Abstract

In 1992 Huffco Brantas initiated a project to study the effects of porosity on seismic velocity and amplitude in three Madura Strait reefs, BD-1, KE-11E and KE-11C. The goal was to establish that carbonate rocks with high and low porosity produce different seismic signatures and amplitudes.

Extensive seismic modelling was employed using the Shuey-Hilterman approximation of Zoeppritz's equation for computing reflection coefficients at zero offset. Specifically the article shows that polarity and amplitude encoded in the normal incidence trace derived via AVO decomposition can help to identify porosity in the carbonate rocks.

The main conclusions from this study are:

1. In a high porosity carbonate, acoustic impedence can be similar and in some cases lower than that of the overlying sediments, whereas, acoustic impedence in a low porosity carbonate is usually much higher than in the overlying rocks.

2. Reflection amplitude from a shale/high porosity carbonate interface to a shale/low porosity carbonate interface may change polarity due to variation of acoustic parameters on either side of the interface.

3. For high porosity rocks the reflection amplitude is normally low at Normal Incidence (NI) and may either increase or decrease in amplitude with offset.

4. In low porosity rocks, the reflection amplitude is high at Normal Incidence (NI) and may remain so on all offsets.

Data processing is a very important part of AVO study. It is essential to recognise the various factors which affect amplitude and apply appropriate corrections before undertaking any AVO study.