Zeitschrift für Geologie und Geophysik
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Analysis of Hydrostatic Pressure Zones in Fabi Field, Onshore Niger Delta, Nigeria

Victor OM*, Ude AE and Valeria AC

The Analysis of hydrostatic pressure of Fabi Field onshore Niger delta was carried out to understand the subsurface depth pressure variations in the field. The well logs were analyzed to identify hydrostatic pressure zones through points of deviation from the compaction trends of the sediments and attribute crossplots while seismic inversion was carried out on the seismic data to obtain the lateral hydrostatic pressure variations in Fabi Field. The well logs exhibited normal compaction trends from depth level of 7200 ft to 8625 ft. At depth level of 8625 ft to 9000 ft, the velocity, density and resistivity logs deviated from the normal compaction trends. This point of deviations from the compaction trends of the sediments were identified as overpressure zones. Through crossplots of velocity and density logs, two overpressure generating mechanism of the Fabi Field such as under compaction and unloading were revealed. The under compaction was characterized by a linear drop in velocity and increase in density with depth while Unloading is characterized by abrupt decrease in velocity and at constant change in density. The results of acoustic impedance inversion revealed the lateral hydrostatic pressure variations of the Fabi Field. The onsets of over pressure zones are found around 1600 ms to about 1630 ms while very high overpressure gradients occur between 1950 ms to 2150 ms. These zones are characterized by very high acoustic impedance. The area extents of the positive anomalies (increase in acoustic impedance) are mostly consistent with the pressure while negative anomalies (low acoustic impedance) are interpreted as reservoir or sand zones.