ABSTRACT

The Omen Lange gas field is located within the scar created by the Storegga Slide. This gigantic submarine slide occurred about 8.200 years ago, and caused a tsunami that also reached the surrounding coasts. Extensive work has been performed to explain the prehstoric sliding and to evaluate the present stability conditions m the vicinity of the Ormen Lange gas field. Based on regional seismic mapping and "geoborings" (combined geotechnical and geological borings to depths of several hundred meters below the seafloor) a geological explanation model for the sliding has been established This model explains the location of the two major slides in the area, the Storegga Slide and the Trienadjupet Slide The two major slides are located m two large depressions in the More-Voring area. These depressions form sediment traps for soft clays continuously transported along the slopes by the North Atlantic current During the short periods of peak glaciations, the Scandinavian ice sheet reaches the shelf edge and rapid deposition of glacial sediments takes place at the outer shelf and upper continental slope. The sudden load from these sediments on the soft clays m the upper margin, increases the pore pressure in the soft clays, which becomes unstable over time. The most likely trigger for the sliding is one or a series of strong earthquakes caused by glacio-isostatic rebound following glacial retreat. This explains the tendency of sliding to occur after glacial periods. The Storegga area has experienced sliding through the last 1.3 mill years, a period dominated by the effects of glacial linter-glacial climatic cyclicity. Slide of similar type and sue as the Holocene Storegga Slide occurred repeatedly in the same area during the last 0.6–0.5 million years Since the soft unstable clays were essentially removed from the Storegga margin during the last slide, it is concluded that a new cycle with sedimentation of fme-gramed marine clays followed by rapid deposition of glacial sediments m the upper slope is needed to create a new large scale unstable situation in the Storegga area. Geotechnical parameters from a number of geoborings together with seismic reflection data and slope geometry are used to calculate the present stability of the steepest slopes in the vicinity of the field development area Potential slide triggers like earthquakes and high pore pressure have been included m this calculation. Based on the stability calculation and the geological model the risk for new sliding in the area is considered very low. However, work is still ongoing to verify the final conclusion of the risk analysis.

INTRODUCTION

The Ormen Lange gas field was discovered in 1997 with the scar of the Storegga Slide. The recoverable gas reserves are 400 billion scm and production is planned to start in 2007. Norsk Hydro is the operator for development and construction and Norske Shell will be the operator for the production. The other partners m the Ormen Lange license are: Statoil, BP, ExxonMobil and Petoro.

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