Framework for AVO gradient and intercept interpretation

Amplitude variation with offset (AVO) interpretation may be facilitated by crossplotting the AVO intercept (A) and gradient (B), Under a variety of reasonable petrophysical assumptions, brine-saturated sandstones and shales follow a well-defined "background" trend in the A-B plane. Generally, A and B are negatively correlated for "background" rocks, but they may be positively correlated at very high V-P/V-S ratios, such as may occur in very soft shallow sediments. Thus, even fully brine-saturated shallow events with large reflection co-efficients may exhibit large increases in AVO, Deviations from the background trend may be indicative of hydrocarbons or lithologies with anomalous elastic properties. However, in contrast to the common assumptions that gas-sand amplitude increases with offset, or that the reflection coefficient becomes more negative with increasing offset, gas sands may exhibit a variety of AVO behaviors. A classification of gas sands based on location in the A-B plane, rather than on normal-incidence reflection coefficient, is proposed, According to this classification, bright-spot gas sands fall in quadrant III and have negative AVO intercept and gradient. These sands exhibit the amplitude increase versus offset which has commonly been used as a gas indicator. High-impedance gas sands fall in quadrant IV and have positive AVO intercept and negative gradient, Consequently, these sands initially exhibit decreasing AVO and may reverse polarity, These behaviors have been previously reported and are addressed adequately by existing classification schemes. However, quadrant II gas sands have negative intercept and positive gradient, Certain "classical" bright spots fall in quadrant II and exhibit decreasing AVO. Examples show that this may occur when the gas-sand shear-wave velocity is lower than that of the overlying formation. Common AVO analysis methods such as partial stacks and product (A x B) indicators are complicated by this nonuniform gas-sand behavior and require prior knowledge of the expected gas-sand AVO response. However, Smith and Gidlow's (1987) fluid factor, and related indicators, will theoretically work for gas sands in any quadrant of the A-B plane.