ABNORMAL PRESSURE PREDICTION USING PRE-STACK DEPTH MIGRATION DATA FROM "X" FIELD, OFFSHORE NIGER DELTA

Abstract

This study presents the results of abnormal pressure prediction of the "X" field, deep offshore Niger Delta, carried out using Pre-Stack depth migration (psdm) of 3D seismic data, to determine the depths to top of overpressure zones in the study area. Due to the complexity of the subsurface environment, psdm is required to build accurate velocity model of the field which will help to properly image the subsurface. A 64 km² of deep marine 3D seismic brute stack data was processed from time domain to depth domain in order to build accurate and geologically reliable velocity model, suitable for subsurface pore pressure analysis. A seismic data processing center was sourced where the 3D raw brute seismic dataset (64 sq. km) was processed from time to depth domain, using Kirchhoff pre-stack depth migration algorithm. The input data was reviewed to ascertain the quality and integrity of the data. The dataset underwent geometry (inline, crossline, offset, fold and azimuth) quality control to produce an accurate subsurface image, with negligible amplitude and phase distortions, and to achieve the highest possible signal to noise ratio (SNR) by attenuating coherent and random noise. Velocity analysis was conducted and the initial model velocity was derived from stacking velocities (RMS velocities) which were converted to interval velocities using Dix's equation. Seismic velocities used during processing were designed to optimize the stack/migration result. The low resolution, layered medium and hyperbolic moveout assumptions of conventional velocity analysis was replaced with reflection tomography – a ray tracing modeling approach. The reflection tomography replaced the common depth point (CMP) gathers of conventional velocity analysis with prestack depth migrated Commom Image Point (CIP) gathers using Snell's law. This gave an improved spatial resolution of the seismic velocity field and thus allowed a more reliable predrill pore-pressure prediction to be obtained. The final derived psdm velocities for nineteen locations were used to make depth versus interval velocity plots. The information extracted from the plots were employed for pore pressure prediction using the Traugott (1997) method and the results correlated with literature. The results are presented in form of seismic sections, which showed improved imaging of psdm processing over pstm, seismic section/velocity overlays, depth versus velocity plots, abnormal pressure gradient map, overburden pressure gradient maps, and depth to top of overpressure zones map of the study area. In all, abnormal pressure prediction was carried out in nineteen (19) locations. The overpressure values computed ranged from 10.67 kpa/m to 14.53 kpa/m (0.47 to 0.64 psi/ft) in the study area which means that the area is prone to mild overpressures. Depth to top of overpressures varies from 940 meters to 3200 meters in the study area. Moreover the computed overburden pressure gradient (OBG) in the area ranged from 20.31 kpa/m to 22.02 kpa/m. The computed overburden values are comparable to that obtained from an offset well North of the study area with OBG values from 19.52 kpa/m to 22.47 kpa/m (0.86 to 0.99 psi/ft) for depth range of 600 m to 3000 m. This is a proof that the velocity attribute volume is accurate and reliable for the pressure computed. The area distribution of depth to top of overpressures has provided idea of the uncertainty in the area in future work. This result is also close to available literature in the Niger Delta using different methods. It is envisaged that this study has provided additional data for proper well design and mitigation of drilling hazards in the study area.