Empirical Investigation of Pore Pressure Prediction Accuracy Using Pre-Drill Seismic Velocity Analysis in Abnormally Pressured Tertiary Sequences, Offshore Calabar, South South Nigeria

Pre-drill pore pressure prediction from seismic interval velocity analysis is a fundamental overpressure risk management tool, but its accuracy in the poorly characterised Tertiary sedimentary sequences offshore South East Nigeria has not been empirically validated against drilling outcomes. This study investigates the predictive accuracy of three seismic velocity-based pore pressure prediction methods applied to Tertiary sequences in wells drilled offshore Calabar, Cross River State. Eaton, Bowers, and Dutta methods were applied to pre-drill seismic velocity profiles extracted from a 3D seismic dataset over an exploration block offshore Calabar. Predicted pore pressure gradients were compared against calibration pore pressures derived from wireline Modular Formation Dynamics Tester measurements and drilling-derived mud weight equivalent circulating density data from five exploration wells drilled in the area between 2015 and 2022. Mean absolute pore pressure prediction errors were computed for each method and each well, with analysis by depth interval to identify systematically over- or under-predicted zones. Results indicate that the Eaton method provides the most consistent performance with a mean absolute error of 0.18 psi per foot, while Bowers significantly underestimates overpressure in intervals where chemical compaction diagenetic mechanisms dominate over mechanical compaction. Transformations calibrated to the South East Nigeria offshore velocity-to-compaction trendlines are developed to improve Bowers method performance in this specific geological setting. Keywords: pore pressure prediction, seismic velocity, offshore Calabar, overpressure, Tertiary sequences.