FRAMEWORK FOR AVO GRADIENT AND INTERCEPT INTERPRETATION

Amplitude variation with offset (AVO) interpretation may be facilitate d by crossplotting the AVO intercept (A) and gradient (B), Under a var iety of reasonable petrophysical assumptions, brine-saturated sandston es and shales follow a well-defined ''background'' trend in the A-B pl ane. Generally, A and B are negatively correlated for ''background'' r ocks, but they may be positively correlated at very high V-P/V-S ratio s, 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 elasti c properties. However, in contrast to the common assumptions that gas- sand amplitude increases with offset, or that the reflection coefficie nt becomes more negative with increasing offset, gas sands may exhibit a variety of AVO behaviors. A classification of gas sands based on lo cation in the A-B plane, rather than on normal-incidence reflection co efficient, is proposed, According to this classification, bright-spot gas sands fall in quadrant III and have negative AVO intercept and gra dient. These sands exhibit the amplitude increase versus offset which has commonly been used as a gas indicator. High-impedance gas sands fa ll in quadrant IV and have positive AVO intercept and negative gradien t, Consequently, these sands initially exhibit decreasing AVO and may reverse polarity, These behaviors have been previously reported and ar e addressed adequately by existing classification schemes. However, qu adrant II gas sands have negative intercept and positive gradient, Cer tain ''classical'' bright spots fall in quadrant II and exhibit decrea sing AVO. Examples show that this may occur when the gas-sand shear-wa ve 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 prio r knowledge of the expected gas-sand AVO response. However, Smith and Gidlow's (1987) fluid factor, and related indicators, will theoretical ly work for gas sands in any quadrant of the A-B plane.