Quantitative petrophysical analysis using AVO and AVA

Abstract

Processing methods that do not only account for kinematic (related to travel times) but for dynamic (related to amplitudes) aspects of seismic reflection data, have become more and more important in recent years. Such methods provide information on the subsurface properties that is useful for reservoir characterization. One technique is based on the amplitude-variation-with-offset (AVO) and amplitude-variation-with-angle (AVA) of seismic reflection data. The technique attempts to explain seismic amplitude anomalies in terms of the nature of the fluid in the pore space of a reservoir. This objective makes it an important exploration tool that greatly improves and expands the applicability of petrophysical interpretation of seismic data. This study provides a review of the theoretical and practical aspects of the AVO /A challenge to elate. Robust inverse modelling using L1 and L2 norm in least square estimation applied to contaminated AVO modelled gathers are compared. L1 norm fitting does much better job than L2 norm fitting with multiple or linear noise existing, whereas the inverse is true for random noise contamination. Uncertainties in AVO /A forward modeling are quantified by crossplotting the AVO /A parameters using the standard deviation method to describe the dispersion, an elliptical region whose properties can be exploited to determine how well AVO /A will work as a hydrocarbon discriminant. Results from this analysis showed that AVO /A has good success in discriminating between brine saturation and oil/gas saturation, and not as successful in discriminate between different hydrocarbon saturation. Overall, the method provides a means by which a quantitative index can be computed for AVO /A risk assessment. Finally, AVO /A attributes are evaluated and studied for hydrocarbon indication in offshore Malta by comparing results with predicted synthetic hydrocarbon related bright spots. The AVA analysis results show that an offshore anomaly is the result of lithology changes unrelated to the presence of hydrocarbons.