Ståle Emil Johansen

Ice-sheet dynamics through the Quaternary on the mid-Norwegian continental margin inferred from 3D seismic data

Reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system, but most detailed palaeo-glaciological reconstructions have hitherto focused on the very recent history of ice sheets. Here, we present a three-dimensional (3D) reconstruction of the changing nature of ice-sheet derived sedimentary architecture through the Quaternary Ice Age of almost 3 Ma. An extensive geophysical record documents a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the mid-Norwegian shelf since the beginning of the Quaternary. Spatial and temporal variability of the FIS is illustrated by the gradual development of fast-flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time. Buried subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and basal thermal regime. Lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine-terminating part of the FIS. This palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

Strike-slip tectonics in the SW Barents Sea during North Atlantic rifting [Swaen Graben, Northern Norway]

This study uses high quality, three-dimensional [3-D] seismic data to investigate the occurrence of strike-slip faults in the Swaen Graben, SW Barents Sea. The Swaen Graben is divided into two principal sub-basins:- SSB1 and SSB2. The along-strike and along–dip displacement variations and scale relationships are analyzed for forty-two [42] faults. The displacement profiles for these faults are complex in the Swaen Graben, showing clear evidence for polycyclic fault growth and marked syn-sedimentary activity. The observed variations in the displacement profiles indicate complex along-strike segmentation, linkage and mechanical interactions at distinct structural levels. Along-dip displacement minima indicate fault reactivation by dip-linkage. Importantly, geometric evidence for strike-slip faulting in the Swaen graben includes the presence of extensional horsetail splay faults, positive flower structures and minor transfer faults. This study shows that the faults in the Swaen Graben developed under extensional regimes during Late Jurassic to Early Cretaceous rifting and were reactivated by regional stresses during the Late Cretaceous. The two principal strike-slip faults in the Swaen Graben reveal sinistral movement and are linked at a shallow depth by minor transfer faults at a relay zone. Our work further demonstrates the occurrence of Late Mesozoic strike-slip movements in the SW Barents Sea, which were induced by regional tectonics, halokinesis and fault block rotation. Importantly, strike-slip faulting in the region extends perhaps into the Cenozoic interacting with extension during the North Atlantic rifting.

Composition of seismic facies: A case study

In this case study, we used simulated seismic data from outcrops on Svalbard to analyze what seismic facies are composed of, what the dominating factors in forming the facies are, and which consequences this has for the interpretation results. Seismic facies analyses can be used to interpret environmental setting, depositional processes, and lithology. Here, we found that noise is the most important factor in forming the seismic facies. Noise is defined as all reflections that cannot be ascribed directly to the reservoir model. Effects from overburden and processing dominated, and the low-frequency content of the seismic section complicated the seismic facies analyses. The main reason for this is that the analysis relies heavily on identified internal patterns and low-angle terminations. Such patterns and terminations are easily created by the seismic method itself, by overburden effects, and by artifacts generated when processing the data. External form, strong amplitudes, and continuous reflections are robust seismic observations, whereas the internal pattern and terminations are commonly deceptive. Identification of boundaries based on predefined patterns of terminations does not work here, and uncritical use of seismic facies analysis in this interpretation case will create wrong reservoir models. Because of the size of the outcrops, the results from this analysis are relevant for reservoir-scale seismic interpretation and detailed interpretation for prospect evaluation in mature basins. For seismic interpretation at a more regional scale, it is probably less relevant.

3D seismic evidence of buried iceberg ploughmarks from the mid-Norwegian continental margin reveals largely persistent North Atlantic Current through the Quaternary.

Over 7500 buried linear and curvilinear depressions interpreted as iceberg ploughmarks were identified within the Quaternary Naust Formation from an extensive three-dimensional seismic dataset that covers ~ 40,000 km2 of the mid-Norwegian continental margin. The morphology and net orientation of ploughmarks were mapped and analysed. These features are up to 28 km long, 700 m wide and are incised up to 31 m deep. On average, ploughmarks are incised 5 m deep, with median width of 185 m and median lengths ranging from 1.2 to 2.7 km for individual palaeo-surfaces. Width to depth ratio ranges from 8:1 to 400:1 and is on average 36:1. The presence of ploughmarks buried deeply within some palaeo-slope surfaces implies the occasional presence of very large icebergs since the middle Quaternary, suggesting that thick ice-sheet margins with fast-flowing ice streams were present in order to calve icebergs of such dimensions into the Norwegian Sea. The wide geographical distribution of ploughmarks suggests unrestricted iceberg drift and an open Norwegian Sea during the periods of iceberg calving since the early Quaternary. Ploughmark trajectory analysis demonstrates that the ocean current circulation, now dominated by the northeasterly flowing Norwegian Atlantic Current (NwAC), has largely persisted throughout the Quaternary. Despite the overall strikingly consistent pattern of iceberg drift, ploughmark mapping also shows evidence for short-lived NwAC reductions possibly related to major phases of iceberg discharge and/or meltwater pulses from the Fennoscandian Ice Sheet during the middle and late Quaternary.

Interpretation of Marine CSEM and Marine MT Data for Hydrocarbon Prospecting

Remote sensing techniques record variations in petrophysical parameters such as acoustic or electric properties. Seismic sounding is by far the most common of such tools. Seismic techniques can detect strata and structure in the subsurface that are potential hydrocarbon (HC) traps. By interpreting the seismic a detailed geological model can be constructed, but seismic data has limitations in direct prediction of pore fluid composition. Given detection of a structural geometry that may contain HC within porous sedimentary rocks, the main remaining uncertainty is normally whether the pore space is filled with saline water or HC. For this reason only 10–30% of exploration wells penetrate commercial oil or gas reserves in many areas.

Multiple Attenuation - a Contractor Evaluation

Six geophysical contractars have been invited to apply their best multiple suppression technology on a seismic data set prepared by Statoil Research.