decrease in magnitude as the fault extends towards the surface. This, plus an identified dip angle of ~70° (east), suggests that it is most likely a normal fault. There is no evidence of topographical features associated with the surface expression of the shallow fault using LIDAR and aerial imagery. Electrical Resistivity Imaging (ERI) results indicate that there are 3 distinct layers in the shallow geology of the Otway site, including a higher resistivity, more clay influenced, 3-5 m thick layer at the surface. The resistivity is also surprisingly heterogeneous over the site, suggesting that the shallow geology is complex. Preliminary hydraulic conductivity measurements confirm that the Port Campbell Limestone is highly permeable in the vicinity of the Otway Project site. The target fault at the CO2CRC Otway Project site appears to be a suitable candidate for a shallow CO2 injection experiment.
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The northern North Sea rift basin developed on a heterogeneous crust comprising structures inherited from the Caledonian orogeny and Devonian postorogenic extension. Integrating two-dimensional regional seismic reflection data and information from basement wells, we investigate the prerift structural configuration in the northern North Sea rift. Three seismic facies have been defined below the base rift surface: (1) relatively low-amplitude and low-frequency reflections, interpreted as pre-Caledonian metasediments, Caledonian nappes, and/or Devonian clastic sediments; (2) packages of high-amplitude dipping reflections (>500 ms thick), interpreted as basement shear zones; and (3) medium-amplitude and high-frequency reflections interpreted as less sheared crystalline basement of Proterozoic and Paleozoic (Caledonian) origin. Some zones of Seismic Facies 2 can be linked to onshore Devonian shear zones, whereas others are restricted to the offshore rift area. Interpreted offshore shear zones dip S, ESE, and WNW in contrast to W to NW dipping shear zones onshore West Norway. Our results indicate that Devonian strain and ductile deformation was distributed throughout the Caledonian orogenic belt from central South Norway to the Shetland Platform. Most of the Devonian basins related to this extension are, however, removed by erosion during subsequent exhumation. Basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters, e.g., in the Stord and East Shetland basins. Prerift structures with present-day dips >15° were reactivated, although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction.
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The Goliat field consists of Middle to Late Triassic reservoirs which exploit an elongate anticline (the Goliat anticline) in the hanging wall of the Troms-Finnmark Fault Complex (TFFC), offshore Norway. The area is affected by a dense network of multiple trending fault populations which historically have inhibited seismic resolution owing to persistent fault shadow. Seismic investigations utilising a multi-azimuth three-dimensional survey (EN0901) allow much crisper delineation of seismic features previously unattainable by vintage single-azimuth surveys. Three dominant fault populations are identified in the area, two of which parallel TFFC segments, the Alke–Goliat (WSW–ENE) and the Goliat–Tornerose (NNE–SSW) segments. The Goliat field is located within a zone of intersection between both segments. A third E-W trending fault population, the Hammerfest Regional population, is likely influenced by the offshore extension of the Trollfjord-Komagelv Fault Complex (TKFZ). A local NW–SE trending fault population, the Goliat Central, affects the Goliat anticline and partitions Alke–Goliat and Goliat–Tornerose subsidiary faults resulting in curvilinear traces. Several cross-cutting relationships between fault populations are observed and may provide fluid compartmentalisation in the reservoirs. Compilation of regional transects and the EN0901 survey provides new insight into the evolution of the Goliat anticline which is underlain by a fault-bound basement terrace that became established in the Late Palaeozoic. The structure is interpreted to have formed due to vertical segmentation of the TFFC and cores the overlying broad anticline. The western limb of the Goliat anticline likely formed by differential compaction, whereas the eastern limb is primarily a result of hanging wall roll-over linked to variable listric to ramp-flat-ramp fault geometry. Rifting took place in the Palaeozoic (Carboniferous to Permian?), and in the Mesozoic, possibly as early as the Late Triassic, with a major event in the Late Jurassic to Early Cretaceous. Minor reactivations continued into the Late Cretaceous, and possibly the Early Cenozoic. Mesozoic syn-kinematic geometries in the hanging wall of the Goliat–Tornerose TFFC segment are consistent with deposition during up section propagation of a blind fault, over which, a monocline was established and later breached. Jogs (abrupt orientation changes) in fault traces, transverse folds (associated with displacement maxima/minima) and vertical fault jogs suggest the TFFC existed as a greater number of segments prior to amalgamation during the Late Triassic to Jurassic. A phase of Barremian inversion created local compression structures above blind extensional faults, and deeper seated buttressing against large faults. Polygonal faults affect the Late Cretaceous to Early Cenozoic successions.
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Fault seal can arise from reservoir/nonreservoir juxtaposition or by development of fault rock having high entry pressure. The methodology for evaluating these possibilities uses detailed seismic mapping and well analysis. A first-order seal analysis involves identifying reservoir juxtaposition areas over the fault surface by using the mapped horizons and a refined reservoir stratigraphy defined by isochores at the fault surface. The second-order phase of the analysis assesses whether the sand/sand contacts are likely to support a pressure difference. We define two types of lithology-dependent attributes: gouge ratio and smear factor. Gouge ratio is an estimate of the proportion of fine-grained material entrained into the fault gouge from the wall rocks. Smear factor methods (including clay smear potential and shale smear factor) estimate the profile thickness of a shale drawn along the fault zone during faulting. All of these parameters vary over the fault surface, implying that faults cannot simply be designated sealing or nonsealing. An important step in using these parameters is to calibrate them in areas where across-fault pressure differences are explicitly known from wells on both sides of a fault. Our calibration for a number of data sets shows remarkably consistent results, despite their diverse settings (e.g., Brent province, Niger Delta, Columbus basin). For example, a shale gouge ratio of about 20% (volume of shale in the slipped interval) is a typical threshold between minimal across-fault pressure difference and significant seal.
Article
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The Lower Jurassic, Johansen Formation sandstone, located in the Northern North Sea, has been proposed as a reservoir candidate for CO2 storage by Norwegian authorities. The objective of this study is to evaluate the reservoir quality of the Johansen Formation, as function of depositional history and architecture. We propose a depositional model comprising an early phase delta progradation, with clinothems building into deep waters, associated with delta front and pro-delta turbidites sourced from river mouths or/and upper delta front collapse. During a subsequent, aggradational stage, thick spit bar deposits developed in the southern, down-current part, sheltering a brackish lagoon, before rapid transgression caused back-stepping and preservation of sandy deposits encased in mud. Considering the depositional model presented, the inferred high porosity spit bar deposits would provide a suitable injection site and reservoir for CO2. Climatic controlling factors, rather than structural, are interpreted to have exerted the major force on the asymmetric sand distributions observed in the Johansen Formation, an architectural style which is repeated in later Jurassic successions on the Horda Platform. On a local scale, accommodation was created by differential compaction above rotated, Permian fault blocks, in addition to regional, post-thermal subsidence and rising sea-level.
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Two cores from the Lunde formation in the Snorre field have been tested at reservoir temperature for enhanced oil recovery by the low saline water injection technique. In both cases, the cores were cleaned by successive flooding with toluene, methanol, and finally with 10 000 ppm NaCl solution. The method to obtain initial saturation of formation water, ≈20%, was different for the two cores. In the case of Core 13, the desiccator technique, and for Core 14, the porous plate method was used. The crude oil was saturated with C02 at 6 bars in order to mimic reservoir conditions regarding initial pH of the formation water. The oil recovery tests were conducted by flooding the cores successively with formation water, seawater, and 500 ppm NaCl solution. The oil recovery by formation water was in the range of 50% of OOIP. A small incremental oil recovery of 1-3% of OOIP was recovered by switching from formation water to seawater, and no further increase in recovery was noticed when flooding with low saline NaCl solution. The results were discussed in terms of the previously suggested chemical mechanism for the low salinity effect, which was based on a local increase in the pH close to the clay-water interface, which should promote desorption of polar components by an acid-base reaction. The results are also related briefly to previously studies performed by SINTEF/Statoil. It was suggested that the adsorption of polar basic components from the crude onto the clay was partly prevented due to the high pH, about 7.5, of the formation water during aging. The high pH was related to the presence of more than 30 wt% of Plagioclase in the core matrix.
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Thick Mesozoic sediments are found offshore Norway and Denmark, and Mesozoic rocks are present and well exposed in Denmark, along the coast of East Greenland and on the arctic islands of Svalbard. During the Mesozoic, Scandinavia and Greenland were subject to major extension in the Late Permian-Early Triassic and Late Jurassic-Early Cretaceous, prior to Cenozoic opening of the North Atlantic. Deep basins developed along the rift zones of the North Sea and between East Greenland and Norway, and were filled with sediments derived from mainland Scandinavia and Greenland. The marginal areas bordering the rift zones suffered less subsidence, as did the epicontinental Barents Sea.
Conference Paper
The presence of Upper Jurassic through Lower Paleogene sealing units in the Horda Platform has been assessed and established for the purpose of derisking the Alpha CO2 storage prospect in the Smeaheia fault block. The nearby Troll East field closure provides as an excellent analogue for evaluating seals surrounding the Alpha closure. Analysis of mapped horizons and trap-bounding faults indicates that all sealing units are required to retain the hydrocarbons trapped at Troll East. Contrastingly, only the Draupne Formation and Cromer Knoll Group are needed to seal the top of Alpha, while the Cromer Knoll is needed to laterally seal the closure. Overall, if the Troll East analogy holds true for Alpha, the arrangement of top and lateral seals appears suitable for CCS at Smeaheia.
Article
The Listafjord–Drangedal Fault Complex is a central structure in the NE-SW-trending Agder–Telemark Lineament Zone that dominates the structural grain and topography of southernmost tip of Norway. The fault can be followed for a distance of more than 170 km from the shelf area off Listafjorden–Fedafjorden in Vest Agder county to Drangedal in Telemark county. It has been analyzed by the use of digital topographic, remote sensing and potential field data, supported by field investigations. At least seven separate left-stepping fault segments have been identified. These are characterized by numerous internal fault lenses, separate fault strands and fault splays, partly displaying contrasting fault attitude and style of deformation. The northeastern termination of the Listafjord–Drangedal Fault Complex consists of fanning fault branches (horse-tailing), whereas its southwestern termination is buried below sediments in the continental shelf and remains obscure. The fault rocks of the various fault segments include cataclasites and mylonites that in places are interlayered with zones of fault gouge. By tentative correlation to the Hunnedalen dyke system in Rogaland, the age of initiation for the Listafjord–Drangedal Fault Complex is suggested to be Late Proterozoic. Parts of the fault complex were affected by at least two stages of faulting including (dextral?) shear and top-to-the-SE extension. The latter stage is assumed to be of post-Caledonian age, and recent seismic activity suggests that this ancient structural grain is still seismically active.
Article
Throw-distance (T-D) and throw-depth (T-Z) plots are widely used by researchers and industry to examine the growth of normal faults. This study uses high-quality three-dimensional (3D) seismic and outcrop information to review the effect of data sampling on the interpretation of normal fault growth. The results show that the accuracy of T-D and T-Z data, and of resulting fault slip tendency and leakage factor analyses, are dependent on the sampling strategy followed by interpreters and field geologists, i.e. on a Sampling Interval/Fault Length Ratio (δ) for discrete structures. In particular, this work demonstrates that significant geometric changes in T-D plots occur when a Module Error (ε i ) for the ratio δ is larger than 6%–9% for faults of all scales and growth histories. This implies that a minimum number of measurements should be gathered on discrete faults to produce accurate T-D and T-Z plots, and that the number of measurements is dependent on fault length. With no prior knowledge of fault segmentation, a δ value of 0.05 should be applied when interpreting faults to fulfil the pre-requisite of a ɛ i < 6–9%. In all faults analysed, slip tendency and leakage factors were systematically misrepresented with increasing δ values. To disregard the limits proposed in this work results in: 1) a systematic underrepresentation of the isolated fault growth model, 2) a systematic misrepresentation of fault geometries and related damage zones, 3) the collation of erroneous fault scaling relationships, and 4) ultimately, unreliable interpretations of fault sealing properties. Hence, this work presents a new tool for interpreters and structural geologists to understand the sampling strategies necessary to obtain accurate fault throw and displacement data at different scales of analysis.
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WA is poised to embark on several major new energy developments. These include $250+ Billion AUD investment in new gas reserves that will begin production in the Carnarvon, Browse and Bonaparte basins. Developing these new gas reserves will require handling many Millions of tons per year in CO2 released as a natural byproduct of the LNG process. To avoid venting this natural CO2 into the atmosphere, which may be bad for both the environment and business, the CO2 will have to be disposed of safely. One of the best available options is to accelerate nature's course by re-injecting and storing CO2 into deep rock formations, termed "geo-sequestration". Geophysical monitoring of producing gas reservoirs will thus play an important role in two ways; (1) enhancing the gas recovery factor of these projects by improving the reservoir model and understanding geologic flow complexity, and (2) monitoring any required CO2 injection to ensure it is being safely stored in the subsurface for the longterm. In addition to petroleum applications, there is a strong interest in developing clean-coal initiatives by capturing the CO2 generated at coal-fired power plants and injecting it into the subsurface. Geophysical techniques will therefore also play a key role in monitoring and verification strategies for clean-coal CO2 sequestration projects.
Article
The Quaternary North Sea Basin, which extends from northwest mainland Europe in the south (52°N) to the Norwegian Sea in the north (62°N), contains a thick (up to 1 km) sedimentary succession that records the changing nature of sediment supply from surrounding land areas during the last c.2.6 Ma. We use an extensive 2D and 3D seismic database to correlate major Quaternary seismo-stratigraphic surfaces and units across the North Sea and reconstruct the broad-scale infill pattern of the entire Quaternary North Sea Basin. The total volume of Quaternary sediments in the North Sea (350 000 km²) is c.140 000 km³, while the area inside the 500 m contour of the base-Quaternary surface has a volume of 109 000 km³. Two largely independent depocentres developed in the North Sea Basin during the early Quaternary: the southern and central sub-basin was infilled by shelf fluvio-deltaic and prodeltaic sediments delivered from the east and south-east, whilst the northern sub-basin was infilled mainly by prograding glacigenic debris-flows deposited from an ice mass centred on the Norwegian mainland. Contour currents were an important mechanism of sediment deposition and reworking, with water circulation in the basin probably occurring in an anti-clockwise direction. Whilst most of the southern North Sea Basin was infilled by around 1.6–1.7 Ma, a depression persisted in the central North Sea until around 1 Ma. The analysis of landforms on 3D seismic data suggests that the Fennoscandian Ice Sheet (FIS) extended intermittently to the palaeo-shelf break in the northern North Sea during the earliest Quaternary and expanded into the central North Sea prior to the excavation of the Norwegian Channel.
Article
Seismic interpretations are, by definition, subjective and often require significant time and expertise from the interpreter. We are convinced that machine-learning techniques can help address these problems by performing seismic facies analyses in a rigorous, repeatable way. For this purpose, we use state-of-the-art 3D broadband seismic reflection data of the northern North Sea. Our workflow includes five basic steps. First, we extract seismic attributes to highlight features in the data. Second, we perform a manual seismic facies classification on 10,000 examples. Third, we use some of these examples to train a range of models to predict seismic facies. Fourth, we analyze the performance of these models on the remaining examples. Fifth, we select the "best" model (i.e., highest accuracy) and apply it to a seismic section. As such, we highlight that machine-learning techniques can increase the efficiency of seismic facies analyses.
Article
In this study we synthesise sedimentological, fault, and Amplitude Versus Angle (AVA) analysis and propose that the Fruholmen and Tubåen formations (Realgrunnen Subgroup) are syn-kinematic deposits that record a previously undocumented early phase of Mesozoic rifting on the Troms-Finnmark fault Complex and within the Hammerfest Basin. The Realgrunnen Subgroup hosts one of two Triassic reservoirs currently being produced in the Goliat field. Here, the subgroup sits unconformably on top of the Storfjorden Subgroup (Carnian Snadd Formation). Away from the Goliat field, which is characterised by a periclinal anticline, the Realgrunnen Subgroup also comprises the Lower–Middle Jurassic Nordmela and Stø formations. Sedimentological analysis of six exploration wells reveals that the Fruholmen Formation was deposited in a prodelta to delta plain environment where tide-influenced and fluvial-dominated distributary channels are represented by clay/siltstones and very fine grained sandstones. The overlying Tubåen Formation is characterised by medium to very coarse-grained deposits (locally conglomeratic) and represents a widespread braid plain with localised alluvial fans. Displacement profiles of faults and along-fault thickness variations demonstrate that an immature fault system was active during deposition of the Realgrunnen Subgroup. A series of unconnected fault segments hosted isolated sub-basins and erosional catchment areas in their hanging and footwalls, respectively. An AVA attribute map generated from a 10 ms interval of the uppermost part of the subgroup reveals gross sand-prone depositional bodies, i.e., individual and amalgamated channels, (some of which show meandering geometries), ox-bow lakes and alluvial fans. Sand bodies frequently show elongate geometries parallel to faults indicative of syn-depositional fault-related subsidence. Driving mechanisms responsible for the Norian to Rhaetian event may relate to contemporaneous rejuvenation of the Fennoscandian hinterland, development of the Novaya Zemlya fold-and-thrust belt and/or the early Cimmerian tectonic phase in northern Europe.
Article
Pockmarks are seafloor craters usually formed during methane release on continental margins. However, the mechanisms behind their formation and dynamics remain elusive. Here we report detailed investigations on one of the World's largest pockmark fields located in the Troll region in the northern North Sea. Seafloor investigations show that >7000 pockmarks are present in a ∼600 km2 area. A similar density of pockmarks is likely present over a 15,000 km2 region outside our study area. Based on extensive monitoring, coring, geophysical and geochemical analyses, no indications of active gas seepage were found. Still, geochemical data from carbonate blocks collected from these pockmarks indicate a methanogenic origin linked to gas hydrate dissociation and past fluid venting at the seafloor. We have dated the carbonates using the U–Th method in order to constrain the pockmark formation. The carbonates gave an isochron age of 9.59±1.38 ka, i.e. belonging to the initial Holocene. Moreover, radiocarbon dating of microfossils in the sediments inside the pockmarks is consistent with the ages derived from the carbonates. Based on pressure and temperature modelling, we show that the last deglaciation could have triggered dissociation of gas hydrates present in the region of the northern part of the Norwegian Channel, causing degassing of 0.26 MtCH4/km2 at the seafloor. Our results stress the importance of external climatic forcing of the dynamics of the seafloor, and the role of the rapid warming following the Younger Dryas in pacing the marine gas hydrate reservoir.
Article
Recent discoveries of oil in deeply buried paleoregolith profiles on the Utsira High, Norwegian North Sea, was the first time basement rocks had been demonstrated to be petroleum reservoirs on the Norwegian continental shelf. The present study aimed to establish the processes responsible for the primary weathering sequence, distinguish them from other phases of alteration, and create a model for the development of reservoir properties in crystalline basement rocks. Hand-specimen and laboratory tests revealed a link between reservoir properties in weathered granitic rocks and alteration facies. Samples were obtained from two distinct paleoregolith profiles on the Utsira High. The core samples were studied in detail by optical microscopy, X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. In the altered coherent rock facies, porosity and permeability were mainly created by joints and fractures prior to subaerial exposure. In the altered compact rock and altered incoherent rock facies, the development of reservoir properties was increasingly affected by physicochemical interactions between the rock and percolating fluids during subaerial exposure and early diagenesis. In well 16/3-4, the altered coherent rock facies contained R0 illite-smectite (I-S), well ordered kaolinite, and a mixture of fine-grained mica and illite, produced in semi-open and closed microsystems. In the altered compact rock and altered incoherent rock facies, disordered kaolinite became more abundant at the expense of R0 I-S, well ordered kaolinite, plagioclase, and biotite, suggesting alteration in semi-open microsystems. The collapse of the rock structure and clogging of mesofractures by clays contributed to reduced permeability in the clay-rich upper part of the altered incoherent rock. In contrast, well 16/1-15 represented a more deeply truncated weathering profile compared to 16/3-4, characterized by open and interconnected mesofractures and moderate formation of clay. R0 I-S was present and kaolinite was rare throughout the profile, suggesting stagnant conditions. During burial, a porosity-reducing serpentinechlorite Ib β = 90º polytype formed in the overlying sandstone and the regolith. Application of these results should improve the success of exploration and production efforts related to hydrocarbon reservoirs in the altered crystalline basement.
Article
Several regional or local unconformities occur in the latest Jurassic-Early Cretaceous sequences of the North Sea and adjacent areas. Each may have been identified locally as the ''late Cimmerian unconformity,'' a supposed major break at the base of the Valhall Formation (or Rodby Formation where the Valhall is locally absent). Although a major hiatus (or a condensed sequence) may occur at basin margins or above structural highs, over most of the North Sea the base of the Valhall Formation is isochronous, and conformable with underlying sediments. It is detected on seismic reflection profiles because it represents a widespread facies change marking the late Ryazanian transgression. Most of the unconformities and associated sedimentary and/or biologic events are of eustatic origin and, even in the tectonically active areas of the North Sea, the effects of eustatic sea level changes were never completely masked by local tectonics. Thus, in the modeling of individual oil fields, the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events.
Article
In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NWeSE-striking segments that are hard- linked by NeS-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NWeSE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-? Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented suprasalt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.
Article
This guide provides a major revision and update of the stratigraphy of the Cromer Knoll, Shetland and Chalk Groups for the UK and Norwegian sectors in the North Sea, and of the Cromer Knoll and Shetland Groups in the Norwegian Sea. The first chapters deal with the paleoceanographic and geologic settings and updated biostratigraphy, followed by the chapters with the new and improved lithostratigraphy. The Cretaceous biostratigraphy calculated for the microfossil record in 37 Norwegian wells integrates over 100 foraminifer, dinoflagellate cyst, diatom and miscellaneous events in nineteen zones, numbered from NCF 1 through NCF 19 (North Sea Cretaceous Micro Fossil Zones 1–19). A literature based Dinoflagellate Cyst Zonation (DCZ), linked to the NCF zones, is also presented with eleven zones and thirty-nine subzones for Cretaceous marine strata in the North Sea. Both zonations are optimized for industrial applications with ditch cuttings samples. The lithostratigraphy of the North Sea, unified for the UK and Norwegian sectors describes 3 groups, 30 formation units and one member. The Cretaceous lithostratigraphy for the Norwegian Sea describes 2 groups, 17 formations and 14 members. This (long overdue) update alleviates misnaming and incidental use of unique names for reservoir units, without documentation and lack of biostratigraphic and correlative insight. The internet site www.nhm2.uio.no/norlex and the CD inserted with this publication provide core archives for the lithostratigraphic units.
Article
The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting.
Article
Recent shallow marine investigations have revealed c150 km2 of Late Jurassic sedimentary bedrock north of Utsira in an area previously mapped as crystalline basement. The rocks, which are characteristically truncated by a marked regional planar angular unconformity further offshore, continue to rise well above this level into shallower waters. This shallower remnant is very unusual in Norwegian coastal waters not just because it survived the full force of the erosional event, but also because it is in lateral continuity with Jurassic rocks on the Continental Shelf. -Authors
Chapter
Faults on the Norwegian continental shelf may be subdivided into groups of differing status, reflecting their significance in the structural history of the region: Both high-angle and low-angle (partly listric) faults have played important roles in the various stages of the structuring, and reactivation of deep zones of weakness have given rise to composite fault geometries. The deeply rooted faults delineate major structural platforms and subplatforms, and have been the most active structural elements throughout the structuring. Their complex geometries and deep roots make them suitable for activation both in the initial, the stretching/crustal thinning, and the thermal cooling/sediment loading stages in graben development. Characteristic of the deeply rooted faults is that they show mixed planar and listric extensional fault geometries (consistent composite faults), reactivation (several stages of growth, more than one level of detachment), and activity associated with different stress situations; inversion, superposition of structural styles (inconsistent composite faults). A preliminary correlation of recent seismic activity to the main fault systems in the northern North Sea Basin, suggests that the areas underlain by deep planar faults are the same areas which are seismically most active.
Article
Linear features (believed to represent fractures of some kind) seen on Landsat satellite images, topographic maps and geological maps have been mapped for the area of southern Norway (roughly the region south of the 65 degree parallel). Special attention is given to the area of the Oslo Graben. The results of the analysis are related to the main geological and geophysical features of the region, of which a brief description is given. A system of NW-SE and NE-SW sets of lineaments is present over most of the region and predominates in the Precambrian area; it is thought to be the oldest structural element. In the Caledonian belt the NE-SW set is predominant, in accordance with the general strike. A roughly N-S trending set is obviously associated with the Oslo Graben but extends well to the north of the latter; a similar pattern is also present near the west coast. An E-W set is locally associated with Devonian movements. The coastlines of southern Norway conform closely to the directions of the dominant fracture pattern in the vicinity. Refs.
Article
The Triassic strata of the northern North Sea were initially sub-divided into three formations: the Smith Bank, Skagerrak and Cormorant formations (Deegan & Scull 1977), defined within the informal "Triassic group". The succession was little understood at that time as few wells had drilled into the Triassic red beds. In an attempt to unify UK and Norwegian Triassic and Jurassic lithostratigraphical nomenclature Vollset & Doré (1984) revised the nomenclature, defining a group with three formations - the Hegre Group with the Teist, Lomvi and Lunde formations. The objective of this paper is to establish a gross nomenclature, which covers the entire northern North Sea. A revised Triassic lithostratigraphic chart for the Northern North Sea Basin is presented.
Chapter
The Triassic extends over 300 000 km2 in the central and northern North Sea, and reaches thicknesses of 4–6 km, but entire sequences, from Lower Jurassic through Triassic to sub-Triassic rocks, are proved in fewer than 15 wells. The Triassic normally rests conformably on Upper Permian strata, except in the north where it overlies crystalline basement. It is overlain conformably by Jurassic rocks in the north, west and south-east, but throughout most of the central region erosion during Middle Jurassic doming and late Jurassic rifting episodes produced a major unconformity.
Article
The tectonic history of the North Sea area can be subdivided into five stages: 1. Caledonian geosynclinal stage (Cambrian-Devonian). Metamorphic and intrusive rocks of Caledonian age form the basement complex for much of the North Sea area. The northeastern boundary of the Caledonian foldbelt at this stage cannot be defined more closely than as an apparent trend from the central North Sea through northern Germany into Poland. 2. Variscan geosynclinal stage. Devonian and Carboniferous deposition transgressed from the south over the eroded Caledonides and reached maximum thickness in the southern North Sea, an area which formed part of the Variscan foredeep. 3. Permian-Triassic intracratonic stage. Following the Variscan orogeny large parts of the North Sea were occupied by the rapidly subsiding intracratonic Northern and Southern Permian basins. These basins contain a thick sequence of clastic and evaporite deposits. 4. Rifting, taphrogenic stage. Development of the North Sea rift system started during the Triassic and dominated the paleogeographic setting of the area during the Jurassic and Cretaceous. The evolution of the North Sea rift is related to the development of the Arctic North Atlantic rift zone. The latter reached the stage of crustal separation in the early Tertiary, at which time the North Sea rift became inactive. 5. Tertiary, postrifting stage of regional basin subsidence. With the termination of rifting movements in the North Sea the area became subject to regional subsidence leading to the development of a symmetrical, saucer-shaped intracratonic basin. The late Tertiary Rhone-Rhine rift system does not extend into the North Sea and postdates the North Sea rift.