Amplitude-versus-offset variations in gas sands

Abstract Seismic reflections from gas sands exhibit a wide range of amplitude-versus-offset (AVO) characteristics. The two factors that most strongly determine the AVO behavior of a gas-sand reflection are the normal incidence reflection coefficient R 0 and the contrast in Poisson's ratio at the reflector. Of these two factors, R 0 is the least constrained. Based on their AVO characteristics, gas-sand reflectors can be grouped into three classes defined in terms of R 0 at the top of the gas sand.Class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for R 0 . Class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of R 0 near zero. Class 3 sands have lower impedance than the encasing shale with negative, large magnitude values for R 0 . Each of these sand classes has a distinct AVO characteristic.An example of a gas sand from each of the three classes is presented in the paper. The Class 1 example involves a Hartshorn channel sand from the Arkoma Basin. The Class 2 example considers a Miocene gas sand from the Brazos offshore area of the Gulf of Mexico. The Class 3 example is a Pliocene gas sand from the High Island offshore area of the Gulf of Mexico.