AVO and Seismic Attributes Analysis of OTG Field, Deep Offshore Nigeria Delta

Abstract

Amplitude variation with offset (AVO) analysis was carried out on OTG field, deep offshore Niger Delta for the purpose of predicting gas in the reservoirs. Exploration for oil and gas are being shifted to more complex and difficult terrain such as the deep offshore, quantitative studies such as AVO analysis are needed to reduce exploration risks associated with this environment. The dataset used for this study includes 3D seismic pre-stack migrated data with near and far offset gathers, full stack 3D seismic and composite well logs including compressional and shear sonics. Methods adopted involved the correlation of wells, generation of synthetic seismograms, surface attribute analysis, estimation of rock attributes, AVO modelling and classification of AVO anomaly type for subsequent prediction of fluids saturating the reservoir. Surface attributes analysis showed amplitude anomalies on the surface around the well tops indicating possible gas distribution. Cross plots of estimated rock attributes showed clusters that distinctively showed the distribution of fluid which are brine, oil and gas. Gradient versus intercept cross plot presented some of the plotted clusters as falling within the third quadrant and this was interpreted as indicative of the presence of gas in the reservoir. The AVO anomaly was classified as being of the class III type. Amplitude of reflection coefficients were observed to decrease with offset on seismic gather. The study concluded that though the reservoir was abundant in oil reserves, it contained significant amount of gas.