Robert Jeffery, Mark Thomsen
A method for predicting lateral changes in reservoir porosity using 3-D seismic amplitudes, calibrated against the amplitude response versus porosity measured at a select number of wells, was implemented and applied to produce a porosity map of a Saudi Arabian carbonate reservoir. The technique relies on the uniform lithologic seismic response of an overlying anhydrite, and thus assigns variations in amplitudes at the reservoir level to changes in reservoir average porosity.
Throughout the study area, reservoir porosity and acoustic impedance logs exhibit a firm linear relationship. As reservoir porosity increases, its acoustic impedance decreases, and the greater contrast with the overlying anhydrite translates into larger seismic amplitudes. Thus, we expect the reservoir's relative amplitude response to also increase linearly with increasing porosity. A check on this hypothesis was provided by computing synthetic seismograms at several wells, and measuring the reservoir's theoretical amplitude response versus porosity averaged over the producing zone within the reservoir. This trend supported a linear seismic amplitude to porosity transform.
Upon verification of the technique's applicability, the reservoir's amplitude response was extracted from the 3-D seismic volume in the vicinity of several wells. These were used in conjunction with porosities averaged over the reservoir to derive the amplitude to porosity transform. This transform was used in converting the mapped reservoir amplitudes into variations in average porosities. The success ratio for predicting porosities in wells not used in the analysis was nearly perfect, and the map continues to correctly predict porosities in subsequently drilled wells.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France