Frequency-dependent Velocity Analysis and Offset-dependent Low-frequency Anomalies from Hydrocarbon-filled Reservoir

Using various spectral decomposition techniques, low-frequency amplitude anomalies at and below hydrocarbon-filled reservoirs were investigated. For this purpose, a 3D seismic dataset situated in the southern North Sea adjacent to the Denmark and United Kingdom sectors was used. Geologically the area lies in the depression between the NW-SE-trending Lindesnes ridge in the NE and the NW-SE-trending Grensen nose in the SW. The observed low-frequency amplitude anomalies were found to be offset-dependent, being strong on the near-offset traces but absent on the far-offset traces. Additionally, horizon velocity analysis was performed along the tops of reservoir and non-reservoir intervals to look for any possible change in velocity with frequency, or other evidence of dispersion. The NMO velocities calculated from the top of non-reservoir events were found to be the same for both the low- and high-frequency filtered CMP gathers. However, where we observed low-frequency amplitude anomalies at and below oil-filled reservoirs, we also observed low velocity for low-frequency and high velocity for high-frequency filtered gathers. Moreover, this change in velocity was not seen in the low-frequency anomalies observed from gas-filled reservoirs