Background

All rock physics models require a number of inputs, including porosity and mineralogy. Then there is a host of models to choose from. The procedure of selecting a model based on a training dataset (e.g., well data) has been discussed and utilized in the previous chapters. Once the model is selected, it allows us to explore various “substitution” scenarios, such as lithology, porosity, and reservoir substitution, usually called “what if” perturbations of the original data.

Perhaps the first rock physics substitution equation is that by Gassmann (1951). It is commonly used now to predict the response from a reservoir with a hypothetical “what if” pore fluid from that measured in the well. But can such fluid substitution be conducted directly on the seismic amplitude? Li and Dvorkin (2012) show that it can be done, at least approximately and within a set of assumptions that include establishing a rock physics model relating the elastic properties to porosity and mineralogy. The question posed is whether there are simple recipes that can guide us in predicting a reflection at the shale/gas-sand interface if the reflection at the shale/wet-sand interface is known (and vice versa).