Lithology color-coded seismic sections: The calibration of AVO crossplotting to rock properties

In the late 1960s, several oil companies noticed that, in environments of young clastic sediments, large seismic amplitudes were associated with gas-saturated sands. This method of correlating lithology to normal incidence (NI) reflectivities was appropriately named the bright spot technique. However, it quickly became apparent that not all large amplitudes were necessarily associated with gas reservoirs and, much to our chagrin, not all gas reservoirs had large amplitudes. For the next decade, geophysicists tried numerous techniques to resolve the ambiguity associated with lithologic identification by means of the seismic attribute NI. This met with various degrees of success. Finally, Ostrander’s work on amplitude variation with offset (AVO) led to the development of another seismic attribute(s) which improved our discrimination of lithologies (see “Plane-wave reflection coefficients for gas sands at nonnormal angles of incidence,” Geophysics 1984). But proper application of this new attribute along with NI reflectivity raised several questions. Among them were “How are the two seismic attributes related to rock properties which can then be correlated to lithology?” and “How can two seismic attributes be combined into a single display parameter?”