Using converted shear waves to image reservoirs with low-impedance contrast

Abstract

Imaging sand/shale sequences is problematic when the impedance contrast between them is very small, a problem commonly encountered in basins with Tertiary clastics. Impedance trends for sand and shale in these basins display higher impedances for shales than for sands at shallow depths; however, sands exhibit higher impedances than shales at greater depths. That implies a crossover between the trends at some depth and that it is going to be difficult to image sands near the crossover depth with traditional seismic surveys. Sometimes stacking far-offset data is a solution because a strong AVO gradient can generate significant reflections at high angles of incidence, even if the normal-incidence reflection coefficient is close to zero. Converted waves can, in some cases, image the reservoir. The question is when because there is no generally accepted insight regarding what conditions favor converted-wave imaging. In other words, why does converted-wave imaging work very well in some reservoirs and not so well in others? Three conditions must exist for converted shear-wave imaging to be preferable to conventional P-P imaging: 1. A very small acoustic impedance contrast results in very weak reflections and thereby poor images. 2. The offset dependence of P-P amplitudes is not useful because (a) a small contrast in Poisson's ratio across the contact between seal and reservoir makes the P-P AVO gradient small, resulting in weak amplitudes at all offsets, or (b) velocity distribution in the overburden makes it impossible to achieve large angles of incidence at the reservoir level. 3. A large shear-wave impedance contrast generates strong converted waves. The first point simply states the obvious—converted-wave imaging is cost effective only when traditional P-P imaging fails, and P-P imaging fails when the acoustic-impedance contrast is close to zero. Point …