Low-frequency seismic analysis and direct hydrocarbon indicators

Abstract

Low-frequency seismic analysis has been used as a possible direct indicator of the presence of hydrocarbons in reservoirs. Laboratory examples, synthetic studies and field examples in oil and gas exploration suggest that hydrocarbons often contribute a low-frequency anomaly (less than 15 Hz). Th ere are a number of algorithms available in interpretation software packages that perform spectral decomposition of reflection seismic arrivals. Using seismic and well data from the Johan Sverdrup field, central North Sea, and the Western Graben, southern North Sea, a low-frequency analysis has been carried out in the context of differences in lithological properties of reservoirs. Spectral decomposition is applied to stacked seismic traces to enable low-frequency analysis and comparison of results from these two oil-bearing reservoirs types. To look for low-frequency anomalies, spectral sections of 5 Hz to 40 Hz have been used. Additionally, in order to analyze anomalous low-frequency responses, so-called frequency gathers (amplitude vs time in various narrow frequency bands) at a single seismic trace or well location have been used. This study focuses on finding a connection between the occurrence or non-occurrence of low-frequency anomalies and the nature – heterogeneous or homogeneous – of the reservoir. Here, Johan Sverdrup is considered a relatively homogeneous reservoir while the 2/7-31 discovery in Western Graben is considered a relatively heterogeneous reservoir.