The use of extended elastic impedance (EEI) as described by Whitcombe et al. (2002) gives promise of a fast and objective way to relate seismic inversion results to different reservoir properties such as, e.g., porosity, fluid content and lithology. This can be a useful tool for exploration, prospect evaluation, and early reservoir characterization. At the same time it is generally less time consuming than other methods such as simultaneous AVO inversion combined with rock physics inversion or lithology/fluid facies classification studies. Two key elements in the EEI workflow are to find the correct chi angles projections used for relating the EEI to different elastic properties and to estimate and select the right wavelet for the inversion of the 0° and 90° EEI.
Herein, we will show how rock physics modeling, fluid substitution, and the selection of input data can be crucial to obtain quality results and also in situations where modeling will not be necessary. In addition, we will demonstrate how the EEI inversion workflow can be constructed to be time efficient and to provide quality results.
An offshore exploration example will illustrate the possibilities and results with this workflow, where the results were used in prospect evaluation, de-risking, quantitative interpretation, and separation of brine vs. hydrocarbon-saturated sands.
The techniques for prestack seismic inversion have envolved from generalized linear inversion (Tarantolla, 1986), elastic inversion (Connolly, 1999) to simultaneous inversion (Ma, 2002). Often the focus is to get the best possible quality (e.g., simultaneous inversion) or small scale details and uncertainties (stochastic inversion). Even though efficient inversion algorithms and powerful computers have reduced the computation time, the time required for data preparation and interpretation seems to be harder to reduce.
However, for some reservoirs and situations, the critical aspect might be how to quickly obtain quantitative interpretation for prospect evaluation, or to limit (and understand) the amount of subjectivity influencing the results. These might be some of the reasons why the EEI method (Whitcombe et al., 2002) has recently gained some popularity. The method can be used for direct indication of or quantification for hydrocarbon saturation and various other reservoir properties, and it utilizes the intercept and gradient as input to the inversion.
In this work we discuss how this technique can be used in an efficient inversion workflow and how to relate the inversion results to reservoir properties. We will also discuss some pitfalls and how to avoid them. In the two-term Zoeppritz linearization (Aki and Richards, 1980) given in equation (1), A refers to the normal incidence reflection coefficient (often called intercept), B refers to the variation at intermediate offsets (often called the AVO gradient, or the gradient), and ? is the angle of incidence. The extended elastic impedance as described by Whitcombe et al. (2002) is an extension of the elastic impedance, where, first, the sin^2 ? term is replaced by the term tan ?.