Abstract

The amplitude-versus-offset (AVO) method has been shown to indicate the presence of gas sands if the reflection amplitude from the seal/reservoir-sand interface, measured in a common midpoint (CMP) gather, increases rapidly with increasing shot-to-geophone distance (or offset). However, in a few instances it has been observed that the seismic reflection amplitude does not increase with offset and may even decrease if there is widespread gas leakage above the hydrocarbon reservoir causing partial gas saturation in the overburden sediments. Gas-charged sediments are known to attenuate seismic energy. Depending on the size and shape of this gas leakage zone, there may be higher attenuation of seismic amplitudes with increasing offset.We present one such case that involves a prominent 'bright-spot' amplitude anomaly corresponding to a 56-ft-thick (17 m-thick) gas sand in the Gulf of Mexico slope. The reflection amplitude for the sand top was found to decrease with increasing offset. There is also evidence of gas leakage into the sediments above the reservoir. Color amplitude displays of the seismic section show a low-amplitude diffused zone above the bright-spot amplitude anomaly, which suggests gas leakage. Further evidence of gas leakage can be inferred from the significant gas content (including heavier hydrocarbons) observed in the mud log. Gas leakage is also confirmed by gather modeling in which the effects of leakage-caused attenuation are accounted for in matching the variation of seismic amplitude with offset.

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