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.

We use rock physics models for poorly consolidated rocks to diagnose reservoir sandstones in the Alvheim Field, North Sea. Geological factors that will control the rock physics and seismic properties include clay content, sorting, diagenesis, mineralogy, and bedding configuration. The various geologic factors will affect the fluid and stress sensitivity in these rocks. We investigate the interrelationships between various geological factors and seismic fluid and stress sensitivity, by combining well log data and rock physics models. Finally, we determine inter-well characteristics in terms of varying geological factors at different locations and discuss the results in terms of expected seismic signatures in the area.

Time-lapse seismic inversion approach to reservoir management has proven to be a vital tool in the industry today because of its effectiveness in tracking the movement of fluid front within the reservoir as well as identifying isolated bypassed accumulations. A base (2001) and three monitor (2003, 2004 and 2006) seismic surveys from the Norne field were inverted during this research. Water and gas have been injected into the reservoir to maintain the initial pressure within the field. These seismic surveys were analyzed for time-lapse impedance changes due to the differences in the produced hydrocarbons and the injected fluids. Check-shot corrected well data as well as interpreted horizons were integrated in the inversion process. Two independent wavelets were extracted from base and monitor surveys and combined to form an all-encompassing frequency and amplitude wavelet. The base and monitors were jointly inverted. This is because of the reduction in inconsistencies that are associated with independent inversions of surveys and the production related changes expected in time-lapse inverted seismic data.The results of the inversion show the impedance difference across the field for the various monitor surveys. Areas surrounding producer wells show slight changes in impedance while great impedance difference are observed around injector wells. A statistical analysis of the inversion results also shows steady increase in impedance across the field for the subsequent monitors. Structural and stratigraphic interpretation of the time-lapse inverted data also confirmed the sealing properties of some formations. This sealing property supported the impedance changes within the field. Fault interpretations as well as its sealing and non-sealing properties were inferred from the impedance differences across various discontinuities. Time-lapse acoustic impedance inversion of the Norne post-stack seismic data has revealed the impacts of production, dynamic fluid changes across main identified geologic structures, fluid front migration, fluid communication across structures and segments and other identified stratigraphic elements.