A New Approach for Low-Resistivity Sand Log Analysis
Authors: Murphy R.P. , Owens W.W. 
Published: December 2007

Certain sands may possess low electrical resistivity and yet may be capable of producing oil. A method for recognizing them involves deriving for each formation an empirical correlation that relates the irreducible water saturation to pore space surface area or formation shaliness. Rock sample analyses and logs are needed to calibrate the interpretation method for any specific application.

Introduction
Some oil-productive horizons have such low electrical resistivities (generally less than 1 ohm-m) that to distinguish them from water-producing zones is difficult, if not impossible, using standard log interpretation procedures. The reason for the difficulty is that these procedures. The reason for the difficulty is that these procedures normally result in the calculation of high procedures normally result in the calculation of high formation water saturation (50 percent or greater). The accuracy of the calculation may be doubtful or its relevance to the formation irreducible water saturation may be unknown. Experience has shown that the high electrical conductivity (low resistivity) of these formations is due to the combined effects of high-salinity or low-resistivity brine in the pore space, high formation porosities, high content of formation shale or clay, and high interstitial water saturations.
A variety of log interpretation procedures have been proposed in the literature to solve this problem of low-resistivity or shaly sand. These procedures have been divided into two broad types.
Type I - The water saturation is calculated from resistivities measured in the mud-filtrate-invaded portion of the sand and from resistivities measured portion of the sand and from resistivities measured in the uninvaded sand, and includes corrections for shaliness usually derived from the SP log. Although water saturations are not usually calculated directly for both the mud-filtrate-invaded zones and the uninvaded zones, the interpretation procedures are based upon the change in water saturation resulting from the mud-filtrate invasion. Therefore, if the water saturations calculated for the invaded and the uninvaded zones are about the same, this suggests that the horizon will be water productive since no hydrocarbon was moved by the invading fluid. However, if the water-saturation near the wellbore is higher, then this indicates that the formation contains movable hydrocarbons (which have been displaced by the mud filtrate) and will likely be oil productive. Some of the log interpretation procedures that fall in this group are the Rocky Mountain method and the procedures proposed by Poupon et al. and by Patchett and Rausch. proposed by Poupon et al. and by Patchett and Rausch. A major disadvantage of log interpretation procedures of this type is that retable measurements of the resistivity of the mud-filtrate-flushed or mud-filtrate-invaded zone must be obtained. Since mud-filtrate invasion into shaly intervals is usually quite shallow, these procedures are frequently ineffective.
Type II - The second type of log interpretation involves calculating the water saturation from resistivities measured in the formation beyond the zone invaded by mud filtrate and includes corrections for shaliness derived from the response of other logs. The successful application of this approach demands a knowledge of the magnitude of the formation irreducible water saturation.
JPT
P. 1302