Prediction of abnormal pressure from AVO velocities over “Safety” field, onshore Niger delta, Nigeria

Abstract

Abnormal pressure prediction was undertaken in ‘‘Safety’’ field, onshore Niger Delta, Nigeria using amplitude variation with offset (AVO) velocities information. Each of the methods used form an integral part of a process that produces AVO Analysis, AVO velocity inversion, extraction of seismic velocity from AVO velocities inversion results and pore pressure prediction. Pore pressure predicted from the seismic velocity has a better resolution than the pressure predicted from the interval transit time. The pore pressure within the field ranges from 14.7psi to 3916psi. Overpressured and underpressured zones were delineated on the field from the pressure predicted from the seismic velocities. Within the field, the over-pressured zones were delineated at depth 6855 ft–7802 ft. Over pressure top was delineated at a depth of 6855 ft with a pore pressure of 3053psi and a corresponding hydrostatic pressure of 2722psi. The under-pressured zones were also delineated at depth 7883 ft–9288 ft. The under pressure top was delineated at a depth of 7883 ft with a pressure of 1093psi and a corresponding hydrostatic pressure of 3122psi. Porosity values within the over pressure zone ranges from 23% to 53% which could be considered as relatively high. This could be as a result of the fact that the pore fluid cannot be expelled rapidly thereby causing the pore fluid to increase rapidly since they are no longer compacted; thus leading to overpressure. As a result of overpressure top which is directly above the reservoir top within the shale zone, drilling this reservoir vertically could not be suggested so as to avoid possible blow out. It was also observed that the primary cause or mechanism of overpressure within this field could be disequilibrium compaction.