PRE-STACK SEISMIC INVERSION AND RESERVOIR PROPERTY PREDICTION OVER ‘JAY’ FIELD, NIGER DELTA

Abstract

Characterization of reservoir heterogeneity is critical to the understanding of reservoir compartments and production optimization of oil and gas. However, very limited knowledge exists on the elastic parameters of subsurface materials. Well log and core data, where available, provide detailed information about the vertical variation of many of the reservoir properties; but they are restricted to the vicinity of the borehole. Conversely, seismic data has lateral resolution and provide information away from the wellbore. Elastic parameters derived from seismic data are valuable information in reservoir characterization since they can be directly related to lithology and to fluid content of a reservoir. This research integrates well logs and prestack 3D surface seismic dataset through angle-dependent simultaneous seismic inversion to estimate elastic and petrophysical parameters. The estimated parameters assist in characterizing lateral variability in reservoir stratigraphy, complimenting the high resolution vertical variability of reservoir properties readily available from well logs in the study area - the ‘JAY’ field, Niger Delta, Nigeria. Rock physics feasibility analysis was carried out to determine the viability of inversion at defining lithology(reservoir / non-reservoir) and detecting pore fluids. The feasibility work involves rock property cross-plots, AVA (Amplitude Versus Angle) forward and fluid substitution models to investigate the sensitivity of seismic response to changes in pore fluid, porosity sweep models to investigate the sensitivity of seismic response to changes in porosity, and tuning thickness models to investigate resolvable bed thickness on seismic reflection data at a dominant frequency of 25-Hz. The feasibility unveils the non-uniqueness of acoustic impedance which made post- stack seismic inversion of no use in the field. Full bandwidth inversion results were achieved using low frequency models (P- and Simpedances and density) built from well logs to complement the band limited seismic data. The low frequency models of P- and S-impedances, and density were perturbed consistently until a good match between the inverted results and theactual log measurements was achieved at the training well. For quality control purposes, inverted results were cross validated using blind well and trace difference between synthetic and the real seismic angle gathers The results of the inversion consist of the P- and S-impedances and density volumes. From these threebasic properties, other elastic properties were estimated. The  (product of Lamé’s incompressibility and density) is significant in that it identified hydrocarbon-bearing reservoirs in the field. The Poisson’s ratio also showed hydrocarbon distribution patterns in reservoirs. Estimated 3D porosity and water saturation ranges from about 10% to 38% and 12% to 100%, respectively. Spatial distribution and variabilities of estimated properties helpedto detect the lateral extent of key hydrocarbon reservoir ‘X’. Based on enhanced reservoir characterization, the estimated rock parameters would help to optimize the location and drilling of more low risk wells in the field.