The addition of the photoelectric index curve to the gamma ray and neutron-density log overlay provides both additional validation of simple lithology picks and the resolution of ambiguities in interpretation of "complex" (multimineral) lithologies. The generalized expectations of log patterns for shales and endmember reservoir lithologies of limestone, dolomite, and sandstone are shown in the figure.
In the examination of the neutron-density log overlay, dolomites and siliceous rocks (either sandstones or cherts) can be recognized by the curve separations. However, the close overlay of the two could be caused either by a limestone or a cherty dolomite (or a cherty dolomitic limestone!). The inclusion of the photoelectric index can be used to choose between these alternatives. Similarly, a dolomite reading on the photoelectric index curve could also be caused by a cherty or sandy limestone. The simultaneous consideration of the neutron-density log overlay resolves the more likely of these two interpretations. An example of the log interpretation of simple and "complex" lithologies is described on the next page.