Sea-state contributions to sea-level variability in the European Seas

Ocean Dynamics - The contribution of sea-state-induced processes to sea-level variability is investigated through ocean-wave coupled simulations. These experiments are performed with a...     

Entropy of viscous Universe models

The cosmological event horizon entropy and the apparent horizon entropy of the ΛCDM and the Bianchi type I Universe model with viscosity has been calculated numerically, and analytically in the large time limit. It is shown that for these Universe models the cosmological event horizon entropy increases with time and for large times it approaches a finite maximum value. The effect of viscosity upon the entropy is also studied and we have found that its role is to decrease the entropy. The bigger the viscosity coefficient is the less the entropy will be. Furthermore, the radiation entropy for the ΛCDM Universe model with and without viscosity is investigated, and together with the cosmological event horizon entropy are used to examine the validity of the generalized second law of thermodynamics, which states that the total rate of change of entropy of the Universe is never negative, in this Universe model.     

Clay mineral diagenesis and quartz cementation in mudstones: The effects of smectite to illite reaction on rock properties

The Late Cretaceous to Early Tertiary sequence of the Vøring and Møre Basins from the Norwegian Sea has been examined with respect to mineralogy based on 319 cutting samples from five wells. A clear relationship between mineralogy and well log data is demonstrated. A significant change with respect to velocity, porosity and density occurs within the depth interval corresponding to 80–90 °C. At shallow depths/temperatures (<2.0 km/70 °C), compaction is mainly mechanical and the physical properties are similar to what has been measured by experimental compaction of mudstones. At greater depths, however, the log derived velocities and densities are higher than those produced by experimental compaction indicating significant chemical compaction. XRD analyses show a progressive alteration of smectite to illite (S–I) within this depth/temperature interval which results in the release of significant amounts of silica into solution. Detrital silt and fine-grained quartz showed no secondary quartz overgrowths. These grains are isolated within a clay matrix and surrounded by clay minerals, thus limiting the available surface area and pore space for quartz overgrowths. Chemical analyses (XRF) indicate that silica is conserved within this depth interval, and the amount released from S–I alteration was locally precipitated. Field emission gun-scanning electron microscopy (FEG-SEM) and cathode luminescence (CL) identified authigenic micro-crystalline quartz cement within the clay matrix at temperatures above ∼85 °C. This is accompanied by an increase in velocity and density indicating that the S–I reaction and the precipitation of authigenic quartz caused a significant change in the rock stiffness.     

Changes in physical properties of a reservoir sandstone as a function of burial depth – The Etive Formation,northern North Sea

Rock physical properties, like velocity and bulk density, change as a response to compaction processes in sedimentary basins. In this study it is shown that the velocity and density in a well defined lithology, the shallow marine Etive Formation from the northern North Sea increase with depth as a function of mechanical compaction and quartz cementation. Physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial. At shallow depths (<2000–2500 m, 70–80 °C) the log derived velocities and densities show good agreement with results from experimental compaction of loose Etive sand indicating that effective stress control compaction at these depths/temperatures. This indicates that results from experimental compaction can be used to predict reservoir properties at burial depths corresponding to mechanical compaction. A break in the velocity/depth gradient from about 2000 m correlates with the onset of incipient quartz cementation observed from petrographic data. The gradient change is caused by a rapid grain framework stiffening due to only small amounts of quartz cement at grain contacts. At temperatures higher than 70–80 °C (2000–2500 m) the velocities show a strong correlation with quartz cement amounts. Porosity reduction continues after the onset of quartz cementation showing that sandstone diagenesis is insensitive to effective stress at temperatures higher than 70–80 °C. The quartz cement is mainly sourced from dissolution at stylolites reflected by the fact that no general decrease in intergranular volume (IGV) is observed with increasing burial depth. The IGV at the end of mechanical compaction will be important for the subsequent diagenetic development. This study demonstrates that mechanical compaction and quartz cementation is fundamentally different and this needs to be taken into consideration when analyzing a potential reservoir sandstone such as the Etive Formation.     

Quartz cementation in Late Cretaceous mudstones,northern North Sea: Changes in rock properties due to dissolution of smectite and precipitation of micro-quartz crystals

Late Cretaceous mudstones from two wells located in the northern North Sea and the Norwegian Sea have been examined with respect to quartz cement. Two different types of quartz cement (Type 1 and Type 2) have been identified using SEM/EDS/CL-analysis of drill-bit cuttings at depths 2370–2670 m (80–85 °C). Type 1 appears as relatively large aggregates (30–100 μm) of depth/temperature related crypto- or microcrystalline to macrocrystalline irregular quartz cement formed by local re-crystallization of biogenic silica. The CL-responses of Type 1 quartz cement give a clear indication of an authigenic origin. Type 2 quartz cement represents relatively high amounts of extremely fine-grained micro-sized (1–3 μm) crystals embedded as discrete, short chains or small clusters/nests within the illitized clay matrix. The CL-responses of micro-quartz crystals indicate an authigenic origin. The micro-quartz is most probably sourced from silica released during the smectite to illite dissolution–precipitation reaction. The petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones. The silica released produce a subtle inter-connected micro-quartz network interlocked with aggregates of micro-quartz and authigenic clay crystals. This micro-quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime. This is indicated by an abrupt increase in well log velocity (Vp) and change in seismic facies close to 2500 m (80/85 °C).     

The importance of wind forcing in fjord wave modelling

Ocean Dynamics - Accurate predictions of surface ocean waves in coastal areas are important for a number of marine activities. In complex coastlines with islands and fjords, the quality of wind...     

Estimating the internal pressure gradient errors in a σ-coordinate ocean model for the Nordic Seas

Internal pressure gradient estimation is problematic in σ-coordinate ocean models and models based on more generalised topography following coordinate systems. Artificial pressure gradients in these models may create artificial flow. In recent literature, several methods for reducing the errors in the estimated internal pressure gradients are suggested. A basin with a bell-shaped seamount in the middle has often been applied as a test case. To supplement the findings from these more idealised experiments, the internal pressure gradient errors in a σ-coordinate ocean model for the Nordic Seas are discussed in the present paper. Three methods for estimating internal pressure gradients are applied in these experiments. The sensitivity of the results to the subtraction of background stratification and to the horizontal viscosity are also investigated. For the extended Nordic Seas case, basin scale modes dominate after a few days of simulation. The errors in the transports across some sections may be larger than 1 Sv (1 Sv = 10⁶ m³ s^− 1) in these studies with 16-km grid resolution. The order of magnitude of the errors in the transports of Atlantic water into the Nordic Seas is approximately 0.5 Sv or between 5 and 10 % of recent transport estimates based on measurements. The results do not indicate that the errors are generally reduced if the background stratification is subtracted when estimating internal pressure gradients in terrain following models. However, the results from the experiments initialised with the background stratification show that the erroneous flows may be reduced considerably by using more recent techniques for estimating internal pressure gradients, especially for higher values of horizontal viscosity.