Seismic Lithology Analysis Using Acoustic Impedance Vs. Poisson Ratio Crossplots

James P. Disiena, Fred J. Hilterman, and Richard W. Verm

Case studies demonstrate predictions of lithology, porosity and pore fluid variations for relatively high acoustic impedance (^rgrV) gas-sands. We utilize amplitude versus offset analysis (AVO) along with crossplotting well log and seismic parameters. These approaches extend application of AVO to non-bright spot sands where gas-sands ^rgrV equals or even exceeds shale ^rgrV. It also utilizes Poisson Ratio estimates (^sgr) from Poisson Reflectivity analysis to overcome limitations in stack seismic inversion. One study investigates a Gulf of Mexico Miocene gas sand. The 110 ft target zone at ~14,000 ft consists of a high velocity, and low-density with respect to the encasing shale. Using a color crossplot display, different colors portray gas-sands, wet-sands and high ^rgrV shale/shale boundaries. Location of the sand colors appears consistent with wells and with sand deposition along growth faults. Another study targets channel sands that lie beneath a Cretaceous shale-chalk section. These high ^rgrV channel sands interfinger with debris-flow facies to reach a thickness of 160 ft, with net pay of 69 ft. Maximum porosity is 22% with pay reservoir quality to 9%. The channel facies provide excellent reservoir quality, while intergranular detrital clay in the debris-flow facies reduces it. A ^rgrV - ^sgr inversion discriminates these facies. Near the productive wells, ^sgr shows a significant drop from the background. Also, crossplot displays reveal that these sands occur within a canyon-like feature. With this definition of geometry and seismic facies, we have the potential to predict well performance in areas where interfingered sands are below seismic resolution.

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