Detecting and Predicting Over Pressure Zones in the Niger Delta, Nigeria: A Case Study of Afam Field.

Abstract

An investigation of the overpressure condition in Afam field was carried out using a suite of borehole logs and 3D seismic data obtained from Shell petroleum Development Company of Nigeria (SPDC) with a view to detecting and predicting abnormal pressure zones in the area.

The methodology employed knowledge of well lithology and more detailed information extracted from inverted seismic traces. The interval velocity data of some shot lines within the study field and its immediate environment were computed to ascertain the pressure variations and the geological model of the less known areas from seismic data. Using well data near the seismic trace for calibration, precise stratigraphic interpretation of the constructed interval velocity section was carried out. The overpressure zones were identified and probable hydrocarbon distribution pattern within the field was established and correlated with the geology of the study area..

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 including their lateral variations. .The plot of the Normal Compaction Trend versus velocities obtained from sonic logs also confirmed the identified overpressure zones. The 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, with thickness varying 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 over pressured shale beds, were delineated at 2657 m, 2804 m, 2916.9 m and 3048 m respectively.

This case study shows how knowledge of well site lithology and detailed information from seismic data could enable prediction match well conditions with high fidelity thereby reducing drilling cost, prospect risk and improving safety..

Keywords: overpressure, compaction, interval velocity, Shale, amplitude inversion