| Abstract: | The quantitative estimate of the anomalies in the seismic signal associated with a realistic subsurface hydrocarbon bearing anticline is discussed. This estimate is derived by the calculation of synthetic seismograms for a dissipative media. The reflection amplitude anomaly associated with gas deposit are as much as three times the background level. However, oil deposits give rise to much weaker anomalies. A decrease of the reflection amplitude by about fifty percent occurs for horizons lying below the gas bearing zone, causing a shadow zone. Time sag anomaly is dependent chiefly on the thickness of the gas bearing zone. The appearance of the anomaly on the time section is modified by the structure, and flat reflectors may be seen over the structure owing to the mutual cancellation of the structural rise and the time sag. Flat spots corresponding to the horizontal fluid interfaces may be expected at the edges of the pool only. The presence of hydrocarbon bearing layers causes a diagnostic character in the composite reflection signal which is very clearly seen in the spectra, the cross correlation functions, and the cross-spectra in profiles crossing the hydrocarbon bearing structures. These are potential diagnostic tools for locating hydrocarbon deposits, particularly when entrapped in thin layers. |
