ALBERT

Description: 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.

Description: Monitoring a time-lapse geophysical anomaly due to variation in the pore-fluid contents of a subsurface reservoir is critical for further development and management. However, it may be difficult to detect the changes in P-wave responses beyond a certain level of saturation. A comprehensive 3D finite-difference time domain (FDTD) forward modeling, based on idealized subsurface resistivity structures, demonstrates the possibility of marine controlled-source electromagnetic (CSEM) surveying to monitor the movement of subsurface $${\mathrm{CO}}_{2}$$ storage. We focused on CSEM sensitivity studies with respect to variations in geometry and/or saturation of $${\mathrm{CO}}_{2}$$ storage, evaluation of the effects of reservoir depths and lithology, and detection of $${\mathrm{CO}}_{2}$$ leakage from the main storage. A 100% lateral expansion from the initial diameter (1500 m) of a typical $${\mathrm{CO}}_{2}$$ plume with thickness and resistivity of 100 m and 80 m, respectively, at 850 m below seafloor, results in 41% electric field (E-field) magnitude increase at 2500 m source-receiver offset for 0.5 Hz source frequency. In comparison, a 300% vertical expansion of a 50 m thick plume with diameter and resistivity of 2500 m and 80 m, respectively, gives only a 7% E-field magnitude anomaly. An increase of the aspect ratio of a $${\mathrm{CO}}_{2}$$ plume with diameter and thickness of 1750 and 130 m, respectively, by 110%, without volumetric change, results in a 20% E-field magnitude increase. The E-field magnitude differences due to $${\mathrm{CO}}_{2}$$ saturation change by 20% in clay-rich (10% clay) and clean reservoirs are almost identical; thus, a time-lapse anomaly in a clay-rich reservoir might be detectable. A gradual inverse variation of offset and frequency can differentiate the responses of a 50 m thick shallow $${\mathrm{CO}}_{2}$$ accumulation, 200 m below the seafloor, from the main $${\mathrm{CO}}_{2}$$ plume at 850 m below the seafloor and allow for early warning of $${\mathrm{CO}}_{2}$$ leakage.

Description: Summary Insights into the spreading evolution of the Knipovich Ridge and development of the Fram Strait are revealed from a recent aeromagnetic survey. As an ultra-slow spreading ridge in an oblique system located between the Svalbard—Barents Sea and the Northeast Greenland rifted margins, the dynamics of the Knipovich Ridge opening has long been debated. Its 90-degree bend with the Mohns Ridge, rare in plate tectonics, affects the evolution of the Fram Strait and motivates the study of crustal deformation with this distinctive configuration. We identified magnetic isochrons on either side of the present-day Knipovich Ridge. These magnetic observations considerably reduce the mapped extent of the oceanic domain and question the present understanding of the conjugate rifted margins. Our analysis reveals a failed spreading system before a major spreading reorganization of the Fram Strait gateway around magnetic chron C6 (circa 20 Ma).

Description: Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.