COMPARISON OF AVO INDICATORS - A MODELING STUDY

A worldwide collection of 25 sets of velocity and density measurements from adjacent shales, brine sands, and gas sands was acquired with full-waveform sonic, dipole sonic, and conventional well logging devices and/or in the laboratory. These data provide theoretical shale over brine-sand and shale over gas-sand P-wave and S-wave normal-incidence reflection coefficients (R(P) and R(S)), AVO intercepts (A), AVO gradients (B), the AVO indicators R(P) - R(S) (reflection coefficient difference), and A * B (AVO product). The reflection coefficient difference is found to be a more universal indicator than the AVO product in elastic stratigraphic intervals. For shale over brine-sand reflections, the average R(P) - R(S) tends to be near zero and relatively invariant with depth. Irrespective of gas-sand impedance, R(P) - R(S) is always negative for shale over reservoir quality gas-sand reflections and more negative than for the corresponding brine-sand reflections. In comparison, the AVO product may be positive, near zero, or negative for gas-sands depending on the impedance contrast with the overlying shale. These measurements also verify that R(P) - R(S) is well approximated by a simple linear combination of A and B.