AVO analyses and spectral decomposition of seismic data from four wells west of Shetland, UK

The first section of the paper focuses principally on the analysis of four wells located west of Shetland, UK to demonstrate that the appropriate use of AVO (amplitude versus offset) analysis forms a worthwhile and valuable tool for exploration. The wells analysed include two with hydrocarbons, the Foinaven oil well 204/24a-2 and the Laggan gas field discovery well 206/1-2, and two wells positioned on amplitude-related prospects (204/17-1 Fleet North Prospect and 204/18-1 Assynt Prospect). Both 204/17-1 and 204/18-1 failed to find hydrocarbons, where the previous work indicated mainly a Class III AVO anomaly. However, these studies demonstrate that both the amplitude anomalies conform to a Class I AVO. For the hydrocarbon-bearing reservoirs encountered in both the 204/24a-2 and 206/1-2 wells, the results confirmed the presence of Class III AVO anomalies.

A second aspect to this paper centres on the application of spectral decomposition to the seismic data relating to these four wells, which provides further evidence that there are also apparent differences in the spectral characteristics between them. Both Foinaven and Laggan exhibit Class III AVO anomalies, which are consistent with the theoretical predictions for frequency-dependent AVO behaviours. However, the results from the 204/17-1 and 204/18-1 wells show rather complicated behaviour in the iso-frequency sections that cannot be explained fully. In summary, though the amplitude anomalies seen in the two prospect wells were interpreted originally to be similar to that in Foinaven, this study demonstrates they are different in both their AVO behaviour and spectral characteristics.