What are the challenges in predicting lithology directly from acoustic impedance?

Question

Seismic inversion methods help to visualize the subsurface layers of the earth in terms of petrophysical properties (e.g. impedance). Still some geophysicists use the final inverted impedance model in company with various seismic attributes (like amplitude envelope, instantaneous frequency, and phase) to predict lithology or paleogeographic features. If seismic inversion transforms the reflection amplitude into acoustic impedance (lithology) than why can't it be used to predict the lithology independently?

Answer 1

Because acoustic impedence != lithology. Lithology is a complex entity, in almost all geological settings. IF you're looking at a zone that has two lithologies, and they are usually thickly bedded and clearly separated with little likelihood of interfingering (thin layers of each, interbedded) then MAYBE you can convert acoustic impedance into lithology with a fair degree of accuracy. But when you start mixing lithologies and with gradational contacts and thin beds all hope of an accurate lithology model from impedance goes out the window. In other words, you can make very expensive (dry hole) mistakes by believing the lithology models generated solely from acoustic impedance.