High Frequencies Attenuation and Low Frequency Shadows in Seismic Data Caused by Gas Chimneys, Onshore Ecuador

Hydrocarbon reservoirs can be sources or sinks for migrating gas, hence gas chimneys have an inherent correlation with HC reservoirs. If frequency effects show a strong correlation between reservoirs and gas chimneys, observing attenuation zones above reservoirs can be considered an equivalent indicator. Loss of seismic data quality is usually the main characteristic of gas chimneys but it is hoped that in the future we can use high frequency observations with CWT as an alternative method to detect gas clouds correlating with reservoirs, even in cases where there is no apparent loss of data quality. In this case study, using spectral decomposition techniques, zones of high frequency attenuation can be observed in seismic data above onshore sandstone oil reservoirs in Ecuador. This attenuation correlates directly with gas chimneys seen in the seismic volume. The amount of attenuation is dependent on the vertical extent of chimneys. Low frequency shadows of reflectors at reservoir depth can also be observed and correlated laterally with the extent of shallower gas chimney zones directly above. Intrinsic attenuation of seismic waves through gas chimneys is proposed as a likely cause of these frequency effects.