Seismic and petrophysical evidence of abnormal pore pressure in western Niger delta

A detailed pore pressure evaluation of Tango Field, western Niger Delta, was carried out with the aim of ascertaining depth to overpressure which enables proper well planning and designing in order to reduce cost of exploration and exploitation. The evaluation was carried out using both petrophysical data consisting of sonic, density and resistivity logs obtained in five wells drilled in the area and high resolution-seismic reflection data from which the average and interval velocities were computed. Crossplots were made for well logs data which were fitted with regression curves, and trend lines in order to establish overpressure patterns. Plots of overburden, fracture and hydrostatic gradients with leak off test (LOT), mud weights and modular dynamic tester (MDT) were made to validate the trends established from the crossplots. The results obtained show that normal compaction trends (NCT) were established for all the data used in the analysis and all the wells except well 5 which showed obvious deviations from these trends. Well 5 encountered overpressure at 8,000 ft which was considered earlier than anticipated, hence, it was abandoned. Top of overpressures for wells 2 and 4 were estimated between 9,000 ft and 12, 000 ft respectively. Porosity versus vertical effective stress from the plots confirmed the above depth to overpressure which reveals that the overpressures are primarily generated by disequilibrium compaction.