Shallow gas AVO analysis in Block 2/4 North Sea

Abstract

The study site block 2/4 blowout area is challenging because of shallow thin sand and limitation of available well data. The high amplitude anomaly with fingering shape pattern was discovered at 520ms. This shallower sand layer is significantly thinner, but shows up clearly on the seismic data. This thesis aims to distinguish observed high amplitude anomaly affected by tuning effects from gas saturation and to estimate sand thickness and extension related to the blowout. The AVO cross-plot created from estimation of intercept and gradient showed the best deviation from the background trend out of the quadrant III, and the anomaly could be classified as a class III AVO anomaly. The capability of interpreting a reservoir is highly dependent of seismic resolution. The limitation of interpreting a high amplitude anomaly sometimes fails to yield a unique solution of hydrocarbon filled. Tuning effect as another main factor is prone to produce constructive interference. The optimization wedge model was applied successfully to conduct tuning thickness survey in an attempt to estimate the thickness of thin gas sand. The relationship of amplitude and tuning thickness between the near and far stack suggests that not only is the observed high amplitude anomaly affected by gas saturation, but also by tuning effect. The P-impedance data in seismic inversion model enhance the interpretability of high amplitude anomaly associated with tuning effects.