A simplified shaly sand analysis that uses a resistivity-one porosity log combination is discussed. The method involves estimation of shaliness by conventional techniques and, if necessary, porosity corrections for gas effects. A "pseudo-q" factor, which compares favorably with q-values from the Density-Acoustic combination, provides a basis for determining cutoff permeability. Computed water saturations compare well with the conventional Swq-equation.
The new method is applicable to minimum logging combinations as frequently run in development wells.
Log analysis of shaly pay sands has received considerable attention. In many investigations, the clays and fluids occupying the interstices of the sand matrix are treated as a slurry, and the effect of both clay and hydrocarbons is taken into account. Based upon the dispersed clay model, the formation water saturation (Swq) has been estimated successfully in the U.S. Gulf Coast area for many years.
Resistivity of the disseminated clay (Rc)is not exactly constant. However, an average value should be useable for a given geological area, since under most circumstances even relatively large changes in the value of Rc cause only small variations in the computed water saturation. Generally, the resistivity of clays dispersed in the pore space of a reservoir rock is less than that of adjacent shales. Values of Rc = 0.5 Rsh for the shallow Wilcox of Louisiana and Mississippi have been given. For the Gulf Coast we recommend the use of Rc = 0.4 Rsh, which closely agrees with the concept of multiple Rw-values proposed previously. previously.
Considering the response of porosity tools, such as the Density and Acoustic, to the shaliness of a formation, it is possible to compute water saturation, with one porosity tool only. In addition, it is then possible to derive a pseudo-q value, which gives an indication for the amount of clay in the formation and, therefore, estimates the likelihood for reservoir producibility. Response of Density and Acoustic logs in shaly, water bearing sands can be expressed such as: