Seismic Characteristics of the Overpressure Area in Southern Pattani Basin, Gulf of Thailand
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Abstract
An overpressure zone is a zone where the formation pressure is higher than hydrostatic pressure. Moragot and Pailin fields in Southern Pattani Basin, Gulf of Thailand, have pore pressures that represent a drilling hazard. In an undrilled location, seismic data is the main information available for pre-drilled prediction of overpressures. Therefore, the seismic velocity data from the stacking velocity cube is an important key to investigate overpressure by converting the stacking velocity to the interval velocity using Dix’s equation. The overpressure area shows slower interval velocity when compared to normal pressure areas. These slower interval velocities were visualized by two methods: isochronal velocity mapping and overpressure volume generation. The interval velocities were mapped to generate the isochronal velocity mappings in 250 m/s intervals starting from 2,950 to 4,200 m/s. The thicker time intervals, representing an area of slower interval velocities, correspond to overpressure areas. This model has an 88% correlation with RFT data. The main uncertainty is below 2,600 mTVDSS due to mapping resolution. The second method developed is to produce an overpressure volume which is a residual velocity anomaly volume. This model is made by subtracting the seismic velocity volume from the created normal pressured velocity volume. This resulting overpressure volume is consistent in indicated overpressure zones. The seismic velocity was further studied to estimate overpressure degree using the Eaton’s method. The result was not conclusive and may need further study using a higher resolution velocity volume. The outcomes from this study can used to identify overpressure by using seismic velocity in the pre-drill phase.
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Copyright © 2008 Department of Geology, Faculty of Science, Chulalongkorn University. Parts of an article can be photocopied or reproduced without prior written permission from the author(s), but due acknowledgments should be stated or cited accordingly.
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