AVO for thin sand bed detection

Abstract

Shelf-deposited sand ridges enveloped in marine shale are common oil traps in the upper Cretaceous sedimentary section of the Powder River Basin, Wyoming. Seismic detection of this class of oil trap is difficult because the reservoir sands average less than 10 m in thickness and they commonly occur at depths of 2500 m or more; they are therefore too thin to image given the frequency content of most seismic data sets. The presence of the sand ridge sequences can be detected however, by means of Amplitude-Variation-With-Offset (AVO) analysis. The AVO effect is a result of the difference in Poisson’ s ratio between clean, fluidfilled sand and marine shale. Both synthetic seismograms and field data recorded over Hartzog Draw field in the Powder River Basin, Wyoming show that reservoir sands that are less than 10 m in thickness produce an observable amplitudevariation-with-offset effect on the reflected wavefield. The results of this work show that amplitude-variation--with-offset techniques can be used as an exploration tool for thin sandsdthat are enveloped in marine shale. The technique also has potential uses for development of newly discovered reserves that are trapped in such sand-shale sequences. Amplitude-Variation-With-Offset (AVO) analysis is a seismic technique by which the lithology of the subsurface is studied by analyzing seismic reflections at various offsets in CDP gathers (Ostrander, 1984). The technique is based on the physical principle that the reflection coefficient at the interface between two rock layers varies with increasing angle of incidence when the ratio of P-wave velocity, Vp, to S-wave velocity, Vs, of one of the layers is significantly different from the Vp/Vs ratio of the other layer (Koefoed, 1955). In order to take full advantage of the AVO technique, the seismic data must be carefully processed so that any variation in amplitude is due solely to reflection coefficient changes and not some physical process such as attenuation or the result of a data processing step (Yu, 1985).