| Each hydrocarbon reservoir has its own characteristic seismic frequency response to seismic signals due to its unique rock and fluid properties in the surrounding environment. Much evidence shows the presence of low frequency spectral anomalies with a high degree of correlation to the location of hydrocarbon reservoirs. To understand the physical reasons causing this phenomenon, and to utilize it as an attribute of hydrocarbon indicator, I categorize the influence factors of seismic frequency into two types: global and local factors. The global factors change the frequency of the entire seismic section and determine the background frequency of the seismic section; the local factors only bring some regional or local frequency variation at the given time and location. Wave equations based on synthetic models can be used to generate local frequency energy anomalies related to local fluid properties, lithology change, and layer thickness variation.;Spectral decomposition analyses a signal in both the time and frequency domain. The choice of an analyzing wavelet function is fundamental to any spectral decomposition method and determines the resolution in the two domains. An orthonormal wavelet optimized to a desired signal in the least square sense is utilized by a hybrid spectral decomposition method which combines the continuous wavelet transform with a non-linear operator. This results in significantly improved frequency resolution and enhances local frequency components. The tool can be used to directly compute seismic frequency attributes from seismic data and identify regions of anomalous frequency caused by gas or fluid as seismic wave propagates through them. This is illustrated for hydrocarbon-bearing sands corresponding to frequency anomalies using deep water Gulf of Mexico field seismic data. |
