Predicting Abnormal Reservoir Pressures using offset Dependent Reflectivity: Theoretical Considerations

The concept of using amplitude variations with offset (AVO) for the direct detection of hydrocarbons was introduced to the oil and gas industry in the early eighties. In the late eighties Rutherford and Williams proposed a model representing different classes in normally pressured gas sand formations. In this paper, a model for the AVO response for overpressured clastic formations is proposed based on the rationale that abnormally high pore fluid pressure leaves its own pronounced effects on clastic formations: such as abrupt increase in porosities, decrease in interval velocities, high fluid content, and lower bulk densities. The implication is that overpressured formations should show a different trend for AVO response compared to normally pressured formations. The AVO response in overpressured formation was modeled using both synthetic well data representing an overpressured sandstone and also typical values of seismic velocities reported from overpressured formations. The results suggest that a characteristic polarity reversal from negative to positive reflection coefficients can be observed as the offset increases.