Geologistudiet - hvor galt står det i grunden til?

I GeologiskNyt 3/04 fremlagde geologerne Ole Christensen, Søren M. Kristiansen og Thomas Varming et interessant debatindlæg, hvor de spørger, om vore kandidater er immobile og/eller geologiuddannelserne er utidssvarende. Nedenstående skulle gerne dokumentere, at geologiuddannelsen ikke er utidssvarende og i høj grad i bevægelse for tiden

Strain Evolution in the Strata Overlying the Kraka Salt Pillow, Inferred from Stratigraphic Reconstruction

In this paper, we employ a novel but simple approach to recover the strain evolution of the chalk overlying the Kraka structure. This approach allows for three-dimensional reconstruction of the flexural strain evolution related to growth of the salt pillow without making strong assumptions about mechanical properties. The strain is estimated from differential subsidence that is in calculated by backstripping, and we distinguish compactional strain (mostly vertical) from flexural strain (mostly horizontal). This reveals an overall radial pattern of extensional strain around the Kraka salt pillow. The method also resolves narrow high-strain zones within the chalk, which are either linear or form concentric patterns around the Kraka salt pillow, typically c.100 m wide and c.300 m apart. While some of the fainter concentric structures may be artefacts, the linear structures and more prominent concentric structures are thought to reflect fracture corridors or small faults, and correspond to features observed on colour-processed and ant-tracked seismic data. When this horizontal strain data is used model the distribution and orientation of natural fractures, the results are remarkably consistent with the fractures observed on borehole images. The 3D strain evolution that can be inferred from differential subsidence analysis may therefore provide a simple proxy that can be used to identify zones of high fracture density

De europæiske inversioner - og Nordatlantens åbning

Geologiske processer i de områder, hvor jordskorpens plader møder hinanden og vekselvirker, er ofte meget voldsomme og destruktive. Det kan derfor været vanskeligt at opnå viden om, hvilke kræfter der satte en given udvikling i gang, og om hvornår det præcist skete

Geomechanical Modelling the Evolution of a Connected Natural Fracture Network to Explain Fluid Flow Variations Across a Fractured Chalk-Marl Reservoir

Natural fractures are frequent in the chalk-marl successions of the Lower Cretaceous deposits constituting the Valdemar Field in the Danish Central Graben. However detailed knowledge on their evolution, nucleation and propagation has not previously been modelled and this study is the first to present numerical discrete fracture network models of the fracture patterns in the Lower Cretaceous strata of the Danish North Sea Basin. These strata are of heterogeneous nature and composed of interbeds of sedimentary facies comprising chalk, slightly marly chalk, marly chalk, chalky marlstone and marlstone. This lithological spectrum results in a range of mechanical properties. The Valdemar Field produces from three main reservoir intervals: the lower Tuxen, the middle-upper Tuxen, and the upper Sola. These intervals comprise a variety of sedimentary facies, and contain differing densities of natural fractures. The sedimentological subdivision of the reservoir correlates with mechanical variations within the different layers, and core studies have shown that the characteristics of the natural fractures vary according to the sedimentary facies. The three reservoir units therefore form the basis for the Discrete Fracture Network simulations, but additional simulations are also carried out on a single 10ft thick clean chalk bed within the upper Tuxen, which may act as a separate mechanical layer. The simulations are carried out by DFM Generator, a code for dynamic fracture modelling developed at Danish Offshore Technology Centre (DOTC) that simulates the growth of fracture networks based on the geomechanical properties of the lithology and the stress and strain history. This study presents geomechanical models of numerical simulations of discrete fracture networks modelled across selected reservoir zones in order to compare the nucleation, evolution, and propagation of fractures. The purpose of this study is to evaluate fracture patterns in the reservoir units and further to conduct a comparative study near two selected wells, one from an area of high productivity at the Jens High and one from an area of lower productivity at the Bo structure. The DFN models illustrate that the well with good production has well connected and dense fracture networks around it to facilitate fluid flow whereas the second well is adjacent to fractures that are widely spaced, less connected and primarily one directional. Thus, we propose that geomechanically based DFN models can act as a proxy of the subsurface conditions and indication of expected fracture growth areas in the reservoir

Discussion of Gabrielsen et al. (2010): Latest Caledonian to Present tectonomorphological development of southern Norway

International audienc

The evolution of western Scandinavian topography: A review of Neogene uplift versus the (isostasy-climate-erosion) hypothesis

International audienceTectonics and erosion are the driving forces in the evolution of mountain belts, but the identification of their relative contributions remains a fundamental scientific problem in relation to the understanding of both geodynamic processes and surface processes. The issue is further complicated through the roles of climate and climatic change. For more than a century it has been thought that the present high topography of western Scandinavia was created by some form of active tectonic uplift during the Cenozoic. This has been based mainly on the occurrence of surface remnants and accordant summits at high elevation believed to have been graded to sea level, the inference of increasing erosion rates toward the present-day based on the age of offshore erosion products and the erosion histories inferred from apatite fission track data, and on over-burial and seaward tilting of coast-proximal sediments. In contrast to this received wisdom, we demonstrate here that the evidence can be substantially explained by a model of protracted exhumation of topography since the Caledonide Orogeny. Exhumation occurred by gravitational collapse, continental rifting and erosion. Initially, tectonic exhumation dominated, although erosion rates were high. The subsequent demise of onshore tectonic activity allowed slow erosion to become the dominating exhumation agent. The elevation limiting and landscape shaping activities of wet-based alpine glaciers, cirques and periglacial processes gained importance with the greenhouse­icehouse climatic deterioration at the Eocene­Oligocene boundary and erosion rates increased. The flattish surfaces that these processes can produce suggest an alternative to the traditional tectonic interpretation of these landscape elements in western Scandinavia. The longevity of western Scandinavian topography is due to the failure of rifting processes in destroying the topography entirely, and to the buoyant upward feeding of replacement crustal material commensurate with exhumation unloading. We emphasize the importance of differentiating the morphological, sedimentological and structural signatures of recent active tectonics from the effects of long-term exhumation and isostatic rebound in understanding the evolution of similar elevated regions