Lithology Prediction Using Rock Physics and Acoustic Impedance for Reservoir Distribution in Northern Pattani Basin, Gulf of Thailand

Abstract

The study area lies in the northern part of the Pattani Basin, Gulf of Thailand. The reservoirs in the basin are fluvial sands. The reservoir sands are compartmentalized and show rapid lateral stratigraphic changes due to their fluvial depositional environment. Imaging of these reservoir sands is not easy by using conventional seismic data. Rock physics analysis and post-stack seismic inversion techniques were applied to map these reservoir sands. According to rock physics analysis, sands and shales have a significant contrast of P-impedance within the stratigraphic interval of Early and Middle Miocene. This contrast decreases in the Oligocene interval, and it is not easy to isolate sands based on P-impedance. Therefore, P-impedance can only discriminate sands in the upper sequences. P-velocity of clean sand and pure shale shows very small or no contrast, but density is different for these lithologies. P-velocity of shale reduces dramatically in a lacustrine section of Oligocene and locally shales have lower P-velocity than sands. Consequently, P-impedance contrast also reduces and makes it difficult to differentiate sand and shale based on P-impedance. The inversion results gave a reasonable match with P-impedance logs at well locations, and blind test results show a good match between actual and predicted P-impedance. Sand bodies were extracted by using computed thresholds of P-impedance based on rock physics cross-plots. Two types of channel belts were observed; 1) high sinuosity and narrow belts within Early Miocene and 2) large and broad channel belts in Middle Miocene. The present study concludes that post-stack P-impedance inversion can successfully map sand reservoirs and determine their connectivity.