Problem on water saturation evaluation of shally sand reservoirs has been widely known amongst log analysts. From 1950 up to now, a large number of models relating resistivity and fluid saturations have been proposed, however, there is no one of them can be applied satisfactorily. The main problem is that the formation factor (F) of shally sand tends to be inconsistent as the water resistivity (Rw) changes. To solve the problem, all previous workers thought that the Rw remain unchanged after penetrated within the rock so that uncertainties of F value is caused by effect of shales to Ro (resistivity rock saturated with water of resistivity Rw). In this study, the writer suggests that water salinity must be changed when occurred in the rock-pores due to chemical dissolution of some shale components especially clay minerals. A short study on formation water carried out by the writer reflects that there is very close relation between Rw and shale content (Vshale). based on this opinion, therefore, there is no shale effect corrections needed to define the intrinsic Formation Factor (F), and more than that, procedure on fluid saturation evaluation can much be simplified with accurate results.
Author : Hastowidodo