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Book Chapter

Chapter 9: Tight Gas Sands

January 01, 1993


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.

Case history lessons
  • 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.

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Figures & Tables


Geophysical Developments Series

Amplitude Variation with Offset: Gulf Coast Case Studies

James L. Allen
James L. Allen
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Carolyn P. Peddy
Carolyn P. Peddy
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Franklyn K. Levin
Franklyn K. Levin
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Society of Exploration Geophysicists
ISBN electronic:
Publication date:
January 01, 1993




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