INTRODUCTION

The reflectivity properties of layers of sand and shale are described by three classes: Class I contains those shale over sand boundaries with high positive P-wave reflection coefficients, Class II has near zero coefficients and Class III has large negative reflection coefficients Rutherford and Williams (1989).

Interfaces at the same CMP position, but at different depths, may show all three effects if there is a crossover in the well log data. A crossover means that if the sand layers nearest the surface have higher porosities than the adjacent shale layers, then the sand layers deepest in the well have lower porosities than the adjacent shale layers. Velocity data exhibit a similar behavior in well logs that cross over. If the P-wave velocity of the sand layers is faster than those of shale at the surface, then the deepest layers have higher P-wave velocities in the shale than in sand layers. In such a well we can study the systematic AVO response as the sand and shale layers change from Class I to Class II and finally to Class III or alternatively from Class III to Class I.

I studied the AVO response of one particular well, which was deep enough ($\approx 5$ km) that crossover was likely. The aim of the project was to study the trends of AVO response for for three different saturating fluids. As the actual well was saturated by only one fluid (water), I approximated the V_p and V_s trends of the same well with the sand saturated with oil and then with with gas. These calculated values were used to model the AVO response with depth for saturation by these other fluids.