Classification of Carbonate Reservoir Rocks and Petrophysical Considerations

Limestone reservoirs found at increasingly greater depths, with accompanying higher per well cost of production, have made the problem of determining net pay thickness of limestone reservoirs more exacting. In discussing the petrophysics of limestones, it is necessary first to classify them in a manner to portray as much as possible the essential pore characteristics of a reservoir. A skeleton classification with symbols is presented and coordinated with petrophysical data, such as porosity, permeability, capillary pressure, and electrical resistivity of water-bearing and oil-bearing limestones. Petrophysical relationships of some limestones are discussed. The application of these relationships to field practice is much more difficult than for sandstones because of the heterogeneity and great amount of dense limestone generally encountered in the gross reservoir interval. This increases the so-called geometrical variables, making well-logging interpretation difficult. The usefulness of the electrical resistivity curve in detecting productive parts of a limestone reservoir will vary, for it is shown that the resistivity of the productive part of a limestone formation may be greater than, less than, or the same as, the resistivity of the dense, non-productive parts. This is due mainly to the variation of porosity (or permeability) with water saturation, i.e., pore-size distribution.