Abstract

Determination of hydrocarbon saturation in fresh water tertiary reservoirs of Upper Assam, Basin has long been a challenge because of the complexities of the electrical conductance mechanism varying in the range from Archie, non-Archie to severely non-Archie behavior of electrical conduction, where conventional shaly sand equation breaks down. This is mainly due to very large facie variations associated with varying grain sizes and presence of mica or combination of feldspars and heavy minerals in addition to quartz and clay. When the reservoir brine is relatively fresh, the evaluation problem is compounded because the effect of grain size/clay in proportional to the resistivity of the brine. Formation water exhibits a very large variation of salinity from ~1.0 kppm in Tipam Sand-1 to ~10 kppm in Sylhet/Kopili formations. Whether variation on resistivity is predominantly controlled by water volume or change of formation resistivity factor, it has to be ascertained beforehand through visual correlation on log motifs. Principal benefits of co-variances of log motifs has been the identification of presence or absence of surface conductance phenomenon associated with grain size variations followed by application of appropriate model for water saturation estimation in these reservoirs.