ABSTRACT

Using 3D broadband seismic data, we have investigated low-frequency seismic amplitude anomalies associated with and below various geologic formations in the Johan Sverdrup oil field situated in the central North Sea. Low-frequency anomalies are observed below the Intra-Draupne and the Heather and Hugin reservoirs, at the Svarte and Tor Fms, and below shallow channels. The Intra-Draupne Fm is the main oil reservoir of interest in the field, and it is relatively homogeneous, with observed low Q values of approximately 25–30. The Heather and Hugin reservoirs, which underlie the Intra-Draupne, are heterogeneous and oil bearing. We performed three-layer elastic modeling on a simple reservoir model based on the properties of the Intra-Draupne Fm, and the results suggest that as the thickness of the middle oil-bearing layer increases the dominant frequency decreases due to the tuning effect. The Svarte and Tor Formations are shallower in the section and are fractured. Low-frequency anomalies associated with these formations seem to indicate a clear correlation with zones of increased fracture density interpreted from high-resolution most-negative-curvature attribute maps. Low-frequency anomalies are also observed below shallow gas channels consisting of stratified sandy and shaly intervals with vertical variations in porosity and permeability. In addition, stacking tests using coarse and fine velocity analysis indicate no noticeable difference in the characteristics of the low-frequency anomalies, in general, at all levels. We conclude, therefore, that the observed low-frequency anomalies are unrelated to stacking issues.

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