Spectral Decomposition Response of Reservoir Fluids in Fluvial sands in Gulf of Thailand; Application of Spectral Decomposition as Hydrocarbon Indicator

Ahmad, Mirza N.; Rowell, Philip
Petroleum Geoscience Program, Chulalongkorn University, Bangkok, Thailand.

he Gulf of Thailand is comprised of a series of Tertiary rift basins. The main hydrocarbon reservoirs in most of the basins are Lower to Middle Miocene fluvial channel sands. Most of the pay zones are encountered along faults within fault bounded structure closures. Some are associated with negative seismic amplitude anomalies cut by the faults. AVO analysis cannot successfully differentiate between hydrocarbon saturated and brine saturated sands. In this study, we applied continuous wavelet transform techniques in Pattani basin of Gulf of Thailand to observe the response of gas and brine saturated sands using synthetic seismograms. These were generated for the zone of interest consisting of multiple gas pay zones within stacked sands using the extracted wavelet from 3D seismic. Fluid substitution was performed to see the amplitude variation due to fluid change and to study the response of spectral decomposition for gas and brine at different frequencies. We applied fluid substitution for two scenarios; 1) one gas pay zone replaced with brine 2) three gas pay zones replaced with brine. Synthetic seismograms indicate that all sands are within the negative amplitudes and amplitudes are more negative in the the case of gas as compared to brine. Both sets of the synthetic seismograms were spectrally decomposed using continuous wavelet transform technique for different discrete frequencies. The synthetic seismograms with gas saturated zones have lower amplitude zones below pay zones at 45~50 Hz as compared to synthetic seismograms with brine saturated zones. This difference of amplitudes in gas and brine saturated sands is more prominent in scenario 2 where multiple pay zones were replaced with brine.

Therefore, synthetic modeling helped to identify the possible frequency range where amplitude contrast between gas sand and brine sand is greater. In order to apply the modeling results to seismic data the 3D volume over the area of interest was processed for 50Hz continuous wavelet transform and horizon slices were created to observe the attenuation phenomena related with gas. Key horizons show low amplitude along structure as compared to surrounding areas whereas horizon slices of the full spectra do not show such phenomena. This study reveals that spectral decomposition techniques can be applied to identify gas pay zones in the Gulf of Thailand.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012