AVO Analysis of High-Impedance Sandstone Reservoirs

Carolyn P. Peddy and Mrinal K. Sengupta

We present case studies of high-impedance gas sands in order to investigate the possibility of using AVO analysis in areas such as the Mid-Continent. The case studies illustrate some of the differences in approach that must be used in order for AVO analysis to be used effectively in high- impedance reservoirs. Sands in the Morrow Formation of the Anadarko basin vary in local properties, significantly affecting the AVO response observable on seismic data. The reflection from the top of a gas-charged Woodbine sandstone shows a strong gas effect that is consistent at four well locations along a seismic line. Aspects of rock properties as they relate to AVO analysis in high-impedance reservoirs indicate that AVO may be a powerful lithology discriminator in higher velocity roc s.

Several issues need to be understood by the explorationist using AVO for high-impedance gas sand exploration. For example, the A*B plot used in AVO interpretation in the Gulf Coast does not provide useful information in the AVO analysis of high-impedance sandstone reservoirs. Normal- incidence reflection coefficients are not direct hydrocarbon indicators as in the Gulf Coast A limitation of AVO analysis for gas detection in high-impedance sands is that there is often very little difference between the wet sand Poisson's ratio and the gas sand Poisson's ratio. As the Vp of a pure sand increases, the Poisson's ratio of the sand decreases. However, in many Mid-Continent trends, sand channels are fully charged, and the exploration problem is finding the location of the sand channel. In th se cases, AVO is an excellent exploration tool, as the difference in Poisson's ratio between rocks of varying lithologies tends to increase as Vp increases.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California