Studying the Tuning Effect from AVO Intercept and Gradient Cross-Plots, Using Pre-Stack Wedge Models

Sbonelo N. Ngcongo
Geological Sciences, University of Houston/University of Cape Town, Rondebosch, South Africa

Estimating AVO attributes for thin beds is problematic because of tuning effect and other factors that affect pre-stack seismic amplitudes. In AVO analysis, two types of tuning effect need to be considered: tuning effect due to thickness of the reservoir and offset-dependent tuning effect due to amplitude variation with offset. Both affect the true AVO expression at an interface and cause the false amplitude variation with offset that is not associated with an individual reflector. Offset dependent tuning effect can be further affected by the differential travel times of adjacent reflections at far offsets. The main purpose of this capstone project is to demonstrate how AVO intercept and gradient are affected by tuning effect as reservoir thickness is reduced.

This study is performed on six gas sand pre-stack wedge models with realistic reservoir properties and encased by shale. Four models have tops and bases of the same AVO classes. The goal is to produce equal strength and opposite polarity reflections from top and base of wedge. The other two models have different tops and bases of AVO classes. Models are as follows: Class III top-Class II base and Class II top-Class I base. The goal is to produce unequal strength and opposite or same polarity reflections events.

In agreement with previous studies, it is found that the tuning effect enhances and reduces the AVO intercept (i.e. normal incident amplitudes) and gradient (i.e. AVO effects). It is noted that these two AVO attributes do not tune at the same thicknesses, except for Class I and Class II top-Class I base models. This causes the two attributes to produce different tuning curves when plotted against CDP number or wedge thickness.

In Class II top-Class I base, as the wedge thins the tuning effect reduces the intercept and enhances the gradient until a certain thickness. Beyond that thickness the intercept is enhanced and gradient is reduced. It is also found that the tuning effect can change the polarity of the AVO gradient for Class III, Class III top -Class II base and Class II top-Class I base responses.
The overall results indicate clearly that the tuning effect disturbs the location of the true AVO response on intercept-gradient cross-plots.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery