Comparison of AVO indicators: A modeling study

Abstract

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‐incedence reflection coefficents [Formula: see text] and [Formula: see text]), AVO intercepts (A), AVO gradients (B), the AVO indicators [Formula: see text] (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 clastic stratigraphic intervals. For shale over brine‐sand reflections, the average [Formula: see text] tends to be near zero and relatively invariant with depth. Irrespective of gas‐sand impedance, [Formula: see text] 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 [Formula: see text] is well approximated by a simple linear combination of A and B.