Chapter 9: Tight Gas Sands
An AVA anomaly was thought to come from a porous channel sand. The test well found instead a tight gas sand. Reprocessing showed that the AVA gathers contained an artifact that produced the AVA anomaly at the well site. The true amplitude of the objective reflection decreased as a function of angle, and a polarity reversal occurred within the CMP gather. Modeling of the well logs agreed that a polarity reversal is the AVO response expected for the tight gas sands in this well.
Angle gathers should be used only in conjunction with CMP gathers in the offset domain. Angle gathers may produce artificial AVO responses through mixing.
Tight, high-impedance gas sands can exhibit a polarity reversal within a gather that can be easily overlooked or disguised by stacking velocity analysis. Horizon-based velocity analysis can be helpful in identifying polarity reversals.
A single pass of surface-consistent amplitude balancing may not be sufficient to remove low- or high-amplitude traces related to surface variations.
Serendipity can turn a technical failure into a commercial success.