DETECTING AND PREDICTING OVERPRESSURE ZONES IN THE NIGER DELTA, NIGERIA: AFAM FIELD CASE STUDY

With the aim of assessing the possibility of detecting overpressure zones in virgin fields lacking well-data information, an investigation of the overpressure condition in Afam field of the Niger Delta was carried out using a suite of borehole logs and 3D seismic data obtained from Shell petroleum Development Company of Nigeria (SPDC). The methodology employed include constructing the interval velocity sections of the shot lines within the study field and its immediate environment, the acoustic impedance sections and the geological model of the less known areas from seismic data. In addition, accurate stratigraphic interpretation from the constructed interval velocity sections, acoustic impedance, and geological model was carried out. Besides, the pore-pressure variation was correlated with the geology of the study area. The overpressure zones were identified and probable hydrocarbon distribution pattern within the field was established. Finally, seismic amplitude inversion was carried out to ascertain the pressure variations using interval velocity values. The results obtained revealed five tops of overpressure (TOV) namely TOV 1, TOV 2, TOV 3, TOV 4, and TOV 5 at 1608.8m, 1884.1m, 2387.3m, 2708.9m and 3001.4m respectively derived from seismic and lithologic logs. Lateral variations in overpressure were also detected at these depths. Also, the plot of the Normal Compaction Trend versus velocities obtained from sonic logs also confirmed the identified overpressure zones. In all, five horizons representing the identified overpressure zones were picked on the 3D seismic sections at 1.42 s, 1.75 s, 2.01 s, 2.11 s and 2.30 s respectively. The thickness of the various identified overpressure zones varied from 4.08 m to 50.58 m. In-addition, the water saturation (Sw) and porosity values calculated showed that the overpressure zones were generally characterized by high water saturation (52 % to 80.36%) and low porosity (16.55 % to 30.80 %). Furthermore, four hydrocarbon-bearing zones, which were overlain by thin overpressured shale beds, were delineated at 2657 m, 2804 m, 2916.9 m and 3048 m respectively.