Quaternary and Neogene Reservoirs of the Norwegian Continental Shelf and the Faroe-Shetland Basin

Glaciogenic reservoirs host important hydrocarbon resources across the globe. Examples such as the Peon and Aviat discoveries in the North Sea show that Quaternary and Neogene reservoirs can be prospective in the region. In this study, we interpret 2D and 3D reflection seismic data combined with borehole information to document unconventional play models from the shallow subsurface of the Norwegian Continental Shelf and the Faroe-Shetland Basin. These plays include (i) glacial sands in ice-marginal outwash fans, sealed by stiff subglacial tills (the Peon discovery), (ii) meltwater turbidites, (iii) contouritic fine-grained glacimarine sands sealed by gas hydrates, (iv) remobilized oozes above large evacuation craters which are sealed by megaslides and glacial muds, and (v) Neogene sand injectites. The hydrocarbon reservoirs are characterized by negative-polarity reflections with anomalously high amplitudes in the reflection seismic data as well as density and velocity decreases in the borehole data. Extensive new 3D reflection seismic data are crucial to correctly interpret glacial processes and distinguish shallow reservoirs from shallow seals. These data document a variety of play models with the potential for gas in large quantities and enable the identification of optimal drilling targets at stratigraphic levels which have so far been overlooked

Structural and fluid-migration control on hill-hole pair formation: Evidence from high-resolution 3D seismic data from the SW Barents Sea

Hill-hole pairs are subglacial landforms consisting of thrust-block hills and associated source depressions. Formed by evacuation of material where ice sheets have been locally frozen to the substrate, they give insights into paleo-ice-sheet dynamics. The aim of this study was to document the relationships between ancient hill-hole pairs identified on a buried glacial unconformity with the structure of the underlying sedimentary deposits, and then to determine if the basin geology and glacial fluid migration pathways promoted local subglacial freeze-on during the hill-hole pair formation. The study is based on seismic geomorphological interpretation of four high-resolution 3D seismic cubes covering an area of 800 km2 in the SW Barents Sea, and fluid seepage data from 37 gravity cores. The seismic datasets allowed the identification of 55 hill-hole pairs along the buried unconformity. The hills are characterized by chaotic to homogenous seismic facies forming up to 19 m high mounds, each covering areas of 2000–644,000 m2. The holes form depressions between 1 and 44 m deep and 2000–704,000 m2 in areal extent, which cut into preglacial Mesozoic bedrock and later infilled by glacial till. The holes are often found above fault terminations. High-amplitude reflections identified along the faults and in the strata below the holes are interpreted as shallow gas migrating upward towards the glacial unconformity. Geochemical data of the seabed sediment cores further indicates an association between hill-hole pair occurrence and present-day thermogenic hydrocarbon seepage. The hill-hole pairs geometries were also used to identify five paleo-ice-flow directions along the glacial unconformity. These ice flows exhibit polythermal regimes, and four of them are parallel to ice-stream flow sets interpreted from glacial lineations. The integrated interpretation supports localized fault-related basal freezing of the Barents Sea Ice Sheet which resulted in the formation of hill-hole pairs when the ice sheet moved. In this context, the faults functioned as migration pathways for deep thermogenic fluids, possibly sourced from leaking Jurassic reservoirs.>p> This study highlights the importance of the underlying geology for ice-sheet dynamics: While hill-hole pairs above glacial till appear to be commonly associated with dispersed gas hydrates, hill-hole pairs above bedrock additionally indicate a link to underlying fault systems and hydrocarbon reservoirs. Freeze-on of underlying bedrock to the basal ice along the strike of faults in sedimentary bedrock explains deeper hill-hole pairs with smaller extents along the glacial unconformity compared to areally larger but shallow hill-hole pairs detected above glacial till on modern seabeds. Such close association between paleo-thermogenic gas seepage and the location of hill-hole pairs strongly support that hill-hole pairs are excellent markers revealing exit points of fluid migration pathways in petroleum system models

Monitoring Of CO2 Leakage Using High-Resolution 3D Seismic Data – Examples From Snøhvit, Vestnesa Ridge And The Western Barents Sea

Source at https://doi.org/10.3997/2214-4609.201802965.Injection of CO2 in subsurface reservoirs may cause overburden deformation and CO2 leakage. The aim of this study is to apply technologies for detection and monitoring of CO2 leakage and deformation above the injection reservoirs. The examples of this study include data from the Vestnesa Ridge natural seep site, the Snøhvit gas field and CO2 storage site region, and the Gemini North gas reservoir. Reprocessing of existing 3D high-resolution seismic data allows resolving features with a vertical and lateral resolution down to c. 1 m and c. 5 m respectively. The current acquisition systems could be modified to image structures down to one meter in both the vertical and horizontal directions. We suggest a monitoring workflow that includes baseline and time-lapse acquisition of highresolution 3D seismic data, integrated with geochemical, geophysical, and geotechnical seabed core and watercolumn measurements. The outcome of such a workflow can deliver reliable quantitative property volumes of the subsurface and will be able to image meter-sized anomalies of fluid leakage and deformation in the overburden

Meltwater sediment transport as the dominating process in mid-latitude trough mouth fan formation

Trough mouth fans comprise the largest sediment deposits along glaciated margins, and record Pleistocene climate changes on a multi-decadal time scale. Here we present a model for the formation of the North Sea Fan derived from detailed horizon and attribute interpretations of high-resolution processed 3D seismic reflection data. The interpretation shows that stacked channel-levee systems form up to 400 m thick sedimentary sequences. The channels are elongated and can be traced from the shelf edge towards the deep basin for distances of >150 km, and document long-distance sediment transport in completely disintegrated water-rich turbidite flows. Downslope sediment transport was a continuous process during shelf-edge glaciations, reaching accumulation rates of 100 m/kyr. Our data highlight that exceptionally large volumes of meltwater may discharge to the slopes of trough mouth fans and trigger erosive turbidite flows. We conclude that freshwater supply is likely an underestimated factor for sedimentary processes during glacial cycles

Meltwater sediment transport as the dominating process in mid-latitude trough mouth fan formation

Abstract Trough mouth fans comprise the largest sediment deposits along glaciated margins, and record Pleistocene climate changes on a multi-decadal time scale. Here we present a model for the formation of the North Sea Fan derived from detailed horizon and attribute interpretations of high-resolution processed 3D seismic reflection data. The interpretation shows that stacked channel-levee systems form up to 400 m thick sedimentary sequences. The channels are elongated and can be traced from the shelf edge towards the deep basin for distances of >150 km, and document long-distance sediment transport in completely disintegrated water-rich turbidite flows. Downslope sediment transport was a continuous process during shelf-edge glaciations, reaching accumulation rates of 100 m/kyr. Our data highlight that exceptionally large volumes of meltwater may discharge to the slopes of trough mouth fans and trigger erosive turbidite flows. We conclude that freshwater supply is likely an underestimated factor for sedimentary processes during glacial cycles

Age determination from sediment core GS14-187-03PC from the Hardangerfjorden system, western Norway

High resolution acoustic data and a 15.7 m long sediment core from the Hardangerfjorden system, western Norway, have been analyzed to increase our knowledge on depositional environments, submarine mass movement trigger mechanisms and submarine mass movement frequencies in high latitude fjord systems. The seismic profiles analyzed show that an up to 160 m thick glacimarine-dominated unit, of probably Younger Dryas age, has been deposited above the acoustic basement. A < 55 m thick unit, comprising stacked mass transport deposits (MTDs) has been deposited atop the glacimarine unit. The identified mass movement events comprise 19 MTDs (MTD1?19), which have transported sediment volumes of up to 0.4 km**3 and initiated turbidity currents resulting in the deposition of up to 13 m thick turbidite layers. The established chronostratigraphical framework reveals high mass movement activity in Hardangerfjorden at 11100?8200 cal. yrs BP (Early Holocene) and at 4100 cal. yrs BP to present (Late Holocene). 14 MTDs have been dated to the Early Holocene, which is a time period characterized by high sedimentation rates (1.1 mm/yr), giving a mass movement recurrence rate of 1/200 years. Several of these failure events are suggested to have been triggered by regional mechanisms such as earthquakes linked to glacioisostatic uplift. Some of the MTDs of that time could potentially be caused by rock avalanches. Furthermore, it seems that the identified 8200 cal. yrs BP MTD5 coincides with the age of the Storegga tsunami, suggesting that processes related to this event may have caused sediment failure in the inner Hardangerfjorden. During the mid-Holocene (8200?4100 cal. yrs BP), a time period which was characterized by low sedimentation rates of 0.1?0.2 mm/yr and a warmer and wetter climate, mass movement events were absent in the study area. The renewed slide activity in the Late Holocene, comprising four MTDs, is probably related to climatic processes, earthquakes and rock avalanches, resulting in a mass movement recurrence rate of 1/1000 years for this time period. This study, thus, underlines the importance of high-latitude fjords, also in a global context, as systems where local, regional and external geological forces interact to impose highly dynamic postglacial depositional environments

Ice-stream dynamics of the SW Barents Sea revealed by high-resolution 3D seismic imaging of glacial deposits in the Hoop area

Recent developments in seismic acquisition systems and seismic data visualization have contributed to improve the imaging of seismic geomorphologies over a broad range of topics. This study focuses on a new high-resolution P-Cable 3D seismic cube located in the Hoop area in the SW Barents Sea. The scientific motivations of the study are (1) to document glacial landforms on a meter-scale, (2) to study the link of these landforms with the subsurface, and (3) to understand ice-stream dynamics by detailed interpretation of main horizons, including the seabed, intra-glacial horizons, and Upper Regional Unconformity (URU). The horizons are assessed using the concept of seismic geomorphology, and compared with interpretations derived from conventional 3D seismic and state-of-the-art multibeam echosounder technologies to discuss the benefits and limitations of the different geophysical technologies. A well-defined intra-glacial seismic reflection is imaged in the eastern part of the P-Cable cube. This reflection has an overall NE-trending ridge-shaped morphology, which is up to 50 m high and 5–10 km wide, and interpreted as a shear margin moraine. The top of the moraine displays well-defined iceberg ploughmarks and clear slope failure events along its eastern flank. The seabed geomorphology, which has been directly shaped by glacial processes related to multiple phases of ice streaming, appears to be influenced by the buried shear margin moraine. Mega-scale glacial lineations (MSGLs) indicative of fast-flowing ice streams characterize the seabed west of the moraine. Infilled MSGLs and different sets of iceberg ploughmarks with large berms dominate the seabed geomorphology above the moraine. The seabed east of the moraine, characterized by a smooth morphology and few pockmarks, is inferred to reflect more slowly-moving ice. Not resolved by other geophysical techniques, seismic attribute maps of P-Cable 3D allowed the identification of smaller ploughmarks and corrugated ridges related to paleo tides. Eroded by the larger iceberg ploughmarks, these small ploughmarks are suggested to be linked to an earlier rapid disintegration of the Barents Sea Ice Stream. The new high-resolution 3D seismic data have led to the development of a revised ice-stream model of the Barents Sea, linking the glacial deposits to different ice stream events

NE Atlantic continental slope stability from a numerical modeling perspective

Trough mouth fans are environments characterized by high sediment supply during glacial stages and the occurrence of large-scale instabilities. The geological record indicates that several of these environments have failed repeatedly resulting in large submarine landslides. The roles of sedimentation rate, weak layers, glacial loading and unloading as well as seismic activity on triggering megaslides in trough-mouth-fan systems is still unclear. A better understanding of the preconditioning factors, triggers and consequences of these landslides is crucial due to the hazard they pose to coastal communities and offshore industries. In this paper, we focus on the North Sea Trough Mouth Fan, which is the result of massive glacial sediment input delivered to the shelf edge through the Norwegian Channel, southeast Nordic Seas margin. The Tampen Slide, one of several large paleo-landslides that have happened within the North Sea Trough Mouth Fan, took place at c. 130 ka (end of MIS 6), and removed an estimated 1800 km3 of sediment. Here, we use boundary conditions from the Tampen Slide and 2D Finite Element Modeling (Abaqus software from Simulia) to evaluate the effects of variations in sedimentation rates as well as sediment properties on the generation of excess pore pressure, fluid flow, and slope stability along the axis of the trough-mouth-fan system. The model domain, 40 km in length and 2 km in height, is dominated by glacigenic debris flows and glacimarine sediment deposits. We use geotechnical data measured on samples of glacigenic and glacimarine sediment deposits from the nearby Ormen Lange gas field area to constrain the model. We evaluate the stability of the slope under various scenarios, including constant sediment loading, episodic changes in sedimentation rates and abrupt pulses in sediment delivery for a 61 kyr period (MIS 6). The models show that increased sedimentation rates during glacial stages do not generate sufficient excess pore pressure to set off a landslide. Furthermore, the simulated overpressures for the different sedimentation scenarios do not significantly differ at the end of the model runs. The results also highlight the importance of a basal glacimarine sediment layer underneath the rapidly-deposited sediments for the build-up of overpressure. Consequently, this glacimarine sediment layer has the inherited potential to act as a weak layer facilitating instability. However, as overpressure due to sediment deposition alone does not result in slope failure, we couple the preconditioned slope with earthquake ground shaking. Based on attenuation models, an earthquake of approximately M6.9 or larger at a short distance from the Tampen Slide headwall could have triggered the landslide. Therefore, we suggest glacial sedimentation and a glacimarine sediment layer to represent preconditioning factors, and seismic shaking as the final trigger mechanism for the Tampen Slide, i.e. similar to the situation that lead to the development of the Storegga Slide in the same area.NE Atlantic continental slope stability from a numerical modeling perspectiveacceptedVersio

Postglacial mass movements and depositional environments in a high-latitude fjord system – Hardangerfjorden, Western Norway

High resolution acoustic data and a 15.7 m long sediment core from the Hardangerfjorden system, western Norway, have been analyzed to increase our knowledge on depositional environments, submarine mass movement trigger mechanisms and submarine mass movement frequencies in high latitude fjord systems. The seismic profiles analyzed show that an up to 160 m thick glacimarine-dominated unit, of probably Younger Dryas age, has been deposited above the acoustic basement. A < 55 m thick unit, comprising stacked mass transport deposits (MTDs) has been deposited atop the glacimarine unit. The identified mass movement events comprise 19 MTDs (MTD1–19), which have transported sediment volumes of up to 0.4 km3 and initiated turbidity currents resulting in the deposition of up to 13 m thick turbidite layers. The established chronostratigraphical framework reveals high mass movement activity in Hardangerfjorden at 11100–8200 cal. yrs BP (Early Holocene) and at 4100 cal. yrs BP to present (Late Holocene). 14 MTDs have been dated to the Early Holocene, which is a time period characterized by high sedimentation rates (1.1 mm/yr), giving a mass movement recurrence rate of 1/200 years. Several of these failure events are suggested to have been triggered by regional mechanisms such as earthquakes linked to glacioisostatic uplift. Some of the MTDs of that time could potentially be caused by rock avalanches. Furthermore, it seems that the identified 8200 cal. yrs BP MTD5 coincides with the age of the Storegga tsunami, suggesting that processes related to this event may have caused sediment failure in the inner Hardangerfjorden. During the mid-Holocene (8200–4100 cal. yrs BP), a time period which was characterized by low sedimentation rates of 0.1–0.2 mm/yr and a warmer and wetter climate, mass movement events were absent in the study area. The renewed slide activity in the Late Holocene, comprising four MTDs, is probably related to climatic processes, earthquakes and rock avalanches, resulting in a mass movement recurrence rate of 1/1000 years for this time period. This study, thus, underlines the importance of high-latitude fjords, also in a global context, as systems where local, regional and external geological forces interact to impose highly dynamic postglacial depositional environments

Ice-stream dynamics of the SW Barents Sea revealed by high-resolution 3D seismic imaging of glacial deposits in the Hoop area

Recent developments in seismic acquisition systems and seismic data visualization have contributed to improve the imaging of seismic geomorphologies over a broad range of topics. This study focuses on a new high-resolution P-Cable 3D seismic cube located in the Hoop area in the SW Barents Sea. The scientific motivations of the study are (1) to document glacial landforms on a meter-scale, (2) to study the link of these landforms with the subsurface, and (3) to understand ice-stream dynamics by detailed interpretation of main horizons, including the seabed, intra-glacial horizons, and Upper Regional Unconformity (URU). The horizons are assessed using the concept of seismic geomorphology, and compared with interpretations derived from conventional 3D seismic and state-of-the-art multibeam echosounder technologies to discuss the benefits and limitations of the different geophysical technologies. A well-defined intra-glacial seismic reflection is imaged in the eastern part of the P-Cable cube. This reflection has an overall NE-trending ridge-shaped morphology, which is up to 50 m high and 5–10 km wide, and interpreted as a shear margin moraine. The top of the moraine displays well-defined iceberg ploughmarks and clear slope failure events along its eastern flank. The seabed geomorphology, which has been directly shaped by glacial processes related to multiple phases of ice streaming, appears to be influenced by the buried shear margin moraine. Mega-scale glacial lineations (MSGLs) indicative of fast-flowing ice streams characterize the seabed west of the moraine. Infilled MSGLs and different sets of iceberg ploughmarks with large berms dominate the seabed geomorphology above the moraine. The seabed east of the moraine, characterized by a smooth morphology and few pockmarks, is inferred to reflect more slowly-moving ice. Not resolved by other geophysical techniques, seismic attribute maps of P-Cable 3D allowed the identification of smaller ploughmarks and corrugated ridges related to paleo tides. Eroded by the larger iceberg ploughmarks, these small ploughmarks are suggested to be linked to an earlier rapid disintegration of the Barents Sea Ice Stream. The new high-resolution 3D seismic data have led to the development of a revised ice-stream model of the Barents Sea, linking the glacial deposits to different ice stream events