Illuminating the shadows: tomography, attenuation and pore pressure processing in the South Caspian Sea

Abstract

A new approach was successfully applied to produce better images through seismic shadow zones. These zones are caused by the existence of shallow low velocity zones and are a problem to interpretation efforts in the South Caspian Sea. A combination of careful wavelet processing with offset domain amplitude compensation and prestack tomographic inversion produced significantly improved images compared to conventional processing. Visco-acoustic modeling was successfully applied to investigate the effects of seismic attenuation in the seismic shadow zones. Forward modeling results with different Q values were compared to the seismic data and the best model fit to the seismic data determined. The final results indicate that the seismic shadow zones have a low Q in the order of 30 compared to 120 outside the anomaly. Q values of 175 were predicted for deeper zones in the seismic data. A detailed interval velocity model derived from the tomographic inversion analysis was also successfully used to make accurate pore pressure and fracture pressure predictions from the seismic data. After calibration to available well information, the pressure predictions were used to construct a mud weight and mud circulation warning display. The information provides drilling with pressure estimates that allow for better planning that can affect both cost and safety in drilling. Knowledge of the difference in pore pressure and rock fracture pressure is useful in designing casing points and deciding on mud weights to use during drilling. Pore pressures in the medium overpressure range which can possibly represent drilling hazard were predicted in this study.