Identification of Hydrocarbon Reservoirs from Seismic Gathers AVO Analysis in the Offshore Niger Delta

A. Adekanle
Federal University of Technology, Akure, Nigeria

Amplitude Variation with Offset (AVO) analysis was carried out in an offshore Niger Delta field. The study consisted of forward modeling from rock parameters measured from well logs and AVO analysis of events on pre-stack time migrated seismic gathers. Forward modeling predicted specific AVO behaviour of hydrocarbon bearing reservoir sands in the field. Density, compressional and shear wave velocity logs, combined with a wavelet extracted from recorded seismic gathers were used to generate synthetic seismic gathers and Gassmann fluid substitution models. These synthetics supported key observations made of the AVO response in the field data. AVO attributes (intercept and gradient) were derived from analysis of Common Depth Point (CDP) gathers obtained from a 3D Ocean Bottom Cable (OBC) seismic survey. The attributes were cross-plotted to establish trends against which anomalous amplitude behaviour were identified. Reflections related to shales and brine sands exhibit a relatively small range of orientations creating a dominant "background trend" against which anomalous events related to hydrocarbon-saturated reservoirs show clear deviations. On the basis cross-plot analysis (reflectivity versus offset / angle and and intercept versus gradient), Class 3 type AVO anomalies are shown to be associated with hydrocarbons in the field. This anomalous amplitude behaviour allows identification of hydrocarbon reservoirs in this area of the Niger Delta.