Lithology and Porosity Estimation

Porosity determination using most of the logging devices presented in earlier chapters relies on a knowledge of the parameters related to the type of rock being investigated. In the case of the density tool, the density of the rock matrix must be known. The matrix travel time is used in interpreting the compressional wave interval transit time. In order to reflect porosity accurately, the matrix setting for the neutron tool must correspond to the rock type for the value of ?n. Determining these parameters is not much of a problem if one has good geological knowledge of the formation and if the lithologies encountered are simple, such as a clean sandstone or limestone reservoir. However, what do you do when you are uncertain of the lithology, or if it is known to vary considerably in its composition, as in the case of limestone formations with variable inclusions of dolomite and anhydrite, or a sandstone with substantial calcite cementing?