An Overpressure Investigation by Sonic Log and Seismic Data in Moragot Field, Gulf of Thailand

Abstract

The investigation of overpressure is important in the subsurface. In the Moragot field, southern part of the Gulf of Thailand, there are 4 wells where obvious overpressure was observed. Equivalent Depth Method and Eaton Method were used to compute pore pressure by using sonic log data in these 4 wells and the results were compared with the RFT log data. Dix's equation was used to convert the seismic post-stack velocities to interval velocities to examine the zone of overpressure. The computed overpressure by Eaton Method with exponential m factor equal to 2 gave the closest result to RFT data. In contrast, the seismic velocity data gave the poorer vertical resolution than sonic log data. Moreover, the seismic velocities were affected by the lithology changes in faulted areas so it was hard to identify the overpressure zone by seismic velocity. However, the deviation of sonic data and seismic velocity in the zone of overpressure do correspond and the overpressure zones in this study area are in the same stratigraphic level. At this level, the northwest part of Moragot field has higher pore pressure than the central part with no elevated pore pressure observed in the southeastern part. The structural map of top of overpressure shows that overpressure zones start at shallower depths in northwest Moragot field than other parts.