AVO for managers: pitfalls and solutions

Jonathan E. Downton, Brian H. Russell, Laurence R. Lines

Amplitude versus offset (AVO) has become an important interpretation tool for the detection of hydrocarbons and reservoir description. It is important to remember when interpreting AVO data the limitations and assumptions behind the approach. This paper explores some of these assumptions and limitations. Further, in interpreting AVO data to make predictions about the geology, it is important to remember there are two inversions or mappings being done. The first inversion is predicting the elastic parameters from the prestack seismic. The second is predicting the rock and fluid properties from the elastic parameter estimates. Each mapping has its own issues of reliability and uniqueness.

Often a linearized approximation of the Zoeppritz equation is used as the model to predict the elastic parameters from prestack seismic. This imposes certain restrictive assumptions. To meet these assumptions the seismic data must be properly processed. Failing to do so will result in AVO anomalies not related to the geology. The elastic parameters that are estimated are bandlimited. This further complicates the analysis of the data and creates a number of pitfalls.

In the literature there are a variety of AVO techniques, which make assumptions about the rock physics and the relationship between the reservoir and the surrounding materials. If these assumptions are incorrect, this can lead to erroneous interpretations. The rock physics of the play must be understood to make reasonable predictions.

Because of these issues, this paper advocates interpreting AVO at various stages; the prestack gathers, the reflectivity sections, and inversions based on the reflectivity sections. This approach includes both forward modeling and inversion.