Using Seismic Inversion Techniques to Delineate Rift-related Miocene Sand Reservoirs in the Gulf of Thailand

Ahmad, Mirza N.; Priyanto, Bagus; Rowell, Philip

The Gulf of Thailand lies on the southern margin of the Eurasian Plate and contains a number of north-south trending Tertiary rift basins. These basins form a series of en echelon grabens and half graben systems. The main reservoirs of the area are Lower to Middle Miocene sands associated with fluvial depositional systems. These reservoir sands are compartmentalized and show rapid lateral stratigraphic changes due to their fluvial nature. Moreover, these reservoir sands are further segmented by intra-basin faults influenced by pre-existing basement fabrics. Rock physics analysis and seismic inversion techniques were applied to map the distribution of reservoir sands and to study the depositional style related to tectonics within the Miocene interval of one of the basins of the Gulf of Thailand.

Rock physics analysis shows that variation in P-impedance can only differentiate porous sands (total porosity > 22-24%) from shales, while lower porosity sands have similar P-impedance to shales. Porous sands have low P-impedance (less than 7000 g/cc*m/s). However, relatively lower porosity sands (total porosity up to 18-22%) can be detected by using P-impedance and Vp/Vs information in combination. A P-impedance volume was computed through post-stack seismic inversion and the results are in reasonable match with the P-impedance logs at well locations for high porosity sands. As well, simultaneous inversion was performed to compute P-impedance and Vp/Vs volumes. The results were checked by different QC parameters. Sand bodies were delineated using computed thresholds of P-impedance and Vp/Vs volumes based on a correlation to well data.

Most of the sand distribution is restricted to the faulted zone which forms the western boundary of the basin. It appears that sedimentation was controlled by N-S structural configuration of the basin and occurred in response to rifting evolution in Miocene. The sand bodies interpreted using the post-stack and pre-stack inversion data volumes indicate that intra-basin faults also break the sand connectivity. Generally, the porosity of sand decreases away from the main rift-bounding fault. This study reveals that seismic inversion techniques can successfully predict reservoir sand depositional systems within the rift basins of the Gulf of Thailand.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013