Examples of shapes encountered in clastic sequences

The seismic amplitude depends not only on the contrasts of the elastic properties but also on the shape of the elastic property distribution in the subsurface which, in turn, is associated with the underlying rock properties, including the porosity and clay content. In Figure 8.1 we show well data from an offshore oil well that encountered a low-GR hydrocarbon reservoir. The shape on the GR curve is blocky at the bottom but has a gradual low- to high-GR at the top. The porosity and density curves mimic this fining-upwards shape and so does the P-wave impedance. As a result, the impedance contrast at the bottom of the oil-filled interval is larger than at its top and, hence, the amplitude at the bottom is stronger.

It exhibits an AVO response with the normal-incidence positive amplitude becoming increasingly positive with the increasing offset. The reason for this behavior is the positive contrast of the P-wave impedance as well as Poisson’s ratio between the sand and underlying shale (Figure 8.1, top). The character of this reflection would be very different if the sand was wet (Figure 8.1, bottom) as the strong impedance and Poisson’s ratio contrasts present in situ virtually disappear. This is the case where a bright spot, especially at far angle incidence, points to a hydrocarbon-filled interval.