Chapter 12: A Shallow Qas Sand
A shallow Frio amplitude anomaly was drilled without benefit of AVO. The well encountered 30 ft of gas sands. The reflection from the gas sands decreases in amplitude with offset, in contrast to the increase in amplitude with offset expected for gas sands. Offset-dependent array attenuation may have changed an increase in amplitude with offset to a decrease for this shallow bright spot.
The reflection from a gas sand did not increase in amplitude with offset, nor did the reflection have an anomalous AVO response relative to a reference reflection.
Array attenuation may have changed an increase in amplitude with offset to a decrease. We recommend that array effects be taken into account during AVO analysis of prospects above about 1 s.