INTRODUCTION Over the last two decades advances in the interpretation of shaly pay sands have been along a broad front. This has been mainly due to (1) new basic research, (2) development of new and advanced logging tools, (3) introduction of major new tech- niques, (4) improvement of known methods, and (5) application of digital data handling for otherwise tedious mathematical computa- tions using sophisticated interpretation methods. Progress in all of these areas is substantial and continuing. Today properly selected logging combinations plus specific interpretation methods allow the log analyst to determine several in situ reservoir pa- rameters on both a qualitative and quanti- tative basis. These parameters include porosity, shaliness, amount and type of hydrocarbons present in the reservoir rock, approximation of in situ permeability and the initial water cut upon completion, etc.