ALBERT

Beschreibung: Deep sea pockmarks underlain by chimney-like or pipe structures that contain methane hydrate are abundant along the Norwegian continental margin. In such hydrate provinces the interaction between hydrate formation and fluid flow has significance for benthic ecosystems and possibly climate change. The Nyegga region, situated on the western Norwegian continental slope, is characterized by an extensive pockmark field known to accommodate substantial methane gas hydrate deposits. The aim of this study is to detect and delineate both the gas hydrate and free gas reservoirs at one of Nyegga's pockmarks. In 2012, a marine controlled-source electromagnetic (CSEM) survey was performed at a pockmark in this region, where high-resolution 3-D seismic data were previously collected in 2006. 2-D CSEM inversions were computed using the data acquired by ocean bottom electrical field receivers. Our results, derived from unconstrained and seismically constrained CSEM inversions, suggest the presence of two distinctive resistivity anomalies beneath the pockmark: a shallow vertical anomaly at the underlying pipe structure, likely due to gas hydrate accumulation, and a laterally extensive anomaly attributed to a free gas zone below the base of the gas hydrate stability zone. This work contributes to a robust characterization of gas hydrate deposits within subseafloor fluid flow pipe structures.

Beschreibung: The Murray Ridge/Dalrymple Trough system forms the boundary between the Indian and Arabian plates in the northern Arabian Sea. Geodetic constraints from the surrounding continents suggest that this plate boundary is undergoing oblique extension at a rate of a few millimetres per year. We present wide-angle seismic data that constrains the composition of the Ridge and of adjacent lithosphere beneath the Indus Fan. We infer that Murray Ridge, like the adjacent Dalrymple Trough, is underlain by continental crust, while a thin crustal section beneath the Indus Fan represents thinned continental crust or exhumed serpentinized mantle that forms part of a magma-poor rifted margin. Changes in crustal structure across the Murray Ridge and Dalrymple Trough can explain short-wavelength gravity anomalies, but a long-wavelength anomaly must be attributed to deeper density contrasts that may result from a large age contrast across the plate boundary. The origin of this fragment of continental crust remains enigmatic, but the presence of basement fabrics to the south that are roughly parallel to Murray Ridge suggests that it separated from the India/Seychelles/Madagascar block by extension during early breakup of Gondwana.

Beschreibung: Fractured rocks are known to exhibit seismic anisotropy and shear wave splitting (SWS). SWS is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type. The presence of partial liquid/gas saturation is also known to affect the elastic properties of rocks. The combined effect of both fractures and partial liquid/gas saturation is still unknown. Using synthetic, silica-cemented sandstones with aligned penny-shaped voids, we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on SWS under partial liquid/gas saturation conditions. The result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model. At high to full water saturation values, SWS decreases as a result of the fluid bulk modulus effect on the quasi-shear wave. This bulk modulus effect is frequency dependent as a result of wave-induced fluid flow mechanisms, which would in turn lead to frequency dependent SWS. This result suggests the possible use of SWS for discriminating between full liquid saturation and partial liquid/gas saturation.

Beschreibung: The seismic properties of rocks are known to be sensitive to partial liquid or gas saturation, and to aligned fractures. P -wave anisotropy is widely used for fracture characterization and is known to be sensitive to the saturating fluid. However, studies combining the effect of multiphase saturation and aligned fractures are limited even though such conditions are common in the subsurface. An understanding of the effects of partial liquid or gas saturation on P -wave anisotropy could help improve seismic characterization of fractured, gas bearing reservoirs. Using octagonal-shaped synthetic sandstone samples, one containing aligned penny-shaped fractures and the other without fractures, we examined the influence of water saturation on P -wave anisotropy in fractured rocks. In the fractured rock, the saturation related stiffening effect at higher water saturation values is larger in the direction across the fractures than along the fractures. Consequently, the anisotropy parameter ‘ ’ decreases as a result of this fluid stiffening effect. These effects are frequency dependent as a result of wave-induced fluid flow mechanisms. Our observations can be explained by combining a frequency-dependent fractured rock model and a frequency-dependent partial saturation model.

Beschreibung: Pore pressure above the hydrostatic (overpressure) is common in deep basins. It plays an important role in pore fluid migration, represent a significant drilling hazard, and is one of the factors controlling slope stability and deformation in seismically active areas. Here, we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring. We minimize a function that contains the misfits between estimates from our forward model and observed values using a non-linear least squares approach. The inverse model allows the introduction of observed seismic and geological constraints such as P -wave velocity ( V p ) and density data, and depth of the layer boundaries, for a better pore-pressure prediction. The model output also provides estimates of: (1) surface porosity, (2) compaction factor, (3) intrinsic permeability at surface conditions, (4) a parameter controlling the evolution of the intrinsic permeability with porosity, (5) the ratio between horizontal and vertical permeability and (6) uncompacted thickness (so sedimentation rate assuming known time intervals), for each sedimentary layer. We apply our inverse approach to the centre of the Eastern Black Sea Basin (EBSB) where the V p structure has been inferred from wide-angle seismic data. First, we present results from a 1-D inverse model and an uncertainty analysis based on the Monte Carlo error propagation technique. To represent the observed rapid change from low V p to normal V p below the Maikop formation, we impose a zero overpressure bottom boundary, and subdivide the layer below the Maikop formation into two sublayers: an upper layer where the rapid change is located and a lower layer where the V p is normal. Secondly, we present the results from a 2-D inverse model for the same layers using two alternative bottom boundary conditions, zero overpressure and zero flow. We are able to simulate the observed V p , suggesting that the low velocity zone (LVZ) at ~3500–6500 m depth below the seabed (mbsf) can be explained by overpressure generated due to disequilibrium compaction (〉90 per cent) and to aquathermal pressuring (〈10 per cent). Our results suggest that the upper sublayer, below the Maikop formation, behaves as a seal due to its low permeability ~0.3–2  x 10 –14 m s –1 . This seal layer does not allow the fluids to escape downwards, and hence overpressure develops in the Maikop formation and not in the layers below. This overpressure was mainly generated by the relatively high sedimentation rate of ~0.29 m ka –1 of the Maikop formation at 33.9–20.5 Ma and an even higher sedimentation rate of ~0.93 m ka –1 at 13–11 Ma. We estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions (*) of ~0.62 at ~5200 mbsf associated with an overpressure of ~42 MPa.

Beschreibung: In the context of seismic imaging of gas/gas-hydrate systems, the fine-scale structure of subseabed gas-related reflections is assessed by taking advantage of the source signature of the deep-towed high-resolution SYSIF seismic device. We demonstrate the value of an original wavelet-based method and associated multiscale seismic attributes, applied to seismic data recently acquired on the western margin of the Arctic archipelago of Svalbard. From analysis in the wavelet domain, we recognize two types of gas-related reflections associated with submetre-scale distribution of gas. We identify a thin gas-charged layer associated with an apparent normal polarity reflection, and we detect gas patches associated with a reverse-polarity bright spot with frequency-dependent elastic properties at small seismic wavelengths. The results provide valuable information on the scale of features through which gas migrates and resolve ambiguities in the interpretation of the seismic data.

Beschreibung: Magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers. We estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below Soufrière Hills volcano, Montserrat. Growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity, which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment. Petrology indicates that before it was reheated, remobilized and erupted, the temperature of the magma residing in the chamber was about 850°C. We ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850°C. In both cases, magma chamber growth over the last 350 years results in tomography anomalies that are too strong, unless the magma was emplaced at an unlikely low melt fraction (〈0·5). Good fits between the modelled and the observed velocity anomaly were obtained with sills 2–5 km in radius emplaced over 6000–150 000 years, depending on the temperature and melt fraction of the emplaced magma. Because of a trade-off between intrusion dimensions and emplacement durations, the volumetric magma fluxes are restricted to 7 x 10 –4 and 5 x 10 –3 km 3 a –1 . The velocity anomaly can be reproduced with a chamber containing high melt-fraction magma or with a mush of crystals and melt. The range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite. This suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non-eruptible mush.

Beschreibung: The Arctic continental margin contains large amounts of methane in the form of methane hydrates. The west Svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone. The presence of bottom simulating reflectors (BSRs) on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area. Resistivity obtained from marine controlled source electromagnetic (CSEM) data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments. We acquired two CSEM lines in the west Svalbard continental slope, extending from the edge of the continental shelf (250 m water depth) to water depths of around 800 m. High resistivities (5–12 m) observed above the BSR support the presence of gas hydrate in water depths greater than 600 m. High resistivities (3–4 m) at 390–600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone (GHSZ) of the continental slope. In addition, high resistivities (4–8 m) landward of the GHSZ are coincident with high-amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas. Pore space saturation estimates using a connectivity equation suggest 20–50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments. A free gas zone beneath the GHSZ (10–20 per cent gas saturation) is connected to the high free gas saturated (10–45 per cent) area at the edge of the continental shelf, where most of the seeps are observed. This evidence supports the presence of lateral free gas migration beneath the GHSZ towards the continental shelf.