AVO Crossplotting: Achieving New and Unbiased Hydrocarbon Indicators

AVO crossplotting has been widely used in the past few years as a way of deriving improved hydrocarbon indicators from seismic data. By crossplotting the standard AVO attributes of intercept and gradient, it is possible to obtain an optimum combination of the two (the fluid factor), which is insensitive to the AVO effect of brine-saturated shales and sands. Any AVO anomaly can then be attributed to hydrocarbons or lithologic factors.However, the background shale trend observed in such crossplots is primarily due to a statistical artifact. In the presence of noise, the standard least-squares regression that is used to estimate intercept and gradient has a tendency to correlate these two attributes. This statistical trend has the same pattern as expected from background shales. Fortunately, since there is some signal in seismic data, the background trend does contain some lithologic information. This information is very limited though, and the fluid factor obtained from this mostly statistical trend is in fact very close to a far offset stack. Although far offset amplitudes are good hydrocarbon indicators, crossplotting is a very complicated way to compute a far offset stack. On the other hand, noise does not introduce a statistical correlation between stack and gradient. Since stack and gradient exhibit the same kind of trend for background shales as intercept and gradient, it is very appealing to crossplot them. Any trend observed in this crossplot would have to be related to lithology and not to a statistical artifact. Unfortunately, when doing this we find that the background trend is generally buried under background noise. However, since this new crossplot is statistically unbiased, it is possible to postulate a background trend, leading to a new definition for the fluid factor. This gives rise to unbiased hydrocarbon indicators that are not limited to far offset stacks.