Abstract: Practical Examples of the Use of Seismic Amplitudes for Predicting Lithology

Alan C. Kemp, Joseph W. Gallagher, Arild K. Jorstad

Seismic amplitude variations have the potential to add significant lithologic information to the interpretation of seismic data. However, the practical approach to optimizing the extraction of such information is usually constrained by the particular geological and geophysical problems of the area in question.

The chance of success with poststack seismic inversion, for example, is heavily dependent on a precise estimate of the wavelet inherent in the seismic data. Experience has shown that this is more likely when the well data used contains several significant breaks in acoustic impedance; otherwise the inevitable errors in both log and seismic data may swamp the wavelet estimate. In certain cases the use of a water-bottom reflector to control the wavelet estimate may considerably improve the final result. However care must be taken to ensure that the water-bottom reflector itself is a true representation of the inherent wavelet and that the projection of this wavelet to depth is realistic.

Volume seismic attribute mapping is becoming more common in the analysis of seismic sequences. Amplitude and frequency, for example, have been used to help delineate a series of channels and fans within a thick sand sequence. In many cases the existing capability of a seismic workstation for handling and display of large data volumes can also be put to good use when analyzing other amplitude attributes derived from seismic data. A map showing the amplitude variation along a picked horizon in CMP-offset space gives a detailed picture of the AVO variation and avoids the averaging and curve fitting simplifications normal with gradient stack and similar approaches.

A further practical step in the analysis of amplitude variation with offset is the use of relative AVO response. By considering the AVO response of the target horizon in relation to a nearby event the high demands of the amplitude processing are relaxed and a more practical approach can be designed.

One of the challenges of using the AVO attribute is balancing the need for high-volume AVO analyses, as typified by the partial cubes and difference cubes, with the necessity of examining other potential sources of AVO anomalies. For example, when an increase in amplitude with offset results from the interference of peg-leg multiples in the far traces, a close examination of the data contributing to the eventual AVO cubes may be necessary to ensure that we understand the source of the anomaly.

Ideally we should quantify the AVO results in terms of possible lithology and fluid models. To this end well data, rock properties modelling, and seismic modelling are used to create the AVO responses of models with differing porosity, fluid saturation and clay content, and to compare these to actual field data.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela