Hydrocarbon indicators derived from AVO analysis

Hong Feng

Numerous approaches have been published which derive fluid indicators, often called direct hydrocarbon indicators (DHI), from AVO (amplitude versus offset) equations. The main idea behind these methods is to use linearized approximations to the Zoeppritz equations to extract petrophysical parameters, such as P-wave and S-wave impedance and velocity, elastic modulii, and then infer the fluid content from cross-plots of these parameters. Russell et al. (2003) used standard poroelasticity theory (Biot, 1941, and Gassmann, 1951) to generalize several of these methods using a scaling factor: (Vp / Vs )2 dry , which is dependent on the properties of the dry rock. The purpose of this study is to study the generalized fluid method proposed by Russell et al. (2003) to find the optimal value for (Vp / Vs )2 dry used in this method.

The effect of clay content, porosity, and pressure on the P-wave and S-wave velocities and (Vp / Vs )dry ratio for sandstones will be investigated in this study. Also, previous fluid methods will be summarized and compared with the generalized fluid method to study which indicator can most easily discriminate a gas or oil sand from the background and which indicator is most sensitive to the pore-fluid content.