Detection of gas-bearing intervals using S-transform and AVO analysis

Abstract

In geophysical studies, attributes are the main tools for hydrocarbon identification and can be applied either pre- or post-stack. The amplitude versus offset (AVO), on the other hand, lies within the inversion methods applicable to pre-stack seismic data specifically tailored to recognize fluid content. A recent method useful for detecting hydrocarbon-bearing intervals in a reservoir is the spectral decomposition and is applied on post-stack data. For any frequency in a time–frequency setting, a single-frequency seismic section can be drawn and the low-frequency shadows related to the hydrocarbon accumulation can be differentiated. In this study, the spectral decomposition and the AVO analysis methods are applied on synthetic and real data, the latter being an example from a gaseous sandstone reservoir. The synthetic data based on the nearby well-logs and the fluid substituted synthetic well-logs were constructed. Results indicate the superiority of spectral decomposition method in detecting gas-bearing intervals (gas saturation above 80 %) in comparison to the AVO method. While the AVO method successfully differentiates the hydrocarbon and water-bearing intervals, its sensitivity in detecting an interval of 80 and 35 % gas is indifferent. We empirically know that the dominant frequency for detecting shadows of low-frequency features is between 10 and 20 Hz. In this study, such shadows are only observed below high gas saturation intervals by which the economical gas-bearing zones could be discriminated from the non-economic ones.