--> --> Abstract: Studying the Tuning Effect from AVO Intercept and Gradient Cross-Plots, Using Pre-Stack Wedge Models, by Sbonelo N. Ngcongo; #90082 (2008)
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Studying the Tuning Previous HitEffectNext Hit from Previous HitAVONext Hit 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 Previous HitAVONext Hit attributes for thin beds is problematic because of tuning Previous HiteffectNext Hit and other factors that affect pre-stack seismic amplitudes. In Previous HitAVONext Hit analysis, two types of tuning Previous HiteffectNext Hit need to be considered: tuning Previous HiteffectNext Hit due to thickness of the reservoir and offset-dependent tuning Previous HiteffectNext Hit due to amplitude variation with offset. Both affect the true Previous HitAVONext Hit expression at an interface and cause the false amplitude variation with offset that is not associated with an individual reflector. Offset dependent tuning Previous HiteffectNext Hit 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 Previous HitAVONext Hit intercept and gradient are affected by tuning Previous HiteffectNext Hit 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 Previous HitAVONext Hit 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 Previous HitAVONext Hit 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 Previous HiteffectNext Hit enhances and reduces the Previous HitAVONext Hit intercept (i.e. normal incident amplitudes) and gradient (i.e. Previous HitAVONext Hit effects). It is noted that these two Previous HitAVONext Hit 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 Previous HiteffectNext Hit 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 Previous HiteffectNext Hit can change the polarity of the Previous HitAVONext Hit 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 Previous HiteffectNext Hit disturbs the location of the true Previous HitAVOTop response on intercept-gradient cross-plots.

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