Mapping of shallow Tunnel Valleys combining 2D and 3D Seismic Data
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In this study 19 tunnel valleys within block 2/4 in the central North Sea basin have been mapped. Furthermore, the possibility for these valleys to act as migration paths for leaked gas has been evaluated. In January 1989 a kick occurred while drilling well 2/4-14 in the area of study, hence the pertinence of evaluating this hypothesis at this locality is evident. The work has been performed using multichannel 2D lines and a conventional 3D seismic survey. The quality of the 2D and 3D data is clearly dissimilar at shallow burial depths, as the 2D data is considered to be high-resolution while the 3D data is low-resolution. However, both data sets have proved to give valuable information on the valley morphology. Great details about the extent and basal morphology have been retrieved from the conventional 3D volume; whereas seismic characteristics of the valley infill have been interpreted from the 2D lines. Tunnel valleys are major, elongated incisions carved into sediments or permeable bedrock during glaciations. They tend to be sinuous in planform, but might also appear as straight valleys. Tunnel valleys often consist of several cut- and fill-structures, both laterally and vertically, and thus form a network of interconnected valleys. This has also been observed in the area of study. No sedimentological logs have been available in the study. Hence, the interpretations of valley fill lithologies are based on the seismic characteristics, and thereby they are quite cautious. The typical fill sequence observed correlates fairly good with similar valleys mapped in the area previously. A lower part of chaotic reflectors, believed to be glaciofluvial sands and gravels, is overlain by sub-horizontal layers of glaciomarine mud. Moreover, velocity pull-up effects are seen in the underburden of some of the valleys. These indicate relatively high velocities of the infill sediments, and hence, it is likely to be clayey tills. Even so, the possibility of gas migration within the tunnel valley system is believed to be conspicuous.