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 [Formula: see text] and the contrast in Poisson’s ratio at the reflector. Of these two factors, [Formula: see text] is the least constrained. Based on their AVO characteristics, gas‐sand reflectors can be grouped into three classes defined in terms of [Formula: see text] at the top of the gas sand. Class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for [Formula: see text]. Class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of [Formula: see text] near zero. Class 3 sands have lower impedance than the encasing shale with negative, large magnitude values for [Formula: see text]. 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.