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.
Figures & Tables
A cursory look at the contents of the book might lead the reader to belive that AVO is not a successful hydrocarbon indicator, as we show many examples of AVO predictions gone awry, and dry holes drilled based on AVO anomalies. However, a closer look at the pages will give the reader the message we are trying to convey: used properly and cautiously, AVO can be a valuable tool for direct detection. The preponderance of unhappy endings in the book is unavoidable, as the goal of this book is to provide information that may prevent other dry holes being drilled. Inevitably, the best lessons are learned from mistakes, a statement that applies to life in general as well as to seismic interpretation.